The Ascophyllum/Polysiphonia/Mycosphaerella Symbiosis. II. 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 (Id = 1.5 to 8.3, significant at p < 0.01). At Tor Bay, at the two sampling heights where 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. lanosa is most successful. The Polysiphonia/Ascophyllum symbiosis is explained using a hypothesis based on recruitment limitation. Introduction Penot 1974> Penot and Penot j 974j Harlin and Craigie Polysiphonia lanosa (Linnaeus) Tandy is a marine red 1975, Turner and Evans 1977, Fries 1979, 1980, Fialga that grows almost exclusively on the fucoid Aslion-Myklebust and Norton 1981, Higgins 1984, Lobcophyllum nodosum (Linnaeus) Le Jolis. It is occa, ban and Baxter 1983, Garbary and Gautam 1989, sionally reported on Fucus species (e. #. Cotton 1912, Pearson and Evans 1989, 1990, 1991), and Fries Feldmann 1954) and there is one report of P. lanosa (1988) has shown that bacteria are also an important on rock (Rattray 1885), although Dixon (1973) quesP of the symbiosis. Despite these studies, there is tioned if there was a complete absence of Ascophyllum no explanation for the host specificity of P. lanosa in such cases. In addition, an ascomycete, Mycö^Aaeand the biological basis for its interaction with A. rella ascophylli Cotton, is always found endophytic nodosum. (parasitic? ) in A. nodosum (Kohlmeyer andKohlmeyer Ascophyllum nodosum and Polysiphonia lanosa are 1972, Garbary and Gautam 1989). Numerous obserwidely distributed and abundant on temperate shores vätions have been made on various aspects of the of the North Atlantic Ocean (Taylor 1957, South and symbiosis mcluding morphological, ultrastructural, Tittley 1986). The western Atlantic distributions of physiological and ecological studies of the symbionts both A. nodosum and P. lanosa extend from New and their interactions (e.g., Webber 1967, Rawlence Jersey to the lower St Lawrence River, and Newfoundand Taylor 1970,1972, Citharel 1972, Rawlence 1972, land. Ascophyllum nodosum extends further, and is Botanica Marina / Vol. 34 / 1991 / Fasc. 5 Copyright © 1991 Walter de Gruyter · Berlin · New York 392 Garbary et al.: Population ecology of Polysiphonia lanosa also found in Hudson Bay, northern Labrador, James Bay, Hudson Strait, Baffin Island and Ellesmere Island (Taylor 1957, Lee 1980). Nova Scotia, with a diversity of oceanographic environments (/. e., Atlantic coast, Bay of Fundy and Gulf of St Lawrence), is roughly in the middle part of the range of P. lanosa, and the species is locally very abundant. For this reason, Nova Scotia is suited for ecological studies of