AME 34:93-104 (2004)  -  doi:10.3354/ame034093

ABSTRACT: The role of transparent exopolymer particles (TEP) and dissolved organic carbon (DOC) for organic carbon partitioning under different CO₂ conditions was examined during a mesocosm experiment with the coccolithophorid Emiliania huxleyi. We designed 9 outdoor enclosures (~11 m³) to simulate CO₂ concentrations of estimated ŒYear 2100¹ (~710 ppm CO₂), Œpresent¹ (~410 ppm CO₂) and Œglacial¹ (~190 ppm CO₂) environments, and fertilized these with nitrate and phosphate to favor bloom development. Our results showed fundamentally different TEP and DOC dynamics during the bloom. In all mesocosms, TEP concentration increased after nutrient exhaustion and accumulated steadily until the end of the study. TEP concentration was closely related to the abundance of E. huxleyi and accounted for an increase in POC concentration of 35 ± 2% after the onset of nutrient limitation. The production of TEP normalized to the cell abundance of E. huxleyi was highest in the Year 2100 treatment. In contrast, DOC concentration exhibited considerable short-term fluctuations throughout the study. In all mesocosms, DOC was neither related to the abundance of E. huxleyi nor to TEP concentration. A statistically significant effect of the CO₂ treatment on DOC concentration was not determined. However, during the course of the bloom, DOC concentration increased in 2 of the 3 Year 2100 mesocosms and in 1 of the present mesocosms, but in none of the glacial mesocosms. It is suggested that the observed differences between TEP and DOC were determined by their different bioavailability and that a rapid response of the microbial food web may have obscured CO₂ effects on DOC production by autotrophic cells.

KEY WORDS: Emiliana huxleyi · Transparent exopolymer particles · TEP · Dissolved organic carbon · DOC · Carbon overconsumption · CO₂ · Redfield ratios · Mesocosms