Evaluation of six relations of the kinetics of uptake by phytoplankton in multi-nutrient environment using JGOFS experimental results

Abstract

The inhibitory effect of ammonium on nitrate uptake, which has received a fair amount of attention, is an example of phytoplankton uptake kinetics in a multi-nutrient environment with nutrient interaction. We examine six relations [Wroblewski, 1977. Journal of Marine Research 35, 357-394; O'Neill et al., 1989. Ecological Modelling 46, 147-163; Walsh and Dugdale, 1971. Deep-Sea Research 22, 201-236; Jamart et al., 1977. Deep-Sea Research 31, 21-37; Frost and Franzen, 1992. Marine Ecology Progress Series 83, 291-303; Yajnik and Sharada, 2003. Current Science 85(8), 1180-1189] governing the uptake of nitrate and ammonium by phytoplankton in the context of the nitrogen dynamics experiments of McCarthy et al. [1999. Nitrogen dynamics during the Arabian Sea northeast monsoon. Deep- Sea Research II 46 (8-9), 1623-1664] in the Indian Ocean. One of the relations [Yajnik and Sharada, 2003. Current Science 85(8), 1180-1189], which is based on the properties of similarity and hyperbolicity, is shown to represent the experimental observations more accurately than others. We then examine the consequences of selection of one of these kinetic relations on 3D simulations of the Indian Ocean ecosystem. The 3D simulations are based on the biological model of Fasham et al. [1990. Journal Marine Research 48, 591-639] coupled to a physical oceanographic model Saramiento et al. [1993. Global Biogeochemical Cycles 7 (2), 417-450] and are performed for three relations [Wroblewski, 1977. Journal of Marine Research 35, 357-394; O'Neill et al., 1989. Ecological Modelling 46, 147-163; Yajnik and Sharada, 2003. Current Science 85(8), 1180-1189]. The values of kinetic parameters are determined from the experiments of McCarthy et al. [1999. Deep- Sea Research II 46 (8-9), 1623-1664]. The spatial variation of annual average primary productivity, especially in the regions of high primary productivity in the northern Indian Ocean, is found to agree most favourably with SeaWiFS data for the relation [Yajnik and Sharada, 2003. Current Science 85(8), 1180-1189] based on similarity and hyperbolicity. The distribution of particle export ratios also appears to be close to present expectations.