Gisela Mayora

Spatial variability of chlorophyll-a and abiotic variables in a river–floodplain system during different hydrological phases

Chlorophyll-a (Chl-a) and abiotic variables were measured in the main channel and floodplain waterbodies of the Middle Paraná River to analyse the system dynamics and to assess their spatial variability during different hydrological phases, including an extreme flood. We wanted to test that the flood does not always have a homogenising effect in a river–floodplain system. An explanatory model for Chl-a was performed according to Akaike’s Information Criterion (AIC), and the relation of water level with the coefficient of variation (CV) among sites for each variable was explored. The model explained 64% of Chl-a variability. Water level, depth:euphotic zone ratio (Zd:Zeu) (inverse correlation) and conductivity (direct correlation) were the significant explicative variables. The CV of Chl-a decreased with flood from the main channel to the floodplain, but for turbidity, Zd:Zeu, pH, dissolved oxygen, soluble reactive phosphorus and Chl-a:pheophytin-a ratio, it increased. However, within the floodplain, CV of turbidity, Zd:Zeu and pH decreased during flood. These suggest that the homogenising effect frequently observed during inundation cannot be generalised and that the floodplain may maintain its identity even during flood. The extreme flood and its overlap with the warm season and sedimentological pulse probably contributed to the heterogeneity in the spatial gradient.

Unravelling the role of determinism and stochasticity in structuring the phytoplanktonic metacommunity of the Paraná River floodplain

One of the ongoing debates around metacommunity ecology is to what extent stochastic and deterministic processes act on community assembly. We explored the influence of both determinism, mediated by environmental filters, and stochasticity, mediated by dispersal and ecological drift, on phytoplankton assembly in a floodplain river. A probabilistic co-occurrence model revealed the presence of 94.1% random and 5.9% non-random species pairwise associations. The latter were higher at both hydrologically isolated (4.42%) and connected environments (2.2%). Variation partitioning analysis showed similar significant explanations by the unique environmental (7.7%, Secchi, conductivity, vegetation, phosphorous) and spatial (7.2%, watercourse distance, longitude) components. Temporal variability was poorly represented (2.4%) because we only considered two low-water periods. Species co-occurrence patterns showed that most taxa coexist randomly. The environmental explanation is in line with niche-assembly models (species sorting), but the similar proportion explained by spatial organisation related to random dispersal guides the evidence to both deterministic and stochastic processes. The higher percentage of random co-occurrence and the larger assemblage variability observed in isolated environments suggests that random dispersal, ecological drift, and priority effects could promote stochasticity. We concluded that both processes affect the structure of phytoplankton metacommunities in a floodplain system and suggest the preponderance of stochastic organisation.

Influence of macrophyte integrity on zooplankton habitat preference, emphasizing the released phenolic compounds and chromophoric dissolved organic matter

Macrophyte health status can influence the composition of their exudates causing different effects on zooplankton behavior and distribution in nature. We hypothesize that: (1) the release of phenolic compounds and chromophoric dissolved organic matter (CDOM) depends on macrophyte species and its health status (broken macrophytes: BM, or healthy macrophytes: HM); (2) the repellency effect depends on zooplankton species, macrophyte species and its health status; and (3) higher concentrations of phenolic compounds and CDOM produce a stronger repellency effect. Phenolic compounds and CDOM were analyzed in exudates of BM and HM of Salvinia sp., Eichhornia crassipes, Pistia stratiotes, Azolla sp. and Ludwigia peploides. Through a flow-through experiment, the repellency produced by these exudates was assessed in two copepods (Notodiaptomus conifer and Argyrodiaptomus falcifer) and one cladoceran (Ceriodaphnia dubia). Our hypotheses were partially validated. The quantity of exudated phenolic compounds and CDOM depended on macrophyte species and, to a lesser extent, on the plant health status. The repellency effect was affected by macrophyte and zooplankton species but not by the health status of plants. Only C. dubia and A. falcifer increased their evasion behavior when phenolic compound and CDOM concentrations increased. In brief, the structuring effect of repellent substances depends on different factors. Under a certain threshold concentration, zooplankton behavior might depend on the information associated with the plant odor (e.g., predation risk, structural complexity) more than on the quantity of the released chemical compounds. Above this threshold, evasion would be the only possible option to avoid damaging effects.

Nutrient Dynamics in Wetlands of the Middle Paraná River Subjected to Rotational Cattle Management

The progressive degradation of wetlands has stressed the need of developing sustainable management strategies for maintaining their ecological character within the context of sustainable development. Rotational cattle grazing is a common grazing strategy in aquatic systems but its effect on nutrient dynamics in lakes is scarcely known. The objectives of this study were to evaluate if cattle produces a significant increase of TN and TP in wetlands, and if the value of these nutrients decrease after cattle are removed. Samples of water, sediment and macrophytes were collected for nutrient, organic matter and other chemical analyses. Results showed a significant increase in water nutrients during the presence of cattle, decreasing during their absence. These changes occurred surprisingly rapid, suggesting a high resilience of these systems to this impact. The presence of submerged vegetation in lakes ameliorated the effect of cattle on water nutrients, suggesting an important role of these plants in ecosystem management. Rotational management would minimize the increase of nutrients in water, maintaining the ecological integrity of wetlands.

Can top-down and bottom-up forces explain phytoplankton structure in a subtropical and shallow groundwater-connected lake?

Bottom-up and top-down control of phytoplankton is one of the most important hypothesis that explains and predicts the structure of aquatic community. Our aim was to elucidate whether predation and resource limitation can control phytoplankton composition and abundance in a subtropical shallow lake with groundwater connection to the river system. During 12 months, the lake was sampled at three points. Physico-chemical parameters, phytoplankton and zooplankton were sampled fortnightly, whereas fish were sampled every 3 months. The results showed that Euglenophyta dominated the total biovolume, followed by Dinophyta and Cryptophyta. As for the species composition, Chlorophyta was the dominant group (80 species recorded), followed by phylum Cyanobacteria (26 species recorded). Redundancy analysis (RDA) indicated that temperature and nitrate + nitrite concentration mainly explained biovolume changes, with zooplankton predation not having any measurable effect on phytoplankton during the high-water (HW) period. During low-water (LW) period top-down by fish was more important. At higher taxonomic resolution (species biovolume), phosphorus was another controlling factor. We concluded that phytoplankton in this lake is mainly regulated by hydrological changes as a macrofactor that affects nutrient availability and other environmental conditions. Even though bottom-up top-down forces do not have a central effect, we found evidence of positive nutrient influences at the HW period and fish effect at the LW period.

Aquatic toxicity of ivermectin in cattle dung assessed using microcosms

Ivermectin (IVM) is a parasiticide widely used for livestock. It is a semisynthetic derivative of avermectin, a macrocyclic lactone produced by Streptomyces avermitilis. This drug is only partly metabolized by livestock; considerable amounts of parent drug are excreted mostly via feces. To simulate exposure of aquatic invertebrates and macrophytes to direct excretion of cattle dung into surface waters, a microcosm experiment with IVM spiked in cattle dung was conducted. The objectives of this study were to characterize accumulation of IVM in water, sediment+dung, roots of the floating fern Salvinia and the zooplankton Ceriodaphnia dubia, the amphipod Hyalella and the apple snail Pomacea; to determine the effect of this drug spiked in cattle dung on life-history traits of these invertebrates; and to evaluate the influence of IVM on aquatic nutrient cycling. Dung was spiked with IVM to attain concentrations of 1150, 458, 50 and 22µgkg-1dung fresh weight, approximating those found in cattle dung at days 3, 7, 16 and 29 following subcutaneous injection. Concentrations found in dung during the first week of excretion were lethally toxic to Ceriodaphnia dubia and Hyalella, whereas no mortality was observed in Pomacea. Concentrations of IVM in roots, sediment + dung and Pomacea increased significantly from the lowest to the highest treatment level. The effect of this drug on decomposition and release of nutrients from dung would have negative consequences for nutrient cycling in water. Increasing concentrations in sediment + dung with days of the experiment suggested that toxic concentrations would persist for an extended period in the water-sediment system. IVM represents an ecological risk for aquatic ecosystems, underscoring the need for livestock management strategies to limit its entry into water bodies.

Phytoplankton-based water quality metrics: feasibility of their use in a Neotropical shallow lake

Urban lakes constitute important recreational areas, but often they are eutrophicated. In this study we discuss the utility of 12 ecological quality metrics to test whether they: (1) can be applied to Neotropical lakes; (2) are sensitive to environmental variations throughout the year; and (3) are affected by heterogeneous spatial distribution of phytoplankton. Phytoplankton and environmental variables (including nutrients) were sampled monthly in an urban lake (four littoral and one limnetic station) throughout 1 year (n=60 samples). Twelve ecological quality metrics were tested using total phosphorus as a proxy of eutrophication through general lineal models. The best adjusted metrics were then transformed to an ecological quality ratio (EQR) to allow comparisons. The Phytoplankton Assemblage Index (Q-index) and the Cyanobacteria Bloom Index (CBI) were the most accurate. Differences in water quality estimation occurred across the year, with an overestimation of water quality in the absence of cyanobacteria blooms. There were no differences due to effects of the spatial distribution of phytoplankton. The Q-index was related to temperature and soluble reactive phosphorus, whereas the CBI was related to conductivity. We conclude that the Q-index is the most accurate metric for monitoring purposes, responding well to variations in phosphorus.

Effects of the hydrosedimentological regime on nitrogen transport and speciation in a large subtropical floodplain river

Abstract Hydrosedimentological conditions of floodplain rivers can affect nutrient delivery processes. This study evaluated the effects of sedimentological and hydrological regimes on nitrogen (N) speciation and transport in the floodplain–river system of the Middle Paraná. Relations of sedimentological and hydrological regimes, assessed through turbidity and hydrometric level, with N speciation and concentration were analyzed. Simple linear regressions were performed to assess whether N load changed within the main channel in response to hydrological and sedimentological regimes. From the main channel to the floodplain, dissolved inorganic N decreased; however, the most isolated lake had the highest N concentration, almost totally in organic form. Turbidity was negatively associated with concentrations of nitrate-N (NO3-N), nitrite-N (NO2-N), and dissolved organic N (DON), and relative contributions of these N forms to total N (TN) but was positively correlated to particulate N (PN) and ammonium-N (NH4-N). The hydrometric level was positively associated with DON concentration and its relative contribution at the main channel but negatively associated with DON, PN, and TN concentrations in the most isolated lake. Simple linear regressions showed that the sedimentological regime significantly explained all N forms but not TN load. Flooding increased TN and mainly DON load. The results show that the hydrosedimentological regime largely affects N transport and speciation. The sediment peak incorporates PN to the system and affects dissolved N speciation, probably through effects of suspended particles on redox reactions. The increase of N inputs to the fluvial system during the flood could be caused by DON exportation from the floodplain.