Water filtration through reflective microtidal beaches and shallow sublittoral sands and its implications for an inshore ecosystem in Western Australia

Abstract

Volumes of seawater filtered through the intertidal zone were measured on three modally reflective microtidal beaches in Western Australia. The filtered volumes were large, 19 m 3 m −1 day −1 and 73 m 3 m −1 day −1 on two ‘clean’ beaches but only 0·4 m 3 m −1 per tidal cycle on a beach covered in kelp and seagrass wrack. The mean residence times of this water in the interstitial system and its percolation paths were both short, 1–7 h and 2–5 m respectively. Water input was greater across a beach cusp horn than across a cusp embayment. Most input occurred in the upper swash zone where the water table was less than 20 cm deep. Tidal variations in input volumes were evident even with tide ranges of only 20 cm. The inshore zone off these beaches filters on average 0·07 m 3 m −2 day −1 at an average depth of 5·5 m under 0·4 m waves of 6·5 s duration. The importance of these procedures in the mineralization of organic materials and the regeneration of nutrients for an inshore ‘lagoon ecosystem’ is estimated and discused.