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Pore Water Exchange Processes in Offshore Intertidal Sandbanks

In recent years blooms of the toxic marine cyanobacteria Lyngbya majuscula have been frequently observed in a system of offshore intertidal sandbanks in Moreton Bay, Australia. Past research suggests that these blooms are linked to the presence of bioavailable forms of iron. Submarine groundwater discharge (SGD) is thought to provide an important nutrient (particularly iron) transport mechanism at locations where L. majuscula blooms are observed. In this study the feasibility of the current conceptual model for SGD to offshore L. majuscula bloom sites was examined using a combination of field measurements and numerical models. Numerical simulations indicated that SGD could occur several kilometres offshore within the sandbank system if a continuous, low hydraulic conductivity aquitard extends from the onshore aquifer to the offshore sandbank. Models that incorporated significant discontinuities or gaps in the aquitard unit resulted in SGD at the location of these discontinuities before reaching the offshore sandbank system. Improved hydrogeological data, particularly in relation to the presence and continuity of any aquitard units, is required to enable a better assessment of SGD mechanisms in this area. Simulated maximum instantaneous vertical flow velocities in the near-surface zone of the sediments around the offshore exit point were an order of magnitude lower the velocities associated with local-scale pore water flow processes that are driven by tidal and subtidal pumping. As such these local-scale flow processes (instead of SGD) are likely to provide the dominant pore water exchange mechanisms in the offshore sandbank systems. Field data was collected from a L. majuscula bloom site at an offshore intertidal sandbank within the Eastern Banks of the Bay to examine these local-scale flow processes. The measurements identified a tidally-induced residual pore water flow system in the sandbank, with seawater entering the upper sandbank platform and discharging through the bank edge. Furthermore the field data revealed concentrations of bio-available iron in pore waters that were more than four times greater than concentrations previously shown to stimulate L. majuscula growth. Upward flow and elevated near-surface dissolved Fe(II) concentrations (> 20 µM Fe(II) at -0.05 m depth) were measured simultaneously in the discharge zones at the sandbank edge. Analysis of dissolved Fe(II) profiles within pore waters at the site showed that measurable concentrations of dissolved Fe(II) were absent at the base (-2 m depth) of all monitoring stations. This result seems to indicate that terrestrially derived SGD is not an iron source at the site. Numerical simulations of the tidally-induced pore water flow at the field site generally showed good agreement with the field data. Simulation results revealed a more complex residual pore water flow system in the sandbank than shown by the field data. Both numerical simulations and field measurements suggest that the residual flow might provide a mechanism for transport of bio-available iron across the sediment-water interface. During periods when these sandbanks are submerged a number of pore water exchange processes, collectively termed subtidal pumping, are likely to be operating at the site. Numerical models were developed to examine one particular form of subtidal pumping, i.e., current-bedform interactions. These models were based on a one-way sequentially coupled model of surface water and porous medium flow. Interactions between unidirectional and bi-directional currents with simple two-dimensional fixed bedforms were considered. Results suggested that while pore water exchange was likely in distinct zones around the sandbank edges, a whole-of-bank scale pore water circulation was unlikely to occur. Smaller scale (< 1.0 m) variations in the sandbank topography might exert a stronger influence on the pore water flow and associated pore water exchange. Simulations showed more complex pore water flow paths and longer pore water residence times in systems under bi-directional flow conditions compared to systems subject to unidirectional flows. The consideration of bi-directional flows is important in modelling current-bedform interactions in tidally influenced environments. The findings of this study have implications for the current conceptual model for L. majuscula blooms in offshore intertidal sandbanks within Moreton Bay. This conceptual model should be updated to include tidal and subtidal pumping as potential delivery mechanisms for bio-available iron. Further investigations are required to fully understand the role of tidal and subtidal pumping in controlling the export of bio-available iron to coastal waters at the field site. In particular there is a need to better assess the link between the pore water flows and the geochemical reactions that might occur along the flow path. Improved data on pore water chemistry as well as the solid phase mineralogy is required to develop reactive transport models for these flow systems. Such models would allow further investigation of the importance of tidal and subtidal pumping as a mechanism for exporting dissolved chemicals across the sediment-water interface.