Factors affecting accumulation and loss of zinc by the aquatic moss Rhynchostegium riparioides (Hedw.) C. Jens

Reciprocal transplants of the moss Rhynchostegium riparioides (Herw.) C. Jens. between populations from streams with high (0.27 mg l −1) and low (0.05 mg l −1) concentrations of Zn showed that Zn uptake over the first 12 h was twice as fast as loss and reached an asymptotic value sooner. However, Zn accumulation was lower in a nutrient-rich (high nitrate and phosphate) than a nutrient-poor stream (4.5 vs. 7.0 mg g −1). Shoots from a high-Zn site (0.70 mg l −1) contained increased Zn concentrations on passing away from the apex and higher concentrations in leaves than stems (6.5 vs. 3.3 mg g −1). Shoots freshly exposed to 1.0 mg l −1 Zn in the laboratory showed similar Zn concentrations at different positions along the stem after exposure for 2 h, but by 24 h the gradient was similar to that found in field material. Ca, Mg, Mn and chelating agents (EDTA, humic acids) all decreased Zn accumulation in the laboratory, but neither nitrate nor phosphate over concentration ranges of two orders of magnitude had any influence. A greater proportion of Zn accumulated over a 24-h period was lost than that accumulated over a 10-day period (66 vs. 45%), even though the total Zn was similar (ca. 3.8 mg g −1) at the beginning of the loss experiments. Studies with NiCl 2 as an eluting agent indicate that this and other observations may be interpreted by assuming that an ‘exchangeable’ Zn fraction becomes converted with time to a ‘residual’ Zn fraction.