Abstract
Estuarine ecosystems are experiencing increasing land‐derived stress, bringing sediments, excess nutrients and contaminants. Once degraded, disturbed soft sediment ecosystems can take years to recover because long‐lived, functionally important species that aid recovery are lost. We undertook an experiment in the Tauranga Harbour, transplanting adult Austrovenus stutchburyi (>18 mm at 800 ind. m −2 ) to nine sites across a stressor gradient of increasing sedimentation, nutrient and heavy metal loading. The goal of this study was to test levels of retention across this gradient and whether transplanting suspension‐feeding clams will improve ecosystem functioning. Measures of ecosystem function (benthic metabolism, organic matter degradation) and sediment characteristics were compared with ambient (non‐addition) plots after 3 months. Transplanted A. stutchburyi retention (28%–75%) varied across sites and was not correlated with the overall stress gradient. The sediment heavy metal load, particularly zinc levels, reduced translocated clam retention despite being well below contamination guideline levels. Ecosystem functions did not differ between treatment and ambient plots; however, when normalised against each site's ambient mean values, organic matter degradation scaled with retention, isolating the effect of the added clams on carbon turnover. This study illustrates that restoration of some functions in degraded soft sediments by A. stutchburyi transplantation is possible, but its success will depend on whether heavy metals have been reduced and the identification of other, less expected, stressors that may reduce clam retention.