Zooplankton biomass of the northernmost Gulf of California

Abstract

To assess the spatial and temporal fluctuations of zooplankton biomass, as dry organic weight per cubic meter (B), in the northernmost Gulf of California, twelve oceanographic cruises were carried out during the period October 1972-October 1973. There was no clear seasonal cycle. However, there are some persistent characteristics. The highest B values (as high as 154 mg m^–3) were always found in the channels around Montague Island (at the Colorado River delta). In general, B values increased westward or northwestward. In the period May-October 1973, there were more uniform gradients, with values at the southeast one order of magnitude smaller than those at the northwest. In general, calanoid copepods were the most abundant taxonomic group, both numerically and in terms of biomass. Without considering B the values found around Montague Island, minimum values were found in October (1-4 mg m^–3) and maximum values in August (25-150 mg m^–3). A principal component analysis shows that the B variance was partly explained by a component whose structure is associated to the bathymetry. The linkage may be S‰. Highest salinities in the Montague Island area indicate higher residence time of water, because they are due to the greater accumulated effect of evaporation in comparison with the more southern areas. Greater residence time may also be related to higher zooplancton biomass in the north. There was a weak ENSO (El Niño-Southern Oscillation) event in the years 1972-73. However, strong local dynamics, mainly upwelling and mixing phenomena associated with tides, maintain sufficiently high nutrient concentrations for primary productivity not to have been significantly affected by ENSO events in the central and northern Gulf (tidal amplitude may be as high as 9 m at the Colorado River delta). Thus, our B data may not reflect any effect of the “El Niño” event in 1972-73.