Salty fertile lakes: how salinization and eutrophication alter the structure of freshwater communities

Lovisa Lind,Brian M Mattes,Devin K Jones,Rick A Relyea,Aaron B Stoler,Matthew S Schuler,William D Hintz

doi:10.1002/ecs2.2383

Abstract

AbstractThe quality of freshwater ecosystems is decreasing worldwide because of anthropogenic activities. For example, nutrient over‐enrichment associated with agricultural, urban, and industrial development has led to an acceleration of primary production, or eutrophication. Additionally, in northern areas, deicing salts that are an evolutionary novel stressor to freshwater ecosystems have caused chloride levels of many freshwaters to exceed thresholds established for environmental protection. Even if excess nutrients and road deicing salts often contaminate freshwaters at the same time, the combined effects of eutrophication and salinization on freshwater communities are unknown. Thus by using outdoor mesocosms, we investigated the potentially interactive effects of nutrient additions and road salt (NaCl) on experimental lake communities containing phytoplankton, periphyton, filamentous algae, zooplankton, two snail species (Physa acuta and Viviparus georgianus), and macrophytes (Nitella spp.). We exposed communities to a factorial combination of environmentally relevant concentrations of road salt (15, 250, and 1000 mg Cl−/L), nutrient additions (oligotrophic, eutrophic), and sunlight (low, medium, and high) for 80 d. We manipulated light intensity to parse out the direct effects of road salts or nutrients from the indirect effects via algal blooms that reduce light levels. We observed numerous direct and indirect effects of salt, nutrients, and light as well as interactive effects. Added nutrients caused increases in most producers and consumers. Increased salt (1000 mg Cl−/L) initially caused a decline in cladoceran and copepod abundance, leading to an increase in phytoplankton. Increased salt also reduced the biomass and chl a content of Nitella and reduced the abundance of filamentous algae. Added salt had no effect on the abundance of pond snails, but it caused a decline in banded mystery snails, which led to an increase in periphyton. Low light negatively affected all taxa (except Nitella) and light levels exhibited multiple interactions with road salt, but the combined effects of nutrients and salt were always additive. Collectively, our results indicate that eutrophication and salinization both have major effects on aquatic ecosystems and their combined effects (through different mechanisms) are expected to promote large blooms of phytoplankton and periphyton while causing declines in many species of invertebrates and macrophytes.

Highlights

Lake ecosystems offer multiple ecosystem services, such as the provisioning of drinking water, water for industry and agriculture, recreation, and fisheries (Malmqvist and Rundle 2002, Keeler et al 2012)
We discovered that altered nutrients, sunlight, and salt concentrations altered the structure of ecological communities by causing direct effects as well as top-down and bottom-up indirect effects that altered the abundance of primary producers and consumers
Eutrophication and salinization are two ecosystem stressors that are being experienced in aquatic ecosystems around the world

Summary

Introduction

Lake ecosystems offer multiple ecosystem services, such as the provisioning of drinking water, water for industry and agriculture, recreation, and fisheries (Malmqvist and Rundle 2002, Keeler et al 2012). The contaminants can dramatically alter the structure and function of freshwater lake ecosystems, triggering a loss of ecosystem services (Hintz et al 2017). It is important to understand whether these co-occurring disturbances interact to affect lakes ecosystems and food webs, and what the implications might be for lake ecosystem services and future mitigation efforts. Over the last two centuries, human activities have enriched freshwater ecosystems with nutrients that have altered the trophic state of systems around the world (Conley et al 2009). Nutrient pollution frequently occurs in human-dominated systems, causing eutrophication, harmful algal blooms, hypoxia, and changes in aquatic food webs (Conley et al 2009, Paerl and Paul 2012). Increased pelagic primary productivity might limit the persistence and growth of benthic primary producers (Scheffer et al 1993)

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