The effects of wood deposition from a coastal log handling operation on the benthos of a shallow sand bed in Saanich Inlet, British Columbia
Date
1977
Authors
Conlan, K. E. (Kathleen Elizabeth)
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Abstract
A log handling ground situated on the northwest coast of Saanich Inlet, B.C., is generating bark debris which has deposited to a thickness of greater than 15 cm over approximately 1 km² of a shallow, subtidal sand bed. Survey of the benthic epibiota by scuba divers and of the infauna by 5 replicated 0.1 m² van Veen grab samples at each of 16 stations showed a marke4 alteration of the biota. Variability in bottom sediment structure and water quality was not great enough to cause the observed faunistic distributions. Application of classification and ordination to the data revealed that infauna responded primarily to a gradient in bark debris thickness, secondarily to depth and least of all to seasonal variation.
The sand bed infauna was dominated by Mysella tumida, Mesochcaetopterus taylori and at greater depth by Phyllochaetopterus prolifica. The community may be characteristic of west Saanich Inlet coastal sands. The sand population was markedly altered by deposited bark debris levels as low as O.8 cm in mean thickness. Suspension feeding organisms were diminished and population biomass and diversity was reduced. Sand bed faunal attributes were lost as bark debris thickened. Small polychaete and crustacean dominants appear ed which utilized the debris for attachment, protection and food. It is hypothesized that, with abatement of fiber deposition, return of the sand bed fauna may not occur until pre-deposition conditions return.
A number of community analytical techniques were applied to the data to differentiate faunal trends objectively. The methods included cluster analysis, Zürich-Montpellier analysis and reciprocal averaging ordination. As well, species-abundance relations, relative diversity and faunistic primary feeding type relations were examined. An evaluation of the analytical methods suggested that the relative merit of each would vary with the degree of heterogeneity of the data. Cluster analysis indicated gross biotic differences , although the Czekanowski coefficient of similarity isolated structural gradients which were not evident in clustering by Jaccard's index or a Zürich-Montpellier analysis. Ordination was more sensitive to faunistic gradients and as well suggested the relative importance of environmental factors which mediate the trends . Species-abundance relations, diversity and trophic structure measures explored community complexity and stability. A combination of the analyses in complementary fashion was found to be the best approach to elucidate and substantiate faunistic trends in this study.