Decomposing macroalgae (Ulva spp.) impacts benthic macrofauna and surface sediment erosion

Abstract

Patches of decomposing macroalgae can be beneficial to intertidal regions, providing habitat and food, or can create anoxic conditions inhospitable to benthic organisms. These opposing outcomes warrant further investigations into biotic-abiotic processes associated with macroalgae. Here, differences in surface sediment erosion measures (erosion threshold (Ʈc; N m−2), erosion rate (ER; g m−2 s−1), and sub-surface erosion rates (me; g N−1 s−1)) were examined at sites dominated by the suspension-feeding clam or deposit-feeding bivalve (Austrovenus stutchburyi and Macomona liliana, respectively) after 30 days of exposure to decomposing macroalgae (Ulva spp.). The Austrovenus site was chosen to represent a species-rich, functionally diverse macrofaunal community. The nearby Macomona site had similar sediment characteristics, yet had a less abundant and diverse faunal community. Despite the equal amounts of Ulva recovered from both sites (< 3% of the initial 3 kg wet weight m−2 added), differences in surface erosion were measured. One day after Ulva removal, an initial increase in surface erosion (−Ʈc and +ER) was measured at the Austrovenus site, but after 14 days there was no difference compared with control plots. At the Macomona site, Ulva addition stabilized sediments (+Ʈc and -ER), an effect that persisted for 14 days. These differences were reflected in the macrofaunal community. At the Austrovenus site, the macrofaunal community recovered within 14 days whereas differences between Ulva and controls persisted at the Macomona site. This work emphasizes how subtle differences in benthic community composition and recovery can result in spatial and temporal variations in sediment erosion potential on intertidal sandflats.