Abstract
Estuaries are biogeochemical reactors able to modulate the transfer of energy and matter from the watershed to the coastal zones and to retain or remove large amounts of terrestrially generated nutrients. However, they may switch from nutrient sink to source depending upon interannual variability of the nutrient supply and internal processes driving whole system metabolism (e.g., net autotrophic or heterotrophic). We tested this hypothesis in the Curonian Lagoon, a hypertrophic estuary located in the south east Baltic Sea, following the budget approach developed in the Land-Ocean Interactions in the Coastal Zone (LOICZ) project. Annual budgets for nitrogen (N), phosphorus (P), and silica (Si) were calculated for the 2013–2015 period. The lagoon was divided in a flushed, nutrient loaded area, and in a confined, less loaded area. The lagoon was always a sink for dissolved inorganic Si and P whereas it was a N sink in the confined area, dominated by denitrification, and a N source in the flushed area, due to dinitrogen (N2) fixation. The net ecosystem metabolism (NEM) indicated that the Curonian Lagoon was mainly autotrophic because of high primary production rates. In this turbid system, low N:P ratio, high summer temperatures, and calm weather conditions support high production of N2-fixing cyanobacteria, suppressing the estuarine N-sink role.
Highlights
River is another tributary directly discharging in Box 1; it generally accounts for 5–8% of the Nemunas River flow, with peaks up to 10–13% in the winter months
The lagoon acted as an efficient filter, in particular for dissolved inorganic phosphorus (DIP) (74% of load), and less for dissolved inorganic nitrogen (DIN) and DSi (10% and 25% of load, respectively). These findings show that DIP retention in the Curonian Lagoon is >30%
Results from this study support the hypothesis that the Curonian Lagoon functioning, which results from the combined biogeochemical processes occurring in this estuary, display large spatial and temporal variability
Summary
Accepted: 5 January 2022Estuaries are highly productive transitional areas between land and the open ocean, contributing with other coastal systems to 30–40% of the global oceanic production [1,2].Their productivity depends on nutrient inputs from upstream catchments and specific hydrometeorological conditions, which in turn regulate nutrient loads to the sea [3,4,5,6,7].Estuaries are frequently considered as important areas for nutrient loads attenuation due to intense productivity, high rates of biogeochemical activity, and elevated water residence time [8,9,10]. The equilibrium between nutrient retention and release can vary at different temporal scales spanning from diel (e.g., light-dependent uptake), seasonal (e.g., primary producers growth and decay), or interannual (e.g., net autotrophic or heterotrophic macro-periods) [4,18]. Other factors such as eutrophication, unbalanced nutrient stoichiometry or altered timing in nutrient transport
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