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A multiproxy analysis of diatoms, chrysophyte cysts, opal phytoliths, and palynomorphs was undertaken in Negra Lagoon, to decipher Holocene changes in paleobotanical proxies associated with sea-level changes and paleolimnological conditions. Before the Holocene transgression (7000 yr BP), a terrestrial system was inferred as no aquatic palynomorphs or biogenic silica remains were recorded. During the sea-level maximum (5200 yr BP), marine/brackish conditions were established as indicated by diatoms, Chenopodiaceae, and a high content of cysts of Peridinioideae. The catchment consisted of grasslands and wetlands as indicated by the opal phytolith data. The Holocene transgression was followed by a sea-level decrease, which led to the onset of brackish/freshwater conditions as inferred from the co-dominance of freshwater and marine/brackish diatoms. This is also supported not only by the concomitant increase in non-siliceous freshwater microalgae and emergent macrophytes, but also by the presence of Myriophyllum. As sea level continued to decrease during late Holocene, a freshwater system was observed because of the complete separation from the ocean. Chrysophyte cysts were consistently recorded and brackish diatoms exhibited reduced abundances. The observed limnological changes are consistent with the alternative states hypothesis of Scheffer’s model (). That is, the clear water phases were explained because of high macrophyte abundances, low phytoplankton frequencies, high cyst to diatom ratios, and increase in frequencies of benthic diatom species. Conversely, turbid phases were likely to occur when low cyst to diatom ratios together with increase in planktonic diatoms and decrease in macrophytes values were detected. This highlights the importance of this approach to detect long-term changes in shallow lake alternative states. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 night-time 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 night-time 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. [ABSTRACT FROM AUTHOR]

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 Niño 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 μg l-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. [ABSTRACT FROM AUTHOR]

A preface for the June 2010 issue of "Hydrobiologia" is presented.