Effect of groundwater-surface interactions on coastal areas hosting aquaculture activities

Abstract

Submarine groundwater discharge (SGD) is crucial for the nutrient budgets of coastal areas and global oceans. However, its role in the function of coastal ecosystems is still poorly characterised. This study aimed to (1) investigate the effect of nutrient fluxes from submarine groundwater discharge (SGD)on Irish coastal ecosystems hosting aquaculture activities and (2) determine how this effect vary during the year, to (3) assess the value of SGD for coastal areas hosting aquaculture activities. Two sites of contrasting hydrogeology and depth were selected to represent the two extreme ranges of the effect of SGD on coastal ecosystems: Kinvarra Bay, a shallow bay connected to a productive aquifer of karstic limestone, and Killary Harbour, a fjord fed by rivers and surrounded by an unproductive aquifer, mainly of impermeable sandstone. Radium isotopes, radon, nutrient, salinity, dissolved oxygen, temperature data and other water chemistry parameters were collected in these two bays and the main water inputs they receive between August 2017 and April 2019. These were combined with modelling to investigate the effect of changes of SGD composition, quantity and variability of flushing on coastal system s nutrient balance, primary production and aquaculture. In Kinvarra Bay, nitrogen inputs from fresh SGD amplified the primary production, directly benefiting the mussel aquaculture activity. Saline SGD can also provide added solute to the system, including limiting nutrient such as phosphorus or radiotracers such as radium, in particular when drought occurs following spring tides. In Killary Harbour, SGD was negligible in this bay by comparison to river flow, except potentially in the most inner part of the bay, where an intertidal area is present. However, solutes fluxes from deep sediment were significant, and likely provides limiting nutrients such as phosphorus for local phytoplankton growth, in addition to the inputs from rivers. This fluxes could be amplified by the resuspension of fine sediments due to cyclic currents in deep waters. In both bays, the transfer of solutes from sediment to surface waters, either through sediment resuspension or SGD, is a key factor driving phosphorus availability. Phosphorus availability affects both phytoplankton biomass and composition, thus the food source and/or ecosystem services providers (maintain water quality, oxygen levels) for aquaculture production. Consequently, solutes inputs from sediments via SGD (in sites with high SGD inputs) or via suspended sediments (in sites with fine sediments) are likely to affect the interannual variability of aquaculture production by controlling in part the limiting nutrients for primary production and the bay nutrient balances.