Effect and timing of increased freshwater runoff into sheltered harbor environments around Auckland City, New Zealand

Abstract

Two short cores of late Holocene, low tidal, estuarine sediment from the sheltered fringes of the Auckland’s Waitemata Harbour, New Zealand, record the following changes through time since human colonization: an abrupt decline and disappearance of marine molluscs, a major decline and virtual disappearance of ostracods, an abrupt decline in calcareous foraminifera (mostly Ammonia spp.), a rapid increase in abundance of agglutinated foraminifera, large diatoms, and freshwater thecamoebians, and an increase in sedimentation rate, but no consistent trend in change of grain-size.

The up-core foraminiferal changes mimic their present day up-estuary zonation, which correlates strongly with decreasing salinity and pH. In both localities the faunal changes can be correlated with the documented local land-use history and increased freshwater runoff over time. At the head of the Waitemata Harbour, in Lucas Creek estuary, three phases of foraminiferal faunal change occurred: minor changes during initial Polynesian forest clearance (1500-1800 AD), a major change in early European times (1840-1870 AD) with clearance of most of the remaining native forest, and another small change in very recent times (~1990s) with urbanization in Lucas Creek catchment. On the eastern, seaward fringes of the Waitemata Harbour, in the smaller Tamaki Estuary, no faunal changes occurred in association with complete forest clearance and establishment of pastoral farming in Polynesian and early European times (before 1950s). Major foraminiferal and other faunal changes occurred in the late European period (1960s-1970s) coincident with the onset of major urbanisation spreading throughout the Tamaki catchment. Our results suggest increased freshwater runoff is the major culprit for many of the observed biotic changes in the urbanized estuaries of New Zealand.