The challenge
Rapidly changing water conditions
Marine managers need to understand changes in water turbidity (caused by particles suspended in the water) because of its influence on the availability of underwater light and aquatic plant growth, especially at the seabed.
This can be difficult because water turbidity is influenced by several biological and physical processes, and can change rapidly over time and place to place.
Measurements that encompass large areas are required in order to understand natural variability, and effectively monitor for future change.
Satellite remote sensing has the potential to provide this information at a greatly reduced cost compared with measurements taken in the field.
Our response
New methods to estimate turbidity using satellite remote sensing
We worked with Curtin University and the Western Australian Department of Parks and Wildlife to estimate how much turbidity there is in Kimberley coastal waters, and how it varies, using satellite remote sensing.
We applied a new method for estimating turbidity, specifically designed for the Kimberley, to 16 years of archived satellite data, providing daily average maps at a spatial resolution of 250 metres.
These remote estimates were first compared with existing field measurements to assess the accuracy of the satellite approach. The satellite-generated data was then analysed to characterise spatial and temporal patterns in turbidity.
We also estimated how turbidity affects the amount of light under water using numerical modelling.
The results
High tide equals high turbidity
We found that turbidity across the region varies in concert with the tide, with the most turbid conditions coinciding with the highest tides. As tidal height increases, so do the associated tidal currents, and this enhances vertical mixing of the water-column.
We also found that turbidity is greatest during the winter months, suggesting that winter heat loss and/or increased wind stress during this part of the year enhances the underlying tidal effect. In contrast, we found that the input of suspended sediment from rivers only has a limited impact on turbidity in the near vicinity of the major rivers.
Variations in seabed depth, sea-surface height and vertical mixing combine to result in highly variable underwater light conditions.
This information will be critical in understanding the distribution and potential vulnerability of plants living on the seabed.