Habitat diversity and type govern potential nitrogen loss by denitrification in coastal sediments and differences in ecosystem-level diversities of disparate N2O reducing communities

In coastal sediments, excess nitrogen is removed primarily by denitrification. However, losses in habitat diversity may reduce the functional diversity of microbial communities that drive this important filter function. We examined how habitat type and habitat diversity affects denitrification and the abundance and diversity of denitrifying and N2O reducing communities in illuminated shallow-water sediments. In a mesocosm experiment, cores from four habitats were incubated in different combinations, representing ecosystems with different habitat diversities. We hypothesized that habitat diversity promotes the diversity of N2O reducing communities and genetic potential for denitrification, thereby influencing denitrification rates. We also hypothesized that this will depend on the identity of the habitats. Habitat diversity positively affected ecosystem-level diversity of clade II N2O reducing communities, however neither clade I nosZ communities nor denitrification activity were affected. The composition of N2O reducing communities was determined by habitat type, and functional gene abundances indicated that silty mud and sandy sediments had higher genetic potentials for denitrification and N2O reduction than cyanobacterial mat and Ruppia maritima meadow sediments. These results indicate that loss of habitat diversity and specific habitats could have negative impacts on denitrification and N2O reduction, which underpin the capacity for nitrogen removal in coastal ecosystems

Denitrification, Nitrogen Uptake, and Organic Matter Quality Undergo Different Seasonality in Sandy and Muddy Sediments of a Turbid Estuary

The interaction between microbial communities and benthic algae as nitrogen (N) regulators in poorly illuminated sediments is scarcely investigated in the literature. The role of sediments as sources or sinks of N was analyzed in spring and summer in sandy and muddy sediments in a turbid freshwater estuary, the Curonian Lagoon, Lithuania. Seasonality in this ecosystem is strongly marked by phytoplankton community succession with diatoms dominating in spring and cyanobacteria dominating in summer. Fluxes of dissolved gas and inorganic N and rates of denitrification of water column nitrate (Dw) and of nitrate produced by nitrification (Dn) and sedimentary features, including the macromolecular quality of organic matter (OM), were measured. Shallow/sandy sites had benthic diatoms, while at deep/muddy sites, settled pelagic microalgae were found. The OM in surface sediments was always higher at muddy than at sandy sites, and biochemical analyses revealed that at muddy sites the OM nutritional value changed seasonally. In spring, sandy sediments were net autotrophic and retained N, while muddy sediments were net heterotrophic and displayed higher rates of denitrification, mostly sustained by Dw. In summer, benthic oxygen demand increased dramatically, whereas denitrification, mostly sustained by Dn, decreased in muddy and remained unchanged in sandy sediments. The ratio between denitrification and oxygen demand was significantly lower in sandy compared with muddy sediments and in summer compared with spring. Muddy sediments displayed seasonally distinct biochemical composition with a larger fraction of lipids coinciding with cyanobacteria blooms and a seasonal switch from inorganic N sink to source. Sandy sediments had similar composition in both seasons and retained inorganic N also in summer. Nitrogen uptake by microphytobenthos at sandy sites always exceeded the amount loss via denitrification, and benthic diatoms appeared to inhibit denitrification, even in the dark and under conditions of elevated N availability. In spring, denitrification attenuated N delivery from the estuary to the coastal area by nearly 35%. In summer, denitrification was comparable (~100%) with the much lower N export from the watershed, but N loss was probably offset by large rates of N-fixation

Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

Nutrient loading to coastal bay ecosystems is of a similar magnitude as that to deeper, river-fed estuaries, yet our understanding of the eutrophication process in these shallow systems lags far behind. In this synthesis, we focus on one type of biotic feedback that influences eutrophication patterns in coastal bays — the important role of primary producers in the ‘coastal filter’. We discuss the 2 aspects of plant-mediated nutrient cycling as eutrophication induces a shift in primary producer dominance: (1) the fate of nutrients bound in plant biomass, and (2) the effects of primary producers on biogeochemical processes that influence nutrient retention. We suggest the following generalizations as eutrophication proceeds in coastal bays: (1)Long-term retention of recalcitrant dissolved and particulate organic matter will decline as seagrasses are replaced by algae with less refractory material. (2) Benthic grazers buffer the early effects of nutrient enrichment, but consumption rates will decline as physico-chemical conditions stress consumer populations. (3) Mass transport of plant-bound nutrients will increase because attached perennial macrophytes will be replaced by unattached ephemeral algae that move with the water. (4) Denitrification will bean unimportant sink for N because primary producers typically out compete bacteria for available N, and partitioning of nitrate reduction will shift to dissimilatory nitrate reduction to ammonium in later stages of eutrophication.In tropical/subtropical systems dominated by carbonate sediments, eutrophication will likely result in a positive feedback where increased sulfate reduction and sulfide accumulation in sediments will decrease P adsorption to Fe and enhance the release of P to the overlying water