Zooplankton abundance in the lower Orinoco River, Venezuela

Zooplankton were sampled quantitatively in the lower Orinoco River and three major tributaries (Apure, Caura, Caroni). Dominant taxa were euplanktonic, which suggests origin from standing waters. In the Orinoco, densities for rotifers (mean, 7.6 ind. liter -I) were greater than those for cladocerans (0.9 ind. liter-I) or copepods (1.2 ind. liter-‘), but cladocerans accounted for more than half oftotal biomass (0.5 kg C liter -I). Average tributary contributions to zooplankton transport in the lower Orinoco (873 kg C d-l) were proportional to their average discharges. Extensive fringing floodplain along the lower Orinoco contributed very little to total transport. At low water in the Orinoco, when there was no contact with the floodplain, the transport of copepods (mainly nauplii) and Bosmina increased downstream by more than could be ascribed to additions from the tributaries or to reproduction in transit. These zooplankton appear to originate along the river margin, within the channel, and arc probably significant as an energy input for food webs in the channel and as an inoculum to floodplain habitats at the time of inundation. Zooplankton are often abundant in the main channels of major rivers. The processes regulating their abundance may not be easily discerned, however, because of advection. Growth and mortality processes that arc primary regulators of zooplankton populations in lakes also affect zooplankton as they are transported downstream by rivers. Superimposed are advective processes that control the influx of zooplankton to rivers from source areas and the continual loss downstream (Rai 1974; Hynes 1970). Reservoirs now occupy the main channcls of most large rivers and often provide continuous supplies of zooplankton to the rivers downstream. The influence of reservoirs on the development of zooplankton in rivers is well known, especially for highly regulated systems such as the Nile (Rzoska et al. 1955; Talling and Rzoska 1967; Brook and Rzoska 1954; Rzoska 1976) and the Murray (Shiel and Walker 1984; Shiel et al. 1982). Impoundment has altered the origAcknowledgments This study is part of a joint Venezuela-North American ecological investigation of the Orinoco River. Logistical support was provided by the Venezuelan Ministerio de1 Ambiente y de 10s Rccursos Naturales Renovablcs. We thank S. Hamilton and S. Sippel for helpful discussions, and Bruce Kimmel and two anonymous reviewers for constructive criticisms. This work was supported by NSF grants DEB 8 l6725, BSR 83-15410, and BSR 86-04655. inal zooplankton dynamics and has obscured the natural sources of zooplankton in rivers. Natural source areas for river zooplankton must have low flushing rates, i.e. they must be essentially lentic habitats. Lentic source areas exist along the periphery of river channels. They may be divided into three categories on the basis of proximity to the channel: channel habitats, side-channel habitats, and floodplain habitats. Variations in river level govern contact of flowing waters with these source arcas (Paggi and Paggi 1974). Channel habitats, which includc eddies, channel pools, and river margins, may continuously contribute zooplankton to the flowing waters, or they may contribute zooplankton mainly in response to lluctuations in river level. Spates may cause the elimination of plankton from channel habitats (des Cilleuls 1928), although smaller but repeated fluctuations in river level may be more significant in this respect (Saunders and Lewis 1988a). Side channels (anabranches and backwaters) may support plankton growth because these habitats have little or no flow when river levels arc low. Seasonal rises in river level greatly increase flow in these habitats and thereby push resident plankton into the main channel (Osmera 1973; Vranovsky 1974; Saunders and Lewis 19883). The difference between channel and sidc-channel habitats lies in the frequency with which