Lúcia H. S. Silva

Cyanoprokaryote assemblages in eight productive tropical Brazilian waters

Cyanoprokaryote assemblages of eight very productive Brazilian lakes are described and the main driving forces of their dominance are considered. Relative abundance of blue-greens is shown to have been positively related to temperature, but not to pH or total-P and to have been negatively associated with light, mixing, NO3, but not with NH4, total N or total N/total P ratio. Both heterocytic and non-heterocytic groups were negatively related to NO3. However, if Cylindrospermopsis species are considered as non-N2-fixing organisms (only 10% of the filaments carried heterocytes), the lakes could be considered as dominated by non-N2-fixing populations during most of the years. In this new scenario, non-N2-fixing were dominant in NO3 (but not NH4) deficient lakes, and in both NO3 and NH4 deficient conditions. Assemblages S, Sn, H, M, X1, as groups of descriptor species of systems having similar features as proposed by Reynolds (1997: Ecology Institute, Oldenburg), were representative of warm, shallow, turbid, enriched and frequently mixed lakes. We propose to move some species from Z (picoplancton of oligotrophic lakes) to X1 assemblage (nanoplankton of eutrophic lakes) and we comment on Microcystis species of M assemblage from mixed shallow lakes in relation to Lm assemblage of end-summer in temperate lakes. S and Sn assemblages, which comprise species which are good-light antennae, were the best represented group in these generally turbid and shallow lakes.

Phytoplankton species composition is more sensitive than OECD criteria to the trophic status of three Brazilian tropical lakes

We compared the structure and function of the phytoplankton community in three tropical systems in Brazil: an oligo-mesotrophic floodplain lake; an eutrophic reservoir; and a hypereutrophic coastal lagoon. Phytoplankton biomass increased along this trophic gradient averaging 2.6, 6.2 and 31.6 mm3 l-1 in the lake, reservoir and lagoon, respectively. Similarly, production increased from 427 to 918 gC m-2 y-1. Along the trophic gradient, diversity and species richness of phytoplankton decreased. There was, however, no relationship between trophy and rates of change of phytoplankton communities or the ratio of maximum/mean biomass. Phytoplankton size varied between systems with small algae (2–20 µm) dominating in the hypereutrophic lagoon and larger algae (20–200 µm) dominating in the other systems. Many of the trophic indices developed for temperate areas could not be applied to these tropical systems since there was no clear relationship between Secchi depth, chlorophyll, or TP and trophic status. Phytoplankton species may be more useful than these trophic indices to the classification of the condition of tropical lakes.

Influence of phytoplankton fractions on growth and reproduction of tropical cladocerans

The influence of two seston fractions, < 20 μm (nanoplankton) and ≥ 20 μm (microplankton), on growth and reproduction of cladoceran species with different sizes, from Lake Monte Alegre, was evaluated through individual growth and life table experiments. Size, shape and other features of the algae in the fractions were described. The procedure of filtering lake water through a 20 μm net for seston fractionation caused mutual contamination. The smallest cladoceran species, Ceriodaphnia cornuta Sars and Moina micrura Kurz, produced larger clutch sizes and exhibited higher intrinsic rates of population growth (r) in the nanoplankton, despite contamination of their food by inedible algae. The largest species, Simocephalus mixtus Sars, produced larger clutch sizes in the microplankton. There were no differences in juvenile biomass growth between treatments for M. micrura and Daphnia gessneri Herbst, but lower value of the exponential growth rate (g) in the microplankton was significant for M. micrura. Fecundity (eggs/total female) of M. micrura was significantly lower in the microplankton, while D. gessneri did not reproduce in this fraction, at the end of growth experiments. Spines, colonies, cenobium, filaments, hard cell wall, and gelatinous sheaths, represented constraints to cladoceran reproductive performance, besides algae size. Microplankton contamination by nanoplanktonic algae, in the experiments, probably minimized the negative effect of inedible algae. Nanoplankton was more suitable for the smallest species and microplankton for the largest one.

Driving forces of the diel distribution of phytoplankton functional groups in a shallow tropical lake (Lake Monte Alegre, Southeast Brazil)

Phytoplankton vertical and diel dynamics in a small shallow lake (Lake Monte Alegre, Ribeirão Preto, state of São Paulo) were investigated in two climatological periods: July 2001 (cool-dry season) and March 2002 (warm-rainy season). Monte Alegre is a eutrophic reservoir, with a warm polymictic discontinuous circulation pattern. The lake was thermally stratified in both periods, although dissolved oxygen varied less in the cool-dry period. Phytoplankton biomass was higher in the warm-rainy season and the vertical distribution was stratified in both seasons. Flagellate groups (L(m), Y, W(1) and W(2)) and functional groups typical of shallow eutrophic environments (J, X(1) and S(n)) were important throughout the study period. The lakes thermal pattern strongly influenced the vertical distribution of the phytoplankton community in both periods. Biomass, functional groups and size classes of phytoplankton also were determined by the presence of more efficient herbivores in the lake, especially during the cool-dry period when phytoplankton biomass decreased.

Cyanobacteria are controlled by omnivorous filter-feeding fish (Nile tilapia) in a tropical eutrophic reservoir

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.

Tilapia rendalli increases phytoplankton biomass of a shallow tropical lake

AIM: This study aimed to experimentally test the influence of a planktivorous filter-feeding fish (Tilapia rendalli) on the phytoplankton dynamics of a small and shallow tropical reservoir (Lake Monte Alegre, Brazil). Adults of T. rendalli of this lake feed preferentially on phytoplankton, and we hypothesize that: I) adults of T. rendalli will decrease the phytoplankton biomass and composition through direct herbivory, and II) as it is a eutrophic system, fish would not have strong influence on phytoplankton through nutrient cycling. METHODS: To evaluate these different effects on algae, a field experiment was performed in the summer period for 15 days, in mesocosms isolated from the sediment, using a control group (no fish) and a treatment group (with one fish in each mesocosm). Physical and chemical variables and phyto- and zooplankton were evaluated at the start, middle, and end of the experiment. RESULTS: At the end of the experiment, it was observed a significant increase in ammonium concentrations and total phytoplankton biomass, Cyanobacteria and Zygnemaphyceae and all size classes except class II (20-30 µm) in the treatment group (with fish). The biomass increase of the potentially toxic cyanobacterium Cylindrospermospsis raciborskii was also observed in the fish treatment at the end of the experimental period. CONCLUSION: This study did not support both initial hypotheses. It supports the assertion that in tropical water bodies, with similar characteristics to the environment studied, planktivorous filter-feeding fish, such as T. rendalli, are not effective in reducing phytoplankton biomass through direct grazing, even when phytoplankton is one of their main food items. T. rendalli can contribute to the increase of phytoplankton biomass and can promote or increase the eutrophication of aquatic systems.

Microbial Food-Web Drivers in Tropical Reservoirs

Element cycling in aquatic systems is driven chiefly by planktonic processes, and the structure of the planktonic food web determines the efficiency of carbon transfer through trophic levels. However, few studies have comprehensively evaluated all planktonic food-web components in tropical regions. The aim of this study was to unravel the top-down controls (metazooplankton community structure), bottom-up controls (resource availability), and hydrologic (water residence time) and physical (temperature) variables that affect different components of the microbial food web (MFW) carbon stock in tropical reservoirs, through structural equation models (SEM). We conducted a field study in four deep Brazilian reservoirs (Balbina, Tucuruí, Três Marias, and Funil) with different trophic states (oligo-, meso-, and eutrophic). We found evidence of a high contribution of the MFW (up to 50% of total planktonic carbon), especially in the less-eutrophic reservoirs (Balbina and Tucuruí). Bottom-up and top-down effects assessed through SEM indicated negative interactions between soluble reactive phosphorus and phototrophic picoplankton (PPP), dissolved inorganic nitrogen, and heterotrophic nanoflagellates (HNF). Copepods positively affected ciliates, and cladocerans positively affected heterotrophic bacteria (HB) and PPP. Higher copepod/cladoceran ratios and an indirect positive effect of copepods on HB might strengthen HB-HNF coupling. We also found low values for the degree of uncoupling (D) and a low HNF/HB ratio compared with literature data (mostly from temperate regions). This study demonstrates the importance of evaluating the whole size spectrum (including microbial compartments) of the different planktonic compartments, in order to capture the complex carbon dynamics of tropical aquatic ecosystems.

Close Link Between Harmful Cyanobacterial Dominance and Associated Bacterioplankton in a Tropical Eutrophic Reservoir

Cyanobacteria tend to become the dominant phytoplankton component in eutrophic freshwater environments during warmer seasons. However, general observations of cyanobacterial adaptive advantages in these circumstances are insufficient to explain the prevalence of one species over another in a bloom period, which may be related to particular strategies and interactions with other components of the plankton community. In this study, we present an integrative view of a mixed cyanobacterial bloom occurring during a warm, rainy period in a tropical hydropower reservoir. We used high-throughput sequencing to follow temporal shifts in the dominance of cyanobacterial genera and shifts in the associated heterotrophic bacteria community. The bloom occurred during late spring-summer and included two distinct periods. The first period corresponded to Microcystis aeruginosa complex (MAC) dominance with a contribution from Dolichospermum circinale; this pattern coincided with high water retention time and low transparency. The second period corresponded to Cylindrospermopsis raciborskii and Synechococcus spp. dominance, and the reservoir presented lower water retention time and higher water transparency. The major bacterial phyla were primarily Cyanobacteria and Proteobacteria, followed by Actinobacteria, Bacteroidetes, Verrucomicrobia, and Planctomycetes. Temporal shifts in the dominance of cyanobacterial genera were not only associated with physical features of the water but also with shifts in the associated heterotrophic bacteria. The MAC bloom was associated with a high abundance of Bacteroidetes, particularly Cytophagales. In the second bloom period, Planctomycetes increased in relative abundance, five Planctomycetes OTUs were positively correlated with Synechococcus or C. raciborskii OTUs. Our results suggest specific interactions of the main cyanobacterial genera with certain groups of the heterotrophic bacterial community. Thus, considering biotic interactions may lead to a better understanding of the shifts in cyanobacterial dominance.

Subaerial eukaryotic algae and cyanobacteria on dripping rocks in the Atlantic Forest of southeast Brazil: composition and abundance

The abundance and composition of subaerial eukaryotic algae and cyanobacteria on dripping rocks are affected by natural and anthropogenic factors. This study evaluated the composition, species richness, and abundance of these microorganisms on two dripping rocks in southeastern Brazil, under different anthropogenic influences. Samples were collected during two seasons (winter and summer) on two dripping rocks in the Atlantic Forest: one located in a protected, and the other in an urban area. Material on the rocks was scraped and quantified by the sedimentation method. Total taxa richness was high (222), and the richness and abundance of both eukaryotic algae and cyanobacteria were affected by the climate, especially in summer. The higher total abundance on the impacted dripping rock was probably related to greater nutrient availability. However, direct exposure to sunlight and less available moisture seemed to affect all groups of microorganisms, favoring large mucilaginous and sheathed cyanobacteria. The protected non-impacted dripping rock supported a lower total abundance, with a larger contribution of diatoms, which are more sensitive to desiccation. The method used in this study to quantify eukaryotic algae and cyanobacteria populations in moist vertical environments was efficient in contributing to our knowledge of the diversity and ecology of these microorganisms in similar systems.