No abstract

Altered nutrient cycles and consumer populations are among the top anthropogenic influences on ecosystems. However, studies on the simultaneous impacts of human-driven environmental alterations on ecosystem functions, and the overall change in system multifunctionality are scarce. We used estuarine tidal flats to study the effects of changes in herbivore density and nutrient availability on benthic microalgae (diversity, abundance and biomass) and ecosystem functions (N2-fixation, denitrification, extracellular polymeric substances -EPS- as a proxy for sediment cohesiveness, sediment water content as a proxy of water retention capacity and sediment organic matter). We found consistent strong impacts of modified herbivory and weak effects of increased nutrient availability on the abundance, biomass and diversity of benthic microalgae. However, the effects on specific ecosystem functions were disparate. Some functions were independently affected by nutrient addition (N2-fixation), modified herbivory (sediment organic matter and water content), or their interaction (denitrification), while others were not affected (EPS). Overall system multifunction remained invariant despite changes in specific functions. This study reveals that anthropogenic pressures can induce decoupled effects between community structure and specific ecosystem functions. Our results highlight the need to address several ecosystem functions simultaneously for better ecosystem characterization and management.

In coastal systems, benthic microalgae are important primary producers that significantly contribute to global primary production. Microphytobenthic (MPB) community structure and the ecosystem functions that it mediates are modulated by biotic and abiotic factors. Through burrowing activity, different types of bioturbators can impact MPB in different ways. Through directed sampling and field experiments performed at the Mar Chiquita coastal lagoon (Argentina), we evaluated the effect of regenerative bioturbation (continuous digging and maintenance of burrows, with sediment transfer from depth to surface) by the intertidal burrowing crab Neohelice (Chasmagnatus) granulata on MPB. We compared sediment properties and MPB attributes between natural burrowed and non-burrowed areas. Moreover, we experimentally manipulated regenerative bioturbation to evaluate if MPB composition and primary production are altered by crab burrowing activity. Field sampling showed that MPB and sediment properties differed between natural burrowed and non-burrowed areas. Experimental results indicated that regenerative bioturbation by N. granulata increased sediment oxygen concentration at each measured depth and also changed MPB composition (due to the reduced total abundance of cyanobacteria), but nearly doubled primary production in comparison with plots without bioturbation. Thus, the net effects of regenerative bioturbation were positive for MPB productivity, reinforcing the idea that this kind of bioturbation is an important biological force that enhances primary production in intertidal systems.