PRIMARY RESEARCH PAPER Zooplankton–phytoplankton relationships in shallow subtropical versus temperate lakes Apopka (Florida, USA) and Trasimeno (Umbria, Italy) Karl E. Havens Antonia Concetta Elia Maria Illuminata Taticchi Rolland S. Fulton III Received: 7 January 2009 / Revised: 27 February 2009 / Accepted: 12 March 2009 Ó Springer Science+Business Media B.V. 2009 Abstract This study compares and contrasts the dynamics of phytoplankton, zooplankton, and nutri- ents in two of the largest shallow lakes in the USA (Lake Apopka, Florida) and Europe (Lago Trasi- meno, Umbria, Italy) and considers particularly the biomass ratio of zooplankton to phytoplankton (BZ:BP) in relation to nutrient levels and in the context of data from other subtropical and temperate lakes. Lake Apopka is hypereutrophic with higher concentrations of total phosphorus (TP), nitrogen (TN), and nearly an order of magnitude higher BP than Lago Trasimeno. However, combined data from the two lakes can be fit to a single log–log regression model that explains 72% of the variability in BP based on TP. In contrast, BZ has a significant positive log–log relationship with TP only for Lago Trasi- meno, and is much lower than expected based on the TP concentrations observed in Lake Apopka. Lake Apopka has a fish assemblage that includes high densities of gizzard shad (Dorosoma cepedianum) and threadfin shad (D. petenense), similar to other eutrophic Florida lakes that also have extreme low BZ. The ratio BZ:BP is below 0.01 in Lake Apopka, 10-fold lower than in Trasimeno and among the lowest values reported in the literature. Although stress of high water temperature and a greater proportion of inedible cyanobacteria may be contrib- uting factors, the collective results support an emerging view that fish predation limits the biomass of crustacean zooplankton in subtropical lakes. Keywords Zooplankton Á Phytoplankton Á Shallow lakes Á Subtropical Á Fish predation Introduction Recent studies suggest that biomass of zooplankton (BZ) is lower in subtropical than in temperate lakes, particularly when considered in relation to biomass of phytoplankton (BP) (Havens et al., 2007; Jeppesen et al., 2007). Furthermore, subtropical lakes generally are dominated by small zooplankton species (Crisman & Beaver, 1990; Dumont, 1994; Havens et al., 2000), and are characterized by the absence of large ( [ 1 mm) species of Cladocera (Crisman & Beaver, 1990; Havens et al., 1996; Gillooly & Dodson, 2000). Different hypotheses have been put forward to explain Handling editor: S. I. Dodson K. E. Havens (&) Florida Sea Grant College Program, University of Florida, Building 803 McCarty Drive, Gainesville, FL 32611-0400, USA e-mail: khavens@ufl.edu A. C. Elia Á M. I. Taticchi University of Perugia, Perugia, Italy R. S. Fulton III St. Johns River Water Management District, Palatka, FL, USA 123 Hydrobiologia DOI 10.1007/s10750-009-9754-4