Influence of Hydrological Connectivity on Plankton Communities in Natural and Reconstructed Side-Arms of a Large New Zealand River

Abstract

We sampled natural and reconstructed side-arms during different stages of hydrological connectivity with a large floodplain river in northern New Zealand, to determine whether re-establishment of connectivity would be an effective strategy for restoring plankton communities in former side-arms. Connectivity between side-arms and the river was moderated by water level and influenced flow rates and closure of inlets and outlets. Physicochemical conditions were more strongly related to the connectivity phase than to habitat type (river, natural or reconstructed side-arm), except during low connectivity when natural side-arms in particular were characterised by higher ammonium (NH4-N) and total phosphorus (P) concentrations, as well as specific conductivity. Dissolved reactive phosphorus (PO4-P), water temperature, conductivity and dissolved oxygen were identified as explanatory variables of phytoplankton and zooplankton community composition, which along with total nitrogen (phytoplankton) or total suspended solids (zooplankton) explained 44–52% of variation. Phytoplankton community composition and the abundance of several dominant or discriminatory taxa were affected by connectivity but not habitat type, whereas habitat and connectivity both had significant effects on zooplankton communities and abundances of the cladoceran Bosmina meridionalis. Significant interactions between connectivity and paired habitat types occurred for abundances of the diatom Asterionella, the cryptophyte Cryptomonas, the rotifer Synchaeta oblonga and cyclopoid copepods, reflecting differential responses to connectivity among habitats by these taxa. Overall, these results underscore the importance of hydrological connectivity between side-arms and rivers in moderating plankton community composition, and highlight unpredictable trajectories of community development and alternative transient states that can occur soon after side-arm reconnection.