Zonation patterns and interspecific relationships of fucoids in microtidal environments

The Northwestern Mediterranean Sea is a microtidal system, where barometric pressure and local wave exposure play the paramount role in sea water level variation. Herein, Cystoseira amentacea var. stricta and Cystoseira compressa are common species, generally co-occurring on exposed shores where they form a fucoid belt that characterise a narrow area called the infralittoral fringe.To better understand the eventual zonation patterns and the interspecific relationships of Cystoseira species, a multi-response manipulative experiment based on cross-transplantation, was set up. The fitness of the two species was assessed in terms of survival, growth and reproductive conditions; the role of a given Cystoseira and of the presence of a hard to measure environmental gradient was assessed on the recruitment of the two species and on the composition of assemblages re-colonising the bare substrate.Our results highlight a different response to the manipulation of the two species: while C. compressa fitness does not seem to be affected by the vertical position on the shore, C. amentacea var. stricta survival, growth and reproduction change in function of the gradient. The height on the shore should therefore be taken into account for a successful restoration of lost habitats. The recruitment of the two species and the early phases of succession in re-colonisation of the substratum are also affected by the gradient, and the dispersal of both species seems to be more limited than it was assumed until now (approximately 20 cm). C. amentacea, able to spread in continuous belts in the infralittoral fringe, may be a better competitor than C. compressa, thriving at a higher level on the shore, with its lower limit potentially confined by competitive exclusion of C. amentacea var. stricta. The two Cystoseira species cannot be considered as ecological analogues and determinism seems to drive their distribution in the infralittoral fringe. Nevertheless, stochastic factors, and in particular the very limited dispersal, can be assumed as major factor in regulating large scale distribution of these species.

► We cross-transplanted two coexisting Cystoseira species in microtidal environment.
► The two species give a different response to the manipulation.
► They are not ecological analogues and determinism drives their distribution.
► A vertical gradient affects fitness, recruitment and colonisation of substratum.
► Their dispersal seems to be lower than generally assumed.