Nina Reuss

Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000–4000 B.P. during the Littorina transgression, (2) ca. 1400–700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions.

The plankton community on Sukkertop and Fylla Banks off West Greenland during a spring bloom and post-bloom period: Hydrography, phytoplankton and protozooplankton

Abstract The plankton community structure was investigated on Sukkertop and Fylla Banks off West Greenland during the spring bloom in May 2000 and the post-bloom period in June 1999. In May a small change in density, clearly illustrated by the profile of potential energy, was sufficient to support a spring bloom in the upper part of the water column. The spring-bloom phytoplankton community displayed high biomass (92 ± 45 mg C m3) dominated by species of the genera Thalassiosira and Chaetoceros. The phytoplankton species composition, ongoing sedimentation and nutrient depletion indicated late spring-bloom conditions. Spring-bloom heterotrophic biomass (17 ± 7 mg C m-3) was dominated by large heterotrophic dinoflagellates (> 20 μm). In June, low autotrophic biomass was present at post-bloom stations (2 ± 1 mg C m-3) and small autotrophic flagellates (< 10 μm), mostly haptophytes, dominated the phytoplankton community. Heterotrophic biomass was low (5 ± 1 mg C m-3) and an important part was comprised by heterotrophic nanoflagellates (24 ±1%). Protozooplankters (heterotrophic dinoflagellates and ciliates) were important grazers of the phytoplankton community in the post-bloom period (estimated grazing impact 112 ± 15% d-1) and in the spring bloom (9 ± 2% d-1).

The Importance of Eolian Input on Lake-Sediment Geochemical Composition in the Dry Proglacial Landscape of Western Greenland

ABSTRACT In proglacial landscapes, such as western Greenland, eolian transport plays an important role for the influx of particulate material to lakes. On the basis of an analysis of a sediment profile and surface sediments from several lakes, we show that eolian activity has a strong influence on sediment deposition in time and space. Principal component analysis revealed that sediments that accumulated during periods with high eolian activity were enriched in zirconium—originating from coarse silt and sand fractions preferentially transported by wind—and depleted in rubidium. In addition, zirconium to rubidium ratios in the surface sediment of four additional lakes decreased with distance from the ice sheet. Finally, previously published data show that pH and alkalinity tend to be higher in lakes close to the front of the ice sheet, which we speculate is coupled to a larger supply of fresh eolian material. These findings demonstrate that lakes in proglacial landscapes may receive a substantial part of their sediment load through eolian deposition, and that this is especially true close to the glacial outwash plains along the ice margin.

Microhabitat influence on chironomid community structure and stable isotope signatures in West Greenland lakes

Most functional feeding types are represented within the species rich group of aquatic chironomids. Thus, we hypothesized that different lake types and microhabitats within lakes would (1) host specific chironomid communities and (2) that the individual communities would show specific δ13C stable isotope signatures reflecting the prevailing origin of food source. To test our hypotheses, five lakes in southwest Greenland were investigated at a high taxonomic resolution and with detailed information on δ13C signature of the chironomids and of individual microhabitats (macrophytes, sediment, stones, and profundal). We found that there was a significant difference in δ13C between the chironomid assemblages of freshwater lakes and oligosaline lakes, while assemblages of the littoral microhabitats did not differ significantly. The δ13C of chironomids reflected the wide variety of habitat signals, particularly in the freshwater lakes. Our results indicate that many chironomid taxa are ubiquitous and are found in several microhabitats, suggesting that they can adjust their feeding strategy according to the habitat. The implication is that chironomid assemblage composition has only limited use as indicator of littoral microhabitats in the Arctic. On the other hand, the δ13C signature of fossil chironomids might have a potential as indicator of microhabitats in freshwater lakes.