The impact of macrophyte mats on biodiversity and ecosystem functioning of benthic communities

Macroalgal blooms, such as Ulva spp., are a common disturbance to estuarine benthic fauna worldwide. As large quantities of macroalgae break free from growing substrates, drifting mats are formed that eventually deposit in low energy environments, including intertidal sandflats. The mats will typically settle on the sediment surface as large sheets. Once these sheets start to decompose, detritus is formed, which is eventually incorporated into the benthic food web; however, the ability to process detritus is dependent on the species present. This thesis examined the impacts of Ulva on the benthic macrofaunal communities and ecosystem functions, at different phases of decomposition; firstly, as large sheets (Chapter 2), then as detritus (Chapter 3), and finally as the Ulva detritus is incorporated and reworked into the sediment and foodweb (Chapters 4 and 5). The impacts of intact macroalgal mats on the sediment characteristics, community composition and ecosystem functions (i.e. benthic primary production, metabolism, and nutrient cycling) associated with an intertidal macrofaunal community in Tauranga Harbour were measured in a manipulative field experiment. Temporal changes and recovery of the community and the ecosystem functions they provide were measured twice over a 14-day period. Subtle treatment effects were observed in the macrofaunal community and sediment characteristics, which in turn resulted in subtle shifts in chlorophyll a (chl a) corrected gross primary production. However, there were no significant impacts on the key benthic species (the suspension-feeder Austrovenus stutchburyi and the deposit-feeder Macomona liliana) at this site, which is likely the reason more significant treatment effects were not observed in the measures of ecosystem function. Significant temporal variation was measured in most sediment properties (all except for phaeophytin), and also in benthic primary production. This study emphasized the importance of temporal variability