Seasonal variation of nitrogen transformations in a subterranean estuary on the Ria Formosa lagoon barrier islands, Portugal

The barrier islands of the Ria Formosa lagoon are known to host substantial land-derived nitrogen (N) originated in the adjacent coastal plain. The permeable subterranean estuary (STE) of the barrier islands may modulate the magnitude of N fluxes derived from submarine groundwater discharge (SGD) into the lagoon. To understand the temporal dynamics of benthic N reactivity of the STE, a seven-month field survey was conducted. Results showed that the STE shifted seasonally between acting as a source and a sink for nitrate (NO₃⁻) and total dissolved nitrogen (TDN). In particular, biogeochemical processes within the sandy sediment that removed reactive N from porewater followed either canonical denitrification or oxygen-limited autotrophic nitrification/denitrification (OLAND). The maximum net removal rates for NO₃⁻ and TDN were observed in May and February, reaching 0.67 mmol/(m³·h) and 13.11 mmol/(m³·h), respectively. On the other hand, the STE could increase concentrations of NO₃⁻ and TDN in the seeped water via remineralisation and nitrification. The maximum net production rate for NO₃⁻ reached 2.44 mmol/(m³·h), while the net TDN addition rate was 21.49 mmol/(m³·h). Compared to the estimated N concentrations in the fresh groundwater endmember, the STE from December to February may lead to an increase of 152% in the magnitude of NO₃⁻ fluxes. In contrast, from March to June, the STE may foster a decrease of 22% of the NO₃⁻ fluxes into the lagoon. These observations suggest that the STE significantly regulates N fluxes and is an important reaction node within coastal systems.