Microphytobenthos and chironomid larvae attenuate nutrient recycling in shallow‐water sediments

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

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Patterns of benthic oxygen uptake in a hypertrophic lagoon: spatial variability and controlling factors

Water temperature, organic matter quality and quantity and macrofauna activity generally regulate the seasonal evolution of benthic oxygen uptake in coastal areas. We hypothesize that highly productive lagoons can represent an exception in this respect, due to alternating sequences of phytoplankton bloom, dystrophy and collapse events, coupled with water anoxia and azoic sediments. In order to verify this assumption, total oxygen uptake (TOU) and diffusive oxygen uptake (DOU) were determined during the ice-free period of 2009 in the sediments of a hypertrophic basin (the Curonian Lagoon, Baltic Sea). Seasonal measurements were carried out via sediment incubation and microprofiling in littoral and pelagic areas. TOU increased from spring to summer, but it remained elevated also in autumn likely due to accumulation of labile organic matter after algal blooms. TOU and DOU closely agreed in pelagic areas, while at littoral sites TOU exceeded DOU, suggesting temporal or local importance of bioturbating organisms. Water chlorophyll a and oxygen saturation were likely the most important driving factors for benthic respiration. Very limited oxygen penetration (<1?mm) over a 6-month period possibly enhances nitrogen removal via denitrification and reactive phosphorus efflux.     

Origin and fate of dissolved organic matter in four shallow Baltic Sea estuaries

Coastal waters have strong gradients in dissolved organic matter (DOM) quantity and characteristics, originating from terrestrial inputs and autochthonous production. Enclosed seas with high freshwater input therefore experience high DOM concentrations and gradients from freshwater sources to more saline waters. The brackish Baltic Sea experiences such salinity gradients from east to west and from river mouths to the open sea. Furthermore, the catchment areas of the Baltic Sea are very diverse and vary from sparsely populated northern areas to densely populated southern zones. Coastal systems vary from enclosed or open bays, estuaries, fjords, archipelagos and lagoons where the residence time of DOM at these sites varies and may control the extent to which organic matter is biologically, chemically or physically modified or simply diluted with transport off-shore. Data of DOM with simultaneous measurements of dissolved organic (DO) nitrogen (N), carbon (C) and phosphorus (P) across a range of contrasting coastal systems are scarce. Here we present data from the Roskilde Fjord, Vistula and Öre estuaries and Curonian Lagoon; four coastal systems with large differences in salinity, nutrient concentrations, freshwater inflow and catchment characteristics. The C:N:P ratios of DOM of our data, despite high variability, show site specific significant differences resulting largely from differences residence time. Microbial processes seemed to have minor effects, and only in spring did uptake of DON in the Vistula and Öre estuaries take place and not at the other sites or seasons. Resuspension from sediments impacts bottom waters and the entire shallow water column in the Curonian Lagoon. Finally, our data combined with published data show that land use in the catchments seems to impact the DOC:DON and DOC:DOP ratios of the tributaries most.

     

The influence of cyanobacteria blooms on the attenuation of nitrogen throughputs in a Baltic coastal lagoon

We combined a mass balance approach with measurements of air–water and sediment–water nitrogen (N) exchange to better understand the mechanisms attenuating N throughputs in a eutrophic coastal lagoon. We were particularly interested in how seasonal shifts in external versus internal N fluxes and the transition from diatom- to cyanobacteria- dominated phytoplankton communities influence N storage and loss to the atmosphere. We found that on an annual basis almost all of the N removed by the lagoon was due to sediment storage following the spring diatom bloom. This period was characterized by high riverine inputs of dissolved inorganic nitrogen, high rates of assimilatory conversion to particulate nitrogen (PN), and net accrual of N in sediments. By contrast, the larger summer bloom was associated with low sediment N storage, which we attribute in part to the presence of positively-buoyant cyanobacteria. Low settling rates during cyanobacteria blooms favored export of PN to the Baltic Sea over sediment accrual in the lagoon. In addition, summer dinitrogen (N₂) fixation by cyanobacteria largely offset annual N₂ losses via denitrification. These findings show that cyanobacteria blooms diminish N attenuation within the lagoon by altering the balance of N exchange with the atmosphere and by promoting export of particulate N over sediment burial.     

Rare but large bivalves alter benthic respiration and nutrient recycling in riverine sediments

Bioturbation studies have generally analyzed small and abundant organisms while the contribution to the benthic metabolism by rare, large macrofauna has received little attention. We hypothesize that large, sporadic bivalves may represent a hot spot for benthic processes due to a combination of direct and indirect effects as their metabolic and bioturbation activities. Intact riverine sediments with and without individuals of the bivalve Sinanodonta woodiana were collected in a reach with transparent water, where the occurrence of the mollusk was clearly visible. The bivalve metabolism and its effects on sedimentary fluxes of dissolved gas and nutrients were measured via laboratory incubations of intact cores under controlled conditions. S. woodiana contributed significantly to O₂ and TCO₂ benthic fluxes through its respiration and to \({\text{NH}}_{4}^{ + }\), SRP and SiO₂ regeneration via its excretion. The bivalve significantly stimulated also microbial denitrification and determined a large efflux of CH₄, likely due a combination of bioturbation and biodeposition activities or to anaerobic metabolism within the mollusk gut. This study demonstrates that a few, large individuals of this bivalve produce significant effects on aerobic and anaerobic benthic metabolism and nutrient mobilization. Random sediment sampling in turbid waters seldom catches these important effects due to low densities of large fauna.