No abstract

The ocean's nitrogen cycle is driven by complex microbial transformations, including nitrogen fixation, assimilation, nitrification, anammox and denitrification. Dinitrogen is the most abundant form of nitrogen in sea water but only accessible by nitrogen-fixing microbes. Denitrification and nitrification are both regulated by oxygen concentrations and potentially produce nitrous oxide (N 2 O), a climate-relevant atmospheric trace gas. The world's oceans, including the coastal areas and upwelling areas, contribute about 30 per cent to the atmospheric N 2 O budget and are, therefore, a major source of this gas to the atmosphere. Human activities now add more nitrogen to the environment than is naturally fixed. More than half of the nitrogen reaches the coastal ocean via river input and atmospheric deposition, of which the latter affects even remote oceanic regions. A nitrogen budget for the coastal and open ocean, where inputs and outputs match rather well, is presented. Furthermore, predicted climate change will impact the expansion of the oceans' oxygen minimum zones, the productivity of surface waters and presumably other microbial processes, with unpredictable consequences for the cycling of nitrogen. Nitrogen cycling is closely intertwined with that of carbon, phosphorous and other biologically important elements via biological stoichiometric requirements. This linkage implies that human alterations of nitrogen cycling are likely to have major consequences for other biogeochemical processes and ecosystem functions and services.

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Macrofaunal samples were collected seasonally from 1978 to 1995 in the subtidal zone off Norderney, one of the East Frisian barrier islands. Samples were taken with a 0.2 m' van Veen grab a t 5 sites with water depths of 10 to 20 m . Interannual variability in biomass, abundance and species number of the blota were related to interannual climate variability using multivariate regression models. Changes in the biota were described in relation to human impact and seasonal and long-term meteorological variability. Our analyses suggest that macrofaunal comnlunities are severely affected by cold winters, whereas storms and hot summers have no impact on the communiti~s It appears that mild meteorological conditions, probably actlng In conjunction with eutrophicatlon, have resulted in an Increase in total biomass since 1989 A multlvariate model found the follo~ving strong relationship: abundance, species number and [less clear) blomass in the second quarter are correlated with the North Atlantic Oscillation (NAO). The med~ator between the NAO and benthos IS probably the seasurface temperature (SST) in late wlnter and early spring. On the basis of our results, w e suggest that most of the interannual variab~l~ty 111 macrozoobenthos can be explained by climate var~ability.

HighlightsWe modelled the surface water salinity in the Baltic from the 1960s to 2100.We studied possible changes in distribution areas of predominant plant, invertebrate and fish species.The results suggest a critical shift in the salinity range 5–7, which is a bottleneck for both marine and freshwater species distribution and diversity.This foreseen salinity change is likely to have large impacts on marine ecology, it́s monitoring, modelling as well as fisheries.