Bel Ombre Lagoon

 First Step: Lagoon Survey

A survey of the Bel Ombre lagoon located was conducted on the 10th -11th and 23rd-24th of September 2015.

Bel Ombre Lagoon

Bel Ombre Lagoon

The principle aims of the survey were:

  • To map the different ecosystems found in the lagoon
  • Evaluate the health of the lagoon
  • Identify existing threats to the health of the lagoon
  • Locate potential areas for a snorkelling trail

The Lagoon:

 Map 2

Coral habitats cover approximately 55 hectares of the lagoon in addition to 20 hectares of sea grass

Four different coral habitats were described:

Site 1 – 57% hard coral cover dominated by massive corals and 57 species of fish Site 1
Site 2 – located just after the reef crest (where the breaking waves can be seen), and is part of the reef flat. Live hard coral covers 21% of the benthos, dominated by robust massive corals (HCM) that can withstand wave energy Site 2
Site 3 – the coral garden is dominated by Acropora sp. branching corals with 61% coral cover and 70 fish species recorded for site 2 an 3 combined. Site 3
Site 4 – by the beach with a mixed benthic composition composed of one third hard corals, one third sand and 21% bare substrate. The dominate coral growth forms are tabular Acropora (HCB) and branching non Acropora (HCD) Site 4

 

Sea grass beds:

The sea grass bed is located in front of the river mouth with four species recorded but dominated by Syringodium isoetifolium. Where the sea grass is dense, Syringodium isoetifolium covers more than 90% of the benthos with less than 10% algae and average height of 13cm. The richness of the sea grass was reflected by the invertebrate diversity and 19 species of fish. Predatorily commercial fish such as barracudas and trevally with an estimated size of over 40cm were observed hunting in this shallow habitat.

Threats

Natural threats:

 

  • Because of the shallow depth of the lagoon, the corals are affected by the tides. Indeed, during low tides, most of the corals tops are emerged thus encouraging horizontal propagation to vertical growth.
Threat 1
  • One instance of coral dieses (probably white band) as well as several crown of thorn starfish (Acanthaster sp) were seen during the surveys. There abundance is currently too low to have any major impact but these threats need to be monitored. Crown of thorn population outbreaks have cause the degradation of many reefs worldwide. However, recent discoveries have found that an injection of household vinegar is very effective at killing the starfish in laboratory conditions.
threat 2.1Threat 2.2
  • No coral bleaching was observed, however bleaching events tend to occur more frequently in summer when water temperature are high (above 29C). However water circulation due to the strong current and close proximity of the reef may help prevent bleaching by keeping the temperature bellow 30C.
  • Several fresh water income and sedimentation from land can also impact corals found in the lagoon. Despite indication of heavy fresh water incomes during rain storms, the corals do not seem to have been affected. This can be explained by the fact that fresh water, being less dense than salt water, floats and most probably flows out of the pass due to the currents.

Anthropogenic impacts:

  • The most visible human impact during the survey was from physical damage of the corals from boats, snorkelers and people walking on the corals. This can be reduced by sensitising visitors and scheduling boat tours based on tide times. The most affected habitats are those close to shore (sea grass and coral) and corals on the contour of boat passages.
Threat 3
  • Boats going fast in the lagoon will also affect shallow marine habitats. The motor creates high energy water movements that disrupt the sea floor. These effects can be reduced by having speed limits in shallow area. While surveying areas for the snorkelling trail, a stretch of dead tabular coral was observed on the side of the sky lane passage. The cause of death is very likely linked to the creation and use of the passage by speed boats.
Threat 4Threat 4bis.
  • These threats should be monitored on a regular basis, especially during and after natural events (storms, cyclones, high water temperatures)…

Partnership with Outrigger Hotels and Resorts and Outrigger Mauritius Beach Resorts

This partnership aims not only to facilitate research in the Bel Ombre lagoon but also to reduce human impacts through training of hotel staff and through sensitisation of tourists and local inhabitants. The goal is to find ways of enjoying the beauty of the lagoon while reducing our impacts and protecting the environment.

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