The algal addition treatment worked according to intentions. No algae colonized control plots, addition plots appeared to contain the same biomass that was added, and we documented clear differences in algal-cover among treatment at the end of the experiment (Table 1; SNK comparisons; control = 0% ± 0 SE < Low = 12% ± 1 SE < high = 35% ± 4 SE). Algal additions significantly decreased the distance to the sulphide horizon (Table 1, Fig. 1A; sulphide horizon of control > low = high) and seagrass leaf densities (Table 1, Fig. 1B; control > low > high), where low and high algal abundances caused ca. 20% and 40% reduction in leaf density, respectively. Algae also changed the multivariate invertebrate community structure (Table 1); control and low treatments and control and high treatments were significantly different (t = 2.48 and 2.05, respectively, p < 0.002) whereas communities of low and high algal treatments were similar (t = 1.25, p = 0.184). Species richness increased with algal density (Fig. 2A, control = low < high; note that richness was, non-significantly, higher in the low than the control plots), whereas evenness decreased with algal density (Fig. 2B, control > low > high). However, algal addition did not affect diversity (Table 1, Fig. 2C).