Importance of different trophic pathways in a nearshore benthic community under upwelling and downwelling conditions

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Abstract
A simulation model is used as a tool to investigate possible ecological effects of upwelling and downwelling water transport on trophic relationships in a kelp bed off the Cape Peninsula, South Africa. The model is based on many years of field and experimental data of energy fluxes between trophic compartments in a community dominated by filter feeders which consume phytoplankton, detritus derived from seaweeds and animal feces. The model is simple, yet incorporates feedback loops via feces and control by predators and is used to calculate the relative amounts of food that filter feeders require from the different sources available to balance their energy needs and maintain a steady biomass. When modeled as a closed system, steady state is maintained by major flows through a feedback loop via feces and bacteria in a detrital food chain. This is analogous to semi-closed systems such as certain estuaries and salt marshes. Under continuous downwelling conditions, phytoplankton from the rich Benguela pelagic zone enters the kelp bed and may form up to 93% of filter feeder food with rapid water exchange. This is similar to many phytoplankton-based nearshore systems. Under continuous upwelling conditions, consumer biomass is limited by food availability since detritus is exported and no phytoplankton is imported. This has been confirmed by observations for which there are few, if any, other published accounts. Under a realistic pulsing regime simulating conditions in the southern Benguela region, consumer biomass reaches steady-state oscillation within 7 yr, increasing in biomass during winter and decreasing during the summer upwelling season. Sensitivity analyses show that pulse duration has less effect than the rate of water transport. The role of a feedback loop in maintaining large animal biomasses in a detrital food chain and the importance of advective water transport in an open ecosystem is demonstrated. The damping control on the system by predators, in this instance mainly the exploited rock lobster, Jasus lalandii, is also shown.

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