Grazer diversity effects in an eelgrass–epiphyte–microphytobenthos system

The dramatic loss of biodiversity and its consequences for ecosystem processes have been of considerable interest in recent ecological studies. However, the complex and interacting processes influencing diversity effects in multitrophic systems are still poorly understood. We used an experimental eelgrass system to study the effects of changing richness of three consumer species on the biomass, diversity and taxonomic composition of both epiphytic and benthic microalgal assemblages. After 1 week, consumer richness enhanced the grazing impact on epiphyte biomass relative to single consumer treatments and a positive effect of consumer richness on prey diversity was found. Moreover, strong effects of consumer species identity on taxonomic composition were found in both microalgal assemblages. However, the effects of consumer richness were not consistent over time. The consequences of high nutrient availability seemed to have masked consumer richness effects

Plankton ecology: The past two decades of progress

This is a selected account of recent developments in plankton ecology. The examples have been chosen for their degree of innovation during the past two decades and for their general ecological importance. They range from plankton autecology over interactions between populations to community ecology. The autecology of plankton is represented by the hydromechanics of plankton (the problem of life in a viscous environment) and by the nutritional ecology of phyto- and zooplankton. Population level studies are represented by competition, herbivory (grazing), and zooplankton responses to predation. Community ecology is represented by the debate about bottom- up vs. top-down control of community organization, by the PEG model of seasonal plankton succession, and by the recent discovery of the microbial food web

Filter mesh-size and retention efficiency for small particles: comparative studies with Cladocera

The ability to utilize very small food particles (picoalgae, bacteria) has been tested for 10 cladoceran species (8 Daphnia species, Simocephalus and Sida) in the laboratory by using a dual-labelling technique. The animals were fed P-32 labelled Synecbococcus cells (about 1.0 x 1.5 μm size) and C-14 labelled Scenedesmus cells (5 x 15 μm) as a reference. For each individual the clearance rates for both particles were compared to the mesh-size distribution of the filtration screen, as measured by SEM. Filter mesh-sizes and food selectivities are correlated: animals with small mesh-sizes have almost the same filtration rates for both Synechococcus and Scenedesmus, whereas animals with larger mesh-sizes show a strong selectivity for the larger food particle

Variability of filter structures in eight Daphnia species: mesh sizes and filtering areas

The morphology of the filtering apparatus of eight Daphnia species (62 individuals of all size classes) was investigated by scanning electron microscopy. In the majority of animals the intersetular distances (=mesh sizes) showed a normal distribution and the mean values were between 0.2 and 1.0 μm. The total filtering area increased approximately proportionally to the second power of the body length. Absolute values were between 0.063 and 2.6 mm2, with open portions of between 64% and 77% of the total area. In the relationship between mesh size and filtering area three types of allometric growth can be distinguished: (i) with increasing body length the mesh size remains small, but filtering area (FA) increases considerably (D. magna); (ii) both mesh size and FA increase with body length (D. pulicaria); and (iii) FA remains small but the mean mesh size becomes larger (D. cucullata)

Strategies of phenotypic low-food adaptation in Daphnia: Filter screens, mesh sizes, and appendage beat rates

Daphnia collects more food at low particle concentrations by increasing its maximum filtering rate either by enlarging the area of its filter screens or by increasing its appendage beat rate (ABR). Various species of Daphnia grown at low levels of food phenotypically enlarge their filter screens. Models of the flow across a filter predict that the energy required to obtain a certain increase in filtering rate is a linear function of the screen area but increases proportionally to the square of ABR. A daphniid should be able to increase its filtering rate without expending more energy if it enlarges the filter and simultaneously reduces ABR. To test the model predictions, we measured morphometric parameters (areas, open space, intersetular distances) of the filters and ABR for Daphnia adapted to high food and to low food. Daphniids adapted to low food not only have larger filter screens, they also have finer meshes. In Daphnia adapted to low food, mean ABR is slightly reduced and there is a negative relationship between filter-screen area and ABR in the low-food treatment. Gains in filtering rate are higher and reductions in ABR are lower than predicted by the equilibrium model, hence Daphnia optimizes input rather than minimizing energy expenditures for filterin

Effects of water column nutrients and grazer diversity on a seagrass-epiphyte-microphytobenthos community within and beneath a Zostera marina meadow.

Seagrass ecosystems have experienced a dramatic decrease in recent years and high water column nutrients as well as changes in grazer diversity are regarded as major drivers of this decline. Few studies have assessed the relative importance of top-down grazer effects in combination with bottom-up nutrient effects on the entire seagrass community. In this study we used indoor mesocosms with constantly high nutrient supply and manipulated grazer diversity. We considered the seagrass habitat as a whole by including the seagrass-epiphyte community and the microphytobenthos beneath a Zostera marina bed. Nutrients significantly affected the composition and diversity of microalgal communities and on the long run eutrophication clearly took on a governing role. Grazer diversity influenced the microalgal communities on the short-term. During the course of the experiment the high food quantities boosted the grazers biomass and reproduction. However, the consumers were unable to control the strong microalgal production. This experiment provides evidence that constantly high nutrient loadings can affect seagrass communities considerably and that grazers at intermediate abundance are unable to sustain the stability at high levels of primary productivity