Chronic exposure to environmental temperature attenuates the thermal sensitivity of salmonids

Metabolism, the biological processing of energy and materials, scales predictably with temperature and body size. Temperature effects on metabolism are normally studied via acute exposures, which overlooks the capacity for organisms to moderate their metabolism following chronic exposure to warming. Here, we conduct respirometry assays in situ and after transplanting salmonid fish among different streams to disentangle the effects of chronic and acute thermal exposure. We find a clear temperature dependence of metabolism for the transplants, but not the in-situ assays, indicating that chronic exposure to warming can attenuate salmonid thermal sensitivity. A bioenergetic model accurately captures the presence of fish in warmer streams when accounting for chronic exposure, whereas it incorrectly predicts their local extinction with warming when incorporating the acute temperature dependence of metabolism. This highlights the need to incorporate the potential for thermal acclimation or adaptation when forecasting the consequences of global warming on ecosystems.

The recovery of European freshwater biodiversity has come to a halt

Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss1. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity2. Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity.

Effects of sewage effluents and seasonal changes on the metabolism of three Atlantic rivers

Sewage inputs on fluvial ecosystems affect benthic communities and alter trophic networks resulting in changes on river functioning. Functional indicators (e.g. river metabolism) have been proposed as a valuable tool to evaluate ecosystem impairment. In the present study we monitored river metabolism in spring (few days after a major flood) and in summer (after 35days of low flow conditions) using both single-station and two-stations methods over a 24h period up and downstream of wastewater treatment plant (WWTP) effluents on three Atlantic river reaches located in northern Spain (Europe). Concurrently with river metabolism, we characterized environmental characteristics (flow, velocity, depth, pH, water temperature, nutrients, etc.), benthic macroinvertebrate communities and biofilm (algae and epilithic biomass). Ecosystem Respiration (ER₂₄) was similar at the different periods and locations, but Gross Primary Productivity (GPP) tended to decrease in impacted reaches (downstream WWTPs) and in summer (except in the Saja River). However, the balance of the metabolic processes showed a trend towards autotrophy in the largest river, while WWTP effluents increased its autotrophy. Chlorophyll a concentration was >4 times larger in spring than in summer in all river reaches, while epilithic biomass followed a similar but less obvious pattern. Increase of invertebrate scraper densities (mainly, Potamopyrgus antipodarum) seems to be a plausible explanation for biofilm biomass temporal patterns in all sites (higher in spring than in summer), altering GPP and ER₂₄ patterns. Thus, metabolism rates show different responses to WWTP effluents depending on season and on the relationships among functional and structural components, with special focus on the composition and structure of macroinvertebrate communities. Increasing our understanding of cause-effect relationships on the impairment of aquatic ecosystems needs to account for both structural and functional components and their interactions.

Modelling macroinvertebrate and fish biotic indices: From reaches to entire river networks

We modelled three macroinvertebrate (IASPT, EPT number of families and LIFE) and one fish (percentage of salmonid biomass) biotic indices to river networks draining a large region (110,000km²) placed in Northern and Eastern Spain. Models were developed using Random Forest and 26 predictor variables (19 predictors to model macroinvertebrate indices and 22 predictors to model the fish index). Predictor variables were related with different environmental characteristics (water quality, physical habitat characteristics, hydrology, topography, geology and human pressures). The importance and effect of predictors on the 4 biotic indices was evaluated with the IncNodePurity index and partial dependence plots, respectively. Results indicated that the spatial variability of macroinvertebrate and fish indices were mostly dependent on the same environmental variables. They decreased in river reaches affected by high mean annual nitrate concentration (>4mg/l) and temperature (>12°C), with low flow water velocity (<0.4m/s) and aquatic plant communities being dominated by macrophytes. These indices were higher in the Atlantic region than in the Mediterranean. This study provides a continuous image of river biological communities used as indicators, which turns very useful to identify the main sources of change in the ecological status of water bodies and assist both, the integrated catchment management and the identification of river reaches for recovery.

Modelling the spatial and seasonal variability of water quality for entire river networks: Relationships with natural and anthropogenic factors

We model the spatial and seasonal variability of three key water quality variables (water temperature and concentration of nitrates and phosphates) for entire river networks in a large area in northern Spain. Models were developed with the Random Forest technique, using 12 (water temperature and nitrate concentration) and 15 (phosphate concentration) predictor variables as descriptors of several environmental attributes (climate, topography, land-uses, hydrology and anthropogenic pressures). The effect of the different predictors on the response variables was assessed with partial dependence plots and partial correlation analysis. Results indicated that land-uses were important predictors in defining the spatial and seasonal patterns of these three variables. Water temperature was positively related with air temperature and the upstream drainage area, whereas increases in forest cover decreased water temperature. Nitrate concentration was mainly related to the area covered by agricultural land-uses, increasing in winter, probably because of catchment run-off processes. On the other hand, phosphate concentration was highly related to the area covered by urban land-uses in the upstream catchment and to the proximity of the closest upstream effluent. Phosphate concentration increased notably during the low flow period (summer), probably due to the reduction of the dilution capacity. These results provide a large-scale continuous picture of water quality, which could help identify the main sources of change in water quality and assist in the prioritization of river reaches for restoration projects.

Competitive removal of pharmaceuticals from environmental waters by adsorption and photocatalytic degradation

This work explores the competitive removal of pharmaceuticals from synthetic and environmental waters by combined adsorption-photolysis treatment. Five drugs usually present in waterways have been used as target compounds, some are pseudo-persistent pollutants (carbamazepine, clofibric acid, and sulfamethoxazole) and others are largely consumed (diclofenac and ibuprofen). The effect of the light source on adsorption of drugs onto activated carbons followed by photolysis with TiO2 was assessed, being UV-C light the most effective for drug removal in both deionized water and river water. Different composites prepared from titania nanoparticles and powdered activated carbons were tested in several combined adsorption-photocatalysis assays. The composites prepared by calcination at 400 °C exhibited much better performance than those synthesized at 500 °C, being the C400 composite the most effective one. Furthermore, some synthetic waters containing dissolved species and environmental waters were used to investigate the effect of the aqueous matrix on each drug removal. In general, photocatalyst deactivation was found in synthetic and environmental waters. This was particularly evident in the experiments performed with bicarbonate ions as well as with wastewater effluent. In contrast, tests conducted in seawater showed adsorption and photocatalytic degradation yields comparable to those obtained in deionized water. Considering the peculiarities of substrate competition in each aqueous matrix, the combined adsorption-photolysis treatment generally increased the overall elimination of drugs in water.