Abstract
The Curonian Lagoon, the largest in Europe, suffers from nuisance cyanobacterial blooms during summer, probably triggered by unbalanced nutrient availability. However, nutrient delivery to this system was never analysed in detail. During 2012–2016, we analysed the monthly discharge, nutrient loads, and ecological stoichiometry at the closing section of the Nemunas River, the main nutrient source to the lagoon. The aim of this study was to investigate seasonal and annual variations of nitrogen (N), silica (Si), and phosphorous (P) with respect to discharge, climatic features, and historical trends. The nutrient loads varied yearly by up to 50% and their concentrations underwent strong seasonality, with summer N and Si limitation. The river discharge (16 ± 4 km3·yr−1) was lower than the historical average (21.8 km3·yr−1). Changes in agricultural practices resulted in similar N export from the river watershed compared to historical data (1986–2002), while sewage treatment plant improvements led to a ~60% decrease of P loads. This work contributes new data to the scattered available information on the most important nutrient source to the Curonian Lagoon. Further P reduction is needed to avoid unbalanced dissolved inorganic nitrogen and phosphorus (DIN:DIP~10) ecological stoichiometry in summer, which may stimulate undesired cyanobacterial blooms.
Highlights
Anthropogenic pressures on ecosystems have led to changes of nutrient genesis, cycling, and transport in watersheds [1,2,3,4,5]
The Nemunas River is the largest river in Lithuania and has a catchment area of 97,864 km2, of which 46,695 km2 is located in Lithuania, 45,389 km2 is located in Belarus, and the rest is located in Latvia, Poland, and the Kaliningrad area [39] (Figure 1)
The discharge from the Nemunas River displayed seasonal and interannual variability, with higher flows generally measured during colder months and lower flows measured during warmer months (Figure 2)
Summary
Anthropogenic pressures on ecosystems have led to changes of nutrient genesis, cycling, and transport in watersheds [1,2,3,4,5]. Climate change, which affects snow cover period, precipitation intensity, and timing, may favour soil erosion and increase short-term transport of particulate phosphorus (P) and silica (Si) to coastal areas [9,10,11]. At regional scales, ongoing socio-economic changes may produce larger impacts on biogeochemical cycles than those produced by climate changes [14]. Socio-economic changes may affect entire productive sectors, with variable price of fertilizers and number of livestock affecting N budgets in watersheds [15,16].
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