Freshwater inflows to estuaries : organic carbon and microbial food webs in south-east Australia

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Freshwater inflows (FWI) play a crucial role in maintaining estuarine processes and productivity. River regulation and extraction have greatly reduced FWI to estuaries. Little attention has been paid to the role FWI has in delivering organic carbon to estuaries. The aim of this thesis was to define the relationship between freshwater inflows, organic carbon, bacteria and zooplankton dynamics. To do this, I performed a series of monitoring and experimental studies on the Bega and Clyde River estuaries, Australia. Discharge on both rivers was highly episodic during the study. On the Bega and Clyde Rivers, increasing dissolved organic carbon (DOC) concentrations were closely coupled with increasing discharge. The bioavailability of DOC increased during FWI events, and in turn bacterial growth rates were also higher during and immediately following inflow events. Bacterial growth was carbon limited most of the time, though during high flows, bacteria often became phosphorus limited. Changing availability of DOC and phosphorus during inflow events was the main reason for shifting resource limitation. On both rivers bacterial biomass was positively related to increasing DOC and phosphorus concentrations. Highly episodic discharge during this study had a major structuring role over carbon and bacteria dynamics. On the Bega River I found strong evidence that allochthonous carbon and bacteria can subsidise zooplankton production following the input of DOC during FWI events. Zooplankton density increased following a flooding event on the Bega River and stable isotope analysis indicated allochthonous terrestrial carbon was the dominant source of carbon utilised by zooplankton. Experimental mesocosms confirmed that allochthonous carbon and bacteria can support increased zooplankton in the presence of high subsidies. The individual studies forming this thesis all contribute new insights to their respective sub-disciplines within aquatic ecology. Viewed together, they present a novel conceptualisation of hydrology and freshwater inflows in the coastal carbon cycle and microbial food webs in south-east Australian estuaries. The results provide a strong case to protect freshwater inflows to estuaries.
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