Habitat effect on spatio-temporal variability in size and density of the introduced algaCaulerpa taxifolia

Caulerpa taxifoliais an introduced green seaweed in the Mediterranean; it is a very fast-spreading species, is able to invade all kinds of substrata and causes regression of some seagrasses.C. taxifoliawas investigated in a small bay of the northwestern Mediterranean, where it occupies 3 distinct habitat types: (1) at the edge ofPosidonia oceanica, (2) withinCymodocea nodosabeds, and (3) on sand and cobbles. To provide a basis for further experimental investigations of the factors affecting its performance, a descriptive study was carried out at this site in which both size and density of blades were measured on 20 dates in all habitats from October 1994 to September 1996. Great temporal fluctuations within the same season were evident for both variables, despite the fact that high variability was found at small spatial and temporal scales (areas within each habitat and time within each season). However, habitat effect in combination with season was evident as a major factor affecting both length and density of blades, suggesting a positive effect of seagrasses onC. taxifolia. The greatest blade length was found in individuals at the edge ofP. oceanica, while withinC. nodosabeds, blade length was intermediate. Reduction of blade density occurred in spring in the sand andC. nodosahabitats, but not in theP. oceanicaone. Patterns were similar throughout the 2 years. Experimental evidence is needed both to highlight mechanisms regulating this kind of interaction (nurse effect vs shade-induced changes) and to investigate whether it affects patterns of invasion and replacement of existing species withC. taxifoliain these habitats. The facilitative effect of seagrasses on the alga observed in the study is also likely to cause an indirect negative effect on seagrasses themselves.