Tropical and temperate differences in the trophic structure and aquatic prey use of riparian predators.

The influence of aquatic resource-inputs on terrestrial communities is poorly understood, particularly in the tropics. We used stable isotope analysis of carbon and nitrogen to trace aquatic prey use and quantify the impact on trophic structure in 240 riparian arthropod communities in tropical and temperate forests. Riparian predators consumed more aquatic prey and were more trophically diverse in the tropics than temperate regions, indicating tropical riparian communities are both more reliant on and impacted by aquatic resources than temperate communities. This suggests they are more vulnerable to disruption of aquatic-terrestrial linkages. Although aquatic resource use declined strongly with distance from water, we observed no correlated change in trophic structure, suggesting trophic flexibility to changing resource availability within riparian predator communities in both tropical and temperate regions. Our findings highlight the importance of aquatic resources for riparian communities, especially in the tropics, but suggest distance from water is less important than resource diversity in maintaining terrestrial trophic structure.

Contrasting the effects of microplastic types, concentrations and nutrient enrichment on freshwater communities and ecosystem functioning.

Microplastics are now ubiquitous in freshwater environments. As most previous research has focused on species-specific effects of microplastics under controlled laboratory conditions, little is known about the impact of microplastics at higher levels of ecological organisation, such as freshwater communities and their associated ecosystem functions. To fill this knowledge gap, an outdoor experiment using 40 freshwater mesocosms, each 1.57 m3, was used to determine the effects of (i) microplastic type: traditional oil-based high-density polyethylene versus bio-based biodegradable polylactic acid, (ii) concentration of microplastic particles and (iii) nutrient enrichment. The two concentrations of microplastics used were equivalent to measured environmentally occurring concentrations and concentrations known to cause toxicological effects under laboratory conditions. Freshwater communities are also at increasing risk from nutrient enrichment, which can alter community composition in favour of competitively dominant taxa. The independent and interactive effects of these treatments on pelagic community structure (phytoplankton standing stock, taxonomic richness, and composition) and ecosystem functioning (periphyton productivity and leaf litter decomposition) were assessed. Taxonomic richness and community composition were not affected by exposure to the experimental treatments and there were no significant treatment effects on phytoplankton standing stock, periphyton productivity, total or microbial leaf litter decomposition. Overall, multiple microplastic exposures, crossed with nutrient addition had little impact on the structure and functioning of semi-natural freshwater ecosystems. These findings indicate that the negative impacts of microplastics predicted from species-specific studies may not be readily realised at the ecosystem scale.

Evidence of potential establishment of pink salmon Oncorhynchus gorbuscha in Scotland.

In spring 2022, pink salmon Oncorhynchus gorbuscha smolts were recorded in the UK. Fish were caught in the Rivers Thurso and Oykel in Scotland between 13 and 17 March. To the authors' knowledge, this is the first observation of O. gorbuscha smolts in Europe outside the Scandinavian and Kola peninsulas, including other tributaries of the White and Barents Seas. It also provides evidence of successful spawning in 2021 and completion of the freshwater phase of the life cycle, and indicates the possibility for potential establishment of an O. gorbuscha population in Great Britain.

High summer macrophyte cover increases abundance, growth, and feeding of juvenile Atlantic salmon.

Aquatic habitats are severely threatened by human activities. For anadromous species, managing freshwater habitats to maximize production of more, larger juveniles could improve resilience to threats in marine habitats and enhance population viability. In some juvenile salmonid habitats, complexity created by large substrates provides resources and reduces competitive interactions, thereby promoting juvenile production. In lowland rivers, which lack large substrates, aquatic plants might provide similar complexity and enhance fish productivity. To test the influence of aquatic plants on juvenile Atlantic salmon and sympatric brown trout in a lowland river, we directly manipulated the cover of the dominant macrophyte, Ranunculus, in nine sites during summer and autumn for two years. We quantified the abundance, site retention and growth of salmon and trout under high, medium or low Ranunculus cover. To investigate the effects of Ranunculus cover on feeding opportunities and interspecific competition, we quantified available prey biomass and body size, fish diet composition and compared dietary niche overlap. Experimentally increased Ranunculus cover supported higher salmon abundance in summer and autumn, and higher site retention and growth of salmon in summer. Trout abundance and growth were not influenced by Ranunculus cover, but trout site retention doubled in high, relative to low, cover sites. Despite the weak effects of Ranunculus cover on prey availability, salmon and trout inhabiting high cover sites consumed larger prey and a higher biomass of prey. Furthermore, dietary niche overlap was lower in high, relative to low, cover sites, suggesting that abundant Ranunculus reduced interspecific competition. This field experiment shows that high Ranunculus cover can support more and better growing juvenile salmon, and facilitate foraging and co-existence of sympatric salmonid species. Maintaining or enhancing natural macrophyte cover can be achieved through sympathetic in-river and riparian vegetation management and mitigating pressures on them, such as sediment inputs and low flows, or through planting. Further research should test whether macrophyte cover benefits propagate to subsequent life stages, particularly juvenile overwintering associated with high mortality. This knowledge, in combination with our findings, would further clarify whether beneficial juvenile habitat can improve the viability of at-risk salmonid populations. Overall, our findings suggest that the aims of river restoration might be achieved through promotion of in-stream aquatic vegetation.

Systematic Analysis of the Relative Abundance of Polymers Occurring as Microplastics in Freshwaters and Estuaries.

Despite growing interest in the environmental impact of microplastics, a standardized characterization method is not available. We carried out a systematic analysis of reliable global data detailing the relative abundance of polymers in freshwaters and estuaries. The polymers were identified according to seven main categories: polyethylene terephthalate, polyethylene, polyvinyl chloride, polypropylene, polystyrene, polyurethane and a final category of miscellaneous plastic. The results show that microplastics comprised of polyvinyl chloride and polyurethane are significantly less abundant than would be expected based on global production, possibly due to their use. This has implications for models of microplastic release into the environment based on production and fate. When analysed by matrix (water, sediment or biota) distinct profiles were obtained for each category. Polyethylene, polypropylene and polystyrene were more abundant in sediment than in biota, while miscellaneous plastics was more frequent in biota. The data suggest that environmental sorting of microplastic particles, influenced by physical, chemical and biological processes, may play a key role in environmental impact, although partitioning among matrices based on density was not realized. The distinct profile of microplastics in biota raises an important question regarding potential selectivity in uptake by organisms, highlighting the priority for more and better-informed laboratory exposure studies.

The Impact of Metal-Rich Sediments Derived from Mining on Freshwater Stream Life.

Metal-rich sediments have the potential to impair life in freshwater streams and rivers and, thereby, to inhibit recovery of ecological conditions after any remediation of mine water discharges. Sediments remain metal-rich over long time periods and have long-term potential ecotoxicological interactions with local biota, unless the sediments themselves are physically removed or replaced by less metal-rich sediment. Laboratory-derived environmental quality standards are difficult to apply to the field situation, as many complicating factors exist in the real world. Therefore, there is a strong case to consider other, field-relevant, measures of toxic effects as alternatives to laboratory-derived standards and to seek better biological tools to detect, diagnose and ideally predict community-level ecotoxicological impairment. Hence, this review concentrated on field measures of toxic effects of metal-rich sediment in freshwater streams, with less emphasis on laboratory-based toxicity testing approaches. To this end, this review provides an overview of the impact of metal-rich sediments on freshwater stream life, focusing on biological impacts linked to metal contamination.