Effects of sediment disturbance by the heart urchin Echinocardium cordatum on the sediment–seawater solute exchange: an exclusion experiment

Spatangoid heart urchins are dominant bioturbators in marine soft-sediment ecosystems worldwide. Their repeated sediment reworking prevents biogeochemical sediment stratification and colonisation by other species, with implications for sedimentary reaction processes that affect the local sediment–seawater solute exchange. Here, we used a simple exclusion experiment to investigate how a subtidal _Echinocardium cordatum_ population (18.2 ± 6.7 individuals m-2), foraging at an individual speed of ~45 cm per day affects the sediment–seawater solute exchange. To do so, we removed all heart urchins from eight one-meter-diameter areas of the 10-m deep seafloor of Man O’War Bay, Hauraki Gulf, New Zealand, and prevented recolonisation and hence sediment reworking for 56 days. Subsequently, we measured the sediment–seawater exchange of O2, NO3–, NO2–, NH4+, and N2 both within and outside the exclusion areas, under light or dark conditions, and found no difference. The absence of a legacy effect of foraging _E. cordatum_ after their removal suggests that, at least in this habitat, their influence on the sediment–seawater solute exchange may be limited to sediment being displaced in the immediate surrounding of the urchin. This unexpected result underlines the importance of evaluating the influence of bioturbators on the sediment–seawater solute exchange in the context of local environmental conditions, animal behaviour, and population characteristics.