Zooplankton abundance in the Caura River, Venezuela

Zooplankton abundance was studied in the Caura River, which drains 47,500 km2 of undisturbed tropical moist forest within Venezuela. During 2 yr of sampling, 72 species were recorded, of which 10 accounted for more than 90 percent of the individuals. Copepods were represented almost exclusively by cyclopoid nauplii. Most of the cladocerans were from planktonic genera; Bosminopsis was the most abundant. The rotifer fauna was dominated by the planktonic genera Filinia, Keratella, Conochilus, and Polyarthra; Lecane, a littoral genus, was also common, especially at low water. The mean density of Crustacea was 2.4 organisms per liter (maximum, 10/liter) and the mean for Rotifera was 5.4 per liter (maximum, 35/liter). The annual export of zooplankton biomass from the watershed was 39,270 kg C. The annual pattern of zooplankton abundance in the Caura River is closely related to hydrologic events. As the river rises early in the wet season and inundates the floodplain, zooplankton densities increase sharply. At peak discharge, densities fall to very low levels. Near the end of the wet season as the river recedes, floodplain lakes drain into the river and zooplankton abundance reaches the annual maximum. Abundance declines when the connection between river and floodplain disappears. There are also small pulses of abundance at minimum flow; these consist chiefly of organisms washed in from channel refuges in response to short-term fluctuations in river level. Reproduction is generally insignificant in the river. Virtually all organisms in the river are swept in from refuges in the channel or the floodplain of the Caura and its lower tributaries. Zooplankton density in the river is regulated by the flow regime and not by biological mechanisms.