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To investigate the potential long-term consequences of environmental warming in subtropical systems, we compare the trophic structure of shallow lakes in tropical and subtropical regions. In total, 25 meso-eutrophic lakes with piscivorous fish were sampled during summer along a latitudinal gradient in South America. The fish catch per unit of effort and the omnivorous fish to zooplankton biomass ratios were significantly lower in the tropical lakes. Despite the lower fish biomass, no significant difference was found in zooplankton or phytoplankton communities or in the zooplankton to phytoplankton biomass ratio between the two sets of lakes. Nevertheless, regression models based on the combined dataset show higher cyanobacteria and total phytoplankton biomass at lower zooplankton to phytoplankton biomass ratio and higher omnivorous fish to zooplankton biomass ratio. Cyanobacteria biomass was dominated by non bloom-forming taxa and was inversely related to the biomass of calanoid copepods suggesting that these herbivores may play an important role in controlling edible cyanobacteria in warm shallow lakes. Overall, our results, however, suggest that warming will have relatively minor impacts on the pelagic trophic structure of shallow subtropical lakes supporting the idea of weaker trophic cascades in warm (sub)tropical lakes in comparison to temperate ones.

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Lacerot, G., Lozoya, J.P., Teixeira de Mello, F. 2020. Plasticos en ecosistemas acuaticos: presencia, transporte y efectos. Ecosistemas 29(3):2122. https://doi.org/10.7818/ECOS.2122

Plastics have been found in marine water and sediments, sea ice, marine invertebrates, and penguins in Antarctica; however, there is no evidence of their presence in Antarctic glaciers. Our pilot study investigated plastic occurrence on two ice surfaces that constitute part of the ablation zone of Collins Glacier (King George Island, Antarctica). Our results showed concentrations of expanded polystyrene (EPS) in the 0.17–0.33 items m−2 range. We registered an atmospheric dry deposition between 0.08 and 0.17 items m−2 day−1 (February 2019). This is the first report of plastic presence in an Antarctic glacier, which was probably transported by wind.

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.

Este artículo contiene 16 páginas, 6 figuras, 3 tablas., 1. Pigments in lake surface sediments integrate both benthic and pelagic primary producer composition at the whole‐lake ecosystem level. 2. We gathered a comprehensive set of water chemistry, morphometric, physical and biological lake variables, catchment and climate characteristics, and geographical descriptors. We used multiple regressions to relate whole‐lake algal pigment concentration to environmental conditions and multivariate statistical analyses to assess the factors most associated with photoautotrophic algal and bacterial community composition. The relative influence of environmental factors and spatial distribution was assessed by variance partitioning analysis among three groups of variables: (1) in‐lake factors, (2) external factors, and (3) geographic descriptors. 3. Pigment concentration and community composition were most sensitive to in‐lake factors. Pigment‐inferred abundance of algae, including cyanobacteria, tended to be highest in shallower systems with high nutrient concentrations, independent of the latitudinal temperature or irradiance gradients. Pigment‐inferred community composition was best explained by nutrients and biotic composition (zooplankton and fish communities). Only in maritime temperate lakes did a link with regional location occur due to their low dissolved inorganic nitrogen to soluble reactive phosphorus ratios (1.5 atomic ratio), suggesting nitrogen limitation of phytoplankton growth; accordingly, the sediment pigments revealed cyanobacteria to be the dominant group, although this may also be a consequence of the overall high nutrient levels in these 4. Despite the large climate gradient covered, in‐lake rather than external factors were associated with the patterns observed in pigment concentration (from benthic and pelagic microorganisms) and inferred composition and abundance in these shallow lakes. Our results suggest that pigment assemblages in sediments, which integrate both benthic and pelagic microbial photoautotrophic community and processes, are valuable indicators of changes in shallow (0.75–12 m depth) lake ecosystems., This research was funded by a post‐doctoral Beatriu de Pinós grant from the Departament d'Universitats, Recerca i Societat de la Informació, Generalitat de Catalunya (T.B.). Further support came from The Netherlands Organization for Scientific Research (NWO) grant W84‐549 and WB84‐586, The National Geographic Society grant 7864‐5; in Brazil by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) grants 480122, 490409, 311427; in Uruguay by PEDECIBA, Maestría en Ciencias Ambientales, Donación Aguas de la Costa S.A., and Banco de Seguros del Estado. N.M. was supported by Sistema Nacional de Investigadores. E.J. was supported by the MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change) and AU Centre for Water Technology (watec.au.dk).

Toxic effects of freshwater cyanobacteria on mesozooplankton partially depend on the feeding strategies, generalist (cladocera) or selective filter-feeders (copepod) and on the type of toxin. Blooms of Cylindrospermopsis raciborskii (Nostocales) are increasingly more common in freshwaters. It can produce neurotoxins (paralytic shellfish poison, PSP), particularly in warm regions of the Americas. The knowledge about the effects of these neurotoxins on the clearance rate of zooplankton from warm regions still lags behind that of other world regions. We compared the toxin profile (using HPLC), morphology and growth rates of two PSP-producing C. raciborskii strains from South America. We evaluated the effect of these two strains on the clearance rate of two medium-sized grazers typical from subtropical and tropical environments, the copepod Notodiaptomus iheringi (~1.1 mm) and the cladoceran Daphnia pulex (~0.8 mm) in 2.5 h experiments. We found differences in the PSP profile, toxicity and morphology of the two C. raciborskii strains. Medium-sized predators were able to remove filaments of both strains in a similar amount despite their morphological and toxin differences. However, cladocera were significantly more affected than copepods in their clearance rates, suggesting differential consequences for their survival and success in warm freshwaters.

We combined the use of polyclonal antibodies against saxitoxin with catalyzed reporter deposition to detect production of saxitoxin by the cyanobacterium Cylindrospermopsis raciborskii. The procedure is simple, allows detection of intracellular saxitoxin in cyanobacteria filaments by confocal laser microscopy and is a promising tool to study toxin production and metabolism.

Metacommunity studies on lake bacterioplankton indicate the importance of environmental factors in structuring communities. Yet most of these studies cover relatively small spatial scales. We assessed the relative importance of environmental and spatial factors in shaping bacterioplankton communities across a >6000km latitudinal range, studying 48 shallow lowland lakes in the tropical, tropicali (isothermal subzone of the tropics) and tundra climate regions of South America using denaturing gradient gel electrophoresis. Bacterioplankton community composition (BCC) differed significantly across regions. Although a large fraction of the variation in BCC remained unexplained, the results supported a consistent significant contribution of local environmental variables and to a lesser extent spatial variables, irrespective of spatial scale. Upon correction for space, mainly biotic environmental factors significantly explained the variation in BCC. The abundance of pelagic cladocerans remained particularly significant, suggesting grazer effects on bacterioplankton communities in the studied lakes. These results confirm that bacterioplankton communities are predominantly structured by environmental factors, even over a large-scale latitudinal gradient (6026km), and stress the importance of including biotic variables in studies that aim to understand patterns in BCC.

It has been suggested that the overwhelming number of species in tropical ecosystems consist largely of "neutral" and functionally equivalent species. In phytoplankton, differences in functionality have been shown to be clearly distinguishable from morphological traits. Here we examine whether the increase in species towards the tropics goes together with an increase in functional richness or not. We analyse the latitudinal distribution of phytoplankton morphology-based functional groups (MBFG), the within-group richness and community morphological traits in 83 shallow lakes across South America (5-55°S). We further looked into explaining environmental variables. Despite the increment in species richness towards the (sub)tropics, the average number of MBFG remained constant. Furthermore, size average and variance decreased towards warmer regions. In warm lakes, phytoplankton communities were species rich but redundant in terms of belonging to a MBFG. Increasing species richness only translated into increasing number of rare species in some of the MBFG. In contrast, cold lakes were species poor but less redundant (i.e. essential to maintain the number of MBFG) and had a higher morphological variability. Our results support the hypothesis of higher functional redundancy in warmer areas and the relevance of increasing herbivory in colder regions as a main driving process of latitudinal patterns.

1. Large zooplankton such as Daphnia play a fundamental role as consumers of phytoplankton in temperate lakes. These organisms are scarce in subtropical lakes where smaller cladocerans or copepods take this niche. However, such smaller grazers appear to be less able to exert an effective top–down control on the phytoplankton community. 2. We experimentally analysed the ability of zooplankton typical of subtropical, nutrient-rich lakes to graze effectively on the phytoplankton community. We conducted two outdoor mesocosm experiments in a hypertrophic lake, with combinations of three different zooplankton densities and three different omnivorous fish densities. In the first experiment, the zooplankton community was dominated by a small-sized cladoceran (Moina micrura) and in the second by a calanoid copepod (Notodiaptomus incompositus). The phytoplankton community also differed between experiments, with dominance of large size classes and less palatable species in the first experiment and edible sizes in the second. 3. In both experiments, the effect of fish on the largest zooplankton was strong and negative, and low fish densities were sufficient to eliminate the larger zooplankton. Fish presence had positive effects on the biovolume of the largest phytoplankton size fraction (30–100 μm) in the first experiment. This effect was more pronounced in combination with high zooplankton biomass, suggesting that nutrient recycling by both fish and zooplankton may have been an important mechanism promoting phytoplankton growth. 4. None of the zooplankton communities tested had significant top–down effects on the phytoplankton community. In view of the phytoplankton species that dominated the communities at the end of both experiments, inedibility, toxicity and antigrazer defences may explain the absence of significant effects of zooplankton grazing. 5. Our results support the idea that in subtropical nutrient-rich lakes, drastic removal of small omnivorous fish may be needed to allow an increase in zooplankton biomass. In addition, our results imply that for such a change to result in effective top–down control of phytoplankton, a shift in zooplankton community composition is essential too, as the experimental increase in small-sized grazers had little effect on the phytoplankton communities.

Even though the suitability of macrophytes to act as a carbon source to food webs has been questioned by some studies, some others indicate that macrophyte-derived carbon may play an important role in the trophic transfer of organic matter in the food web of shallow lakes. To evaluate the importance of macrophytes to food webs, we collected primary producers-macrophytes and periphyton-and consumers from 19 South American shallow lakes and analyzed their carbon stable isotopes composition (d13C). Despite the diversity of inorganic carbon sources available in our study lakes, the macrophytes' d13C signatures showed a clear bimodal distribution: 13C-depleted and 13C-enriched, averaging at -27. 2 and -13. 5‰, respectively. We argue that the use of either CO2 or HCO3 - by the macrophytes largely caused the bimodal pattern in d13C signals. The contribution of carbon from macrophytes to the lake's food webs was not straightforward in most of the lakes because the macrophytes' isotopic composition was quite similar to the isotopic composition of periphyton, phytoplankton, and terrestrial carbon. However, in some lakes where the macrophytes had a distinct isotopic signature, our data suggest that macrophytes can represent an important carbon source to shallow lake food webs. © 2012 Springer Science+Business Media Dordrecht

Global warming and eutrophication in fresh and coastal waters may mutually reinforce the symptoms they express and thus the problems they cause.

A series of surface sediments from 82 lakes of variable water depth and size was analyzed for glycerol dialkyl glycerol tetraethers (GDGTs) in order to investigate the potential of the MBT/CBT (methylation ratio/cyclization ratio of branched tetraethers) as a continental palaeothermometer in lacustrine environments. Branched GDGTs dominated in most sediments, as indicated by the high branched vs. isoprenoid tetraether (BIT) values. We observed that CBT and MBT varied substantially. Mean annual air temperature (MAAT reconstructed) and pH values were calculated using the CBT and MBT values and the calibration from the global soil data set. The MBT/CBT inferred temperatures were considerably lower than measured values. Nevertheless, there was a significant correlation between MAAT reconstructed and MAAT observed on site, although there was still considerable scatter (r(2) 0.47). Lacustrine sediments integrate organic remains of organisms in a lake and its drainage basin, thereby offering a unique opportunity for calibrating MBT and CBT, as small scale variability is averaged out. Since the source of the branched GDGTs in the lake sediments is ambiguous, it is not clear whether only temperature and pH in the catchment area are the driving factors. Therefore, even in lake sediments with high concentrations of branched GDGTs the straightforward application of MBT/CBT as palaeoproxies may be difficult given the uncertainties regarding their source and origin.

It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (430% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2mgL � 1 . Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1‐2mgL � 1 , except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes’ ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered.

SUMMARY 1. Shallow lakes and ponds contribute disproportionally to species richness relative to other aquatic ecosystems. In-lake conditions (e.g. presence of submerged plants) seem to play a key role in determining diversity, as has been demonstrated for temperate lakes. When water quality deteriorates and turbidity increases, conditions in such lakes are affected drastically resulting in a loss of diversity. However, it is not clear whether subtropical lakes show the same pattern and whether the richness of all groups reacts similarly to environmental changes. 2. Our aim was to analyse the main factors explaining patterns of species richness in plankton, fish and submerged macrophyte assemblages in both turbid and clear subtropical shallow lakes. We analysed abiotic and biotic features of 18 subtropical, small- to mediumsized, shallow lakes along the Uruguayan coast. We compared both turbid and clear ecosystem states and evaluated the relative variance explained by the factors measured. 3. Variables describing lake and catchment morphology, as well as the percentage of the water column occupied by submerged macrophytes (%PVI) and water turbidity, had strong effects on taxon richness. Interestingly, individual biotic groups had dissimilar richness patterns. Macrophyte %PVI decreased with increasing lake area, while fish species richness showed the opposite pattern. Phytoplankton species richness increased with macrophyte %PVI, while the zooplankton richness pattern varied depending on the taxonomic group considered. 4. Overall, our results indicate that, as found for temperate lakes, a greater submerged plant cover promotes higher species richness in several groups, and that this may overwhelm the otherwise expected positive effect of lake size on species richness. On the other hand, small-bodied zooplankton predominated in lakes with high plant abundance. Our findings concur with recent studies, indicating that refuge capacity of aquatic plants might be weaker in (sub)tropical than in temperate shallow lakes.

Synthetic plastics have become an indispensable component of modern life, and the amount of plastics disposal has increased dramatically as a result. With human population increasing, it is expected that the prevalence of plastic debris in the environment will also increase, unless sustainable daily habits are incorporated, waste management improved, and new alternative materials are discovered and popularized. To date, several reports show negative effects of plastic debris on marine and freshwater fauna (e.g. invertebrates, birds, turtles, marine mammals). Plastic are ubiquitous in the water column, deposited in fine sediments and enter the guts, respiratory structures and tissues of different aquatic species, and are incorporated into food webs via ingestion. This global problem also affects beach and near-shore activities (e.g. tourism, fisheries, and aquaculture) with negative social and economic consequences. The research of plastics effect on aquatic environments in Uruguay is only incipient. With the aim of contributing to scientific knowledge, decision-making and the management of plastic debris, here we reviewed the available information on plastic debris sources, their impacts on biodiversity, and policy issues in Uruguayan aquatic systems. Moreover, we reviewed and systematized community outreach initiatives, and examined national policies and management initiatives. We found that Uruguayan aquatic systems (freshwater, coastal marine and benthic) are affected by plastic pollution, both from land-based and sea-based activities, although national-level policies are modern and well suited for minimizing the impacts of plastic pollution. We reviewed biota-plastic debris interactions, and found evidence for impacts on a number of aquatic taxa, including the poorly reported ingestion of microplastics in freshwaters fishes and the association with plastic benthic debris. Global and regional hydrographic settings (Río de la Plata Estuary), as well as idiosyn-cratic ecological, socio-economic and cultural issues, make Uruguay a valuable test-site for this topic. Our ultimate goal is to minimize the effects of this widespread environmental, economic, health and aesthetic problem. Produtos plásticos tem se tornado um componente indispensável na vida moderna, como conseqüência disso a quantidade de plásticos descartados tem dramaticamente aumentado. É esperado que com o aumento da população de seres humanos a prevalência de descartes plásticos também aumente, a menos que hábitos cotidianos sustentáveis sejam incorporados, e novos materiais alternativos sejam descobertos e popularizados. Até o momento, diversas pesquisas têm mostrado os efeitos negativos de descartes plásticos sobre a fauna marinha e dulcícola (p. ex.: invertebrados, aves, tartarugas e mamíferos marinhos). Em ambientes aquáticos, plásticos estão em toda parte: na coluna de água, depositados em sedimento fino e dentro de estômago, estruturas respiratórias e tecidos de diversas espécies aquáticas, e são incorporadas nas teias alimentares através da ingestão. Este problema global também afeta atividades realizadas na praia e próximas à costa (p. ex.: turismo, pesca, e aqüicultura) com conseqüências sociais e econômicas negativas. Pesquisas que abordem os efeitos de plásticos sobre ambientes aquáticos no Uruguai é ainda incipiente. Com o objetivo de contribuir para o conhecimento científico, a tomada de decisões e o manejo de descartes plásticos, revisou-se a informação disponível acerca de fontes de descartes plásticos, observando os impactos delas sobre a biodiversidade e sobre questões políticas em sistemas aquáticos uruguaios. Além disso, revisou-se e sistematizou-se o alcance de iniciativas da comunidade, e examinou-se a política nacional de iniciativas de manejo. Descobrimos que os sistemas aquáticos uruguaios (de água doce, costeiros e marinhos bentônicos) são afetados pela poluição de plástico, pelas atividades terrestres e pelas atividades ligadas ao mar, embora as políticas a nível nacional sejam modernas e bem adaptadas para minimizar os impactos dessa poluição. Revisamos as interações biota-detritos plásticos, e encontramos evidência de impactos sobre diversos taxa aquáticos, incluindo a ingestão de microplástico per peixes de água doce, e da associação que existe entre biota e os detritos plásticos bentônica, geralmente pouco relatados. As configurações hidrográficas globais e regionais (Estuário Rio de la Plata), bem como questões ecológicas idiossincráticas, sócio-econômico e culturais fazem do Uruguai um valioso lugar-teste para este tópico. O objetivo final dessa pesquisa é minimizar os efeitos deste amplamente difundido problema de saúde, ambiental, econômico e estético.

Lake Rodo (Montevideo, Uruguay) is a small, urban, hypertrophic lake undergoing restoration. In this study, we evaluated the nutrient removal efficiency and water quality improvement attributable to a water recirculation system, consisting of the lake and three connected pools converted to artificial wetlands dominated by free-floating hydrophytes. Eichhornia crassipes and Spirodela intermedia dominated the hydrophyte community during summer and winter, respectively, with the biomass production being maintained throughout the year. The maximum production values of E. crassipes were 11.3 and 5.6 g DW m ‐2 d ‐1 in the summers of 1998 and 2000, respectively, while those of S. intermedia were 2.7 and 0.8 g DW m ‐2 d ‐1 in the summers of 1999 and 2000, respectively. The aquatic plant community reduced the concentration of nutrients in the water column but did not significantly affect the sediment concentrations. Harvesting the hydrophytes removed the equivalent of 58‐88% and 39‐78% of the nitrogen (N) and phosphorus (P) load associated with the water column, respectively. In contrast, the harvests accounted for only 1‐2% of the N and P load associated with the sediments. In the pools, the combination of water recirculation and hydrophytes generally diminished the algal biomass and the associated N and P, compared to that observed for the lake. The combined use of adequate aquatic plant harvests and hydraulic management increased the efficiency of the system and, therefore, seems to be a useful tool for restoring small, shallow lakes in tropical and subtropical regions.

Submerged plants are thought to negatively affect phytoplankton crops in the temperate zone by a number of mechanisms, including nutrient and light limitation, and enhancement of top-down control by offering diurnal refuge for zooplankton against visual predation, and by favouring piscivores. In 1997–1998, Lake Blanca (34 ◦ 54 � S, 54 ◦ 50 � W), a yellow-brownish shallow lake in Uruguay, suffered a severe water level reduction (associated with El Nino events between 1995–1997) that resulted in a massive fish kill and an extensive colonisation by Egeria densa. A clear water phase is established nowadays in the system (Secchi depth > 1 m), despite a fish community restricted to two small omnivorous–planktivorous fish: Jenynsia multidentata and Cnesterodon decemmaculatus. We studied the effects of E. densa on bottom-up and top-down controls on phytoplankton by comparing physical, chemical, and biological characteristics between submerged plant beds and sites without plants, from autumn 2000 to autumn 2001. The water column had low to intermediate nutrient concentrations, and phytoplankton community was highly diverse with a low to moderate biomass (mean Chl-a = 10.6 µ gl −1 ). The water level, recovered during the study, promoted a dilution process that explained the temporal pattern of many chemical variables. Macrophyte PVI represented 28–39% of the lake volume (annual mean biomass = 174 g DW m −2 ). The zooplankton community was generally dominated by copepods in terms of biomass. Fish and zooplankton were significantly associated with submerged plant beds. In spite of the high biomass and density of omnivorous-planktivorous fish (115 kg ha −1 , 13 ind m −2 ), zooplankton strongly affected phytoplankton spatial and temporal variation. The most important differences of algal biomass between zones coincided with a high herbivorous zooplankton biomass and/or with plants occupying the entire water column during the low level period. Medium-sized zooplankton declined with fish reproduction. The consequent stronger predation of juvenile fish seemed to decrease macrophyte efficiency as a zooplankton refuge in summer. E. densa bottom-up mechanisms would also be present, contributing to maintaining clear water. Besides the usually described nutrient and light limitation, the internal production of humic substances could enhance the observed top-down effect.

1. Lake Rodo is a turbid system, a condition attributed to algal biomass. The proximal source of the eutrophication was stormwater discharges from an ill-defined urban area. This paper describes an attempt to restore the water quality of Lake Rodo, the first time this has been done in Uruguay. In spring 1996 it was drained, sediments were removed and stream inputs were diverted. Groundwater was used to re-fill the lake. Due to its high nutrient concentration a re-circulation system was designed, pumping water from associated pools covered with free-floating plants. 2. After the lake was refilled, the system was characterized by oxygen saturation or over-saturation, neutral to basic pH, and high phosphorus, nitrogen and silicate concentrations. Ratios of total nitrogen (TN):total phosphorus (TP) and chlorophyll a (Chl a):TP indicated that phosphorus was the primary limiting nutrient during the period of groundwater supply. Once groundwater pumping had ceased, there was a decrease in TN:TP and Chl a:TP ratios, suggesting N-limiting conditions prevailed in some periods. 3. Before restoration, the phytoplankton community was dominated year-round by Planktothrix agardhii; since restoration the community has been more diverse. This change has favoured grazing by mesozooplankton, and the onset of clear-water phases in spring. 4. Abundant populations of small omnivorous fish maintained a high predation pressure on zooplankton, restricting the abundance of large-bodied herbivores, which, in turn, allowed an increase in phytoplanton biomass and a decrease in water transparency. Based on this observation, together with the phosphorus concentration and the low abundance of filamentous cyanobacteria compared with previous studies, we suggest that top-down control has played a key role in increasing transparency in Lake Rodo. 5. A nutrient reduction programme, by the mechanical harvest of floating plants, and a removal of small omnivorous fishes and stocking strictly with piscivores, could be key factors in the achievement of a stable clear-water phase. However, if blooms of Microcystis or other similar genera occur in summer, additional measures (e.g. reduction of the hydraulic residence time) will be needed to improve water transparency. Copyright © 2001 John Wiley & Sons, Ltd.

We compared the species richness and abundance of fish, zooplankton and phytoplankton in nine mesotrophic coastal shallow lakes (Northeastern Brazil) with and without the exotic predator cichlid tucunaré or ‘peacock bass’ (Cichla cf. ocellaris). We hypothesized that the introduction of tucunaré would lead to decreased abundance and species diversity of native fish assemblages and cause indirect effects on the abundance and species diversity of the existing communities of zooplankton and phytoplankton and on water transparency. Our hypotheses were only partly confirmed. Although fish richness and diversity were, in fact, drastically lower in the lakes hosting tucunaré, no significant differences were traced in total fish catch per unit of effort, zooplankton and phytoplankton biomass, plankton diversity or the zooplankton:phytoplankton biomass (TZOO:TPHYTO) ratio. However, zooplankton biomass and TZOO:TPHYTO tended to be higher and the phytoplankton biomass lower in lakes with tucunaré. Our analyses therefore suggest that the introduction of tucunaré had marked effect on the fish community structure and diversity in these shallow lakes, but only modest cascading effects on zooplankton and phytoplankton.

Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.

We explored systematic patterns in predictability of phytoplankton species from 83 lakes over a gradient ranging from subpolar to tropical regions in South America. We estimated the explained variance (proxy of predictability) of the presence and biomass (estimated as biovolume) of species using multiple regressions from commonly measured environmental variables such as nutrient levels, light, mixing depth, temperature, and zooplankton biomass. Both the presence and biomass of species occurring at least in 10 lakes were quite well predicted from the environmental variables, with average values of 35% and 58%, respectively. Predictability was not systematically related to phylogenetic affiliation or particular functional groups as defined by morphology. However, biomass predictability decreased with increasing occurrence, and improved with larger species size (maximum linear dimension). Species that were predictable in terms of biomass (R 2 $ 0.5, p # 0.05) had, on average, a larger volume, and were relatively more frequent in lakes from warmer regions, with high water temperature, low chlorophyll a, low nutrient concentrations, and low total zooplankton biomass. Although we cannot diagnose the mechanisms involved, our finding that the number of predictable species increases towards warmer regions resembles situations where competition for nutrients and grazing are likely to be less severe, and may imply that in a future warmer world phytoplankton will be easier to predict.

Shallow lakes, one of the most widespread water bodies in the world landscape, are very sensitive to climate change. Several theories predict changes in community traits, relevant for ecosystem functioning, with higher temperature. The space-for-time substitution approach (SFTS) provides one of the most plausible empirical evaluations for these theories, helping to elucidate the long-term consequences of changes in climate. Here, we reviewed the changes at the community level for the main freshwater taxa and assemblages (i.e. fishes, macroinvertebrates, zooplankton, macrophytes, phytoplankton, periphyton and bacterioplankton), under different climates. We analyzed data obtained from latitudinal and altitudinal gradients and cross-comparison (i.e. SFTS) studies, supplemented by an analysis of published geographically dispersed data for those communities or traits not covered in the SFTS literature. We found only partial empirical evidence supporting the theoretical predictions. The prediction of higher richness at warmer locations was supported for fishes, phytoplankton and periphyton, while the opposite was true for macroinvertebrates and zooplankton. With decreasing latitude, the biomass of cladoceran zooplankton and periphyton and the density of zooplankton and macroinvertebrates declined (opposite for fishes for both biomass and density variables). Fishes and cladoceran zooplankton showed the expected reduction in body size with higher temperature. Life history changes in fish and zooplankton and stronger trophic interactions at intermediate positions in the food web (fish predation on zooplankton and macroinvertebrates) were evident, but also a weaker grazing pressure of zooplankton on phytoplankton occurred with increasing temperatures. The potential impacts of lake productivity, fish predation and other factors, such as salinity, were often stronger than those of temperature itself. Additionally, shallow lakes may shift between alternative states, complicating theoretical predictions of warming effects. SFTS and meta-analyses approaches have their shortcomings, but in combination with experimental and model studies that help reveal mechanisms, the “field situation” is indispensable to understand the potential effects of warming.

The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. Daphnia and Simocephalus) appeared in fishless mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of small-sized zooplankton in warm lakes. Fil: Iglesias, Carlos. University Aarhus; Dinamarca. Universidad de la República; Uruguay Fil: Mazzeo, Néstor. Universidad de la República; Uruguay Fil: Meerhoff, Mariana. University Aarhus; Dinamarca. Universidad de la República; Uruguay Fil: Lacerot, Gissell. Universidad de la República; Uruguay Fil: Clemente, Juan M.. Universidad de la República; Uruguay Fil: Scasso, Flavio. Universidad de la República; Uruguay Fil: Kruk, Carla. Universidad de la República; Uruguay Fil: Goyenola, Guillermo. Universidad de la República; Uruguay Fil: García Alonso, Javier. Universidad de la República; Uruguay. Natural History Museum; Reino Unido Fil: Amsinck, Susanne L.. University Aarhus; Dinamarca Fil: Paggi, Juan Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina Fil: Jose, Susana Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina Fil: Jeppesen, Erik. University Aarhus; Dinamarca. Greenland Institute of Natural Resources; Groenlandia. Sino-Danish Centre for Education and Research; China

Background: Free-living microorganisms have long been assumed to have ubiquitous distributions with little biogeographic signature because they typically exhibit high dispersal potential and large population sizes. However, molecular data provide contrasting results and it is far from clear to what extent dispersal limitation determines geographicstructuring of microbial populations. We aimed to determine biogeographical patterns of the bloom-forming freshwatercyanobacterium Microcystis aeruginosa. Being widely distributed on a global scale but patchily on a regional scale, this prokaryote is an ideal model organism to study microbial dispersal and biogeography.Methodology/Principal Findings: The phylogeography of M. aeruginosa was studied based on a dataset of 311 rDNAinternal transcribed spacer (ITS) sequences sampled from six continents. Richness of ITS sequences was high (239 ITS typeswere detected). Genetic divergence among ITS types averaged 4% (maximum pairwise divergence was 13%). Preliminary analyses revealed nearly completely unresolved phylogenetic relationships and a lack of genetic structure among all sequences due to extensive homoplasy at multiple hypervariable sites. After correcting for this, still no clear phylogeographic structure was detected, and no pattern of isolation by distance was found on a global scale. Concomitantly, genetic differentiation among continents was marginal, whereas variation within continents was high and was mostly shared with all other continents. Similarly, no genetic structure across climate zones was detected.Conclusions/Significance: The high overall diversity and wide global distribution of common ITS types in combination with the lack of phylogeographic structure suggest that intercontinental dispersal of M. aeruginosa ITS types is not rare, and that this species might have a truly cosmopolitan distribution.

A positive feedback between submerged vegetation and water clarity forms the backbone of the alternative state theory in shallow lakes. The water clearing effect of aquatic vegetation may be caused by different physical, chemical, and biological mechanisms and has been studied mainly in temperate lakes. Recent work suggests differences in biotic interactions between (sub)tropical and cooler lakes might result in a less pronounced clearing effect in the (sub)tropics. To assess whether the effect of submerged vegetation changes with climate, we sampled 83 lakes over a gradient ranging from the tundra to the tropics in South America. Judged from a comparison of water clarity inside and outside vegetation beds, the vegetation appeared to have a similar positive effect on the water clarity across all climatic regions studied. However, the local clearing effect of vegetation decreased steeply with the contribution of humic substances to the underwater light attenuation. Looking at turbidity on a whole-lake scale, results were more difficult to interpret. Although lakes with abundant vegetation (>30%) were generally clear, sparsely vegetated lakes differed widely in clarity. Overall, the effect of vegetation on water clarity in our lakes appears to be smaller than that found in various Northern hemisphere studies. This might be explained by differences in fish communities and their relation to vegetation. For instance, unlike in Northern hemisphere studies, we find no clear relation between vegetation coverage and fish abundance or their diet preference. High densities of omnivorous fish and coinciding low grazing pressures on phytoplankton in the (sub)tropics may, furthermore, weaken the effect of vegetation on water clarity.

Large herbivorous zooplankton and spring clear-water phases occur rarely in tropical lakes. It has been hypothesized that this can be explained by latitudinal differences in the structure of the trophic cascade. Many tropical fish species reproduce throughout the year and are omnivorous. Both properties could reduce the chances for large zooplankton to cause a clear-water phase. Continuous reproduction leads to permanent presence of planktivorous young individuals, and thus a continuously high predation pressure on large zooplankton. On the other hand, omnivory could allow fish to be more abundant, leading to a higher predation pressure on zooplankton. Data to distinguish between effects of continuous reproduction and omnivory boosted abundance are scarce. Here, we use a simple model to indicate that continuous fish reproduction in the tropics by itself can reduce the chances for top–down control of phytoplankton by Daphnia even if the carrying capacity for small fish would not be boosted by omnivory. The model predicts that only at very low fish densities, clear-water phases are possible, but these tend to occur at irregular times of the year. We also analyzed the effect of change in the mean annual temperature. This model analysis suggests that in temperate regions where fish reproduction is pulsed, climatic warming would tend to promote the chances for top–down induced clear-water phases.

During ecosystem succession, phytoplankton species composition is hard to predict and although it is tempting to use taxonomic groups for predictive purposes, the conditions favouring their development are often cross-phyletic and, frequently, overlap. Another alternative is to consider functional groups. Reynolds (1997) proposed phytoplankton associations according to functional criteria, based upon identified coherent morphological and ecological properties. Here we apply data from the phytoplankton community of Lake Rodo in Montevideo, Uruguay (small, shallow, polymictic and hypertrophic lake under restoration) to test and quantify the effectiveness of the approach. The phytoplankton species were sorted into their main taxonomic groups and into the associations proposed by Reynolds. A canonical variate analysis was use to test the non-random occurrence of these classification schemes and to determine their discriminatory power. Both classification schemes, taxonomic and functional, showed a significant result, but classification into functional associations had a higher discriminatory power. The eigenvalue for the canonical correspondence analysis first axis for the functional associations was 0.708 and the cumulative explained variance for the species-environmental relationship was 78. 6%. The environmental factors showed similar patterns between associations and individual species. Our data indicate that the scheme using functional associations does indeed capture much of the ecology of the phytoplankton.