Effects of salinity on the decay of the freshwater macrophyte, Triglochin procerum

We examined the effects of increasing salt concentrations on the decay of the common aquatic angiosperm, Triglochin procerum R. Br. (Juncaginaceae) from a freshwater wetland close to Gippsland Lakes, eastern Vic., Australia. Rate of decay, measured as leaf mass loss, and microbial enzymatic activity, used as a surrogate for microbial activity, were measured on leaves placed in mesocosms ranging in electrical conductivity from 100 to 45,000 EC. The rate of leaf mass loss was up to three times slower in salt concentrations of 45,000 EC (∼69% ash-free dry leaf weight remaining after 21 days), compared to salt concentrations of 100 EC (∼23%). Enzymatic activity on the leaves at 45,000 EC (0.56, A490) was about one-half that on leaves in 100 EC (1.00, A490). A second experiment measured the same variables for leaves placed in solutions of NaCl, marine salt, or an organic osmoticum, polyethylene glycol (200 Da molecular weight). Results indicated that the inhibition of leaf mass loss was ∼1.5 times greater in NaCl (∼39% remaining after 21 days) than an organic osmoticum, polyethylene glycol (∼24% remaining after 21 days), indicating a role for ionic toxicity in the salt effects. Enzymatic activity on leaves was significantly inhibited in NaCl (0.50, A490) compared with marine salt (0.74, A490) or polyethylene glycol (0.72, A490). Our findings suggest several implications for the effects of acute secondary salinisation on organic matter decomposition. Inhibition of decay rates due to acute increase in salt concentration is related to decreased enzymatic activity on decaying leaves. This relationship has ramifications for microzoan food webs based on a microbial loop of bacterial production and consumption and availability of degraded organic matter entering metazoan food webs.