FY 2023

Seasonal dynamics of primary production in the eastern Bering Sea assessed using continuous temporal and vertical dissolved oxygen and chlorophyll-a measurements

Nielsen, J.M., N.A. Pelland, S.W. Bell, M.W. Lomas, L.B. Eisner, P.J. Stabeno, C. Harpold, and C.W. Mordy

J. Geophys. Res., 128(5), e2022JC019076, doi: 10.1029/2022JC019076, View article online at AGU (external link) (2023)

The Bering Sea and other high-latitude systems are experiencing unprecedented changes related to climate warming and the consequent loss of sea ice. Understanding how such changes influence primary production is a pressing question. Here, we quantify primary production rates in the southeastern Bering Sea over 4 years (2016–2019) at daily to weekly time resolution. A moored high-resolution Profiling Crawler was used in combination with in situ sampling to collect physical and biological data in the ocean's upper 50 m. We used dissolved oxygen (O₂) data to estimate gross and net primary production (NPP), and chlorophyll-a (Chl-a), temperature, and irradiance data to model NPP rates. We then assessed seasonal variation in phytoplankton production rates, phytoplankton community growth rates (p^b_opt) and explored to what extent summer phytoplankton community growth rates may be influenced by nitrogen limitation. Our analyses revealed that the majority of gross primary production (GPP) and net community production occurs in association with the spring phytoplankton bloom. After the bloom, the water column generally experienced low GPP and net biological carbon consumption. Phytoplankton growth rates were commonly suppressed in late summer due to apparent nitrogen depletion. Using high temporal resolution vertically resolved measurements of O₂ and Chl-a, our analyses provides important insight into how biogeochemical cycles, phytoplankton community growth rates, and carbon available for export vary seasonally in the southeastern Bering Sea.

Plain Language Summary The Bering Sea is experiencing unprecedented changes related to climate warming and the consequent loss of sea ice. How such changes affect the growth and composition of phytoplankton at the base of the food web is a pressing concern. Here, we quantify phytoplankton and community production rates in the surface ocean in the southeastern Bering Sea over four years (2016–2019), using a moored Profiling Crawler that samples temperature, salinity, oxygen, and chlorophyll-a data over the top 50 m. We then assessed seasonal and shorter-term variations in phytoplankton production rates. Our analyses showed that the majority of phytoplankton production and organic carbon available for leaving the upper ocean occurs during the phytoplankton spring bloom. After the bloom, phytoplankton production was low for most of the summer, with phytoplankton growth rates limited by nutrient availability in late summer. Our high temporal resolution and vertically resolved measurements from the Profiling Crawler provide new insight into the seasonal primary production cycle in the southeastern Bering Sea.