Vertical gradients of meiofauna and bacteria in subtidal sandy sediments from two high-energy surf zones in Algoa Bay, South Africa

Abstract

Subtidal sediments off two South African sandy beaches of different wave exposure were sampled using SCUBA and long cores. Kings Beach had fine, well sorted sand and Sundays River beach had medium, moderately well-sorted sand decreasing in particle size offshore. The sand from these beaches had a high content of CaCO3. Subsieves ranged from 2,1 to 7,7 per cent. Organic matter was in the range 1,5–2,6 per cent and porosity 21,6–33,9 per cent. Permeability was correlated with grain size. The less exposed sediments of Kings Beach had much higher chlorophyll a and phaeopigment values, but supported a poorer benthic diatom population than did the sediments of the Sundays River beach. Nutrient levels in both the sea and the sediment were high. The pH and oxygen saturation decreased with depth into the sediment, but no anoxic conditions were encountered. Meiofaunal biomass, to a depth of 60 cm, averaged 1 119 and 656 mg·m−2 at Kings and Sundays River beaches respectively. Nematodes dominated virtually throughout, whereas harpacticoid copepod numbers were low. Bacterial numbers ranged from 12 × 106 to 130 × 106·cm−3, and at both beaches increased in an offshore direction. Bacterial biomass averaged 6 197 and 5 234 mg·m−2 at Kings and Sundays River beaches respectively. Significant numbers of meiofauna and bacteria were still encountered in the 50–60 cm depth layer and it was therefore concluded that use of long cores when sampling in high energy systems is essential. © 1985 Taylor & Francis Group, LLC.