Foraminiferal record of human impact on intertidal estuarine environments in New Zealand's largest city

Abstract

Fossil foraminiferal faunas were studied in four, short, late Holocene cores (two localities) from the low tidal, estuarine fringes of the Waitemata Harbour, which is surrounded by New Zealand's largest city, Auckland. All cores record similar major changes in their fossil content since the arrival of humans (ca. 1300 AD), with faunal changes continuing through to the 1970s. Molluscs have disappeared from all cores, and the foraminiferal faunas have switched from dominantly calcareous ( Ammonia association) to dominantly agglutinated ( Textularia– Schlerochorella, Miliammina– Haplophragmoides associations). A two-step change is evident, with an intermediate mixed calcareous-agglutinated faunal zone (1950s–1970s), characterised by peak abundances (13–20%) of Elphidium gunteri and Elphidium excavatum s.l. The faunal changes in each core since human colonisation replicate the faunal zonation in a low tidal transect of surface samples going up the Rangitopuni Estuary at the head of the Waitemata Harbour. Canonical correspondence analyses of the foraminiferal and environmental proxy data from the cores and the modern estuary transect indicate that faunal changes can be largely attributed to decreasing salinity, and additionally lowered pH (causing carbonate dissolution) in the more brackish Rangitopuni Estuary core locality. There is a weak correlation with increased nutrients (TOC, N, P), but sediment grain size and increasing heavy metal concentrations played no major part in producing the faunal changes. In the estuary transect, complete dissolution of calcareous foraminiferal shells varies between years, but occurs upstream where pH falls below ∼7.5. The absence of deformed foraminiferal shells in any cores suggests that neither natural environmental stress nor heavy metal concentrations (Pb 40–100 ppm; Zn 130–250 ppm) were sufficient for these to be generated. All cores contain a major hiatus between the prehuman and late European (post 1950) periods, coincident with mollusc shell layers and the first major change in foraminiferal faunas. The decrease in salinity, indicated by faunal changes, was probably a result of increased freshwater runoff associated with forest clearance in Polynesian (ca. 1300–1840) and early European (1840–1900) times. Faunal changes were more significant because of more severe salinity decreases in Rangitopuni Estuary cores with its larger catchment. Sedimentation has increased throughout the harbour since the 1950s, as a result of land modifications associated with accelerating urban subdivisions. More faunal changes linked to further decreased salinity, occurred in the 1970s, probably attributable to an increase in impervious surfaces (roofs and paved areas) with the rapid growth of Auckland city.