Rapid reworking of subtidal sediments by burrowing spatangoid urchins

The mixing and displacement of sediment by benthic macrofauna (bioturbation) has major biogeochemical implications, and can control rates of organic matter degradation and carbon burial. Large, abundant, mobile macrofauna often dominate sediment bioturbation, and heart urchins of the genus Echinocardium are regarded as key sediment bioturbators in marine systems throughout the world. To better understand the bioturbation potential and functional role of Echinocardium, we developed a mathematical model and parameterized it with field data from six locations in northern New Zealand in order to estimate bioturbation rates in these places. Although urchin sizes and densities were measured in consecutive years at all six locations, we obtained a third model parameter, urchin movement rate, from one time and place only (Site OB5). Because confidence in model output was greatest at OB5, and since OB5 had the highest sediment reworking rate of all sites, our model yielded a good upper bound estimate for the bioturbation potential of Echinocardium in the areas examined. The volume of sediment displaced by Echinocardium populations reached 20,000 cm³ m⁻² d⁻¹ at OB5, suggesting that surface sediment is reworked about every 3 days at sites where Echinocardium is abundant. Experimental work with a fluorescent tracer at OB5 suggested limited downward particle movement as a result of Echinocardium bioturbation, though vertical profiles of chlorophyll a and organic matter content indicated well mixed sediment. The loss of Echinocardium because of broad-scale anthropogenic disturbance to the seabed could have major consequences on marine ecosystem functioning.