Phosphorus and aquatic bryophytes in the Swale–Ouse river system, north-east England. 1. Relationship between ambient phosphate, internal N:P ratio and surface phosphatase activity

A study was made of the phosphorus content and surface phosphomonoesterase activities (PMEase) of two aquatic mosses, Fontinalis antipyretica and Rhynchostegium riparioides, in relation to the water chemistry of the River Swale–Ouse, NE England. The samples were taken from a headwater stream and three sites down the main river. Water and mosses (2-cm tips) were sampled monthly for 1 year from October 1995. Total inorganic combined nitrogen concentration and phosphate dissolved in the water both increased on passing downstream. Variability in concentrations of the various N and P fractions was greatest at the headwater site. The mosses showed their lowest concentrations of N and P at the headwater site, with both elements increasing on passing downstream. However, there was a marked shift in the N:P ratio (by mass), decreasing from 14.9 to 6.8 for F. antipyretica and from 12.5 to 5.5 for R. riparioides. PMEase assayed with 4-methylumbelliferyl phosphate as substrate was always greater at pH 5.5 than at pH 7.5 or 9.5. Differences in PMEase between sites were broadly similar for the two mosses, but F. antipyretica showed almost twice the activity (per unit mass) of R. riparioides. Material from the headwater site showed the most activity, though there were clear seasonal differences, with the highest activity in late summer; PMEase at all three pH values decreased on passing downstream. There was a marked increase in PMEase when the tissue P content was below about 0.3% or when the tissue N:P content was above about 9:1; the data were insufficient to establish which is the better predictor of PMEase. However, both mosses showed detectable activity at downstream sites. It is suggested that a combined programme of water analysis and moss PMEase assays provides the most efficient way of studying the nutrient status of streams showing highly variable nutrient conditions.