Kelp is in trouble. Warming oceans, storms, increasing flows of nutrient-poor tropical water from the north and phase shifts thanks to the rise of black urchins are conspiring to put our beautiful kelp beds under pressure. Whilst we have little data from NSW, in Tasmania scientists have estimated that giant kelp cover has fallen by up to 95% in some areas (Edgar 1997).
Kelp is essential for many processes in our temperate and sub-tropical marine ecosystems. It provides habitat (for weedy seadragons, for example), absorbs CO 2 and produces oxygen like all plants, provides food for herbivores and buffers against storm impacts. So if it’s in trouble, we need to know. Unfortunately, we have little long-term data on kelp cover in NSW so it’s difficult to quantify the problem.
A couple of us at URG including Kris O’Keeffe have been discussing this with scientists and have started a trial of a method for recording kelp cover at key sites. At some point in future, these data may become important to assess the health of our kelp beds around Sydney. We need to record the appearance (and hence health) of kelp, and coverage for an area which can be re-visited accurately in future. To do this, we’re using underwater video synchronised with GPS.
This presents technical challenges. GPS receivers work by detecting signals sent from a network of satellites in orbit around the earth, and comparing the timing of these signals to determine latitude and longitude. GPS units can be inhibited by tall buildings, thick tree cover and they just don’t work at all indoors. It’s therefore understandable that GPS won’t work underwater; the water changes the angle, speed and strength of the signals.
To use GPS tracking underwater, therefore, we have to keep the GPS unit on the surface whilst we are down below. Enter stage left – the towed GPS rig. We need a surface float tied to a sealed container that houses a high sensitivity GPS unit, a reel and line, and a digital camera.
The method involves firstly synchronising the camera and the GPS. We do this by photographing the GPS unit once it’s turned on. We then take the camera and towed GPS with us on our dive, and record video pointing downwards at the kelp as we swim along.
Back at home, we can then take screen grabs of the video at random points, and locate these using the GPS track information. Google Earth is helpful for this, as we can just drag the GPS *.gpx file into the program then use a slider to follow our track on the screen. Once extracted, the photos can be analysed for kelp % cover and health, for example damaged or encrusted fronds.
The technique has other uses too, for example for tracking the exact location of rare or threatened species encountered on a dive such as grey nurse sharks, blue devil fish and weedy seadragons.
We’ll be discussing the results of our trial soon with scientists, but at this stage the data look pretty good.