Between December 17th and 18th of 1998the Soloncy Moura Research Vessel (CEPSUL/ IBAMA) was stationed for 24 hours at the eastern margin of the Arvoredo island, inside of the Arvoredo Biological Marine Reserve (27°19,6'-27°8,6' S and 48°27,1'-48°9,1' W). During this time the temperature and salinity profiles were obtained using a CTD, and water samples were collected for ammonium determination. The biomass in wet weight of the size classed zooplankton, was collected by vertical trawls in 3 strata in the water column. Incubations were executed at 5 meters depth in DBO flasks, of the Salpidae Thalia democratica Oozooids and the phytoplankton collected in the same place. The filtration rates were estimated by the increase of feopigments and growth rates by blastozooids liberation. The temperature and salinity data measured along the 24 hours didn’t show a significant temporal variation, with the Tropical Water occurring at surface and the Subtropical Water below 20 meters depth. The zooplankton biomass presented a clear vertical migration behaviour, with larger concentrations in the intermediary stratum (10 to 20 m) during the day and with a displacement for the superficial stratum (0 to 10 m) at sunset. The ammonium concentration exhibited a similar behaviour in the water column indicating probable regeneration processes and bioconvection of this nutrient. The species Thalia democratica was the dominant organism in the fraction larger than 1000 µm and it was responsible for the highest zooplankton biomass in the samples. The incubation experiments of this species presented an average filtration rate of 176.4 mL.day-1 and growth rate of blastozooids from 8 to 10.3% in length per hour.