The dependence of intertidal consumers on kelp-derived organic matter on the west coast of South Africa

Abstract

In this study we explored the trophic relationships between intertidal and subtidal ecosystems of a rocky-shore community. We selected the most abundant intertidal primary consumers, principally invertebrate grazers and filter-feeders, and investigated their trophic connections with in situ intertidal production (internal) and/or subtidal kelp production (external). Using two stable isotopes (nitrogen and carbon), we examined the degree to which filter-feeders depend trophically on kelp detritus or pelagic phytoplankton, and to which intertidal grazers depend on subtidal kelps or intertidal micro- and macroalgae. The filter-feeders used kelp-derived detritus as their main source of organic carbon and nitrogen. Similarly, two abundant and highly specialized grazers also used subtidal kelps as their main source of carbon and nitrogen, although other more generalized intertidal grazers relied mainly on the in situ epilithic algal production. We determined the extent to which kelp detritus contributes to the total particulate organic matter (POM) on exposed and sheltered habitats during different tidal phases. Kelp-derived detritus represented more than 65% of POM, being consistently high all year round and during both high and low tides, but was particularly high on exposed shores. Phytoplankton was seldom found and contributed no more than 6% of total intertidal POM. We conclude that kelp-derived detritus represents a steady source of food for filter-feeders, upon which they can rely all year round. A short-term experiment revealed that the water turnover on exposed shores was about seven times greater than on sheltered shores; the consequent increase in the supply of food may explain why filter-feeder biomass is high on wave exposed shores. Finally, a simple steady-state model was built to simulate mussel standing stock (biomass per unit area) as a function of food supply and water turnover. The model predicts significantly lower mussel standing stock on sheltered than exposed shores. We conclude that replenishment of food by wave action is sufficient to explain the observed differences of filter-feeder biomass between exposed and sheltered shores. It is also concluded that subtidal macrophyte production greatly influences the structure of intertidal rocky-shore communities on the west coast of South Africa.