Freshwater Inflows to Estuaries: Terrestrial Resource Inputs and Planktonic Food Web Responses

Abstract

Freshwater inflows are crucial to estuarine processes, regulating habitats and delivering important resources. However, river regulation has substantially reduced freshwater inflows to estuaries, affecting them negatively. This thesis aimed to enhance understanding of 1) how estuarine planktonic food webs responded to inflows in a regulated estuary; and 2) the importance of terrestrial carbon resources to estuarine food webs. These are important questions in estuarine flow-ecology and water resource management. To achieve this a suite of monitoring and manipulative studies were carried out in the Williams and Manning River estuaries.

A 4.5-year monitoring study comparing the unregulated Manning with the regulated Williams demonstrated the potential impacts of river regulation with results from the Williams contrasting strongly with the Manning. The Williams estuary was characterised by high concentrations of DOC and nutrient, regardless of flow conditions, with zooplankton communities consistently dominated by copepod nauplii, displaying no seasonal trends and responding positively to inflows and inputs of terrestrial DOC. Fine-scale monitoring of an inflow on the Williams estuary showed the importance of inflows in delivering terrestrial carbon and phosphorus resources and in stimulating bacterial and algal productivity. However, this did not translate to increased zooplankton production, relative to pre-flow periods.

Strong evidence was found for the importance of terrestrial carbon resources delivered to estuarine food webs by inflows. Analysis of zooplankton stable isotopes across a variable flow period showed that terrestrial carbon resources were being utilised by zooplankton following inflows, suggesting its importance in upper estuarine areas. Experimental results also indicated that terrestrial carbon in mesozooplankton food webs could contribute to the growth of juvenile fish of an endemic estuarine species. Together these results highlighted the importance and role of terrestrial carbon in estuarine food webs and productivity.

Individually my studies contribute new knowledge to important ecological and management questions in estuaries. Combined as a thesis they provide strong evidence for the importance of FWIs to estuaries and their food webs, highlighting the importance of terrestrial carbon as an energetic resource and contributor to estuarine productivity. This knowledge contributes to the wider literature recognising the importance of freshwater inflows to estuaries and supports their protection.