Abstract
A detailed analysis of the internal stoichiometry of a temperate latitude shelf sea system is presented which reveals strong vertical and horizontal gradients in dissolved nutrient and particulate concentrations and in the elemental stoichiometry of those pools. Such gradients have implications for carbon and nutrient export from coastal waters to the open ocean. The mixed layer inorganic nutrient stoichiometry shifted from balanced N:P in winter, to elevated N:P in spring and to depleted N:P in summer, relative to the Redfield ratio. This pattern suggests increased likelihood of P limitation of fast growing phytoplankton species in spring and of N limitation of slower growing species in summer. However, as only silicate concentrations were below potentially limiting concentrations during summer and autumn the stoichiometric shifts in inorganic nutrient N:P are considered due to phytoplankton nutrient preference patterns rather than nutrient exhaustion. Elevated particulate stoichiometries corroborate non-Redfield optima underlying organic matter synthesis and nutrient uptake. Seasonal variation in the stoichiometry of the inorganic and organic nutrient pools has the potential to influence the efficiency of nutrient export. In summer, when organic nutrient concentrations were at their highest and inorganic nutrient concentrations were at their lowest, the organic nutrient pool was comparatively C poor whilst the inorganic nutrient pool was comparatively C rich. The cross-shelf export of these pools at this time would be associated with different efficiencies regardless of the total magnitude of exchange. In autumn the elemental stoichiometries increased with depth in all pools revealing widespread carbon enrichment of shelf bottom waters with P more intensely recycled than N, N more intensely recycled than C, and Si weakly remineralized relative to C. Offshelf carbon fluxes were most efficient via the inorganic nutrient pool, intermediate for the organic nutrient pool and least efficient for the particulate pool. N loss from the shelf however was most efficient via the dissolved organic nutrient pool. Mass balance calculations suggest that 28% of PO43−, 34% of NO3− and 73% of Si drawdown from the mixed layer fails to reappear in the benthic water column thereby indicating the proportion of the nutrient pools that must be resupplied from the ocean each year to maintain shelf wide productivity. Loss to the neighbouring ocean, the sediments, transference to the dissolved organic nutrient pool and higher trophic levels are considered the most likely fate for these missing nutrients.
Highlights
For example, the net offshelf carbon flux is associated with a fixed carbon to nitrogen (C:N) or carbon to phosphorous (C:P) ratio that is equal to the oceanic inflow a persistent net carbon export due to atmospheric inputs would lead to long-term N and P limitation on the shelf
The widespread occurrence of high particulate organic carbon (POC):POP and POC:particulate organic nitrogen (PON) particulate matter, relative to the Redfield ratio, in surface waters during an autumn survey of the Hebrides Shelf and neighbouring ocean may initially be suggestive of N or P deficiency under strict adherence to the Redfield ratio of 106C:16N:1P (Redfield et al, 1963)
Whilst mixed layer NO3−:PO43− was < 16:1, indicating PO43− depletion relative to NO3−, concentrations of both NO3− and PO43− were above potentially limiting thresholds of ∼0.5 μmol NO3− L−1 (Eppley et al, 1969), and ∼0.03 μmol PO43− L−1 (Riegman et al, 2000; Laws et al, 2011; Grant et al, 2013)
Summary
The coastal ocean is responsible for 10–20% of global marine primary production, 8–15% of net oceanic CO2 uptake, and ∼40% of global particulate carbon sequestration yet covers only ∼7% of global ocean area (Wollast, 1998; Thomas et al, 2004; Muller-Karger et al, 2005; Simpson and Sharples, 2012; Chen et al, 2013; Schlesinger and Bernhardt, 2013; Laruelle et al, 2014). If there is plasticity in C:N or C:P a net offshelf carbon flux can be sustained by internal biogeochemical processes that elevate C:N or C:P in the outflowing waters relative to oceanic waters flowing onto the shelf Which of these two scenarios is correct is important to understand as a fixed ratio assumption may require the identification of additional nutrient sources to support shelf wide productivity, whilst the variable ratio assumption does not. An answer to this question can subsequently inform the wider debate about the internal biogeochemical functioning of shelf sea systems, numerical model development, and whether there are missing nutrient source terms or not
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