Predicting critical thresholds of aquaculture waste loading to coastal sediment

Abstract

The use of cages in the ocean to support finfish aquaculture is increasing as an important food supply industry. However, local environmental impacts directly below and around cages can include the degradation of water and sediment quality. In particular, high rates of organic matter deposition to the sea floor beneath the cage, primarily from the deposition of faeces and uneaten food, drives dissolved oxygen consumption and hydrogen sulfide production, ultimately leaving the sediment environment uninhabitable for benthic macrofauna. The aim of this study was to develop and apply a vertically-resolved sediment biogeochemical model (Figure 1) to simulate the response of marine sediment to organic matter loading from fish cages. After an initial spin-up period, the model simulations assume aquaculture operations for a five year period, followed by seven years of recovery. The model is run within a Monte Carlo framework to assess sensitivity to key parameters. A series of simulations were then undertaken to define how changes in the extent of loading impact upon sediment chemical concentration profiles and sediment-water fluxes of variables such as oxygen, nutrients and sulfide. We then defined threshold-loading rates, based on sulfide accumulation in the surficial sediment, that correspond to a low, medium or large impact to sediment quality. The recovery time for sediment experiencing five years of fish-waste input is then computed, with an associated range of uncertainty. Recovery time is defined as the time taken from the end of aquaculture operations to the time when oxygen in the surficial sediment recovers to 85% of its base concentration. For each rate of loading, we categorize whether the top 5 cm of sediment would recover in less than 1 year, between 1 and 5 years, or >5 years. The results of the analysis may be interpreted by managers and operators to assess the likelihood of sediment condition, and the potential for recovery once farming has ceased, for a range of stocking densities. Further the model approach may be applied in conjunction with hydrodynamic model assessments of waste dispersion and deposition to assess the potential footprint of cage installations.