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Bathymetric and Seasonal Patterns in the Sublittoral Megafauna off Central Chile

The sublittoral off central Chile represents a dynamic border separating an oxic surface equatorward current and an oxygen-poor poleward undercurrent. This investigation dealt with whether megafaunal assemblage structure and its temporal and bathymetric distribution were affected by this feature. The problem was approached by investigating the empirical relations between an assemblage index of abundance and physical factors of the environment. The megafauna off Conceptión Bay, central Chile (36°32′S) was thus sampled with an Agassiz trawl on eight cruises performed during 1991 and 1992 at three fixed stations (40, 64 and 96 m depth). Simultaneous reference samples of sea-water were taken to measure temperature, salinity, and dissolved oxygen. Descriptive assemblage attributes, diversity indices, and principal component analysis (PCA) for assemblage study were used. Moreover stepwise linear regression, with the first principal component of the specific abundance matrix as dependent variable was used to study relationships between temperature, salinity and dissolved oxygen and an assemblage of 23 species. Species richness, biomass and abundance decreased significantly with depth while diversity was highest in the intermediate depth station. Discontinuities were thus evident between the shallow, intermediate and deep stations. The grouping of samples in the plane formed by the first and second principal components showed a similar bathymetric discontinuity. In the Euclidian plane, a seasonal cycle of variation of the shallow sampling site was evident. Between 46 and 64 m depth lies a boundary separating different states of the same assemblage, the shallow seasonally variable and the deep constant. The intermediate station shares characteristics of both these stations reflecting a more variable environment. The overall bathymetric and the shallow seasonal patterns of variation were statistically linked to dissolved oxygen.