The influence of connectivity on the functional role of a natural and re-constructed side-arm in the lower Waikato River

Abstract

This thesis was a comparative study of a natural and a recently re-constructed (artificial) side-arm in the lower Waikato River. A twelve monthly temporal investigation into (i) the physico-chemical variables, nutrient and suspended solid dynamics; and (ii) phytoplankton and zooplankton abundance and community composition was carried out in order to compare each side-arm with the river, and any patterns occurring within the side-arm longitudinally. The study was based on three observed phases of connectivity of the side-arms with the river flow: Phase 1 - side-arms connected with the river (late spring-summer); Phase 2 - side-arms disconnected from the river (autumn); Phase 3 - side-arms re-connected with the river (winter). Functional roles described for side-arms in the literature were used to assess whether the natural and artificial side-arms functioned as predicted for hydraulic retention zones during disconnection, a period of high water retention and water age. Both side-arms were influenced by seasonal variation during the late spring-summer and winter connection with the river. High nutrient and suspended solid concentrations were observed in the natural side-arm during disconnection in contrast to predictions, and it was speculated that pest fish bio-turbation was at least in part responsible for this. Zooplankton community composition was dominated by rotifers and changed as expected in the natural side-arm between connection and disconnection with a 20-fold increase in abundance during disconnection. A considerable increase in Copepoda abundance was also observed, which suggested a positive influence of side-arm disconnection on planktivorous fish communities. The artificial side-arm removed nutrients and suspended solids from the water column as expected during disconnection. Phytoplankton biomass showed some increase within the artificial side-arm as expected, with a longitudinal decrease within the side-arm, which was speculated to be caused by biotic control by zooplankton grazing. Differences between the artificial and natural side-arms were likely caused by physical differences in substrate, riparian vegetation and zooplankton accumulation of diapausing eggs caused by establishment age of the side-arms. The artificial side-arm functioned as predicted for hydraulic retention zones during disconnection, suggesting potential for side arm re-construction as a restoration method to increase lateral habitat and improve local habitat health in the lower Waikato River. Ongoing monitoring of the artificial side-arm would be beneficial to see whether increased establishment age would allow the artificial side-arm to develop functional roles indicative of a naturally occurring side-arm in the lower Waikato River.