Seasonal and inter-annual variability of dissolved inorganic nutrients along the western Antarctic Peninsula: is the Southern Annular Mode a potential driver?

We investigated long-term time series data to improve our understanding of phytoplankton bloom dynamics and its regulation by climate variability on the western Antarctic Peninsula (wAP). Specifically, we examined seasonal and inter-annual variability in utilization of three major inorganic nutrients (nitrate and nitrite, phosphate, silicate) by spring phytoplankton blooms and potential drivers responsible for shaping those variations, using a 20-year time series (1992-2012) near Palmer Station on the wAP. Seasonal and inter-annual covariability in the 50-m depth integrated nutrients and chlorophyll-a (Chl) was analyzed using principle component analysis. We found no linear trends in any of the nutrients or Chl, but there were strong year-to-year fluctuations in seasonal nutrient drawdown and phytoplankton blooms. On average, Chl peaked around days 290-310, stayed high for ca. 40 days, and gradually decreased to the end of the growing season (March-April). Nitrate and nitrite and phosphate behaved in the opposite way of chl-a indicating biological utilization by phytoplankton, but silicate was not utilized in all years, implying non-diatom blooms. Annual anomalies showed a cyclic pattern in Chl, with a maximum occurring every four to six years on average. These years were found to be associated with the negative phase of the Southern Annular Mode (SAM) in preceding winter. Our results suggest large-scale climate variability as a potential driver of biological utilization of nutrients and phytoplankton blooms.