Phosphorus dynamics in the swale - Ouse River system

Abstract

A study was made of the phosphorus ecology of the Swale - Ouse river in northern England. It ranges from a stream draining a peat moorland to a mature river influenced by urbanisation (estimated population 250 000) and intensive agriculture. The aims were to assess the concentration and variability of aqueous N and P on spatial and temporal scales, and the response of two common mosses, Fontinalis antipyretica and Rhynchostegium riparioides, to those changes. Key aspects included analysis of water chemistry, internal nutrient contents of mosses, and 'surface' phosphatase activity. The rate of activity of m situ plants was measured over an annual period, in conjunction with short-term studies of transplanted populations. Studies of phosphatase activity in the water also were carried out to assess the biological cycling of phosphorus. Aqueous total phosphorus and total dissolved nitrogen concentration increased on passing down the river. In the upper reaches, total phosphorus comprised equal contributions of dissolved organic and inorganic phosphorus, which were almost certainly derived from diffuse sources. Further downstream, total phosphorus was almost entirely comprised of inorganic phosphorus from point source inputs. The nitrogen and phosphorus content of Fontinalis antipyretica and Rhynchostegium riparioides increased on passing downstream, consistent with the water chemistry. The rate of phosphomonoesterase activity of both mosses was high in the upper reaches of the river, and was inversely related to nutrient content. Fontinalis antipyretica sampled from streams draining peat moorland was shown to have a high phosphodiesterase : phosphomonoesterase ratio. A possible explanation for this is that peat is a potentially rich source of phosphodiester substrate, although increased phosphodiesterase activity may be a response to extreme phosphorus limitation. Transplantation of F. antipyretica showed that internal nutrient content and phosphatase activity respond to changes in ambient nutrients. Aqueous phosphomonoesterase activity was studied over a 12-month period. Laboratory and field studies suggest it plays an important role in the. phosphorus dynamics of the Swale - Ouse river system.