Influence of acute and chronic disturbance on macrophyte landscape zonation

Abstract

ABSTRACT: Although the roles of physical disturbance and successional recovery from such distur-bances in structuring natural communities are well known, recent studies have begun to uncover thepotential for alternate outcomes or climax states in a number of systems. Here, we examine the dis-tribution of tropical macrophytes at a site with heavy wave exposure and explore equilibrium (micro-habitat) and non-equilibrium (patch-dynamic) hypotheses to explain the observed pattern. The exis-tence of a large and distinct zone of prolific macroalgae, undescribed 30 yr earlier, situated between2 zones with a relatively high abundance of seagrass, challenges the classic successional regimewithin Caribbean macrophyte beds. Significantly smaller mean size and lower frequency of acutedisturbances within the macroalgal-dominated zone, as compared to the outer, mixed macrophytezone, appears to contradict the highly disturbed environment of classic colonizers. The dominance ofmacroalgae in the mid-shore zone may be enhanced by the presence of large sediment sizes that areaccumulated through the effect of chronic wave stress. Potential causes of overall changes in thehydrodynamic forces at the study site include rise in sea level, increased wave exposure and associ-ated erosion of reefs protecting the coast. We propose a model of macrophyte bed development anddistribution that includes the important chronic ‘stress’ category of wave energy and potential domi-nance by macroalgae. Increased average wave energy can impose an elevated level of stress leadingto modified patterns of macrophyte distributions and changes in the deterministic endpoint of thesuccessional sequence. KEY WORDS: Succession · Colonization · Competition · Waves · Stress · Gaps · Seagrass ·Macroalgae