Examining the effects of nutrients on the composition and size of phytoplankton using unaltered deep-sea waters

Effects of nutrients on the size and composition of phytoplankton and the resulting changes in ecosystem structure were investigated using incubations of unaltered deep-sea waters from 400 m and 700 m depths of Suruga Bay Japan. This new approach allowed us to observe the succession of phytoplankton communities from conditions of high to low nitrogen availability and explore the effects of N source and availability on the composition and size of diatoms under low grazing pressure. Diatoms dominated the phytoplankton biomass throughout the duration of experiments in both incubations. However, among diatoms, the genus Thalassiosira dominated the abundance and biomass when nutrients were replete, and the genus Chaetoceros dominated the abundance and biomass when nitrate became depleted. Our results show that diatoms can dominate other phytoplankton groups not only under nutrient replete conditions, but also (particularly for Chaetoceros) under low concentrations of recycled nitrogen if silica and phosphate are not deficient. Diatoms < 15 μm dominated the biomass in the 700 m incubation whereas those > 15 μm dominated the biomass in the 400 m incubations during the formation of blooms under nitrate-replete conditions. After nitrate depletion, diatoms > 15 μm dominated the biomass in both incubations. Our results showed that availability of N determined the composition of diatoms if Si and P are not deficient. However, for size the results showed that N source and availability did not determine the size of phytoplankton. Our results also showed that the molar ratio of Si:N uptake was < 1 for diatoms under nutrient replete conditions. Further the Si:N drawdown ratio decreased with the increasing abundance of smaller diatoms (< 15 μm).