Variability on phytoplankton size structure in the western Antarctic Peninsula (1997–2006)

The temporal and spatial variability of phytoplankton size structure of waters west of the Antarctica Peninsula (WAP) was investigated between 1997 and 2006. Time series of satellite-derived (phytoplankton size structure index or γ b bp , chlorophyll a concentration or chlT, and sea-ice extent) and shipboard (temperature, salinity, nutrients, and mixed-layer depth) variables were generated during spring–summer in slope, middle shelf and inshore waters and analyzed in relation to atmospheric anomalies (El Niño Southern Oscillation, ENSO, and Southern Annular Mode, SAM). The sampling design included stations north (northern, 62°S) and within (central and southern, 64–68°S) the Pal-LTER (Palmer Long Term Ecological Research) study site. It is hypothesized that contribution of ‘small’ phytoplankton (<20 μm) has increased in the last decade in WAP waters due the ongoing regional climate change. Relationships between γ b bp , the spectral slope of particle backscattering, and environmental parameters were explored based on non-parametric trends (Mann–Kendall test) and cross-correlation coefficients (Spearman matrix). Three types of temporal patterns were detected in satellite-derived phytoplankton size distributions: (1) inter-annual variations of spring–summer γ b bp related to monthly sea-ice extent, (2) abrupt transitions toward dominance of ‘small’ (<20 μm) phytoplankton cells (high γ b bp ) and low chlT values (<1 mg m−3) during 1998 and 2003 summer seasons, and (3) positive or negative trends (decrease vs increase of mean cell size) in specific domains of central and northern stations. Temporal transitions in cell size coincided with a switch on ENSO and SAM anomalies as well as increase of heat content of shelf waters over the WAP region. The lack of offshore spring bloom and summer shelf bloom most likely explains the dominance of relative small phytoplankton cells during the 1998 and 2003 summer seasons. A greater frequency of southerly winds during spring and autumn is expected to favor the dominance of ‘small’ (<20 μm) phytoplankton cells over WAP waters. Conversely, the greater intensification of the Antarctic Circumpolar Current interaction with the WAP shelf-break during SAM+ years is expected to intensify topographically induced upwelling and favor the dominance of ‘large’ (>20 μm) phytoplankton cells on slope waters of central stations. The well-described 50-year warming trend in the Antarctic Peninsula has not resulted in a consistent trend in phytoplankton size structure, as originally hypothesized, but a mosaic of trends attributed to anomalous mesoscale changes of sea-ice extent and circulation patterns.