Growth limitation and growth rates of (pico)phytoplankton in the banda sea during two different monsoons

Abstract

The types of growth limitation and growth rates of phytoplankton and picocyanobacteria in the Banda Sea were examined in August 1984 (southeast monsoon, upwelling, expected nutrient-rich) and February–March 1985 (northwest monsoon, down-welling, expected nutrien-poor). The sensitive indicator used to discriminate between the different types of phytoplankton growth limitations was the ratio of the initial saturated uptake rate (Vm) and the specific nutrient uptake rate (q). In addition, the ratio of carbohydrates and proteins was used. Specific growth rate was assessed from the ratio of Vm of the non-limiting nutrient and cellular nutrient content. In August 1984 the phytoplankton was not limited by nutrients, but most probably by light, except at one station (St. 53, to the south of Irian Jaya) where the surface phytoplankton was nitrogen-limited and growing at a very low rate. During February 1985 the surface phytoplankton was nitrogen-limited. In the middle part of the Banda Sea and N-limitation extended to deep water layers. However, heterogeneity in phytoplankton growth-limitation with depth was also found at other stations, where the deep small-sized photoautotrophs (mainly red-pigmented picocyano-bacteria) were growing at considerably higher rates, which often exceeded the growth rate of larger-sized species by a factor of three. These results support the view that picoplankton forms an important component of primary productivity in tropical waters. The assessment of growth limitation by the use of physiological indicators, rather than by the use of nutrient concentrations in seawater, will be discussed.