Small-scale disturbance and increased nutrients as influences on intertidal macrobenthic assemblages: experimental burial of wrack in different intertidal environments

Wrack (dead, washed-up seaweed and seagrass) buried in soft substrata causes increased organic content and alters the physical structure of sediments. These effects may influence the types and relative abundances of species in the sediment. Such influences can be expected to vary according to the type and organic content of the sediment. In this study, wrack was buried in 3 mudflats and 2 intertidal sandflats (with coarser sediments) in Botany Bay (New South Wales, Australia). The experiments tested the hypotheses that burial of wrack would (1) increase the content of total nitrogen (N) and organic carbon (OC) of sediments; (2) increase the biomass of micro-algae; and (3) alter the abundances of macrofauna in the sediments. Simulated wrack (equivalent amounts of plastic ribbon, without any organic content) was added to other experimental plots to test the above hypotheses when only physical disturbance, not organic enrichment, occurred. Total OC, N and chlorophyll a (chl a) were sampled every 2 wk for 6 wk and macrofauna were sampled after 6 wk. Some taxa (capitellid, orbinid and nereid worms) increased abundances where wrack was added. Nereidae, however, responded only in muddy sediments. The physical structure associated with wrack also caused an increase in numbers of Oligochaeta and increased patchiness in numbers of the soldier-crab Mictyris longicarpus, but only in sites where they were already abundant. Wrack represents a source of food for some taxa. Physical disturbance due to burial of wrack also adds to patchiness and abundance of taxa on sandy and muddy intertidal shores. The major responses of fauna predicted from other studies did not occur.