Conan, Pascal, Morten Søndergaard, Theis Kragh, Frede Thingstad, Mirielle Pujo-Pay, Peter J. le B. Williams, Stiig Markager, Gustave Cauwet, Niels Henrik Borch, Dylan Evans, and Bo Riemann
Limnol. Oceanogr., 52(2), 2007, 753–765
We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11-m3 marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 µg L-1, an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (-Si). Addition of inorganic nutrients in a N: P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom-dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N-limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was recovered as new DON in the N-replete bags, while the DON production was very low during N limitation. The community structure had no effects on nitrogen partitioning. Diatom dominance (+Si bags) resulted in P sequestration to particles and a constant low net production of dissolved organic phosphorus (DOP) across the nutrient gradient. The production of DOP was low in the P-limited (-Si) bags; however, with a surplus of inorganic P, most of the assimilated P (74% to 85%) was recovered as new DOP. The consequence was a huge range in stoichiometric ratios for newly produced dissolved organic matter (DOM). With N limitation, the C:N ratio of new DOM was from 40 to 100, but it was below 40 under N-replete conditions. The C: P ratio of new DOM in the -Si bags traced the P availability, and values approached 500 in P-deficient bags to values between 17 and 58 in the P-replete bags. The C: P ratio of new DOM in the +Si bags was about 300 at all dosing regimes. Consequently, the range in N: P ratios was also large, with values from below 1 to about 30. Carbon-rich DOM in oceans and coastal waters is not necessarily a function of a slow diagenetic ‘‘maturation’’ process but can be produced almost immediately. Both the nutrient regime and phytoplankton community composition affected the production and composition of new DOM in this experiment.