Title: Effects of Groundwater-Surface water Interactions (GSI) on the biogeochemistry of coastal areas hosting aquaculture activities
Researchers: Maxine Savatier, Dr Carlos Rocha
Why this research: The contribution of coastal ecosystems to human welfare and the economy was estimated to be 12.5 Trillion $US (dollars) globally at the end of the 20th century, and compared roughly with the annual GDP of 18 trillion/year . However, this value is progressively reduced , as additional nutrient inputs to rivers and groundwater from human activities impact on coastal ecosystem health in diverse ways. Observed changes involves inter-alia, more frequent development of toxic algae blooms , low oxygen (hypoxic) zones, overgrowth of nuisance or harmful micro or macroalgae, overdevelopment of specific species such as jellyfish.
These events are detrimental for fishing and aquaculture industries, coastal communities, tourism, and in some cases for marine and coastal infrastructure. While nutrient inputs to coastal areas from channeled freshwater flows such as rivers are relatively well gauged, the role of subsurface sources on the coastal nutrient cycle (ex: submarine groundwater discharge, or SGD) is more difficult to quantify in coastal areas. Fresh groundwater fluxes were estimated to represent between 6 to 10% of the surface waters discharging into the ocean. As a result, groundwater/sea water interactions have been frequently ignored in ocean nutrient balance in comparison with river fluxes.
However, seminal studies conducted over the past two decades have increasingly shown the significance of subsurface water sources and pore water exchange for coastal ecosystem biogeochemical budgets and dynamics . Indeed, water fluxes crossing the sea floor are not only composed of land derived groundwater, but also in a large part of sea water, recirculated through the sea floor and mixed with land derived groundwater. This occur at short time and space scale, by wave and tidal driven recirculation through coastal sediments , or at a larger scale through the repositioning of the salt/freshwater interface within groundwater by seasonal changes and tides . For this reason, Burnett et al. (2003) define submarine groundwater discharge (SGD) as any flow of water across the sea floor, regardless of fluid composition or driving force. The reactions involved when meteoric and sea water mix and travel through porous media significantly alter the composition of the discharging water with respect to both original contribution.
As a result, the rates of transfer, mixing and the original water chemical signatures are changed in a non-uniform way at the system scale . This makes any estimation of SGD-driven nutrient fluxes, from a series of individual nutrient measurements, challenging without a characterization and understanding of the mixing patterns timing and reactions involved within the aquifer. In Ireland, around 35 sites with potentially SGD related temperature anomalies have been identified using sea surface temperature (SST) anomalies derived from Landsat ETM+TIR . However, only Kinvara Bay have been studied extensively to determine the potential ecosystems impacts of SGD. A multi-¬tracer approach (combining pH, Electric conductivity (EC) and radon measurement) differentiated fresh SGD from surface waters input , while a combination of tracers with biogeochemical methods during the summer 2010, 2011 and 2013 was used to build one of the first nutrient balances in Ireland for an SGD dominated coastal system.
The derived estimate of SGD flow rates confirmed quantitatively the fresh water flow predictions derived from hydrogeological modelling and water balance approaches, and SGD derived dissolved inorganic nitrogen inputs were proven as the major driver of eutrophication during summer. Nevertheless, several questions are still arising, regarding: the effect of sea water recirculation within aquifers on the system, the potential role of local aquaculture activities to offset the effect of groundwater pollution by agriculture, and the optimal management strategy to follow to: (1) maintain a suitable suite of coastal ecosystem services and (2) protect the local aquaculture-derived revenue in the spirit of the EU’s Marine Strategy Framework Directive. The current research will use radon 222, Radium isotopes natural levels along with nutrients levels measurements and O, H, N isotopes to trace the nutrient sources, pathways and transformation in two contrasting bays with respectively major river and groundwater inputs (Killary Harbour and Kinvara Bay respectively, Western Ireland).
-Groundwater and recirculated sea water‐borne nutrients influence the local primary production and the local aquaculture in irish coastal areas.
- Combining Radium and radon measurements with solute distribution is a reliable and cost effective way to determine and describe fluxes and retention of major nutrients in coastal areas of restricted exchange.
-Residence time are seasonally variable in river and groundwater influenced coastal environments. Ignoring these seasonal changes as often currently done in coastal science is likely to create a bias when describing coastal nutrient cycles.
-The recirculation of sea water within a limestone karst systems have an effect on Radium and Radon levels and nutrient transformations within coastal aquifers.
Objectives: To identify and quantify nutrient exchange and reaction pathways between groundwater and surface water in Irish coastal systems hosting aquaculture and their controlling factors, in order to assess the contribution of groundwater‐borne nutrients to aquaculture and its potential to offset flow externalities associated with nutrient pollution caused by agriculture.
Aims a) to evaluate and quantitatively describe pathways followed by groundwater-borne nutrients into the food chain through aquaculture products, considering the potential retentive role of local biogeochemical processes; b) to develop nutrient budgets for selected coastal areas in receipt of SGD, comprising the quantitative discrimination of its contribution to fulfil demand from in-situ nutrient‐dependent industries, such as algae mariculture and oyster/mussel aquaculture; c) to evaluate the mitigation potential of aquaculture activities, and therefore economic effect at regional scale, in the process of recycling of excess nutrients (nitrogen in particular) produced by agricultural activities on land back into the food chain