Ecological effects of multiple stressors on freshwater ecosystems
Freshwaters provide a resource essential for life, yet they are critically affected by a multitude of human-induced pressures. These pressures rarely occur alone and typically interact to produce the effects that are difficult to predict based on their individual effects. Most knowledge of the effects of stressors is derived from the effect of single stressors and the application of this knowledge for assessing ecological risks of multiple stressors to freshwater ecosystems, particularly at higher levels of biological organisation (population, community, and ecosystem) remain challenging.
The main aim of this study was to explore the individual direct, indirect, and combined effects of multiple stressors on the structure and function of a model freshwater ecosystem. Experiments were conducted in outdoor experimental ponds under environmentally realistic exposure conditions with sediment-bound Cu as a chemical stressor, and common carp (Cyprinus carpio L.) as a biological stressor. Ponds were monitored for six months with Cu as a sole stressor. Carp was added to half of the ponds and were monitored for a further six months. To check the recovery of the system, ponds were monitored another six months after carp removal. The research reported in this thesis focuses on the effects to benthic communities, decomposition, and productivity, and examines their resistance to both stressors and the resilience (recovery) of the pond communities after carp removal. Benthic communities were characterised using microscopy-based counts of invertebrates, and environmental DNA targeting eukaryotes using the 18S rDNA gene, and prokaryotes using the 16S rDNA gene. Primary productivity was assessed by measuring the plant growth, chlorophyll a concentration and periphyton cover, while organic matter decomposition was assessed by mass loss of leaf litter, and tensile strength of cotton strips deployed in and on the pond sediments.
Copper as a sole stressor directly reduced primary production and decomposition in a concentration dependent manner. There was clear difference in macrophyte growth between the controls and Cu treated ponds, which is a likely direct effect of the Cu-contaminated sediment. Carp alone caused a direct effect on aquatic plant biomass by removing plants outside the protective cages but caused an increase in the macrophytes growth (in caged plants), periphyton cover, and Chlorophyll a concentration. Ponds with Cu and Carp had no macrophyte growth within the cages caused by the combined effects of Cu toxicity and increased turbidity from carp bioturbation. Cotton strip decomposition was affected by both Cu and carp, and responses varied between surface and buried strips, and were also unpredictable.
Copper alone altered the benthic prokaryotic and eukaryotic communities in both the low and high Cu treatments. The impact of Carp on biodiversity were not as strong as those of Cu. However, community composition of Cu ponds changed further with the addition of carp. The prokaryote, eukaryote, and invertebrate assemblages all responded differently to the stressor regime, which highlights the complexity of multiple stressor effects in natural systems.
The impacts of Cu exceeded the ecosystem’s capacity to resist change, and effects were concentration dependent. Carp addition changed the water quality, periphyton cover and eukaryotes (18S rDNA), while prokaryotes (16S rDNA) were resistant and remained unchanged following carp addition. All resistance parameters changed significantly in ponds where both Cu and carp were present. After the removal of carp, water quality improved, and within six months the microbial communities in the Low Cu + Carp and High Cu + Carp ponds had recovered, demonstrating resilience in those communities. However, over the short post-carp study period, there was no recovery in periphyton cover or eukaryote communities.
The outcomes of this study have deepened our understanding of the impacts of multiple stressors on freshwater ecosystems, and the resilience of these systems to Cu and C. carpio, which are both globally significant stressors in freshwater systems. The aims and objectives of the research have been met, and in doing so the research has demonstrated the importance of the assessment of structural and functional endpoints, and at different levels of biological organisations in an environmentally realistic exposure scenario. Importantly, the research has emphasised the unpredictability of ecological responses to multiple stressors and the need to understand stressor interactions to reliably assess ecological risk. Furthermore, assessing the resistance and resilience of a system provides valuable insights that will inform better management policies and decision making, risk assessments, and further experimental studies such as those reported here are necessary to achieve that goal.