David C. Aldridge

Microplastics in freshwater systems: a review of the emerging threats, identification of knowledge gaps and prioritisation of research needs.

Plastic contamination is an increasing environmental problem in marine systems where it has spread globally to even the most remote habitats. Plastic pieces in smaller size scales, microplastics (particles <5 mm), have reached high densities (e.g., 100,000 items per m(3)) in waters and sediments, and are interacting with organisms and the environment in a variety of ways. Early investigations of freshwater systems suggest microplastic presence and interactions are equally as far reaching as are being observed in marine systems. Microplastics are being detected in freshwaters of Europe, North America, and Asia, and the first organismal studies are finding that freshwater fauna across a range of feeding guilds ingest microplastics. Drawing from the marine literature and these initial freshwater studies, we review the issue of microplastics in freshwater systems to summarise current understanding, identify knowledge gaps and suggest future research priorities. Evidence suggests that freshwater systems may share similarities to marine systems in the types of forces that transport microplastics (e.g. surface currents); the prevalence of microplastics (e.g. numerically abundant and ubiquitous); the approaches used for detection, identification and quantification (e.g. density separation, filtration, sieving and infrared spectroscopy); and the potential impacts (e.g. physical damage to organisms that ingest them, chemical transfer of toxicants). Differences between freshwater and marine systems include the closer proximity to point sources in freshwaters, the typically smaller sizes of freshwater systems, and spatial and temporal differences in the mixing/transport of particles by physical forces. These differences between marine and freshwater systems may lead to differences in the type of microplastics present. For example, rivers may show a predictable pattern in microplastic characteristics (size, shape, relative abundance) based on waste sources (e.g. household vs. industrial) adjacent to the river, and distance downstream from a point source. Given that the study of microplastics in freshwaters has only arisen in the last few years, we are still limited in our understanding of 1) their presence and distribution in the environment; 2) their transport pathways and factors that affect distributions; 3) methods for their accurate detection and quantification; 4) the extent and relevance of their impacts on aquatic life. We also do not know how microplastics might transfer from freshwater to terrestrial ecosystems, and we do not know if and how they may affect human health. This is concerning because human populations have a high dependency on freshwaters for drinking water and for food resources. Increasing the level of understanding in these areas is essential if we are to develop appropriate policy and management tools to address this emerging issue.

Non-indigenous invasive bivalves as ecosystem engineers

Several non-indigenous bivalve species have been colonising aquatic ecosystems worldwide, in some cases with great ecological and economic impacts. In this paper, we focus on the ecosystem engineering attributes of non-indigenous invasive bivalves (i.e., the capacities of these organisms to directly or indirectly affect the availability of resources to other species by physically modifying the environment). By reviewing the ecology of several invasive bivalves we identify a variety of mechanisms via which they modify, maintain and/or create habitats. Given the usually high densities and broad spatial distributions of such bivalves, their engineering activities can significantly alter ecosystem structure and functioning (e.g., changes in sediment chemistry, grain size, and organic matter content via bioturbation, increased light penetration into the water column due to filter feeding, changes in near bed flows and shear stress due to the presence of shells, provision of colonisable substrate and refuges by shells). In addition, changes in ecosystem structure and functioning due to engineering by invasive bivalves often have very large economic impacts. Given the worldwide spread of non-indigenous bivalves and the varied ways in which they physically modify habitats, their engineering effects should receive more serious consideration in restoration and management initiatives.

Conservation status of freshwater mussels in Europe: state of the art and future challenges.

Freshwater mussels of the Order Unionida provide important ecosystem functions and services, yet many of their populations are in decline. We comprehensively review the status of the 16 currently recognized species in Europe, collating for the first time their life‐history traits, distribution, conservation status, habitat preferences, and main threats in order to suggest future management actions. In northern, central, and eastern Europe, a relatively homogeneous species composition is found in most basins. In southern Europe, despite the lower species richness, spatially restricted species make these basins a high conservation priority. Information on freshwater mussels in Europe is unevenly distributed with considerable differences in data quality and quantity among countries and species. To make conservation more effective in the future, we suggest greater international cooperation using standardized protocols and methods to monitor and manage European freshwater mussel diversity. Such an approach will not only help conserve this vulnerable group but also, through the protection of these important organisms, will offer wider benefits to freshwater ecosystems.

Horizon scanning for invasive alien species with the potential to threaten biodiversity in Great Britain

Invasive alien species (IAS) are considered one of the greatest threats to biodiversity, particularly through their interactions with other drivers of change. Horizon scanning, the systematic examination of future potential threats and opportunities, leading to prioritization of IAS threats is seen as an essential component of IAS management. Our aim was to consider IAS that were likely to impact on native biodiversity but were not yet established in the wild in Great Britain. To achieve this, we developed an approach which coupled consensus methods (which have previously been used for collaboratively identifying priorities in other contexts) with rapid risk assessment. The process involved two distinct phases: Preliminary consultation with experts within five groups (plants, terrestrial invertebrates, freshwater invertebrates, vertebrates and marine species) to derive ranked lists of potential IAS. Consensus-building across expert groups to compile and rank the entire list of potential IAS. Five hundred and ninety-one species not native to Great Britain were considered. Ninety-three of these species were agreed to constitute at least a medium risk (based on score and consensus) with respect to them arriving, establishing and posing a threat to native biodiversity. The quagga mussel, Dreissena rostriformis bugensis, received maximum scores for risk of arrival, establishment and impact; following discussions the unanimous consensus was to rank it in the top position. A further 29 species were considered to constitute a high risk and were grouped according to their ranked risk. The remaining 63 species were considered as medium risk, and included in an unranked long list. The information collated through this novel extension of the consensus method for horizon scanning provides evidence for underpinning and prioritizing management both for the species and, perhaps more importantly, their pathways of arrival. Although our study focused on Great Britain, we suggest that the methods adopted are applicable globally.

Estimating the financial costs of freshwater invasive species in Great Britain: a standardized approach to invasive species costing

Both ecological and economic impacts factor into invasive alien species (IAS) management considerations; however, economic impacts are often difficult to assess, much less quantify. Studies frequently aggregate identified financial costs as a proxy for IAS economic impacts, but these aggregate figures are often generated in an ad hoc fashion. Such estimates typically sum disparate costs, which might vary with respect to precision, accuracy, and scope. A standardized approach for IAS costing would better enable the comparison of cost estimates between taxa and across studies by controlling for surveying and scaling inconsistencies. This study develops a simple, survey-based approach to generate economic cost estimates for non-native freshwater invasive species (FIS) in Great Britain. The approach scales an average cost for each species by a ratio of management effort, thereby estimating the actual, annual expenditures incurred by a variety of stakeholders. The Great Britain-wide cost of controlling FIS is estimated to be approximately £26.5 million year−1; however, the costs of control could total £43.5 million year−1 if management efforts were undertaken at all FIS infested locations. Cost estimates are highest for Canadian pondweed (Elodea canadensis), a particularly widespread species, and for the zebra mussel (Dreissena polymorpha), which adversely impacts both industrial water users and boaters. This assessment of the relative economic impacts between species provides policy-makers with a monetary basis for rank-ordering species’ economic impacts and prioritizing management efforts. In addition, the cost assessment approach developed in this study could serve as a model for IAS economic impact assessments elsewhere.

Phenotypic plasticity and genetic isolation-by-distance in the freshwater mussel Unio pictorum (Mollusca: Unionoida)

Freshwater mussels (Unionoida) show high intraspecific morphological variability, and some shell morphological traits are believed to be associated with habitat conditions. It is not known whether and which of these ecophenotypic differences reflect underlying genetic differentiation or are the result of phenotypic plasticity. Using 103 amplified fragment length polymorphism (AFLP) markers, we studied population genetics of three paired Unio pictorum populations sampled from two different habitat types (marina and river) along the River Thames. We found genetic differences along the Thames which were consistent with a pattern of isolation by distance and probably reflect limited dispersal via host fish species upon which unionoid larvae are obligate parasites. No consistent genetic differences were found between the two different habitat types suggesting that morphological differences in the degree of shell elongation and the shape of dorso-posterior margin are caused by phenotypic plasticity. Our study provides the first good evidence for phenotypic plasticity of shell shape in a European unionoid and illustrates the need to include genetic data in order properly to interpret geographic patterns of morphological variation.

Vectors and timing of freshwater invasions in Great Britain.

Nonindigenous freshwater species cause large ecological and economic impacts in Great Britain. In response the government is in the process of implementing a broad, new nonindigenous species strategy. We assembled a list of all nonindigenous freshwater species that are or were established in Great Britain, their date of first record, and their vector of introduction. This list provides a baseline against which the success of new policies can be assessed. Because the biota of Great Britain has been well recorded, our results provide a highly resolved case study of the vectors and drivers of species transport and establishment. A total of 117 nonindigenous freshwater species are currently established in Great Britain; a further 17 species were once established but are now extirpated. Between 1800 and 2000 the number of established species increased at an accelerating rate, and this increase correlated with the growth in human population and gross domestic product. The construction of large reservoirs in Great Britain occurred over a short period and overlapped high rates of new species establishment, indicating that habitat modification may have been an important driver of establishment. Nonindigenous species now account for 24% of fish, 12% of plant, 54% of amphibian, and 88% of decapod crustacean freshwater species richness in Great Britain. The ornamental trades have been responsible for the greatest percentages of intentionally (73%) and unintentionally (34%) introduced species that have become established. Shipping and aquaculture have also been strong vectors. These vectors should be prioritized for management within the new nonindigenous species strategy.

Size-dependent effects of low level cadmium and zinc exposure on the metabolome of the Asian clam, Corbicula fluminea

The toxic effects of low level metal contamination in sediments are currently poorly understood. We exposed different sized Asian clams, Corbicula fluminea, to sediment spiked with environmentally relevant concentrations of either zinc, cadmium or a zinc-cadmium mixture for one week. This freshwater bivalve is well suited for sediment toxicity tests as it lives partly buried in the sediment and utilises sediment particles as a food resource. After one week, the whole tissue composition of low molecular weight metabolites was analysed by nuclear magnetic resonance spectroscopy (NMR) and gas chromatography-mass spectrometry (GC-MS). The condition index (ratio of tissue dry weight to volume inside the shell valves) was also measured. Small and large clams were clearly differentiated by their metabolic composition and the two size classes showed opposite responses to the mixture spiked sediment. No effects of zinc alone on the metabolome were found and cadmium only influenced the smaller size class. The main perturbations were seen in amino acid and energy metabolism, with small clams using amino acids as an energy resource and larger clams primarily drawing on their larger storage reserves of carbohydrates. Our study demonstrates that metabolomics is a useful technique to test for low level toxicity which does not manifest in mortality or condition index changes. The differing effects between the two size classes stress that it is important to consider age/size when conducting metabolomic and ecotoxicology assessments, since testing for the effects on only one size class makes it more difficult to extrapolate laboratory results to the natural environment.

The Importance of the Human Footprint in Shaping the Global Distribution of Terrestrial, Freshwater and Marine Invaders

Human activities such as transport, trade and tourism are likely to influence the spatial distribution of non-native species and yet, Species Distribution Models (SDMs) that aim to predict the future broad scale distribution of invaders often rely on environmental (e.g. climatic) information only. This study investigates if and to what extent do human activities that directly or indirectly influence nature (hereafter the human footprint) affect the global distribution of invasive species in terrestrial, freshwater and marine ecosystems. We selected 72 species including terrestrial plants, terrestrial animals, freshwater and marine invasive species of concern in a focus area located in NW Europe (encompassing Great Britain, France, The Netherlands and Belgium). Species Distribution Models were calibrated with the global occurrence of species and a set of high-resolution (9×9 km) environmental (e.g. topography, climate, geology) layers and human footprint proxies (e.g. the human influence index, population density, road proximity). Our analyses suggest that the global occurrence of a wide range of invaders is primarily limited by climate. Temperature tolerance was the most important factor and explained on average 42% of species distribution. Nevertheless, factors related to the human footprint explained a substantial amount (23% on average) of species distributions. When global models were projected into the focus area, spatial predictions integrating the human footprint featured the highest cumulative risk scores close to transport networks (proxy for invasion pathways) and in habitats with a high human influence index (proxy for propagule pressure). We conclude that human related information–currently available in the form of easily accessible maps and databases—should be routinely implemented into predictive frameworks to inform upon policies to prevent and manage invasions. Otherwise we might be seriously underestimating the species and areas under highest risk of future invasions.

Invasion Science: A Horizon Scan of Emerging Challenges and Opportunities

We identified emerging scientific, technological, and sociopolitical issues likely to affect how biological invasions are studied and managed over the next two decades. Issues were ranked according to their probability of emergence, pervasiveness, potential impact, and novelty. Top-ranked issues include the application of genomic modification tools to control invasions, effects of Arctic globalization on invasion risk in the Northern Hemisphere, commercial use of microbes to facilitate crop production, the emergence of invasive microbial pathogens, and the fate of intercontinental trade agreements. These diverse issues suggest an expanding interdisciplinary role for invasion science in biosecurity and ecosystem management, burgeoning applications of biotechnology in alien species detection and control, and new frontiers in the microbial ecology of invasions.