Zooplankton community structure, micro-zooplankton grazing impact, and seston energy content in the St. Johns river system, Florida as influenced by the toxic cyanobacterium Cylindrospermopsis raciborskii Jeremy A. Leonard* & Hans W. Paerl University of North Carolina-Chapel Hill Institute of Marine Sciences, 3431 Arendell St. Morehead City, NC 28570, USA (*Author for correspondence: E-mail: jeremy.leonard@duke.edu) Received 17 February 2004; in revised form 4 August 2004; accepted 24 August 2004 Key words: carbohydrates, Cylindrospermopsis, lipids, micro-zooplankton, proteins Abstract Zooplankton can influence the phytoplankton community through preferential grazing. In turn, nui- sance cyanobacteria may affect zooplankton community structure by allowing certain species to out- compete others. We examined zooplankton-phytoplankton interactions, micro-zooplankton (<200 lm) grazing, and biochemical components of the seston in the St. Johns River System (SJR), Florida in the presence and absence of the toxin-producing cyanobacterium Cylindrospermopsis raciborskii. We tested whether this cyanobacterium would cause a decrease in the size structure of the zooplankton com- munity and postulated a resultant decline in the metabolic energy and carbon available to higher consumers (i.e. fish). When numbers of C. raciborskii were low or undetectable, zooplankton were more diverse and were comprised of larger species. Rotifers were the dominant zooplankton, and their numbers relative to other zooplankton increased as C. raciborskii concentrations increased. Micro- zooplankton grazing was higher in times of C. raciborskii abundance, suggesting competitive and predatory exclusion by larger zooplankton in times of higher phytoplankton diversity. Total caloric content of the seston was higher in times of C. raciborskii abundance. However, essential fatty acids and phosphorus may be lacking in the seston, or nutrients may potentially be sequestered by the cyanobacteria and remain as organic matter in the water column. In such cases, higher trophic levels would not be able to obtain optimal energy requirements. Overall, there was a greater impact of micro- grazers on phytoplankton in the presence of C. raciborskii and apparent negative effects on the larger zooplankton species, suggesting a potential for changes in zooplankton and higher trophic level community structure. Introduction Human population expansion in coastal water- sheds has led to increased nutrient inputs to many aquatic ecosystems, resulting in eutrophication that favors nuisance cyanobacteria over other phytoplankton species (Fogg, 1969; Paerl, 1988). Both N 2 -fixing species and non-N 2 -fixing species of cyanobacteria are often abundant in eutrophic lakes, including unicellular/colonial genera like Microcystis and filamentous genera like Anabaena, Oscillatoria, and Aphanizomenon. Cyanobacteria can affect zooplankton grazers by their colonial and filamentous morphology (Fulton & Paerl, 1987), toxicity (Fulton & Paerl, 1988), or lack of nutritional value (Von Elert & Wolffrom, 2001). Changes in zooplankton com- munity structure associated with the presence of filamentous and colonial cyanobacteria include the replacement of large cladocerans by smaller Hydrobiologia (2005) 537: 89–97 Ó Springer 2005