Net community production on the middle shelf of the eastern Bering Sea

Abstract

To estimate temporal changes of nutrients and calculate the seasonal net community production (NCP) on the eastern shelf of the Bering Sea, hydrographic sampling along the 70-m isobath of the middle shelf was conducted in spring (2007–2009), summer (2008–2009), and fall (2007). These were cold years, with sea ice covering much of the eastern Bering Sea in April. Each spring, there was a region with relatively low nitrate in the middle portion (59°–60°N) of the transect prior to the spring phytoplankton bloom. This water appeared to have originated in the coastal domain and was advected offshore into the middle domain. Seasonal NCP (mean±standard deviation) in this region was low (26±12gCm−2), and may be indicative of a portion of the middle shelf ecosystem that is chronically short of fixed carbon in spring. In 2007, the post-bloom cruise occurred during the fall transition when deep mixing, remineralization, and denitrification/anammox compromised seasonal estimates of NCP. In other years (2008–2009), the post-bloom cruise occurred in summer. On those cruises, the euphotic zone, elevated chlorophyll fluorescence, and oxygen supersaturation were occasionally deeper than the pycnocline, and there was a seasonal loss of nitrate and phosphate in the bottom layer. In 2008, preferential uptake of ammonium may have sustained sub-surface production in the north. Therefore, seasonal estimates of NCP were not only evaluated in the upper mixed layer, but throughout the water column. During summer, denitrification/anammox in bottom waters did not appear to compromise seasonal estimates of NCP. Seasonal NCP averaged for 2008 and 2009 was slightly but significantly higher (p<0.0041) in the south (47±9gCm−2, n=80) than in the north (41±16gCm−2, n=78). In the south, interannual variability of seasonal NCP was related to the wind mixing in spring rather than the presence or absence of ice.