Alexander Hayward

Marine heatwaves (MHWs) are prolonged anomalies of warm sea surface temperature (SST) that can disrupt marine ecosystems, physical climate processes, and human coastal activities. MHW definitions vary due to different stakeholders requirements, such as ecological scientists and climate scientists having differing yet specific thresholds and metrics. Here we introduce a new global database of daily MHW metrics: climatological baselines, threshold exceedances, SST anomalies, and categorical event classifications of severity, derived from the European Space Agency SST Climate Change Initiative (ESA SST CCI) climate data record (CDR; 1982–2021) version 3.0 and an extension from 2022–2024 provided as an interim climate data record (iCDR). Building on the widely used definition of MHWs, periods in which SST exceeds the local 90th percentile for 5 or more days, our database extends this framework by incorporating multiple baseline climatologies (including fixed 30-year periods and rolling 30-year windows, as well as the period for reanalysis 1993–2022), varied percentile thresholds (90th, 95th, 99th), and both raw and linearly detrended SST anomalies. We also implement alternative event duration criteria (minimum 10 and 30 d persistence) to classify longer-lasting warm events. In addition to warm extremes, we provide marine cold spell (MCS) indices derived from the 1st, 5th and 10th percentiles, enabling analysis of cold as well as warm ocean extremes within the same framework. All data products are provided at daily resolution on a 0.05° (∼ 5 km) grid, with outputs including daily climatological percentiles, SST anomalies and binary MHW flags with severity category indices. This comprehensive database provides a consistent foundation for detecting and analysing MHWs across time and space, enabling researchers to assess how methodological choices affect MHW characterisation. By offering multiple definitions in parallel, the database facilitates intercomparison studies and supports applications from climate monitoring and model evaluation to marine ecological impact assessment, thereby providing users with pre-made indices for extremes, with the ambition to be rolled out into the future with new data releases. The dataset is available at https://doi.org/10.5281/zenodo.21835325 (Hayward et al., 2026).

Climate change has dramatically altered the Arctic seas with significant decrease in sea ice extent and thickness and warming water temperature. The ecological impacts of such change have been described for many parts of the Arctic Ocean, but long-term records of biological indicators are still missing. Among those, photosynthetic and accessory pigments are one of the key tools that aid quantification of phytoplankton and sea–ice algae biomass and characterisation of community composition. To address this gap, we present the first pan-Arctic compilation of in situ algal pigment data obtained exclusively by High-Performance Liquid Chromatography (HPLC), containing 10 798 samples collected across 77 Arctic research cruises between 2000 and 2024. As a result of large-scale collaborative effort, this database covers both open water and sea–ice environments across coastal, shelf and open domains. The database (https://doi.org/10.11583/DTU.29445104, Heidemann et al., 2026) includes measures of up to 26 pigments, with 8 major marker/accessory pigments being considered in this study, namely Alloxanthin (Allo), 19'-Butanoyloxyfucoxanthin (But-fuco), Chlorophyll a (Chl a ), Chlorophyll b (Chl b ), Fucoxanthin (Fuco), 19'-Hexanoyloxyfucoxanthin (Hex-fuco), Peridinin (Peri), and Zeaxanthin (Zea). This publicly available database provides crucial data that can be used to assess phytoplankton dynamics, validating remote sensing observations and can serve as a resource for future Arctic ecological- and modelling studies.

Phytoplankton communities in the Southern Ocean (SO) play a central role in regulating global climate and ecosystems through the biological carbon pump (BCP), which is influenced by ocean warming and sea-ice variability. Here, we combine an expanded circumpolar surface pigment dataset with satellite ocean-colour observations to develop a SO-diagnostic pigment framework that resolves phytoplankton community composition for austral spring and summer between 1998 and 2024. We compare multi-year mean composition for 1998-2014 and 2015-2024 periods separated by a ~ 7% step decrease in September sea-ice extent. Between these two periods, the mean diatom contribution to total chlorophyll increased by up to ~ 10% during spring in sea-ice-influenced regions but decreased by up to ~ 3% over the Antarctic shelf during summer. These findings suggest a seasonally and spatially variable reorganisation of phytoplankton community composition that may alter bloom dynamics, carbon-export efficiency, and the functioning of the SO BCP under continued climate-driven sea-ice variability.

Climate change has dramatically altered the Arctic seas with significant decrease in sea ice extent and thickness and warming water temperature. The ecological impacts of such change have been described for many parts of the Arctic Ocean, but long-term records of biological indicators are still missing. Among those, photosynthetic and accessory pigments are one of the key tools that aid quantification of phytoplankton and sea-ice algae biomass and characterisation of community composition. To address this gap, we present the first pan-Arctic compilation of in situ algal pigment data obtained exclusively by High-Performance Liquid Chromatography (HPLC), containing 10.798 samples collected across 77 Arctic research cruises between 2000 and 2024. As a result of large-scale collaborative effort, this database covers both open water and sea-ice environments across coastal, shelf and open domains. The database (10.11583/DTU.29445104) includes measures of up to 26 pigments, with 8 major marker/accessory pigments being considered in this study, namely Alloxanthin (Allo), 19’-Butanoyloxyfucoxanthin (Butfuco), Chlorophyll a (Chl-a), Chlorophyll b (Chl-b), Fucoxanthin (Fuco), 19’-Hexanoyloxyfucoxanthin (Hexfuco), Peridinin (Peri), and Zeaxanthin (Zea). This publicly available database provides crucial data that can be used to assess phytoplankton dynamics, validating remote sensing observations and can serve as a resource for future Arctic ecological- and modelling studies.

Marine heatwaves (MHWs) are prolonged anomalies of warm sea surface temperature (SST) that can disrupt marine ecosystems, physical climate processes, and human coastal activities. MHW definitions vary due to different stakeholders requirements, such as ecological scientists and climate scientists having differing yet specific thresholds and metrics. Here we introduce a new global dataset of daily MHW metrics: climatological baselines, threshold exceedances, SST anomalies, and categorical event classifications of severity, derived from the European Space Agency SST Climate Change Initiative (ESA SST CCI) climate data record (CDR; 1982–2021) version 3.0 and an extension from 2022–2024 provided as an interim climate data record (iCDR). Building on the widely used definition of MHWs, periods in which SST exceeds the local 90th percentile for 5 or more days, our dataset extends this framework by incorporating multiple baseline climatologies (including fixed 30-year periods and rolling 30-year windows, as well as the period for reanalysis 1993–2016), varied percentile thresholds (90th, 95th, 99th), and both raw and linearly detrended SST anomalies. We also implement alternative event duration criteria (minimum 10-day and 30-day persistence) to classify longer-lasting warm events. All data products are provided at daily resolution on a 0.05° (~5 km) grid, with outputs including daily climatological percentiles, SST anomalies and binary MHW flags with severity category indices. This comprehensive dataset provides a consistent foundation for detecting and analysing MHWs across time and space, enabling researchers to assess how methodological choices affect MHW characterisation. By offering multiple definitions in parallel, the dataset facilitates intercomparison studies and supports applications from climate monitoring and model evaluation to marine ecological impact assessment, thereby providing users with pre-made indices for extremes.

Combined observations of phytoplankton abundance, community structure, and trace metals in winter are scarce but crucial for understanding Southern Ocean biogeochemistry. This study provides the first early winter insights of phytoplankton‐micronutrient dynamics in the Indian sector of the Southern Ocean. Depth‐resolved measurements of chlorophyll‐ a , phytoplankton composition, and micronutrients iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) reveal low phytoplankton abundance (0.27–0.08 μg L ⁻¹ chlorophyll‐ a ) and variable phytoplankton–micronutrient dynamics between the zones. In the Subtropical Zone, cyanobacteria, and haptophytes dominated, driven by higher temperatures, limiting macronutrients, Fe and Zn, and shaped Co and Mn through biological uptake. In the Subantarctic and Polar Frontal Zones, cyanobacteria contributions declined but continued to play a key role in shaping Co dynamics. Diatoms were limited by Mn, Zn, and silicic acid, enabling the dominance of prasinophytes. Other groups such as coccolithophores may have benefitted from the ability to substitute Zn for Cd and contributed to the formation of particulate Cd. Coccolithophores along with Phaeocystis also contributed to particulate Cd in the Antarctic Zone, where Fe and Mn limited the abundance of the dominating diatoms. Diatoms, in turn, strongly contributed to particulate Zn. This perspective on phytoplankton‐micronutrient relationships shows that despite their low productivity in the harsh low light wintertime Southern Ocean, phytoplankton continue to influence trace metal distribution and that there are strong phytoplankton group‐specific associations with various essential metals, rather than Fe alone, that vary latitudinally.

Regional taxonomic variation of phytoplankton communities in the Southern Ocean remains largely uncharacterised despite the distinct trophic and biogeochemical roles of different taxa in anthropogenic carbon uptake, biogeochemical processes, and as the primary source of energy for marine ecosystems. Here we analysed 26 years of pigment data (14,824 samples between 32°S and the Antarctic coast) from over 50 voyages (1996 – 2022), using the phytoclass software. The analysis confirms that the Antarctic Polar Front (APF) is a circumpolar phytoplankton class boundary, separating haptophyte dominated communities to the north from diatom domination of chlorophyll a in the south, and thereby a biological analogue corresponding to the Biogeochemical Divide. Furthermore, community composition was remarkably similar in different zones south of the APF despite substantial spatial variation in biomass. This circumpolar characterisation of the geospatial distribution of phytoplankton community composition will contribute to improved modelling and projection of future change in ecosystems and carbon in the Southern Ocean.