Composting as Sustainable Managing Option for Seaweed Blooms on Recreational Beaches

Seaweed blooms are an irregular but frequent phenomenon in many coastal areas and during the touristic season, can cause limitations for recreational use of beaches. Here, we propose composting of seaweed removed from the shoreline as alternative to current management practices (e.g. disposal in landfills or incineration). A co-composting strategy with garden prune waste was chosen for the treatment of seaweed blooms removed from the beaches. Composting was performed in windrows of 10 m long, 1.5 m wide and 1 m high using different ratios of algae and garden prune residues (1:2; 1:1 and 2:1) at a composting facility in Rota (Cadiz, Spain). Characteristic temperature profiles consisting of a very brief mesophilic phase, a 15–20-day thermophilic phase (maximum of 50–70 °C) and a second mesophilic phase (40–35oC) after 30–50 days were observed. After curing for more than 160 days, a stabilized non-phytotoxic composted material (50–70 g kg−1 C; 2–3 g kg−1 N; C/N ratio of 21–27 and electrical conductivity between 1.8–2.4 mS cm−1) was obtained. Composting worked best using a 2:1 ratio (prune waste:algae). The composted organic material showed larger presence of stabilized lignin-derived compounds as assessed via nuclear magnetic resonance. No odors were detected during the composting process suggesting that open-air treatment is feasible. The co-composting strategy provides a safe, local and sustainable approach to deal with seaweed blooms during peaks of recreational use and in situations where algae have to be removed.