S E C T I O N S

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 consist 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.  

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

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

Site Nitrate-Nitrogen Phosphate BOD5 Ile Aux Benitiers ND 0.014 0.38 Bel Ombre ND 0.014 0.44 Bambous Virieux ND 0.019 0.32 Trou d'Eau Douce ND 0.015 0.52 Anse La Raie 0.02 0.013 0.51 Trou Aux Biches ND 0.014 0.52 Balaclava ND 0.019 0.49 Pointe Aux Sables ND 0.022 1.09 Tombeau Bay 0.16 0.052 1.73 Port Louis Harbour ND 0.040 1.05

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

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.