Changes in water quality and subsequent ecosystem responses to freshwater inflow events can occur rapidly in estuaries. Despite this, most flow-response studies are typically sampled at weekly to monthly intervals. This leaves a significant knowledge gap regarding rapid changes occurring in ecological communities following inflow events. To address this, a flow-response study was conducted n the Williams River estuary, NSW, Australia, upstream and downstream of a weir which controls flow between freshwater and mesohaline zones. It monitored for changes to dissolved organic carbon, nutrients and bacterial cell concentrations every 4 hours for the first four days of a flow event followed by additional monitoring of algae and zooplankton responses every 3-5 days for six weeks. Rapid increases in dissolved organic carbon and phosphorous concentrations were observed during the inflow which were tightly coupled with increases in bacterial concentrations. Following the inflow event, while the weir remained open, increased algal concentrations occurred, dominated by euglenoids. In the same period juvenile copepod populations, which had initially been reduced by the inflow, began to increase in concentration, as did freshwater cladocerans probably advected to the estuary. However, following the closure of the weir gates the mesozooplankton community was substantially reduced in concentration relative to the preceding community while the gates were open and the pre-flow community. Fine-scale monitoring suggested that inflows are key in stimulating basal resources for planktonic food webs and that rapid changes to flow conditions can quickly shift the composition of mesozooplankton communities.