Production and decomposition of new DOC by marine plankton communities: carbohydrates, refractory components and nutrient limitation

The accumulation and biodegradation of dissolved organic carbon (DOC) and dissolved and particulate combined neutral sugars (DCNS, PCNS) were followed during a period of 22 days in experimental marine phytoplankton incubations. Five different growth regimes were established in 11 m(3) coastal mesocosms to test whether an alternate Redfield ratio with either N or P depletion and +/- diatom dominance would induce accumulation of refractory DOC (RDOC) and DCNS. The highest accumulation of DOC, DCNS and PCNS was found in the diatom dominated mesocosms. Sixteen percent of the newly accumulated DOC in the mesocosms with diatoms dominating could be explained by DCNS, while only 6% was explained in the mesocosms with few diatoms. PCNS composition was similar in all mesocosms and with dominance of glucose and mannose, while DCNS were more evenly distributed with the following mole percentages fucose 15, rhamnose 14, arabinose 6, galactose 27, glucose 20 and mannose 18%. The DCNS composition did not reflect the PCNS composition at any time during the experiment. Accumulated DCNS were quickly degraded and only 1% of the new RDOC was explained by DCNS. RDOC accumulated after day #17 in the two mesocosms driven towards the most severe P limitation both with and without silicate. This shows that RDOC can be produced directly by the phytoplankton or indirectly in food web processes during the later stages of a bloom where the phytoplankton is P limited.