Late spring nitrate distributions beneath the ice-covered northeastern Chukchi Shelf

Abstract

Measurements of late springtime nutrient concentrations in Arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult. During the SUBICE (Study of Under-ice Blooms In the Chukchi Ecosystem) cruise in May–June 2014, an extensive survey of hydrography and prebloom concentrations of inorganic macronutrients, oxygen, particulate organic carbon and nitrogen, and chlorophyll a was conducted in the northeastern Chukchi Sea. Cold (<−1.5°C) winter water was prevalent throughout the study area, and the water column was weakly stratified. Nitrate (NO3−) concentration averaged 12.6 ± 1.92 μM in surface waters and 14.0 ± 1.91 μM near the bottom and was significantly correlated with salinity. The highest NO3− concentrations were associated with winter water within the Central Channel flow path. NO3− concentrations were much reduced near the northern shelf break within the upper halocline waters of the Canada Basin and along the eastern side of the shelf near the Alaskan coast. Net community production (NCP), estimated as the difference in depth-integrated NO3− content between spring (this study) and summer (historical), varied from 28 to 38 g C m−2 a−1. This is much lower than previous NCP estimates that used NO3− concentrations from the southeastern Bering Sea as a baseline. These results demonstrate the importance of using profiles of NO3− measured as close to the beginning of the spring bloom as possible when estimating local NCP. They also show that once the snow melts in spring, increased light transmission through the sea ice to the waters below the ice could fuel large phytoplankton blooms over a much wider area than previously known.