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

Abstract

The 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.

A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters

“…Another study showed 1,000 mg Cl − /L reduced chlorophyll a content and biomass of the macroalgae Nitella sp. (Lind et al, 2018). Some evidence suggests that road salt can have a stimulating effect on phytoplankton abundance (Fay & Shi, 2012;Hintz et al, 2017).…”

Competition alters species’ plastic and genetic response to environmental change

“…We used a full-factorial design of species kept in the absence and presence of competing species along five different salinity environments (adding appropriate levels of NaCl to the protist medium to reach concentrations of 0, 0.5, 1, 2, and 4 g/l respectively). These NaCl concentrations correspond to a natural, unpolluted freshwater pond or lake (0 g/l), as well as realistic and currently experienced as well as future predicted environmentally relevant NaCl concentrations 64 , 65 . The experiment was conducted in 50 ml centrifuge tubes.…”

Concurrent improvement and deterioration of epilimnetic water quality in an oligotrophic lake over 37 years

“…Although increasing, present‐day concentrations of sodium and chloride from road salts are unlikely to affect the biota of Lake George. Some of the most sensitive lake organisms can tolerate much higher salt concentrations before being impacted (Evans and Frick ; Elphick et al ; Roe and Patterson ; Hintz et al ; Hintz and Relyea ; Lind et al ; Sinclair and Arnott ). For instance, low water hardness increases the toxicity of chloride from road salts triggering negative effects around 64 mg Cl − L −1 for sensitive zooplankton species (Elphick et al ).…”