CONTRASTING EFFECTS OF HABITAT STRUCTURE ON THE RECRUITMENT AND MORTALITY OF AN EPIBIOTIC MACROALGA

Field surveys over 2 years in contiguous beds of the seagrasses Zoster a capricorni and Posidonia australis showed that the green alga Codium duthieae was consistently more abundant in Z. capricorni than in P. a ustralis. In 1 year, mature plants were also more abundant at the boun dary between the seagrass beds than in either bed. Field experiments a nd programmes of sampling were used to investigate three potential exp lanations for the unusual distribution of this alga: (1) that the avai lability of substrata suitable for attachment of the alga differed bet ween the two seagrass beds; (2) that mortality of mature C. duthieae d iffered between the seagrass beds; and/or (3) that the intensity of re cruitment was different in the two seagrass beds. C. duthieae plants w ere exclusively epibionts of the bivalve Anadara trapezia. Detailed sa mpling showed that the abundance of A. trapezia was similar in both se agrass beds and that the distribution of bivalves suitable as substrat a for C. duthieae plants was not obviously related to proximity to the boundary between the beds. Two experiments investigated the survival of C. duthieae plants in each bed. In the first, mature C. duthieae pl ants transplanted into the P. australis bed suffered similar rates of mortality to plants which were disturbed and moved within the Z. capri corni bed or which were left undisturbed in the Z. capricorni bed. Few er of the host bivalves were recovered from the Z. capricorni bed, how ever, indicating that the mechanism of mortality differed between the beds, hosts being more frequently dislodged in the Z. capricorni bed. Removal of the leaves of the seagrasses had consistently greater effec ts on near-bottom current velocities in the Z. capricorni bed than in the Ft australis bed and significantly increased mortality of C. duthi eae in the Z. capricorni bed. Survival of plants was greater in plots of artificial leaves of P. australis placed in the Z. capricorni bed t han in plots of the natural Z. capricorni leaves or plots where the na tural leaves were removed. Most mortality in the Z. capricorni bed was due to dislodgement of the alga and its bivalve substratum. Correspon ding manipulations of leaves in the P. australis bed had consistently smaller effects on survivorship of both the alga and its host. Pattern s in the recruitment of the alga most clearly reflected the distributi on of adults. C. duthieae recruits were 5 times more abundant in the Z . capricorni bed and at the boundary between the two beds than in the Fl australis bed. The results demonstrate how habitat structure, provi ded by the canopy of leaves of the two species of seagrass, can have c ontrasting effects on the recruitment and mortality of a macroalga. In the case of C. duthieae, it appears that the differential pattern of recruitment is the primary determinant of the distribution of adult pl ants.