Seabed Effects
The accumulation of fine-grained organic particles (mussel faeces and pseudofaeces) can change the seabed below mussel farms, along with the deposition and accumulation of live mussels and mussel shells. This material settles on the seafloor and can alter the physical, chemical and biological nature of the seabed. Farm structures may also shade the seabed under some conditions and so potentially affect light-dependent species like algae.
In New Zealand, it is usual to site Greenshell™ mussel farms over muddy habitats that are adapted to naturally higher rates of deposition. This minimises habitat modification and protects biodiversity of more sensitive habitats.
Sedimentation and Deposition
The effects of deposition on the seabed depend on the mussel farm’s management practices (e.g. stocking densities, line orientation, harvesting techniques); and environmental characteristics at the site (e.g. current, water depth, wave action, seabed habitat, freshwater influences).
As part of the ecological assessment for a mussel farm application, scientists check whether there are any unusual habitats or species at or near the site. Proposed farms are more likely to be declined if the sites contain habitats or species of special conservation value or ecological importance
In their assessment of environmental effects, mussel farm applicants must also provide an assessment of the depositional footprints of the proposed mussel farms. This is typically done by scientists using a predictive model, which estimates the distance and direction that mussel wastes could travel before reaching the seabed. It takes into account local water current speed, water depth, and the time it takes for particles to settle to the seabed. The model also estimates the amount of deposition that would be likely to occur at increasing distances from the farm.
It is general practice in New Zealand to site mussel farms over muddy habitats that are adapted to higher rates of deposition, to minimise habitat modification and protect biodiversity of more sensitive habitats. Although particulate waste deposition from marine farms often has little effect on mud or silt habitats, it can cause significant changes to habitats with coarser substrates such as sand, pebbles, cobbles, and rocks. The discharge of fine waste materials may increase the turbidity of the water and potentially smother benthic species. Local abundances of species adapted to the coarser substrate may decline as a result.
Because of this, scientists study the composition of the sediments beneath the proposed site. This usually includes collecting samples inside and outside the proposed site, to investigate the proportion of fine mud, sand, and shell gravel, the organic matter content of the sediment, and the REDOX depth (the depth at which sediment becomes anoxic). These values are compared to the average values for other sediments in the region.
Based on observations of existing small mussel farms throughout the Marlborough Sounds (a major mussel farming region of New Zealand), measurable seabed effects are usually limited to the area directly beneath the farm and the immediate surrounding area (within metres of the farm boundaries).
Habitat Creation from Shell Accumulation
Larger material falling from the farm - like live mussel clumps and empty shells - can encourage the development of reef-type communities. Such communities observed under existing farms include species like sea squirts, sponges, sea cucumbers, and tube worms,; as well as mobile predatory species like starfish, crabs and fish.
An increase in predators may help maintain a balance in the ecosystem with respect to the increased number of prey species (farmed mussels).
In other situations, mussel clumps and shell can remain relatively clear of reef-type communities (refer to the Biodiversity Effects section).
