Seasonal variation in hyperiid amphipod abundance and diversity and influence of mesoscale structures off Baja California

Hyperiid amphipods collected during 4 IMECOCAL (Spanish acronym for Mexican Investigations of the California Current) cruises in January, April, July, and October 2005 were analyzed for seasonal variation in abundance and diversity and to determine the influence of surface circulation. The number of species was high in all seasons (between 79 and 92), but abundances showed remarkable differences. The total number of amphipods was lowest in January and highest in October (geometric means were 82 and 606 ind. 1000 m(-3) respectively). Multivariate analysis based on similarity indices demonstrated a tight coupling between species composition and mesoscale structures. The largest cluster joined 67 stations, with 40% of similarity being explained by 4 species (Vibilia armata, Lestrigonus schizogeneios, Eupronoe minuta, and Primno brevidens). The geographic position of these stations coincided with the main California Current (CC) flow from spring to autumn. Subgroups were defined by minor seasonal differences. In contrast, the CC flow during winter was reduced to a meander off north Baja California, and was surrounded by oligotrophic water as indicated by the presence of the hyperiids Anchylomera blossevillei and Phrosina semilunata plus 3 of the dominant species (V. armata, E. minuta, and P. brevidens). To the south of this front, other clusters with high oceanic influence were formed. In 2005, 3 eddies were detected in the area: one anticyclonic eddy in April and 2 cyclonic eddies in July. The anticyclonic eddy located in the southwest had its own faunal cluster that had high diversity but low population density. The main species in the anticyclonic eddy was Platyscelus ovoides. Only one of the cyclonic eddies had a unique faunal identity, being strongly dominated by Vibilia armata; this was located in the north between the CC flow and the coast. The other eddy was large and situated in the middle of the area, with a faunal array analogous to that in the CC flow. Correlation between hyperiid and salp species abundances suggested that the increase in salps (Salpa fusiformis, Thalia orientalis, Cyclosalpa bakeri) in spring could be relevant, as many hyperiid species showed a strong increase at this time. However, the most correlated salp was Cyclosalpa danae, which occurred only in October. Chains of C. danae aggregates were firmly attached in the samples. This would promote the aggregation and survival of amphipod hosts, particularly small species such as Lestrigonus schizogenios and L. bengalensis.