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.

Filter‐feeding mussels couple benthic and pelagic environments and create biogeochemical hot spots. Mussels may exert either top‐down control (via filtration) or bottom‐up stimulation (via biodeposition and excretion) of primary producers. Mussel metabolism may be species‐specific and the disappearance of native species or their replacement by invasive species may affect ecosystem functioning, notably gas exchange, nutrient fluxes, and stoichiometry at the sediment–water interface. In this study, we tested experimentally how native (unionids) and invasive (dreissenids) mussels, singly and in association, affect benthic fluxes of dissolved gas and nutrients in the light and in the dark, and indirectly phytoplankton growth from a freshwater estuary, the Curonian Lagoon, where the two species of mussel coexist. We show that native and invasive mussels stimulated O₂ consumption and the production of total dissolved inorganic carbon and N₂ increasing sediment heterotrophy and nutrient regeneration, with species‐specific effects on nutrient stoichiometry and algal growth. Ammonium and SiO₂ exchanges were similar for both species, while soluble reactive phosphorus fluxes and excretion rates were significantly higher in sediments with dreissenids. Phytoplankton growth was also significantly higher in the presence of dreissenids as compared to unionids. Dreissenid mussels decreased the dissolved inorganic N to P ratio of regenerated nutrients and may favour the growth of cyanobacteria. Hence, the replacement of native with invasive mussels may produce large changes in benthic nutrient cycling and in phytoplankton growth and community composition.

Sediment-water oxygen, ammonium and soluble reactive phosphorus fluxes in a turbid freshwater estuary (Curonian lagoon, Lithuania): evidences of benthic microalgal activity

The diversity and spatial distribution of microscopic invertebrates in the Arctic have yet to be studied in detail. Knowledge of this is especially important in the context of glacier shrinking, one of the most visible environmental consequences of climate change. To understand how time since deglaciation shapes the communities of limno-terrestrial microfauna, we analysed samples of moss collected during the summer of 2021 in forefields of Trygghamna (Svalbard), where glaciers thought to have started to recede at the beginning of the twentieth century. We estimated the taxonomic and trophic composition and abundance of all microfauna groups (with a specific focus on little-known bdelloid rotifers) in two areas, which correspond to the different stages of the glacial retreat. The impact of 14 other environmental parameters (distance from the sea, moisture, moss structural complexity, soil nutrient, and isotopic composition) was considered. Thirty-seven microfauna taxa were found, wherein Dorylaimida nematodes (Dorylaimida, Nematoda) dominated both in frequency (present at all sites) and abundance (50 ± 21% from all individuals). Less prevalent bdelloid rotifers (Bdelloidea, Rotifera) were followed by tardigrades (Tardigrada), which, in contrast to others, were more abundant in the later deglaciated area. In general, environmental parameters explained 81.8% of the microfauna distribution. Time since deglaciation was the most significant factor (9.5%). Among the variables that act at a fine scale, the most important were moisture in the habitat (6%), presence of ground in the samples (4%), and δ¹⁵N (4%). The moss structural complexity did not have a significant effect, and neither did most of the variables characterising soil nutrient and isotopic composition. However, some variability was observed for different lower taxa.

The cryosphere encompasses a wide range of habitats that support cold-adapted communities. Seasonal snow patches (SSPs) in lowlands are underexplored cryohabitats, characterised by a short persistence period (late winter–early spring) and the presence of trees around, in contrast to large vegetation-free high-mountain and polar ecosystems. To provide the first assessment of organisms from SSPs, we focused on invertebrate diversity and densities from 40 sites in the Baltic States in relation to physicochemical parameters (i.e., suspended solids, chlorophyll a, nutrients), microalgae, and surrounding landscape features (i.e., tree canopy cover). SSPs appeared to be an important spot for bdelloid rotifers (Bdelloidea), tardigrades (Tardigrada), and nematodes (Nematoda), which together accounted for 60 %–100 % of all invertebrates, reaching densities >7000 ind m−2. Acari and Insecta were less abundant, whereas other invertebrates occurred only sporadically. The community was strongly determined by surroundings (trees), which supply snow ecosystems with organic and inorganic matter. Chlorophyll a, particulate phosphorus, total suspended solids and organic debris were strongest predictors of invertebrate distribution. The canopy cover also influenced invertebrate communities, highlighting the importance of the organic deposition from trees and also suggesting that trees may be a source of microscopic invertebrates to the snow. Results demonstrate the importance of SSPs as overlooked ephemeral habitats and can be used as a baseline for future changes in snow communities in temperate regions.