Abstract

The vertical distributions of 49 species (53 taxa) of calanoid copepods were determined in cyclonic and anticyclonic hydrographic features in the western Gulf of Mexico for three seasons of the year. The relative intensities of the features varied among seasons, while within seasons the physical structure between features was different. Since the same copepod species were present in both features for all seasons sampled, the contrasting hydrography provided a natural experiment in which to study the mechanisms, biological or physical, acting to influence species vertical distribution patterns. Hierarchial classification analysis revealed that groups of samples characterized by relatively homogeneous biotic characteristics were related more to depth of the sample than to the season or location (hydrographic feature) in which the sample was taken. Species groups defined by the classification analysis also tended to occupy different depth zones, but the species groups overlapped strongly in the vertical dimension. Significant species structure was found as well, with stability of the rank order of species abundances. Both species and vertical structure persisted over time and were resistant to nonseasonal hydrographic variability. A weighted version of multiple discriminant analysis was used to relate the species patterns to the environmental patterns. The most important extrinsic variable correlated with where species were likely to occur was depth, although the depth distributions of chlorophyll and nitrate cannot be ignored. In the absence of corresponding abiotic patterns, biotic factors are suggested as the dominant causative agents of copepod vertical distribution patterns. It is proposed that on a broad scale the observed constancy of copepod species and vertical spatial structure is related to the “nutrient-limited” and “light-limited” physiological regimens documented for oceanic phytoplankton species.

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