Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences

Foraminiferal record of ecological impact of deforestation and oyster farms, Mahurangi Harbour, New Zealand

Hugh R. Grenfell A C , Bruce W. Hayward A and Mark Horrocks B
+ Author Affiliations
- Author Affiliations

A Geomarine Research, 49 Swainston Rd, St Johns, Auckland 1072, New Zealand.

B Microfossil Research, 31 Mont Le Grand Rd, Mt Eden, Auckland 1024, New Zealand.

C Corresponding author. Email:

Marine and Freshwater Research 58(5) 475-491
Submitted: 31 August 2006  Accepted: 8 March 2007   Published: 22 May 2007


The microfossils in eight short sediment cores in Mahurangi Harbour, northern New Zealand, record major ecological changes since colonisation by Europeans. During the period of catchment deforestation (1850s–1900) these changes include increased sedimentation rates, decline of soft-shore molluscs, increased diatoms and seaward migration of brackish, agglutinate-dominated, foraminiferal associations, inferred to be due to increased freshwater runoff and decreased pH. The interval of most rapid change (1950s–1980s) occurred before and during the establishment of oyster farms, which therefore are not implicated. Comparison of the microfossil content of surface sediment samples taken inside and outside of four oyster farms indicates variable effects dependent on farm location. In less saline sites, the presence of oyster-shell debris in sediment beneath oyster farms appears to have buffered the foraminiferal faunas from some of the impacts of lowered salinity and pH. At more tidally flushed, saline sites, the oyster farms influence a wider surrounding area that has lower relative abundance of three species of Elphidium and other calcareous Foraminifera, attributed to slightly muddier sediment, higher nutrients and consequently lower oxygen. A newly arrived exotic foraminifer, Elphidium vellai, appears to be an indicator species for the enhanced nutrient, carbonate-rich conditions beneath oyster farms.

Additional keywords: benthic Foraminifera, freshwater runoff, pH, salinity.


We thank Shungo Kawagata for assistance in the field; Ashwaq Sabaa and Rhiannon Daymond-King for assistance in the laboratory and Barrie Forrest and Andrew Swales for critically reading the manuscript and suggesting improvements. This research was funded by the New Zealand Foundation for Research, Science and Technology.


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Appendix 1.  Taxonomic reference list
Dominant foraminiferal taxa cited in the text and figures are included, together with citations of figured specimens that reflect the taxonomic concept followed here. Species classification largely follows Hayward et al. (1999). The original descriptions of these species can be found in the Ellis and Messina world catalogue of foraminiferal species on

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