Predicting impacts and recovery of marine-farm sites in Stewart Island, New Zealand, from the Findlay-Watling model

Data collected at marine farm sites on Stewart Island, New Zealand, were us ed to test the model for prediction of benthic impact of salmon net-pens pr oposed by Findlay and Watling [Findlay, R.H., Watling, L., 1997. Prediction of benthic impact for salmon net-pens based on the balance of benthic oxyg en supply and demand. Mar. Ecol.: Frog. Ser., 155, 147-157.]. The model use s the ratio of oxygen supply to the benthos (based on Fickian diffusion and current velocities above the seabed) to oxygen demand (based on rate of in put of organic carbon to the benthos) as a predictor of benthic response to input of organic matter from farms. Estimates of the rate of input of carb on and supply of oxygen to the seabed below an operating salmon farm in thi s study provided predictions of the severity of impact that compared reason ably well with the observed anoxic nature of the sediment and presence of p atches of Beggiatoa-like growths. At a site where farming of salmon has rec ently ceased, the model predicted rates of mineralisation of organic carbon in the sediments of 279-908 mmol m(-2) d(-1) (depending on the method of s ummarising data on current velocity). This was in reasonable agreement with the rates derived from the observed decrease in the depth of the layer of waste over the period of a year (476-1206 mmol m(-2) d(-1)) and with rates derived from measurements of fluxes of several chemical species from the se diments (146-525 mmol m(-2) d(-1)). These results suggest that the model ha s utility beyond the locality for which it was originally developed and tha t it may also be useful for predicting rates of recovery of farm sites. Acc umulation of zinc and copper (derived from food and antifoulants) in sedime nts below farms may significantly affect recolonisation of sediments by ben thic organisms. (C) 2000 Elsevier Science B.V. All rights reserved.