Effects of macroalgal mats and hypoxia on burrowing depth of the New Zealand cockle (Austrovenus stutchburyi)

Macroalgal mats commonly occur in estuaries and sheltered embayments where they are thought to affect the oxygen conditions in the sediment, influence the geochemical process and influence the burrowing activity of bivalves. Laboratory experiments evaluated the effects of sediment hypoxia and algal mats on the burrowing ability and survival of the New Zealand cockle Austrovenus stutchburyi at 15 °C. Both dissolved oxygen concentration and time affected the burial depth of the cockles over the 12 days of the experiment. In hypoxic conditions (<2 mg L^?1), cockles migrated to the sediment surface after 3.5 days and mortality occurred after 11 days. Bivalves exposed to oxygen concentrations of 2–3 mg L^?1 buried closer to the sediment surface than those in the other treatments. Using a simulated tidal regime, in a mesocosm, burrowing behaviour of the cockle and pore-water oxygen conditions in the sediment were measured on exposure to experimental mats of Gracilaria chilensis and Ulva spp. for over 6 days. Algal mats on the surface of the sediment significantly lowered the dissolved oxygen concentration of the sediment pore-water and this effect was greater for the Ulva spp. treatment than the G. chilensis treatment. Cockles were buried more deeply in the control treatment without algae than in either of the two algal treatments. It is concluded that reduced oxygen conditions (<3.5 mg L^?1) develop under macroalgal mats and that this reduces the burial depth of cockles. The potential harmful effects of the mats can depend on the species forming the mat and these effects are likely to be greater in the field than they are in controlled laboratory conditions.