Resistance of Posidonia oceanica seagrass meadows to the spread of the introduced green alga Caulerpa cylindracea: assessment of the role of light

Abstract

Posidonia oceanica seagrass meadows are one of most resistant Mediterranean habitats to invasion by the green alga Caulerpa cylindracea. We evaluated the hypothesis that light reduction caused by the seagrass canopy can limit algal photosynthesis and growth and hence potentially explain this resistance. To this end, we analysed light regimes and C. cylindracea biomass and photoacclimative variables measured outside and within P. oceanica meadows at different sites and during contrasting times. The success of photoacclimatory responses was assessed using an ecophysiological, carbon balance approach. C. cylindracea abundance significantly varied depending on the sampling site and time, but its biomass was always 10- to 50-fold higher outside the meadow. Outside the canopy, C. cylindracea showed characteristic morphological and photosynthetic plasticity closely related to the spatio-temporal variation in light regimes, which varied as expected with depth and season. Under these conditions, the alga was able to perform successful photoacclimation, although some degree of light limitation was observed at the deepest sites and in winter conditions, as indicated by near-zero carbon balance and lower algal abundances. Within the P. oceanica canopy, light was reduced by 60–89 %relative to that outside and was at its lowest levels recorded (1–7 % of the sub-surface irradiance), close to the minimum light requirements for growth. Light limitation was evident inside the canopy in the winter sampling, when the photosynthetic plasticity of the alga appears to be exceeded and when carbon balances were clearly negative. Therefore, light appears to play a key role in the apparent incapacity of C. cylindracea to penetrate within P. oceanica meadow edges.