Effect and timing of increased freshwater runoff into sheltered harbor environments around Auckland City, New Zeland |
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Authors: | Bruce W Hayward Hugh R Grenfell Ashwaq T Sabaa Margaret S Morley Mark Horrocks |
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Institution: | 1. Geomarine Research, 49 Swainston Road, St Johns, Auckland, New Zealand 2. Microfossil Research, 31 Mont Le Grand Road, Mt Eden, Auckland, New Zealand
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Abstract: | Two short cores of late Holocene, low tidal, estuarine sediment from the sheltered fringes of the Auckland's Waitemata Harbor,
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 (mostlyAmmonia 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 Harbor, 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 the Lucas Creek
catchment. On the eastern, seaward fringes of the Waitemata Harbor, 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 urbanization 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. |
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