Seasonal variation of atmospheric and terrestrial nutrients and their influence on primary production in an oligotrophic coastal system-southeastern Brazil

Abstract

In an oligotrophic coastal zone, land drainage and atmospheric precipitation can temporarily modify the concentrations of specific dissolved compounds in local surface waters, mainly nutrient salts, setting up conditions for "new" primary production of phytoplankton. The Ubatuba region (23º30'S -45º06'W) is considered an oligo-mesotrophic region subject to a high average annual precipitation (- 2,000 mm). The small rivers flowing into Palmas' Inlet present outflow patterns linked to the precipitation cycle. Rain and river waters show nitrate, nitrite, ammonium, phosphate and silicate concentrations that are very variable often higher than in surface seawater. When one considers the total nutrient inventory ofPalmas' Inlet (total volume) in each sampling period, the relative contribution of nutrients ftom rain water presents the following seasonal variations: DIN (0.1-21.0 %), nitrate (0.06-15.86 %), phosphate (0.01 - 2.75 %), silicate (0.01 - 0.50 %); and the contribution ftom river waters varies within the following limits: DIN (0.02 - 0.36 %); nitrate (0.01 - 0.88 %); phosphate (0.01 - 0.11 %); silicate (0.04 - 4.70 %). Prime production in this oligotrophic zone showed seasonal variations with up to 17.83 mgC m-3 h- in the winter of 1992. When fertilization caused by the South Atlantic Central Water (SACW, T-S characteristics -14ºC and S -35.5 PSU) intrusion occurred in the during of 1991, the average value (all depths) of primary production was significant (7.00 mgC m-3 h- ). The atmospheric source of dissolved nitrogenous compounds is very important to this region on account of the high pluviosity and the fact that this input occurs directly into the surface seawater, and thus to the euphotic zone.