The ubiquity of anthropogenic litter (AL) in the environment is a significant and increasing problem, with implications for terrestrial, freshwater and marine ecosystems. Despite recorded densities of AL in freshwater systems being comparable to marine systems, research examining the impact of AL in freshwaters is relatively scarce. In addition, research investigating the colonisation of AL by freshwater biota is minimal. However, a small number of studies have indicated that AL is used as a novel habitat by macroinvertebrate taxa in freshwater systems. The primary aim of this study was to examine the effects of AL on macroinvertebrate diversity and community composition in two urban freshwater ecosystems: canals and rivers. To achieve this aim, the diversity and composition of macroinvertebrate communities inhabiting AL was compared to bed sediments in rivers, and mesohabitats (open water and emergent vegetation) in canals. This study is the first to examine the impact of AL on macroinvertebrates in canal habitats and to examine effects at a landscape scale across two freshwater ecosystems. Two urban river reaches and two urban canal reaches were surveyed for macroinvertebrates and environmental variables between October and November 2021. In the two river reaches, 18 macroinvertebrate samples were collected from riffle habitats, and 81 pieces of AL were sampled. In the two canal reaches, 18 mesohabitats (open water and emergent vegetation) and 133 pieces of AL were sampled for macroinvertebrates. The AL items in the river and canal reaches varied in material and composition, although plastic was the dominant material type in both freshwater systems.In the river reaches, a total of 2617 individuals (37 taxa) were recorded inhabiting bed sediments, compared to 3538 individuals (63 taxa) on AL. Bed sediments supported significantly greater alpha diversity compared to AL, and distinct macroinvertebrate communities were recorded between bed sediments and AL. In addition, alpha diversity was significantly higher for fabric AL items compared to plastic and other AL items. However, no differences in community composition occurred between different types of AL (i.e., plastic, fabric, metal, rubber, and other) in the studied urban river reaches. In the canal reaches, a total of 831 individuals (31 taxa) were recorded in mesohabitats, relative to 1908 individuals (49 taxa) inhabiting AL. Alpha diversity was significantly greater for mesohabitats relative to AL, and significant differences in community composition between mesohabitats and AL were recorded. No significant differences in macroinvertebrate alpha (taxonomic richness) and beta (community composition) diversity were found between different AL material types (i.e., plastic, polystyrene, metal, and other) within urban canals. This study also compared the diversity and community composition of macroinvertebrates inhabiting AL between the urban river and canal reaches. This study showed that alpha diversity was significantly greater for AL retrieved from the river reaches than the canal reaches, and that the communities inhabiting AL in each freshwater system were distinct. The findings of this study highlight the importance of AL in supporting distinct macroinvertebrates communities relative to those occurring in bed sediments in rivers and open water and emergent vegetation in canals. This study contributes towards a greater understanding of the role of AL as a physical habitat in lotic freshwater systems and indicates that AL removal may lead to a local loss of macroinvertebrate diversity. To counteract any potential macroinvertebrate diversity loss, restoration actions, such as gravel augmentation and installation of large wood or boulders in urban rivers, and the construction of shallow zones and backwater areas to encourage macrophyte colonisation and flow diversity in urban canals, are recommended. In addition, it is advised that AL items are rinsed within the water column prior to removal to dislodge any inhabiting macroinvertebrates and allow taxa to colonise alternative habitat. In conclusion, the findings of this study are valuable in informing AL management and clean-up and initiatives, as well as physical habitat restoration strategies.
|Date of Award
|25 May 2023
|Tory Milner (Main Supervisor) & Matthew Hill (Main Supervisor)