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

Abstract

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 222Rn, 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 224Ra and 223Ra were employed in parallel to the freshwater fraction method to determine the timescale of freshwater retention in the system. Based on 222Rn mass balances (n = 4), the mean groundwater (±SE) discharge into Kinvara Bay was 10.4 ± 6.3×104 m3 days−1, delivering average loads of 376 ± 67 kg Si day−1 (as DRSi), 1.6 ± 0.2 kg P day−1 (as TDP) and ~280 kg N day−1 of dissolved nitrogen (272 ± 49 DIN, essentially as NO3−, and 8.2 ± 1.6 DON), which correspond to ~98.8, 49.1 and ~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 ~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 (~7 days) contributes ~50 % to local biological N fixation. Nutrient availability ratios are N:P ~173 and Si:P ~503, indicating that primary production is P-limited while the carbon yield (~3.01 × 105 mol C day−1, or ~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.