No 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.

Sanvicente-Añorve, L., Alatorre, M. A., Flores-Coto, C., and Alba, C. 2007. Relationships between fish larvae and siphonophores in the water column: effect of wind-induced turbulence and thermocline depth. – ICES Journal of Marine Science, 64: 878–888. The relationship between the abundance of fish larvae and siphonophores in relation to wind-induced turbulence and thickness of the mixed layer in the southern Gulf of Mexico were studied during two periods of different wind conditions: April (5.25 m s−1) and October (6.5 m s−1). The Spearman correlation between fish larvae and siphonophores revealed a random relationship in the 0–10 m layer during April and in the 0–20 m layer in October. This structure presumably persists while turbulent forces remain at sufficient strength. Positive patterns were observed deeper in the water column. Whereas thermocline position did not correspond with the depth separating random and positive relationships, low turbulence values did. Observations indicate that turbulent kinetic energy values above 4 × 10−4 J kg−1 might promote a random distribution between potential prey and predator zooplankton taxa. In surface waters, contact rates between siphonophores and fish larvae showed that turbulence enhances encounters 2.5 (1.2) times in April and 3.3 (1.3) times in October for prey velocities of 0.003 (0.011) m s−1. The positive relationship between fish larvae and siphonophores could be caused by a high degree of spatial overlap, enough food for both groups, and limited predation on larvae in the presence of alternative prey. Seasonal variability in the vertical structure of distribution patterns was attributed mainly to aggregative feeding behaviour of organisms and disruption of patches as a consequence of small-scale water turbulence.

Two oceanographic cruises were taken during the winter (SAV I, November and December 2007) and summer (SAV II, July and August 2008) across the mouth of the Papaloapan River in the Gulf of Mexico. Surficial sediment samples were collected from shallow (16-30 m), intermediate (30 to 80 m), and deeper areas (≥300 m). Shallow water sediments are coarser, better-sorted, and primarily composed of sands during the winter, while those found in the summer are finer. At depths greater than 30 m, sediments are primarily fine-grained no matter the season. Major element analysis from shallow areas indicates higher SiO2 concentrations during the windy season with negative correlation against Al2O3 during both seasons, following the respective abundances of sand and muds. High organic carbon content was observed in shallow areas during the summer. Trace metals V, Ni, Cu, Zn, Pb, Li, Cr, Co, and Ba were evaluated. The first six metals showed higher average concentration in the deeper areas, although the highest values at some individual sampling sites for Cr, Co, Cu, and Ba were observed in the coastal area. Factor and cluster analysis were used to explain the sediment distribution pattern and the factors that determine the sediment characteristics within the study area. In shallow areas, four clusters were observed during the winter and five during the summer. The geochemical characteristics of the samples in each cluster suggest association with fluvial sediment input, textural characteristics, heavy minerals, and Cu and Ba concentration. To evaluate the variations in heavy metal concentration, metal enrichment factors (EFs) were calculated. Enrichment in V, Cr, Co, Zn, Ba, and Pb was detected at certain sites, whereas Cu behaved differently. The distribution of Cu enrichment suggests that it may be of natural origin, associated with the lithology of the volcanic continental area. The minor enrichment observed for other elements may be associated with river discharge. According to sediment quality guidelines, trace metal concentrations of Cu, Pb, and Zn present occasional risks to aquatic organisms.