Normalized abundance spectra of fish community reflect hydro-peaking on a Mediterranean large river

The European Water Framework Directive requires the integration of body size characters as an important part of fish-based bioassessment tools for freshwaters ecological status determination. The study of the entire fish community size-structure provides valuable information about food web capacity, food web stability and ecological efficiency of aquatic ecosystems. One of the most used representations of community size structure is the Normalized Abundance Spectra (NAS) that provides an approximation of the total fish abundance and food web capacity (through the parameter y-intercept) and an estimation of food web efficiency (through the slope of the linear regression). In this study we explored NAS of the lower Ebro River fish community by integrating data from monthly electrofishing samplings during a whole year (November 2014- October 2015). We found that the percentage of total alien and alien-prey individuals were directly related with y-intercept and inversely related with slope of NAS. This is because the bulk of the community consists of introduced species of small body length. Furthermore, we detected significant relationships between NAS-related parameters and the hydrological variables describing diel flow oscillations and daily flow variability. Based on this, we suggest that high flow variability conditions and, above all, high hydro-peaking conditions, caused a diminution of the total abundance of fish and a decrease of the proportion of small sized fish (i.e. lower y-intercept and flatter slopes of NAS, respectively). Finally, a significant interaction between hydro-peaking and the percentage of alien-predators suggests that high hydro-peaking conditions benefit predation by facilitating predator-prey encounters. This is reflected by strong linear relationship between NAS parameters and percentage of piscivorous at high hydropeaking conditions that disappears at low hydro-peaking fluctuations. We concluded that the high proportion of alien fishes and the presence of a hydropower generation plant that operates by hydro-peaking are important factors determining fish size structure in the lower Ebro River.info:eu-repo/semantics/acceptedVersio

Plankton Taxonomic and Size Diversity of Mediterranean Brackish Ponds in Spring: Influence of Abiotic and Biotic Factors

In this study, performed in Mediterranean brackish ponds during spring season, we assessed the effects of biotic interactions and abiotic factors on the size and taxonomic structure of the phytoplankton and zooplankton. We used a taxonomic and a size diversity index as a descriptor of the community structure. We predicted that the size diversity of each trophic level would be mainly related to biotic interactions, such as size-based fish predation (in the case of zooplankton) and food resource availability (in the case of phytoplankton), whereas taxonomic diversity would be more affected by abiotic variables (e.g., conductivity, pond morphology). Our results showed a negative relationship between phytoplankton size diversity and food resource availability leading to low size diversities under food scarcity due to dominance of small species. Conductivity also negatively affected the phytoplankton size diversity, although slightly. Regarding zooplankton size diversity, none of predictors tested seemed to influence this index. Similar fish size diversities among ponds may prevent a significant effect of fish predation on size diversity of zooplankton. As expected, taxonomic diversity of phytoplankton and zooplankton was related to abiotic variables (specifically pond morphometry) rather than biotic interactions, which are usually body size dependent, especially in these species-poor brackish environments

Is Microtox® toxicity related to potentially harmful algae proliferation in Mediterranean salt marshes?

Is Microtox® toxicity related to potentially harmful algae proliferation in Mediterranean salt marshes? Mediterranean salt marshes are ecosystems that are highly influenced by sea changes and freshwater inputs from runoff. In these ecosystems, toxic and nontoxic algae blooms often produce Jarge and unpredictabJe biomasses of phytoplankton. The Microtox@) test has been described as a successful, quick method for detecting toxicity in various phytoplankton taxa. Our study sought to test the efficiency of Microtox® in detecting toxic HAB in Mediterranean salt marshes. The results showed that the Microtox® test was able to detect toxic substances in the particulate matter of several lagoons in the Emporda salt marshes. This Microtox® toxicity coincided with periods when potentially harrnful cyanobacteria, dinoflagellates and haptophytes had a high biomass. The results suggest that potentially harmful phytoplankton cannot be ruled out as a source of Microtox® toxicity or as a source of other organism kills and subacute effects in Mediterranean salt marshes.Las marismas mediterráneas son ecosistemas altamente injluenciados por la entrada de agua marina y de agua dulce, procedente de la escorrentía. En estos ecosistemas, las proliferaciones de algas tóxicas y no tóxicas sin patrón temporal o espacial, alcanzan altas e impredecibles biomasas de fitoplancton. El ensayo Microtox® ha sido descrito como un rápido y eficiente método para detectar la toxicidad de diferentes taxones de fitoplaneton. Nosotros hemos hecho una primera aproximación para determinar la eficiencia del test Microtox® en la detección de proliferaciones algales tóxicas en las marismas mediterráneas. Se ha podido concluir que hay substancias tóxicas, en la materia particulada de diversas lagunas de las marismas del Emporda, que el método Microtox® es capaz de detectar. Esta toxicidad detectada por Microtox® se da en periodos en que cianobacterias, dinojlagelados y haptófitos potencialmente tóxicos presentan alta biomasa. Los resultados sugieren que el fitoplancton potencialmente tóxico no puede ser descartado como causante de la mortalidad o de efectos subagudos a otros organismos de las marismas mediterráneas

Empirical Correspondence Between Trophic Transfer Efficiency in Freshwater Food Webs and the Slope of Their Size Spectra

The density of organisms declines with size, because larger organisms need more energy than smaller ones and energetic losses occur when larger organisms feed on smaller ones. A potential expression of density-size distributions are Normalized Biomass Size Spectra (NBSS), which plot the logarithm of biomass independent of taxonomy within bins of logarithmic organismal size, divided by the bin width. Theoretically, the NBSS slope of multi-trophic communities is exactly 1.0 if the trophic transfer efficiency (TTE, ratio of production rates between adjacent trophic levels) is 10% and the predator-prey mass ratio (PPMR) is fixed at 104 . Here we provide evidence from four multi-trophic lake food webs that empirically estimated TTEs correspond to empirically estimated slopes of the respective community NBSS. Each of the NBSS considered pelagic and benthic organisms spanning size ranges from bacteria to fish, all sampled over three seasons in 1 yr. The four NBSS slopes were significantly steeper than 1.0 (range 1.14 to 1.19, with 95% CIs excluding 1). The corresponding average TTEs were substantially lower than 10% in each of the four food webs (range 1.0% to 3.6%, mean 1.85%). The overall slope merging all biomass-size data pairs from the four systems (1.17) was almost identical to the slope predicted from the arithmetic mean TTE of the four food webs (1.18) assuming a constant PPMR of 104 . Accordingly, our empirical data confirm the theoretically predicted quantitative relationship between TTE and the slope of the biomass-size distribution. Furthermore, we show that benthic and pelagic organisms can be merged into a community NBSS, but future studies have yet to explore potential differences in habitat-specific TTEs and PPMRs. We suggest that community NBSS may provide valuable information on the structure of food webs and their energetic pathways, and can result in improved accuracy of TTE-estimates

Effects of fish predation on density and size spectra of prey fish communities in lakes

Planktivorous and benthivorous fish have been documented to influence the density and size structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey fish communities in the same way and sought to find evidence for such predation effects from a comparison across 356 lakes located in nine European ecoregions. We categorized individual fish as being either piscivore, non-piscivore or prey of piscivores, depending on species and individual size. We calculated piscivore, non-piscivore and piscivore prey densities, respectively, and fit linear abundance size spectra (SS) on lake-specific piscivore, non-piscivore and piscivore prey size distributions. Multiple linear regressions were calculated to quantify the effect of piscivore density and SS slopes on non-piscivore and piscivore prey densities and SS slopes, by accounting for potentially confounding factors arising from lake morphometry, productivity and local air temperature. Piscivore density correlated positively with piscivore prey density, but was uncorrelated to density of non-piscivores. Across a subset of 76 lakes for which SS slopes of piscivores were statistically significant, SS slopes of piscivores were uncorrelated with SS slopes of either non-piscivores or piscivore prey. However, densities of piscivores, non-piscivores or piscivore prey were a significant negative predictor of SS slopes of the respective groups. Our analyses suggest that direct predation effects by piscivorous fish on density and size structure of prey fish communities are weak in European lakes, likely caused by low predator-prey size ratios and the resulting size refuges for prey fish. In contrast, competition may substantially contribute to between-lake variability in fish density and size

Can size distributions of European lake fish communities be predicted by trophic positions of their fish species?

An organism's body size plays an important role in ecological interactions such as predator-prey relationships. As predators are typically larger than their prey, this often leads to a strong positive relationship between body size and trophic position in aquatic ecosystems. The distribution of body sizes in a community can thus be an indicator of the strengths of predator-prey interactions. The aim of this study was to gain more insight into the relationship between fish body size distribution and trophic position in a wide range of European lakes. We used quantile regression to examine the relationship between fish species' trophic position and their log-transformed maximum body mass for 48 fish species found in 235 European lakes. Subsequently, we examined whether the slopes of the continuous community size distributions, estimated by maximum likelihood, were predicted by trophic position, predator-prey mass ratio (PPMR), or abundance (number per unit effort) of fish communities in these lakes. We found a positive linear relationship between species' maximum body mass and average trophic position in fishes only for the 75% quantile, contrasting our expectation that species' trophic position systematically increases with maximum body mass for fish species in European lakes. Consequently, the size spectrum slope was not related to the average community trophic position, but there were negative effects of community PPMR and total fish abundance on the size spectrum slope. We conclude that predator-prey interactions likely do not contribute strongly to shaping community size distributions in these lakes

Un mundo cada vez más salado

La actividad humana está provocando la salinización de ríos, lagos y otros sistemas acuáticos. Este fenómeno afecta a la calidad de las aguas y tiene graves consecuencias para los seres vivos