Primary production in the Chukchi Sea with potential effects of freshwater content

Abstract. The in situ primary production rates and various environmental variables were investigated in the Chukchi Sea during the RUSALCA expedition, which was conducted in 2012, to identify the current status of primary production. A ¹³C–¹⁵N dual-tracer technique was used to measure the daily primary production rates, which ranged from 0.02 to 1.61 g C m⁻² d⁻¹ (mean ±SD  =  0.42 ± 0.52 g C m⁻² d⁻¹). The primary production rates showed large regional differences, with the southern region (0.66 ± 0.62 g C m⁻² d⁻¹) producing approximately 5 times as much as the northern region (0.14 ± 0.10 g C m⁻² d⁻¹), which was primarily due to the differences in phytoplankton biomasses induced by regional nutrient conditions. The primary production rates in the Chukchi Sea were averaged using data acquired during the three different RUSALCA expeditions (2004, 2009, and 2012) as 0.33 g C m⁻² d⁻¹ (SD  =  0.40 g C m⁻² d⁻¹), which was significantly lower than previously reported rates. In addition to strong seasonal and interannual variations in primary production, recent decreases in the concentrations of major inorganic nutrients and chlorophyll a could be among the reasons for the recent low primary production in the Chukchi Sea because the primary production is mainly affected by nutrient concentration and phytoplankton biomass. The nutrient inventory and primary production appear to be largely influenced by the freshwater content (FWC) variability in the region due to the significant relationships between FWC, nitrate inventory (r  =  0.54, p < 0.05), and primary production rates (r  =  0.56, p < 0.05). Moreover, we found highly significant relationships between the nutrient inventory and the primary production rates (r  =  0.75, p < 0.001). In conclusion, the primary production in the Chukchi Sea is primarily controlled by nutrient availability, which is strongly related to the FWC variability. Our results imply that the predicted increase in freshwater accumulation might cause a decrease in primary production by lowering the nutrient inventory in the euphotic zone of the Chukchi Sea.