A Review of Key Features and Their Implementation in Unstructured, Structured, and Agent-Based Population Models for Ecological Risk Assessment.

Population models can provide valuable tools for ecological risk assessment (ERA). A growing amount of work on model development and documentation is now available to guide modelers and risk assessors to address different ERA questions. However, there remain misconceptions about population models for ERA, and communication between regulators and modelers can still be hindered by a lack of clarity in the underlying formalism, implementation, and complexity of different model types. In particular, there is confusion about differences among types of models and the implications of including or ignoring interactions of organisms with each other and their environment. In this review, we provide an overview of the key features represented in population models of relevance for ERA, which include density dependence, spatial heterogeneity, external drivers, stochasticity, life-history traits, behavior, energetics, and how exposure and effects are integrated in the models. We differentiate 3 broadly defined population model types (unstructured, structured, and agent-based) and explain how they can represent these key features. Depending on the ERA context, some model features will be more important than others, and this can inform model type choice, how features are implemented, and possibly the collection of additional data. We show that nearly all features can be included irrespective of formalization, but some features are more or less easily incorporated in certain model types. We also analyze how the key features have been used in published population models implemented as unstructured, structured, and agent-based models. The overall aim of this review is to increase confidence and understanding by model users and evaluators when considering the potential and adequacy of population models for use in ERA. Integr Environ Assess Manag 2021;17:521-540. © 2020 SETAC.

Comparative Toxicity of Nitrate to Common and Imperiled Freshwater Mussel Glochidia and Larval Fishes.

Growing human populations and increasingly intensive agriculture have resulted in widespread aquatic nitrate pollution. Freshwater mussel populations have been in decline for decades but often are underrepresented in data used for the development of ambient water quality criteria and acute toxicity of nitrate to mussel glochidia has not yet been established. Additionally, toxicity testing with aquatic species often is limited to a few model species; however, relatively little is known about how representative model species are of imperiled species. Therefore, to better define the acute toxicity of nitrate to common and rare aquatic species, we conducted 24-h nitrate acute toxicity tests with glochidia of four species of freshwater mussels, including a federally threatened species (Hamiota altilis) and 7-day tests with larval fish of three species: fathead minnow (Pimephales promelas), tricolor shiner (Cyprinella trichroistia), and tilapia (Oreochromis spp.), across a range of water hardness. Median effective concentrations (EC50s) in freshwater mussel glochidia ranged from 524 to 904 mg/L NO3-N in moderately hard water. In fish, median lethal concentrations (LC50s) ranged from 228 to 1725 mg/L NO3-N and varied with water hardness. Of the species tested, generally sensitivity of the common species was similar to the rare species, although relative sensitivity varied with water hardness. Based on these results, we can conclude that acute lethal effects are unlikely for the fish and mussel species considered here at current environmental levels, but the results of these standardized tests are useful for the development of ambient water quality criteria and other regulatory and management decisions regarding acute nitrate exposures.

Effects of nitrate on freshwater mussel glochidia attachment and metamorphosis success to the juvenile stage.

Water quality and contaminants have been frequently identified as critical stressors for freshwater mussels, many species of which are highly imperiled throughout North America and the world. Nutrient pollution, specifically nitrate, has become one of the most prevalent causes of water quality degradation globally, with increasing anthropogenic input from suburban and agricultural runoff, municipal wastewater, and industrial waste. Nitrate acute toxicity is generally low for aquatic species, but the potential effects of nitrate exposure are largely unknown for freshwater mussels, particularly during the parasitic stage of their complex lifecycle. Therefore, this study was designed to determine the effects of short-term nitrate exposure at environmentally relevant concentrations on juvenile production in two freshwater mussel species. Lampsilis siliquoidea and L. fasciola glochidia were exposed to nitrate (0, 11, or 56 mg NO3-N/L) for 24 h before inoculation on a primary host, Largemouth Bass (Micropterus salmoides). Glochidia attachment, metamorphosis success, and total number of juveniles produced were monitored on individual fish. Exposure of L. siliquoidea glochidia to 56 mg NO3-N/L nitrate resulted in a significant (p = 0.02) 35% reduction of total juveniles produced, a combined result of moderate decreases in both glochidia attachment and metamorphosis success. A similar trend (28% reduction; p = 0.06) was evident with 11 mg NO3-N/L. No effects were apparent for L. fasciola, suggesting species-specific differences in responses even among closely related species. These results are the first to suggest that glochidia exposure to nitrate may adversely affect juvenile recruitment in some species. Findings from these studies are important for improving characterization of the hazards of nitrate pollution to aquatic life and this work will help better define the role of water quality in assessing habitat suitability for mussel conservation efforts.

Chronic nitrate exposure alters reproductive physiology in fathead minnows.

Nitrate is a ubiquitous aquatic pollutant that is commonly associated with eutrophication and dead zones in estuaries around the world. At high concentrations nitrate is toxic to aquatic life but at environmental concentrations it has also been purported as an endocrine disruptor in fish. To investigate the potential for nitrate to cause endocrine disruption in fish, we conducted a lifecycle study with fathead minnows (Pimephales promelas) exposed to nitrate (0, 11.3, and 56.5 mg/L (total nitrate-nitrogen (NO3-N)) from <24 h post hatch to sexual maturity (209 days). Body mass, condition factor, gonadal somatic index (GSI), incidence of intersex, and vitellogenin induction were determined in mature male and female fish and plasma 11-keto testosterone (11-KT) was measured in males only. In nitrate-exposed males both 11-KT and vitellogenin were significantly induced when compared with controls. No significant differences occurred for body mass, condition factor, or GSI among males and intersex was not observed in any of the nitrate treatments. Nitrate-exposed females also had significant increases in vitellogenin compared to controls but no significant differences for mass, condition factor, or GSI were observed in nitrate exposed groups. Estradiol was used as a positive control for vitellogenin induction. Our findings suggest that environmentally relevant nitrate levels may disrupt steroid hormone synthesis and/or metabolism in male and female fish and may have implications for fish reproduction, watershed management, and regulation of nutrient pollution.