An ecosystem model for evaluating the effects of introduced Pacific salmon on contaminant burdens of stream-resident fish

Trophic transfer and biomagnification of microplastics through food webs in coastal waters: A new perspective from a mass balance model

Since the 1950s, plastic pollution and its risk have been recognized as irreversible and nonnegligible problems as global plastic production has increased. In recent years, the transport and trophic transfer of microplastics (MPs) in biotic and abiotic environment have attracted extensive attention from researchers. In this study, based on the Ecotracer module from Ecopath with Ecosim (EwE) model, the marine ranching area of Haizhou Bay, Jiangsu Province, China, was taken as a case study by linking the environmental plastic inflow with MPs in organisms to simulate the variation of MPs in the marine food web for 20 years, as well as its potential trophic transfer and biomagnification. We found that the concentration of MPs in top consumers first increased when the concentration of MPs in the environment increased, while that in primary consumers first decreased when the concentration of MPs in the environment decreased. Moreover, high TL consumers had a stronger ability to accumulate MPs, and pelagic prey fishes was the opposite. From the perspective of the food web, all functional groups showed significant trophic magnification along with the trophic level and no biodilution. Generally, there is a direct relationship between the MPs in marine organisms and environmental inflow. If the pollutants flowing into the environment can be reduced, the MP pollution problem in coastal waters will be effectively alleviated. Our research can further provide a scientific basis for ecological risk assessment and management of MPs and biodiversity protection in marine ecosystems.

Pacific salmon as vectors of environmental contaminants: An experimental test confirms synoptic surveys in natural streams

Pacific salmon transfer large quantities of material to tributaries during their spawning migrations, including carcass tissue and labile nutrients but also persistent organic pollutants (POPs) and heavy metals. We conducted a Before-After-Control-Intervention experiment by adding salmon carcasses and eggs to a Michigan (USA) stream that had never received inputs from non-native salmon to understand the bioaccumulation and persistence of biotransported contaminants. Our experimental outcomes were compared to previous studies using meta-analysis. Coincident with the introduction of salmon, the PCB and DDE burden of resident trout significantly increased. However, we did not observe changes in total mercury (Hg). Two years after the salmon addition experiment concluded, resident trout POP concentrations had returned to pre-addition levels, with no difference between the treatment and control reaches. Analysis of effect sizes suggested that the contaminant response observed in our experiment is consistent with field survey observations. Our study suggested that the consumption of salmon eggs drove the increase in POP burden of resident trout while Hg bioaccumulation was influenced by watershed sources. Critically, our study suggests that ecosystems are capable of quickly recovering from POP inputs from species migrations if contaminant sources are removed.

Exploring multiple stressor effects with Ecopath, Ecosim, and Ecospace: Research designs, modeling techniques, and future directions

Understanding the cumulative effects of multiple stressors is a research priority in environmental science. Ecological models are a key component of tackling this challenge because they can simulate interactions between the components of an ecosystem. Here, we ask, how has the popular modeling platform Ecopath with Ecosim (EwE) been used to model human impacts related to climate change, land and sea use, pollution, and invasive species? We conducted a literature review encompassing 166 studies covering stressors other than fishing mostly in aquatic ecosystems. The most modeled stressors were physical climate change (60 studies), species introductions (22), habitat loss (21), and eutrophication (20), using a range of modeling techniques. Despite this comprehensive coverage, we identified four gaps that must be filled to harness the potential of EwE for studying multiple stressor effects. First, only 12% of studies investigated three or more stressors, with most studies focusing on single stressors. Furthermore, many studies modeled only one of many pathways through which each stressor is known to affect ecosystems. Second, various methods have been applied to define environmental response functions representing the effects of single stressors on species groups. These functions can have a large effect on the simulated ecological changes, but best practices for deriving them are yet to emerge. Third, human dimensions of environmental change - except for fisheries - were rarely considered. Fourth, only 3% of studies used statistical research designs that allow attribution of simulated ecosystem changes to stressors' direct effects and interactions, such as factorial (computational) experiments. None made full use of the statistical possibilities that arise when simulations can be repeated many times with controlled changes to the inputs. We argue that all four gaps are feasibly filled by integrating ecological modeling with advances in other subfields of environmental science and in computational statistics.

Investigating the dynamics of methylmercury bioaccumulation in the Beaufort Sea shelf food web: a modeling perspective

High levels of methylmercury (MeHg) have been reported in Arctic marine biota, posing health risks to wildlife and human beings. Although MeHg concentrations of some Arctic species have been monitored for decades, the key environmental and ecological factors driving temporal trends of MeHg are largely unclear. We develop an ecosystem-based MeHg bioaccumulation model for the Beaufort Sea shelf (BSS) using the Ecotracer module of Ecopath with Ecosim, and apply the model to explore how MeHg toxicokinetics and food web trophodynamics affect bioaccumulation in the BSS food web. We show that a food web model with complex trophodynamics and relatively simple MeHg model parametrization can capture the observed biomagnification pattern of the BSS. While both benthic and pelagic production are important for transferring MeHg to fish and marine mammals, simulations suggest that benthic organisms are primarily responsible for driving the high trophic magnification factor in the BSS. We illustrate ways of combining empirical observations and modelling experiments to generate hypotheses about factors affecting food web bioaccumulation, including the MeHg elimination rate, trophodynamics, and species migration behavior. The results indicate that population dynamics rather than MeHg elimination may determine population-wide concentrations for fish and lower trophic level organisms, and cause large differences in concentrations between species at similar trophic levels. This research presents a new tool and lays the groundwork for future research to assess the pathways of global environmental changes in MeHg bioaccumulation in Arctic ecosystems in the past and the future.

Coupling isotopic analysis and Ecopath model to detect the ecosystem features based on functional groups of the southwestern Yellow Sea, China

The paper evaluates the species richness, material transfer, energy flow, and system function of the southwestern Yellow Sea Ecosystem (SYSE) indicating intensive human intervention affecting this large marine ecosystem. Twenty functional groups were chosen to represent the basic components of the SYSE for Ecopath modeling based on offshore surveys, annual bird observations, and the China Fisheries Statistical Yearbooks. Forty-nine species based on 15 functional groups of Ecopath model were assessed by stable isotopic analysis (SIA) to verify ecosystem features, energy flow, and trophic structure of the SYSE derived from Ecopath model. Results showed there was a clear correlation of the estimated trophic structure calculated from SIA and the Ecopath model with R²=0.7184. The SYSE Ecopath model was still at an immature and unstable stage according to outputs of the modeling parameters. This paper provides a verification method of detecting the ecosystem features and maturity, stability, and resilience of marine ecosystems by comparing outputs from Ecopath models with SIA.

Using a dynamic bioenergetics-bioaccumulation model to understand mechanisms of uptake and bioaccumulation of salmon-derived contaminants by stream-resident fish

Ecosystem linkages created by migratory organisms such as Pacific salmon (Oncorhynchus spp.) facilitate the transfer of ecologically beneficial resource subsidies and environmentally damaging contaminants to recipient food webs. In the Laurentian Great Lakes, introduced Pacific salmon accumulate large contaminant burdens that they disperse to streams during spawning in the form of carcass and gametic tissue, with uncertain consequences for stream food webs. Here, we describe a coupled bioenergetics-bioaccumulation model parameterized using empirical and literature-sourced data to predict the dual effect of Pacific salmon on stream-resident brook trout (Salvelinus fontinalis) growth and contaminant bioaccumulation. Within the model, we developed four unique scenarios to ascertain how the (1) trophic pathway to contamination, (2) level of salmon egg consumption, (3) intensity and duration of salmon exposure, and (4) age of first exposure to salmon, affected growth and contaminant bioaccumulation in brook trout. Our model demonstrated that salmon egg consumption increased brook trout growth and PCB bioaccumulation while reducing Hg tissue concentrations. Other trophic pathways, including direct carcass consumption and an indirect food web pathway, did not strongly influence growth or contaminant bioaccumulation. Our model also demonstrated that variation in the magnitude and temporal duration of salmon egg consumption mostly strongly influenced the growth and contaminant concentration of younger brook trout. Overall, our model highlighted that Pacific salmon transfer energy and contaminants but this balance is dictated by the food web pathway and plasticity in the diet of stream-resident fish. Our mechanistic, model-based evaluation of salmon contaminant biotransport can be extended to predict the impact of other migratory fishes on recipient food webs.

Modeling the impact of biota on polychlorinated biphenyls (PCBs) fate and transport in Lake Ontario using a population-based multi-compartment fugacity approach

Organisms have long been treated as receptors in exposure studies of polychlorinated biphenyls (PCBs) and other persistent organic pollutants (POPs). The influences of environmental pollution on organisms are well recognized. However, the impact of biota on PCB transport in an environmental system has not been considered in sufficient detail. In this study, a population-based multi-compartment fugacity model is developed by reconfiguring the organisms as populated compartments and reconstructing all the exchange processes between the organism compartments and environmental compartments, especially the previously ignored feedback routes from biota to the environment. We evaluate the model performance by simulating the PCB concentration distribution in Lake Ontario using published loading records. The lake system is divided into three environment compartments (air, water, and sediment) and several organism groups according to the dominant local biotic species. The comparison indicates that the simulated results are well-matched by a list of published field measurements from different years. We identify a new process, called Facilitated Biotic Intermedia Transport (FBIT), to describe the enhanced pollution transport that occurs between environmental media and organisms. As the hydrophobicity of PCB congener increases, the organism population exerts greater influence on PCB mass flows. In a high biomass scenario, the model simulation indicates significant FBIT effects and biotic storage effects with hydrophobic PCB congeners, which also lead to significant shifts in systemic contaminant exchange rates between organisms and the environment.