Cross-shelf and vertical distribution of siphonophore assemblages under the influence of freshwater outflows in the southern Gulf of Mexico

Siphonophores are exclusively marine cnidaria and their predatory role in plankton food-webs is well recognised. In this study, we analyse the structure and the spatial extent of siphonophore assemblages in relation to changes in freshwater outflows and food availability in the southern Gulf of Mexico during a high (October) and a low (April) outflow periods. A total of 149 samples were collected using a 505 μm multiple closing net at 1-6 levels (0-100 m) of the water column, depending on the bathymetry. Data on siphonophore species biovolumes (ml 100 m-³) were treated by means of the Bray-Curtis Dissimilarity Index, and two distinctive assemblages were identified: the 'inner' and the 'outer' assemblages, located over the inner and outer shelves. Temperature, salinity, zooplankton biomass, and siphonophore species were included in a Principal Component Analysis (PCA) to identify the factors associated with each assemblage. Geographical distribution of the assemblages practically remained the same during both seasons and its cross-shelf variability was stronger than the vertical one. Seasonally, diversity values were higher in October, when the highest river discharges occur. Spatially, the lowest diversity and mean siphonophore biovolumes values were registered in the 'inner assemblage', where the highest and lowest salinity values were recorded. We suggest that even when extreme salinity values (>36.5 or <34) might depress siphonophore populations in the coastal area, enough food availability in the concerned areas might mitigate the negative effect of salinity, since a positive and significant (p < 0.05) relationship was found between siphonophores biovolume and zooplankton biomass. Bassia bassensis, Diphyes dispar and Enneanogum hyalinum, present in both assemblages during both seasons, were able to survive in a wide range of salinity values, following perhaps, their prey. All the 23 species here registered were found in the 'outer assemblage'; however, Abylopsis eschscholtzi, Chelophyes appendiculata and Diphyes bojani were more associated with the 'outer' group according to the PCA results. Enneagonum hyalinum was the only species frequently encountered and abundant in the 'inner assemblage' during both seasons and, supporting previous observations, this species might be considered as an indicator of nearshore waters.