Expansive covers of turf-forming algae on human-dominated coast: the relative effects of increasing nutrient and sediment loads

Abstract

Turf-forming algae form more extensive habitat on subtidal rock adjacent to urban than non-urban coast of South Australia. This pattern is frequently observed on the world’s temperate coasts and is variously considered to be a result of enhanced concentration of nutrients or rates of sediment deposition on urban coasts. We experimentally tested which of three components of environmental change (increased nutrients in water, increased nutrients in sediments and increased sediment deposition) best explain the expansive covers of turf-forming algae on urban coasts. All three treatments had independent and positive effects on the percentage cover of turf-forming algae. The addition of nutrients from the water column had the largest influence (ω 2=0.55), which was more than six times greater than the effect of nutrients added to sediments (ω 2=0.08). An increase in rate of deposition of sediments had substantial effects (ω 2=0.35), which were about one third less than those of water-borne nutrients. Importantly, the combined effect of all three treatments caused a 77% increase in percentage cover of turf-forming algae, which is comparable to the observed difference in covers between urban and non-urban coast in South Australia (93%). These results suggest that human activities that reduce water quality in both nutrient and sediment loads account for major change observed on human-dominated coasts. Despite this knowledge, we still lack complete information on the mechanisms that switch the primary subtidal habitat from canopy-forming algae to turf-forming algae on human-dominated coasts.