CIRCADIAN BIOMASS AND ABUNDANCE CHANGES OF 5 EUPHAUSIID SPECIES ALONGTHE WEST-COAST OF BAJA-CALIFORNIA MEXICO, DECEMBER 1993

Bongo net trawls were used to determine circadian biomass changes of m acrozooplankton of the most abundant euphausiids seaward of the contin ental shelf off the west coast of Baja California, Mexico (24 degrees to 31 degrees N). Eight transects perpendicular to the coast and 18 km long were made in December 1993. Each transect was monitored during 2 4 h. Euphausiid biomass, one of the most important components of the m acrozooplankton (size > 1000 mu m); showed significant differences by latitude and over 24 h. They were more abundant at night. The most abu ndant euphausiid species in the area were Nyctiphanes simplex Hansen a nd Nematoscelis difficilis Hansen. Nyctiphanes simplex had a low bioma ss contribution because only individuals < 12 mm were found. The large r species, like N. difficilis and Euphausia eximia Hansen, were import ant contributers to the macrozooplanktonic biomass. Euphausia pacifica Hansen and Thysanoessa spinifera Holmes were found only in the northe rn area (30 degrees to 31 degrees N) and are a potential seasonal sour ce of biomass in the area. These five species were mainly responsible for the high daily variability of the macrozooplankton biomass (> 1000 mm) along the west coast of Baja California and play an important rol e in transferring energy from the deep to the surface layer during the ir nocturnal vertical migrations. The energy transfer was generally on e to five times the mg dry weight greater at night than day, but at ti mes more than 36 times. The implication of this finding is the food av ailable for daytime predators is lower than for nighttime predators.