Antonie M. Verschoor

Climate‐driven changes in the ecological stoichiometry of aquatic ecosystems

Advances in ecological stoichiometry, a rapidly expanding research field investigating the elemental composition of organisms and their environment, have shed new light on the impacts of climate change on freshwater and marine ecosystems. Current changes in the Earths climate alter the availability of carbon and nutrients in lakes and oceans. In particular, atmospheric CO2 concentrations will rise to unprecedented levels by the end of this century, while global warming will enhance stratification of aquatic ecosystems and may thereby diminish the supply of nutrients into the surface layer. These processes enrich phytoplankton with carbon, but suppress nutrient availability. Phytoplankton with a high carbon-to-nutrient content provide poor-quality food for most zooplankton species, which may shift the species composition of zooplankton and higher trophic levels to less nutrient-demanding species. As a consequence, climate-driven changes in plankton stoichiometry may alter the structure and functioning of entire aquatic food webs.

Climate change and the future of freshwater biodiversity in Europe: a primer for policy-makers.

Abstract Earths climate is changing, and by the end of the 21st century in Europe, average temperatures are likely to have risen by at least 2 °C, and more likely 4 °C with associated effects on patterns of precipitation and the frequency of extreme weather events. Attention among policy-makers is divided about how to minimise the change, how to mitigate its effects, how to maintain the natural resources on which societies depend and how to adapt human societies to the changes. Natural systems are still seen, through a long tradition of conservation management that is largely species-based, as amenable to adaptive management, and biodiversity, mostly perceived as the richness of plant and vertebrate communities, often forms a focus for planning. We argue that prediction of particular species changes will be possible only in a minority of cases but that prediction of trends in general structure and operation of four generic freshwater ecosystems (erosive rivers, depositional floodplain rivers, shallow lakes and deep lakes) in three broad zones of Europe (Mediterranean, Central and Arctic-Boreal) is practicable. Maintenance and rehabilitation of ecological structures and operations will inevitably and incidentally embrace restoration of appropriate levels of species biodiversity. Using expert judgement, based on an extensive literature, we have outlined, primarily for lay policy makers, the pristine features of these systems, their states under current human impacts, how these states are likely to alter with a warming of 2 °C to 4 °C and what might be done to mitigate this. We have avoided technical terms in the interests of communication, and although we have included full referencing as in academic papers, we have eliminated degrees of detail that could confuse broad policy-making.

Inducible colony formation within the Scenedesmaceae: adaptive responses to infochemicals from two different herbivore taxa

We studied the occurrence of colony formation within 40 different strains of Scenedesmaceae (Chlorococcales, Chlorophyta) in response to grazing‐released infochemicals from the herbivorous zooplankters Brachionus calyciflorus Pallas (Rotifera) and Daphnia magna Strauss (Cladocera). With the exception of two strains, all strains showed similar responses to both B. calyciflorus and D. magna infochemicals, either no response or inducible colony formation. Colony size was found to increase with B. calyciflorus infochemical concentration and could be described by a sigmoid function. The increase in colony size was more pronounced in the Scenedesmus species tested than in Desmodesmus species, which was probably due to higher threshold infochemical concentrations for colony induction in Desmodesmus. Therefore, the adaptivity of colony formation to the herbivory threat only holds above the threshold concentration for colony induction and as long as maximum colony size has not been attained. Taking this into account, our results suggest that inducible colony formation is a common adaptive response of many Scenedesmaceae to the threat of herbivory.

INDUCED DEFENSES IN HERBIVORES AND PLANTS DIFFERENTIALLY MODULATE A TROPHIC CASCADE

Inducible defenses are dynamic traits that modulate the strength of both plant-herbivore and herbivore-carnivore interactions. Surprisingly few studies have considered the relative contributions of induced plant and herbivore defenses to the overall balance of bottom-up and top-down control. Here we compare trophic cascade strengths using replicated two-level and three-level plankton communities in which we systematically varied the presence or absence of induced defenses at the plant and/or herbivore levels. Our results show that a trophic cascade, i.e., significantly higher plant biomass in three-level than in two-level food chains, occurred whenever herbivores were undefended against carnivores. Trophic cascades did not occur when herbivores exhibited an induced defense. This pattern was obtained irrespective of the presence or absence of induced defenses at the plant level. We thus found that herbivore defenses, not plant defenses, had an overriding effect on cascade strength. We discuss these results in relation to variation in cascade strengths in natural communities.

Functional response of Anodonta anatina feeding on a green alga and four strains of cyanobacteria, differing in shape, size and toxicity

We studied the functional response of the freshwater unionid bivalve Anodonta anatina, feeding on five phytoplankton strains differing in food quality: the small green alga Scenedesmus obliquus, a toxic and a non-toxic strain of the filamentous cyanobacterium Planktothrix agardhii and a toxic and a non-toxic strain of the coccoid cyanobacterium Microcystis aeruginosa. On S. obliquus, A. anatina had a type II functional response with a maximum mass-specific ingestion rate (IRmax) of 5.24 mg C g DW−1 h−1 and a maximum mass-specific clearance rate (CRmax) of 492 (±38) ml g DW−1 h−1, the highest values for all the phytoplankton strains that were investigated. On toxic and non-toxic P. agardhii filaments, A. anatina also had a type II functional response, but IRmax and CRmax were considerably lower (IRmax 1.90 and 1.56 mg C g DW−1 h−1; CRmax 387 (±97) and 429 (±71) ml g DW−1 h−1, respectively) than on S. obliquus. Toxicity of P. agardhii had no effect on the filtration rate of the mussels. On the non-toxic M. aeruginosa (small coccoid cells), we also observed a type II functional response, although a type I functional response fitted almost as good to these data. For the colonial and toxic M. aeruginosa, a type I functional response fitted best to the data: IR increased linearly with food concentration and CR remained constant. CRmax and IRmax values for the (colonial) toxic M. aeruginosa (383 (±40) ml g DW−1 h−1; 3.7 mg C g DW−1 h−1) demonstrated that A. anatina filtered and ingested this cyanobacterium as good as the other cyanobacterial strains. However, on the non-toxic M. aeruginosa we observed the lowest CRmax of all phytoplankters (246 (±23) ml g DW−1 h−1, whereas IRmax was similar to that on toxic M. aeruginosa. The high maximum ingestion rates on S. obliquus and M. aeruginosa indicate a short handling time of these phytoplankton species. The high clearance rates on S. obliquus, toxic M. aeruginosa and P. agardhii reflect a high effort of the mussels to filter these particles out of the water column at low concentrations. The low clearance rates on non-toxic M. aeruginosa may be explained by the small size and coccoid form of this cyanobacterium, which may have impaired A. anatina to efficiently capture the cells. Although A. anatina had relatively high maximum clearance rates on non-toxic and toxic P. agardhii, this cyanobacterium does not seem to be a good food source, because of the observed high rates of pseudofaeces production and hence low ingestion rates.

Application of stable isotope tracers to studies of zooplankton feeding, using the rotifer Brachionus calyciflorus as an example

We present a protocol and calculation methods for the determination of zooplankton ingestion and assimilation rates with stable isotope tracers. These methods have been developed from experiments with the rotifer Brachionus calyciflorus that had been fed 13C-labelled Scenedesmus obliquus. Stable isotope tracers offer the same advantages as radioisotopes. These include the possibility for direct and accurate quantification of ingestion and assimilation rates, short sample analysis times and low animal densities requirements. However, the use of stable isotope tracers requires relatively long sample preparation times and specialist equipment and is, thus, relatively costly for most laboratories. The application of stable isotope tracers in zooplankton feeding studies offers several advantages in comparison with radioisotopes. Firstly, they do not emit harmful radiation and can therefore be applied safely both in the laboratory and in the field. Secondly, the samples can be dried for safe storage and easy transportation. Thirdly, no aggressive chemicals are required for sample analysis.

The Limnotrons: a facility for experimental community and food web research

Experiments with multi-species communities are essential in order to get more insight in the complex interactions between organisms and their biotic and abiotic environment. The Limnotrons are aquatic model ecosystems that have been developed at the NIOO-KNAW Centre for Limnology to study pelagic community dynamics. They are suitable for the controlled study of multi-species interactions at larger spatial and temporal scales. The Limnotrons do not mimic any particular ecosystem, but should be used for the exploration of basic ecological principles. The temperature and mixing conditions in the Limnotrons can be set within narrow limits, whereas light conditions at the water surface are fixed. We show some results of system performance: mixing time, temperature control, light quantity and quality and development of a Cladocera community in a prototype of the Limnotrons. We provide results of an experiment done in four Limnotrons with the chlorophyte Scenedesmus obliquus. All trophic levels (decomposers, primary producers, and secondary producers) could be maintained in the Limnotrons for at least several weeks. Both abiotic and biotic data from the phytoplankton experiment show remarkably similar patterns through time, but had too low statistical power to prove that they are identical. We calculated the numbers of samples required for sufficient power for biomass data from two plankton experiments, and calculate required effect sizes for certain powers for a future set-up with 2×4 Limnotrons. We show that the power of the data is dependent on: the number of samples, the sample volume, the choice of the measurement method and the type of data transformation.