Human activities create threats that have consequences for freshwater ecosystems and, in most watersheds, observed ecological responses are the result of complex interactions among multiple threats and their associated ecological alterations. Here we discuss the value of considering multiple threats in research and management, offer suggestions for filling knowledge gaps, and provide guidance for addressing the urgent management challenges posed by multiple threats in freshwater ecosystems. There is a growing literature assessing responses to multiple alterations, and we build off this background to identify three areas that require greater attention: linking observed alterations to threats, understanding when and where threats overlap, and choosing metrics that best quantify the effects of multiple threats. Advancing science in these areas will help us understand existing ecosystem conditions and predict future risk from multiple threats. Because addressing the complex issues and novel ecosystems that arise from the interaction of multiple threats in freshwater ecosystems represents a significant management challenge, and the risks of management failure include loss of biodiversity, ecological goods, and ecosystem services, we also identify actions that could improve decision-making and management outcomes. These actions include drawing insights from management of individual threats, using threat attributes (e.g., causes and spatio-temporal dynamics) to identify suitable management approaches, testing management strategies that are likely to be successful despite uncertainties about the nature of interactions among threats, avoiding unintended consequences, and maximizing conservation benefits. We also acknowledge the broadly applicable challenges of decision-making within a socio-political and economic framework, and suggest that multidisciplinary teams will be needed to innovate solutions to meet the current and future challenge of interacting threats in freshwater ecosystems.

Because humans have such strong physical and emotional connections to water in the environment, it follows that individuals and groups will have strong ideas and expectations of that environment that must be related to visible features. Optical water quality describes the scattering and absorption of light in water as governed by its dissolved and particulate composition. Mineral suspended sediment, dissolved organic matter, and living and nonliving particulate organic matter have different influences on the behavior of light in water. The physical sciences endeavor to define variability in water quality, be it natural or anthropogenic, for use in water resource management. However, a more holistic approach to water resource management is becoming increasingly advocated, which requires investigation of the human dimension. The directly visible attributes of color and clarity of water are among the strongest influences on human perceptions of water quality. The observed environment is perceived through cognitive processes that are defined by spiritual and cultural beliefs and evolve with personal experience, creating the mental model. Water color and clarity can provide a critical link between water quality and human perceptions. Ethnohydrology is the study of culturally constructed knowledge and understanding of water. An interdisciplinary approach, integrating optical water quality measurement and ethnohydrology methods, may help achieve better awareness, communication, participation, and support in water resource management and sustainability endeavors.

Rivers are revered worldwide for their ecologic, scenic, and recreational value. The capacity to communicate effectively among human groups with vested interest in rivers hinges on understanding the nature of human perceptions of water quality and the extent to which they vary intraculturally. Recognizing the intersection between measured water quality and the characteristics of rivers that influence human perceptions facilitates potential for better communication across disciplines and among stakeholders. We conducted interviews and a pile‐sort task with water quality experts and nonexperts. Our analysis suggested human evaluation of water quality is guided by culturally constructed criteria, regardless of respondent expertise, experience, or demographics. Cluster analysis results implied that measured physical and chemical parameters of rivers were directly related to the visible attributes used in human judgments. We suggest that, regardless of variability among individual stakeholders, observable characteristics may be the foundation for a common understanding of water quality in rivers.

This study compared horizontal black disk visibility (BDV), a measure of visual water clarity, to turbidity and total suspended solids (TSS) concentrations in five rivers of the southwestern Ozarks of Arkansas, U.S. to assess its usefulness in evaluating optical water quality. We investigated correlative relationships between BDV vs. turbidity and BDV vs. TSS, and compared them to those reported in similar studies. Our results indicated that replicate measures of horizontal BDV were subject to less error than replicate turbidity (coefficients of variation were 5.0% and 6.4%, respectively), and BDV was more strongly correlated with TSS concentrations than was turbidity (correlations coefficients were −0.97 and 0.92, respectively). We also assessed the validity of the black disk method as a surrogate for TSS concentrations and derived a predictive model for each river in the study. The benefits of the horizontal black disk method are both scientific and practical. Horizontal BDV provides a means of quantifying the beam attenuation coefficient. And, the low cost, ease of use, and usefulness in shallow rivers make the black disk method a versatile tool in water quality assessment. We ultimately conclude that the horizontal black disk method should be added to routine monitoring programs in the U.S., and that it be adopted by public groups involved in volunteer monitoring and water quality communication that may be limited by equipment availability or budget.

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