Composition, density, and biomass of salpidae and chaetognatha in the southwestern Atlantic Ocean (34.5°S-39°S)

Abstract

Salps and chaetognaths constitute an important fraction of the macrozooplankton and have a prominent role in the marine food web. In our study, we analyzed the species composition, density, and biomass in an area of the southern Atlantic Ocean during the austral winters of 1999, 2000, and 2001. The most abundant and frequent species were the salpids Ihlea magalhanica (Apstein, 1894) and Iasis zonaria (Pallas, 1774), and the chaetognaths Parasagitta friderici (Ritter-Zahony, 1911) and Serratosagitta tasmanica (Thomson, 1947). Chaetognaths were found in over 80% of the stations throughout the three winters, reaching up to 67 individuals (ind) m -3. Salps were found surviving at low population densities in 2000 and 2001, but in 1999, there were mass occurrences of I. zonaria and I. magalhanica, reaching densities of 301 and 123 ind m -3, respectively. To estimate biomass in C units, the relationship between dry weight and size was calculated for S. tasmanica and for solitaries and aggregates of I. zonaria and I, magalhanica. The biomass of salps and chaetognaths (as mg C m -3) over the shelf during the three consecutive winters was strongly related to prevailing physical and biological conditions. In 1999, the greatest contribution to macrozooplankton biomass corresponded to salps, while in 2000 and 2001, chaetognaths dominated the biomass. In swarm conditions, like in 1999, I. zonaria and I. magalhanica widely dominated over copepods and chaetognaths, producing an increase in the quantity of available C of up to 60 times in relation to the periods with very low population densities.