Retention and fate of groundwater-borne nitrogen in a coastal bay (Kinvara Bay, Western Ireland) during summer

The magnitude of submarine groundwater discharge (SGD) and its contribution to nitrogen biogeochemistry in a small embayment in the Western Coast of Ireland subject to occasional hypoxia were investigated during summer. Time series (24 h) of Rn-222, NO3 (-), NO2 (-), NH4 (+), dissolved reactive silicate (DRSi) and salinity (June 2010, July and September 2011 and July 2013), total dissolved phosphorus (TDP) (September 2011 and July 2013) and dissolved organic nitrogen (DON) (July 2013) were measured at the mouth of the bay and coupled with relevant sediment-water fluxes and input loadings to derive nutrient budgets. In-situ activity ratios of the naturally occurring radium isotopes Ra-224 and Ra-223 were employed in parallel to the freshwater fraction method to determine the timescale of freshwater retention in the system. Based on Rn-222 mass balances (n = 4), the mean groundwater (+/- SE) discharge into Kinvara Bay was 10.4 +/- A 6.3x10(4) m(3) days(-1), delivering average loads of 376 +/- A 67 kg Si day(-1) (as DRSi), 1.6 +/- A 0.2 kg P day(-1) (as TDP) and similar to 280 kg N day(-1) of dissolved nitrogen (272 +/- A 49 DIN, essentially as NO3 (-), and 8.2 +/- A 1.6 DON), which correspond to similar to 98.8, 49.1 and similar to 93.5 % of total allochthonous nutrient inputs respectively. Expressed on an areal basis and annual scale the exogenous N summer loading of Kinvara is equivalent to 25.9 g N m(-2) year(-1). Our biogeochemical budgets indicate that tight benthic-pelagic coupling contributes to the very high retention levels of N within the bay with subtidal sediments acting as a link in the internal N cycle via DNRA, while similar to 18 % of the exogenous N load is removed by benthic denitrification. Rapid cycling of DON into bioavailable forms of N within the timescale of freshwater retention in the system (similar to 7 days) contributes similar to 50 % to local biological N fixation. Nutrient availability ratios are N:P similar to 173 and Si:P similar to 503, indicating that primary production is P-limited while the carbon yield (similar to 3.01 x 10(5) mol C day(-1), or similar to 0.313 kg C m(-2) year(-1)) suggests the bay is eutrophic during the summer. SGD-borne Nitrogen loading is therefore the major driver of eutrophication in Kinvara Bay. Our biogeochemical characterization is consistent with the observed phytoplankton community composition and species succession and justifies the local observation of HAB's. In addition, the relative magnitude of DNRA-promoted N retention compared to N removal by denitrification, coupled with seasonal hypoxia, suggests that the system is advanced in the chronological sequence of eutrophication effects on shallow coastal systems.