Accumulation of degradable DOC in surface waters: Is it caused by a malfunctioning microbial loop?

Recent literature indicates that dissolved organic carbon (DOC) may accumulate in productive surface waters. Such accumulation will allow export of DOC to the aphotic zone by diffusion and downwelling. As an alternative to models based on low degradability, we here propose a mechanism where bacterial carbon consumption is restricted due to food web mechanisms controlling both growth and biomass of the bacteria: growth rate is kept low by bacteria-phytoplankton competition for mineral nutrients, and biomass is kept low by bacterial predators. With such a mechanism, otherwise degradable material may accumulate and become subject both to chemical transformation and vertical transport. The steady-states of a model describing the interactions between heterotrophic bacteria, phytoplankton, and bacterivorous protozoa is used to explore how the balance between DOC production and consumption shifts along a gradient from oligotrophy to eutrophy.