River outwelling plumes: good or bad places to farm mussels?28/06/2007
|Motueka River mouth|
Aquaculture in New Zealand is entering a phase where the potential for development of large (i.e. 100s of hectares) farms positioned 3 to 10 or more kilometres offshore can be considered. The shellfish farming industry now actively pursues development of sites in areas provisionally designated by councils as Aquaculture Management Areas or AMAs. Applications are often based on the assumption that growth potential and water quality conditions will be appropriate. Whether or not this assumption is valid, depends on the location of the proposed development and the degree to which the farm would be affected by the highly mobile and often ill–defined river outwelling plumes. Without some preliminary investigation into these outwelling plumes, it can be somewhat of a gamble because some of the effects can be positive and others negative.
Full details of this can be found in an article by Paul Gillespie in the New Zealand Marine Farm Association. June Newsletter. p 5, 6 & 10.
Re–defined catchment area
In order to address the potential coastal problems associated with land runoff, environmental managers and marine farm developers need to expand their definition of “catchment” to include the region that is significantly influenced by the river outflow. This emphasises the need for coastal activities like aquaculture and fishing to be considered alongside land catchment uses, like forestry and dairy farming, so that they can be managed in an integrated way.
River plume effects
Terrestrial contributions that can have an effect on shellfish resources and aquaculture growing conditions include plant nutrients, suspended sediments and a range of potential microbial and chemical contaminants. Tasman Bay and Golden Bay are good examples of regions that can be significantly affected by land use activities within contributing catchments. Research carried out through the Motueka Integrated Catchment Management (ICM) programme (http://icm.landcareresearch.co.nz/) has shown that, after a moderate rainfall event, the surface salinity plume from the Motueka River mouth can extend to more than 20 km into Tasman Bay encompassing a majority of the regions designated for mussel farming and/or shellfish spat harvesting. Cawthron’s Nelson Bays hydrodynamic model indicates that, after a major flood event, the much expanded wind and tide driven plume can even push out around Separation Point and into Golden Bay.
During rainfall events, the river outflow delivers sufficient nutrients (i.e. inorganic nitrogen and phosphorous) from the catchment to stimulate phytoplankton production within the western side of Tasman Bay. At the same time, the freshwater plume, which is less dense than seawater, floats out over the Bay setting up stratified conditions in the water column.
What lessons have we learned?
It is clear that river plumes from even moderately–sized catchments can affect proposed aquaculture sites located considerable distances offshore. Some of the effects can be beneficial but there are also risks of adverse effects. The take home message for shellfish farmers is to consider how adjacent catchments may affect growing conditions and product quality at a potential farm location. Since terrestrial, freshwater and coastal catchment uses will undoubtedly change over time, it is essential that they be managed in an integrated way.
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Estuarine & marine microbial ecology