9 – Interstitial Ecology

This chapter provides an overview of the interstitial ecology of sandy shores. In contrast to the wave-swept surface sand inhabited by most of the macrofauna, the lacunar interstitial system is complex and three-dimensional, often having great vertical extent in the sand body. This porous system averages about 40% of the total sediment volume. Its inhabitants include small metazoans forming the meiofauna, as well as protozoans, bacteria, diatoms, and other microorganisms. On most beaches, the interstitial fauna is rich and diverse, even exceeding the macrofauna in biomass in some cases. On sandy beaches there may be as many as 25 interstitial species for every macrofauna species. The reason for this diversity is the greater stability and complexity of the interstitial habitat. The interstitial environment experiences a continuum of conditions ranging between chemically and physically controlled extremes. The chemical extreme is represented by sheltered beaches of fine sand, where water filtration through the sand is negligible and organic inputs may be high. The physical extreme occurs on coarse-grained steep beaches subject to strong wave action, where huge volumes of water are flushed through the beach face and drain rapidly back to the sea because of high-sand permeability. The interstitial system here is always highly oxygenated and drains out during the low tide.