Effects of Planktonic Copepods on Transparent Exopolymeric Particles (TEP) Abundance and Size Spectra

Diatoms exude considerable quantities of polymers, mainly polysaccharides, that play an important role in the process of sestonic particle aggregation in the sea. We investigated the impact of copepods on transparent exopolymeric particles (TEP) generated by the diatom Thalassiosira weissflogii. Grazing experiments with 14C-labelled algae exudates demonstrated that copepods typical of the Baltic Sea were not actively filtering TEP. Control experiments showed that ‘uptake’ of radioactivity could be ascribed to passive uptake, such as adsorption of radioactively-labelled particles to the body surface. Furthermore, we tested the effect of copepods on TEP size spectra. The abundance and size distribution of TEP (from 1.4 to 180 μm of Equivalent Spherical Diameter) were analysed in a 4 h incubation experiment. In the presence of copepods, the proportion of larger TEP was higher. An increase in total volume of TEP in jars containing copepods (~2 × 107 μm ml–1) compared with control jars without copepods (~0.5 × 107 μm3 ml–1) was also observed. The process of aggregation of TEP demonstrated in this work, whereby copepods increase downward particle flux without consuming carbon, can have far-reaching consequences for carbon fluxes along the water column and for copepods feeding dynamics

Molecular identification and first demographic insights of sharks based on artisanal fisheries bycatch in the Pacific Coast of Colombia: implications for conservation

The Pacific coast of Colombia is characterized by mangrove ecosystems which play a crucial role as possible nurseries for juvenile sharks. However, trophic food webs from coastal ecosystems are heavily disturbed by increased fishing pressure, which affects numerous shark species. In this region of the Eastern Tropical Pacific (ETP), fisheries’ data from coastal areas are scarce and unspecific, as most sharks from artisanal fisheries are landed decapitated and finless, making their morphological identification difficult. For the establishment and implementation of effective regional conservation and management policies, information on the diversity and population dynamics of shark species is crucial. We therefore sequenced the mitochondrial NADH2 gene of 696 samples taken from fishermen’s landings of shark’s bycatch along the Colombian north Pacific coast. We were able to identify 14 species of sharks, two of the most abundant species were Sphyrna lewini and Carcharhinus falciformis, both evaluated on IUCN the Red List of Threatened species (Critically Endangered and Vulnerable) and CITES regulated. We found low genetic diversity in the sampled area increasing the concern for both species in the region, even more considering that the majority of individuals were juveniles. Our results showed the importance of genetic markers for first population genetic insights as a complementary tool during the decision-making process in management plans. For this specific region, strategies such as the delimitation of conservation priority areas or the regulation of fishing gears could help improve the sustainability of shark populations in the Colombian Pacific.publishedVersio

From molecular manipulation of domesticated Chlamydomonas reinhardtii to survival in nature

In the mid-20th century, the unicellular and genetically tractable green alga Chlamydomonas reinhardtii was first developed as a model organism to elucidate fundamental cellular processes such as photosynthesis, light perception and the structure, function and biogenesis of cilia. Various studies of C. reinhardtii have profoundly advanced plant and cell biology, and have also impacted algal biotechnology and our understanding of human disease. However, the 'real' life of C. reinhardtii in the natural environment has largely been neglected. To extend our understanding of the biology of C. reinhardtii, it will be rewarding to explore its behavior in its natural habitats, learning more about its abundance and life cycle, its genetic and physiological diversity, and its biotic and abiotic interactions

Exposure pathways matter: Aquatic phototrophic communities respond differently to agricultural run-off exposed via sediment or water

1. Small shallow ponds are widespread but understudied water bodies in agricultural landscapes. Agricultural run-off (ARO) transports pesticides and nutrients into adjacent aquatic ecosystems where they occur dissolved in the water column or are bound to sediments. Consequently, aquatic communities are affected by ARO via different exposure pathways. We hypothesize that sediment-bound ARO mainly affects submerged rooted macrophytes, while phytoplankton and periphyton are more prone to ARO in water. These primary producers compete for resources resulting in a regime shift between alternative stable states of macrophyte or phytoplankton dominance. We hypothesize that warming increases nutrient release from sediments and thereby facilitates the occurrence of phytoplankton dominance. 2. Using a full-factorial microcosm design, we exposed aquatic primary producers to either sediment or water application of a mixture of common pesticides (terbuthylazine, pirimicarb, tebuconazole and copper) and nitrate at two concentrations and two temperatures (22°C and 26°C) for 4 weeks. Initial and final concentrations of pesticides and nitrate, final biomass of macrophytes, periphyton and phytoplankton, pesticide accumulation in macrophytes and changes in carbon, nitrogen and phosphorus content and selected exoenzyme activities in the sediment were measured. 3. We found lower final macrophyte biomass for both ARO treatments compared to controls, indicating a prevalence of negative effects by herbicides and competition for light with other phototrophs. In contrast, phytoplankton and periphyton biomass increased, but only when exposed to ARO via the water column, indicating a prevalence of positive effects by nutrient supply. Microbial carbon and nutrient cycling in sediments was not affected by ARO. Higher temperature mitigated ARO-related effects on macrophytes under sediment exposure. 4. Synthesis and application. ARO poses a strong risk of submerged macrophyte loss and establishment of turbid conditions with phytoplankton dominance in aquatic ecosystems. In conclusion, exposure pathways as well as indirect and interacting effects of multiple stressors need to be considered when designing appropriate mitigation measures. Under climate change, we suggest to prioritize local measures as buffer strips a reduced use of pesticides and fertilizers, and sediment removal as appropriate measures to protect these vulnerable but widespread aquatic systems, which are highly relevant for biodiversity in agricultural landscapes

Trophic switches in pelagic systems

Ecological studies need experimentation to test concepts and to disentangle causality in community dynamics. While simple models have given substantial insights into population and community dynamics, recent ecological concepts become increasingly complex. The globally important pelagic food web dynamics are well suited to test complex ecological concepts. For instance, trophic switches of individual organisms within pelagic food webs can elongate food webs or shift the balance between autotroph and heterotroph carbon fluxes. Here, we summarize results from mesocosm experiments demonstrating how environmental drivers result in trophic switches of marine phytoplankton and zooplankton communities. Such mesocosm experiments are useful to develop and test complex ecological concepts going beyond trophic level–based analyses, including diversity, individual behavior, and environmental stochasticity

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the -Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the +Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency

The role of yolk protein dynamics and predator kairomones for the life history of Daphnia magna

Resource allocation for reproduction of daphniids is strongly determined by the production of yolk protein. It is the main process channeling material into the reproductive pool of the animals. My investigations of yolk protein dynamics provided evidence that Daphnia are able to exhibit flexible brood reduction behavior and that individuals do not use all of the produced yolk protein for reproduction. The animals show parental optimism, meaning that they start to produce more material for offspring than they actually release. Yolk protein dynamics were influenced by predator-released kairomones. Fish kairomones influenced the beginning of yolk production resulting in a shorter time until first reproduction. In addition, daphniids exposed to fish kairomones used more of the produced yolk protein for reproduction than did control animals. The fish kairomone did not influence the rate of yolk production. In contrast, Chaoborus kairomones influenced the rate of yolk production, but did not change either the beginning of yolk production or the proportion of yolk that was used for egg production. The yolk protein dynamics of daphniids and their ability to exhibit flexible brood reduction behavior can help to explain kairomone-induced life history modifications of daphniids in numerous life history experiments

Prey diversity and prey identity affect herbivore performance on different time scales in a long term aquatic food-web experiment

The question whether and how diversity-mediated productivity at the base of food-webs influences adjacent trophic levels is still unclear. Experiments revealed negative effects on consumers due to the increasing dominance of inedible species under grazing pressure, and positive effects due to a greater variety of prey resources. We experimentally investigate two more hypotheses, which have not been addressed in detail so far: first, more diverse primary producer communities potentially use limiting resources more efficiently, and are, therefore, more productive. This effect can be considered functionally similar to a direct enrichment with limiting resources, potentially resulting in a higher stochastic risk of herbivore extinction (paradox of enrichment'). Second, in a stable environment, enclosed primary producer communities should evolve towards a climax state', eventually dominated by one or few prey species. Therefore, long-term diversity effects in producer communities should more likely result from the specific traits of the dominating species, than from complementarity. To address these hypotheses, we conducted long-term laboratory experiments, exposing the freshwater grazer Daphnia magna to a gradient of algal species richness (1, 2, 4 or 8 edible chlorophyte species). The experiments were run in batch cultures, without exchange of growth medium after the start of the experiment. Six parameters related to Daphnia population demography, biomass accrual, and stability were followed and determined over a period of up to 263 days. Producer diversity exhibited strong positive effects on the short-term performance of grazers (first reproduction, first population peak), and on grazer mean standing stocks. However, herbivore long-term dynamics (day of extinction and temporal stability) depended on prey species identity, namely the presence of Chlamydomonas reinhardtii. Our experiments suggest that both prey diversity and identity can have positive effects on consumer performance, but act on different time scales

The influence of fish-exuded chemical signals on the carbon budget of Daphnia.

Respiration and carbon assimilation rates were measured in juvenile instars of Daphnia magna cultivated either in fish-treated or in control medium without fish exudates. Respiration and carbon assimilation rates increase proportionally to Daphnia body size. This relationship did not differ between fish-influenced and control daphnids throughout juvenile development. The results imply that observed Life-history shifts in Daphnia exposed to fish exudates are facultative changes and not the result of changes in the carbon budget induced by other predator-induced defense mechanisms or experimental setups