Sand beach energetics: An ecosystem approach towards a high energy interface

Abstract

General results of a study of energetics on open sandy beaches in South Africa are presented. These sand beaches are considered to interact with adjacent terrestrial environments via the sand dune system and with the sea via the surf zone. A food web is given for the macrofauna showing all known interactions from the supply of food material to the beach, mainly from the sea, to the removal of the macrofauna by birds and fishes. An energy circuit diagram is presented quantifying the main energy flows through this system of filter feeders and scavengers. The interstitial biota of these beaches is considered separate from the macrofauna and consists of bacteria, protozoa and meiofauna feeding on dissolved and particulate organics flushed into the beaches by wave and tide action. Interstitial energy flow and nutrient cycling rates are quantified in an energy circuit diagram. It is suggested that nutrients regenerated by this latter system in the intertidal and surf zone, as well as by the activities of the macrofauna, have sufficient residence times in the surf zone to cause blooms of surf zone phytoplankton which in turn are the main food for the intertidal filter feeders. In this respect the beach and surf zone may represent a more closed system than previously thought. A combined energy circuit diagram is given depicting the beach and surf zone as an ecosystem with the surf zone phytoplankton the producers, the macrofauna the consumers and the interstitial fauna the decomposers. Main imports and exports as well as the consequences of this ecosystem approach are discussed.