Abstract
The flow regime governs river and floodplain biodiversity by controlling spatiotemporal heterogeneity in hydrologic connectivity and habitat characteristics. However, river regulation has drastically modified natural flow patterns, resulting in reduced flow magnitude and an altered frequency of overbank flow events. Although river-floodplain connectivity is known to provide important breeding cues and basal resources for consumers, how these events shape aquatic invertebrate communities in regulated systems remains poorly understood. This includes zooplankton, which are pivotal in aquatic food webs for the transfer of energy from primary producers to higher trophic levels. I sought to understand how flow variation influences zooplankton abundance and diversity in the Lachlan River, NSW, a regulated river in Australia’s Murray-Darling Basin. I did this by combining analysis of long-term zooplankton observations, a two-year field study of zooplankton responses to a large-scale flood event, and mesocosm and laboratory experiments.Zooplankton abundance during spring and summer was positively related to flow magnitude across a nine-year period that comprised flows ranging from very low discharge to extensive flooding. Increases in abundance were particularly evident during overbank flooding events, especially for rotifers, suggesting benefits of floodplain resource subsidies and egg bank emergence. Rotifer diversity also increased at higher flows, whereas the diversity of copepods and cladocerans declined under these conditions.
The potential mechanism underlying these trends was investigated by examining the influence of patch-scale water velocity on zooplankton communities across a range of small-scale habitat patches (e.g., snags, mid-stream, and littoral habitats) before, during, and after a significant flood event. Zooplankton communities exhibited considerable overlap in composition among patches, but microcrustaceans were relatively more abundant in benthic habitat patches, perhaps due to these habitats providing flow refugia and biofilms as a food resource. Patch-scale water velocity did not exert a strong control on zooplankton communities. Instead, zooplankton abundance and diversity were positively related to river discharge, regardless of habitat patch type.
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Experimental incubation of floodplain and riverbank sediments at warm and cool water temperatures suggested that flood extent and seasonality will impact zooplankton emergence from the egg bank. A higher abundance of zooplankton emerged under warmer conditions, with emergence peaking after 3-4 weeks of inundation. Floodplain sediments generally contained a greater abundance and diversity of emerging zooplankton than did the riverbank sediments.
Overall, my results show that zooplankton community structure and abundance tracked the flow regime, underscoring the significance of maintaining flow variability. Floodplain connection events were especially influential, indicating that egg-bank emergence and enhanced resource availability drive zooplankton dynamics in regulated lowland rivers. Further, during these periods of elevated flows the effects of habitat differentiation at small scales on zooplankton communities may be diminished. My findings reinforce our understanding of the boom-bust nature of river-floodplain ecosystems and highlight the importance of considering the extent, duration, and seasonal timing of inundation events to predict zooplankton responses to future environmental changes. Environmental managers can use these findings to plan environmental flows for productivity responses in similar river systems.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Darren GILING (Supervisor), Ross THOMPSON (Supervisor) & Fiona DYER (Supervisor) |