Large rivers, such as the Waikato River, receive inputs of carbon from a complex range of sources such as (i) upstream reservoirs which may act as hotspots of phytoplankton production, (ii) tributary sources draining floodplain wetlands and lakes, (iii) seasonal leaf-fall from exotic willow trees, (iv) internal phytoplankton and zooplankton production within the main river, and (v) migrating fish and shrimps which import marine-derived nutrients. Furthermore, inundation of floodplains can mobilise terrestrial carbon and also lead to the generation of algae which are exported to the river as floodwaters subside. Understanding the relative significance of these various inputs and trophic pathways that lead to the nutrition of consumers, such as fish, is essential knowledge for enhancing productivity and maintaining ecological health of river-floodplain ecosystems.
Figure 1. Simplified food-web structure in the Waikato River ecosystem.
Ongoing work is investigating food-webs in floodplain lakes which support modified biotic communities through the proliferation of introduced fish. We are also currently analysing food-web data from side-arms of the main river to understand the significance of these lateral habitats for the transfer of carbon to the main river.
Contact: Kevin Collier
LERNZ students have investigated the significance of different energy sources fuelling fish production in the lower Waikato River. This involved quantifying food resources and energy flow through food webs, determining the relative significance of algal production compared terrestrial inputs, and understanding the roles of tributary junctions and side-arms in food web resilience.