Contrasting structural patterns of the mesozooplankton community result from the development of a frontal system in San José Gulf, Patagonia

The structural patterns of the mesozooplankton community of San José Gulf (SJG), a one of a kind semi-enclosed basin located in the northern margin of the Valdés Peninsula (Argentine Patagonia), were studied by means of semi-automatic image analysis. Samples were obtained bimonthly during the warm season (October 2012–April 2013) at stations distributed on a regular grid. Cluster analyses outlined two spatially consistent groups of stations showing similar patterns when based on physical (SST, Chl-a, and bottom depth) and community structure (abundance, biomass, and slope of the size spectra) descriptors, each broadly distributed on one of two previously described hydrographic domains formed by the interaction of tidal circulation, coastal morphology, bottom topography, and seasonal vertical stratification. Examination of the slope of the mesozooplankton size spectra revealed that the stratified waters of the East Domain sustain higher proportions of small organisms and higher biomass, suggesting potentially low trophic transfer efficiency compared to its vertically-mixed West counterpart. Three-clustering analyses resulted in still spatially consistent groups of stations when based on the community descriptors, revealing the existence of a transition zone, presumably resulting from intermediate hydrodynamic conditions. Principal component analysis revealed that biomass of cladocerans modulates the size spectra and is related with distances from sampling stations to SJG's mouth. Overall, a consistent spatial correlate was found between physical conditions of water masses and the structure of the mesozooplancton community contained within.