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

Abstract

Siphonophores are exclusively marine cnidaria and their predatory role in plankton foodwebs 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 lm 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.

Distribution of planktonic cnidarian assemblages in the southern Gulf of Mexico, during autumn

“…These distribution patterns coincided with those observed by LomanRamos et al (2007) and additionally suggest that the planktonic cnidarian assemblages vary markedly from the coast to deeper waters (offshore variability) in agreement with the observations of Sanvicente-Añorve et al (2007b).…”

“…abund = average abundance, δ = average similarity between stations in each group, SD = standard deviation of δ, Contrib% = percentage contribution of each species to δ, Cum.% = cumulative percentage of contribution to δ, contributors up to a cumulative 90% for each group are shown. , 1999Sanvicente-Añorve et al 2007b;Segura-Puertas et al 2009;Pugh and Gasca 2009), sometimes being the most abundant component of the gelatinous zooplankton community (Segura-Puertas 1992;Segura-Puertas and Ordóñez-López 1994;Loman-Ramos et al 2007). …”

“…Knowledge of this group of organisms in the Gulf of Mexico is as yet limited, despite several recent studies that have informed on the relationship between the assemblages of medusae or siphonophores and the local oceanographic conditions (Loman-Ramos et al 2007;Sanvicente-Añorve et al 2007a, 2007b. Considering the intense water mass transportation that takes place in the Campeche Bay during the autumn-winter, it would be expected that the structure and composition of the planktonic cnidarian assemblages reflect the oceanographic conditions.…”

“…Several environmental factors have also been pointed out as determining the distribution patterns of planktonic cnidarian populations in other parts of the world. In general, these factors vary in relation to the temporal and spatial scales under study; however, it is possible to say that upwelling phenomena, as well as oceanic gyres, currents, the distance off the coast, the depth of the mixing layer and the effect of freshwater input, have been recognised by various authors (Gasca 1999;Suárez-Morales et al 2002;ThibaultBotha et al 2004;Sanvicente-Añorve et al 2007b) as important factors that determine spatial-temporal changes in planktonic cnidarian assemblages.…”

“…Depth at each sampling station, on the other hand, reflects the position of the station with respect to the coast, and as such has been previously considered an important parameter for the definition of groups of stations in the Gulf of Mexico (Gasca 1999) and elsewhere (Biggs et al 1981;Mackie et al 1987). Salinity has been previously related to the abundance and richness of medusae and siphonophores in several parts of the world (Santhakumari et al 1999;Thibault-Botha et al 2004;Zakaria 2004), among them is the Gulf of Mexico (Sanvicente-Añorve et al 2007b. Furthermore, the presence of saline fronts is commonly associated with the formation of planktonic cnidarian assemblages (Graham et al 2001), the same way the patterns observed in the south-eastern Gulf of Mexico suggest that the input of freshwater (mainly from the Grijalva-Usumacinta system) and the salinity gradient it generates are very important factors that determine the dynamics of the zooplankton populations in this region.…”