Underwater Mapping on Saba

This article is from BioNews 9 – October 2013. See all BioNews issues here.

– by Dr. Jennifer Rahn, coastal geomorphologist at Samford University, Alabama, USA

Geomorphology is the scientific study of landforms and the processes that shape them. The need to understand the complex interactions of geomorphic and biotic processes nearshore and on coral reef areas is growing in importance due to climate change and other hazards.

basic dataCoral reef biology is a function of the depth of the ocean and the size and shape of the formations on which coral grow. For example, research has shown that fish aggregate on specifically shaped underwater structures. Just as changes in elevation on land help determine the type of vegetation growth, the depth of the ocean affects what type of corals will grow and the abundance of associated organisms. Elevation mapping or topography specifically involves the recording of relief or terrain, the three-dimensional quality of the surface, and the identification of specific landforms. Under water, this type of mapping is called bathymetry. Both include collecting data (elevation or depth) at points and making maps from this data. The raw data is then synthesised into contour lines that are based on generalisations from the point data. Whereas on land the contour lines can be seen on topographic maps, underwater contour lines show seafloor relief (called depth contours or isobaths) and this is called a bathymetric map.

This summer, the seafloor depths of the major coral reef systems on Saba were mapped. The ideal time to conduct bathymetric mapping is during the summer months, when the wind and waves are the calmest. The data points needed to make the bathymetric maps were acquired using a GPS and a single beam sonar device, while travelling along transect lines, and consist of an X (latitude), Y (longitude) and Z (depth) coordinate. The boat moves slowly back and forth over the area to be mapped to get as many points as possible.

jenniferrahn
Basically, the data collection process is much like
mowing a lawn
.”

— Dr. Jennifer Rahn —

The more points are collected, the more accurate the eventual maps will be. These points are then processed and entered into a Geographic Information System (GIS) to make the required maps of the depth and shape of each reef. The areas mapped include the popular Tent Reef (where 26% of all Saba dives take place), The Pinnacles, Ladder Bay, and Wells Bay. These maps are important for recreational divers as well as researchers. The maps will allow divers to “see” the ocean floor features at each dive site. Besides being a valuable addition to the Saba National Marine Park and the Dutch Caribbean Biodiversity Database (DCBD), visiting scientists will be able to use the maps to support their research, including the current IMARES benthic habitat mapping on Saba.

The recent history of underwater mapping on Saba begins in 1991, when Dr. Rahn and M. Claveau created a detailed map of the spectacular dives sites on The Pinnacles. This map was created using a depth gauge and was drawn by hand. The contour lines are 3 metres (10 feet) apart. In the 1990s the Dutch Navy surveyed the whole island to the 60-metre depth contour. The information generated by this survey is used on the current Saba Marine Park Map, but is very coarse. The contour lines are 10 metres (30 feet) apart so it is difficult to see detailed sea floor features. The data collected this summer by Dr. Rahn, allows for maps with contour lines of 0.3 metres (1 foot). In the example shown here, the contours are 3 metres (10 feet) apart in order to be able to compare it to the 1991 map. The Dutch Navy will soon be completing a new, more detailed bathymetric map of the waters surrounding Saba whereby the whole island is once again surveyed.


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