Interannual variability of Si and N cycles at the time-series station KERFIX between 1990 and 1995 - A 1-D modelling study

Interannual variability of nutrients and plankton cycles were studied at th e time-series station KERFIX (50 degrees 40'S, 68 degrees 25'E) using a 1-D coupled physical-biogeochemical model that is descended from that of Ponda ven et al. (1998). At KERFIX, a high half saturation constant for silicic a cid uptake (K-Si) and a high Si/N uptake ratio are required to reproduce th e Si and N cycles. Although very high in comparison with most data from tem perate systems, these values are consistent with K-Si and Si/N uptake ratio s measured in the Indian sector of the Southern Ocean. Past and recent find ing on the role of light and iron limitation on nutrient consumption ratios might explain these "unusual" silicon uptake kinetic parameters. Compariso n of model results with observations show that the model correctly reproduc es the observed interannual variability of nutrients and plankton cycles at KERFIX between 1992 and 1995. Characteristic features of this region are a spring phytoplankton bloom of 1.0-1.5 mg Chlorophyll a m(-3) and a net exc ess of silicic acid utilisation over that of nitrate. This high silicic aci d utilisation leads to low Si concentrations in late summer and subsequent Si limitation of diatom growth. The interannual variability of production o f silicon and nitrogen predicted by the model is 1.93 +/- 0.04 mol Si m(-2) yr(-1) and 1.35 +/- 0.07 mol N m(-2) yr(-1) ( +/- SD). In parallel, the pr edicted export is 1.12 +/- 0.04 mol Si m(-2) yr(-1) and 0.06 +/- 0.01 mol N m(-2) yr(-1). It is shown that diatoms may contribute significantly to exp ort if diatom sinking is taken into account. An interannual variability of the predicted Si and N cycles is detected. This variability is associated w ith changes in the mixed layer properties, which have been documented to be linked to the Pacific El Nino Southern Oscillation or displacement of the Polar Front. (C) 1999 Published by Elsevier Science Ltd. All rights reserve d.