Lúcia Ribeiro Rodrigues

Food-web structure in a subtropical coastal lake: how phylogenetic constraints may affect species linkages

To describe a food-web structure is a main goal in any attempt to understand ecosystem functioning. In the present study, we analysed the isotopic composition (δ13C and δ15N) of primary producers and consumers in the large subtropical Mangueira Lake in southern Brazil. On the basis of the δ13C and δ15N values and analysis of stomach contents of several fish species, we provide a description of the food-web structure and trophic positions of the dominant fish species. Analysis of nitrogen isotope ratios indicated the existence of two consumer trophic levels. Isotopic signatures of primary consumers were compatible with those of producers, indicating a food web sustained by autochthonous carbon. Nevertheless, when food items were classified in larger groups by relative size and source, the combined analysis of isotopic signature and feeding preferences revealed a phylogenetically structured arrangement for the fish species of Mangueira Lake. This indicates that the main feeding niches are shared by taxonomically related species.

Dynamics of plankton and fish in a subtropical temporary wetland: Rice fields

Rice fields are temporary wetlands that harbor many of the same species that breed in natural temporary ponds. These systems have a complex limnology, characterized by rapid physical, chemical, and biological changes. The goal of this study was to evaluate the role of nutrients in the plankton and fish dynamics during a production cycle, based on the auto-ecology of the species related to their adaptations to environmental temporal changes in an irrigated rice field in Southern Brazil. The principal components analysis (PCA) indicated a temporal gradient driven by nutrient availability, grouping sampling periods according to the production cycle. ANOVA indicated temporal differences in the limnological variables during the development of the rice field production cycle. Linear regression showed a positive relationship between chlorophyll a, nutrients and biomass of small and medium filter-feeders. In contrast, planktivorous fish biomass was inversely related to chlorophyll a. This study showed both top down and bottom up processes simultaneously regulating the primary production in the rice field wetland along a temporal gradient the rice production cycle.

Temporal variability determines phytoplankton structure over spatial organization in a large, shallow, heterogeneous, subtropical lake

Abstract Spatial heterogeneity is associated with the temporal variability of the habitat and may affect the structural patterns of biological communities. This study evaluated the influence of spatial and temporal heterogeneity on the phytoplankton structure in a large shallow subtropical lake, Lake Mangueira in southern Brazil (Zmax= 6 m; length = 90 km; width = 3–10 km). The lake is continuous warm polymictic, oligo-mesotrophic, under the direct influence of an adjacent wetland on the north, and has extensive aquatic macrophyte beds. Samples for abiotic and biological analyses were taken quarterly for 2 years at 19 points at the water subsurface, comprising the pelagic and littoral zones and the southern, central, and northern areas of the lake. The phytoplankton structure was analyzed with measurements of chlorophyll , biomass, richness, diversity, and descriptor species. Of the 117 phytoplankton species identified, Cyanobacteria (e.g., , , Chroococcus limneticus, Aphanocapsa conferta, Aphanothece smithii, and Planktolyngbya contorta) was the major group in all areas and both zones of the lake. Total biomass levels were highest in the northern area, whereas the richness and the descriptor species showed no clear spatial differences. A redundancy analysis (RDA) indicated strong temporal organization of the species according to the abiotic conditions, indicating that the high level of temporal variability due to local hydrodynamics was the main factor structuring the phytoplankton community in Lake Mangueira during the study period.

Environmental dissimilarity over time in a large subtropical shallow lake is differently represented by phytoplankton functional approaches

The aim of the present study was to understand how different phytoplankton functional approaches responded to environmental variability in a large shallow lake, namely Lake Mangueira, in southern Brazil. This coastal lake has a maximum depth of 7m and is ~90km long and ~3–10km wide. Physical and chemical variables, maximum linear dimension, life forms, functional and morphofunctional groups of phytoplankton were analysed. The results showed that the phytoplankton were primarily comprised Cyanobacteria (63.9%), followed by middle-sized algae (21–50μm; 46.7%) and colonial non-flagellated taxa (63%). The highest percentage of total biomass was accounted for by the functional group ‘K’ (as classified by Reynolds et al. 2002; 36.3%), large mucilaginous colonies (46.79%) according to morphologically based functional classification and other large colonies (mostly non-vacuolated; 36.7%) according to morphofunctional classification. Dissimilarity analysis indicated a significant correlation between abiotic data and functional approaches (P=0.001). The dissimilarity in the functional compositions of phytoplankton was related to nutrient and light conditions, especially to silicon content and water transparency, in all functional approaches. The highest correlation with abiotic variables was seen for morphologically based functional group community structure, although the functional group composition of phytoplankton sensu Reynolds et al. seemed to be the most effective system in describing environmental variability in Lake Mangueira over the long term.

Development of temporary subtropical wetlands induces higher gas production

Temporary wetlands are short-term alternative ecosystems formed by flooding for irrigation of areas used for rice farming. The goal of this study is to describe the development cycle of rice fields as temporary wetlands in southern Brazil, evaluating how this process affect the gas production (CH4 and CO2) in soil with difference % carbon and organic matter content. Two areas adjacent to Lake Mangueira in southern Brazil were used during a rice-farming cycle. One area had soil containing 1.1% carbon and 2.4% organic matter, and the second area had soil with 2.4% carbon and 4.4% organic matter. The mean rates of gas production were 0.04 ± 0.02 mg CH4 m−2 d−1 and 1.18 ± 0.30 mg CO2 m−2 d−1 in the soil area with the lower carbon content, and 0.02 ± 0.03 mg CH4 m−2 d−1 and 1.38 ± 0.41 mg CO2 m−2 d−1 in the soil area with higher carbon content. Our results showed that mean rates of CO2 production were higher than those of CH4 in both areas. No statistically significant difference was observed for production of CH4 considering different periods and sites. For carbon dioxide (CO2), however, a Two-Way ANOVA showed statistically significant difference (p = 0.05) considering sampling time, but no difference between areas. The results obtained suggest that the carbon and organic matter contents in the soil of irrigated rice cultivation areas may have been used in different ways by soil microorganisms, leading to variations in CH4 and CO2 production.

Integrating ontogenetic shift, growth and mortality to determine a species' ecological role from isotopic signatures

Understanding species linkages and energy transfer is a basic goal underlying any attempt at ecosystem analysis. Although the first food-web studies were based on gut contents of captured specimens, the assessment of stable isotopes, mainly δ13C and δ15N, has become a standard methodology for wide-range analyses in the last 30 years. Stable isotopes provide information on the trophic level of species, food-web length, and origin of organic matter ingested by consumers. In this study, we analyzed the ontogenetic variability of δ13C and δ15N obtained from samples of three Neotropical fish species: silver sardine (Lycengraulis grossidens, n=46), white lambari (Cyanocharax alburnus, n= 26), and the red-tail lambari (Astyanax fasciatus, n=23) in Pinguela Lagoon, southern Brazil. We developed a new metric, called the Weighted Isotopic Signature (φ 15N or φ 13C, ‰), that incorporates ontogenetic variability, body growth, and natural mortality into a single number.

Short-Term Photochemical and Biological Unreactivity of Macrophyte-Derived Dissolved Organic Matter in a Subtropical Shallow Lake

Macrophytes have been associated with low bacterial metabolism in the littoral zones of lake Mangueira, but an explanation for this pattern is largely unknown. In this study, macrophyte-derived DOM was incubated in situ for the measurement of the effect of grazers, bacteria, and light on its degradation in three experiments. The water was separated in bulk, bacterial, and control (+ HgCl2) fractions and exposed to or hidden from sunlight for 120 h. Unchange in bacterial variables in the bulk fraction suggested a combined control of radiation and grazing on bacteria. Light treatment increased bacterial density but not biomass and biovolume, while bacterial density decreased in the dark. Significant fading of water color in the bacterial fraction only occurred after light exposure, indicating a complementary pathway of light and bacteria. DOC and the Abs250 : 365 ratio did not change with incubation, indicating no net change of DOC pool and reactivity. Due to continuous carbon loading from macrophytes and low UV irradiance, the very low rates of DOM degradation provide the mechanistic explanation for the observed impacts of macrophytes in lake’s carbon metabolism in littoral zones.