DOI: 10.5061/dryad.xpnvx0khq
title: Historical kelp forests in California over multiple centuries

description: Kelp forests have deteriorated globally due to anthropogenic stressors.
 There is an urgent need to extend baselines, to understand the processes
 that underlie the persistence and recovery of kelp forests, and to
 distinguish the normal range of ecosystem variability from more extreme
 changes. Using a mixed-method, historical ecology approach we integrate
 archival data, oral histories, and contemporary ecological data to examine
 the dynamics of kelp forests over a multi-decadal to multi-century time
 period in central California. We focus on sea otters, sunflower seastars,
 sea urchins, kelp cover, kelp species dynamics, and climate. From 1826 to
 2020 kelp was highly variable. There were seven periods of low kelp cover
 and two periods of exceptionally low kelp cover (1896-1899; 2014-2016)
 following El Niño-Southern Oscillations (ENSOs). Exceptionally low kelp
 cover did not occur when two predators – seastars and sea otters – were
 present. In all cases, kelp recovered following times of extremely low
 cover, with a lag, which was extended by the duration of warm water
 anomalies. We present the concept of an ENSO Recovery Lag - a metric
 indicating the time it takes for kelp to recover following ENSO events.
 Kelp remained low for approximately two years following 80% of ENSOs. The
 greatest kelp decline (12-fold) was in Santa Cruz (northern Monterey Bay).
 Herbivore populations (sea urchins) were highly variable over the past
 century and exhibited short and long-term changes in abundance. Sunflower
 seastars were present in low, stable abundances prior to seastar wasting
 disease (1938-2013 mean density: 0.02/m2) when they declined by 97.5%.
 Insights from this reconstruction indicate that kelp recovery following
 extended warm water anomalies exhibits a lag, and occurs over multiple
 years.
description: Study Area Our research focused on kelp
 forest ecosystems in central California, USA (here, Año Nuevo
 (37°07'35.4"N, 122°19'35.4"W) in the north to Big Sur
 (36°06'16.2"N, 121°37'19.8"W) in the south. This cold
 water, upwelling-driven ecosystem is characterized by high biodiversity.
 The social-ecological history of central California has been documented
 for over two hundred years by historical and contemporary sources due to
 the region’s history of Tribal nations, exploration, European
 colonization, maritime trade, and Western science. We divided central
 California into three regions, north to south: Santa Cruz, Monterey
 Peninsula, and Big Sur. Data Sources
 To identify historical baselines and longitudinal dynamics of
 kelp forest ecosystems, we assembled a multi-decadal to multi-century
 dataset that integrated historical and contemporary information focused on
 central California largely from the early 1800s onward. We evaluate trends
 in the relative abundance of sea otters (<em>Enhydra lutris
 nereis</em>; hereafter ‘otters’), predatory sunflower seastars
 (<em>Pycnopodia helianthoides</em>), herbivorous purple
 urchins (<em>Strongylocentrotus purpuratus</em>, hereafter
 ‘urchins’), and two species of canopy forming kelp (giant kelp
 <em>Macrocystis pyrifera</em> and bull kelp
 <em>Nereocystis luetkeana</em>; hereafter ‘kelp’).  We did not
 include red urchins (<em>Mesocentrotus franciscanus</em>) in
 formal analyses over historical time periods because there was
 insufficient information about their abundances over a multi-decadal
 period. Instead, we qualitatively discuss changes in their
 abundances. Qualitative and quantitative data sources
 include narrative accounts (1602 onward), archival maps (1852-1934), 48
 oral histories (1939-2020), ENSO reconstructions (1820-2020), kelp harvest
 records (1931-1999), and ecological surveys (1985-2020). We use
 archeological records and Indigenous knowledge as reference where
 available, but focus on integrating sources onward from the exploration
 and colonization of California by Europeans. Sources for these datasets
 are avaliable in the article's supplementary materials.
 <strong>Archival Documents</strong>
 Diverse historical accounts provide early narrative descriptions
 of the social-ecological system of central California. We searched
 collections that focused on west coast history for accounts of early
 explorers, traders, and religions figures who were active in central
 California. Next, we searched early natural history and scientific books,
 field notes, newspaper articles, historical art, and reports that provided
 accounts of natural history and environmental conditions. Since historical
 documents are not indexed for ecological terms, historical sources were
 identified using the expert knowledge of librarians at Stanford
 University’s Hopkins Marine Station (Pacific Grove, California) and
 authors (JP, JTC, TT), and were supplemented with searches for historical
 literature referencing the three regions and four focal species. We
 reviewed identified documents to assess if they contained descriptions of
 natural history.   We considered four types of
 abundance values: (1) presence only; (2) relative abundance; (3)
 quantitative abundance; and for kelp, (4) percent cover (methods described
 in the Kelp Map sections below) (Table S7). First, when qualitative
 descriptions of species’ existence could not be translated to relative
 abundances, we considered these records to represent presence-only data.
 We used presence-only data to triangulate against other data sources and
 used the information in qualitative assessments of change. Second, we
 developed a standardized rubric for translating qualitative descriptions
 of abundance to relative abundance values (scale: 0-5). Third, for
 quantitative abundance records (e.g., ecological transects, published
 data) we assigned densities to relative abundances (range: 0-5) based on
 the mean and standard error values for all records of that species (Table
 S4). When data were only available in graphs, we estimated the density
 values from the figures. We considered relative abundances derived from
 both qualitative and quantitative sources to be consistent within species,
 but not comparable across species. <strong>El
 Niño-Southern Oscillations and Marine Heat Waves</strong>
 To assess long term ENSO and MHW patterns, we used historical
 ENSO time series (1770 – 1983) and continued the historical series with
 contemporary MEI (multivariate ENSO index) estimates (1984 – 2020) and MHW
 records (Quinn et al. 1987; Gergis and Fowler 2009; NOAA 2020). We
 considered that a year was influenced by an ENSO when the year had a MEI
 2.0 value &gt;1.5, or when the ENSO was classified
 as Medium + or greater by historical analyses (Quinn et al. 1987; Gergis
 and Fowler 2009; NOAA 2020). <strong>Sea Surface
 Temperature Records</strong> To assess changes
 in SST we used daily sea surface temperature measured at Hopkins Marine
 Station since 1919 (Breaker and Miller 2023).
 <strong><em>Kelp
 Maps</em></strong> <em>Kelp Maps:
 Archival Sources.<strong> </strong></em>
 We used a total of seven archival maps (five nautical charts and
 two kelp surveys), which we considered to represent presence-only data
 (Costa et al. 2020). Nautical charts which documented kelp were surveyed
 from 1856 to 1934 (Fig. 1; Table S6). The US Department of Agriculture
 conducted two surveys of kelp (1911, 1912), which included maps with
 species-level resolution (Table S6). For all archival maps, we documented
 the map name, number, scale, year of the last survey, survey date (where
 available), and first year of publishing (Table S6). For charts published
 in multiple years, we only considered the first edition.
 To extract kelp data from archival maps, we first photographed
 paper maps and downloaded maps that had been previously scanned by
 libraries and NOAA. Second, we georectified digital maps using satellite
 images and control points from stable features (e.g., rock outcrops) to
 reduce root-mean-square error (ArcGIS 10.7.1) (Costa et al. 2020). Third,
 we demarcated kelp areas using on-screen digitization. We traced the edges
 of the kelp symbology to create polygons depicting the area of kelp beds
 (Fig S2). Where species information was available, we assigned kelp bed
 polygons to one of three groups: giant kelp
 (<em>Macrocystis)</em>; bull kelp
 (<em>Nereocystis)</em>; or mixed beds (similar amounts of
 <em>Macrocystis</em> and
 <em>Nereocystis</em>). <em>Kelp
 Maps: Contemporary Sources.<strong>
 </strong></em> The contemporary spatial
 extent of canopy forming kelp was documented by the California Department
 of Fish and Game (now California Department of Fish and Wildlife, CDFW)
 during a series of aerial kelp surveys (1989–2016), available as
 shapefiles (Table S6; Movie S1). When both canopy and subsurface canopy
 data were available in later years (2008-2016) we restricted maps to
 canopy to improve data consistency through time (R. F. Miller, CDFW, pers.
 comm., 2020). We attempted to find earlier aerial photos taken by CDFW
 (e.g., 1972-1977), but the agency no longer maintained these records (R.
 F. Miller, CDFW, pers. comm., 2020). We documented methods and survey
 months when such data were available in shapefile metadata.
 <em>Kelp Maps: Estimates of Error.<strong>
 </strong></em> To assess the locational
 accuracy of the archival maps we set a reliability criterion that
 considered the local depth in which kelp was located. We used a depth
 threshold of 40 m to account for known depth ranges, kelp movement with
 currents and tides, residual processing errors, and cartographic
 variability (e.g., line thickness) (Costa et al. 2020). We overlaid the
 polygons on a 10m resolution bathymetry map of California. We classified
 map reliability based on the percentage of kelp that was mapped in shallow
 (&lt; 40m) vs deep (&gt; 40m) areas (four classes: Very High
 (&gt;99% of kelp mapped in shallow areas), High (&gt;80%), Medium
 (&gt;60% mapped in shallow areas), and Low (&gt;40% mapped in
 shallow areas)). <em>Kelp Maps: Estimates of
 Kelp Cover.<strong> </strong></em>
 To account for the fact that different extents were mapped across
 years, we restricted our analysis of kelp canopy cover to areas where the
 kelp canopy was documented by archival maps for a minimum of three years
 (Fig. 1). We calculated kelp area as the total area
 where kelp-forming canopy was documented in one year
 (<em>t</em>) in the spatial extent covered by the mapped area
 (<em>m</em>) in each Region
 (<em>r</em>), <em>(i)            
 </em>Kelp Area<sub>tmr</sub> = Total area of
 canopy-forming kelp in time (t) in the mapped area (m) in each region
 (r) To standardize estimates of kelp area, we next
 identified the maximum area of kelp canopy that was ever mapped in any
 year
 (<em>t<sub>1</sub>-t<sub>i</sub></em>)
 using any method for each region (<em>r),</em>
 <em>(ii)           </em>Maximum Area
 t<sub>1</sub>-t<sub>i</sub>
 <sub>r</sub> = maximum area of kelp mapped across all years
 (t<sub>1</sub>-t<sub>i</sub>) in each region
 (r) Finally, for each map, we calculated the proportion
 of maximum area in time (<em>t</em>) as the Kelp Area in time
 (<em>t</em>) divided by the maximum area of the kelp canopy
 that was ever mapped in a region (r),
 <em>(iii)         </em>Proportion of Maximum
 Area<sub>t</sub> = Kelp Area<sub>tmr </sub>/
 Maximum Area t<sub>1</sub>-t<sub>i</sub>
 We used the proportion of maximum area as the standardized
 measure of kelp cover in all quantitative analyses. Additionally, we
 calculated a second standardized kelp cover metric as the maximum extent
 to which kelp occurred in any year (Maximum Extent) within the mapped area
 (m). This yielded similar trends, and we thus proceeded with the method
 described above. <strong>Oral Histories
 </strong> To document changes in coastal marine
 species and conditions of the coastal ecosystem we conducted
 semi-structured oral history interviews (n = 48 respondents). We used
 snowball sampling to select respondents who had first-hand experience with
 the marine biota and ocean conditions of the central Californian coast.
 Initial respondents were identified through personal networks of the
 authors (JP, JC, JTC, TT and FM) who had worked in central California for
 decades. We included individuals who fell into two categories of
 ocean-knowledge: people who had worked in the region for (a) a long-term
 period (e.g., scientists, historians based in the region) or (b) a finite
 period (e.g., former ocean-focused graduate students). We obtained
 informed consent from all participants. Although several respondents had
 not lived in central California for many years, the ocean in the region
 made a strong impression on their memories. Most respondents (54%) resided
 along the central California coast for less than ten years (duration:
 range=3-54 years; median=8 years), placing their memories within discrete
 date ranges. We conducted interviews both in person and
 virtually through Zoom (stanford.zoom.us). We also received written survey
 responses from six individuals. Interview and survey data (hereafter ‘oral
 history data’) were transcribed and respondents were provided the
 opportunity to make corrections. Final data were thematically coded using
 grounded theory. We used NVivo and documented the presence, abundance, and
 location of species, including observations of changes over time (Table
 S8). Oral history data also included distribution information regarding
 both species of canopy-forming kelp, <em>Macrocystis</em> and
 <em>Nereocystis</em>. Species density estimates from oral
 histories were typically qualitative (e.g., ‘a lot’; ‘hardly any’).
 Following the methods for historical data (see above), we standardized
 responses by assigning specific descriptions to different abundance levels
 (range: 0-5) (Table S3). For kelp records, we weighted observations from
 three kelp experts higher than other respondents, which allowed us to
 capture their specific experiences with ENSO dynamics that were not
 mentioned by other respondents. Abundance levels were relative to each
 species. All interviews were reviewed for accuracy by JP and
 JCS. <strong>Contemporary Ecological
 Data</strong> We compiled contemporary
 ecological data (sea otters, purple urchins, sunflower seastars, kelp
 cover, kelp species) from several sources: USGS otter surveys (1985-2019),
 unpublished field data (2002, F.M.), published field data (various), PISCO
 database (1999-2020), ReefCheck database (2006-2019), CDFW kelp harvest
 data (1916-2001), and CDFW aerial kelp surveys (2003-2016) (sample sizes
 in supplement). To facilitate data comparison, we converted density data
 (e.g., purple urchins per m<sup>2</sup>) to relative abundance
 values (range: 0-5). For kelp maps we used equal intervals to set relative
 abundances. For contemporary datasets, we set relative abundances based on
 the mean and standard deviation of survey values for each species
 considered. We then integrated data from archival sources, oral histories,
 and contemporary ecological surveys.
description: # Historical kelp forests in California over multiple centuries
 [https://doi.org/10.5061/dryad.xpnvx0khq](https://doi.org/10.5061/dryad.xpnvx0khq) For this research, we integrated hundreds of datasets to look at the historical changes in kelp forests through time. The data sets we considered ranged from 1602 to 2020. | FILE | TYPE | DESCRIPTION | YEARS | | :---------------------------------------------- | :----------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | :-------- | | coding\_themes\_categories\_oral\_histories.csv | analysis | themes used to code oral history data | NA | | contemporary\_data\_sources.csv | sources | contemporary ecological and ENSO data sources | 1770-2020 | | enso\_recovery\_lag.csv | dataset | data about the ENSO recovery lag. This metric integrates published sources about the intensity of El Nino Southern Oscillations and Marine Heat Waves in central California | 1800-2020 | | extent\_historical\_map.gpkg | gis  | extent of historical maps in specific years | 1852-2016 | | historical\_collections\_searched.csv | sources | collections searched for data | NA | | historical\_organism\_abundances.csv | dataset | values from archival sources documenting the presence and abundance of kelp, sea otters, sunflower seastars, and urchins in central California | 1602-2017 | | kelp\_data\_sources.pdf | sources | historical and contemporary maps used to determine the spatial extent of kelp | 1856-2016 | | kelp\_historical\_expanded\_20230612.csv | dataset | historical and descriptive data about the abundance of canopy forming kelp | 1774-1967 | | kelp\_shallow\_final\_canopy\_only.gpkg | gis  | kelp canopyfrom all sources in years under study | 1852-2016 | | otter\_historical\_location3.csv | dataset | historical and descriptive data about the abundance of sea otters | 1815-1914 | | otter\_historical\_long\_20230702.csv | dataset | historical and descriptive data about the abundance of sea otters | 1602-1929 | | otter\_kelp\_oh\_subset\_long.csv | dataset | oral history observations about kelp forests in central California  | 1939-2020 | | pycno\_contemporary\_density\_post-SSWD.csv | summary statistics | density estimates for sunflower seastars after sea star wasting disease (2013) | 2013-2020 | | pycno\_contemporary\_density\_pre-SSWD.csv | summary statistics | density estimates for sunflower seastars before sea star wasting disease (2013) | pre-2013 | | pycnopodia\_historical\_long\_20240308.csv | dataset | historical data about the abundance of sunflower seastars (Pycnopodia) | 1835-1978 | | qualitative\_abundance\_species.csv | analysis | examples of qualitative descriptions of species abundances and their assigned relative abundance values | NA | | respondentsYear.csv | dataset | number of oral history respondents in each year | 1939-2020 | | urchins\_data\_micheli\_2002.csv | dataset | survey data of sea urchin abundances | 2002-2003 | | urchins\_data\_published.csv | dataset | published sea urchin densities  | 1964-1989 | | urchins\_historical\_20240308b.csv | dataset | historical and descriptive data about the abundance of purple and red sea urchins | 1854-1961 | ## Sharing/Access information Links and/or references to original datasets and sources can be found in: \ \+ contemporary_data_sources.csv\ \+ kelp_data_sources.csv\ \+ historical_organism_abundances.csv\ \+ are referenced in other .csv files ## Code/Software Code is available on GitHub and Zenodo (links in the related works section). The driver.R file provides the overview of the inputs, outputs, and goals of each code chunk.