Biogeochemical processes in permeable estuarine sediments following benthic community change: implications for system dynamics & Lyngbya majuscula blooms in Deception Bay, Queensland

Abstract

Estuaries are productive and dynamic ecosystems that provide a wide range of environmental services to human society, many of which contribute to people’s livelihoods and wellbeing. However, many of these coastal ecosystems have become degraded by the impacts of human activity. Such degradation is often manifested in changes to an ecosystem’s structure and function. Such changes impact on the resilience of these ecosystems to recover from natural disasters and maintain essential services that human and ecological communities rely upon. In shallow-water estuarine ecosystems, sediment-based processes often play a significant role in ecosystem function, such as the biogeochemical cycling of carbon and nutrients. This thesis examines the importance of benthic community composition on key biogeochemical processes that underpin local phenomena such as blooms of the toxic cyanobacterium Lyngbya majuscula and materials cycling at ecosystem-wide scales. The research presented here shows that changes in benthic community composition have implications for system-wide functions such as biogeochemical cycling. In Deception Bay, 100 % of the seagrass community dominated by Syringodium isoetifolium was lost after a major flood event in January 2011. The loss of S. isoetifolium from Deception Bay was shown to fundamentally change the structure and function of the seagrass meadow from a net heterotrophic seagrass dominated benthic community to a net autotrophic seagrass-microphytobenthic community. Such a change in function has implications for system-wide biogeochemical budgets. This study has quantified a benthic release of Fe(II) from sediments sampled from northern Deception Bay. To our knowledge, this study represents the first documented benthic release of Fe(II) from permeable sandy sediments of a shallow subtropical embayment. The results suggest that sandy sediments that are relatively low in organic carbon are capable of supplying bioavailable Fe at levels significant for the growth of benthic organisms. The results of this research show that the post-flood sediments of northern Deception Bay are capable of supplying >1700 % of the daily Fe demands and up to 10.6 % of daily P requirements for the growth of L. majuscula blooms. This result suggests that the growth of L. majuscula blooms in Deception Bay is likely to be P limited and that future management actions that are targeted towards reducing L. majuscula blooms in Moreton Bay need to include a strategy for reducing P loads to the Bay.