Macronutrient and carbon supply, uptake and cycling across the Antarctic Peninsula shelf during summer

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’.

Sea Ice Meltwater and Circumpolar Deep Water Drive Contrasting Productivity in Three Antarctic Polynyas

“…These trends were confirmed by looking at the distribution of N* (= NO 3 − − 16*PO 4 3− ) and Si* (= Si (OH) 4 – NO 3 − ). N* decreased from the surface to 100 m in the Mertz and Ninnis polynyas, similarly to observations from the Antarctic Peninsula where diatoms are dominant (Henley et al, ), and unlike in the Dalton (Figure e). The increase of Si* from the surface to 100 m in the Mertz polynya (Figure f) might indicate a faster remineralization of organic matter than the dissolution of biogenic silica (Henley et al, ).…”

Variability and change in the west Antarctic Peninsula marine system: Research priorities and opportunities

“…Vertical nutrient fluxes from the modified CDW source into the surface ocean vary substantially in space and time. Vertical nitrate fluxes during summer in Marguerite Bay and along Marguerite Trough were estimated as 0.18 ± 0.17 mmol NO 3 m -2 d -1 with a maximum of 0.56 mmol NO 3 m -2 d -1 (Henley et al 2018). The mean summertime vertical nitrate flux for the Palmer LTER grid for 1998-2007 was estimated as 1.36 ± 1.79 mmol NO 3 m -2 d -1 (Pedulli et al 2014), although the latter study used a uniform value for the vertical eddy diffusivity (K z ) that is likely to overestimate fluxes.…”

“…Nitrate, phosphate and inorganic carbon are progressively enriched in subsurface waters as CDW moves across the shelf, and 15 N 18 O) isotope composition of nitrate ( Figure 6) and nutrient stoichiometry show that this is driven by local remineralisation of organic matter and nutrient recycling in the upper (Henley et al 2017(Henley et al , 2018. Nitrification (regeneration of nitrate via ammonium oxidation) occurs in the deeper darker part of the euphotic layer, making the regenerated nitrate contribution to surface ocean primary production sensitive to whether the base of the euphotic layer resides within the mixed layer.…”

The marine system of the West Antarctic Peninsula: status and strategy for progress

“…While bringing together this unprecedented dataset, our understanding would be improved even further if it was possible to include data from throughout the year, in addition to summer snapshots. Primary production along the WAP is strongly linked with sea-ice duration, extent and timing of spring melt, and the impact of these factors on water column stability and light availability [3][4][5] and inputs of key macronutrients and trace elements from a variety of sources, linked with glacial meltwater and sediment interactions [6][7][8]. 'Productivity hotspots' of high phytoplankton standing stocks, such as Palmer Deep and Marguerite Bay, provide an example of extreme heterogeneity in WAP primary production.…”

“…Kohut et al [10] use high-frequency radar to show that the residence time of phytoplankton is much shorter than their growth rate, implying that phytoplankton are concentrated in 'hotspots' by physical transportation rather than by growing in situ, and canyons are 'conveyor belts' rather than 'incubators'. There is increasing evidence that shelf sediments may be a more important source of macronutrients and trace elements than deep water [6,8], in addition to water column modification due to organic matter remineralization [7]. New geochemical analyses from Sherrell et al [8] show that there is a considerable supply of key trace elements, such as iron, by horizontal advection of particulate and dissolved species from shallow sediments (constituting a 'bathtub ring') providing localized rather than regional supplies from water masses [8].…”