|dc.description.abstract||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 when measuring ecosystem functions in shallow intertidal environments.
A second manipulative field experiment examined the impact of different quantities of Ulva detritus (low [60 g dw m-2], medium [120 g dw m-2] and high [240 g dw m-2]) on the sediment properties, benthic community composition and ecosystem functions of an estuarine benthic community. Plots were sampled on three occasions over an 8-week period, to examine changes in macrofauna and/or ecosystem functions over time. The only significant treatment effect was less gross ammonium flux from the sediment in procedural controls compared to low density treatments. Although no further significant treatment effects were detected, important temporal variation was observed. The macrofaunal community varied significantly between all three the sampling dates, while gross primary production, sediment oxygen consumption, and nutrient efflux from the light and dark chambers varied between at least two of the sampling dates. The results from this study again highlighted the important temporal changes that can be observed over relatively short time scales, and the importance of measuring temporal variation, particularly when measuring ecosystem functions such as primary production and benthic metabolism.
Finally, in a laboratory study, I examined the density dependent effects of two key intertidal bivalve species (A. stutchburyi and M. liliana) and their associated communities on the breakdown, loss, burial and uptake of Ulva detritus. Results showed that the site dominated by A. stutchburyi (AS) had higher overall chl a biomass and chl a was distributed evenly throughout the sediment profile, whilst less labelled Ulva was retained in the sediment. Both chl a biomass and the amount of Ulva recovered were correlated with the density of M. liliana, but only in cores collected from AS sites. Cores collected from the M. liliana dominated site (ML) showed an exponential decline in chl a with depth, and chl a biomass was negatively correlated with species richness. In ML cores (which had very low A. stutchburyi abundances), M. liliana densities showed no correlation with chl a biomass or the amount of Ulva that was recovered, which suggested that the community present in cores from AS is important in facilitating the impact by M. liliana on the mixing and the reworking of organic matter. These results emphasize the value of considering whole communities, and not just key species, when trying to understand sediment mixing and organic matter processing.
Although the addition of Ulva did not result in large and obvious shifts in community composition or ecosystem function, small subtle shifts were observed. These results underpinned the complex nature of biotic and abiotic interactions in dynamic systems like estuaries, but show that for the most part, ecosystem functions are robust. Temporal variations in ecosystem functions appeared to be largely driven by environmental conditions such as light availability, and were a more prominent driver in the differences observed in ecosystem function compared to treatment effects of Ulva addition. The results also highlighted the importance of community composition in the processing and reworking of macroalgal detritus, which further emphasized the complexity of estuarine systems. The results from this research suggest that on intertidal sandflats, providing the abundances of key species remain intact, the benthic community composition can shift, and species can be lost, without a significant loss or shift in overall ecosystem function.||