Reliability of estimating chlorophyll a concentrations in Antarctic waters by measurement of in situ chlorophyll a fluorescence

Using an extensive data set acquired in Antarctic waters in February-March of 1998, algorithms have been formulated which permit reliable estimation of chlorophyll a (chl a) concentrations in the upper 100 m of the water column. The algorithms were derived from 105 oceanographic stations where an instrumented CTD-rosette profiling unit was deployed from the surface to 750 m depth. During each upcast 9 water samples were obtained at depths between 5 and 100 m. The values for in situ chl a fluorescence and solar irradiance were recorded simultaneously with collection of water at each depth. The chl a concentrations at each of the 9 depths were determined by standard laboratory procedures after extraction of the photosynthetic pigments into absolute methanol. Analysis of the data indicated that separate algorithms had to be formulated for coastal waters as contrasted to pelagic, low-biomass, waters. Each of these 2 algorithms also required 2 equations to compensate for the inhibitory effect of solar radiation on the fluorescence yield per unit chl a. Water column data obtained during January 1998 were used to test the algorithms. The results showed that the profiles of estimated and measured chl a concentrations were similar throughout the upper 100 m of the water column, but that the estimated values were lower than the actual values by an average of 19% (n = 528, r 2 = 0.83). It is concluded that the use of a calibrated in situ fluorometer can, with the proper algorithms, provide the investigator the detailed profile of chl a distribution as well as realistic estimations of the actual chl a concentrations in the upper 100 m of the water column.