Watersheds deliver numerous pollutants to the coastline of oceans and lakes, thereby jeopardizing ecosystem services. Regulatory frameworks for stressors often focus on loading rates without accounting for the physical dynamics of the receiving water body. Here, we use a three-dimensional hydrodynamic model to simulate the transport of a generic tributary-delivered anthropogenic pollutant within Lake Michigan based on the location and timing of loading. Simulating pollutant plumes from 11 rivers, and their intersections with coastal ecosystem services, reveals strong mediation of potential impacts by lake physics. Trapped pollutants accumulate in nearshore waters during spring peak flows, and become diluted by spreading offshore during the summer. The threat to coastal ecosystem services posed by pollutant loading differs sharply among rivers; high potential impact arises from the spatiotemporal coincidence of tributary input rates, lake mixing dynamics, and multiple human uses of the shoreline. Simultaneous pollution from multiple rivers yields overlapping plumes, creating a second way in which lake hydrodynamics can amplify potential impacts on coastal ecosystem services. Our simulations demonstrate that the physical dynamics of large water bodies can create a dynamic stressor landscape arising from multiple independent sources of non-point-source pollution. The design and implementation of pollution regulations rarely account for spatial and temporal complexities of load processing in receiving waters, yet the resulting variability is likely to strongly mediate impacts on society. As hydrodynamic models improve, our analytical framework could be applied to a wide range of pollutants and waterbodies to enhance the sustainable use of coastal ecosystems.

Co-occurrence of environmental stressors is ubiquitous in ecosystems, but cumulative effects are difficult to predict for effective indicator development. Individual stressors can amplify (synergies) or lessen (antagonisms) each other's impacts or have fully independent effects (additive). Here we use the Laurentian Great Lakes, where a multitude of stressors have been studied for decades, as a case study for considering insights from both a systematic literature review and an expert elicitation (or structured expert judgment) to identify stressor interactions. In our literature search for pairs of stressors and interaction-related keywords, relatively few studies (9%, or 6/65) supported additive interactions with independent stressor effects. Instead, both antagonisms (42%, or 27/65) and synergies (49%, or 32/65) were common. We found substantial evidence for interactions of invasive dreissenid mussels with nutrient loading and between pairs of invasive species (predominantly dreissenids x round goby), yet both sets of records included mixtures of synergies and antagonisms. Complete quantification of individual and joint effects of stressors was rare, but effect sizes for dreissenid mussels x nutrient loading supported an antagonism. Our expert elicitation included discussion in focus groups and a follow-up survey. This process highlighted the potential for synergies of nutrient loading with dreissenid mussels and climate change as seen from the literature review. The elicitation also identified additional potential interactions less explored in the literature, particularly synergies of nutrient loading with hypoxia and wetland loss. To stimulate future research, we built a conceptual model describing interactions among dreissenid mussels, climate change, and nutrient loading. Our case study illustrates the value of considering results from both elicitations and systematic reviews to overcome data limitations. The simultaneous occurrence of synergies and antagonisms in a single ecosystem underscores the challenge of predicting the cumulative effects of stressors to guide indicator development and other management and restoration decisions.

Summary Phytoplankton communities can experience nonlinear responses to changing nutrient concentrations, but the nature of species shifts within phytoplankton is not well understood and few studies have explored responses of pelagic assemblages in large lakes. Using pelagic phytoplankton data from the Great Lakes, we assessed phytoplankton assemblage change‐point responses to nutrients and invasive Dreissena, characterising community responses in a multi‐stressor environment and determine whether species responses to in situ nutrients can be approximated from nutrient loading. We demonstrate assemblage shifts in phytoplankton communities along major stressor gradients, particularly prominent in spring assemblages, providing insight into community thresholds at the lower end of the phosphorus gradient and species‐stressor responses in a multi‐stressor environment. We show that responses to water nutrient concentrations could not be estimated from large‐scale nutrient loading data likely due to lake‐specific retention time and long‐term accumulation of nutrients. These findings highlight the potential for significant accumulation of nitrates in ultra‐oligotrophic systems, nonlinear responses of phytoplankton at nutrient concentrations relevant to current water quality standards and system‐specific (e.g. lake or ecozone) differences in phytoplankton responses likely due to differences in nutrient co‐limitation and effects of dreissenids.

A comprehensive inventory of ecosystem services across the entire Great Lakes basin is currently lacking and is needed to make informed management decisions. A greater appreciation and understanding of ecosystem services, including both use and non-use services, may have avoided misguided resource management decisions in the past that have resulted in legacies inherited by future generations. Given the interest in ecosystem services and lack of a coherent approach to addressing this topic in the Great Lakes, a summit was convened involving 28 experts working on various aspects of ecosystem services in the Great Lakes. The invited attendees spanned a variety of social and natural sciences. Given the unique status of the Great Lakes as the world's largest collective repository of surface freshwater, and the numerous stressors threatening this valuable resource, timing was propitious to examine ecosystem services. Several themes and recommendations emerged from the summit. There was general consensus that 1) a comprehensive inventory of ecosystem services throughout the Great Lakes is a desirable goal but would require considerable resources; 2) more spatially and temporally intensive data are needed to overcome our data gaps, but the arrangement of data networks and observatories must be well-coordinated; 3) trade-offs must be considered as part of ecosystem services analyses; and 4) formation of a Great Lakes Institute for Ecosystem Services, to provide a hub for research, meetings, and training is desirable. Several challenges also emerged during the summit, which are discussed in the paper.