Walking the Beaches – The mystery of the lagoon

When we eventually got out on the water looking for the algal mats that were the source of material on the strand line became a priority.  This coastline was a problem to navigate in; whereas elsewhere the lagoons were wide and extensive, here there were several rocky barriers that meant we had to use either different boats in each section or transport the boat by road from ramp to ramp to obtain full access.  Even so, one or two of the smaller lagoons at the southern end were inaccessible by any boat.  Also, the prevailing wind direction for Mauritius was from the south east at this time of year, which impacted strongly on the Belle Mare area; whereas Grand Baie had been sheltered, the outer reef was bashed by high waves almost continually, and where the reef was broken up by deep water gaps called passe, the currents rushed in causing a significant hazard for a small open boat.

Again we were lucky to use glass bottom boats, and had a charming Creole captain who was much more courageous and lagoon-savvy than his Grand Baie counterpart.  We were afraid we were going to find the whole lagoon smothered in a green algal covering, choking the reef to death.  On the satellite imagery I had interpreted there were large dark green areas throughout the lagoon and the characteristic colours and textures of reef were rarely present.

The nearshore area was sandy bottom, with the occasional rocky substrate, as we expected.  When we reached the first of these dark green areas, we were astounded.  Yes there was algae tangled around the reef but the reef itself was very much alive, and if anything expanding.  On the staghorn coral, at the ends of lots of little yellow branches appeared almost fluorescent blue patches of recent growth, and there were not just the large ancient stands of coral heads, but plenty of tiny baby corals starting to branch out.  In fact, it was the liveliest and most densely packed area of coral organisms I had seen anywhere in Mauritius. Tangled up amongst it was all this algae, though.

We trawled extensively over the lagoon trying to pick up clues on the original source of the algae and the impact on the reef but we continually saw that the reef and other habitats here were generally in broad health, despite the slimy covering.  Then the boat engines cut.  The captain had underestimated the amount of running around we were going to do and the boat’s fuel tank was empty.  The boat started to drift northwards along the lagoon parallel to the beach.  Fortunately when the power was lost, the boat was less than thirty metres from the shore, and given our captain was local he spotted a friend on the beach.  It happened to be a Sunday and the beach was busy enough.  Although he spoke in Creole, we got the Captain’s drift; he was asking his friend to fetch some more fuel from a nearby resort.  As in many places the water was shallow and our helper splashed out with some more outboard motor fuel in a large water bottle.  Our captain gave him a grubby rupee note and then looked at the water bottle – he looked at us all in the boat waiting for him to start again and smiled.  He handed the bottle to me and reached below the outboard motor to release the fuel intake pipe.  He dipped it in the open bottle, then fired up the outboard.  He took back the bottle and settled it on the seat beside him then with a huge grin on his face recommenced the trip.

Walking the Beaches – The First Boat Survey

Jeremy was very clever to find a highly motivated, organised, intelligent and willing partner from the University of Mauritius,  We visited his office at the Reduit campus in the middle of the island and he not only offered his expertise but gave the services of a number of students who were willing enough to drop over the side of a small boat and explore the substrate in the lagoon.

We decided Grand Baie was the best place to start our field survey.  Not only was it close to Calodyne, but it was an intensely busy bay and the smallest of the pressure zones.  I had prepared the initial interpretation from some satellite imagery back at the house and chosen  a route around the bay where we could investigate all the patches I had identified.  By using the GIS to mark those locations, their coordinates could be uploaded into a GPS and then it was a matter of sailing around the bay guided by the unit.  I found this relatively easy to do using the GOTO function on the GPS – it warned you how far you way from your destination and in which direction.  Fairly quickly I learnt that you could read the currents and wind by looking at the water surface and if we approached from the direction of the current we got a good chance of standing over the spot we wanted.  In most of the bay the current was fairly mild so the boatmaster could use the engine to fight against the current and keep us stable over the patch of sea.  It helped in Grand Baie that we had found a glass bottom boat.  Glass bottom boat tours are common around many of the touristy areas to give visitors a chance of seeing under the water without getting too wet.  They helped us enormously as I could keep the map and survey materials dry.  Our colleagues from the University of Mauritius  were very keen to keep popping in to the water to take a look around.  With over a hundred points to survey we could not spend huge amounts of time analysing each frond of coral, but we did want to ensure that we were not overgeneralizing, so we did allow them early on to investigate any new cover class in a bit more depth.  In particular we were keen to record where algal growth had choked the coral, or for any evidence of bleaching, an aptly named process where the animal parts of coral migrate out of the rocky skeleton, effectively taking the colour out of the coral.

On the first day in Grand Baie we made slow progress.  It took time to explain the methodology to the new field workers, and also we were trialling it ourselves for the first time, so we had to see what was necessary and what could be streamlined.  But although the bay looked complicated on the satellite image, large areas of the central portion were a grey sandy bottom with occasional mats of seagrass or algae and could be covered quickly.  The slight colouration differences on the image proved not to be a substrate issue but related to the depth of the water.  Around the fringes of the bay and at its mouth was the reef itself and on the satellite image this looked like a myriad tapestry of shapes and colours.  Even here ,with a bit of understanding of the morphology of coral reefs, one can see the patterns repeated time and time again; the head reef, the long thing parallel lines where countless waves have ground a path through, the scattering of patch reefs behind.  Once these classes of substrate have been confirmed by the field survey, over 80 percent of the job is complete.  The challenge is to seek out the exceptions to those rules – different textures or colours on the image that do not conform to this morphology.  Usually the solution to those teasers are fairly obvious once seen, which shows the value of spending time on the boat.

The sands and the seagrass beds and even most of the coral patches were easy enough to navigate over, but coral reef has a tendency to try and grow up to the light and much of the reef was too shallow for the boat to go over, in some places even breaking the surface.  We had to be extremely careful if we needed to get in amongst the sharp reef heads to investigate deeper water beyond.  Over our numerous trips, our captains varied in their courage levels, or maybe it was their boat was uninsured, I never dared ask.  Even when I would show them print outs of the satellite image and demonstrate a clear way in (and more importantly out ) of the reef they would be sceptical.  Their experience had told them that reef bad, deep water good – why would anyone want to navigate a boat against that hard jagged thing that could rip your bottom apart on your way out.  Indeed we had many nervous moments when all you could hear was the scratching of reef against the fibre glass and the sucking of teeth from our captain.

Our surveying had to extend out into the open water beyond the bay and too look at the forereef.  Out here we were still not in the ocean but the incredible lagoon that surrounds almost the whole of Mauritius.  The waves were larger and the currents stronger here, and it took all the skill of the captain to be able to still navigate to our survey locations and hold the boat still while we observed under the water.  I’ve got better at keeping my nausea to a minimum in small boats, but this started to affect me; Jeremy was looking the worse for wear and he retreated into the depths of his rain jacket and lowered his wide brimmed hat over his eyes.  We did not spend too long out there but there was nothing difficult about the interpretation in the open water; it was a mixture of hard reef and rubble kicked up by the wave energy.  Anyway, on the satellite imagery much was obscured by the white foam and breaking waves themselves so I could not do much with any mapping.  So we headed back to the shore.