Introduction to a Mesocosm Approach for Establishment of Phosphorus Gradient Experiments

In the late 1980s and early 1990s, the major question being asked about the Everglades was: What concentration of phosphorus could be added to the ecosystem without resulting in an imbalance of flora and fauna? While ecological studies had been undertaken along known P gradients in areas like Water Conservation 2A (Koch and Reddy 1992; Craft and Richardson 1993b) it was also clear that the ecological responses along this gradient were also influenced by other nutrient additions, including Na, Ca, N, and S; thus, the shifts in species and changes in water and soil chemistry could not be totally attributed to P additions. Moreover, it became essential to determine if a P threshold existed in the Everglades, i.e., to determine at what total P (TP) concentration significant shifts in ecosystem structure and function occurred. This chapter outlines the design, construction, and operational features of a replicated dosing system created to establish a gradient of known water TP concentrations. Importantly, the dosing facility site was selected from among many sites tested for uniform low background concentrations of TP. The final selection of the two replicate undisturbed site locations in the lower part of WCA-2A was based on sites with uniform slough species distributions, low P concentrations (< 400 mg kg⁻¹ TP) in the soil, and TP background water concentrations of ≈10µg L⁻¹ TP. Importantly, the dosing site was allowed to equilibrate from construction effects for 9 months prior to dosing trials. Test trials on the system were run to provide information on improving system water pumping efficiency and consistency as well as uniform phosphorus distributions and concentrations in the water column prior to the commencement of the 6-year P dosing experiment (1993–1998).