Ecology and energetics of slacks in the Alexandria coastal dunefield

This paper summarises information on the structure and function of dune slacks as discrete ecosystems within the Alexandria coastal dunefield. A typical slack, with a total area of 1900 m² and floor area of 8833 m², harbours four dominant species of pioneer plants in a succession spanning 5 years, corresponding to the interval between a section of slack floor being uncovered by a retreating dune and smothered by an advancing dune. Vegetation biomass totals 221.1 g m⁻² and detritus mass 40.1 g m⁻², with 31% and 3% above ground, respectively. Annual plant production is estimated at 190 g m⁻² using a combination of theoretical calculations and vegetative growth estimates. Animal biomass has been quantified for interstitial meiofauna at 1.0 g m⁻² and all arthropod and vertebrate macrofauna at 0.2 g m⁻². Based on estimated decomposition coefficients of 0.5 year⁻¹ and theoretical consumption estimates for the fauna, 14% of plant production is grazed directly (93% by plant parasitic nematodes, 7% by macrofauna) and 86% enters the detritus pool. Of the latter, 2% is consumed by macrofauna detritivores, 63% is decomposed by the interstitial fauna and 35% accumulates to add to the detritus pool. Vegetation succession and production in the slack thus results in a net build-up of organic matter in the sand. This is finally decomposed when the slack is smothered by an advancing dune ridge, thus maintaining a long term equilibrium level of soil organics. These slacks require minimal management as long as free dune movement and limited human access are maintained.