Ecosystem modelling expands population modelling by integrating the biological and
physical components of the environment into a single interactive system (Smith & Smith
1998; Pastor 2008). This is consistent with the process-functional approach in ecology.
The interaction of living (biological component) and non-living (physical) components
occurs through nutrient flows. All nutrients flow from the non-living to the living and
back to the non-living components of the ecosystem in a circular path known as a biogeochemical
cycle. This process is called internal cycling and represents a recycling of
nutrients within the ecosystem. It is an essential feature of all ecosystems (Smith & Smith
1998). Animals and other consumers gain their nutrients by eating producer organisms
or each other. When an organism dies, its remains are broken down by decomposers. The
components of their cells and tissues are utilized by decomposers and later returned to
the environment and recycled (Karleskint 1998). All biological entities require nutrients
(matter) for their construction and energy for their activities (Begon et al. 2006). Therefore,
nutrient and energy flows play a vital role in marine ecosystems. Naturally, each
unit of energy can be used only once, whereas chemical nutrients can be used again and
repeatedly recycled as the building blocks of biomass (Begon et al. 2006)