The significance of body size and foraging mode in the structuring of marine soft-bottom polychaete assemblages

Gagnon, Jean-Marc (1990) The significance of body size and foraging mode in the structuring of marine soft-bottom polychaete assemblages. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Functional approaches are introduced to study the structure of marine soft-bottom communities. Results of these approaches, based on the feeding type, microhabitat preference, motility pattern and body size of benthic polychaetes, were compared to one based on taxonomy (families). The material came from grab samples collected from the continental shelf and upper slope off the east coast of Canada, in depths of 85 to 622 m ($\bar x$ = 247 m). Various multivariate analyses are used to identify recurrent biological patterns in macrofaunal assemblages. These patterns are then correlated with environmental variables via canonical analysis. No distinct faunal associations are identified from the various approaches. Instead, there was a gradual shift from one type of faunal grouping to another. -- The value of each approach in characterizing community structure is evaluated in part by the amount of variation explained by the relationship between the biological and environmental data, and by the statistical significance of this relationship as determined by Monte Carlo tests. The recognition of homogenous groupings of samples based on faunal composition and environmental conditions, however, tends to weigh more heavily in this evaluation. With one exception, all functional and taxonomic approaches show statistically significant patterns. The approach based on feeding microhabitat indicates that this biological attribute of marine benthic polychaetes may not play an important role in the structuring of their communities. The functional approach that comprises all functional attributes (i.e. foraging attributes and body size) provides the most meaningful ecological characterization of community structure. In this functional approach, groups of samples along the faunal gradients are strongly associated with large-scale topographic features of the Labrador continental shelf and upper slope. Although the taxonomic approach yields similar results, it does not appear to be as efficient in distinguishing between sample points in multivariate analyses. In functional approaches, interpretation of the results in terms of community structure is greatly facilitated by the direct use of ecological attributes such as foraging mode and body size. -- About half the variation in the biological data can be explained by variables such as water depth, current regime, sediment grain size and benthic biomasses which are associated with two major environmental gradients extracted by Redundancy Analysis. The effect of other biotic and abiotic factors on benthic community structure is discussed. The remaining variability in the biological data unaccounted for in this study may be explained by these factors, other unidentified processes and noise. The spatial scales at which the biological and environmental data are collected may also influence the outcome of a multivariate analysis and its interpretation.

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