THE ASCOPHYLLUM-POLYSIPHONIA-MYCOSPHAERELLA SYMBIOSIS .2. ASPECTS OF THE ECOLOGY AND DISTRIBUTION OF POLYSIPHONIA-LANOSA IN NOVA-SCOTIA

The population ecology and distribution of Polysiphonia lanosa were studied at five sites along the Atlantic and Bay of Fundy coasts of Nova Scotia. Polysiphonia lanosa occupies a restricted portion of the Ascophyllum nodosum zone which is site dependent, and P. lanosa is not evenly distributed within its zone. Using five separate measures of Polysiphonia abundance, highest values at all sites occurred toward the lower end (but not at the lower limit) of the A. nodosum zone. The differential abundance and regular pattern of distribution of P. lanosa suggests that there is an optimal physical and/or competitive environment for the epiphyte in this part of the intertidal zone. The distribution pattern of P. lanosa on A. nodosum at all sites is highly contagious, as shown by high values for Morisita's index of dispersion (I(d) = 1.5 to 8.3, significant at p < 0.01). At Tor Bay, at the two sampling heights were P. lanosa is most abundant, 31.7% and 14.5% of the A. nodosum fronds have no P. lanosa, and 10% of the A. nodosum fronds host 55.9% and 41.0% of the individuals of P. lanosa, respectively. Transplant studies suggest that the absence of P. lanosa in the southern Gulf of St Lawrence results neither from genetic features of the local A. nodosum populations, nor from an inability of of P. lanosa to survive environmental conditions in the southern Gulf. This absence may be accounted for by low A. nodosum densities in that part of the intertidal region where P. lonosa is most successful. The Polysiphonia/Ascophyllum symbiosis is explained using a hypothesis based on recruitment limitation.