Many arid and semiarid regions are likely to become warmer and drier by the end of this century, due to human-induced climate change. We hypothesize that a reduction in water level caused by droughts ...will aggravate eutrophication, leading to higher cyanobacteria biomass and dominance in tropical regions. To test this hypothesis, we analyzed physical and chemical variables and plankton communities of 40 man-made lakes in warm semiarid northeastern Brazil at the end of the wet and dry seasons. We also constructed a predictive model of cyanobacteria biovolume in these lakes. The lakes had significantly lower water volume, transparency, and CO
2
concentrations but higher water temperature, water column stability, electrical conductivity, pH, suspended solids, ammonium, total nitrogen concentrations, bacteria biomass, phytoplankton biomass, and cyanobacteria biomass and dominance in the dry than in the wet season. Our regression model suggested that cyanobacteria biovolume was positively related to water column stability, pH, and total nitrogen and negatively related to water transparency and concentrations of inorganic suspended solids. These results suggest that the projected warmer and drier climate in the future will reduce water quantity and quality of man-made lakes in the region, increasing the risks of salinization, anoxia, eutrophication, and cyanobacteria blooms.
Climate change is affecting the global hydrological cycle and is causing drastic changes in the freshwater hydrological regime. Water level (WL) reduction caused by drought tends to increase the ...concentration of nutrients favoring the dominance of cyanobacteria. We hypothesized that the WL reduction favors the dominance of cyanobacteria at regular dry conditions, but at extremely dry events mixotrophic algae would thrive because of light limitation due to increased resuspension of sediments on the water column. To test our hypothesis, we compared phytoplankton traits and water quality variables between two sets of reservoirs located in two watersheds with contrasting precipitation regimes within the Brazilian semi-arid. The reservoirs were compared in a dry period and in an extremely dry period to evaluate the response of the variables to an extreme drought. Drought intensification decreased the reservoirs’ WL and water transparency and increased the total phosphorous. Cyanobacteria dominated in the dry period, and the contribution of mixotrophic algae increased in the extremely dry period. Thus, phytoplankton with mixotrophic potential was favored by the extreme drought. This result suggests that this can be one possible scenario for phytoplankton communities in reservoirs of semi-arid regions if extreme droughts become more frequent because of climate change.
The energetic demand of consumers increases with body size and temperature. This implies that energetic constraints may limit the trophic position of larger consumers, which is expected to be lower ...in tropical than in temperate regions to compensate for energy limitation. Using a global dataset of 3635 marine and freshwater ray‐finned fish species, we addressed if and how climate affects the fish body size–trophic position relationship in both freshwater and marine ecosystems, while controlling for the effects of taxonomic affiliation. We observed significant fish body size–trophic position relationships for different ecosystems. However, only in freshwater systems larger tropical fish presented a significantly lower trophic position than their temperate counterparts. Climate did not affect the fish body size–trophic position relationship in marine systems. Our results suggest that larger tropical freshwater fish may compensate for higher energetic constraints feeding at lower trophic positions, compared to their temperate counterparts of similar body size. The lower latitudinal temperature range in marine ecosystems and/or their larger ecosystem size may attenuate and/or compensate for the energy limitation of larger marine fish. Based on our results, temperature may determine macroecological patterns of aquatic food webs, but its effect is contingent on ecosystem type. We suggest that freshwater ecosystems may be more sensitive to warming‐induced alterations in food web topology and food chain length than marine ecosystems.
We investigated the roles of a benthivorous (
Prochilodus brevis,
Steindachner 1875) and a planktivorous (
Oreochromis niloticus,
Linnaeus, 1758) fish in translocating phosphorus from the benthic to ...the pelagic habitat of a tropical eutrophic shallow lake and its impact on phytoplankton biomass and water transparency. We performed two field experiments in 20 mesocosms (6 m
3
) with a 2 × 2 factorial design. Fish presence/absence was manipulated in combination with the presence/absence of a fish cage (4 m
3
) that prevented fish accessing the sediment. Benthivorous fish increased total phosphorus and chlorophyll a concentrations and decreased water transparency, but only when they had access to the sediment. Planktivorous fish increased the concentration of chlorophyll a without changing total phosphorus concentrations, whether or not they had access to the sediment. Results suggest that only the benthivorous fish increased phytoplankton biomass by translocating phosphorus from benthic to pelagic habitats. However, the planktivorous fish increased phytoplankton biomass by removing zooplankton and recycling nutrients within the pelagic zone. We conclude that removal of either fish species can improve the water quality of eutrophic shallow lakes in tropical regions, but only the removal of benthivorous fish will reduce the internal loading of phosphorus.
Omnivorous filter-feeding fish are common in tropical lakes and reservoirs, and can potentially reduce phytoplankton biomass in eutrophic systems. The goal of this study was to evaluate direct ...grazing or indirect increase in phytoplankton biomass through the trophic cascade and fish-mediated nutrient recycling produced by Nile tilapia. Natural phytoplankton assemblages were incubated in permeable chambers placed inside mesocosms with and without fish. Outside these chambers (mesocosms), phytoplankton was exposed to effects from nutrient recycling by zooplankton and fish, and to grazing by these consumers. Inside the permeable chambers, phytoplankton was exposed only to nutrient recycling by zooplankton and fish. Our results showed that in mesocosms, cyanobacteria biomass was significantly reduced by fish; water transparency and ammonium concentrations also increased, but did not affect soluble reactive phosphorus concentrations or zooplankton biomass. Fish-mediated nutrient recycling did not enhance phytoplankton growth inside permeable chambers, because phytoplankton growth was limited in this study by phosphorus availability. The estimated grazing rates showed that tilapia were able to reduce approximately 60% of phytoplankton biomass (mostly cyanobacteria). Our data indicated that fish grazing was the mechanism controlling cyanobacteria biomass. This study provides evidence that Oreochromis niloticus has the potential to reduce cyanobacteria community in eutrophic reservoirs.
The response of zooplankton assemblages to variations in the water quality of four man-made lakes, caused by eutrophication and siltation, was investigated by means of canonical correspondence ...analysis. Monte Carlo simulations using the CCA eigenvalues as test statistics revealed that changes in zooplankton species composition along the environmental gradients of trophic state and abiogenic turbidity were highly significant. Brachionus calyciflorus, Thermocyclops sp. and Argyrodiaptomus sp. were good indicators of eutrophic conditions, whereas B. dolabratus, Keratella tropica and Hexarthra mira were good indicators of high turbidity due to suspended sediments. Overall, our results showed that changes in the water quality of man-made lakes in a tropical semi-arid region have significant effects on the structure of zooplankton assemblages that can potentially affect the functioning of these ecosystems.
Classical models of phytoplankton-zooplankton interaction show that with nutrient enrichment such systems may abruptly shift from limit cycles to stable phytoplankton domination due to zooplankton ...predation by planktivorous fish. Such models assume that planktivorous fish eat only zooplankton, but there are various species of filter-feeding fish that may also feed on phytoplankton. Here, we extend these classical models to systematically explore the effects of omnivory by planktivorous fish. Our analysis indicates that if fish forage on phytoplankton in addition to zooplankton, the alternative attractors predicted by the classical models disappear for all realistic parameter settings, even if omnivorous fish have a strong preference for zooplankton. Our model also shows that the level of fish biomass above which zooplankton collapse should be higher when fish are omnivorous than when fish are zooplanktivorous. We also used the model to explore the potential effects of the now increasingly common practice of stocking lakes with filter-feeding fish to control cyanobacteria. Because omnivorous filter-feeding fish forage on phytoplankton as well as on the main grazers of phytoplankton, the net effect of such fish on the phytoplankton biomass is not obvious. Our model suggests that there may be a unimodal relationship between the biomass of omnivorous filter-feeding fish and the biomass of phytoplankton. This implies that to manage for reductions in phytoplankton biomass, heavy stocking or strong reduction of such fish is best.
Although some efforts have been made to understand the role of hydrology in structuring aquatic systems, little is known about how rainfall affects the relative roles of local environmental and ...spatial processes in structuring plankton metacommunities in tropical drylands. We hypothesize that the role played by spatial variables is more important in the dry than in the rainy season, because drought increases lakes isolation in semiarid regions. To test our hypothesis, we compared the variation in plankton structure between seasons attributable to local and spatial predictors by using variation-partitioning techniques. We used data for phytoplankton and zooplankton communities as a whole and separately by their size groups (nanoplankton, microplankton and mesoplankton) from 40 man-made lakes in northeastern Brazil. Our results showed that rainfall homogenized limnological variables and reduced total phytoplankton and nanophytoplankton beta diversity, but no effect of season was observed for microphytoplankton and zooplankton communities. Overall, in the dry season, both environmental and spatial variables were important structuring factors for the total phytoplankton, total zooplankton and microzooplankton communities, concurring with both niche- and neutral-based models. In the rainy season, spatial variables were neither important for shaping the phytoplankton nor the zooplankton metacommunities, confirming our hypothesis. The main ecological implication of our findings is that both niche- and neutral-based processes might play an important role on phytoplankton and zooplankton metacommunities dynamics in a future warmer and drier climate in tropical semiarid regions.
Global warming may intensify eutrophication of shallow lakes by affecting nutrient loading, evaporation rates, and water level and thus produce major changes in food webs. We investigated to what ...degree food webs in tropical humid lakes differed from those in more eutrophic semi-arid lakes of the same latitude. Our results indicate that the catchment area-to-lake area ratio, nutrients, chlorophyll
a
, suspended solids, abundances of phytoplankton, zooplankton, and omnivorous fish as well as total fish catch per unit effort were all higher in the semi-arid lakes, whereas inlet water-to-evaporation ratio (proxy for water balance), water transparency, percentage macrophytes cover, and the piscivores:omnivores ratio were higher in the humid lakes. Our results suggest that reduced inlet water-to-evaporation ratio will increase lake eutrophication, which, in turn, as in temperate regions, will alter trophic structure of the freshwater community.
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