Tīmata whakaora, kickstarting recovery - Using bivalves to bioremediate degraded estuarine sediments

Abstract

Estuarine soft sediment ecosystems worldwide are increasingly threatened by anthropogenic, land-derived and marine stressors that compromise their health and functionality with cascading effects on ecosystem services that they provide. Once degraded, natural recovery can take years due to the loss of long-lived, functionally important benthic species. These species are important in maintaining internal feedback loops, thereby facilitating resilient communities that underpin critical ecosystem functions. In this thesis, I evaluated the role of two functionally distinct bivalve species, Austrovenus stutchburyi, a surface-dwelling suspension feeding bioturbator, and Macomona liliana, a deep-burrowing porewater-pressuriser, in supporting recovery of estuarine function following disturbance. Two field experiments in Tauranga Harbour, Aotearoa New Zealand were conducted, first a controlled disturbance-recovery translocation trial (after acute disturbance) and second, a large-scale translocation across a gradient of environmental (chronic) stress. In the first experiment bivalves were added to defaunated plots in single and combined species treatments and compared to ambient and defaunated no-addition controls. Measurements were taken over a period of 389 days (one year) and included influence of bivalve additions on sediment properties, nutrient cycling, benthic metabolism (Chapter 2), and community composition recruitment (Chapter 3).

Results from the first research chapter (Chapter 2) demonstrate that A. stutchburyi consistently enhanced ecosystem function proxies, reducing sediment mud content, increasing oxygen consumption, and stimulating ammonium flux, even when survival was low, compared to unaided recovery. In contrast, M. liliana showed limited direct effects on the measured ecosystem functions and the co-addition of both species did not yield synergistic effects.

The second research chapter (Chapter 3) elucidates that the presence of A. stutchburyi also altered macrofaunal recovery trajectories and moderated the proliferation of opportunistic species, particularly in the absence of M. liliana. In contrast, M. liliana only treatments showed limited impact on functional recovery metrics but contributed to expected post-disturbance recruitment patterns by opportunists. Juveniles of both bivalve species settled in all treatments, M. liliana juveniles were enhanced in all defaunated treatments, whereas A. stutchburyi decreased but approached ambient after one year in all but M. liliana only treatments. While all treatments trended toward ambient community states over the course of one year, differences in recruitment patterns and functional diversity suggest that species additions only subtly altered recovery trajectories.

In the third research chapter (Chapter 4), A. stutchburyi was translocated across 9 sites within the Tauranga Harbour. Results showed that translocation success was not dependent on the overall stress-gradient or ambient A. stutchburyi densities. Translocation success did, however, vary with heavy metal (zinc) contamination after three months, even when concentrations were well below guideline thresholds. Effects of translocations on measures of ecosystem productivity could only be discerned in translocation sites with >44% clam retention, showing that increases in benthic metabolism and organic matter degradation are dependent on bivalve survival.

The synthesis of these chapters offers insights into the potential of using adult ecosystem engineering bivalves, particularly A. stutchburyi, to facilitate estuarine recovery and places this work in the broader context of restoration ecology with management implications. These findings highlight the importance of early reintroduction of ecosystem engineers to re-establish complex facilitatory feedbacks and support estuarine ecosystem recovery. However, successful restoration depends on environmental context, particularly the extent of stressor reduction needed to ensure translocation survival.