Recent Trends (2012–2016) of N, Si, and P Export from the Nemunas River Watershed: Loads, Unbalanced Stoichiometry, and Threats for Downstream Aquatic Ecosystems

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.

Stoichiometry of regenerated nutrients differs between native and invasive freshwater mussels with implications for algal growth

“…Cyanobacteria blooms are regularly occurring in the Curonian Lagoon, which is a large freshwater estuary (Gasiūnaitė, Daunys, Olenin, & Razinkovas, ). The large nutrient inputs of the Nemunas River to the Curonian Lagoon (44,208 ± 12,677 tons total N/year, 59,048 ± 10,770 tons total Si/year and 1,547 ± 266 tons total P/year) largely contribute to the hypertrophic status of this estuary (Vybernaite‐Lubiene, Zilius, Saltyte‐Vaisiauske, & Bartoli, ). Besides dissolved nutrients, the Nemunas River delivers large amounts of phytoplankton, with an estimated annual load of ~350 tons chl a/ year (Vybernaite‐Lubiene et al., ).…”

“…Such loads undergo profound seasonal variations, with summer minima for inorganic nutrients and summer peaks for particulate forms. Interestingly, the ecological stoichiometry of dissolved inorganic N, Si, and P displays a pronounced and steep drop in the critical period between the spring and the summer, when N and Si limitation establishes for 4–5 months (Vybernaite‐Lubiene et al., ). Such limitation was generally considered to be the main reason favouring cyanobacteria blooms in the Curonian Lagoon.…”

“…In the present study, the benthic system of the Nemunas River deltaic area did not release any dissolved reactive Si, regardless the presence of mussels. This suggests either a strong Si limitation of the benthic primary producers and a retention/translocation of Si at the sediment–water interface or the dominance in late summer of not siliceous algae feeding the mussels (Vybernaite‐Lubiene et al., , ).…”

The Effect of Chironomid Larvae on Nitrogen Cycling and Microbial Communities in Soft Sediments

“…Within the estuarine N cycle, permanent N removal via dissimilative processes such as denitrification is of particular interest, as well as undesired dissimilative nitrate reduction to ammonium (DNRA)-N recycling, which is favoured under eutrophic conditions [8,9]. Processes leading to permanent N removal counteract the excessive loads of reactive N to coastal areas, resulting from anthropogenic activities such as agriculture and animal farming and inducing eutrophication, loss of biodiversity and the deterioration of ecosystem health [10,11].…”

Estuarine Macrofauna Affects Benthic Biogeochemistry in a Hypertrophic Lagoon

“…In estuarine systems, the understanding of the role of macrofauna communities on benthic N-cycling is a keystone, due to its large inputs from catchments and potentially large macrofauna-mediated microbial N losses [27][28][29][30][31]. It is well known that macrofauna actively contribute to the translocation and transformations of N within and among different compartments of aquatic ecosystems, stimulating microbial processes [32,33].…”