NUTRIENT LIMITATION IN RELATION TO PHYTOPLANKTON CAROTENOID CHLOROPHYLL-A RATIOS IN FRESH-WATER MESOCOSMS

The 480/665 nm absorption ratio for pigment extracts was evaluated as an indicator of nutrient limitation using freshwater plankton communit ies and a mesocosm approach. The mesocosm design was used to create va rious nutrient regimes for the phytoplankton communities, and nutrient limitation was detected by means of growth response assays (C-14 fixa tion and pigment increase following addition of nutrients) and the 480 /665 ratio. Results from four periods in 1993 and one period in 1994 s howed that high 480/665 ratios (indicative of nutrient limitation) wer e only found at chlorophyll a concentrations <20 mu g l(-1), where nut rient limitation was not detected. Periods with potential nutrient lim itation exhibited low 480/665 ratios. Consequently, the absolute value s of the 480/665 ratio could not be used to predict nutrient limitatio n. However, when nutrients were added to nutrient-limited phytoplankto n communities in growth response assays, the ratio decreased, indicati ng that the 480/665 ratio can be used as a relative index to detect nu trient limitation in bioassays.