Abstract
The West Antarctic Peninsula shelf is a region of high seasonal primary production which supports a large and productive food web, where macronutrients and inorganic carbon are sourced primarily from intrusions of warm saline Circumpolar Deep Water. We examined the cross-shelf modification of this water mass during mid-summer 2015 to understand the supply of nutrients and carbon to the productive surface ocean, and their subsequent uptake and cycling. We show that nitrate, phosphate, silicic acid and inorganic carbon are progressively enriched in subsurface waters across the shelf, contrary to cross-shelf reductions in heat, salinity and density. We use nutrient stoichiometric and isotopic approaches to invoke remineralization of organic matter, including nitrification below the euphotic surface layer, and dissolution of biogenic silica in deeper waters and potentially shelf sediment porewaters, as the primary drivers of cross-shelf enrichments. Regenerated nitrate and phosphate account for a significant proportion of the total pools of these nutrients in the upper ocean, with implications for the seasonal carbon sink. Understanding nutrient and carbon dynamics in this region now will inform predictions of future biogeochemical changes in the context of substantial variability and ongoing changes in the physical environment.This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.
Highlights
Southern Ocean biogeochemical processes play a critical role in the redistribution of nutrients and other chemical species between the major ocean basins, in air–sea CO2 exchange, and in modulating global climate over seasonal, interannual and millennial time scales [1,2,3,4]
High nutrient utilization in surface waters over the high-productivity West Antarctic Peninsula (WAP) shelf allows us to use nitrate isotopic signatures to estimate the local contribution of nitrate regenerated by organic matter remineralization and nitrification to the upper ocean nitrate pool, as opposed to that supplied from Circumpolar Deep Water (CDW)
We show enrichments of nitrate, phosphate, inorganic carbon and silicic acid in subsurface waters as warm, nutrient- and carbon-rich CDW crosses the WAP shelf
Summary
Southern Ocean biogeochemical processes play a critical role in the redistribution of nutrients and other chemical species between the major ocean basins, in air–sea CO2 exchange, and in modulating global climate over seasonal, interannual and millennial time scales [1,2,3,4]. The Antarctic continental shelves are important for the biological uptake of CO2 due to higher area-normalized primary production rates than any other Southern Ocean region [5]. Primary production is paced by the annual sea ice cycle, being negligible over winter and maximal during summer, when large phytoplankton blooms can develop under favourable upper ocean conditions where demands for light, iron and macronutrients are met [8,9]. Increased or longer-duration sea ice cover leads to higher primary production, by sheltering the upper ocean from winddriven mixing during winter and spring, resulting in a shallow well-lit mixed layer favourable for phytoplankton growth during summer [12,13,14]. Because changes in primary production have strong consequences for higher trophic levels, this sea ice-driven variability can influence the functioning of the entire ecosystem [15,16]
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