The Lagoon & Coral Formation

Under Construction

Return to: Main Page / The Environment / Marine Resources / Coastal Zones

coin de mire

A view of the lagoon near Cap Malheureux

Introduction

What is a Lagoon?

Corals

Stresses on the Lagoons


Introduction

The island of Mauritius is surrounded in most parts by lagoons created by the formation of either barrier reefs or fringing reefs. The area of the lagoons is 243 square kilometres. They are vitally important as sources of food through artisanal fishery, as places of leisure for both the population and tourists. Unfortunately, development and industrialisation are taking their toll, and dearly so too.

What is a Lagoon?

Lagoons are areas of relatively shallow water situated in a coastal environment and having access to the sea but separated from the open marine conditions by a barrier. The barrier may consists of sandbars, barrier islands or coral reefs. There are two types of lagoons: (1) elongated or irregular stretches of water that lie between coastal barrier islands and the shore line known as coastal lagoons and (2) circular or irregular stretches of water surrounded by coral atoll reefs or protected by barrier coral reefs from direct wave action known as coral lagoons.

The second form of lagoons are restricted to tropical open seas that provide the conditions necessary for coral growth.
In Mauritius, most lagoons are of the coral type, though a few lagoons exist which are of the second type. For example, along the western and south western coasts, coastal processes were at work in the formation of lagoons. Along those coasts, long shore currents move coastal sediments along the mouths of streams resulting in marshy lagoons. Such coastal lagoons can be seen at Wolmar, La Priairie, and Anse St Martin.



Stresses on the Lagoons

Human activities which have an impact on the lagoons are: hotel development, sand extraction, artisanal fisheries, operation of leisure boats, industrial sewage, domestic sewage and solid waste disposal. Please follow the hyper links for further information.

The Albion Fisheries Centre has carried out a number of measurements on the water quality on the lagoon at selected sites. Each site was visited 6 times over the course of a year.

The laboratory analyses consisted of the following measurements: Nitrate-Nitrogen, Phosphate and BOD5.

The normal values for the above are as follows:

Nitrate Nitrogen: 0.1 - 0.2 mg/l

Phosphate : 0.02 - 0.04 mg/l

BOD5 : 0 - 1.0 mg/l

For 1996, the average values (in mg/l) of the parameters analysed for the different sites are as follows:

SiteNitrate-Nitrogen PhosphateBOD5
Ile Aux BenitiersND 0.0140.38
Bel OmbreND 0.0140.44
Bambous VirieuxND 0.0190.32
Trou d'Eau DouceND 0.0150.52
Anse La Raie0.02 0.0130.51
Trou Aux BichesND 0.0140.52
BalaclavaND 0.0190.49
Pointe Aux SablesND 0.0221.09
Tombeau Bay0.16 0.0521.73
Port Louis Harbour ND0.040 1.05

Source: Ministry of Cooperatives, Fisheries and Marine Resource Development, Annual Report 1996

The centre at Albion also carried out bacteriological analyses of sea water near Albion itself, they did report the presence of coliform bacteria at the mouth of Belle Eau river that discharges in the Albion Lagoon.

The annual report mentions a number of abnormal events that occurred during 1996. Fish mortality was reported twice, once at Bain Des Dames and once at Tombeau Bay, both localities being situated close to Port Louis Harbour. Outpourings of sewage leading to eutrophication and high BOD loads were thought responsible.

In the north, at Trou Aux Biches and Mon Choisy, several occurrences of red tides were reported. The causes of theses red tides are not known. In 1992 too, there were occurrences of red tides which had caused massive fish mortality.



Last Update: Thursday, February 05, 1998