Abstract

Biodiversity loss in aquatic ecosystems is accelerating, yet whether the effects of different biodiversity facets on biogeochemical processes are consistent or environmentally contingent remains poorly understood. Benthic invertebrate bioturbators drive nutrient exchange at the sediment–water interface of shallow aquatic ecosystems, but how sediment trophic status modulates the effects of their richness, identity, and composition on benthic–pelagic nutrient cycling is unclear. We conducted a factorial microcosm experiment using three functionally and phylogenetically distinct species—Campsurus melanocephalus (Ephemeroptera), Heteromastus similis (Polychaeta), and Heleobia australis (Gastropoda)—assembled as monocultures, bicultures, and tricultures across a gradient of sediment organic matter concentration ([OM]) representative of natural variability in a neotropical coastal lagoon. Species richness produced a consistent, saturating enhancement of NH3 fluxes that was independent of sediment trophic status, while species identity strongly differentiated monoculture effects in a pattern that was likewise stable across the [OM] gradient. In contrast, although biculture compositions did not differ from one another across the [OM] gradient, the overall magnitude of biculture-mediated NH3 fluxes increased with sediment trophic status, suggesting that multispecies assemblage effects are more responsive to environmental context than those of single species. These findings reveal that biodiversity facets diverge not only in how strongly they affect ecosystem functioning, but also in how sensitive their effects are to environmental variation. We therefore conclude that predicting benthic–pelagic nutrient exchange in shallow coastal ecosystems requires considering both the biological structure of bioturbator assemblages and the organic matter context of the sediment.

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