Excessive growth of macroalgae: A symptom of environmental disturbance

Excessive growth of seaweeds is a response of the biocenose, in a particular biotope, to a supply of nutrients. This supply may be natural or of anthropogenic origin. In the latter case, excessive growths and drifts of algae are consequences of the disturbance in the environment. Such growths are becoming increasingly widespread as the result of the progressive eutrophication of coastal waters, and now occur in at least twenty five countries worldwide. For example, particularly significant accumulations of seaweed are observed annually in Italy (1 000 000 t fresh weight), France (100 000-200 000 t), and Australia (100 000-600 000 t). The densities range from 0.2 to 400 kg m(-2), the thickness of the mats from 2 to 100 cm. The first part of the present review describes the worldwide phenomenon and includes information on the localities affected and the density, biomass, annual production and harvesting time of the excessive algal growths. The second part tries to analyse the phenomenon and the problems posed by seaweed decay. Lagoons, bays and shallow estuaries near industrial, agricultural and urban areas are most affected by these increases in algal growth. In temperate waters, excessive growth of macroalgae is generally observed during the summer, while in tropical waters. it occurs during the winter. At first, the development of opportunist and tolerant seaweeds acts as a purifying system. Then, when the seaweed is stranded or the environmental conditions become unfavourable, the plants die and decompose. At this stage, a large biomass can become troublesome. In summary, the impact on the environment is as follows: an increase in the herbivore population, competition or toxicity towards flora and fauna, alteration of the sediment, recycling of nutrients and pollutants in the ecosystem, nuisance for local residents and reduction of tourism. Cast weed is sometimes collected from the beaches and dumped inland, but this method of disposal merely constitutes a transfer of pollution. Consequently, different ways of treatment were developed, including composting and methanisation.