Modification of the vertically generalized production model for the turbid waters of Ariake Bay, Southwestern Japan

Abstract

The vertically generalized production model (VGPM), which was designed for open ocean waters (Behrenfeld and Falkowski, 1997a; henceforth BF), was evaluated using in situ measurements of primary productivity (PP) in the characteristically turbid coastal waters of Ariake Bay, southwestern Japan, to develop a regionally modified version of the model. The euphotic depth (Zeu)-integrated PP (IPP) calculated from the VGPM using in situ chlorophyll a (Chl a) and sea surface temperature (SST) was significantly overestimated (by factors of 2–3), but 52% of the observed variability was explained. The weak correlation could have partially resulted from overestimations by the sub-models embedded in the original VGPM model for estimation of Zeu (Morel and Berthon, 1989; henceforth MB) and the optimal Chl a-normalized PP (). The sub-model estimates of and Zeu with in situ and Zeu showed significant improvement, accounting for 84% of the variability and causing less overestimation. Zeu was the most important parameter influencing the modeled IPP variation in Ariake Bay. Previous research suggested that the Zeu model, which was based on surface Chl a, overestimated in situ Zeu by a factor of 2–3, resulting in weak correlation between the modeled and in situ IPP. The Zeu sub-model was not accurate in the present study area because it was basically developed for clear (case 1) waters. A better estimation of Zeu could be obtained from the in situ remote sensing reflectance (Rrs) using a quasi-analytical algorithm (QAA) in this turbid water ecosystem. Among the parameters of PP models, is conventionally considered the most important. However, in this study was of secondary importance because the contribution of to the variation in modeled IPP was less than the contribution of Zeu. The modeled and in situ were weakly correlated with 50% of the data points that overestimated the in situ values. The estimation of Chl a was improved by optimizing the Chl a algorithm with in situ Rrs data. Incorporating the QAA-based Zeu and the optimized Chl a and constant (median) value led to improved performance of the VGPM for the study area. Thus, even though the VGPM is a global open ocean model, when coupled with turbid water algorithms for Zeu and Chl a and constant (median) , it provided realistic estimates of IPP in the turbid water ecosystem of Ariake Bay.