1. Introduction
Culture of finfish in cages and of shellfish on longlines in coastal waters has a number of effects on the underlying seabed. Among the most widely documented are the
impacts of organic waste from the farms on the character of the sediment and Ž
consequent effects on communities of benthic organisms see Brown et al., 1987; Gowen and Bradbury, 1987; Nature Conservancy Council, U.K., 1989; Washington
State Department of Fisheries, 1990; British Columbia Environmental Assessment .
Office, 1997 for reviews . In the most severely impacted areas, these changes may cause substantial reductions in the diversity and abundance of benthic organisms. Numerous
models have been developed to predict the dispersal and deposition of waste on the Ž
. seabed e.g., Hargrave 1994; Hevia et al. 1996; Panchang et al. 1997 . Predicting the
effects of the waste on benthic communities, other than in general terms, has proved less tractable due to, among other things, the spatial and temporal variability in the
assimilative capacity of the sediments and in the distribution and response of benthic
Ž .
organisms Findlay et al. 1995 . Ž
. Findlay and Watling 1997 proposed a simple model of benthic impact based on the
balance of benthic oxygen supply and demand. The anoxic conditions that develop within the sediment when demand exceeds supply, the subsequent loss of benthic
macrofauna, and the occurrence of mats of sulphide-oxidising bacteria, such as Beggia- toa species, were taken as criteria for unacceptable impact. The forcing functions of this
Ž model were simplified into the variables of current velocity above the seabed which
. determines rate of oxygen supply and rate of input of organic carbon; both of which are
relatively easy to measure. This simplification was based on correlations between benthic metabolic rates and rates of carbon flux to the sediment, derived from field
studies at sites with different hydrodynamic conditions. It was also assumed that rate of supply of oxygen is determined by Fickian diffusion in response to current velocity,
temperature, and concentration of oxygen in the overlying water.
There is limited published information on rates of recovery of sites impacted by Ž
organic waste from marine farms Mattsson and Linden, 1983; Edwards, 1988; Nature
´
Conservancy Council, U.K., 1989; Johannessen et al., 1994; British Columbia Environ- .
mental Assessment Office, 1997 ; however, as with studies of impacts, there has been no attempt to synthesize site-specific information into predictive models of recovery. The
first stage in the process of recovery is the mineralisation of accumulated waste on the seabed, and only when this has occurred can the chemical environment of the sediment
return to its pre-impact state and benthic macrofauna recolonise. Potentially, the Findlay–Watling model can be used to predict rates of decomposition of accumulated
waste, based on the rate of supply of oxygen as determined by local current velocities.
In this study, we tested two hypotheses derived from Findlay and Watling’s model. First, that the ratio of oxygen supplyrdemand at sites where waste had accumulated on
the seabed would be much less than one. To do this, we measured the rate of carbon input to the seabed below an operating salmon farm, where accumulation of waste has
occurred, and compared it to rates of oxygen supply estimated from measurements of current velocity. Second, that at a disused farm site where waste had accumulated in the
past, the rate of mineralisation of organic waste is determined by the rate of oxygen
supply, and therefore, by measuring the latter, we should be able to predict the rate of decrease in the amount of waste. This predicted rate of recovery was compared with a
rate derived from in situ measurements of fluxes of dissolved oxygen, nitrogen, and hydrogen sulphide from the sediments below the farm. As an adjunct to the study of
recovery of farmed sites, we also measured concentrations of heavy metals in sediments because they may affect recolonisation by benthic organisms.
Ž .
Findlay and Watling 1997 apparently collected current velocity data over a period of 2 years and suggested that the minimum 2-h average velocity was an appropriate
summary value for incorporation into their model. They chose this summary statistic because current velocity varies with tidal stage and published information suggested that
2-h exposures to reduced oxygen and increased hydrogen sulphide concentrations may
Ž cause permanent damage to the gills of sensitive infauna Findlay and Watling 1997, p.
. 152 . In our study, we explicitly do not test the appropriateness of the minimum 2-h
average velocity relative to other summaries of long-term data, for two reasons. First, testing has already been done during the original development of the model. Second, for
the model to be useful in practical application, it must be capable of producing reliable predictions using the limited amount of information on near-bed current velocities that is
generally available from monitoring programmes. Long-term collection of such informa- tion is a specialist task, and therefore, the information available for model calibration is
likely to be collected over relatively short periods. We used minimum 2-h velocities derived from current meters deployed for periods of 24–145 h. For purposes of
comparison, however, we also derived rates of oxygen supply from minimum velocities observed during our study.
2. Materials and methods
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