Soft sediment environments include gravely, sandy, silty, muddy areas of the sea floor which are home to a variety of organisms including a wide variety of invertebrates (e.g. worms, crabs, shellfish), bottom dwelling fish and bacteria. The majority of salmon farms in Tasmania are located over soft sediment environments.
The release of un-eaten salmon feed, salmon faeces, and other nutrient rich organic matter (e.g. biofouling debris) into the water column occurs as a result of current marine salmon farming practices in Tasmania. Some of this organic matter is dissolved in the water column while some of it is in the form of particles which float in the water column, sinking to the sea floor (at varying rates depending on their size and water current speeds). The release of dissolved and particulate waste has the ability to interact with soft sediment environments in a number of ways:
- Organic enrichment
The impacts of these interactions tend to be greatest immediately below actively farmed salmon cages, decreasing with distance from the farm. The magnitude of interactions depends on the intensity and management practices of farming, the environmental conditions at the farm site (e.g. depth of water below cages, the energy of the environment) and the sensitivity of the soft sediment ecosystem.
How can extra organic matter and sedimentation interact with soft sediment ecosystems?
- In most situations the benthic ecology (animals that live within the sediments) will adapt to assimilate and process increased organic matter, and so long as the inputs do not exceed the assimilative capacity of the animals to breakdown this material the system will achieve a new equilibrium.
- However, if the deposition exceeds that assimilative capacity, there can be a build up of organic matter on the sediment surface and oxygen levels in the sediments or in the water just above the sediments can be reduced. This is often evident in the presence of bacterial mats (e.g. Beggiatoa) as the organic material is now broken down by microbes. These microbes use sulphide in the breakdown process which can result in outgassing from the sediment surface; another indicator that deposition is exceeding the assimilative capacity of the environment.
- Benthic monitoring programs will often include some or all of these indicators (e.g. infaunal community structure (animals in and on the sediments), oxygen levels and evidence of outgassing) to determine environmental condition and support management practices that avoid deleterious outcomes.
For more detailed information on the interaction between salmon farming and soft sediment environments browse through our research outputs on our publications page.