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 mystery of the east coast algae

Our work on the coast at Belle Mare and Palmar was split in a similar way to before – trips in the lagoon to look at the coral reef and seagrass beds, and walking the coast to look at defences and other issues.  A major new issue here was the washing up of huge amounts of algal material onto the shoreline day after day.  The Department of the Environment for  whom we were working had done some preliminary investigations.  The Beach Authority were trying to tackle the result.  In a few days, a layer of algae would coat most of the beaches from one end of the lagoon to the other, and the rate would increase if the weather had been turbulent.  Having decaying alga on your beaches is severely detrimental to your tourist image (you would not put it in your brochures!) so the hotels employed people to rake it off the beaches every day.  That was not so bad where a resort had , say 100m of beach, but the larger resorts and the public beaches had kilometres to clear.  In some places the work was on an industrial scale, with diggers coming down on the beach to scoop up the strandline, and trucks towing away the smelly , dirty loads.  One day, when exploring the back of the district to look for possible reasons for why the algae was blooming in the lagoon, we discovered where they were dumping the problem – along a disused forest track there was several hundred metres worth of rotting algae over 3 metres high in places.  It was posited with Environment that they could be looking at this as a valuable source of fertiliser is the salt could be washed out.

Before commencing the detailed survey we looked around the area to try and find the reason why the lagoon was starting to choke with algae.  Pollution seemed the obvious culprit – enriching the lagoon with either agricultural inputs or human waste, but there were no natural run off channels along this part of the coast.  Instead the coast was a sandy berm higher than the immediate hinterland.  Behind the sand, in an intensely agricultural zone predominated by onion growing,  were several freshwater lakes.  We took a close look at them but could not see any concentrated sources where fertiliser could be running off – so unless it was the combined effort of all the onion growers or the large catchment from the island’s interior covered in sugar cane fields,  we could not fathom it out.  If it were just the drip drip effect of so much agriculture – why was the algae prevalent in the lagoon here but not elsewhere in the island where similar intensity of agriculture was in the catchments that fed into them.

We did work out how any pollutants could potentially be transferred from these lakes into the sea, though.  A common feature of both sandy coastlines and ones where permeable rocks exist is that freshwater can leak through and appear at or below the shoreline.  While we were walking the coast, we noticed damp patches in the sand, even pools in some places, and in a couple of instances could see water bubbling up in a spring.  A quick finger in the water and taste test determined that the water was sweet and we duly noted these locations in case we found some pattern of these against the distribution of the algae in the lagoon itself.  Only after a few taste tests like this did we suddenly think – what if this is a sewage leakage of some sort?  Well, we spat out the sample as soon as we had our verdicts, and I never did get diarrhoea during this period, so it must have been “safe” – at least in a field working manner if not covering health and safety rules.

The other potential source was human effluent, but the resort hotels were meticulous about their sewage treatment – why would they not be – there was no point dumping waste in the lagoon you wanted your guests to swim, dive and sail in.  Occasional transgressions may occur when some failure in the system caused a leak, but it would never be on the scale to cause such algal blooms.

The other potential source could be offshore.  I know many beaches in the Caribbean can suffer from huge builds up of seaweed, thought to have been carried in on wind and wave from the Sargasso Sea, but I knew of no equivalent in the nearby Indian Ocean.