Reduced macrofauna diversity and abundance in response to red macroalgal detritus

Macrophyte detritus from exogenous sources can play an important role in structuring benthic communities. Macrofaunal responses to seagrass wrack, mangrove leaf litter, and detritus from brown and green macroalgae have previously been examined through enrichment experiments. Yet effects from red macroalgal detritus, which is a major component of algal drift in many regions, are not well understood. In this study, enrichment experiments were performed on a shallow subtidal sandflat in Puget Sound, Washington, USA with detritus from three species of red macroalgae to assess: (1) whether macrofaunal assemblages were affected by two different “dosages” of red macroalgal enrichment (100 ml vs 500 ml per 0.079 m3 of sediment); (2) whether macrofaunal response differed between one-time and repeated (weekly) additions of red macroalgae; and (3) whether responses to red macroalgal enrichment changed over time. There appeared to be little or no effect on macrofauna from one-time enrichment regardless of the amount/dosage of algae added. However, weekly additions of red macroalgae led to negative responses across macrofaunal taxa. These responses occurred rapidly, within the first 3 weeks, and were largely unchanged after 7 weeks, intensifying for only 2 of the 10 most common taxa. No opportunistic responses to weekly additions were observed. Frequent influx of some types of red macroalgae may degrade the quality of sedimentary habitats by leaching chemically defensive compounds or through other mechanisms, which should be investigated in future studies. Although weekly enrichment treatments were informed by previous estimates of detrital delivery rates near hard-bottom habitats in the same locality, further research is needed to assess detrital influx frequency and community response in other regions where red macroalgal detritus is common, and to understand the broader implications of exogenous detritus from red macroalgae on benthic ecosystems.