Abteilung Oberflächengewässer

Over the last decades, the chromium (Cr) stable isotope system (referred to as δ⁵³Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ⁵³Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg⁻¹ (n = 68) and δ⁵³Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ⁵³Cr show only a weak correlation (δ⁵³Cr vs. Ln([Cr]) R² = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ⁵³Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.

The removal of organic particulate matter, predominantly phytoplankton, in eutrophic coastal seas and estuaries is considered an ecosystem service performed by large bivalve assemblages. Mussel farming has been proposed as a measure to mitigate eutrophication, as filtration directly reduces the concentration of chlorophyll a (Chl a), a primary ecological indicator. Seston depletion is typically assessed by in situ investigation, which generally lacks spatiotemporal coverage of features relative to greater ecosystem dynamics. To assess the scale and structure of this service, the present study couples multiple measurement approaches, including moored stations, synoptic transect surveys, flow cytometry, a preliminary drone survey technique, and satellite remote sensing within and around a large mussel farm. Significant depletion patterns were observed with all methods, and mixing gradients could be detected hundreds of meters beyond the farm, with repeatable patterns but distinct findings between methods. The intensity of the depletion signal was correlated with mussel biomass loads, ambient conditions, and hydrodynamic regimes, ranging from 5% to 91% relative Chl a depletion and Secchi depth increases of up to 2 m. Changes in particle size distributions were impacted in all downstream areas, as well as phytoplankton diversity. Observed depletion gradients with satellite imagery were consistent with other measurements and can be used to complement in situ field measurements. The findings of this study, will inform carrying capacity assessments, farm configuration, and development of impact assessment programs on seston removal for bivalve aquaculture.

Biodiversity loss is increasing worldwide, necessitating effective approaches to counteract negative trends. Here, we assessed aquatic macroinvertebrate biodiversity in two river catchments in Switzerland; one significantly degraded and associated with urbanisation and instream barriers, and one in a near-natural condition. Contrary to our expectations, environmental heterogeneity was lower in the near-natural stream, with enhanced productivity in the degraded system resulting in a greater range of environmental conditions. At face value, commonly employed alpha, beta and gamma biodiversity metrics suggested both catchments constituted healthy systems, with greater richness or comparable values recorded in the degraded system relative to the near-natural one. Further, functional metrics considered to be early indicators for anthropogenic disturbance, demonstrated no anticipated differences between degraded and near-natural catchments. However, investigating the identity of the taxa unique to each river system showed that anthropogenic degradation led to replacement of specialist, sensitive species indicative of pristine rivers, by generalist, pollution tolerant species. These replacements reflect a major alteration in community composition in the degraded system compared with the near-natural system. Total nitrogen and fine sediment were important in distinguishing the respective communities. We urge caution in biodiversity studies that employ numerical biodiversity metrics alone. Assessing just one aspect of diversity, such as richness, is not sufficient to track biodiversity changes associated with environmental stress. We advocate that biodiversity monitoring for conservation and management purposes must go beyond traditional richness biodiversity metrics, to include indices that incorporate detailed nuances of biotic communities that relates to taxon identity.