Date of Award
Doctor of Philosophy
Faculty of Communications, Health and Science
Dr Paul Lavery
Dr Graham O'Hara
This study documented the macroalgal assemblages of the Swan-Canning Estuarine System (SCES) over a two year period, and the influences of several environmental parameters on the assemblages. In addition, the Impacts of unattached macroalgal accumulations on benthic nutrient fluxes and microbial communities were investigated. Benthic macroalgal assemblages and physico-chemical regimes were monitored in the SCES, to determine temporal and spatial changes in macroalgal communities and the influence of environmental factors in these changes. Physico-chemical regimes demonstrated strong seasonal changes, which revolved around the onset and cessation of freshwater flows in winter (May to September). In the months after freshwater flows, strong spatial variability in physico-chemical profiles was observed. However, by summer the system was essentially marine. Macroalgal biomass and species richness was lowest in winter. Species number was maximal during periods of greatest hydrological variability in the estuary (spring and autumn). It may be inferred from results of statistical analyses that substrate type (i.e. hard/soft) and waterflow were the most Influential factors over temporal and spatial distribution of macroalgal species in the SCES. These factors ware reflected by the patchiness of macroalgal distribution in the system- attached macroalgal species distributed unevenly according to availability of limited hard substrate and presence/absence of unattached macroalgal species corresponding to seasonal freshwater flows. One species, Gracileria comosa, dominated macroalgal biomass and was the most widespread species and commonly occurred as extensive, unattached accumulations. As G. comosa was the most abundant unattached macroalga, accumulations of this species were investigated to determine the characteristics and behaviour or accumulations in the Swan-Canning Estuarine System. Accumulations were characterised by seasonally measuring height and biomass of accumulations in three regions or the estuarine system over one year. The height of accumulations was generally between 5 and 25cm, regardless of water depth, location, or season. Biomass was highly variable, but generally between 100 and 500 dw/m2 . The persistence of macroalgal accumulations was monitored at 28 sites within 10 estuarine regions, over a three month period, during which the first freshwater flows were recorded. Accumulations persisted between one week and one month, depending on the region, with accumulations persisting for longer periods in areas of low flow such as embayments and the regulated Canning River, and for shorter periods In regions of higher flow such as the channalised Swan River. Field and laboratory studies were performed to determine If the presence of G. comosa accumulations had an Impact on sediment-water nutrient exchange. Field studies established that accumulations affected benthic nutrient fluxes within a 24 hour period. However, this effect was site-dependent, occurring at an estuarine site of relatively high sediment organic content, but not at a site of relatively low sediment organic. Diurnal changes in water quality inside algal accumulations corresponded to photosynthetic/respiratory activity of the macroalgae - most notably, Increases In orthophosphate and ammonium fluxes from the sediment after approximately 8h of darkness. Since this effect was on time scales less than the period of persistence (weeks to months), It was concluded that macroalgal accumulations have an impact on benthic nutrient fluxes from sediments of relatively high organic content in the system. Laboratory studies investigated the effect of depth and density of an algal layer on sediment- water nutrient exchange. The experimental results concurred with field observations; water column concentrations of inorganic nutrients were significantly higher in sediment cores overlain by an algal layer over a 7 day period. In addition, Inorganic nutrient concentrations increased With Increasing height of the layer and ammonium concentrations increased with increasing density of the algal layer. Additional laboratory experiments tested the effect of an algal layer on sediment denitrification rates, and the composition and distribution of benthic microbial populations, Benthic nitrogen (N2) release rates were low irrespective of the presence of macroalgae and sediment types (less than 1mmo N/m2/d). However, release rates were significantly higher in sediment cores covered by algae than in comparable bare sediment cores, provided the algal layer was relatively high (5cm in height} and sediment organic content was high. The presence of an algal layer did not have a significant effect on the composition or distribution of microbes in the sediment. In all cases, microbial populations contained relatively few denitrifiers/nitrate reducers compared to nitrifiers and ammonifiers. High ammonium release rates from the sediment to the water column, and the low release rates of elemental nitrogen, suggested that even II the nitrate reducing bacteria were active they were not reducing nitrate to nitrogen, suggesting the possibility of Dissimilatory Nitrate Reduction to Ammonium (DNRA). Subsequent analysis confirmed that the nitrate reducers were reducing nitrate to nitrite, a result compatible with the hypothesis that the main microbial processes occurring were ammonification, nitrification, and DNRA, but not denitrification. These processes, regardless of the presence of a benthic algal layer, contribute to high ammonium flux rates from the sediment and provide a mechanism of internal inorganic nitrogen regeneration. In conclusion, this study has established that unattached macroalgal accumulations are a prominent component of the macroatgal community in the Swan-Canning Estuarine System. Accumulations may remain within an estuarine region for up to one month, particularly in regions of low water flow. In seasons and regions of relatively high water flows (e.g. the Swan River), accumulations become highly transient, if present at all. At times, and in regions where they may persist, algal accumulations of 5cm or more in depth have an impact on benthic nutrient fluxes. In particular, their presence over sediments of high organic content appears to exacerbate the release of ammonium from the sediment to the overlying water column. Of note, the benthic process Dissimilatory Nitrate Reduction to Ammonium appears to dominate in summer while denitrification rates are minimal, regardless of the presence of a macroagal layer. From these findings, it is recommended that high fluxes of ammonium in the system be recognised In water quality management and nutrient budgets for the system, as It appears that Internal ammonium regeneration Is a large source of Inorganic nitrogen for organisms In the overlying water body, and may support algal blooms In summer. In addition, it appears that the most appropriate method of managing macroalgal distribution and biomass in the system is ensuring strong freshwater flushes during winter periods when macroalgal biomass is largely removed. If seasonal flushes were inhibited, it is predicted that macroalgal biomass and distribution would increase, extending the period that thsy can influence benthic nutrient cycles. The physical removal of macroalgae as a management option in such a scenario would require much time and effort, as the Swan-Canning Estuarine System is such a large system, and macroalgae are spread throughout. Therefore, in modifying river flows into the estuarine system, the quantity, composition and distribution of macroalgae, and possibly other flora and fauna, will be altered. This is already evident in the Canning River, which is regulated and suffers management problems, such as altered species composition, bathymetric changes, toxic algal blooms, and eutrophication.
Astill, H. L. (2000). The role of benthic macroalgae in sediment-water nutrient cycling in the Swan-Canning estuarine system, Western Australia. Retrieved from https://ro.ecu.edu.au/theses/1344