Sea ice impacts on spring bloom dynamics and net primary production in the Eastern Bering Sea

Abstract

[1] In the Eastern Bering Sea, changes in sea ice have been implicated in recent major upper-trophic level shifts. However, the underlying relationships between sea ice and primary producers have not been well tested. Here, we combine data from multiple satellite platforms, reanalysis model results and biophysical moorings to explore the dynamics of spring and summer primary production in relation to sea ice conditions. In the northern Bering Sea, sea ice consistently retreated in late spring, leading to ice-edge phytoplankton blooms in cold (0-1 C) waters. However, in the southeastern Bering Sea, sea ice retreat was far more irregular. Although this did not significantly alter bloom timing, late retreat led to blooms at the ice-edge while early retreat led to blooms in open waters that were warmer (≤5.4 C) and >70% more productive. Early sea ice retreat also led to higher productivity in summer, likely due to weaker thermal stratification. Overall, annual net primary production during warm years of early sea ice retreat was enhanced by 40-50% compared to years with late sea ice retreat in the southeastern Bering Sea. These findings suggest the potential for future sea ice loss to enhance overall carrying capacity of the southeastern Bering Sea ecosystem. Consistently warm blooms in the future may also channel more energy flow toward the pelagic, rather than benthic, environment. To date, however, neither sea ice extent nor the timing of its retreat have undergone long-term changes in the Eastern Bering Sea.

Modeling spatial patterns of limits to production of deposit-feeders and ectothermic predators in the northern Bering Sea

“…There is undoubtedly appreciable error from applying values of biomass, P:B, and Q:B for meiofauna, protists, and bacteria from published studies of other systems. However, even allowing for such error, our model estimates of the needed annual inputs of fresh microalgae far exceed estimates of annual primary production based on satellite measurements of near-surface chlorophyll (Brown and Arrigo, 2013). Direct local measurements of the biomass, P:B, and Q:B of meiofauna and especially bacteria, as affected by temperature and organic matter supply, are critical to understanding trophic controls on these food webs and how they might respond to environmental change.…”

“…Consequently, we raised fresh microalgal inputs (gC m À2 ) above standing stocks from 10 to 147 in the Chirikov Basin, 16 to 176 in the East sector, and 36 to 201 in the West sector. By estimating total primary production throughout the year based on remote sensing imagery, Brown and Arrigo (2013) calculated mean (±1 SD) annual primary production in a region that overlapped our West sector of 101 ± 31 gC m À2 y À1 . In that study, primary production below depths detected by satellites was accounted for by extrapolating chlorophyll concentrations measured in the top 5e6 m of the water column (maximum euphotic depth ÷ 4.6, Morel and Berthon, 1989) throughout an assumed mixed layer.…”

“…One possible change is in organic matter supply. Such change could result from altered sea ice patterns which affect the timing, duration, or magnitude of microalgal blooms (ice algae and phytoplankton), or from shifts in wind-driven currents that redistribute settled bloom material Brown and Arrigo, 2013;Lovvorn et al, 2013b). Another expected change is increased water temperature, which could alter the species composition, abundance, or metabolic demands of predators or prey.…”

“…Thus, direct field measurements of primary production are specific to locations and periods of particular oceanographic cruises (Cooper et al, 2002;Lomas et al, 2012). Satellite images of chlorophyll concentrations near the water surface are available over large areas at regular intervals throughout the year (Brown and Arrigo, 2013). However, in highly productive waters, such sensors often penetrate to depths of only 5e6 m (maximum euphotic depth ÷ 4.6, Morel and Berthon, 1989), whereas chlorophyll maxima during the spring on Arctic shelves are commonly at depths of 20e40 m .…”

Under-ice measurements of suspended particulate matters using ADCP and LISST-Holo

“…Satellite observations, beginning in 1979, also support the contention that the extent of summer sea ice is declining by around 13% per decade (NSDIC 2012). Sea ice is an important factor controlling the physical and biogeochemical processes in the upper water column, since its year-to-year extent and retreat timing directly influence stratification and nutrient availability (Brown and Arrigo 2013). Sea ice is also regarded as a main "delivery vehicle" for transporting significant loads of total suspended particulate matter (SPM) from shelf regions to open ocean.…”