Macrofaunal Functional Diversity Provides Resilience to Nutrient Enrichment in Coastal Sediments
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The degradation of ecosystems is often associated with losses of large organisms and the concomitant losses of the ecological functions they mediate. Conversely, the resilience of ecosystems to stress is strongly influenced by faunal communities and their impacts on processes. Denitrification in coastal sediments is a process that may provide ecosystem resilience to eutrophication by removing excess bioavailable nitrogen. Here, we conducted a large-scale field experiment to test the effect of macrofaunal community composition on denitrification in response to two levels of nutrient enrichment at 28 sites across a biologically heterogeneous sandflat. After 7 weeks of enrichment, we measured denitrification enzyme activity (DEA) along with benthic macrofaunal community composition and environmental variables. We normalised treatment site specific DEA values by those in ambient sediments (DEACN) to reveal the underlying response across the heterogeneous landscape. Nutrient enrichment caused reductions in DEACN as well as functional changes in the community; these were both more pronounced under the highest level of nutrient loading (on average DEACN was reduced by 34%). The degree of suppression of DEACN following moderate nitrogen loading was mitigated by a key bioturbating species, but following high nitrogen loading (which reduced the key species density) the abundance and diversity of other nutrient processing species were the most important factors alleviating negative effects. This study provides a prime example of the context-dependent role of biodiversity in maintaining ecosystem functioning, underlining that different elements of biodiversity can become important as stress levels increase. Our results emphasise that management and conservation strategies require a real-world understanding of the community attributes that facilitate nutrient processing and maintain resilience in coastal ecosystems.
Keywordsdenitrification benthic community nutrient processing eutrophication enzyme assay intertidal functional traits sandflat
We thank all those involved in field sampling and laboratory work, especially Barry Greenfield, Sarah Hailes, Lisa McCartain, Katie Cartner, Kelly Carter, Rosalie Carter, Carsten Dormann, Chris Eager, Laura Hines and Rebecca Gladstone-Gallagher. Special thanks to Judi Hewitt for advice with statistical analysis and two anonymous reviewers whose constructive comments improved the manuscript. This work was funded by the Marsden Fund, Royal Society of New Zealand (NIW1102) to SFT, a Marie-Curie International Outgoing Fellowship (FP7-PEOPLE-2011-IOF) to CK and CO1x1515 4.2.1 to SFT and CAP. EJD was funded by a University of Waikato Doctoral Scholarship. These funders had no role in the study design, interpretation or writing of this manuscript.
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Conflict of interest
The authors declare that they have no conflict of interest.
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