Evaluation of nitrate fluxes into a coastal lagoon through Submarine Groundwater Discharge (SGD)

Abstract

Submarine Groundwater Discharge (SGD) delivers substantial land-derived nitrate (NO3-) to coastal regions, which potentially influences the environmental status and ecological balance in the receiving water. A systematic study of NO3- discharge from SGD on a regional scale is however limited. In this thesis, distribution, turnover and circulation of SGD-borne NO3- in one semi-enclosed coastal system, the Ria Formosa lagoon (Portugal) were studied. The research in this thesis comprises an examination of NO3 - transformation within the subterranean estuary, a regional quantification of NO3- input from SGD to the lagoon, and an evaluation of nutrient ratios and limiting nutrient in the case of N, P, and Si in SGD. A combination of in-situ sampling and laboratory simulation experiments under controlled conditions with deterministic modelling techniques was employed in order to quantify NO3- reduction/accumulation rates and to parameterise key factors affecting N processing in the benthic compartment. In particular, to separate the effects of chemical reactions from those of physical transport on porewater concentrations of N at the subterranean estuary, a 2-D Advection-Reaction model was used to describe the in-situ N-transformation rate. Results indicated that the subterranean estuary in the Ria Formosa lagoon is an important biogeochemical reactor, with clear effects on N turnover on a regional scale. The transformation rates varied from -0.69 mmol m-3 h-1 to 2.44 mmol m-3 h-1 for NO3- in different seasons. As a potential electron donor to both NO3- reductions and benthic respiration, labile dissolved organic matter (DOM) may play an important role in the modulation of SGD-borne NO3-. Flow-Through Reactor (FTR) experiments with sediment collected at a sandy beach site within the Ria Formosa lagoon were performed under different labile DOM and NO3- input conditions. Results suggest that more than 70% of glucose added to porewater was utilised by aerobic respiration, while the stimulation of NO3- reduction in the sediment was limited compared with the enhancement of respiration. This suggests that an enrichment of labile DOM in the subterranean estuary may lead to an increase in the magnitude of NO3- fluxes in SGD due to the significant stimulation of respiration and presence of nitrification. The regional survey based on the NO3- concentration in endmembers and 222Rn activity revealed that NO3 - fluxes associated with SGD may reach 1.3(±1.0)×103 kg day-1, even though a substantial amount of NO3- might be reduced in the subterranean estuary prior to discharge. Compared to the entire NO3- input into the lagoon, the magnitude of NO3- fluxes from SGD accounted for 89%. Furthermore, in the total NO3-budget of SGD, approximately 98% NO3- was derived from fresh groundwater. The exogenous NO3- from SGD was rapidly removed from the lagoon system. In particular, the reduction rate reached 69.9±68.5 μmol N-NO3- m2 h-1. Such high reduction may reflect the assimilation potential from the lagoon algae. As an important nutrient source in coastal regions, nutrient structure in SGD may influence ecological balance in the receiving water. To understand variations of nutrient structure in SGD, database established by previous annual field survey in the barrier island at the Ria Formosa lagoon was revaluated. Limiting nutrient along the mixing gradient was examined via a combination of frequency analysis and Dirichlet regression. Results showed that limiting nutrient in SGD can vary between N and P in both spring term and autumn term. Furthermore, in the autumn survey, the occurrence of Si-limitation in SGD was observed which may result from an increase of NO3- concentration in terrestrial groundwater and the presence of a spectrum of biogeochemical reactions in the subterranean estuary. To constrain the impact of SGD on the ecological balance in the lagoon, two possible regulation methods, further reduction of the NO3- concentration in terrestrial groundwater and proper management of groundwater sources, should be included in future regulation.