Trophic effects of sponge feeding within Lake Baikal's littoral zone. 1. Insitu pumping rates

Demosponges dominate the littoral zone of Lake Baikal, Russia. During August 1993, we measured the in situ pumping activity of a globose sponge, Baikalospongia bacillifera, common at depths of 8–50 m. Excurrent flow speed, surface area, and oscular diameter were measured with fluorescein dye release, underwater video, and image processing for nine individual sponges; diel deployments of a heated thermistor flowmeter were made on another three individuals. Mean pumping rates for oscula ranged from 0.2 to 3.3 cm s −1 , with instantaneous measurements as high as 25 cm s −1 . Although between‐sponge variability is large, oscula from an individual maintain speeds within 1 cm s −1 of each other. Volume fluxes for oscula measured from all individuals range from 0.01 to 0.60 cm 3 s −1 . Fluxes for individuals indicate that a sponge processes its volume in 17–24 s, comparable to values obtained for marine species. Oscula are not continuously active and often reduce pumping for several hours at irregular intervals. Two of three individuals monitored over a diel cycle exhibited negative correlations between the ambient current and oscular excurrent flow. Boundary layer measurements and diffusivity calculations demonstrate that sponges are capable of depleting picoplankton near the bottom.