Vertical distribution patterns of calanoid copepods in the western Gulf of Mexico

Abstract

The vertical distributions 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 intensity of the hydrographic features varied between 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 hydrographic regimes provided a natural experiment in which to study relationships between copepod and environmental patterns of variation. Similarity between samples on the basis of copepod species composition was determined by classification analysis. Sample similarity was related more to depth of the sample than to the location or season in which the sample was taken. A weighted discriminant analysis was used to relate the species patterns to the environmental patterns. The species separated best along a depth gradient, although chlorophyll was also important. Species tend to occupy different depth zones, but the species groups defined by the classification analysis overlap in space. A set of samples taken over small time and space scales while following a surface buoy drogued at depth yielded similar results. Spatial autocorrelation analysis of 14 species indicated that the species have preferred depths. The species either actively maintain these depths, or the horizontal spatial scales of ecological factors affecting the species distributions are quite large. Groups of species were found to have similar variation patterns and similar correlograms within each group, and dissimilar variation patterns and more or less dissimilar correlograms among groups...