Experimenting with ecosystem interaction networks in search of threshold potentials in real-world marine ecosystems.

Thresholds profoundly affect our understanding and management of ecosystem dynamics, but we have yet to develop practical techniques to assess the risk that thresholds will be crossed. Combining ecological knowledge of critical system interdependencies with a large-scale experiment, we tested for breaks in the ecosystem interaction network to identify threshold potential in real-world ecosystem dynamics. Our experiment with the bivalves Macomona liliana and Austrovenus stutchburyi on marine sandflats in New Zealand demonstrated that reductions in incident sunlight changed the interaction network between sediment biogeochemical fluxes, productivity, and macrofauna. By demonstrating loss of positive feedbacks and changes in the architecture of the network, we provide mechanistic evidence that stressors lead to break points in dynamics, which theory predicts predispose a system to a critical transition.

Changes in benthic community structure and sediment characteristics after natural recolonisation of the seagrass Zostera muelleri.

Macrofauna are important contributors to estuarine ecosystem services within and outside of seagrass beds. Here we documented the natural recolonisation of a temperate seagrass (Zostera muelleri) community over 15 years in an urban estuary (Waitemata Harbour, North Island, New Zealand). We also investigated the change in macrofaunal communities in relation to seagrass cover over time, from transition from bare sandflat to seagrass. Colonisation by seagrass was associated with an increase in macrofaunal species diversity (from an average of 32 species per core in 2001 to 46 species per core in 2015) and abundance (from 482 to 2273 individuals per core), as well as an increase in sediment mud (from 4.09% to 12.37%) and organic matter content (from 0.90% to 1.41%). The most abundant species within both seagrass and adjacent unvegetated sandflat were similar, the polychaetes Heteromastus filiformis, Aricidea sp., and Prionospio aucklandica, and the amphipod Paracalliope novizealandiae. The difference in macrofaunal community structure between seagrass and unvegetated sandflat was primarily associated with higher abundance of P. novizealandiae and lower abundance of Pseudopolydora sp. in seagrass. A successional effect was observed in macrofaunal communities over the 15 years following seagrass expansion, primarily associated with an increase in the abundance of Aricidea sp., H. filiformis, and P. novizealandiae, and a reduction in the abundance of the bivalve Linucula hartvigiana. This study is the first to document long-term changes in seagrass and their associated communities during a natural recolonisation event, providing insight into timeframes required both for the regrowth of a seagrass meadow from initial colonisation of individual patches, as well as the trajectories and timeframes of change from a sandflat to a seagrass-associated macrofaunal community. This research enhances our understanding of how changes in seagrass distributions due to seagrass loss or restoration may affect macrofaunal community composition and ultimately ecosystem function.