Environmental patterns and biomass distribution of gelatinous macrozooplankton. Three study cases in the South-western Atlantic Ocean

Abstract

Periodic swarms or blooms of gelatinous macrozooplankton have a negative effect on many human activitiessuch as tourism, fisheries, and industry, but for several reasons (sampling procedures, underestimation of their real abun-dance, etc.), they have often been neglected in the local literature. The “high spatial resolution” exercise of the South-west-ern Atlantic anchovyEngraulisanchoitaRecruitment Project (SARP) was therefore also suitable for estimating standingstocks of “jelly” macrozooplankton, attempting to establish particular environmental patterns exerting control on the spatialdistribution of these facultative carnivorous predators in coastal frontal environments. These studies were carried outthrough a sampling programme on board the German R/V “Meteor” in three different systems, convergence and divergent,in the South-western Atlantic Ocean: Region A (42°S) on the Argentine shelf, characterised by tidal mixing fronts; RegionB (36°S), the freshwater outflow from Río de la Plata; and Region C (28°S), under upwelling events in subtropical waterson the Brazilian shelf. In general, a dominance of gelatinous macrozooplankton, compared with the other fraction of macro-zooplankton and micronekton was observed. Mean standing stock of the gelatinous zooplankton was always greater than50% of organic carbon (org. C) in every section analysed. The lobate ctenophore Mnemiopsis leidyidominated the zoo-plankton biomass in Region A, Argentina. It represented 60% of total org. C and was more abundant at the stratified zoneof the front. Ctenophores were also dominant in Region B, Río de la Plata, where the related species Mnemiopsis mccradyiand the cydippid ctenophore Pleurobrachia pileuscomprised 81% of total org. C. Mnemiopsiswas most common in areasof vertical thermal and saline stratification, while Pleurobrachiawas dominant in the less stratified areas. Gelatinous zoo-plankton was also the principal component of the macrozooplankton biomass in Region C, Brazil. The hydromedusae Rha-costoma atlanticaand Olindias sambaquiensisdominated both the total and gelatinous biomass (68% and 7% of total org.C), being always more abundant under lower thermal stratification. It was found that, both in convergent and divergent localsystems, gelatinous plankton tended to aggregate in areas where the presence of isolines outcropping to the surface (associ-ated with production processes) was observed. These results are discussed in the context of existing hypotheses regardingecosystem production and food webs.