Mortality of shoots of Posidonia oceanica following meadow invasion by the red alga Lophocladia lallemandii

Abstract

Abstract Lophocladia lallemandii, an Indo-Pacific alga that is currently invading several Mediterranean areas, is especially aggressive when colonizing meadows of Posidonia oceanica in Formentera (Balearic Islands). L. lallemandii settles on rhizomes and old leaves situated mainly at meadow edges and in low density Posidonia patches, growing rapidly, producing disc-like holdfasts along the thalli that allow formation of a mat of the red algal filaments intermingled with P. oceanica leaves. This mat can get so thick and dense that new leaves become confined inside the web of red algal filaments. The seagrass leaves display chlorosis and sometimes die. Invasion by L. lallemandii significantly reduces seagrass shoot size, leaf biomass, and percentage of living shoots. A lower content of sucrose in shoots from invaded plots points to a reduction of photosynthetic activity. However, other causes such as enhanced sediment trapping, organic and nutrient enrichment inside the algal carpet, and increased oxygen consumption may jointly or individually contribute to observed seagrass shoot demise.