EVOLUTION OF PHYTOPLANKTON PIGMENTS IN MESOCOSM EXPERIMENTS

Changes in pigments, biominerals and particulate organic carbon (POC) were investigated in nutrient controlled mesocosms dominated by diatom s and Emiliania 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 phospha te 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 the E. huxley i cells were producing significant levels of fucoxanthin in addition t o hexanoyloxyfucoxanthin. There were concomitant increases and decreas es in POC and CaCO3, and in SiO2 in the N/P/Si-enriched mesocosms. Phy toplankton-carbon/POC ratios showed that the phytoplankton accounted f or 60-90% of the POC during exponential growth, whereas this proportio n was < 50% in the decline phase. High fucoxanthin/ hexanoyloxyfucoxan thin (fuc/hex) ratios in the N/P/Si-enriched mesocosms suggested that a significant fraction of the biomass increase was due to diatoms, whi le the smaller ratios in the N/P-enriched mesocosms were associated wi th the dominance of coccolithophores. Mean rates of increase in pigmen ts, POC and biominerals were estimated for each mesocosm and compared with the rate of uptake of nutrients. (C) 1998 Academic Press Limited.