RECOGNITION OF NUTRIENT AND LIGHT LIMITATION IN TURBID MIXED LAYERS - 3 APPROACHES COMPARED IN THE PARANA FLOODPLAIN (ARGENTINA)

The potential difficulties of testing for nutrient limitation in turbid mixed layers are illustrated by comparing the outcome of three different approaches (in vitro bioassays, limnocorrals, and whole-lake fertilization) in the subtropical Parana floodplain. Bioassays of the planktonic communities consistently suggested acute N limitation, no limitation, or P limitation, depending on the time at which we chose to terminate the experiment. Enclosures (17 m2) deployed in one lake indicated N limitation but increased sedimentation and water transparency in the enclosures. Although short-term bioassays consistently indicated limitation by N, whole-lake fertilization produced only a small and transient increase in chlorophyll a and primary production or no response at all. Some lakes are mildly limited by N during summer; others appear to be limited by light. In the latter, bioassays and whole-lake fertilization lead to inconsistent conclusions. A steady state model of phytoplankton biomass (as Chl a) under excess nutrient concentrations is proposed to facilitate the diagnosis of light limitation in shallow and turbid water bodies. Model predictions are consistent with mean column critical irradiance (PAR) of 1.6 MJ m-2 d-1 (7.36 Einst m-2 d-1) previously reported for net phytoplankton growth in marine and freshwater systems.