Interactive effects of sedimentation and microtopography on the abundance of subtidal turf-forming algae

We quantified the percentage cover of turf-forming algae among locations spanning > 1000 km of continuous South Australian coastline and found that they formed one of the most extensive subtidal habitats on reefs devoid of macroalgae. We then experimentally tested the hypothesis that sediment deposition and microtopography of substratum interact to control turf alga abundance (percentage cover and biomass). Both microtopography (topographically simple and complex) and sedimentation (natural and reduced) were manipulated orthogonally to test their independent and interactive effects on subtidal turf-forming algae. This field experiment demonstrated that reduced rates of sedimentation have positive effects on the percentage cover and biomass of turf-forming algae (Feldmannia spp.) and that a very small increase in topographic complexity (microtopography) can have large positive effects (> 36% increase) on the biomass of turf-forming algae under conditions of reduced sedimentation. These results support new models that suggest differences in microtopography may account for observed variation in the effects of sedimentation on the abundance of turf-forming algae. Importantly, heavy rates of sediment deposition can negate the positive effects of greater topographic complexity at fine spatial scales (i.e. microtopography). These results raise serious concerns about gaps in our knowledge of the effects of sedimentation on turf-forming algae and its consequences for current declines in the diversity and cover of algae in disturbed coastal habitats.