According to an estimate, a mangrove forest supports 100 species of plants and about 500 speci
es of animals, birds and fish. These forests also act as filters for nutrients from water; the fine soil sediments act as sinks for a variety of effluents. Thus these forests have long been used as convenient sites for sewage disposal. It was believed to be good for their growth.
A study by Catherine Lovelock of the University of Queensland, Australia, and her colleagues shows that nutrients from the effluents, in fact, contribute to mangrove tree deaths. Excessive nutrients promote excessive growth of the plant’s aerial parts (shoots, leaves, stems) over roots. The roots then find it difficult to balance the oversized tree. This makes the plant susceptible to particularly strong winds and tropical storms.
The research was carried out for a period of three to 12 years at 12 sites spread over New Zealand, the west and east coasts of Australia and in two provinces located within the Caribbean basin and in the Indo-Pacific region. Holes were dug around the trees and in each site, six to nine trees were fertilised with 200-300 g of urea or phosphorous—the major components of sewage—annually or biennially. Around other trees used as control, holes were dug but no fertilizer was added.
Tree growth was measured by assessing the growth of five twigs per tree in sunlit positions annually or biennially. Salinity of the water extracted from the soil at the base of the tree was measured. The data were correlated using a mathematical model.