DOI: 10.5061/dryad.pk0p2ngwp
title: Heatwave responses of Arctic phytoplankton communities are driven by combined impacts of warming and cooling

description: Marine heatwaves are increasing in frequency and intensity as climate
 change progresses, especially in the highly productive Arctic regions.
 Although their effects on primary producers will largely determine the
 impacts on ecosystem services, mechanistic understanding on phytoplankton
 responses to such extreme events is still very limited. We experimentally
 exposed Arctic phytoplankton assemblages to stable warming, as well as to
 repeated heatwaves, and measured temporally resolved productivity,
 physiology and composition. Our results show that even extreme stable
 warming increases productivity, while the response to heatwaves depends on
 the specific scenario applied, and are not predictable from stable warming
 responses. This appears to be largely due to the underestimated impact of
 the cool phase following a heatwave, which can be at least as important as
 the warm phase for the overall response. We show that physiological and
 compositional adjustments to both, warm and cool phases drive overall
 phytoplankton productivity, and need to be considered mechanistically to
 predict overall ecosystem impacts.
description: We exposed natural spring communities from coastal Svalbard
 (Norway) for 2-3 weeks to stable temperature treatments (2°C, 6°C, 9°C),
 where 2°C acted as a control treatment, as well as to repeated 5-day
 heatwaves of differing intensity (6°C and 9°C, Figure 1). By excluding
 grazers and ensuring nutrient replete and stable light conditions, we
 focused on the effect of temperature only. To understand the dynamics and
 mechanisms during changing temperature regimes, we explicitly investigated
 the different phases of a heatwave towards their impact on the community.
 At several timepoints, we measured an extensive set of parameters,
 including growth and productivity assays, stoichiometry, photophysiology,
 as well as species and intraspecific population composition.
description: # Heatwave responses of Arctic phytoplankton communities are driven by
 combined impacts of warming and cooling
 [https://doi.org/10.5061/dryad.pk0p2ngwp](https://doi.org/10.5061/dryad.pk0p2ngwp) We exposed natural spring communities from coastal Svalbard (Norway) for 2-3 weeks to stable temperature treatments (2°C, 6°C, 9°C), where 2°C acted as a control treatment, as well as to repeated 5-day heatwaves of differing intensity (6°C and 9°C, Figure 1). By excluding grazers and ensuring nutrient replete and stable light conditions, we focused on the effect of temperature only. To understand the dynamics and mechanisms during changing temperature regimes, we explicitly investigated the different phases of a heatwave towards their impact on the community. At several timepoints, we measured an extensive set of parameters, including growth and productivity assays, stoichiometry and photophysiology. ## **Description of the data and file structure** Physiological data are summarized in a single excel file containing different tabs for different datasets. Details of Methodology on all data can be found in the publication "Heatwave responses of Arctic phytoplankton communities are driven by combined impacts of warming and cooling" by Wolf et al. 2024. **Tab1: weighted means (Fig.2,S1,S9)** This tab contains mean values of the different treatments over the time of the experiment, weighted by the number of days passed between each timepoint and the next up to the final timepoint (t3 for stable temp. treatments, t4 for heatwave treatments). The add-on .wm behind parameters signifies that the value is a weighted mean over time. Data were used to produce Figure 2, as well as Supplementary Figures S1 and S9. up.to.timepoint = timepoint up to which the mean of the treatment was calculated Treatment = Treatment of incubation (2°C, 6°C, 9°C, HW6°C, HW9°C) Replicate = Bottle replicate within treatment (A,B,C) Growth.C.wm = growth rate normalized to POC [(µg C) (µg C)-1 (day)-1] Growth.Chl.wm = growth rate normalized to Chl a [(µg C) (µg Chl a)-1 (day)-1] 14C.Chl.wm = 14C-based Net Primary Production (NPP) normalized to Chl a [(µg C) (µg Chl a)-1 (day)-1] 14C.C.wm = 14C-based NPP normalized to POC [(µg C) (µg C)-1 (day)-1] O2.net.Chl.wm = O2-based NPP normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1]  O2.net.C.wm = O2-based NPP normalized to POC [(µmol O2) (µg C)-1 (h)-1] O2.gross.Chl.wm = O2-based Gross Primary Production (GPP) normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1] O2.gross.C.wm = O2-based GPP normalized to POC [(µmol O2) (µg C)-1 (h)-1] O2.resp.Chl.wm = O2-based dark respiration normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1] O2.resp.C.wm = O2-based dark respiration normalized to POC [(µmol O2) (µg C)-1 (h)-1] Chl.POC.wm = Chl a: POC ratio [µg µg-1] isETR.wm = in-situ relative electron transport rate (isETR, no unit) Fv.Fm.wm = Fv/Fm, maximum potential quantum efficiency of Photosystem II (no unit) tau.wm = reoxidation time at PSII [ms] **Tab2: Over.time (Fig.3)** This tab summarizes parameters at temporal resolution over the course of the experiment. Data was used to produce Figure 3 (A-D). unique.ID = unique ID description of each value Treatment = Treatment of incubation (2°C, 6°C, 9°C, HW6°C, HW9°C) Replicate = Bottle replicate within treatment (A,B,C) Timepoint = temporal identifier within experiment Temperature = incubation temperature since the last timepoint Day = numbered day of experiment Date = Date of measurement [dd/mm/yyy] Days.passed = number of days passed since last measurement growth.POC = growth rate normalized to POC [(µg C) (µg C)-1 (day)-1] growth.Chl  = growth rate normalized to Chl a [(µg C) (µg Chl a)-1 (day)-1] NPP.POC = 14C-based NPP normalized to POC [(µg C) (µg C)-1 (day)-1] NPP.Chl = 14C-based Net Primary Production (NPP) normalized to Chl a [(µg C) (µg Chl a)-1 (day)-1] POC = Particulate Organic Matter [(µg C) (L)-1] Chla = Chl a [(µg Chl a) (L)-1] Chla.POC = Chl a: POC ratio [µg µg-1] C.N = POC: PON (Paticulate Organic Nitrogen) ratio [µg µg-1] **Tab3: O2 Data over.time** This tab summarizes O2-measurement based parameters at temporal resolution over the course of the experiment. Data was used to produce Figure 3 (E+F). ID = unique ID description of each value Treatment = Treatment of incubation (2°C, 6°C, 9°C, HW6°C, HW9°C) Replicate = Bottle replicate within treatment (A,B,C) Measurement = type of measurement (either based on two measurents (start 0h, end 24h) or continuous (every few minutes for 24h) Temperature = incubation temperature since the last timepoint Timepoint = temporal identifier within experiment Day = numbered day of experiment Date = Date of measurement [dd/mm/yyy] O2.net.POC = O2-based NPP normalized to POC [(µmol O2) (µg C)-1 (h)-1] O2.net.Chl = O2-based NPP normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1] grossO2.POC = O2-based GPP normalized to POC [(µmol O2) (µg C)-1 (h)-1] grossO2.Chl = O2-based GPP normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1] Resp.POC = O2-based dark respiration normalized to POC [(µmol O2) (µg C)-1 (h)-1] Resp.Chl = O2-based dark respiration normalized to Chl a [(µmol O2) (µg Chl a)-1 (h)-1]  **Tab4: FRRF timepoints** This tab contains photophysiological data measured in different treatments at different timepoints of the experiment via variable Chl *a* fluorescence of photosystem II, using a fast repetition rate fluorometer (FRRf, FastOcean PTX; Chelsea Technologies, UK) in combination with a FastAct Laboratory system (Chelsea Technologies). Most parameters are measured at either experimental light conditions (.is ~30µmol photons/m²/s) or at high, over-saturating light (HL ~600µmol photons/m²/s). All parameters (except tau) are unitless, for details see Method section and Schuback et al. 2021. Treatment = Treatment of incubation (2°C, 6°C, 9°C, HW6°C, HW9°C) Timepoint = temporal identifier within experiment Day = numbered day of experiment Replicate = Bottle replicate within treatment (A,B,C) Date = Date of measurement [dd/mm/yyy] Fv.Fm  = Fv/Fm, maximum potential quantum efficiency of Photosystem II isETR = relative electron transport rate at in-situ light (30 µmol photons m-2 s-1) NPQis = Non-photochemical quenching at in-situ light (30 µmol photons m-2 s-1) NPQHL = Non-photochemical quenching at high light (600 µmol photons m-2 s-1) YNOis = Quantum yield of non-regulated energy pathways through ChlF and non-regulated heat dissipation at in-situ light (30 µmol photons m-2 s-1) YNPQis = Quantum yield of regulated energy dissipation processes (NPQ) at in-situ light (30 µmol photons m-2 s-1) YNOHL = Quantum yield of non-regulated energy pathways through ChlF and non-regulated heat dissipation at high light (600 µmol photons m-2 s-1) YNPQHL = Quantum yield of regulated energy dissipation processes (NPQ) at high light (600 µmol photons m-2 s-1) connectivity = connectivity of photosystems antenna = antenna size tau = reoxidation time at PSII [ms] Fm = maximum flourescence in dark state FmHL = maximum flourescence at high light (600 µmol photons m-2 s-1) JVPII.is = PSII photochemical flux per unit volume at in-situ light (30 µmol photons m-2 s-1) phi.is = Absorption cross-section for PSII at in-situ light (30 µmol photons m-2 s-1) phi.HL= Absorption cross-section for PSII at high light (600 µmol photons m-2 s-1) **Tab5: FRRF ramps HW9 (Fig.S7)** This tab contains photophysiological data (as tab 4) measured in treatment HW9 during the heating (.Up) and cooling (.Down) phase of the two consecutive heatwaves (R1, R2) throughout the experiment. This data was used to produce Supplementary Figure S7. All parameters and units are identical to those in Tab 4. **Sharing/Access information** The community composition data from the experiment based on 18S rRNA gene metabarcoding will be shared on the ENA platform under the title "Metabarcoding dataset (18S) of incubation experiment of phytoplankton community from Kongfjorden (Svalbard) under heatwave scenarios".