THE INTERSTITIAL ENVIRONMENT OF SANDY BEACHES

The interstitial system of sandy beaches is lacunar and has its dimens ions defined by the sand granulometry. It can be described by features such as pore size, porosity, permeability, and water content. The mos t important process occurring in this system, water filtration, is dri ven by inputs of freshwater from groundwater discharge, and inputs of seawater by tides, wave run-up, and subtidal wave pumping. Reflective beaches have seawater input effected mainly by waves; they filter larg e water volumes with short residence times. Dissipative beaches displa y the opposite patterns, slowly filtering small volumes input by tides . Flow patterns and their effects on interstitial climate are describe d. The water table of the beach moves in response to groundwater disch arge, tides, and waves and influences erosion/accretion processes on t he beach face: a high water table promotes erosion. A series of moistu re zones can be recognised from the dry surface sand at upper tide lev els, to permanently saturated sand below the low tide water table, nam ely: a stratum of dry sand, a stratum of retention, a stratum of resur gence, and stratum of saturation. Interstitial chemistry is briefly de scribed in terms of salinity changes, organic loads, oxygen content, a nd nutrient cycling. It is concluded that the interstitial environment of sandy beaches spans a continuum between physically and chemically controlled extremes: the former condition occurs on coarse sand reflec tive beaches, which experience low organic inputs and high filtration rates of large water volumes - resulting in powerful hydrodynamic forc es; the latter occurs on dissipative beaches of fine sand, which are s ubject to high organic inputs and low filtration volumes - resulting i n stagnation and steep vertical chemical gradients. Many intermediate situations occur and these are more favourable to interstitial life th an either of the extremes.