Intertidal macroalgae influence macroinvertebrate distribution across stress scenarios

Intertidal macroalgae can create or modify habitats, potentially influencing ecosystem dynamics by altering the abundance and distribution of species within their community. However, the engineering ability of these organisms and the relative importance of their bioengineering potential may change rapidly in response to environmental stress. To better understand how bioengineering might influence macroinvertebrate community composition, several scenarios were assessed. First, we evaluated the effect of different macroalgal assemblages composed of single-species cultures at different densities on the attenuation of temperature, irradiance and water loss. Subsequently, we assessed the effect that these modifications had on the abundance and distribution of macroinvertebrates. Results showed that intertidal macroalgae differentially ameliorated physical factors depending on the composition of macroalgal aggregations and the level of environmental stress. Higher macroalgal densities attenuated physical factors the most during daytime low tides, modulated by macroalgal species composition. Moreover, macroalgal species composition was found to influence the community composition of macroinvertebrates under certain environmental baseline conditions. During the day, invertebrates were abundant underneath high-density canopies, regardless of the macroalgal species. At night, however, invertebrates showed specificity towards particular macroalgal species and densities. Bioengineers can influence the strength and nature of interspecific interactions in a variety of ways. Here, the differences in the abundance and distribution of macroinvertebrates associated with macroalgae show that the presence or absence of a bioengineer can result in completely different outcomes for associated organisms (feeding vs. shelter), which may, in turn, have feedback implications at many trophic and spatial scales in the intertidal ecosystem.