Water Column Effects
Water Clarity and Quality
Effects of GreenshellTM mussel farming on water clarity and quality are thought to be highly site specific, and usually minor.
How great these effects are depends on farm management practices, as well as where the farm is located, the density of farms in a given bay, and seasonal variations and natural weather events.
It is important to note that sediment and organic run-off from land typically has a greater impact on water quality and clarity than mussel farming.
Mussels filter plankton (small free floating plants and animals), nutrients, and other suspended particles from the water column. This may theoretically result in localised increased water quality and clarity around farm structures due to the removal of particulate matter.
Mussels, however, also produce faeces and pseudofaeces, which are released into the water column. These can reduce water quality and clarity, particularly if currents swirling around farm structures keep them suspended in the water column. Decreased water clarity can mean less light reaches the seabed, particularly in shallower areas. In theory this could affect the nature of these seabed communities.
Water quality is hugely important to the GreenshellTM mussel farming industry, as they rely on good water quality to grow a premium product and have fewer restrictions on harvesting. During periods of high rainfall, when runoff from land and rivers causes an increase in siltation, turbidity and contaminants, mussel harvesting restrictions may apply to protect the consumer (see Food Safety section).
Nutrient Cycling
GreenshellTM mussels filter plankton and other organic matter from the water column. Some of this is converted to tissue by the shellfish; the rest is released as soluble nutrients into the water column or builds up as organic sediment deposits on the seafloor.
Good water circulation distributes and dilutes waste products and soluble nutrients over a wide area, and encourages oxygen and nutrient exchange between the seabed and the water body. As water movement is relatively high in most areas where mussel farming occurs in New Zealand, nutrient cycling in these places has been little affected by marine farming.
Marine farming in some situations can alter nutrient concentrations at a local scale (e.g. increased ammonium levels below mussel farms in sheltered waters). However, long-term effects on macro-scale nutrient cycling are less likely to occur.
Harvesting the mussel crop removes nutrients from the marine environment. However, this effect is thought to be relatively insignificant, given the amount of nutrients entering the system from the ocean and from land runoff.
Part of the science required for a marine farm application in New Zealand involves looking at nutrient enrichment at the proposed site. This can help predict the likely effects of the proposed farm on nutrient cycling.
To get a general indication of nutrient enrichment, researchers take sediment samples from the site and measure them for sediment size, organic matter content and REDOX depth (the depth at which sediment becomes anoxic).
Enrichment values at the site are also compared to regional values to be able to assess the site in the context of the wider area.
Water Flow
Mussel farming structures create obstructions in the water column that can alter local current speed and flow patterns around a farm. This can potentially affect the ecology of the surrounding area by altering the rate of plankton replenishment or waste dispersion, or increasing the nutrients available.
How great these effects are depends on the particular hydrodynamic characteristics and the scale of marine farm development in the area. In general, unless farms are very large and densely stocked or closely grouped, the risk of significant changes to water flow and subsequent effects on the ecology tends to be low.
The assessment of environmental effects which must accompany a consent application for a new mussel farm usually describes the hydrodynamic patterns at the proposed farm site. This can include modelling of the current speeds and directions, flushing times, and water dispersion patterns on both the ebb and flood tides.
Potential changes to water flow from a proposed farm can also be estimated by modelling data on the local current speed and the size and density of the farm. This information will show whether water flow at the site would be greatly affected if the farm were approved.
