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Feature

Filtering Out the Murk

A horse mussel's view of life in our harbours can be a murky one.

Greig Funnell

With the development of the catchment areas around our harbours and estuaries, sediment run off from the land is increasing. Elevated sediment levels are an important factor in an estuary's health, affecting the ecology and productivity of the harbour and recreational resource values such as water clarity.

Effects of sediment loading have been the focus of recent studies conducted by NIWA (the National Institute of Water and Atmospheric Research) in a publicly funded research programme on estuarine ecosystems. The importance of finding a method that can monitor both the short- nd long-term effects of this increase in sediment on harbour health, has led to investigations into the possibility of using the horse mussel, Atrina zelandica, as a bio-indicator species.

One of the potential long-term effects from the increase in sediment input is the effect on suspension-feeding bivalves such as Atrina, which filter their food out of the water column. Any change in food quality or quantity would be expected to have some effect on the condition of these water-filtering animals. Past studies have shown that Atrina remove up to 80% of the sediment in the water column. In laboratory studies we have also shown that the more sediment there is in the water column, the less time Atrina can keep removing sediment. Atrina substantially alter the water flow over the seabed. This in turn alters the transport and deposition of sediment, having a myriad of effects on the structure and diversity of fauna in and around the seafloor.

Filtering Out the Murk Figure A (27KB)

To investigate the potential of using Atrina as a bio-indicator species for assessing ecological health of our estuaries it is necessary to link the amount of sediment flux (the amount of sediment settling on the seabed) with the condition of the Atrina.

Initial surveys in Mahurangi harbour in the north of the North Island have shown that there are differences in the condition of Atrina in areas with varying levels of sediment loading. Increasing sediment load causes a decrease in Atrina condition. Condition is a measure of the health of an animal and is used in many applications, such as in the shellfish farming industry. The results mentioned here use the ratio between dry weight of flesh to the shell length, as the measure of condition. To establish if a cause-effect relationship exists, that is, the condition of Atrina is caused by differences in sediment load rather than some other factor, we conducted a transplant experiment.

Transplanting Mussels

Mahurangi harbour was chosen as the study site as it is an estuary that has had changes in land use in its catchment area and varying levels of sediment inputs, along with extensive areas of horse mussel beds.

Filtering Out the Murk Figure B (10KB)

Using Atrina from a source in a relatively clean water location, Atrina were transplanted into seven sites with different levels of sediment loading. Each plot consisted of a 1m by 1m plot of mussels with two sediment traps extending into the water column on diagonal corners. The plots were established in July and the Atrina were left for three months. Each month, the sediment traps were emptied of sediment and analysed for dry weight, particle size and organic matter.

Results from the sediment traps show an increase in sediment settled into the traps (flux) with distance from the estuary mouth (ranging from 23g compared to 108g per month), and similar patterns for amount of organic matter.

The condition analysis showed that Atrina from the upper-most site (site12) had the poorest condition, whereas the rest of the sites had little difference between them. Apart from site 12, all sites had similar Atrina condition after the experiment to those from the collecting site. The results indicate that increased sediment flux has a negative effect on the condition of transplanted Atrina, confirming what was found in previous work on ambient Atrina in different sediment flux conditions. We need now to understand the relevance of changes in the condition indexes of Atrina to the ecology of the harbour by relating it to mortality, growth and reproductive output of the Atrina, and community function.

Land Use Effects

To use Atrina as an indicator species we need to go through a series of steps relating land use to the physical condition of the Atrina. Once sediment loading is established from different land uses, we need to investigate further the relationship between sediment load and the settling flux.

By using current catchment models for Mahurangi to get estimates of sediment load, and regressing these to monthly measurements of settling flux from our experiments, it may be possible to get a good relationship that can be used to predict changes in Atrina condition with changing land use scenarios. There are some problems with this, however, in that the range of data used to derive the relationships in the model may not reflect the entire range of input conditions that are of interest. The relationship is also restricted to the areas where the monthly settling flux is measured.

Potentially we may be able to use existing hydrodynamic models of Mahurangi harbour to refine relationships between sediment runoff and settling flux over the entire estuary. From this model it may be possible to predict distribution of suspended sediment concentration over the estuary during different sized events, such as floods.

If we can link the catchment land use with water sediment flux and thus Atrina condition (and overall harbour health), it would mean we could then predict what the effect might be on Atrina condition due to changes in catchment land use scenarios.

There are a number of factors that may influence these relationships and many of these are currently being studied as part of NIWA research programmes. This research is at an early stage and the need to develop and test the above relationships and models may mean that it will another few years before it is possible to reliably use Atrina as an indicator species.

The aim of our research is to provide a good, cost effective way to monitor increases in sediment loading in our harbours, and the effect that this has on community function. The use of filtering bivalves as biomonitors has been used successfully before and looks promising for Atrina as well.

Greig Funnell is a Marine Benthic Ecology Technician with NIWA in Hamilton.