Upwelling conditions associated to the distribution of zooplankton in San Hipolito, B.C.S.

Abstract

From June 22 to 24, 1989, temperature, salinity, dissolved oxygen, nitrate, phosphate (0 to 300 m depth), wind speed and direction were measured in a grid of 35 hydrographic stations in the coastal Pacific off Bahía San Hipólito, B.C.S. Zooplankton biomass, abundance and diversity of the copepod community were also determined. The average wind direction and speed were 309º NW and 6.7 m s^–1, respectively. The correlation coefficient between sigma-t and NO₃^–1 in the 25.0 to 26.0 density range was 0.67 (r, 95% confidence level). In general, high concentrations of zooplankton biomass were found at the coolest coastal stations. Thirty-three species of copepod were identified, dominated by Calanus pacifìcus (91%) and, in lesser abundance, by Pleuromama abdominalis (6.2%) and Calanus minor (0.7). A taxonomic analysis of the composition of copepods indicated a predominantly transitional fauna with some species of temperate-tropical origin. Using wind velocity and nitrate distribution, we estimate that 80 millimoles of NO₃^–1 m^–2 d^–1 are transported into the euphotic zone, which is equivalent to a new production of approximately 6.4 g C m^–2 d^–1. This value is considered high in comparison to other upwelling areas and apparently maintains high zooplankton biomass at inshore stations.