New Zealand Journal of Marine and Freshwater Research abstracts
Foraminiferal record of sewage outfall impacts on the inner Manukau Harbour,
Auckland, New Zealand
Andrew Matthews
Geology Department
University of Auckland
Private Bag 92 019
Auckland, New Zealand
Hugh R. Grenfell
Bruce W. Hayward
Geomarine Research
49 Swainston Rd, St Johns
Auckland, New Zealand
email: b.hayward@geomarine.org.nz
Mark Horrocks
Microfossil Research
31 Mont Le Grand Rd, Mt Eden
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.
Keywords New Zealand; Manukau Harbour; Mangere Inlet;
foraminifera; sewage outfalls; pollution
M04133; Online publication date 18 February 2005
Received 25 August 2004; accepted 2 December 2004
New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39:
193–215
0028-8330/05/3901-0193 © The Royal Society of New Zealand 2005
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