Response and resilience of estuarine benthic ecosystems to anthropogenic pressures
Human activities are altering coastal environments at local and global scales with largely unknown consequences. Catchment land-use changes increase the rates of delivery of fine sediments and nutrients to estuaries, resulting in increased suspended sediment concentrations (SSC) and eutrophication; this thesis investigates the response and resilience of soft-sediment intertidal ecosystems to these two anthropogenic stressors. Increased SSC elevates water-column turbidity and reduces the light available for benthic primary producers that sustain estuarine food webs. In intertidal habitats, this turbidity stressor is removed during low tide as the water-column uncovers the seafloor. In other words, turbidity is a temporally displaced stressor in intertidal environments. Photosynthesis during low tide periods of emergence may therefore provide resilience against elevated turbidity during periods of submergence. However, the role of low tide primary production (PP) in estuarine benthic foodwebs has largely been overlooked. Emerged and submerged benthic PP was measured in adjacent seagrass and microphytobenthos-dominated (sandflat) soft-sediment habitats at three locations along a turbidity gradient. Benthic chambers were used to measure the flux of CO₂ across emerged sediments, and dissolved O₂ across submerged sediments to derive net (NPP) and gross (GPP) PP. Emerged NPP and GPP were higher (2–16 times) than submerged in all instances (p < 0.01), and when standardised by mean incident photosynthetically active radiation, the difference between emerged and submerged seagrass PP increased with site turbidity (from 2 to 26 times greater). Emerged PP may be crucial for providing resilience against benthic productivity losses in turbid environments. Eutrophication results in the deposition of excess organic material to soft-sediments and stimulates microbial respiration; this consumes O₂, potentially leading to hypoxia, and releases CO₂, causing localised acidification. I enriched intertidal sediments with increasing quantities of organic matter to identify the effects of eutrophication-induced acidification on benthic structure and function, and assessed whether biogenic calcium carbonate (CaCO₃) would alter the response. Declines in macrofauna biodiversity (abundance and species richness), reduced benthic NPP and impaired nutrient cycling occurred along the eutrophication gradient. CaCO₃ did not alter the macrofaunal response, but significantly reduced negative effects on function (e.g. net autotrophy occurred at higher levels of organic enrichment in CaCO₃-treated plots than controls (1400 vs 950 g dw m-2)). This study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO₃ in moderating responses. Ecological theory states that the resilience of ecosystem functions to environmental disturbance depends on the biodiversity that underpins them, but field validations of this are lacking. I explored the shifts in sandflat macrofaunal community composition following organic matter enrichment, and the consequences for the representation and composition of functional groups (FG). Taxa-specific sensitivities to enrichment resulted in significant response diversity which provided resilience to FG, but substantial declines in the abundance of dominant taxa meant they became functionally extinct nevertheless. Density compensation occurred in one FG, but overall community abundance decreased by 80 % between low and high levels of enrichment. Evidence of density-dependent relationships between ecosystem function and biodiversity were discovered, highlighting the need to conserve macrofaunal biodiversity to preserve function despite the existence of stabilising mechanisms. This thesis demonstrates the existence of mechanisms that provide resilience against increased SSC and eutrophication in benthic ecosystems, but highlights that the structure and function of these systems remain sensitive nevertheless.
ecosystem function, disturbance, biodiversity-ecosystem function, intertidal
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