Late summer/early fall phytoplankton biomass (chlorophyll a) in the eastern Bering Sea: Spatial and temporal variations and factors affecting chlorophyll a concentrations

The spatial and temporal variability of late summer/early fall phytoplankton biomass estimated from in situ chlorophyll a (Chia) concentrations was investigated over a 10-year time period from 2003-2012 in the eastern Bering Sea, encompassing both warm (2003-2005) and cold (2007-2012) temperature regimes. Warm temperature regimes were characterized by above average water temperatures and low seasonal sea-ice extent and by below-average temperatures and high seasonal sea-ice extent. The highest phytoplankton Chla was observed near the Pribilof Islands and the southeastern shelf break where nutrient concentrations were high due to onshore flow from Pribilof and Bering Canyons. The lowest Chla was observed on the northeastern middle and inner shelf, north of Nunivak and St. Matthew Islands and south of St. Lawrence Island (similar to 61-63 degrees N). Stations north of St Matthew Island (61 degrees N) did not show significant variations in Chla between temperature regimes. To the south, total phytoplankton Chla was significantly higher in warm compared to cold years on the south-outer shelf and on portions of the middle shelf. Large phytoplankton Chla was higher in warm years over most of the southern middle shelf. For the entire southeastern Bering Sea shelf (similar to 30-200 m bathymetry, south of Nunivak Island), the highest Chla was seen in 2005 and lowest in 2007 and 2008. On the south-middle shelf, wind mixing and temperature below the pycnocline had strong positive associations with Chla (total and large-size fraction) integrated over the top 50 m, explaining 85% of the variability in mean Chla. This indicates that Chla in summer and early fall is positively affected by wind-induced upwelling of nutrients to the surface and possibly by other bottom up effects such as temperature-mediated growth. Higher bottom temperature is related to reductions in sea-ice extent, which may elicit ecosystem responses such as reduced biomass of large crustacean zooplankton grazers, potentially due to the removal of ice algae, an important food resource for zooplankton in early spring. This, in turn, could reduce or alter the grazing pressure on phytoplankton later in the growing season. Overall, spatial and temporal variations in phytoplankton Chla are due to a combination of factors, from local inputs of nutrients related to mixing or advection, up to large-scale ecosystem effects. Published by Elsevier Ltd.