Phytoplankton Community Structure at Subsurface Chlorophyll Maxima on the Western Arctic Shelf: Patterns, Causes, and Ecological Importance

Abstract

Subsurface chlorophyll maxima (SCM) layers are becoming more important to the Arctic shelf ecosystem as phytoplankton growing season and ice‐free water increase. We measured size‐fractionated pigments and environmental variables on the western Arctic shelf during the open‐water season in 2014, to study the spatial pattern and mechanisms of the phytoplankton community at the SCM. The SCM layers accumulated approximately 50% of the chlorophyll a (Chl a) standing stock in the water column, and the contribution increased to >70% for high‐biomass areas, suggesting the biological importance of the summer SCM in the shelf ecosystem. The spatial difference in phytoplankton biomass was caused mainly by bioavailable nitrogen concentrations, and the spatial difference in diatom size may be the result of differences in predominant species. High biomass and large diatoms dominated in the SCM layers, making the south and north of the Chukchi shelf a biological hotspot for the ecosystem's food chain and benthos of the western Arctic shelf in summer.

Extreme Nitrate Deficits in the Western Arctic Ocean: Origin, Decadal Changes, and Implications for Denitrification on a Polar Marginal Shelf

“…The nitrate deficit in the Arctic Ocean arises mainly due to the Pacific origin of upper halocline water (Chen et al., 2003; Shimada et al., 2005), as this Pacific water already has a large nitrate deficit upon entering the Arctic Basin (Mordy et al., 2010; Yamamoto‐Kawai et al., 2006). Furthermore, the Bering–Chukchi Shelf is a hotspot for benthic denitrification (Chang & Devol, 2009; Devol et al., 1997; Granger et al., 2011), especially in areas where diatom accumulation and organic matter deposition rates are high (Zhuang et al., 2014, 2020). The shelf area maintains high biological productivity (Grebmeier et al., 2006, 2015) and effective particle removal (Chen et al., 2012) throughout the year, which is believed to fuel denitrification.…”

Spatial Distribution of Colored Dissolved Organic Matter in the Western Arctic Ocean

“…DOM sources for the shelf area include inputs from Pacific inflow and sediments, and internal production by biological activity [36]. However, due to the low pelagic consumption limited by low temperatures of the Arctic shelf, the diatom biomass reaches mainly benthic communities [37] and, compared with low-latitude waters, CDOM concentrations The shelf area of the western Arctic Ocean is one of the most productive ocean areas globally [34], with a predominance of diatoms in the phytoplankton community [35]. DOM sources for the shelf area include inputs from Pacific inflow and sediments, and internal production by biological activity [36].…”

“…DOM sources for the shelf area include inputs from Pacific inflow and sediments, and internal production by biological activity [36]. However, due to the low pelagic consumption limited by low temperatures of the Arctic shelf, the diatom biomass reaches mainly benthic communities [37] and, compared with low-latitude waters, CDOM concentrations in the western Arctic Ocean may not be intrinsically related to overall biological production [38]. Rather, CDOM variations may be related to internal production by a microbial carbon pump that is more active in the basin area than on the shelf.…”

Seasonal Water Mass Evolution and Non‐Redfield Dynamics Enhance CO2 Uptake in the Chukchi Sea

“…DO was measured onboard using the spectrophotometric method based on Winkler titration with a precision of ±1 μmol kg −1 . See more descriptions of sampling and analysis of nutrients and Chl a samples in Zhuang et al (2020). DIC and TA samples were collected following similar procedures described above, and then were analyzed at the University of Delaware, USA.…”