The Antarctic Peninsula's marine ecosystem has experienced substantial changes since the 1950s, primarily due to climate change-induced increases in atmospheric and oceanic temperatures. These changes have led to glacier and ice shelf retreat and a decrease in sea ice extent, thickness, and volume. These alterations impact all trophic levels, including primary producers (phytoplankton). While some studies indicate increased phytoplankton biomass and cell size in the southern-mid West Antarctic Peninsula (WAP), others report decreases in the northern WAP. However, the northern Peninsula has been less studied, resulting in a less comprehensive understanding of its phytoplankton dynamics. This study addresses this knowledge gap by leveraging satellite ocean color remote sensing, which provides a valuable tool for understanding both short-term and long-term phytoplankton patterns. Previous applications of satellite data to Antarctic coastal waters have been hindered by challenges like persistent cloud cover, sea ice, and low solar angles. Furthermore, global satellite algorithms have often underestimated chlorophyll-a (Chl-a, a proxy for phytoplankton biomass) in the region. To overcome these limitations, this study employs the OC4-SO regional algorithm, specifically calibrated for the Antarctic Peninsula using a robust in-situ dataset. This allows for a more accurate assessment of phytoplankton dynamics and the influence of climate change on primary production in this crucial region of the Southern Ocean.

Existing research on the impacts of climate change on the West Antarctic Peninsula's phytoplankton community presents a mixed picture. Studies using in-situ data have shown increases in phytoplankton biomass and cell size in the southern-mid WAP, south of Anvers Island. Conversely, studies in the northern Peninsula suggest decreases in phytoplankton biomass and cell size. However, data limitations and the uneven distribution of research effort between the northern and southern WAP hinder a comprehensive understanding of the regional patterns. This study contributes to resolving this by examining long-term satellite data and using a newly calibrated algorithm to improve the accuracy of Chl-a estimations.

This study utilized 25 years (1998–2022) of continuous multi-sensor remote sensing data from the ESA Ocean Colour Climate Change Initiative (OC-CCI) v6.0 product. Chlorophyll-a (Chl-a) concentrations, a proxy for phytoplankton biomass, were estimated using the newly developed OC4-SO regional algorithm, specifically calibrated for the Antarctic Peninsula to improve accuracy. Satellite sea surface temperature and sea ice concentration data were obtained from the Operational Sea Surface Temperature and Ice Analysis (OSTIA) product. Photosynthetically active radiation (PAR) data came from the ESA GlobColour project, and monthly Southern Annular Mode (SAM) index values were acquired from the NOAA Centre for Weather and Climate Prediction Centre. In addition to satellite data, the study incorporated in-situ surface measurements of Chl-a collected across the Western Antarctic Peninsula, combining published data (from Valente et al. and Palmer LTER) and unpublished data collected by the Brazilian High Latitude Oceanography Group (GOAL-FURG). These combined datasets, from 1998-2020, provide a comprehensive record of phytoplankton dynamics. Hierarchical clustering analysis was used to identify five marine subregions along the Antarctic Peninsula with distinctive seasonal phytoplankton patterns. These regions, from north to south, were: DRA (Drake Passage), BRS (Bransfield Strait), WEDN (Weddell Sea), GES (Gerlache Strait), and WEDs (Weddell Sea). For each subregion, several metrics were calculated, including bloom initiation, termination, peak, and duration, along with the annual integral of Chl-a. Linear trend analysis was performed to assess statistically significant changes in phytoplankton biomass and bloom phenology from 1998-2022. Pixel-wise linear regressions were conducted to examine spatial variations in Chl-a trends. The analysis also considered in-situ Chl-a data to evaluate trends within specific regions. Finally, correlation analyses were used to examine relationships between Chl-a, SAM, and sea ice extent.

The study revealed a significant increase in mean phytoplankton biomass (September-April) in the West Antarctic Peninsula (WAP) between 1998 and 2022, particularly in the Bransfield Strait. The biomass increase was most pronounced in early spring and early autumn. Analysis showed a positive correlation between phytoplankton biomass and the decline in sea ice coverage, particularly during the late autumn-early winter transition. This suggests that the reduction in sea ice is a primary factor enabling increased phytoplankton growth in both early spring and autumn. However, regional differences in biomass trends were observed, with the more statistically significant trends occurring in the northernmost and more offshore areas of the WAP. This regional variability was partly attributed to the recent intensification of the Southern Annular Mode (SAM) since 2010. A positive correlation was observed between SAM index and chlorophyll-a concentration in the Southern Drake Passage and the northern tip of the Antarctic Peninsula but not in the southernmost coastal areas (GES). The intensification of SAM, linked to stronger winds, may increase vertical mixing, potentially enhancing micronutrient availability (like iron) and contributing to higher biomass in less ice-influenced areas. Conversely, in ice-influenced areas, an increase in biomass in the autumn is observed; this could be driven by the earlier sea ice retreat increasing the light availability for phytoplankton growth. Analysis of in-situ chlorophyll-a data from the BRS and GES regions corroborated the satellite-derived trends, showing a significant increase in phytoplankton biomass during the same period. In the WAP, the proportion of total biomass attributed to the autumn has increased from 2001-2010 to 2011-2020. The proportion of biomass attributed to spring decreased slightly, but the summer proportion remains unchanged, suggesting a shift toward the autumn.

The findings address the research question by demonstrating a significant positive link between climate change, as manifested by reduced sea ice and intensifying SAM, and enhanced phytoplankton biomass and bloom duration in the WAP. The results highlight the complex interplay between environmental change and phytoplankton responses in this climatically sensitive region. The observed increase in autumn biomass is particularly noteworthy, as it cannot be directly explained by earlier sea ice retreat, indicating other mechanisms at play (e.g., increased nutrient availability due to enhanced mixing). The regional differences, explained by the SAM intensification, illustrate the importance of considering large-scale climate patterns when assessing regional ecological changes. These findings have broad implications for the understanding of carbon cycling in the Southern Ocean and the structure of the Antarctic food web. The shift in phytoplankton community composition, potentially from larger diatoms to smaller cryptophytes, under positive SAM conditions may have consequences for krill populations (a key species in the Antarctic food web) and, consequently, higher trophic levels.

This study provides robust evidence for substantial changes in phytoplankton biomass and bloom phenology in the West Antarctic Peninsula driven by climate change, particularly the decline in sea ice extent and the recent intensification of the Southern Annular Mode. The increased autumn biomass, alongside regional variations, highlights the complexity of the interactions between climate processes and ecological responses. Future research should focus on more detailed in-situ studies, particularly during the spring and autumn, and incorporate investigations into shifts in phytoplankton community composition and their cascading effects on the Antarctic food web. This will help to refine our understanding of the long-term consequences of climate change on this vital ecosystem and its role in global carbon sequestration.

While the study utilizes a large dataset and a regionally calibrated algorithm to improve the accuracy of Chl-a estimations, some limitations exist. Data scarcity in certain regions, particularly the eastern sector of the Antarctic Peninsula, hampered a complete analysis of biomass trends across the entire peninsula. The analysis primarily relies on satellite data, which, despite the improvements, can still be affected by cloud cover and other atmospheric conditions. Furthermore, the study focuses primarily on Chl-a as a proxy for phytoplankton biomass, with limited direct information on species composition and size structure, limiting the analysis of community shifts. Finally, the study has focused on a particular period, which limits the generality of the results.