Diversity of habitats and bacterial communities support landscape-scale multifunctionality differently across seasons

In this study, we demonstrate how changes in the diversity of habitat and bacterial communities affect landscape multifunctionality. Habitat diversity may beget species diversity by increasing niche availability and resource complementarity. Species diversity, in turn, generally promotes multifunctionality, i.e. the simultaneous performance of multiple ecosystem functions. However, the relationship between habitat diversity and functioning remains to be explicitly explored. In order to test the relationship between habitat diversity and multifunctionality we constructed experimental landscapes of four different habitats common in shallow-water sediment ecosystems: cyanobacterial mats, Ruppia maritima meadows, silty mud and sandy beach. We manipulated the diversity of these habitats over three consecutive seasons and measured bacterial diversity, benthic microalgal diversity and four functions related to marine nitrogen cycling (gross primary production, nitrogen fixation, denitrification and uptake of dissolved inorganic nitrogen). Our results showed that higher habitat and bacterial diversity, but not benthic microalgal diversity, increased landscape multifunctionality. However, the relative importance of habitat and bacterial diversity varied with season. Habitat diversity was generally the strongest driver, affecting multifunctionality directly in summer and indirectly via bacterial diversity in autumn. In spring, neither of the two aspects of diversity was important. Our study demonstrates the importance of considering temporal differences in both habitat and species diversity for landscape multifunctionality, and the importance of direct and indirect effects in mediating ecosystem functions. Habitat homogenization in concert with loss in biodiversity can thus be a driving force of declining ecosystem functioning and the services they underpin.