Aquatic invertebrates are highly sensitive to changes in the concentrations of sodium in their environment, increasingly struggling to maintain osmotic and ionic balance in their bodies with greater departure of environmental sodium concentrations from their biological needs. Nonetheless, they require sodium for innumerable cellular functions. The Sodium Subsidy-Stress (SSS) hypothesis predicts that organisms are subsidized by sodium addition to the water when availabilities are lower than an organism’s needs, are healthiest at an “optimum point” of sodium availability near the concentration in hemolymph, and then experience stress when water ion concentrations exceed internal levels. Individual aquatic invertebrates (i.e., body mass) and, by extension, invertebrate communities (i.e., diversity) are expected to display SSS along this scarcity-optimum-overabundance curve. To test SSS, a paired terrestrial-aquatic mesocosm experiment was used to analyze the response of aquatic invertebrate diversity and body mass to a sodium addition gradient with six weekly treatments: 0 (controls), 100, 200, 300, 400, 500 mg/L NaCl addition (n=5 per treatment). Ten additional mesocosms received 300mg/L NaCl addition to streams (n=5) or to terrestrial soils (n=5). Aquatic invertebrates were sampled monthly using benthic cores. All the organisms from a subset of 12 samples have been identified to order, and body length, a proxy for biomass, was measured. So far, 18 orders have been identified. Oligochaeta was most frequently encountered, followed by Ostracoda. Classes Insecta and Clitellata, as well as the subphylum Crustacea accounted for ~20%, ~40%, and ~25% of individuals, respectively. Typically sensitive taxa such as Ephemeroptera and Plecoptera were infrequently collected in sampled replicates. Notably, a number of terrestrial (e.g., Acari: Oribatida) or amphibious (e.g., Clitellata) taxa were collected in experimental streams and provide a measure of terrestrial subsidies to stream systems. From the organisms most represented in preliminary samples, Oligochaeta had the greatest body sizes in control mesocosms, whereas Ostracoda have slightly longer body lengths in relatively elevated [NaCl]. These unique, paired, and experimental riparian mesocosms will enable us to examine links between systems and how communities change in response to both terrestrial and aquatic salinization.