Steven Declerck

Land-water transition areas play a significant role in the functioning of aquatic ecosystems. However, anthropogenic pressures are posing severe threats on land-water transition areas, which leads to degradation of the ecological integrity of many lakes worldwide. Enhancing habitat complexity and heterogeneity by restoring land-water transition areas in lake systems is deemed a suitable method to restore lakes bottom-up by stimulating lower trophic levels. Stimulating productivity of lower trophic levels (phytoplankton, zooplankton) generates important food sources for declining higher trophic levels (fish, birds). Here, we study ecosystem restoration project Marker Wadden in Lake Markermeer, The Netherlands. This project involved the construction of a 700-ha archipelago of five islands in a degrading shallow lake, aiming to create additional sheltered land-water transition areas to stimulate food web development from its base by improving phytoplankton quantity and quality. We found that phytoplankton quantity (chlorophyll-a concentration) and quality (inversed carbon:nutrient ratio) in the shallow waters inside the Marker Wadden archipelago were significantly improved, likely due to higher nutrient availabilities, while light availability remained sufficient, compared to the surrounding lake. Higher phytoplankton quantity and quality was positively correlated with zooplankton biomass, which was higher inside the archipelago than in the surrounding lake due to improved trophic transfer efficiency between phytoplankton and zooplankton. We conclude that creating new land-water transition areas can be used to increase light and nutrient availabilities and thereby enhancing primary productivity, which in turn can stimulate higher trophic levels in degrading aquatic ecosystems.

The growth rate hypothesis (GRH) posits that the relative body phosphorus content of an organism is positively related to somatic growth rate, as protein synthesis, which is necessary for growth, requires P-rich rRNA. This hypothesis has strong support at the interspecific level. Here, we explore the use of the GRH to predict microevolutionary responses in consumer body stoichiometry. For this, we subjected populations of the rotifer Brachionus calyciflorus to selection for fast population growth rate (PGR) in P-rich (HPF) and P-poor (LPF) food environments. With common garden transplant experiments, we demonstrate that in HP populations evolution toward increased PGR was concomitant with an increase in relative phosphorus content. In contrast, LP populations evolved higher PGR without an increase in relative phosphorus content. We conclude that the GRH has the potential to predict microevolutionary change, but that its application is contingent on the environmental context. Our results highlight the potential of cryptic evolution in determining the performance response of populations to elemental limitation of their food resources.

Brachionus calyciflorus is a recently described monogonont rotifer species complex that comprises four species. The observation of hybridisation between two of these species challenges this species delimitation. The mechanisms of reproductive isolation essential to the maintenance of species integrity remain unclear. Here, we conducted upscaled hybridisation experiments to obtain large numbers of hybrid and non-hybrid dormant propagules. Through hatching assays, we compared the zygote viability of hybrid with non-hybrid dormant propagules. Furthermore, we investigated populations of F1 hybrid clones and assessed their clonal growth rate and the ability to reproduce sexually. Our results demonstrated higher rates of morphological abnormality and associated mortality in dormant propagules of hybrids compared to non-hybrids yet hatching rates of healthy-looking propagules proved similar. F1 hybrids exhibited high clonal population growth rates, nevertheless, we also observed strong differences between clones and a strong influence of parental genotype identity. Two-thirds of the F1 hybrid clones showed a low incidence of sexual reproduction and almost never produced dormant propagules. Clones with high population growth rates seemed to invest less in sexual reproduction. Our results clearly demonstrate the existence of intrinsic postzygotic barriers caused by relatively high mortality of dormant propagules. Furthermore, the low ability of most hybrid clones to engage in sexual reproduction may reduce the long-term fitness of hybrid clones. These postzygotic barriers probably impede genetic exchange between parental species and contribute to the maintenance of their integrity.

Using capture fishery-derived fish oil and fishmeal in aquafeeds is unsustainable. This study mimicked semi-intensive shrimp ponds, including primary producers, in mesocosm tanks. Fatty acid mass balances were computed to distinguish between diet-based and primary production-based LC-PUFA contributions to shrimp (Litopenaeus vannamei) production and meat quality. Performance and body fatty acid composition were compared of shrimp fed a commercial diet containing fish oil and fishmeal (control), with a fishmeal- and fish oil-free diet (low LC-PUFA diet: LOW). Six mesocosms were each stocked with 60 juvenile shrimp and randomly assigned to the two diets. After an 8-week grow-out period, biomass production, survival and proximate body composition were similar between diets. Control shrimp contained twice as much LC-PUFA and omega-3 fatty acids than LOW shrimp. Large quantitative losses (85%) were found in both treatments of the LC-PUFA-precursors alpha-linolenic acid (ALA) and linoleic acid (LA) that were being used as energy source by the shrimp instead for LC-PUFA synthesis. Whereas losses were also observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the control group, there was a gain for these components in the LOW tanks. LOW shrimp sourced at least 32% of their total EPA gain and 15% of their total DHA gain from the algal-based food web. This quantitative analysis of the fate of major dietary fatty acids strongly suggests that the pond's primary production can provide shrimp additional LC-PUFA. Finding a balance between LC-PUFA contribution through formulated feed and natural production seems possible and deserves further research.

Recently, community ecologists are focusing on the relative importance of local environmental factors and proxies to dispersal limitation to explain spatial variation in community structure. Albeit less explored, temporal processes may also be important in explaining species composition variation in metacommunities occupying dynamic systems. We aimed to evaluate the relative role of environmental, spatial and temporal variables on the metacommunity structure of different organism groups in the Upper Paraná River floodplain (Brazil). We used data on macrophytes, fish, benthic macroinvertebrates, zooplankton, periphyton, and phytoplankton collected in up to 36 habitats during a total of eight sampling campaigns over two years. According to variation partitioning results, the importance of predictors varied among biological groups. Spatial predictors were particularly important for organisms with comparatively lower dispersal ability, such as aquatic macrophytes and fish. On the other hand, environmental predictors were particularly important for organisms with high dispersal ability, such as microalgae, indicating the importance of species sorting processes in shaping the community structure of these organisms. The importance of watercourse distances increased when spatial variables were the main predictors of metacommunity structure. The contribution of temporal predictors was low. Our results emphasize the strength of a trait-based analysis and of better defining spatial variables. More importantly, they supported the view that “all-or- nothing” interpretations on the mechanisms structuring metacommunities are rather the exception than the rule.