Evolution of Phytoplankton Pigments in Mesocosm Experiments

Changes in pigments, biominerals and particulate organic carbon (POC) were investigated in nutrient controlled mesocosms dominated by diatoms andEmiliania huxleyi. A rapid increase in pigments was observed in the first 3–6 days of the experiment after the mesocosms were enriched with nitrate, phosphate and silicate (N/P/Si), or nitrate and phosphate only (N/P). Pigment concentrations then declined steadily to Day 17–19, after which a secondary increase was again monitored in the final 10 days. High concentrations of fucoxanthin were measured in all the mesocosms that were sampled, and the data indicated that theE. huxleyicells were producing significant levels of fucoxanthin in addition to hexanoyloxyfucoxanthin. There were concomitant increases and decreases in POC and CaCO3, and in SiO2in the N/P/Si-enriched mesocosms. Phytoplankton-carbon/POC ratios showed that the phytoplankton accounted for 60–90% of the POC during exponential growth, whereas this proportion was <50% in the decline phase. High fucoxanthin/hexanoyloxyfucoxanthin (fuc/hex) ratios in the N/P/Si-enriched mesocosms suggested that a significant fraction of the biomass increase was due to diatoms, while the smaller ratios in the N/P-enriched mesocosms were associated with the dominance of coccolithophores. Mean rates of increase in pigments, POC and biominerals were estimated for each mesocosm and compared with the rate of uptake of nutrients.