A MULTIVARIATE STUDY OF BIOTIC AND ABIOTIC RELATIONSHIPS IN A SUBTIDAL ALGAL STAND

The distribution patterns of dominant macrophytes (Gelidium sesquipedale, Saccorhiza polyschides and Laminaria ochroleuca) in relation to local abiotic conditions (depth, sediment loading and substrate topography) were investigated in a commercial stand of G. sesquipedale off Cape Espichel, Portugal, using principal component analysis. Observed patterns were analysed using multiple regression models to assess the strength of the relationship between G. sesquipedale and the different biotic and abiotic factors. Both multivariate and multiple regression analyses revealed significant positive relationships between G. sesquipedale cover and substrate slope, and negative relationships among G. sesquipedale, sediment loading and S. polyschides density. The main effects of these variables on the abundance of G. sesquipedale are of the same relative magnitude. Within the depth range studied (0 to 18 m), S. polyschides abundance was the only variable correlated with depth. The only interaction significantly correlated with G. sesquipedale local distribution was that between L. ochroleuca density and depth. Data suggest that steep slopes and reduced sediment loading favour G. sesquipedale development. On the contrary, S. polyschides dominated in shallow zones with low slope surfaces and high sediment loading. L. ochroleuca by itself was not significantly correlated to the distribution of G. sesquipedale, but its interaction with depth suggests that at greater depths it dominates G. sesquipedale. This is supported by analysis of the correlations between the 3 species in shallow versus deeper zones. Potential mechanisms are discussed for the significant relationships found, and testable causal hypotheses are suggested.