Small Island Developing States (SIDS) and the sea: creating high resolution habitat maps to support effective marine management in St. Lucia

This study, conducted under the auspices of the Commonwealth Marine Economies Programme (CMEP), involved the characterisation of geomorphology and benthic assemblage groups around the previously unmapped western St. Lucian coastline. A high-resolution (2 x 2 m) seabed map was then produced, specifically focussing on the economically important regions.

Two hundred twelve video tow transects were acquired by CEFAS and associated to Multibeam echosounder (MBES) data collected by the United Kingdom Hydrographic Office (UKHO). Through object-based image analysis, MBES data and derivatives were categorized into eight basic morphological classes (e.g. slope, flat), with manual intervention required to further discriminate more complex forms (e.g. anthropogenic scours). Percentage coverage of epibenthic biota and substrate type data were extracted from still seabed images using the CATAMI[1] morphological classification system and a randomised point count approach. Benthic community assemblages were then defined based on K-means clustering. A random forest model was used to predict benthic community groups. Predictive layers included MBES-derived physical properties, geomorphological classes and wave exposure (using GREMO[2]). The resulting model had a predictive accuracy of 80%.

St. Lucia is characterised by gently sloping exposed plateaus (~30 m deep), extending for several km offshore in the north and south of the island. These appear to be mostly covered by an algal-dominated gravelly substratum. Bioturbated mud and sand with pockmarks occur typically in sheltered bays, near river mouths or within paleochannels that incise the seabed. Seagrass patches were more difficult to predict (50% producer’s accuracy) but are generally limited to very shallow coastal waters habitats. Finally, coral and sponge-dominated reef communities appear to generally be associated with raised “staircase” platforms and bommie features close to the coast, however, these were also observed further from shore and in deeper waters.

The seabed habitat maps produced will support the St. Lucian government to manage their shallow seabed resources. In particular, the maps will assist the systematic characterisation of the coral reef habitats and, therefore, improve delineations of marine reserves, especially around the town of Soufrière and the Pitons UNESCO world heritage site.

 

[1]           Althaus, F., N. and many others. 2015. A standardised vocabulary for identifying benthic biota and substrata from underwater imagery: The CATAMI classification scheme. PLoS ONE 10:1–18.

[2]           Pepper, A. and Puotinen, M. L., 2009. GREMO: A GIS-based generic model for estimating relative wave exposure. The 18th World IMACS Congress and MODSIM09 Int. Congress on Modelling and Simulation (pp. 1964-1970). Cairns, Australia.