Assessing anthropogenic impacts on reef communities: patterns, indicators and processes

Estuaries have been favoured for human settlement, and since then have slowly deteriorated as a consequence of multiple and interacting anthropogenic impacts in their unique sheltered environments. The capacity to predict which urban impacts are most severely affecting benthic communities is essential for estuarine biological conservation. Understanding urban effects and key biological responses may help to determine where impacts take place, if effects are reversible, and how pollution affects flow through the estuarine ecosystem. In this thesis, I apply observational surveys and experimental manipulation to examine patterns in sessile communities in the marine component of estuaries, and to link changes in the ecosystem to pollution sources. In one of the first studies to consider large scale patterns of rocky reef communities across multiple estuaries, I investigated the influences of environmental and anthropogenic factors in influencing the composition, cover and dominance of macro-algae and sessile invertebrates. Reef Life Survey underwater transect protocols were conducted in three large urban estuaries. I utilised information from photo-quadrats to disentangle natural and human impact variables and describe an environmental baseline for current sessile community patterns within these ecosystems. Statistical model selection was conservative, with environmental variables entered into the models first, and then human effects accounted for. I then tested for significant patterns for functional groups. Heavy metals and proximity to ports appeared to be the major anthropogenic drivers of patterns of temperate reef sessile biota, with functional group response consistent and generalizable for two of the three estuaries. Comparison between the three capital city estuaries highlighted a clear effect of intense urbanisation and the complex nature of historical and contemporary pollution effects on biota. To further disentangle the effects of different pollution types on sessile reef assemblages, I conducted a manipulative experiment in one of the most heavily polluted estuaries worldwide (Derwent Estuary, Hobart). I translocated healthy sessile communities grown on concrete pavers to locations adjacent to marinas, sewerage outfalls, fish farm cages, and stormwater discharges, each with associated controls. Reef communities subjected to chronic levels of pollution in the most heavily urbanised area differed from those outside this area, with perennial Laminariales largely replaced by stress-tolerant species. Pollution types differed in their effects on transplanted communities, with marinas showing greatest negative impact, with significant losses in canopy and foliose macroalgae. Communities near fish farms, marinas and storm water drains were characterised by abundant filamentous algae. A concurrent experiment using bare pavers assessed the effects of the four different pollution sources on recruitment of native and non-indigenous algae and sessile invertebrates on rocky reefs in the degraded Derwent estuary over a one year period. Non-indigenous and cryptogenic species showed significantly higher cover on experimental pavers near marinas and sewerage drains compared with associated control sites. The cover of opportunistic species was significantly higher near fish farms and sewerage outfalls, and the cover of some native species was amplified at sewerage outlets relative to the control sites. Colonisation of less desirable algal communities seems to be accelerated by some urban impacts. Results suggest that careful consideration of urban drainage is required to reduce introductions of invasive algal species. Redirection of outlets offshore into better flushed areas and relocation of dense fish farm leases away from partially enclosed areas may help reduce some of the negative changes to sessile communities. Impacts of urban pollution on benthic assemblages can be direct or indirect, through ecological interactions. I analysed invertebrate macro and mesograzer abundances from pavers used for the reef transplantation experiment to explore the potential mediation of pollution impacts through effects of pollution on mobile fauna. Log response ratios and structural equation modelling indicated that observed responses of algal groups and grazers were directly affected by pollution, rather than through trophic pathways involving interactions between these groups. Overall, this study provides important information to improve management in estuarine systems with macroalgal-dominated reefs. Urbanisation has clearly led to large-scale decline in abundance of ‘healthy’ sessile benthic species, with persisting species presumably living near their tolerance limits. Current pollution loads need to be minimised if we are to maintain and restore the unique and undervalued reef communities in the increasingly urbanised environment of estuaries.