Integrating Taxonomic Meta-Analysis and Mussel Physiology to Assess Secondary Salinization in Appalachian Headwaters and Urban Streams

Abstract

Secondary salinization from fertilizer application, mining, urbanization, and de-icing salts like sodium chloride (NaCl), is elevating chloride (Cl⁻) and sodium (Na⁺) in streams. While Cl⁻ effects on streams are well-studied, Na⁺ effects are less known. Elevated Cl⁻ levels in the environment can surpass physiological thresholds for some aquatic organisms, affecting nutrient cycling in ecosystems. A recent global review showed salinization negatively impacted freshwater animals in 78% of non-arthropod invertebrates, 68% of zooplankton, 62% of amphibians and insects, 56% of mollusks and crustaceans, and 51% of fishes. However, the diverse study designs and ionic concentrations hindered generalizable comparisons across taxa. My first objective was to expand the existing review by calculating population-level effect sizes for crustaceans, zooplankton, and mollusks. Despite the initial prediction that mollusks would exhibit the strongest negative response due to their low internal salt concentration, my findings indicated zooplankton were the most sensitive, followed by mollusks, while crustaceans exhibited more positive effects. The limited representation of unionid mussel population responses in the literature precluded their inclusion in our analysis. However, studies have noted that elevated NaCl can influence mussel filtering behavior, potentially impacting other ecosystem processes. My second objective was to examine how unionid mussel Elliptio complanata's energy storage, filtering, excretion, and biodeposition changed with elevated NaCl across a 28-day laboratory study. I anticipated reduced filtering rates, attributed to avoidance behavior (i.e. clamming-up), leading to decreased ammonium excretion and biodeposition, thereby altering nutrient cycling. I found that E. complanata energy storage remained stable across salt treatments, their excretion rate increased halfway through the experiment before declining by day 28, and clearance and biodeposition rates declined in all treatments and the control over time. My results suggest that E. complanata can live at a high-salt urban gradient due to their salt-induced avoidance behaviors, but these sublethal responses affect their ecosystem functions, thereby affecting mussel-mediated nitrogen cycling. Examining these effects is vital for understanding the ecosystem services provided by mussels amid increasing secondary salinization.