The effects of the Patagonian shelf-break front on copepod abundance, biodiversity, and assemblages

Fronts play an important role in ecological processes, having multiple effects on the structure of plankton communities. The Argentine Sea is characterized by the presence of the large and very productive Patagonian shelf-break front (PSBF). This is a permanent front, with high thermohaline gradients defined between the relatively warmer and fresher shelf waters and those of the Malvinas Current. We studied the effects of the PSBF in the vertical and horizontal structure of the copepod community along two transects crossing the front. For all the variables studied, the outer shelf plus the front behaves as a unity, different of the offshore oceanic region. The highest copepod abundances were recorded on the outer shelf and shelf-break front, while the highest diversity was verified in the oceanic regime. Three copepod assemblages were identified associated to different regions: outer shelf and front (OS-PSBF), upper (UMW) and deeper (DMW) Malvinas waters, each one characterized by its own set of indicator species, having different reproductive and feeding strategies. Sac-spawners, ambush or cruise-feeders, and detritivore or omnivore species, occurred in DMW, while broadcasters, filter-feeders and mainly herbivores dominated at UMW and OS-PSBF. The PSBF acts as a leaky boundary between the oceanic regime (UMW and DMW) and the outer shelf-front regime (OS-PSBF), the first one characterized by higher copepod diversity and several functional groups, and the last one depicted by higher copepod abundance and only one functional group. In addition to its high primary production, the PSBF and surroundings are important to provide diversity of pelagic habitats that promote the existence of different assemblages, which are characterized by species having similar functional traits and that could play a similar role in the ecosystem.