Effects of varied nitrate and phosphate supply on polysaccharidic and proteinaceous gel particle production during tropical phytoplankton bloom experiments

Abstract. Gel particles such as the polysaccharidic transparent exopolymer particles (TEP) and the proteinaceous Coomassie stainable particles (CSP) play an important role in marine biogeochemical and ecological processes like particle aggregation and export, or microbial nutrition and growth. So far, effects of nutrient availability or of changes in nutrient ratios on gel particle production and fate are not well understood. The tropical ocean includes large oxygen minimum zones, where nitrogen losses due to anaerobic microbial activity result in a lower supply of nitrate relative to phosphate to the euphotic zone. Here, we report of two series of mesocosm experiments that were conducted with natural plankton communities collected from the eastern tropical North Atlantic (ETNA) close to Cape Verde in October 2012. The experiments were performed to investigate how different phosphate (experiment 1, Varied P: 0.15–1.58 μmol L⁻¹) or nitrate (experiment 2, Varied N: 1.9–21.9 μmol L⁻¹) concentrations affect the abundance and size distribution of TEP and CSP. In the days until the bloom peak was reached, a positive correlation between gel particle abundance and Chl a concentration was determined, linking the release of dissolved gel precursors and the subsequent formation of gel particles to autotrophic production. After the bloom peak, gel particle abundance remained stable or even increased, implying a continued partitioning of dissolved into particulate organic matter after biomass production itself ceased. During both experiments, differences between TEP and CSP dynamics were observed; TEP were generally more abundant than CSP. Changes in size distribution indicated aggregation of TEP after the bloom, while newly formed CSP decomposed. Abundance of gel particles clearly increased with nitrate concentration during the second experiment, suggesting that changes in [DIN] : [DIP] ratios can affect gel particle formation with potential consequences for carbon and nitrogen cycling as well as food web dynamics in tropical ecosystems.