Zooplankton abundance and evidence for its reduction by macrophyte mats in two Orinoco floodplain lakes

Zooplankton populations were sampled over one annual cycle in two floodplain lakes of the Orinoco River, Venezuela, in an attempt to establish the relationship between abundance patterns and the hydrology and morphometry of the lakes. One of the lakes (Tineo) is relatively large with a gently sloping basin; the other one (Aguilera) is smaller and channel-shaped. The hydraulic residence time of Lake Aguilera during inundation by the river is shorter (<1 day) than the minimum generation times of crustacean (4–12 days) and rotiferan (2.5 days) zooplankton. For Lake Tineo, residence time during inundation (7 days) is longer than generation times for all taxa except copepods. Although Lake Aguilera receives water from Lake Tineo during inundation, zooplankton densities were greatly reduced during passage through a large bed of the floating aquatic grass Paspalum repens located near the outlet of Lake Tineo. This retention was not size-selective and affected phytoplankton as well as zooplankton. In the Orinoco floodplain zooplankton densities are affected not only by hydraulic residence times but also by passage of water between lakes, which exposes populations to large losses within macrophyte beds. Retention of plankton by floating macrophyte beds is potentially important to the trophic ecology of tropical floodplain lakes because it results in the concentration of planktonic production in epiphytic and benthic habitats, where it can readily support food webs consisting of macroinvertebrates and fishes. Export of plankton from floodplain waterbodies to the river is also reduced by this mechanism.