Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient

1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danis h lakes with contrasting total phosphorus concentrations (summer mean 0.02- 1.0 mg P L-1), we describe how species richness, biodiversity and trophic s tructure change along a total phosphorus (TP) gradient divided into five TP classes (class 1-5: < 0.05, 0.05-0.1, 0.1-0.2, 0.2-0.4, > 0.4 mg P L-1). 2. With increasing TP, a significant decline was observed in the species ri chness of zooplankton and submerged macrophytes, while for fish, phytoplank ton and floating-leaved macrophytes, species richness was unimodally relate d to TP, all peaking at 0.1-0.4 mg P L-1. The Shannon-Wiener and the Hurlbe rt probability of inter-specific encounter (PIE) diversity indices showed s ignificant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotife rs, phytoplankton and fish. 3. At low nutrient concentrations, piscivorous fish (particularly perch, Pe rca fluviatilis) were abundant and the biomass ratio of piscivores to plank ti-benthivorous cyprinids was high and the density of cyprinids low. Concur rently, the zooplankton was dominated by large-bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged mac rophyte abundance high. 4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10-fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased f rom 0.6 in class 1 to 0.10-0.15 in classes 3-5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis br ama) decreased two-threefold. Simultaneously, small cladocerans gradually b ecame more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 mug in clas s 1 to 1.5 mug in class 5, and the biomass ratio of zooplankton to phytopla nkton from 0.46 in class 1 to 0.08-0.15 in classes 3-5. Conversely, phytopl ankton biomass and chlorophyll a increased 15-fold from class 1 to 5 and su bmerged macrophytes disappeared from most lakes. 5. The suggestion that fish have a significant structuring role in eutrophi c lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manip ulation. In these lakes, studied for 8 years, a reduction in planktivores r esulted in a major increase in cladoceran mean size and in the biomass rati o of zooplankton to phytoplankton, while chlorophyll a declined substantial ly. In comparison, no significant changes were observed in 33 'control' lak es studied during the same period.