Improved estimation of f-ratio in natural phytoplankton assemblages

Statistical properties in non-linear regression models of f-ratio versus ni trate relationships were investigated using a case study conducted at the E uropean continental margin (Project OMEX). Although the OMEX data fit withi n the family of empirical models introduced by Platt and Harrison (1985, Na ture 318, 55-58), it is shown that the discrepancy between experimental and fitted values is larger than expected, assuming that the data are accurate . Since the ambient ammonium concentration plays a leading role in regulati ng new production, the present analysis was extended to include explicitly the effect of ammonium on f-ratio versus nitrate plots. Experimental result s based on controlled ammonium additions were used to express the f-ratio a s a function of both nitrate concentration and ammonium inhibition, i.e, f* ((NO3,alpha)).I-(NH4,I-beta). The estimation behaviour of the data set/mode l combination was analysed by testing the appropriateness of various model functions for f* and I. The best fit was obtained with a sigmoid curve, for which mean values of the random fluctuation are almost commensurate with t he estimated uncertainty of the measurements with natural phytoplankton ass emblages. Precision of the f-ratio estimates was further assessed from cont our diagrams of constant likelihood. The significance, validity limits of t he estimated parameters, and the relevance of the proposed model as a predi ctive tool are discussed. Overall, this empirically determined model result s in improved precision off-ratio estimations. (C) 1999 Elsevier Science Lt d. All rights reserved.