Danish coastal waters were severely affected by eutrophication in the 1980s, prompting the implementation of national nutrient reduction plans. The aim of this study was to explore the recovery of Danish coastal waters and report trends for selected indicators of eutrophication. Dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), chlorophyll a, light penetration depth, eelgrass main depth limit, a benthic...

Parasites are important members of ecosystems, and recent metabarcoding efforts have revealed an abundance of microbial eukaryotic parasites in copepods. However, a putative linkage to host community biomass and composition has not been addressed. We performed zooplankton tows monthly for 2 yr at five stations in the Oslofjord, Norway. The samples were cleared of larger, non‐copepod zooplankton and contained mainly copepods an...

Climate change is projected to cause elevated precipitation in northern Europe, leading to increased runoff of terrestrial matter to coastal areas. The consequences for food web production and ecosystem function remain unclear. A mesocosm experiment was performed to investigate the impacts of elevated terrestrial matter input, using a natural plankton community from the northern Baltic Sea with added young‐of‐the‐year perch as...

Nitrogen fixation, the microbial reduction of dinitrogen to ammonia, is increasingly recognized to occur in the Arctic Ocean. However, knowledge about the composition, biogeography, abundance, and ecology of nitrogen‐fixing organisms (diazotrophs) is poor. This ultimately hinders the prediction of ecosystem productivity fueled by nitrogen fixation in this rapidly changing and predominantly nitrogen‐limited ocean. We assessed t...

Microbe-mineral interactions, such as mineral substrate utilization and aggregate formation, have played a key role in the cycling of elements through Earth evolution. In water, soils, and sediment biogeochemistry modulates microbial community composition and mineral formation over spatial and temporal scales. Plastic is a new material that is now widespread in the environment. Both microbial and mineral associations with plas...

The oceanic dissolved organic phosphorus (DOP) pool is mainly composed of P-esters and, to a lesser extent, equally abundant phosphonate and P-anhydride molecules. In phosphate-limited ocean regions, diazotrophs are thought to rely on DOP compounds as an alternative source of phosphorus (P). While both P-esters and phosphonates effectively promote dinitrogen (N 2 ) fixation, the role of P-anhydrides for diazotrophs is unknown....

Estuaries receive substantial anthropogenic nitrogen loading and are mainly considered net nitrogen sinks. While several studies have identified diverse diazotrophic communities in estuarine sediments, the role of pelagic diazotrophs in these systems is not well understood. We investigated the links between diazotrophic community composition, nitrogenase ( nifH ) gene expression, N 2 fixation, and environmental conditions in N...

Summary Bacteria are major drivers of biogeochemical nutrient cycles and energy fluxes in marine environments, yet how bacterial communities respond to environmental change is not well known. Metagenomes allow examination of genetic responses of the entire microbial community to environmental change. However, it is challenging to link metagenomes directly to biogeochemical process rates. Here, we investigate metagenomic respon...

Summary Nitrification is important in nitrogen (N) cycling of aquatic environments, but knowledge about its regulation and importance is sparse. Here we examined nitrification and ammonia oxidizers in the Baltic Sea. We investigated two sites with different catchment characteristics (agricultural and forest), the Bay of Gdánsk (south) and the Öre Estuary (north), and measured pelagic nitrification rates and abundance, composit...

Summary The number of icebergs produced from ice‐shelf disintegration has increased over the past decade in Antarctica. These drifting icebergs mix the water column, influence stratification and nutrient condition, and can affect local productivity and food web composition. Data on whether icebergs affect bacterioplankton function and composition are scarce, however. We assessed the influence of iceberg drift on bacterial comm...

Specific regions of otherwise oligotrophic oceans seem to attract fish spawning and sustain significant abundances of fish larvae. The Sargasso Sea in the North Atlantic subtropical gyre is known as the spawning area of the Atlantic eels, but numerous other fish species also spawn in the area. In order to evaluate spatial variability of larval fish in the region, we examined species diversity, composition and abundances at eig...

Summary Due to climate change, sea ice experiences changes in terms of extent and physical properties. In order to understand how sea ice microbial communities are affected by changes in physicochemical properties of the ice, we used 454‐sequencing of 16 S and 18 S rRNA genes to examine environmental control of microbial diversity and composition in A ntarctic sea ice. We observed a high diversity and richness of bacteria, whi...

Summary The community composition of N 2 ‐fixing microorganisms (diazotrophs) was investigated in copepods (primarily A cartia spp.) in parallel to that of seawater in coastal waters off D enmark ( Ø resund) and N ew E ngland, USA . The unicellular cyanobacterial diazotroph UCYN ‐ A was detected from seawater and full‐gut copepods, suggesting that the new N contributed by UCYN ‐ A is directly transferred to higher trophic leve...

Summary The wide distribution of diverse nitrogenase ( nifH ) genes affiliated with those of heterotrophic bacteria in marine and estuarine waters indicates ubiquity and an ecologically relevant role for heterotrophic N 2 ‐fixers (diazotrophs) in aquatic nitrogen ( N ) cycling. However, the lack of cultivated representatives currently precludes an evaluation of their N 2 ‐fixing capacity. In this study, microoxic or anoxic N ‐...