Modelling of marine ecosystems : a viral solution to the DOC enigma

Abstract

The role and dynamics of the ocean's pool of dissolved organic carbon (DOC) in both the marine carbon cycle and marine ecosystems is not well known. A large imbalance between net export and measured loss of carbon in the upper 150m of Bermuda remains unexplained. Knowledge of the processes that control the source and rate of production of DOC in the upper ocean, as well as its subsurface remineralisation rate and lifetime are required to explain the ocean's global distribution of nutrients. May viral lysis yield the carbon required to account for the fluxes into the semi-labile DOC pool of biogeochemical cycles? The role of marine viruses in ecosystem dynamics is determined and their impact on overall biological production rates and DOC production is analyzed. Viral induced collapse of plankton blooms is investigated. This is achieved through the development of an epidemics model based on a compartmental ecosystem model of the upper ocean. Inter-compartmental flows and fluxes to the deep ocean are plotted daily. A leapfrog integration method with a timestep of two hours is used. The model is run over the period of ten years for the Bermuda Station "S" area. Two thousand simulations are analyzed. Phytoplankton primary production and bacterial production are diverted by viral lysis into the synthesis of new viruses which are inactivated by natural UV radiation and then decay into DOC slowly. The remaining cell debris of lysed hosts flow directly into the detritus and DOC pools respectively. The effect of differing levels of contact rate, inactivation rate, decay time of inactivated viruses and sinking speed of detritus on overall production rates and biomass are investigated. Results show that marine viruses act as regulators of the size and timing of plankton blooms and are important partners in bacterial trophodynamics. The stability of the ecosystem is extremely sensitive to the contact rate between virus and host cells which provides the underlying mechanism for non linear epidemics. Increased levels of the contact rate determine higher viral infectivity and increase viral lysis which forces phytoplankton and bacterial collapses. The epidemics result in a considerable growth of bacterial production and hence viral lysis which increase with the shortening in decay time and the slowing of the detritus sinking speed. Variations in the level of the inactivation rate are found to have minor effect on the overall annual production rates but have large effect on the maximum daily flow into lysis during blooms. The model results on viral production foster support for the viral source of the high-molecular weight fraction of the semi-labile DOC, identify bacterial viruses as its main contributor and show a much improved fit to the observed data in comparison to previous modelling attempts without epidemics.

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