Mechanisms of stability of rhodolith beds: sedimentological aspects

Rhodolith beds are highly diverse benthic communities organized around the physical structure and primary productivity of red coralline algae. Despite a worldwide distribution and growing recognition that rhodolith beds are important calcium carbonate (CaCO3) bio-factories, little is known of the factors and processes that regulate their structure, function, and stability. One prevalent, largely untested paradigm is that beds develop in environments where water motion is strong enough to prevent burial by sediments. Observations over 7 mo and 3 wk in the centre and near the upper and lower margins of a Newfoundland (Canada) rhodolith Lithothamnion glaciale bed, as well as a laboratory mesocosm experiment with rhodoliths and dominant macrofauna from the bed, were used to characterize, parse, and model spatial and temporal variation in rhodolith sediment load (RSL) and movement among presumably important abiotic and biotic factors. RSL and rhodolith movement were largely mediated by a few dominant benthic invertebrates. Hydrodynamic forces were insufficient to move rhodoliths. Daisy brittle stars Ophiopholis aculeata and small common sea stars Asterias rubens contributed to dislodgement of sediment from rhodoliths. Large green sea urchins Strongylocentrotus droebachiensis easily moved rhodoliths in mesocosms. Results provide the first quantitative demonstration that rhodolith beds need not be exposed to threshold hydrodynamic forces to avoid burial. Beds can simply occur in areas where burial is unlikely because of low sedimentation rates. In such cases, select resident bioturbators operating simultaneously at different spatial scales (within and outside rhodoliths) appear to suffice to maintain RSL below lethal quantities, contributing to stability of beds.