Circadian biomass and abundance changes of five euphausiids along the west coast of Baja California, Mexico. December 1993

Bongo net trawls were used to determine circadian biomass changes of macrozooplankton of the most abundant euphausiids seaward of the continental shelf off the west coast of Baja California, Mexico (24° to 31° N). Eight transects perpendicular to the coast and 18 km long were made in December 1993. Each transect was monitored during 24 h. Euphausiid biomass, one of the most important components of the macrozooplankton (size > 1000 mm), showed significant differences by latitude and over 24 h. They were more abundant at night. The most abundant euphausiid species in the area were Nyctiphanes simplex Hansen and Nematoscelis difficilis Hansen. Nyctiphanes simplex had a low biomass contribution because only individuals < 12 mm were found. The larger species, like N. difficilis and Euphausia eximia Hansen, were important contributers to the macrozooplanktonic biomass. Euphausia pacifica Hansen and Thysanoessa spinifera Holmes were found only in the northern area (30° to 31° N) and are a potential seasonal source 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 role in transferring energy from the deep to the surface layer during their nocturnal vertical migrations. The energy transfer was generally one to five times the mg dry weight greater at night than day, but at times more than 36 times. The implication of this finding is the food available for daytime predators is lower than for nighttime predators