Freshwater plankton: Links. If you're intrigued by this small and microscopic pond life, you should scope the "Pond life identification kit" at the "Microscopy-UK" website.
At Molecular Expressions Video Gallery "Pond Life" you can find digital videos of many freshwater pond plankton and creatures of the biofilm, protists of many kinds, rotifers, algae, flatworms etc, right up to the small crustaceans and insect larvae, all taken from a North Florida pond. Your web browser needs RealPlayer software, but there's a free download of it from Real.com linked from the site.
The sheer variety of freshwater tropical plankton is suggested at "Microscopic critters of Guam," Brenna Lorenz's description of what she finds under the microscope, looking at rich green water from the Madog River on Guam. Her pencil sketches of ciliates and rotifers and such are evocative, and she describes with vivid enthusiasm what these creatures are actually doing.
A serious but perfectly readable article, archived on the web by Asia-Pacific Fishery Commission, is A. Thienemann, "Tropical freshwater plankton", a lecture about deep freshwater lakes in Indonesia delivered in 1954 but still valid.
Iain M. Suthers and David Rissik, Plankton: A Guide to Their Ecology and Monitoring for Water Quality (2009) concentrates on the plankton of temperate waters. But I'm intrigued by a book edited by Constantine H. Fernando that seems worth looking into: it's A Guide to Tropical Freshwater Zooplankton: Identification, Ecology and Impact on Fisheries (Leiden: Backhuys), 2002: it's reviewed by Richard P. Lin, Sydney University of Technology here. Good luck finding a copy at your local library, eh; a price of £95.00 / $152/€108 at NHBS is holding me back, though.