Salt in our streams: even small sodium additions can have negative effects on detritivores

Large, pulsed sodium chloride (NaCl) additions can increase mortality of aquatic biota, but longer-term effects from low-level additions are less understood. Small ionic increases may alleviate sodium (Na) limitation or osmoregulatory stress, thereby increasing microbial respiration and macroinvertebrate consumption and growth. We manipulated NaCl levels in microcosms containing just sweetgum (Liquidambar styraciflua L.) leaves with associated microbes, or leaves, microbes, and one of two macroinvertebrate detritivores (Tipula abdominalis Say in Experiment I and Lirceus sp. in Experiment II). In Experiment I, microcosms had either ambient or elevated NaCl (3 or 7 mg Na l(-1), respectively). Contrary to predictions, microbial respiration did not significantly differ between treatments after 4 weeks. However, after 2 weeks, T. abdominalis marginally decreased leaf consumption in elevated treatments without change in growth. Experiment II had three NaCl treatments: low (ambient), medium, and high (3, 14, and 140 mg Na l(-1), respectively). After 6 weeks, microbial respiration averaged 15% lower in medium and 29% lower in high than in low treatments. Throughout, Lirceus sp. ate and grew similarly in low and medium treatments. However, Lirceus sp. growth was 12% slower in high than in low treatments. Lirceus sp. ate 74% more leaves in high than medium treatments, but growth and assimilation did not differ. Therefore, we infer possible osmoregulatory stress. Even low-level NaCl inputs may negatively impact some detritivores, which could alter stream processes.