The relative importance of salinization in lowland stream zooplankton: Implications of the ecosystem nutrient status.

Salinization of aquatic systems is predicted to increase due to climate and land use changes. Nevertheless, community responses may be different according to the ecosystem characteristics and contextual scenarios. Small flowing waters are particularly vulnerable to salinization, which may impact on the biodiversity and ecosystem processes, but this remains unclear. We conducted a study in 42 lowland streams characterized by overall high nutrient levels along a salinity gradient between 2 and 160 g L-1 to analyze changes in zooplankton structural and functional metrics, and the grazing effects of zooplankton on phytoplankton affecting the energy transfer. Generalized additive models revealed that the analyzed metrics were relatively influenced by salinity, with factors related to trophic conditions playing an important role as well. Total abundance and biomass decreased along the salinity gradient while increasing at intermediate soluble reactive phosphorous concentrations (SRP) in the former and with a linear increase in the SRP in the latter case. Taxonomic richness decreased with salinity and dissolved inorganic nitrogen, with species replacement toward saline-tolerant ones according to the compositional and optimums analyses. In opposite, functional richness did not display any specific trend within the environmental gradients. This explains why zooplankton compositional changes were not reflected into shifts in the grazing pattern on phytoplankton, which was in turn driven by SRP and dissolved oxygen concentrations. Further research is a critical requirement in these poorly studied ecosystems for planning mitigation actions to the co-occurrence of eutrophication and salinization in a fast changing world.

Effects of bifenthrin on microcrustaceans grazing behavior on a phytoplankton assemblage dominated by Cyanobacteria.

Insecticides are widely used for pest control and frequently reach aquatic systems, imposing a risk to the biota. In this work, the effect of environmental concentrations of bifenthrin on the grazing capacity of Simocephalus vetulus (Cladocera) and Argyrodiaptomus falcifer (Copepoda) on phytoplankton was evaluated. Fifteen microcrustacean individuals and a natural phytoplankton assemblage dominated by Cyanobacteria were exposed during 46 h to three concentrations of bifenthrin (C0 0 µg L-1, C1 0.02 µg L-1, and C2 0.05 µg L-1). A significant decrease in both microcrustaceans grazing rates on total phytoplankton was observed in C2 compared to C0 and C1. The filtration rate (ml ind-1 h-1) of S. vetulus decreased significantly for the cyanobacteria Anabaenopsis arnoldii, Dolichospermum circinale, and Glaucospira sp. in C2 compared to C0 and C1. The ingestion rate (org ind-1 h-1) of A. falcifer decreased significantly in C1 and C2 compared to C0 only for A. arnoldii. Regarding phytoplankton morphological groups, the filtration rate of S. vetulus decreased in C1 and C2 compared to C0 for Colonies and Coenobiums in C2 concerning C0 and C1 for Filaments and in C2 compared to C0 for Silicified. For A. falcifer, the ingestion rate was reduced in C2 compared to C0 for Silicified, Flagellated, and Sessile. The results showed that bifenthrin affected both microcrustaceans grazing capacity on phytoplankton, especially at the highest insecticide concentration.

Integrating field and satellite monitoring for assessing environmental risk associated with bacteria in recreational waters of a large reservoir.

In a large South American Reservoir (750 km2, limit between Uruguay and Argentina), we characterized the environmental risk posed by cyanobacteria proxies (abundance, toxin concentration, chlorophyll-a) and Escherichia coli abundances, integrating field (six sites, summers 2011-2015) and satellite (750 km2, summers 2011-2017) monitoring. We further assessed how well field cyanobacteria quantitative proxies (abundance, toxin concentration, chlorophyll-a and scum formation) used to build a local risk communication system for recreational (bathing) use of waters named "cyano-traffic-light", ongoing since 2011, reflected its outcome. Cyanobacteria abundance in the field ranged from moderate (>20,000 to <100,000 cells mL-1) to high-risk (>100,000 cells mL-1), and its abundance was positively related to toxin (microcystin) concentration. Mean microcystin concentrations was within the low (≤2 µg L-1, 50% sites) or moderate (>2 < 10 µg L-1, 50% sites) risk categories. On rare occasions, toxin concentration posed a high-risk for human health. E. coli abundance was within the high-risk category (>126 CFU 100 mL-1) for human health, mostly in the northern part of the reservoir. Cyanobacteria proxies (abundance and toxins) and E. coli abundance were, however, unrelated. The predictive model showed that, out of the four cyanobacteria proxies used to construct the cyano-traffic-light only cyanobacteria abundance (p < 0.05) explained the outcome of the reports, yet with low explanatory power (41%). The satellite monitoring allowed delimiting the extent and magnitude of the environmental risk posed by cyanobacteria at landscape scale (highest risk in the meander parts of the Argentinean side of the reservoir) and producing risk maps that can be used by water management agencies. Based upon our results we propose including E. coli abundances and satellite derived cyanobacteria abundances in the building of the cyano-traffic-light, among other modifications.

Responses of stream zooplankton diversity metrics to eutrophication and temporal environmental variability in agricultural catchments.

Eutrophication of rivers and streams in agricultural lands is one of the main threats for biodiversity and ecosystem functions. This study was focused on seven subtropical streams where agriculture is the predominant land use. We tested the hypothesis that (i) eutrophication causes a decrease in taxonomic and functional diversity of zooplankton, leading to potential consequences for the ecosystem integrity. Furthermore, given that the temporal variability in the environmental conditions of each stream may influence the species sorting mechanisms, we also hypothesized that (ii) streams with higher temporal environmental variability have greater taxonomic and functional alpha (α) and temporal beta (ßt) diversity measures regardless of the trophic state. Thus, we characterized the streams according to their trophic state and analyzed the zooplankton composition, α and ßt by using taxonomic and functional perspectives. We found differences in the zooplankton composition between mesotrophic and eutrophic streams. However, eutrophic streams supported similar taxonomic and functional α diversity and similar taxonomic ßt diversity to mesotrophic ones. These results were mainly explained by the occurrence of rare species occupying different temporal niches in eutrophic systems. On the contrary, functional ßt diversity was lower in the eutrophic streams, being nestedness the ecological mechanisms underlying the variability in the zooplankton functional groups. Streams with higher temporal environmental variability supported greater α taxonomic diversity. However, the ßt diversity metrics showed no correlation with the environmental variability, suggesting that the environmental filters of the studied systems were the overriding determinants of species turnover. Our study suggests that both taxonomic and functional perspectives should be considered to improve our knowledge on the biotic responses to environmental changes. Also, among all metrics analyzed on the zooplankton community, functional ßt diversity was the most sensitive indicator of the eutrophication impact.

Interactive effects of fish predation and sublethal insecticide concentrations on freshwater zooplankton communities.

Stresses imposed by insecticides and predators are possibly the most rigorous filters to which aquatic organisms are exposed in rivers and lakes associated with agricultural lands. However, their interactive effects on zooplankton communities are still unclear. This study elucidated the zooplankton community response to fish predation, the insecticide chlorpyrifos (CLP), and a combination of both factors, using a 30-day mesocosm experiment. The zooplankton assemblage was influenced by fish presence prior to CLP toxicity. Fish predation reduced microcrustacean density leading to a community dominated by microzooplankton (i.e.: rotifers and copepod nauplii). CLP decreased the species richness in treatments with and without fish, yielding an increase in the abundance of bdelloid rotifers, in the genera Lepadella and Trichocerca. The zooplankton:phytoplankton (<20 µm) ratio decreased substantially when the two stressors, fish predation and insecticide toxicity, were combined. Although CLP dissipated relatively rapidly in the aqueous phase and accumulated in sediment and fish tissue, zooplankton richness was unable to recover. A possible explanation for this could be the inhibitory effect of CLP on resting stage hatchings in the sediment. Therefore, the combined effects of fish predation and CLP might influence zooplankton richness, leading to an assemblage dominated by rotifers that appeared to be resistant to both factors, with a limited capability to control phytoplankton growth. Thus, the effects of natural and anthropogenic stressors should be considered together when assessing community dynamics in aquatic ecosystems.

Water quality assessment of a neotropical pampean lowland stream using a phytoplankton functional trait approach.

The aim of this study was to test whether the water quality phytoplankton assemblage index adapted for rivers (Qr index) is useful to characterize the water quality of a neotropical stream. We were interested also in inferring the main pollutants through a phytoplankton functional trait characterization and assessing the phytoplankton groups which may influence the Qr index final estimations. Monthly sampling of environmental variables and phytoplankton were done in three sites (S1, S2, and S3). Phytoplankton was classified according to Reynolds Functional Groups (RFG) and water quality estimation was performed using the Qr index. Principal coordinates (PCO) and PERMANOVA were applied to identify the main pollutants through the RFG. RFG linkage to Qr values was assessed by general linear models (GLM). "Moderate" water quality was found in S1 the whole year, in all sampling stations during the winter, and in summer-autumn in S2. "Regular" water quality was found in S3 during the summer-autumn, and S2-S3 during the spring. S1 and S2 showed eutrophic, standing, or mix waters whereas S3 had high organic matter content and eutrophic conditions. Despite some RFG (X1 and MP) being linked to high Qr values and some other (M, S1 and Z) to low, their dominance did not influence water quality estimation performed by the Qr. We conclude that the Qr index was useful for assessing the water quality. Though RFG were valuable for inferring eutrophication, organic pollution, and mixing, but their dominance does not necessarily have a direct effect on the final Qr estimation.