ALBERT

Description: Conceptual models predict a unimodal effect of consumer abundance on prey diversity with the highest diversity at intermediate consumer abundance (intermediate disturbance hypothesis). Consumer selectivity and prey productivity are assumed to be further important determinants. Preferential grazing on dominant prey species favoured by high nutrient supply is supposed to increase prey diversity, whereas the effect of consumers on prey diversity may be negative under low nutrient conditions (grazer reversal hypothesis). We tested the effect of four common consumers the isopod Idotea baltica, the amphipod Gammarus oceanicus, and the gastropods Littorina littorea and Rissoa membranacea on diversity and composition of epiphytes growing on eelgrass Zostera marina. Consumer density was manipulated (four levels: grazer free control, low, medium, high) based on abundances observed in eelgrass systems. Additionally, we manipulated nutrient supply (three levels) and the presence of Idotea in a factorial experiment. The impact of consumer abundance on epiphyte diversity varied depending on consumer identity and epiphyte evenness was affected rather than species number in this short-term experiment. Idotea reduced epiphyte diversity (Shannon-Wiener index H') and Gammarus increased epiphyte diversity. Littorina had no effect at low and medium abundance, but a negative effect in the high density treatment. Only Rissoa supported the conceptual models as it caused the proposed unimodal pattern in epiphyte diversity. The varying species-specific selectivity of the studied consumers is likely to explain their diverse impact on epiphyte diversity. Nutrients enhanced epiphyte diversity at medium enrichment, whereas higher nutrient supply reduced epiphyte diversity. The effect of Idotea changed from negative at low nutrient concentration to positive at higher nutrient supply, supporting the grazer reversal hypothesis. This study implies that consumer species identity and nutrient concentrations are important in controlling prey diversity and composition. Different consumer selectivity and changes in selectivity with growing consumer abundance and nutrient concentration are the causal factors for this effect.

Description: The European water framework directive and the marine strategy framework directive have the objective to establish at least ‘good’ ecological status in all European waters by 2015. Therefore a classification system has to be established in each European country, in order to assess their water bodies. For the German coastal waters of the Baltic Sea, a classification system for phytoplankton based on the abundance of Cyanophyceae and Chlorophyceae was already presented. This system has been successfully adapted in regions with low salinity levels (〈10 PSU). With this study, we present the results trying to develop a classification system for our German region of the Baltic Sea characterized by higher salinity (〉15 PSU). All present taxonomic groups, most common species and functional groups were tested. It could be shown that all tested correlations to nitrogen concentration as eutrophication descriptor are relatively week. Nevertheless, the biovolume of Cryptophyceae was found to be the most reliable phytoplankton composition indicator, which could serve as future assessment criterion. Furthermore, as proposed by some experts, the use of maximal dissolved winter nitrogen concentration as eutrophication descriptor might be an advantage over using the total nitrogen concentrations in summer.

Description: Traditionally, consumer–prey interactions have been considered as purely negative, but herbivores may have positive effects on plants and their productivity. Grazing may enhance prey biomass-specific productivity by directly or indirectly reducing the competition for light, nutrients, and space. We studied the effect of 4 common mesograzers, the isopod Idotea baltica, the amphipod Gammarus oceanicus, and the gastropods Littorina littorea and Rissoa membranacea on epiphytes in an eelgrass Zostera marina L. system. Eelgrass was grown in laboratory mesocosms for a set of experiments manipulating mesograzer species identity, mesograzer density and nutrient concentration. We measured epiphyte biomass-specific productivity via incorporation of radioactive carbon. Herbivore effects on epiphyte photosynthetic capacity were strongly positive for R. membranacea, moderately positive for L. littorea and I. baltica and zero for G. oceanicus under low nutrient supply. Both gastropods increased the nitrogen content of epiphytes, especially the small R. membranacea, and enhanced epiphyte growth. The crustacean species did not increase epiphyte nutrient content, but I. baltica probably enhanced epiphyte productivity by removing the overstory of algal cells, and thus reducing competition for light, nutrients, and space. The positive effect of the 2 gastropod species disappeared under higher nutrient supply, implying the importance of nutrient limitation for this interaction. The positive effect of I. baltica remained at moderate grazer densities despite the higher nutrient concentrations.

Description: Increasing levels of environmental stress due to global warming and eutrophication, and concerns about an unparalleled global diversity loss, have triggered new interest in the question whether the stability of ecosystem properties depends on population dynamics of dominant species or on compensatory growth of rare species. Recent meta-analyses suggest that compensatory dynamics are rare in natural systems. Experimental results, however, indicate that the interdependence of stressor regime, species traits, and species richness determines which mechanisms stabilise communities. Stability will depend on population dynamics of dominant species, if they remain the best performers regardless of disturbance. If dominant species become rare or lost, compensatory growth of rare species will insure natural communities against complete failure. Salinity is an important stressor governing growth and distribution of phytoplankton in brackish ecosystems, and its impact on coastal aquatic ecosystems is likely to change due to global warming. We performed two short-term experiments to investigate the effects of salinity stress on community structure and biomass production of natural phytoplankton communities collected in tidally influenced and polymictic Lake Waihola (New Zealand). The lake was brackish when the inoculum for the first experiment was collected. The inoculum for the second experiment originated from a fresh water situation. In both experiments, the phytoplankton assemblage was exposed to a salinity gradient ranging from 0 to 5. To assess the importance of dominance and compensatory growth, we determined biomass production, species richness, diversity, evenness and dominance indices, and species specific growth rates. Biomass production in our experiments was determined by dominant species. Anabaena flos-aquae dominated in the first experiment, and Asterionella formosa in the second experiment. Despite the importance of these species, we found significant growth responses of rare and abundant species. Even if these species showed high growth rates, biomass production was carried by the dominant species as long as the salinity level allowed them to grow. When the salinity level was detrimental to the growth of the dominant species, reduced dominance and increased diversity indices emphasised the importance of compensatory growth of rare species. The salinity stress applied in our experiments was strong enough to change the hierarchy of successful functional traits, which affected community structure and biomass production of the plankton communities. If the predicted sea water rise, increasing frequency of storm tides, rising water temperatures, and altered precipitation and run-off cause the salinity of coastal aquatic ecosystems to change, major changes in community composition, diversity and dominance structure of planktonic primary producers might be expected.

Description: Multiple stable isotope and fatty acid analyses were applied to examine food web dynamics in an eelgrass Zostera marina L. system in the western Baltic Sea. Samples of eelgrass, epiphytic algae, sand microflora, red algae, phytoplankton and main consumer species were collected in June 2002. delta C-13 values of primary producers ranged from -9.6%. for eelgrass to the most depleted value of -34.9%. for the most abundant red alga, Delesseria sanguinea, Epiphyte delta C-13 (-11.3 parts per thousand.) was close to the value for eelgrass, whereas sand microflora and phytoplankton showed intermediate values (-20.0 and -22.6 parts per thousand, respectively). delta C-13 values of consumer species varied from - 12.2 parts per thousand in the gastropod Lacuna vincta to -23.9 parts per thousand in the amphipod Amphitoe rubricata. Epiphytes, sand microflora and phytoplankton had relatively similar fatty acid signatures, indicating a dominance of diatoms. Fatty acid composition of the main consumers included the biomarker fatty acids for diatoms and red algae, whereas those for eelgrass were negligible. The stable isotope data in combination with the results of the fatty acid analysis strongly indicated that the food web in this eelgrass community was based on epiphytes, sand microflora and red algae. Additionally, the continuous distribution of delta N-15 values implied a food web that was characterized by a large proportion of generalist feeders in every group of consumers and by a high degree of omnivory.

Description: The effect of in situ acidification on the stable isotope ratios of carbon and nitrogen was tested in several invertebrates living in an eelgrass system. Dried and ground samples of individuals were weighted in silver cups and treated in situ with 10% HCl. Control samples were measured without acidification. This treatment to remove inorganic carbon significantly decreased the δ13C values. The δ15N values were not affected by this method of acidification. In contrast to the acid washing method the tested procedure seems suitable to remove inorganic carbon in small invertebrate species.