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Foraminiferal record of sewage outfall impacts on the inner Manukau Harbour, Auckland, New Zealand

Abstract The fossil foraminiferal faunas preserved in intertidal sediment in six cores document the impact of human activities on the ecosystems of the north‐east arm of the Manukau Harbour, Auckland, New Zealand. They indicate that Polynesian forest clearance and horticulture had negligible impact on the harbour biota. Before 1960, organic‐rich discharges from four large meat works around the head of Mangere Inlet produced changes in nearby foraminiferal faunas but did not affect faunas in the more seaward part of the arm. Initial early European increases in organic nutrients from the meat works resulted in the development of weak hypertrophic zones around the outfalls, characterised by higher absolute abundances of benthic foraminifera and ostracods and possible benthic algal blooms. Higher organic nutrient levels in one core closer to an outfall produced eutrophic and anoxic conditions with no foraminifera (abiotic zone). In 1960 the Mangere Sewage Treatment Plant (MSTP) was opened and the meat works and other outfalls progressively closed. This change coincided with a sharp increase in zinc (Zn) and lead (Pb) levels in sediments throughout the arm, attributed to a rapid increase in motor vehicle numbers and stormwater run‐off. The MSTP discharged vast quantities of nutrient‐ and mud‐rich fresh water into the seaward part of the arm, resulting in an order of magnitude increase in sedimentation rates throughout most of the arm and a widespread change in the foraminiferal faunas. Foraminiferal abundance generally decreased and species diversity increased with the appearance of numerous juvenile foraminiferal shells carried in by the incoming tide from subtidal habitats nearer the harbour mouth. Also appearing in large numbers in the post‐1960 sediment are planktic foraminifera, large diatom valves, and sponge sterrasters, all of which are inferred to have been transported in. The sudden increase in displaced microfossils may be a result of a combination of fresh water‐induced flocculation and increased “stickiness” of the substrate from the growth of benthic algal mats, resulting from the high nutrient output from the MSTP.