Estuarine zooplankton responses to flood pulses and a hypoxic blackwater event

Flood pulses in estuaries following storms and rainfall events, are short-lived but important moments for a range of ecosystem processes including the delivery of resources and promoting productivity. Conversely some flood pulses can lead to adverse outcomes such as poor water quality conditions. The aim of this study was to determine how zooplankton abundance and community composition responded to flood pulses and if they responded differently during a flood pulse that led to hypoxic conditions. To do this we conducted a two-year observational study in the Hunter River estuary, Australia, monitoring zooplankton communities monthly for a period that covered two major flood pulse events including one that caused widespread hypoxia and a major fish kill. The results showed zooplankton abundance was higher or no different following the 2012 flood when dissolved oxygen remained stable compared to pre-flood conditions. During the 2013 flood when hypoxia occurred the abundance of copepods, nauplii and rotifers were at their lowest for the study period. Zooplankton assemblages were not distinctly different following the 2012 flood pulse compared to the pre-flood period but were different during the hypoxic 2013 flood, though quickly returned to resemble pre-flood conditions in the proceeding months. The study provides useful insights in how zooplankton populations may respond to flood events and recover after hypoxic conditions in estuarine ecosystems.