Phytoplankton abundance and biomass can be explained as a result of spatial and temporal changes in physical and biological variables, and also by the externally imposed or self-generated spatial segregation. In the present study, we analyzed contrasting-season changes in the phytoplankton communities of five subtropical shallow lakes, covering a nutrient gradient from oligo-mesotrophy to hypereutrophy, using a morphologically based functional approach to cluster the species. Six environmental variables accounted for 46% of the total phytoplankton morphological groups variance, i.e., turbidity (Secchi disk), conductivity, total phosphorus, total nitrogen, total zooplankton abundance, and herbivorous meso:microzooplankton density ratio. The differences in resource availability and zooplankton abundance among the systems were related with important changes in phytoplankton composition and structure. Within phytoplankton assemblages, adaptations to improve both light and phosphorus/nitrogen uptake were important in nutrient-poor systems; while grazing-avoidance mechanisms, such as colonial forms or bigger individuals, seemed relatively important only in eutrophic Lake Blanca, where light was not a limiting factor. However, this was not observed in the nutrient-rich Lake Cisne, where low light availability (due to clay resuspension and dark water color) was identified as the main structuring factor. Our results suggest that the composition of phytoplankton morphologically based functional groups appear to reliably describe the trophic sate of the lakes. However, other factors, such as nonbiological turbid condition, or zooplankton composition, may interact rendering interpretations difficult, and therefore, deserve further studies and evaluation.

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Fish play a key role in the functioning of shallow lakes. Simultaneously, fish are affected by physical in-lake factors, such as temperature and water transparency, with potential changes in their cascading effects on other communities. Here, we analysed the fish community structure and fish activity in four subtropical shallow lakes, varying in trophic state and water transparency, to assess changes promoted by temperature (i.e. summer and winter) and the light regime (i.e. day and night). We used a passive method (gillnets) during the day-and at nighttime to detect changes in fish activity, but also sampled the littoral zone (during night) by point sample electrofishing to obtain a better description of the fish assemblage and habitat use. We observed different fish assemblages in the two seasons in all lakes. We captured more fish species and also obtained higher numbers (CPUE with nets) in summer. Contrary to our expectations, the visually oriented Characiformes were the most captured fish regardless of water transparency, at both day-time and night-time. We also found higher fish CPUE at nighttime in all lakes. However, the differences between night and day decreased with decreasing transparency, being lower in the least clear lake, Lake Cisne. The nocturnal increase in fish CPUE (including visually oriented species) suggests that darkness serves as a good refuge for fish in shallow subtropical lakes, even at the likely cost of a lower feeding efficiency during the night. The importance of darkness seems to decrease with decreasing water transparency. We also argue that cascading effects of changes in the activity of piscivorous fish (seasonal changes in piscivores CPUE), when omni-planktivorous fish are indeed affected, may eventually reach the zooplankton, but may not be strong enough to reach the phytoplankton, regardless of water transparency.

While the cascading effect of piscivorous fish on the pelagic food-web has been well studied in north temperate lakes, little is known about the role of native piscivores in warm lakes. Here, the fish communities are typically characterized by high abundances of small, omnivorous fish exerting a high predation pressure on the zooplankton. We conducted a 1-month replicated mesocosm experiment at subtropical conditions to test the effects of piscivorous (Hoplias malabaricus) fish on phytoplankton biomass and water transparency. Our experimental design comprised two (phytoplankton ? zooplankton), three (phytoplankton ? zooplankton ? planktivores) and four (phytoplankton ? zooplankton ? planktivores ? piscivores) trophic levels. We designed two different four trophic level treatments, one with juveniles of H. malabaricus (\15 cm) and the other with adults ([30 cm), to evaluate the strength of the effects of juveniles and adults. A major trophic cascade response was observed. In the planktivores treatment, chlorophyll a (Chl a) and turbidity significantly increased, while total zooplankton abundance (especially Daphnia obtusa) and water transparency decreased. In both H. malabaricus treatments and in the two trophic levels control, the opposite pattern was observed; thus, Chl a and turbidity decreased, while zooplankton abundance and water transparency increased. The differences observed reflected the strong control on the planktivore Jenynsia multidentata by both sizes of H. malabaricus, propagating down through the trophic web. Hoplias malabaricus is widely distributed in South America and may, therefore, be a good candidate for restoration by biomanipulation in eutrophic lakes of subtropical and tropical regions. However, detailed investigations at whole-lake scale are needed to determine its potential.