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 ( Z eu )-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 Z eu ( Morel and Berthon, 1989; henceforth MB) and the optimal Chl a-normalized PP ( p opt B ). The sub-model estimates of p opt B and Z eu with in situ p opt B and Z eu showed significant improvement, accounting for 84% of the variability and causing less overestimation. Z eu was the most important parameter influencing the modeled IPP variation in Ariake Bay. Previous research suggested that the Z eu model, which was based on surface Chl a, overestimated in situ Z eu by a factor of 2–3, resulting in weak correlation between the modeled and in situ IPP. The Z eu sub-model was not accurate in the present study area because it was basically developed for clear (case 1) waters. A better estimation of Z eu could be obtained from the in situ remote sensing reflectance ( R rs ) using a quasi-analytical algorithm (QAA) in this turbid water ecosystem. Among the parameters of PP models, p opt B is conventionally considered the most important. However, in this study p opt B was of secondary importance because the contribution of p opt B to the variation in modeled IPP was less than the contribution of Z eu . The modeled and in situ p opt B 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 R rs data. Incorporating the QAA-based Z eu and the optimized Chl a and constant (median) p opt B 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 Z eu and Chl a and constant (median) p opt B , it provided realistic estimates of IPP in the turbid water ecosystem of Ariake Bay.