Climate change is associated with higher phytoplankton biomass and longer blooms in the West Antarctic Peninsula

Domain: earth_science
Published: 2024-08-02
Editorial outcome: minor_revision

Abstract
The Antarctic Peninsula (West Antarctica) marine ecosystem has undergone substantial changes due to climate-induced shifts in atmospheric and oceanic temperatures since the 1950s. Using 25 years of satellite data (1998-2022), this study presents evidence that phytoplankton biomass and bloom phenology in the West Antarctic Peninsula are significantly changing as a response to anthropogenic climate change. Enhanced phytoplankton biomass was observed along the West Antarctic Peninsula, particularly in the early austral autumn, resulting in longer blooms. Long-term sea ice decline was identified as the main driver enabling phytoplankton growth in early spring and autumn, in parallel with a recent intensification of the Southern Annular Mode (2010-ongoing), which was observed to influence regional variability. Our findings contribute to the understanding of the complex interplay between environmental changes and phytoplankton responses in this climatically key region of the Southern Ocean and raise important questions regarding the far-reaching consequences that these ecological changes may have on global carbon sequestration and Antarctic food webs in the future.

Introduction
Marine ecosystems along the coast of the Antarctic Peninsula (West Antarctica; Fig. 1) have been undergoing significant change over the past few decades. Among the most important is the rise of atmospheric and oceanic temperatures due to global warming, particularly during the winter, which has led to the generalised retreat of glaciers, ice shelves and to a decrease in the extent, thickness, and volume of sea ice. All marine trophic levels have been impacted by these changes. Concerning primary producers, alterations in phytoplankton biomass, composition, and cell size have been reported, with potential consequences for the West Antarctic food web. Nevertheless, phytoplankton communities appear to be affected differently along the Western Antarctic Peninsula (WAP). In the southern-mid WAP (south of Anvers Island; 64°33′S), studies have shown increases in phytoplankton biomass and cell size, while in the northern Peninsula, the current consensus is that phytoplankton biomass and cell size are decreasing. However, it should be noted that the phytoplankton dynamics in the northern part of the Peninsula have been less studied overall and are currently much less understood than the southern-mid WAP.

While the continuous in-situ observation of marine phytoplankton in the WAP has been undertaken since the second half of the 20th century, the latter was mostly restricted to coastal sites along the WAP in the proximity of active research stations and to oceanographic expeditions during the austral summer. This raises difficulties in understanding whether rapid biological changes are already taking place in the region. Ocean colour remote sensing provides a complementary tool to in-situ sampling, which has already proven to crucially contribute to the unravelling of short- and long-term phytoplankton patterns in the ocean at both regional and global scales. Nonetheless, applications of satellite data to Antarctic coastal waters have been hampered by limited data availability due to persistent cloud cover, sea ice, and low solar angles, inter alia. Additionally, global satellite algorithms have consistently underestimated chlorophyll-a (Chl-a; a proxy for phytoplankton biomass) in the Antarctic Peninsula by up to a factor of 2.

In this study, we implement the new OC4-SO regional algorithm for Chl-a, which was specifically calibrated for the Antarctic Peninsula using the largest high-quality in-situ regional dataset to advance existing knowledge of phytoplankton dynamics and the impact of climate change on primary production in this critical Southern Ocean region. We use 25 years of continuous multi-sensor remote sensing data (ESA OC-CCI) to disentangle patterns of phytoplankton biomass and bloom phenology throughout the Antarctic Peninsula marine ecosystem in relation to regional atmospheric and oceanographic changes related to ongoing climate change. We observe enhanced phytoplankton biomass along the WAP, particularly in the early austral autumn, resulting in longer blooms. Long-term sea ice decline appears to be the main driver enabling growth in early spring and autumn, in parallel with a recent intensification of the Southern Annular Mode (SAM; 2010-ongoing).

Results
Phytoplankton biomass dynamics along the Antarctic Peninsula
Five marine subregions with distinctive seasonal phytoplankton patterns along the Antarctic Peninsula were identified after applying a hierarchical clustering analysis to key phytoplankton biomass and bloom phenology metrics (Fig. 2). From north to south: (i) DRA, which includes waters in the southern Drake Passage, north of the Southern Antarctic Circumpolar Current Front (SACCF; Fig. 2a), (ii) BRS, which extends from Elephant Island to the offshore waters south of Anvers Island, including the Bransfield Strait and the South Shetland Islands (Fig. 2b), (iii) WEDN, which is situated between the Bransfield Strait and the outer NW Weddell Sea, off the Northern Antarctic Peninsula (Fig. 2c), (iv) GES, which contains the coastal waters stretching south from the Gerlache Strait (Fig. 2d), and (v) WEDS, the NW Weddell Sea coastal waters east of the Antarctic Peninsula, within the Larsen A and B embayments (Fig. 2e).

Phytoplankton blooms off the Antarctic Peninsula generally began in late October or November, with peaks in productivity occurring between December and February and ending in late March or April, coastal locations showing later blooms and higher Chl-a peaks (Fig. 2; Supplementary Table 1). DRA, the northernmost and most open ocean subregion, was where phytoplankton started to bloom earlier, right after austral winter. The low biomass levels and early short-lived biomass peak observed in this subregion suggest that micronutrient limitation (potentially iron) may occur shortly after December, corroborating previous in-situ studies that have reported lower biomass in the southern Drake Passage associated with iron limitation, a characteristic of the more oceanic waters off the WAP during summer. The nature of phytoplankton dynamics among subregions is also related to their physical environment. Regions within the WAP exhibited generally warmer waters, lower sea ice coverage, and stronger winds than the regions in the eastern sector of the Antarctic Peninsula (Supplementary Fig. 1; Supplementary Table 2). Sea ice concentration was the primary factor influencing the timing of bloom initiation around the Peninsula (Supplementary Fig. 2; p value < 0.05), as regions with more sea ice during spring were often associated with later blooms (e.g., WEDN, GES, and WEDS). The importance of environmental conditions in shaping phytoplankton dynamics was also evidenced in BRS, as seen by its high variability in both bloom phenology and biomass. Despite its relatively low sea ice concentration, the Bransfield Strait, at the centre of BRS, exhibits a complex current system that includes cyclonic and anticyclonic eddies, surface and subsurface thermal fronts, as well as intrusions of warmer waters from the west (the Circumpolar Deep Water; CDW) and colder waters from the northeast (from the NW Weddell Sea), an overall complexity that translates to high spatial and temporal variability.

Enhanced biomass and longer blooms in the WAP
Results from a linear trend analysis indicated that the mean phytoplankton biomass from September to April (austral spring to autumn) has significantly increased between 1998-2022 in the WAP, particularly in the Bransfield Strait (p value < 0.05; Fig. 3a, Supplementary Fig. 3). At the same time, it is not clear from our results whether biomass also changed in the eastern sector of the Antarctic Peninsula, which may be a consequence of the low number of satellite observations in this sector and lower accuracy caused by the high sea ice coverage. The phytoplankton biomass increase observed in the WAP seem to have occurred mainly in the early spring and early autumn (Table 1). This is evidenced by the increases of +0.003 mg m−3 Chl-a year−1 in the DRA and of +0.009 mg m−3 Chl-a year−1 in the BRS during September (early spring). This was even more the case during early autumn (March and April), when biomass trends were nearly three times higher compared with the early spring: +0.009 mg·m−3 Chl-a year−1 in DRA, +0.026 mg m−3 Chl-a year−1 in BRS, and +0.061 mg m−3 Chl-a year−1 in GES. Therefore, most of the biomass increase in the WAP occurred in early autumn, which corroborates the higher average autumn biomass observed in 2011–2020 compared to 2001–2010 (Fig. 2a, b, d).

The increase in biomass seen in the GES partially corroborates the findings of previous in-situ and satellite studies, although for different temporal periods. For instance, Moreau et al. observed an increase in the annual integrated primary production from 1997 to 2010 using exclusively satellite data but found no clear trend in satellite Chl-a. These authors attributed this increase in primary production to the long-term trend towards earlier sea ice retreats and its effects on phytoplankton growth during the spring. Therefore, they did not consider the large phytoplankton growth in autumn that we observed here, which cannot be related to the earlier sea ice retreat. However, Turner et al. using satellite Chl-a from 1997 to 2022, also observed an increase in phytoplankton biomass in the autumn in northern WAP. Previous studies using in-situ data, such as Montes-Hugo et al. (from 1978 to 2006) and Brown et al. (from 1993 to 2017), also observed a similar decadal enhancement of phytoplankton biomass in this region, yet, unlike our satellite-detected trends, they only observed it for the summer (the only period for which the current availability of in situ data allows for more accurate long-term trend assessments). These studies suggested that this biomass increase was a consequence of a more stable upper mixed layer during the summer due to the meltwater resulting from climate-induced sea ice decline and glacier retreat. While our region-wise satellite observations do not show a clear increasing trend of biomass during summer in the mid/southern WAP (GES), the average summer biomass between 2011 and 2020 does appear to have increased in comparison to the previous decade (Fig. 2d). Moreover, a slight increasing biomass trend was also observed for December (although only at a p value = 0.09; Table 1) and pixel-wise trend analyses for the summer do suggest that biomass might be increasing in coastal waters south of the Gerlache Strait, although the significant increase is locally observed (Supplementary Fig. 3). A reanalysis of in-situ Chl-a data collected along the WAP (from Palmer LTER and the Brazilian High Latitude Oceanography Group) also suggests that summer biomass has increased in the BRS (+0.06 mg m−3 year−1) and GES (+0.18 mg m−3 year−1) (Fig. 3b; Supplementary Fig. 4). Therefore, there may have been, in truth, an increase in summer biomass that satellite data did not accurately capture.

Our findings also help to shed some light on the current understanding of the northern WAP (DRA and BRA regions). Our results contradict the hypothesis proposed by Montes-Hugo et al. in 2009, who suggested that regional phytoplankton biomass is decreasing due to the deepening of the upper mixed layer caused by less sea ice concentration and enhanced wind stress, although it is important to note that this study compared the period between 1978–1986 and 1998–2006. Other phytoplankton ecology studies have since echoed this hypothesis, although no further study has confirmed it using long-term datasets. Here, however, we demonstrate that phytoplankton biomass is increasing since 1998, an increase concurrent with wind stress (Supplementary Fig. 5), which suggests that phytoplankton growth has not been hindered by a long-term deepening of the mixed layer resulting from stronger winds. Our observations is supported by recent studies reporting localized long-term increases in summer biomass in Potter Cove (King George Island; 62.2382°S, 58.6673°W) and the region between the South Shetland Islands and the Elephant and Clarence islands, although neither of these works report increases in autumn biomass. Contrarily, Turner et al. observed a recent increase in autumn biomass in the northernmost waters of the WAP, yet did not report long-term increase in annual Chl-a.

The changes in phytoplankton biomass have led to changes in bloom phenology (Table 2), with blooms ending later in two of the three studied sub-regions of the WAP (+0.4 weeks year−1 in GES and +0.1 weeks year−1 in DRA). As a result, blooms have become longer in both regions, thus resulting in higher total biomass production, particularly at the DRA (p value < 0.05). While the timing of the bloom termination changed, there were no apparent shifts in the timings of bloom initiation or bloom peak. Surprisingly, no trends were observed for BRS despite also being a region where biomass increased significantly in the autumn. This was likely due to the high interannual variability associated with bloom timing in this region, which might prevent the identification of linear trends within the 25 years of our dataset. Previous long-term studies on phytoplankton bloom phenology in the WAP are rare due to the scarcity of data that characterizes the region. Using +20-year data (1992-2016) from three different stations along the WAP, Kim et al. showed that the northernmost station (Carlini Station; BRS region) exhibited a longer and less pronounced (i.e., with a lower biomass peak) bloom than the other stations (Palmer and Rothera), in line with our results. However, these authors did not assess long-term changes in bloom timing. Recently, Thomalla et al. suggested that there has been an overall tendency between 1998 and 2022 for blooms in the more heavily ice-influenced regions of the Southern Ocean to initiate earlier and have longer durations, which contradicts our results for the WAP. It is important to mention that Thomalla et al. focused on the entire Southern Ocean, thus being less detailed at a regional scale, unlike the present study for the WAP. Another recently published study, focused on the WAP, also reported that phytoplankton blooms in the marginal ice zone and the continental shelf have shifted later, corroborating our results and reinforcing the importance of regional studies for the understanding of phytoplankton phenology in complex regions.

Sea ice decline is enabling phytoplankton growth
The changes observed in phytoplankton biomass and bloom dynamics in spring and autumn can be mainly attributed to the long-term decline in sea ice coverage seen along the WAP over the past four decades, particularly during the late autumn – early winter transition. A decrease in the mean sea ice extent was seen in both spring and autumn periods from 1982–1997 to 1998–2022 for each region of the WAP (DRA, BRS, and GES; Fig. 4a; Supplementary Fig. 6). A key consequence of this decline is that sea ice retreat (in spring) and advance (in autumn) are occurring increasingly earlier and later, respectively, leading to an increase in the number of sea ice-free days (Fig. 4a). This allows for enhanced phytoplankton growth during spring and autumn in areas previously occupied by sea ice since the additional space and the improved light conditions are now more favourable for phytoplankton accumulation. The observed sea ice decline across the entire WAP likely explain why the phytoplankton growth in the autumn was widespread along the region.

On average, the autumn phytoplankton biomass in the WAP increased from 29% to 42% of the total biomass from September to April, while spring and summer biomass contribution slightly decreased (Fig. 4b). Several reasons can explain this difference. Firstly, the rate at which sea ice advance is becoming later is higher than the rate at which sea ice retreat is becoming earlier, i.e., the amount of the new ice-free days caused by the long-term warming is higher in the autumn than in the spring. Secondly, the advance of sea ice in the WAP is delayed by persistent northerly winds that occur during the autumn, meaning that sea ice coverage tends to be lower in the autumn than in the early spring. As such, there is more new space for phytoplankton to grow and accumulate biomass in the autumn than in the spring. Moreover, the lower sea ice coverage in the autumn may allow for higher light penetration in the upper layers of the water column, promoting phytoplankton growth in waters where biomass could not previously accumulate under the sea ice. Finally, phytoplankton biomass typically reaches much higher concentrations in the late summer compared to the early spring (Fig. 3a, b, d). Apart from the higher temperatures and lower sea ice coverage, a key reason for this is that the phytoplankton biomass already accumulated in the late summer can be maintained throughout the early autumn if enough nutrients are available, while phytoplankton in the early spring must grow from the typically very low winter biomass.

These findings put into perspective the future trends for sea ice extent in Antarctica. In the long term, sea ice is expected to decrease as warming continues, similar to what is also projected for the Arctic. The resulting decrease in the duration of the sea ice season is likely to be followed by enhanced phytoplankton growth at the limits of the sea ice season (spring and autumn). Nevertheless, even if all required nutrients for growth are available, the duration of phytoplankton blooms will ultimately be limited by the low light availability that characterizes the austral winter in Antarctica. In the Arctic, Ardyna et al. showed that recent sea ice loss have triggered fall blooms spatially analogous to what we observe in the WAP, with wind-driven vertical mixing playing an important role in supporting phytoplankton growth. More high-resolution in-situ studies focused on the austral spring and, particularly, autumn are essential for a better understanding of how phytoplankton dynamics will change. In addition, complementary sampling techniques to in-situ sampling expeditions such as multi-parametric (biogeochemical) floats and underwater gliders are becoming more frequent and are expected to contribute significantly to the acquisition of in-situ observations from other seasons besides summer.

SAM as a key driver of regional variability
While the overall declining trend in sea ice explains most of the findings observed in our work, it does not explain some of the regional differences observed along the WAP. For instance, it does not explain why the more statistically significant pixel-wise biomass trends were found in the northernmost and more offshore areas of the WAP (Fig. 2a). This can be mainly attributed to an intensification in the Southern Annular Mode, a key climate mode modulating the marine environment of the WAP, particularly its northernmost sector. Under positive SAM conditions, circumpolar westerlies strengthen, bringing warmer air into the WAP and increasing cloud coverage. In the marine ecosystems of the WAP, positive SAM conditions are typically associated with warmer surface temperatures and reduced sea ice. Since the 1950s, the SAM index has shown a generally upward trend linked to anthropogenic climate change, although a plateau was observed between the late 1990s and the 2000s. Since 2010, however, the SAM index has intensified again and is becoming increasingly positive at a fast rate, as seen by the large increase in the cumulative SAM index (Fig. 5a).

We hypothesize that this recent intensification of SAM may have contributed to drive the observed region increase in phytoplankton biomass as well as the reason why this was more prevalent in the northwestern Peninsula. We observed a positive and significant pixel-wise correlation (considering the average between September-April period) between the average Chl-a concentration and SAM index in the waters along most of the Western Antarctic Peninsula, particularly for the southern Drake Passage and the northern tip of the Antarctic Peninsula (Fig. 5b). As previously mentioned, wind intensification associated with positive SAM results in deeper mixed layers along the Antarctic Peninsula, especially in offshore waters. The increased vertical mixing caused by stronger winds could be increasing micronutrient availability, particularly iron (Fe), for the DRA and the non-sheltered areas of the BRS, which are less affected by sea ice conditions and have a stronger oceanic influence. This would help prevent nutrient limitation in these areas, especially in the autumn, when a greater amount of nutrients has already been consumed, coinciding with the period when most of the biomass enhancement was observed in our study (Table 1). Accordingly, Tagliabue et al. observed that Fe supply from winter mixing is crucial to offshore primary production in the Southern Ocean. Furthermore, the stronger westerlies associated with positive spring SAM can also enhance warm, nutrient-rich CDW intrusions over the shelf during summer. Such intrusions propagate along the shelf, eventually reaching the northern Antarctic Peninsula and its more offshore areas. Several studies have already shown high phytoplankton biomass linked to CDW intrusions, suggesting that these ocean circulation processes coupled with positive SAM trends could be another contributing factor to the increased biomass in these regions.

Contrastingly, we do not observe a positive correlation between SAM and Chl-a in the southernmost coastal WAP (GES; Fig. 5b). Enhanced CDW intrusions have been reported to force and increase glacier retreat due to its warmer temperatures, leading to more frequent and stabilized water column structures during spring and summer months. While such changes in the upper ocean physical compartments may have initially led to an increase in summer biomass, the recent intensification of SAM could be driving shifts in phytoplankton community composition and size structure and, consequently, on biomass accumulation. Recent studies along the WAP have reported frequent shifts from large centric diatoms to smaller-sized cryptophytes in regions under localized glacier meltwater input during the austral summer. As a result, this shift could decrease the total summer phytoplankton biomass accumulation under positive SAM periods, explaining the slightly negative correlation between SAM and Chl-a for the coastal regions observed here. Moreover, it will also be essential to consider the influence the El-Niño Southern Oscillation can have on the interannual dynamics of phytoplankton, as summers following extreme El-Niño events have already been associated with large diatom blooms.

A preferential feeder on large diatoms, Antarctic krill (Euphausia superba) is a key species in the Southern Ocean ecosystem and an essential link between phytoplankton and higher trophic levels such as whales, seals, and penguins. Therefore, localized shifts from diatoms to cryptophytes could likely contribute to shaping the phytoplankton dynamics along the coastal WAP by alleviating the grazing pressure from the krill on phytoplankton biomass accumulation, while also potentially impacting the krill stocks. Nevertheless, it is still unclear what impacts such changes in phytoplankton community composition and size structure may have on the WAP marine ecosystems. Further in situ studies are required to understand if a phytoplankton community shift to cryptophytes will significantly weaken biological carbon uptake or lead to structural changes in the WAP coastal marine food web.

Methods
A summary of all datasets used, including their temporal coverage, spatial and temporal resolution, units, product name and relevant references is available in the Supplementary Table 3. The following subsections present further detail on each dataset and presents all methodologies used. All analysis were performed in Python 3.8.8.

Satellite data
Daily satellite remote sensing reflectance data with 4 km spatial resolution was extracted from the ESA Ocean Colour Climate Change Initiative (OC-CCI) v6.0 product for 1998-2022. Chlorophyll-a (mg m−3) was estimated by applying the OC4-SO algorithm, a regional Chlorophyll-a algorithm specifically devised for the Antarctic Peninsula region to increase the accuracy of Chlorophyll-a measurements. Satellite sea surface temperature (°C) and sea ice concentration (%) fields with daily temporal resolution and ~5 km spatial resolution were acquired from the Operational Sea Surface Temperature and Ice Analysis (OSTIA), available at the Copernicus Marine Environment Monitoring Service (https://marine.copernicus.eu/). The sea ice concentration was used to calculate daily sea ice extent (i.e., the area occupied by sea ice) for the clusters DRA, BRS and GES by multiplying the area of each pixel (~25 km2) by the number of pixels covered by sea ice (using a minimum threshold of 15% of sea ice concentration per pixel). For each cluster, the annual sea ice duration, day of sea ice advance, and day of sea ice retreat were calculated following Stammerjohn et al. Following this definition, the day of sea ice advance corresponded to the first day in which sea ice concentration is higher than 15% for at least 5 consecutive days. The day of sea ice retreat is the inverse, i.e., the first day after the winter when the sea ice concentration drops below 15% (a 5 consecutive day window is also used). Daily, 4 km resolution photosynthetically active radiation (PAR; Einstein m−2 d−1) data were retrieved from the ESA GlobColour project. Monthly Southern Annular Mode (SAM) index values were retrieved from the NOAA Centre for Weather and Climate Prediction Centre (https://www.cpc.ncep.noaa.gov/).

In-situ data
HPLC-derived in-situ surface measurements of Chl-a collected across the Western Antarctic Peninsula were used. The full dataset (N=4322) included published data available in Valente et al. and Palmer LTER, as well as unpublished data collected by the Brazilian High-Latitude Oceanography Group (GOAL-FURG).

The GOAL-FURG in-situ Chl-a measurements were collected during 12 austral summer expeditions to the WAP aboard the Brazilian vessels NP Almirante Maximiano and NP Ary Rongel (N=533). Seawater samples (ranging from 0.5 to 2.5 L) were filtered under low vacuum using GF/F filters (25 mm diameter, 0.7 μm pore size) and subsequently stored at −80°C. Prior to analysis, the filters were placed in screw-cap centrifuge tubes with 3 mL of 95% methanol (2% ammonium acetate) containing 0.05 mg L–1 trans-β-apo-8’-carotenal as an internal standard. Samples were sonicated for 5 min, stored at −20°C for one hour and then centrifuged at 1100 × g for 5 min at 3°C. The supernatants were filtered through PTFE membrane filters (0.2 μm pore size) to ensure no residues were inserted in the HPLC system. For pigment analysis, a monomeric C8 column and a mobile phase containing pyridine was used. Before injection, 1 mL of sample was mixed with 0.4 mL of Milli-Q water in 2.0 mL glass sample vials and placed in the HPLC cooling rack of a Shimadzu Prominence LC-20A Modular HPLC System. The method’s detection and quantification limits have been calculated and discussed by Mendes et al. Pigment standards from DHI (Institute for Water and Environment, Denmark) were used for calibration of the HPLC system. Pigments were identified based on their absorbance spectra and retention times from the signals in the photodiode array detector (SPD-M20A; 190–800 nm; 1 nm wavelength accuracy) or fluorescence detector (RF-10AXL; Ex. 430 nm/Em. 670 nm). Peaks were integrated using the LC-Solution software, but all peak integrations were manually verified and corrected if necessary. A quality assurance threshold procedure, through the application of limits of quantification and detection, was applied to the pigment data to reduce the uncertainty of pigments found in low concentrations. Pigment concentrations were normalized to the internal standard to account for losses and volume changes.

Combined, the three databases result in a comprehensive dataset of in situ Chl-a measurements spanning from 1998 to 2020. In-situ atmospheric wind speed measured at Palmer Station were extracted from the ‘Palmer Station Weather—Daily Averages (#28)’ dataset, available in EDI. In-situ atmospheric wind speed measured at Marambio and Great Wall stations were extracted from the Global Historical Climatology Network daily (GHCNd).

Clustering areas with coherent phytoplankton seasonal patterns
Due to the extent and complexity of the marine environments off the Antarctic Peninsula, an agglomerative hierarchical clustering analysis was first used to cluster areas within the Antarctic Peninsula with coherent phytoplankton seasonal patterns using remote sensing data from 1998 to 2022. First, for each pixel, the following metrics were calculated: (i) the first month when phytoplankton growth (i.e., satellite Chl-a) is, on average, detected after the winter, (ii) the last month when phytoplankton growth is, on average, detected after the summer, (iii) the average month when the peak satellite Chl-a is detected, and (iv) the numerical integration of mean Chl-a concentrations during the austral summer (November-February), calculated using Simpson’s rule. All metrics were normalized. Finally, the spatial resolution of the pixels was reduced to 10 km to avoid performance issues when running the clustering analysis. The distance and linkage metrics used for clustering were the Manhattan distance and average linkage, respectively. Four clusters were initially identified. After closer inspection, one cluster was divided in two due to a partial influence of sea ice, which could affect phytoplankton bloom phenology, thus bringing the total number of clusters to 5. All pixels that were, on average, covered with sea ice during the entire year (i.e., sea ice concentration above 15%) were discarded. Subsequently, pixels indicative of noise (e.g., pixels of a cluster located inside another cluster without a logical reason) were manually removed. Finally, the spatial resolution of the resulting map of clusters was upscaled to 4 km. All analyses were performed using Scipy and Sklearn modules in Python 3.8.8.

Phytoplankton bloom phenology
For each of the five identified clusters, phytoplankton bloom phenology metrics were derived from satellite Chl-a. First, daily satellite Chl-a was temporally averaged to an 8-day week frequency and spatially averaged within each cluster. Subsequently, the phytoplankton austral spring-summer bloom was identified as the event, no shorter than 2 weeks, in which Chl-a surpasses a threshold of 5% of the annual Chl-a median. Five bloom phenology metrics were then calculated. The bloom initiation and bloom termination dates were defined as the first and last week of the bloom, respectively (i.e., when Chl-a rises above and subsequently falls below the 5% threshold). Bloom duration corresponds to the number of weeks between bloom initiation and bloom termination. The bloom peak date is the week within the blooming period when the maximum Chl-a value is found.

Analysis of phytoplankton trends
To understand if phytoplankton biomass and bloom phenology metrics in the Western Antarctic Peninsula have changed since 1998, the time series for each previously defined cluster were assessed for statistically significant trends or environmental shifts. For each year, the mean Chl-a during each month between September and April (full austral spring-summer period) was calculated, as well as the mean Chl-a during this entire period (September-April) and during only the peak of the summer (December-February). The regions within the eastern sector of the Antarctic Peninsula (WEDi and WEDo) were excluded from these analyses since satellite observations are very limited due to the persistence of sea ice that characterizes this sector. Since low data availability and sea ice adjacency is likely to affect the validity of trend analyses. Linear trends were estimated by fitting linear regressions to each time series and extracting the slope of the resulting line. Outliers were excluded by removing all Chl-a values over (under) the mean Chl-a + (−) 3 standard deviations. Pixel-wise linear regressions were also performed for the mean Chl-a during September-April in order to produce maps showcasing the linear trend for Chl-a. In order to understand if a similar trend was observed in in-situ data, the mean Chl-a during September-April was also calculated using only in-situ data from the GES and BRS regions (the only regions of the WAP where data was abundant enough to accurately run trend analyses). To understand how the trends in phytoplankton biomass may have changed between decades in the WAP, the average Chl-a concentration for each season (spring, summer, and autumn) was calculated for 2001–2010 and for 2011–2020. Data from the three regions of the WAP (DRA, BRS and GES) were used for this comparison. The proportion of the Chl-a attributed to each season to the total Chl-a (the sum of the average spring, summer, and autumn mean Chl-a) was then calculated. Finally, to better understand the relationship between Chl-a and SAM, a pixel-wise linear regression was performed for the full WAP region.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of “Phytoplankton trends in a changing climate: higher biomass and longer blooms\nin the West Antarctic Peninsula” submitted to Nature Communications by Ferreira et al.\nIn this paper, the authors look at 25 years of satellite derived ocean color in the waters of\nthe west Antarctic Peninsula to understand the impact of climate change on the\nphytoplankton phenology and biomass. They find an interesting result in that the changes in\nthe sea ice cover linked to the impacts of climate change have mainly led to an increase in\nfall phytoplankton blooms, something that is new to the Southern Ocean. This is an\ninteresting study and could be worth considering in Nature Communications if some of the\ncaveats I detail below are corrected.\nMy first major comment is that it is not clear enough throughout the manuscript what\nperiod is used for the analyses. In the abstract, the authors state they ran their analyses\nfrom 1998 to 2022, but the final year 2021 is mentioned in several places in the manuscript,\nfigures, and supplementary figures. The authors need to give precisely what years and\nmonths they have used for their analyses. See my further comments below.\nFor my second major comment, I believe that the referencing in the present paper is not\nfully correct and leads to some flawed argumentation around the interpretation of the\nauthors’ results. I explain why I think this is a problem in detail here. In the introduction and\nthroughout their paper, the authors refer to previous seminal papers that have studied a\nrelatively similar subject, climate change in the waters west of the Antarctic Peninsula and\nits impacts on the marine ecosystems. However, these studies are almost all 10 or more\nyears old. And the climatic trends that these papers presented seemed to have changed. In\nfact, the most conspicuous climatic trend until 2010 was an earlier sea ice retreat and a later\nice advance as reported in, for example, Stammerjohn et al. 2008. However, the trend has\nchanged, and the sea ice retreat has not particularly become earlier since 2010 (see the\nauthors’ Figure 4a for example). Therefore, the authors’ reasoning regarding the role of the\nsea ice retreat and advance cannot be fully verified at this stage. I would suggest that the\nauthors present the sea ice phenological changes focused on the period they study in their\npaper (1998-2022) and not for the entire time series available for satellite derived sea ice\ndata (starting in 1978 or 1980 as chosen by the authors). Only by doing this, will the authors\nbe able to study the possible role of changes in sea ice phenology on the observed changes\nin phytoplankton biomass and phenology. The main result of this paper, the change in\nphytoplankton biomass that the authors mainly observe in autumn is an interesting result,\nbut as of now, the supporting data are not well enough articulated. The authors should also\nacknowledge throughout their paper that the time series they study is different than the\nones previously studied for this region, explaining the different results obtained.\nIn parenthesis, this reminds me very much of an Arctic Ocean paper that found similar\nphenological changes following the well-known sea ice changes that have happened in the\nArctic Ocean, in case the authors did not know of it: Ardyna, M., M. Babin, M. Gosselin, E.\nDevred, L. Rainville, and J.-É. Tremblay (2014), Recent Arctic Ocean sea ice loss triggers\nnovel fall phytoplankton blooms, Geophys. Res. Lett., 41, 6207–6212,\ndoi:10.1002/2014GL061047\nAt the moment, I suggest the authors to consider my suggestions and those of other\nreviewers, to improve the quality of their study for consideration of publication in Nature\nCommunications.\nI wish the authors the best of luck with the review process.\nBest regards\nSpecific comments:\nLine 20: in this place, the authors mention that they are going to present their analyses over\n25 years of data (1998-2022), but the final year of analyses is presented as 2021 in many\nother places of the manuscript. So I would suggest the authors to be very precise and also\ninclude the month for the start and end dates, since we are dealing here with Austral\nprimary productivity seasons.\nLine 27: the end year for the “recent intensification of the Southern Annular Mode” is not\ngiven in this sentence.\nLines 29-30: “region” is repeated twice in this sentence. Please remove one occurrence.\nLine 30: “raise important critical questions”. Please choose between “important” and\n“critical”, no need to use both adjectives in this sentence.\nLines 34-38: In the first lines of the introduction, where the references #1, #2, #4 and #5 are\nused to describe the environmental changes that have happened in the Peninsula over the\nlast decades, the authors must bring more recent references of the physical changes that\nhave occurred in the WAP since the most recent of these references (i.e. Montes-Hugo et al.\n2009). I will come back to this question later, but I think it is essential to follow the\nargumentation of the authors that we also know what physical changes have happened in\nthe WAP since these seminal studies were published.\nLine 85: when mentioning Figure 2 here, there also seems to be inconsistencies regarding\nthe dates which are given as “1998-2022” in the Figures panels and as “1998-2021” in the\nFigure legend (line 450). Please correct throughout the manuscript.\nLines 86-90: so does that mean that the other regions are less likely to experience iron\nlimitation? The authors should comment on this in the paper and bring previous references.\nLine 108: here the period stated is 1997-2021, contrary to what is indicated in the abstract.\nIn the legend of Figure 3 (line 458), the period indicated is 1997-2022. And in the legend of\nSupplementary Figure 3 (line 16), the period indicated is also 1997-2022. The authors should\nbe consistent throughout the paper regarding the exact period they ran their analyses on.\nLine 119: “2011-2011” should be “2011-2021” I believe. However, the authors should make\nsure the period studied is consistent throughout the paper.\nLines 134-138: the authors’ Supplementary Figure 3b also suggests this increase in\nphytoplankton biomass in the Gerlache Strait in summer.\nLines 150-151: this paragraph by the authors is about the increase in phytoplankton biomass\nin the northern WAP (BRS and DRA). However, this sentence and the Supplementary Figure\n5 present data from the Southern (or mid-) WAP (i.e. Palmer Station). Therefore, this\nargumentation is not valid and the authors should present wind data trends for the whole\nWAP to feed their argumentation.\nLines 144-157: I think that the major problem in this paragraph and in other places of this\npaper is that the authors compare their long-term study (1998-2022) with other long-term\nstudies that were performed using data from different periods. For example, the Montes-\nHugo et al. (2009) study compares two very different decades (1978-1986 and 1998-2006)\nthan the present paper. However, the physical dynamics of the WAP have not evolved\nlinearly during the period presented in Montes-Hugo and the period presented in this paper.\nThis is the case of sea ice phenology for example. Therefore, the present study is not\ncomparable to these previous seminal studies and the authors should be more cautious\nwhen comparing their results with these previous studies.\nLine 185: in the legend of Figure 4a, only the equation for the linear regressions for the sea\nice advance is given. The authors should also give the equation for the linear regressions for\nthe sea ice retreat which they also plot.\nIn Figure 4, I think that this would be important to highlight the trends that correspond to\nthe period that the authors use to study the change in phytoplankton biomass and\nphenology, so 1998-2022. This will be important when trying to interpret the role of sea ice\nphenology changes in phytoplankton dynamics. What happened in the 2 decades prior to\n1998 is interesting but not relevant regarding the data presented in this paper.\nLine 187: I guess that the authors should also explain somewhere in the manuscript,\nperhaps here, that sea ice presence acts to decrease light penetration to the upper ocean.\nEven though relatively known, this may still be useful to some readers.\nLine 191: The statement that sea ice declining rates are higher in the Gerlache Strait\ncompared to the Drake Passage and Bransfield Strait needs to be backed up by showing\nrelevant sea ice trends in the aforementioned regions or by presenting a reference that has\nshown this previously, if possible, a recent reference.\nLine 193: “while spring and summer biomass contribution slightly decreased…” add missing\nword.\nLine 194-196: since the authors only give the equation of the linear regression for sea ice\nadvance, the reader cannot judge if this statement is true. Again, here, only the period\n1998-2022 will be relevant to the present study.\nLine 201-204: while I agree with the authors’ statement that phytoplankton stocks are\nimportant here, I think that the authors should still consider and mention nutrients’\nlimitation for autumnal primary productivity.\nLine 204-206: as of now, this statement cannot be verified since the authors do not show\nthe trend regarding sea ice retreat between 1998 and 2022, and instead use a much longer\ntime series starting in 1980. This statement also contradicts the information the authors\ngive in their Supplementary Figure 2.\nLine 238: this is not entirely clear to me if the authors tested the correlation between the\nannual(?) SAM index and the annual(?) average Chl-a concentration for each pixel of the\nregion, or they used an interannual(?) mean. I would think that the first is true but perhaps\nthe authors could clarify.\nLine 269: Should this sentence read “following strong El-Nino…”?\nSupplementary Figures:\nLine 5: why is the period used for these data (1997-2021) different than the period used for\nthe overall study (1998-2022)?\nLine 10: why is the period used for these data (1997-2021) different than the period used\nfor the overall study (1998-2022)?", "reviewer_2": "The paper “ Phytoplankton trends in a changing climate: higher biomass and longer blooms\nin the West Antarctic Peninsula” by Ferreira and colleagues describes a satellite-based study\nof the changes in phytoplankton biomass and phenology in waters around the Antarctic\nPeninsula. Although the title refers to the west side of the peninsula, the east side was\nconsidered as well. The paper provides a nice description of the changes over the last few\ndecades and provides reasonable explanations for them. I found the paper to be informative\nand interesting and their methods robust. I only have a few minor issues that the authors\nshould consider when revising their manuscript. I describe them below.\nLine 22. What is meant by climate change here? Is this referring to anthropogenic or natural\nclimate changes? This should be made clear here.\nLine 26-27. Do the authors claim it is anthropogenic climate change or the SAM that is\ndriving phytoplankton phenology? Or to they believe that anthropogenic climate change\ndriving the SAM? They need to clarify if these changes are part of a natural climate cycle or\nsomething induced by human activities.\nLine 90-91. What differences are being referred to here? The authors have only described\none subregion at this point.\nLine 114. These numbers need more specific units. I assume that this is chlorophyll, but it\nneeds to be specified.\nLine 127-128. Could these observations be related to later ice freeze-up?\nLine 137-138. Is this increase statistically significant? If the cutoff for significance is 0.05,\nthen it is not statistically significant, and the authors should not say that there was an\nincreasing trend.\nLine 152-153. Has a long-term deepening of the mixed layer been observed, or is this only\nassumed from the stronger winds?\nLine 188-189. I'm not sure what generalized means. Is it that phytoplankton growth was the\nsame throughout the WAP?\nLine 209-211. Do nutrients not play a role at all? Eventually, the iron will be consumed and\nthe bloom will end, unless there are mechanisms to mix new iron to the surface. A longer\ngrowing season will not necessarily guarantee higher production. Plus, it is new production\nthat is important to the ecosystem. Continuing a bloom for longer on recycled nutrients\nprovides fewer benefits for the ecosystem and does not sequester carbon.\nLine 260. Maybe change “compartments” to “components” ?\nLine 268-270. If it is important to consider ENSO, then why hasn't it been considered in this\nstudy?\nLine 272-273. Wouldn't the bigger issue be the decline in krill and consequently the animals\nthat rely on them for food?\nLine 510. Insert space (5 km)\nLine 512. Change “photosynthetic” to “”photosynthetically\nLine 513. Should be “data were”\nLine 518. What is the sea ice concentration cutoff used to separate the sea ice season from\nthe open water season? It may be described later, but should be given here first.\nLine 542. Insert space (10 km)\nLine 551. Insert space (4 km)"}, "author_response": "Point by point response to reviewers\nAll comments by Reviewers are presented in black.\nAll responses by the Authors are present in purple.\nPlease note that all line numbers included in this response correspond to the manuscript with\ntracked changed.\nReviewer #1:\nReview of “Phytoplankton trends in a changing climate: higher biomass and longer blooms in\nthe West Antarcfic Peninsula” submifted to Nature Communicafions by Ferreira et al.\nIn this paper, the authors look at 25 years of satellite derived ocean color in the waters of the\nwest Antarcfic Peninsula to understand the impact of climate change on the phytoplankton\nphenology and biomass. They find an interesfing result in that the changes in the sea ice cover\nlinked to the impacts of climate change have mainly led to an increase in fall phytoplankton\nblooms, something that is new to the Southern Ocean. This is an interesfing study and could be\nworth considering in Nature Communicafions if some of the caveats I detail below are\ncorrected.\nAuthors: We are grateful to Reviewer #1 for taking the fime to review the manuscript. We will\nnow respond to each major comment sequenfially.\nMy first major comment is that it is not clear enough throughout the manuscript what period is\nused for the analyses. In the abstract, the authors state they ran their analyses from 1998 to\n2022, but the final year 2021 is menfioned in several places in the manuscript, figures, and\nsupplementary figures. The authors need to give precisely what years and months they have\nused for their analyses. See my further comments below.\nAuthors: We acknowledge that the references to the temporal extent of the datasets used in\nthe study were not sufficiently clarified in the original submifted version of the manuscript.\nThis was primarily due to the use of mulfiple datasets from various sources, some of which did\nnot originally encompass data up to 2022 at the fime of the inifial analyses. For instance,\nalthough the used OC-CCI Chl-a dataset already spanned from 1998 to 2022, certain\nenvironmental datasets (SST, Sea Ice concentrafion, and PAR) only contained data up to\nDecember 2021 when analysed.\nThis issue has been addressed in the new and revised version of the manuscript, for which we\nhave:\n1. Updated the satellite SST, Sea Ice, and PAR datasets to now also include data up to\nDecember 2022 and subsequently re-runed the analyses with the updated datasets. It\nshould be highlighted that, not only this did not alter our originally submifted findings\n(refer to the updated Supplementary Table 2), but has provided even more consistency\nto our study.\n2. Ensured that all references to dataset periods in the text and figures accurately reflect\nthe fimeframe of 1998-2022 and, when not, clearly indicafing it in the text and figures\n(e.g., lines 110, 503, 510, 518, 544).\n3. Introduced a new Supplementary Table that succinctly lists each dataset ufilized,\nincluding their temporal coverage, spafial and temporal resolufion, units, product\nname and relevant references (new Supplementary Table 3).\nFor my second major comment, I believe that the referencing in the present paper is not fully\ncorrect and leads to some flawed argumentafion around the interpretafion of the authors’\nresults. I explain why I think this is a problem in detail here. In the introducfion and throughout\ntheir paper, the authors refer to previous seminal papers that have studied a relafively similar\nsubject, climate change in the waters west of the Antarcfic Peninsula and its impacts on the\nmarine ecosystems. However, these studies are almost all 10 or more years old. And the\nclimafic trends that these papers presented seemed to have changed. In fact, the most\nconspicuous climafic trend unfil 2010 was an earlier sea ice retreat and a later ice advance as\nreported in, for example, Stammerjohn et al. 2008. However, the trend has changed, and the\nsea ice retreat has not parficularly become earlier since 2010 (see the authors’ Figure 4a for\nexample). Therefore, the authors’ reasoning regarding the role of the sea ice retreat and\nadvance cannot be fully verified at this stage. I would suggest that the authors present the sea\nice phenological changes focused on the period they study in their paper (1998-2022) and not\nfor the enfire fime series available for satellite derived sea ice data (starfing in 1978 or 1980 as\nchosen by the authors). Only by doing this, will the authors be able to study the possible role of\nchanges in sea ice phenology on the observed changes in phytoplankton biomass and\nphenology. The main result of this paper, the change in phytoplankton biomass that the\nauthors mainly observe in autumn is an interesfing result, but as of now, the supporfing data\nare not well enough arficulated. The authors should also acknowledge throughout their paper\nthat the fime series they study is different than the ones previously studied for this region,\nexplaining the different results obtained.\nAuthors: We acknowledge Reviewer #1’s concerns and provide a thorough feedback to his/her\ncomments below.\nIndeed, the discussion of our manuscript revolves around key seminal papers, several of which\npublished prior to 2010. We chose to emphasize these studies because they have crucially\ncontributed to the current understanding of climate change impacts on the ecosystems of the\nWestern Antarcfic Peninsula (WAP). Nevertheless, we understand how important it is that the\nclimate trends previously seen for the WAP remain true after 2010. We also agree with the\nreviewer that the original Figure 4a does appear to hint that the long-term trends in sea ice\nadvance and retreat may have plateaued or even reverted recently, although no stafisfical\nsignificance was found after tesfing. Due to the important role sea ice can have on phytoplankton\ngrowth, parficularly in the spring and autumn, we have redone the analysis.\nThe first thing we would like to clarify is that the sea ice phenology metrics plofted in the original\nFigure 4a were acquired from the Palmer LTER programme (i.e. the long-term monitoring\nprogramme run in the U.S. Antarcfic Palmer Stafion), which monitors a large region within the\nsouthern-mid WAP (hftps://pallter.marine.rutgers.edu/research-groups/research/), stretching\nfrom 64°S to 70°S. Although we originally used this dataset since it is one of the most renowned\nand readily available sea ice datasets in the WAP, we have further inspected the data and it is\nnow clear that the region does not accurately match the clusters we idenfified in this work. See\nthe following Figure (Fig. R1) for a comparison between the Palmer LTER grid and the clusters\ndelineated in our work:\n[figure redacted]\nFigure R1: Comparison between the Palmer LTER regional grid (left; extracted from the Palmer\nLTER website) and the clusters idenfified in our work (right; from Figure 2).\nTherefore, we recalculated the sea ice phenology metrics, as well as sea ice extent, for each of\nthe three clusters in the WAP where the changes in phytoplankton biomass and bloom\nphenology are occurring (DRA, BRS, and GES). The new Supplementary Figure 6 shows how the\nsea ice advance and retreat have changed since 1982 for the area of each cluster. We also added\na regression line for the period since 2010-, to show how each metric has changed in recent\nyears.\nSup. Fig. 6: Interannual variability of Sea Ice Advance (blue) and Retreat (red) dates for the\nregions of the WAP - DRA (a), BRS (b) and GES (c) - during 1980 to 2022. Full lines correspond to\nthe linear regression for the 1982-2022 period, while the dashed lines correspond to the 2010-\n2022 period. These regions where chosen since they were the only ones that exhibited trends in\nbiomass and/or bloom phenology (see Tables 1 and 2).\nWe can see that, for all clusters, the linear regression of the dates of sea ice advance (the start\nof the sea ice season; after the austral summer) since 2010 appears to follow the long-term trend\n(i.e. becoming later), even if only slightly. Nevertheless, we would like to stress that neither of\nthe linear regressions are stafisfically significant, which is likely due to the high variability which\ncharacterizes sea ice phenology. This is in line with our results that show that phytoplankton\nbiomass in the autumn appears to have increased in all three clusters (Table 1), as well as with\nthe later bloom terminafions observed in DRA and BRS (Table 2).\nInteresfingly, the linear regression of the dates of sea ice retreat (i.e. the end of the sea ice\nseason; after the austral winter) for 2010 onwards show differences between the clusters. The\nnorthernmost regions (DRA and BRS) exhibit a trend towards earlier sea ice retreats, which\nfollows the long-term trend that has long been described. In the southernmost, coastal region\n(GES), however, sea ice retreats appear to be becoming later. Although none of these trends are\nstafisfically significant, they appear to fit our results. For instance, we observed stafisfically\nsignificant increases in the biomass of spring months for both DRA and BRS, which are likely\nrelated to the earlier sea ice retreat and the posifive effect it has on phytoplankton growth (Table\n1).\nYet, we do not observe the same stafisfically significant increase in GES (Table 1). When wrifing\nthe original version of the manuscript, we assumed this could be a consequence of the high\ntemporal and spafial variability in chlorophyll-a that characterizes onshore waters of the WAP.\nHowever, it is possible that this may due to a potenfial reversal of the long-term trends towards\nearlier sea ice retreat, as seen in Sup. Fig. 6c, although the lack of stafisfical significance prevents\nany definite conclusions. The GES region is also the closest to the Palmer LTER grid, overlapping\nwith Palmer Stafion and the coastal systems sampled by the Palmer LTER programme. This\noverlap is coherent with the fact that, as pointed out by the reviewer, the sea ice retreat does\nnot appear to become earlier since 2010 for the original Figure 4a, which uses Palmer LTER sea\nice metrics. A recent study (Eayrs et al. 2021; Figure R2) also appears to support this disfincfion\nbetween northern and southern WAP, since it shows a recent (2015-2018) sea ice concentrafion\ndecline in northern WAP (which matches the 1979-2015 trend), yet an increase in sea ice\nconcentrafion towards the southern WAP. It is possible that is related to the fact that\natmospheric warming trends at the Antarcfic Peninsula may have paused (or reversed, in some\nplaces) since 2000, although the overall consensus is that this is a case of natural interannual\nvariability superimposed on the longer-term climate-change-driven trends (Turner et al., 2016).\n[figure redacted]\nFigure R2 (extracted from Eayrs et al. 2021): Variability in Sea Ice Concentrafion derived from\npassive microwave remote sensing. b, Mean annual SIC 1979–2018. c, Mean annual SIC changes\nfrom 1979 to 2015. d, Mean annual SIC changes from 2015 to 2018.\nWhile we have included Supplementary Figure 6 in the Supplementary Material, we have also\nopted to redo and recontextualize Figure 4a (Line 525). Our original goal for Figure 4a was to\noffer context for the reader regarding the sea ice trends that have been described for the WAP.\nThe new Figure 4a is now clearer, more complete, and more adequate for our work. It now\npresents sea ice extent data specifically for each of three subregions where impacts were\nobserved (DRA, BRS, and GES). Along with the sea ice extent, the mean dates of sea ice retreat\nand advance are also shown. Furthermore, it unequivocally shows that, for all idenfified\nsubregions, the 1998-2022 period is characterized by mean lower sea ice extent, earlier sea ice\nretreat and later sea ice advance than the previous years between 1982-1997. We sfill redirect\nthe reader in the main text to the new Supplementary Figure 6 when discussing the phenology\nof sea ice, but we think that the Figure 4a is more informafive and clear this way. Sea ice\nphenology can be a limited metric, as it only indicates the first and last day of the sea-ice-free\nseason in a given region. Therefore, it does not consider the extent of sea ice in that region nor\nthe spafial distribufion of sea ice, which can vary greatly along the Antarcfic Peninsula coast due\nto the existence of bays, straits, fjords, as well as other sources of ice, such as glaciers and ice\nshelves.\nNew Figure 4a: Yearly average cycle of the Sea Ice Extent (103 km2) in each region (DRA, BRS,\nGES) between 1982-1997 (blue) 1998-2022 (red). For each period, verfical lines indicate the\naverage date of sea ice advance (SIA) and sea ice retreat (SIR; dashed).\nWe have also included the following more recent references in the main text that report\nobservafions related to ice loss across the WAP: i) Lin et al. 2021 (decline in sea ice well into the\n2000s); ii) Flexas et al. 2022 and Davison et al. 2024 (widespread increase in glacier runoff and\nmelt rates); iii) Andreasen et al. 2023 (ice shelves decline by over 6000 km2 between 2009 and\n2019) (Lines 39, 193). The main text has been revised to accommodate for all the changes made\nto Figure 4a (e.g. Lines 193-195). We have also ensured that all comparisons of our results with\nones from previous studies clearly indicate the studies’ analysed fime periods, thus offering\nbefter context to the reader (e.g. Lines 131-133, 156-157, 178).\nNote that Figures R1 and R2 are exclusive to this point-by-point response and only serve to\nsupport our responses to the comments provided by the Reviewer 1.\nIn parenthesis, this reminds me very much of an Arcfic Ocean paper that found similar\nphenological changes following the well-known sea ice changes that have happened in the\nArcfic Ocean, in case the authors did not know of it: Ardyna, M., M. Babin, M. Gosselin, E. Devred, L.\nRainville, and J.-É. Tremblay (2014), Recent Arcfic Ocean sea ice loss triggers novel fall\nphytoplankton blooms, Geophys. Res. Left., 41, 6207–6212, doi:10.1002/2014GL061047\nAt the moment, I suggest the authors to consider my suggesfions and those of other reviewers,\nto improve the quality of their study for considerafion of publicafion in Nature Communicafions.\nI wish the authors the best of luck with the review process.\nBest regards\nAuthors: We are grateful for the suggesfion of this highly relevant paper and have cited it in the\ntext as an analogous example of fall blooms in a polar region arising from climate change (Lines\n235-238).\nSpecific comments:\nLine 20: in this place, the authors menfion that they are going to present their analyses over 25\nyears of data (1998-2022), but the final year of analyses is presented as 2021 in many other\nplaces of the manuscript. So I would suggest the authors to be very precise and also include\nthe month for the start and end dates, since we are dealing here with Austral primary\nproducfivity seasons.\nAuthors: We agree that we must be clear about the periods analysed in our study. As\nmenfioned earlier, we now provide all major informafion regarding the datasets ufilized in the\nnew Supplementary Table 3. Furthermore, we have carefully revised the text to prevent any\nconfusion for the reader when comparing datasets with different temporal coverage (see our\nresponse to the first major comment).\nLine 27: the end year for the “recent intensificafion of the Southern Annular Mode” is not\ngiven in this sentence.\nAuthors: Thank you for bringing this to our aftenfion. We have added “ongoing” instead of\nspecifying an end year, as the intensificafion of SAM is ongoing (Lines 28).\nLines 29-30: “region” is repeated twice in this sentence. Please remove one occurrence.\nAuthors: Removed.\nLine 30: “raise important crifical quesfions”. Please choose between “important” and\n“critical”, no need to use both adjecfives in this sentence.\nAuthors: Done. Thank you.\nLines 34-38: In the first lines of the introducfion, where the references #1, #2, #4 and #5 are\nused to describe the environmental changes that have happened in the Peninsula over the last\ndecades, the authors must bring more recent references of the physical changes that have\noccurred in the WAP since the most recent of these references (i.e. Montes-Hugo et al. 2009). I\nwill come back to this quesfion later, but I think it is essenfial to follow the argumentafion of\nthe authors that we also know what physical changes have happened in the WAP since these\nseminal studies were published.\nAuthors: We fully agree with the reviewer that more updated context should be provided\nregarding the enfire period under study, for which we have added references to the following\nrecently published works that convey the fact that the physical changes to sea ice are currently\nongoing: Siegert et al. 2019, Lin et al. 2021, Flexas et al., 2022, Andreasen et al., 2023, Davison\net al. 2024 (e.g. Lines 39, 193).\nLine 85: when menfioning Figure 2 here, there also seems to be inconsistencies regarding the\ndates which are given as “1998-2022” in the Figures panels and as “1998-2021” in the Figure\nlegend (line 450). Please correct throughout the manuscript.\nAuthors: Thank you, the analysed period for Figure 2 has been corrected to 1998-2022.\nLines 86-90: so does that mean that the other regions are less likely to experience iron\nlimitafion? The authors should comment on this in the paper and bring previous references.\nAuthors: Indeed, iron limitafion is less likely to occur in the other regions due to their coastal\nnature, thus being more associated with higher sea ice coverage and surrounding glaciers.\nGlacial melt, in parficular, serves as a key source of iron in the Antarcfic Peninsula, thereby\nmifigafing iron limitafion during the austral summer. We have added one reference to ensure\nthis is clear for the reader. (Annet et al. 2017; Lines 91).\nLine 108: here the period stated is 1997-2021, contrary to what is indicated in the abstract. In\nthe legend of Figure 3 (line 458), the period indicated is 1997-2022. And in the legend of\nSupplementary Figure 3 (line 16), the period indicated is also 1997-2022. The authors should\nbe consistent throughout the paper regarding the exact period they ran their analyses on.\nAuthors: Thank you, we have uniformized the dates.\nLine 119: “2011-2011” should be “2011-2021” I believe. However, the authors should make\nsure the period studied is consistent throughout the paper.\nAuthors: This has been corrected.\nLines 134-138: the authors’ Supplementary Figure 3b also suggests this increase in\nphytoplankton biomass in the Gerlache Strait in summer.\nAuthors: Added to the text (Lines 143-146).\nLines 150-151: this paragraph by the authors is about the increase in phytoplankton biomass in\nthe northern WAP (BRS and DRA). However, this sentence and the Supplementary Figure 5\npresent data from the Southern (or mid-) WAP (i.e. Palmer Stafion). Therefore, this\nargumentafion is not valid and the authors should present wind data trends for the whole WAP\nto feed their argumentafion.\nAuthors: We used wind data from Palmer Stafion as an example of the increasing winds trend\nseen across the Western Antarcfic Peninsula, including its northern sector. However, we agree\nthat presenfing wind data for different regions contribute to strengthen our discussion.\nTherefore, wind data from the Marambio (on the northern fip of the Antarcfic Peninsula) and\nGreat Wall (South Shetland Islands, in the Bransfield Strait) have now been added to the\nSupplementary Figure 5. All three show a similar stafisfically significant increasing trend in wind\nspeed.\nLines 144-157: I think that the major problem in this paragraph and in other places of this\npaper is that the authors compare their long-term study (1998-2022) with other long-term\nstudies that were performed using data from different periods. For example, the Montes-Hugo\net al. (2009) study compares two very different decades (1978-1986 and 1998-2006) than the\npresent paper. However, the physical dynamics of the WAP have not evolved linearly during the\nperiod presented in Montes-Hugo and the period presented in this paper. This is the case of\nsea ice phenology for example. Therefore, the present study is not comparable to these\nprevious seminal studies and the authors should be more caufious when comparing their\nresults with these previous studies.\nAuthors: We have already addressed this concern earlier (see the response to the second\nmajor comment). Nonetheless, we agree that directly comparing our results with the ones\nfrom Montes-Hugo et al. (2009), as well as any of the other studies covering different periods\nfrom ours, should be done carefully. Therefore, we have revised this part of the main text to be\nmore prudent (e.g. Lines 125, 156-157).\nLine 185: in the legend of Figure 4a, only the equafion for the linear regressions for the sea ice\nadvance is given. The authors should also give the equafion for the linear regressions for the\nsea ice retreat which they also plot.\nAuthors: We have now redone Figure 4a, as explained in the second major comment.\nIn Figure 4, I think that this would be important to highlight the trends that correspond to the\nperiod that the authors use to study the change in phytoplankton biomass and phenology, so\n1998-2022. This will be important when trying to interpret the role of sea ice phenology\nchanges in phytoplankton dynamics. What happened in the 2 decades prior to 1998 is\ninteresfing but not relevant regarding the data presented in this paper.\nAuthors: We have now redone Figure 4a, as explained in the second major comment.\nLine 187: I guess that the authors should also explain somewhere in the manuscript, perhaps\nhere, that sea ice presence acts to decrease light penetrafion to the upper ocean. Even though\nrelafively known, this may sfill be useful to some readers.\nAuthors: We have now added this informafion (Lines 218-220).\nLine 191: The statement that sea ice declining rates are higher in the Gerlache Strait compared\nto the Drake Passage and Bransfield Strait needs to be backed up by showing relevant sea ice\ntrends in the aforemenfioned regions or by presenfing a reference that has shown this\npreviously, if possible, a recent reference.\nAuthors: We have removed this sentence from the text.\nLine 193: “while spring and summer biomass contribufion slightly decreased…” add missing\nword.\nAuthors: Done. Thank you.\nLine 194-196: since the authors only give the equafion of the linear regression for sea ice\nadvance, the reader cannot judge if this statement is true. Again, here, only the period 1998-\n2022 will be relevant to the present study.\nAuthors: The new Figure 4a, along with the Supplementary Figure 6, supports the statement\nmade. We also would like to menfion that several studies have shown that the rate at which\nsea ice advance is becoming later is higher than the rate at which sea ice retreat is becoming\nearlier (e.g. Stammerjohn et al. 2008; Moreau et al., 2015).\nLine 201-204: while I agree with the authors’ statement that phytoplankton stocks are\nimportant here, I think that the authors should sfill consider and menfion nutrients’ limitafion\nfor autumnal primary producfivity.\nAuthors: We agree and have menfioned the potenfial influence of nutrient limitafion on the\nautumn (Lines 225).\nLine 204-206: as of now, this statement cannot be verified since the authors do not show the\ntrend regarding sea ice retreat between 1998 and 2022, and instead use a much longer fime\nseries starfing in 1980. This statement also contradicts the informafion the authors give in their\nSupplementary Figure 2.\nAuthors: We have removed this sentence to avoid confusing the reader.\nLine 238: this is not enfirely clear to me if the authors tested the correlafion between the\nannual(?) SAM index and the annual(?) average Chl-a concentrafion for each pixel of the\nregion, or they used an interannual(?) mean. I would think that the first is true but perhaps the\nauthors could clarify.\nAuthors: We have menfioned in the Methods and in Line 263 that we tested the correlafion\nbetween the mean September-April SAM index and the mean September-April Chl-a\nconcentrafion for each pixel. Nevertheless, we have adjusted the sentence to make it clearer\n(Line 263).\nLine 269: Should this sentence read “following strong El-Nino…”?\nAuthors: Correct. This has now been corrected.\nSupplementary Figures:\nLine 5: why is the period used for these data (1997-2021) different than the period used for the\noverall study (1998-2022)?\nLine 10: why is the period used for these data (1997-2021) different than the period used for\nthe overall study (1998-2022)?\nAuthors: This has been corrected now, see our response to the first major comment above.\nOnce again, we would like to thank Reviewer #1 for the insighfful comments. We are confident\nthat they have strengthened the quality and rigor of our paper. We also believe that the\ncomments have contributed to the overall clarity of the manuscript.\nFull list of the references included in this response:\n Andreasen, J.R., Hogg, A.E. and Selley, H.L., 2023. Change in Antarcfic ice shelf area\nfrom 2009 to 2019. The Cryosphere, 17(5), pp.2059-2072.\n Davison, B.J., Hogg, A.E., Moffat, C., Meredith, M.P. and Wallis, B.J., 2024. Widespread\nincrease in discharge from West Antarcfic Peninsula glaciers since 2018. EGUsphere,\n2024, pp.1-22.\n Flexas, M.M., Thompson, A.F., Schodlok, M.P., Zhang, H. and Speer, K., 2022. Antarcfic\nPeninsula warming triggers enhanced basal melt rates throughout West Antarcfica.\nScience advances, 8(31), p.eabj9134.\n Lin, Y., Moreno, C., Marchefti, A., Ducklow, H., , O., Delage, E., Meredith, M., Li, Z.,\nEveillard, D., Chaffron, S. and Cassar, N., 2021. Decline in plankton diversity and carbon\nflux with reduced sea ice extent along the Western Antarcfic Peninsula. Nature\ncommunicafions, 12(1), p.4948.\nMontes-Hugo, M., Doney, S.C., Ducklow, H.W., Fraser, W., Marfinson, D., Stammerjohn,\nS.E. and Schofield, O., 2009. Recent changes in phytoplankton communifies associated\nwith rapid regional climate change along the western Antarcfic Peninsula. Science,\n323(5920), pp.1470-1473.\n Moreau, S., Mostajir, B., Bélanger, S., Schloss, I.R., Vancoppenolle, M., Demers, S. and\nFerreyra, G.A., 2015. Climate change enhances primary producfion in the western\nAntarcfic Peninsula. Global change biology, 21(6), pp.2191-2205.\n Siegert, M., Atkinson, A., Banwell, A., Brandon, M., Convey, P., Davies, B., Downie, R.,\nEdwards, T., Hubbard, B., Marshall, G. and Rogelj, J., 2019. The Antarcfic Peninsula\nunder a 1.5 C global warming scenario. Fronfiers in Environmental Science, 7, p.102.\n Stammerjohn, S.E., Marfinson, D.G., Smith, R.C., Yuan, X. and Rind, D., 2008. Trends in\nAntarcfic annual sea ice retreat and advance and their relafion to El Niño–Southern\nOscillafion and Southern Annular Mode variability. Journal of Geophysical Research:\nOceans, 113(C3).\n Turner, J., Lu, H., White, I., King, J.C., Phillips, T., Hosking, J.S., Bracegirdle, T.J., Marshall,\nG.J., Mulvaney, R. and Deb, P., 2016. Absence of 21st century warming on Antarcfic\nPeninsula consistent with natural variability. Nature, 535(7612), pp.411-415.\nReviewer #2:\nThe paper “ Phytoplankton trends in a changing climate: higher biomass and longer blooms in\nthe West Antarcfic Peninsula” by Ferreira and colleagues describes a satellite-based study of\nthe changes in phytoplankton biomass and phenology in waters around the Antarcfic\nPeninsula. Although the fitle refers to the west side of the peninsula, the east side was\nconsidered as well. The paper provides a nice descripfion of the changes over the last few\ndecades and provides reasonable explanafions for them. I found the paper to be informafive\nand interesfing and their methods robust. I only have a few minor issues that the authors\nshould consider when revising their manuscript. I describe them below.\nAuthors: We are grateful to Reviewer #2 for the thoughfful and construcfive feedback. We will\nnow address each of the minor issues raised.\nLine 22. What is meant by climate change here? Is this referring to anthropogenic or natural\nclimate changes? This should be made clear here.\nAuthors: We are referring to anthropogenic climate change. We have now added\n“anthropogenic” to this line to make it clear for the reader.\nLine 26-27. Do the authors claim it is anthropogenic climate change or the SAM that is driving\nphytoplankton phenology? Or to they believe that anthropogenic climate change driving the\nSAM? They need to clarify if these changes are part of a natural climate cycle or something\ninduced by human acfivifies.\nAuthors: We claim that the trends seen in phytoplankton phenology are a consequence of long-\nterm environmental changes likely to be associated with anthropogenic acfion (e.g. ocean\nwarming and decrease of sea ice along the Western Antarcfic Peninsula). The mulfidecadal rising\ntrend in SAM has also been linked to anthropogenic climate change (e.g. Fogt & Marshall, 2020).\nWe have made this clearer in the abstract and in the main text (Lines 23, 256). Nevertheless, we\nsought to be prudent throughout the text when aftribufing our results to anthropogenic climate\nchange since this is a very rapid changing region, and it is always difficult to disfinguish between\nnatural and anthropogenic, parficularly at shorter fime scales.\nLine 90-91. What differences are being referred to here? The authors have only described one\nsubregion at this point.\nAuthors: This sentence has been rephrased (Lines 91-92).\nLine 114. These numbers need more specific units. I assume that this is chlorophyll, but it\nneeds to be specified.\nAuthors: It is indeed chlorophyll a. This has been corrected.\nLine 127-128. Could these observafions be related to later ice freeze-up?\nAuthors: Ours results do suggest that long-term trends towards later sea ice advance (i.e. ice\nfreeze-up) are related to higher producfion in the autumn (Lines 190-228).\nLine 137-138. Is this increase stafisfically significant? If the cutoff for significance is 0.05, then it\nis not stafisfically significant, and the authors should not say that there was an increasing\ntrend.\nAuthors: While the increase in December biomass in the GES region is not stafisfically\nsignificant at 0.05 (although its p-value is less than 0.1), we think that it is important to retain\nthis observafion in the manuscript. GES is a region that exhibits high seasonal and interannual\nvariability in phytoplankton biomass, which can make achieving stafisfical significance harder.\nDespite the near-significant trend, there are several evidence suggesfing an increase in\nsummer phytoplankton biomass in GES: i) the average summer biomass between 2011-2020\nappears to have increased compared to the previous decade (Fig. 2d); ii) pixel-wise increasing\nbiomass trends were observed for the GES region, both during the summer period and from\nSeptember to April (Fig. 3a; Supplemental Fig. 3b). However, we agree with Reviewer #2 that\nwe should be more accurate, for which it is now menfioned in the sentence that the\nobservafion is not stafisfically significant using a p-value threshold of 0.05 (Lines 143; p-value =\n0.094 and R=0.34).\nLine 152-153. Has a long-term deepening of the mixed layer been observed, or is this only\nassumed from the stronger winds?\nAuthors: In this study, we do not present mixed layer data because we would need the physical\nprofile (in situ temperature and pracfical salinity) to esfimate the seawater potenfial density\nand, subsequently, the mixed layer depth. However, most of the open access data available for\nthis region do not present physical profiles, making it unfeasible to calculate and consequently\nuse this parameter. Nevertheless, we can assume this based on the wind trends\n(Supplementary Figure 5), as the deepening of mixed layer in open ocean regions is strongly\ndriven by wind pafterns. For the WAP, several studies have already reported the deepening of\nthe mixed layer depth associated with strong winds (e.g., Schofield et al. 2018).\nLine 188-189. I'm not sure what generalized means. Is it that phytoplankton growth was the\nsame throughout the WAP?\nAuthors: We have changed ‘generalized’ to ‘widespread’.\nLine 209-211. Do nutrients not play a role at all? Eventually, the iron will be consumed and the\nbloom will end, unless there are mechanisms to mix new iron to the surface. A longer growing\nseason will not necessarily guarantee higher producfion. Plus, it is new producfion that is\nimportant to the ecosystem. Confinuing a bloom for longer on recycled nutrients provides\nfewer benefits for the ecosystem and does not sequester carbon.\nAuthors: We agree that nutrients can play a significant role in limifing phytoplankton growth.\nWe see this in the DRA region, which is the most open ocean region in our study. While we\nobserve a longer growing season in the DRA, the summer bloom ends relafively early (mid-\nFebruary, on average), most likely due to nutrient limitafion. However, based on previous\nstudies, macronutrient availability in more coastal regions, such as the BRS, can sfill be high in\nlate February and March, potenfially fuelling phytoplankton growth in the early autumn (e.g.\nMonteiro et al. 2023). In addifion, onshore waters of the WAP have also been described as\nhighly abundant in iron, even in late summer and autumn (Anneft et al. 2017; Sherrell et al.\n2018; Pan et al. 2020).\nLine 260. Maybe change “compartments” to “components” ?\nAuthors: We are grateful for the suggesfion. However, in this case, we think that the use of\ncompartments (i.e. any of the enclosed parts into which a space (…) is divided) is more\nappropriate than components in this context (i.e. one of the parts of a system, process, or\nmachine). Definifions were taken from the Cambridge Dicfionary website.\nLine 268-270. If it is important to consider ENSO, then why hasn't it been considered in this\nstudy?\nAuthors: While ENSO can indeed play a significant role in shaping the interannual variafion in\nthe coastal WAP (e.g. summers following strong El-Niño may yield large blooms; Costa et al.\n2020, 2021), our study focuses on the long-term trends in phytoplankton, with SAM being\ntypically regarded as the key climate mode linked with long-term environmental change in this\nregion. In addifion, it is important to highlight that the effects of the ENSO are sfill not well\nunderstood for the WAP, especially the delay of ENSO effects reaching the WAP after its inifial\nexpression in the Equatorial Pacific Ocean. Nonetheless, we have revised the sentence to make\nclear for the reader that ENSO is likely to hold a more interannual importance (Lines 296-299).\nLine 272-273. Wouldn't the bigger issue be the decline in krill and consequently the animals\nthat rely on them for food?\nAuthors: Of course, we agree that a decline in krill can be devastafing for the Antarcfic food\nweb. In this sentence, we meant to stress how important the relafionship between\nphytoplankton and krill is and how a change in size composifion of phytoplankton may affect\nphytoplankton predafion by krill. We have revised the text to make this clearer (Lines 300-305).\nLine 510. Insert space (5 km)\nAuthors: Done.\nLine 512. Change “photosynthefic” to “”photosynthefically\nAuthors: Done.\nLine 513. Should be “data were”\nAuthors: Done.\nLine 518. What is the sea ice concentrafion cutoff used to separate the sea ice season from the\nopen water season? It may be described later, but should be given here first.\nAuthors: The sea ice concentrafion cutoff is 15%. The day of sea ice advance (first day of the\nsea-ice season) is the first day where sea ice concentrafion is higher than 15% for at least 5\nconsecufive days. The day of sea ice retreat is the inverse, i.e., the first day after sea ice season\nwhen the sea ice concentrafion drops below 15%. All details can be found Lines 579-588, in the\nMethods.\nLine 542. Insert space (10 km)\nAuthors: Done.\nLine 551. Insert space (4 km)\nAuthors: Done.\nReference list:\n Anneft, A.L., Fitzsimmons, J.N., Séguret, M.J., Lagerström, M., Meredith, M.P.,\nSchofield, O. and Sherrell, R.M., 2017. Controls on dissolved and parficulate iron\ndistribufions in surface waters of the Western Antarcfic Peninsula shelf. Marine\nChemistry, 196, pp.81-97.\n Costa, R.R., Mendes, C.R.B., Tavano, V.M., Dofto, T.S., Kerr, R., Monteiro, T., Odebrecht,\nC. and Secchi, E.R., 2020. Dynamics of an intense diatom bloom in the Northern\nAntarcfic Peninsula, February 2016. Limnology and Oceanography, 65(9), pp.2056-\n2075.\n Costa, R.R., Mendes, C.R.B., Ferreira, A., Tavano, V.M., Dofto, T.S. and Secchi, E.R.,\n2021. Large diatom bloom off the Antarcfic Peninsula during cool condifions associated\nwith the 2015/2016 El Niño. Communicafions Earth & Environment, 2(1), p.252.\n Fogt, R.L. and Marshall, G.J., 2020. The Southern Annular Mode: variability, trends, and\nclimate impacts across the Southern Hemisphere. Wiley Interdisciplinary Reviews:\nClimate Change, 11(4), p.e652.\n Monteiro, T., Henley, S.F., Pollery, R.C.G., Mendes, C.R.B., Mata, M., Tavano, V.M.,\nGarcia, C.A.E. and Kerr, R., 2023. Spafiotemporal variability of dissolved inorganic\nmacronutrients along the northern Antarcfic Peninsula (1996–2019). Limnology and\nOceanography, 68(10), pp.2305-2326.\n Pan, B.J., Vernet, M., Manck, L., Forsch, K., Ekern, L., Mascioni, M., Barbeau, K.A.,\nAlmandoz, G.O. and Orona, A.J., 2020. Environmental drivers of phytoplankton\ntaxonomic composifion in an Antarcfic fjord. Progress in oceanography, 183, p.102295.\n Schofield, O., Brown, M., Kohut, J., Nardelli, S., Saba, G., Waite, N. and Ducklow, H.,\n2018. Changes in the upper ocean mixed layer and phytoplankton producfivity along\nthe West Antarcfic Peninsula. Philosophical Transacfions of the Royal Society A:\nMathemafical, Physical and Engineering Sciences, 376(2122), p.20170173.\n Sherrell, R.M., Anneft, A.L., Fitzsimmons, J.N., Roccanova, V.J. and Meredith, M.P.,\n2018. A ‘shallow bathtub ring’of local sedimentary iron input maintains the Palmer\nDeep biological hotspot on the West Antarcfic Peninsula shelf. Philosophical\nTransacfions of the Royal Society A: Mathemafical, Physical and Engineering Sciences,\n376(2122), p.20170171.\n Stammerjohn, S.E., Marfinson, D.G., Smith, R.C., Yuan, X. and Rind, D., 2008. Trends in\nAntarcfic annual sea ice retreat and advance and their relafion to El Niño–Southern\nOscillafion and Southern Annular Mode variability. Journal of Geophysical Research:\nOceans, 113(C3)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review of “Phytoplankton trends in a changing climate: higher biomass and longer blooms\nin the West Antarctic Peninsula” submitted to Nature Communications by Ferreira et al.\nI read the new version of the paper and the response to the reviewers’ comments by the co-\nauthors, and I am satisfied with the authors' response. They have addressed all the\ncomments satisfyingly and improved the robustness of the paper, especially regarding the\ntiming of the trends they observe and compared to previously published results.\nI find the results of this paper interesting and contributing significantly to our knowledge of\nthis climate-sensitive and important region of the Southern Ocean. The paper is well written\nand interesting. Therefore, I recommend the paper for publication in Nature\nCommunications.\nI congratulate the authors on a nice and interesting paper.\nBest regards\nSpecific comments:\nLine 262: “Tagliabue et al.” the last name is miss-spelled here", "reviewer_2": "The authors have adequately addressed all of my concerns."}, "author_response": ""}]

Decision letter