Improved estimation of f-ratio in natural phytoplankton assemblages
Elskens, M × Goeyens, Leo Dehairs, F Rees, A Joint, I Baeyens, W #
Pergamon-elsevier science ltd
Deep-sea research part i-oceanographic research papers vol:46 issue:10 pages:1793-1808
Statistical properties in non-linear regression models of f-ratio versus nitrate relationships were investigated using a case study conducted at the European continental margin (Project OMEX). Although the OMEX data fit within the family of empirical models introduced by Platt and Harrison (1985, Nature 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 regulating new production, the present analysis was extended to include explicitly the effect of ammonium on f-ratio versus nitrate plots. Experimental results based on controlled ammonium additions were used to express the f-ratio as 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/model 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 the estimated uncertainty of the measurements with natural phytoplankton assemblages. Precision of the f-ratio estimates was further assessed from contour diagrams of constant likelihood. The significance, validity limits of the estimated parameters, and the relevance of the proposed model as a predictive tool are discussed. Overall, this empirically determined model results in improved precision off-ratio estimations. (C) 1999 Elsevier Science Ltd. All rights reserved.