Evaluation of Beach Wrack for Use as an Organic FertilizerTemporal Survey in Different Areas

Resumo

We analysed the chemical composition of beach wrack (drift seaweed) to evaluate its potential for use as an organic fertilizer. For this purpose, two very different areas were selected for collecting samples of the material: a partially enclosed bay that is subjected to a high level of anthropogenic pressure and an open sea area. We sampled 13 beaches in these sites over a period of 1 year and analysed both the bulk (unseparated) samples and the constituent species. Beach wrack appears to be a potentially good material for producing high quality organic fertilizers, and the concentrations of N and K (especially in brown seaweeds) and of micronutrients (such as B) were particularly high. However, systematic collection of drift seaweed is difficult because of the seasonal variability in its presence. Furthermore, the low concentrations of P and the presence of extraneous matter in beach wrack collected in areas affected by anthropogenic pressure may limit use of the material. The temporal variability in the concentrations of certain nutrients must also be taken into account when using the beach wrack as a fertilizer. The concentrations of toxic metals in the material must be evaluated, particularly in areas subjected to high levels of anthropogenic pressure. Finally, the sustainability of beach wrack exploitation must be considered, as the material fulfils an ecological function as a habitat and resource for some beach-dwelling species.