Siphonophores off a riverine system in the southern Gulf of Mexico: factors affecting their distribution and spatial niche breadth and overlap

Abstract

Environmental factors determining the spatial pattern of the whole siphonophore community, as well as the hydrological occurrence ranges, small-scale vertical distribution, and spatial niche breadth and overlap of 23 siphonophore species collected off a riverine system in the southern Gulf of Mexico were analysed. A total of 149 zooplankton samples were collected in different strata of the water column (from 0 to 100 m) over a grid of 23 oceanographic stations during low (April) and high (October) rainy periods. Temperature and salinity measurements were taken with a CTD probe. Considering both seasons, salinity fluctuated between 30.7 and 37.0, and temperature between 18.2 and 29.0°C. Under these conditions, the hydrological occurrence ranges of species were analysed. Data on siphonophore biovolumes in the upper 30-m layer were subjected to a regression tree (RT) analysis taking the zooplankton biomass, the distance to the shore, the temperature, and the salinity as predictable variables. Results of the RT analysis showed that the distance from the shore, food availability, and temperature were among the most important factors affecting siphonophore spatial distribution. Food availability had a positive influence on the siphonophores distribution, whereas temperatures higher than 28.1°C seemed to depress most siphonophore populations. Diphyes dispar moderately dominated the community and represented 30% of the total biovolume. The calycophorans Abylopsis tetragona, A. eschscholtzi, Diphyes bojani, and Chelophyes appendiculata were the most generalist species as revealed by their niche breadth values, and Enneagonum hyalinum endured the widest salinity conditions (30.7–37.0) and was mainly distributed in coastal waters. The second most abundant species, the physonect Agalma okeni, exhibited a low mean niche overlap value with the calycophoran species. We hypothesise that differences in diet composition between physonects and calycophorans are the main cause of those low niche overlap values.