Water Column Effects
Water Clarity and Quality
Intertidal oyster farms in New Zealand are very unlikely to have any significant effect on water clarity and quality in New Zealand estuaries. Although sedimentation occurs below farms and in the immediate area, this effect is localised and small in scale relative to the wider ecosystem.
Adverse water quality effects from oyster farming are more likely where farms are overstocked and sited in poorly flushed environments. This can be avoided by appropriate site selection, and by ensuring farm structures have a minimal effect on flushing processes. Both these factors are taken into account when planning oyster farm developments or extensions in New Zealand.
It is critical to the oyster farming industry that good water quality is maintained around oyster farms, so farms can 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, oyster harvesting restrictions may apply to protect the consumer (see Food Safety section).
Nutrient Cycling
In relation to oyster farm development in New Zealand, alterations to nutrient cycling have been recognised, but are not regarded as a significant ecological issue. The issue still needs to be considered, along with other environmental effects, when assessing oyster aquaculture development.
Oysters 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 becomes organic deposits on the seafloor. Nutrients are removed from the system when the oyster crop is harvested. However, this effect is thought to be relatively insignificant given the amount of nutrients entering the system from the ocean and from land runoff.
Nutrient cycling processes are complex and often influenced by various environmental characteristics including water temperature, water clarity, and flushing processes. For example, good water circulation minimises the impacts associated with the potential concentration of nutrients due to marine farming by distributing and diluting waste products and soluble nutrients over a wide area, and enhancing oxygen and nutrient exchange between the seabed and the water body.
Part of the assessment of effects provided with a marine farm application in New Zealand often 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, scientists 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
Oyster farming structures can physically alter local water currents and wave action within and around a farm. This is likely to have some affect on ‘flushing’ characteristics of the site and may contribute to erosion or sediment build-up (see Seabed Effects section), or changes to plankton dispersal in the area.
These effects can be managed by locating oyster farms in appropriate places and making sure any intertidal farm racks are oriented parallel to (rather than at right-angles to) currents and wave action.
The assessment of environmental effects which must accompany a consent application for a new oyster farm usually describes the patterns of water flow at the proposed farm site. Scientists can measure currents directly and, using these measurements, model the current speeds and directions, flushing times, and water dispersion patterns as the tide goes in and out.
This information is used to help decide whether the proposed farm site and orientation of structures are appropriate to the area and conditions.
