AN IMPROVED MODEL OF CARBON AND NUTRIENT DYNAMICS IN THE MICROBIAL FOOD-WEB IN MARINE ENCLOSURES

A description of an improved dynamic simulation model of a marine encl osure is given. New features in the model are the inclusion of picoalg ae and mixotrophs; the ability of bacteria to take up dissolved inorga nic nutrients directly; and, for the phytoplankton functional groups, the inclusion of luxury uptake and the decoupling of the nutrient upta ke dynamics from carbon-assimilation dynamics. This last feature impli es dynamically variable phosphorus/carbon and nitrogen/carbon ratios. The model was calibrated with experimental results from enclosure expe riments carried out in Knebel Vig, a shallow microtidal land-locked fj ord in Denmark, and verified with results from enclosure experiments i n Hylsfjord, a deep and salinity-stratified Norwegian fjord. Both obse rvations and model simulations showed dominance of a microbial food we b in control enclosures with low productivity. In Nand P-enriched encl osures a classical food web developed, while an intermediate system wa s found in N-, P-and Si-enriched enclosures. Mixotrophic flagellates w ere most important in the nutrient-limited control enclosures where th ey accounted for 49% of the pigmented biomass and about 48% of the pri mary production. Lumping the mixotrophs in the simulation model with e ither the autotrophic or the heterotrophic functional groups reduced t otal primary production by 74%. Model-derived, time-averaged phosphoru s budgets suggested that bacteria competed with algae for orthophospha te in the control enclosure, but not in the enclosure to which N and P had been added, where bacteria functioned as net mineralisers of phos phate. In the N, P and Si enclosure, bacteria took up only 10% of the amount of orthophosphate taken up by the primary producers, passing mo st of the organic phosphorus on to their grazers, the heterotrophic na noflagellates, and mineralising only a small fraction directly. Inclus ion of luxury nutrient uptake affected the simulation of the nutrient- enriched enclosures, while the decoupling of carbon and nutrient dynam ics affected the simulation of the control enclosure. Without these 2 processes it was not possible to simulate the carbon and nutrient dyna mics in the different enclosures adequately with the same parameterisa tion.