A steady-state analysis of the 'microbial loop' in stratified systems

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Abstract
Steady state solutions are presented for a simple model of the surface mixed layer, which contains the components of the ''microbial loop'', namely phytoplankton, picophytoplankton, bacterioplankton, microzooplankton, dissolved organic carbon, detritus, nitrate and ammonia. This system is assumed to be in equilibrium with the larger grazers present at any time, which are represented as an external mortality function. The model also allows for dissolved organic nitrogen consumption by bacteria, and self-grazing and mixotrophy of the microzooplankton. The model steady states are always stable. The solution shows a number of general properties; for example, biomass of each individual component depends only on total nitrogen concentration below the mixed layer, not whether the nitrogen is in the form of nitrate or ammonia. Standing stocks and production rates from the model are compared with summer observations from the Celtic Sea and Porcupine Sea Bight. The agreement is good and suggests that the system is often not far from equilibrium. A sensitivity analysis of the model is included. The effect of varying the mixing across the pycnocline is investigated; more intense mixing results in the large phytoplankton population increasing at the expense of picophytoplankton, microzooplankton and DOC. The change from phytoplankton to picophytoplankton dominance at low mixing occurs even though the same physiological parameters are used for size fractions. The F-ratio falls abruptly at low mixing rates. Estimates of microbial food web efficiency made with this model show that bacteria are not important, a result confirmed by excluding bacteria from the system. The model therefore does not support the ''microbial loop'' hypothesis. The model solutions and parameters values are presented in full in 3 appendices.