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Benthic diatoms as bioindicators of a point source thermal discharge to an estuary


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Ingleton, TC 2012 , 'Benthic diatoms as bioindicators of a point source thermal discharge to an estuary', PhD thesis, University of Tasmania.

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The work presented here describes a novel approach for determining past and present effects
of a power station thermal plume discharge on an estuary. The technique utilised benthic
diatoms, combining palaeoecological and contemporaneous multi-taxa approaches, to
demonstrate that diatoms were not only responding to the plume in the modern day
environment, but also indicated a changing temperature regime within the receiving water bay
over time. The approach provided new information and demonstrated a range of techniques
that may be employed for improved management of thermal discharges to enclosed coastal
water bodies.
To determine the spatial distribution of benthic diatoms in relation to a power station
cooling field, the discharge at Vales Point, Lake Macquarie, was selected as the study site and
first sampled in 2003. The initial fieldwork sampled sediments along a thermal gradient with
increased distance from the discharge point. This spatial pattern was then replicated within
two other southern lake control embayments. Multi-variate analyses determined that
temperature and a number of other variables associated with the plume explained gradients in
the diatom flora of the receiving bay. Satellite imagery and high resolution logger data then
provided detailed plume and lake temperature information and indicated that thermal loading
of lake water and thus the potential for ecological effects were expected to be greater in
autumn/winter compared to summer.
To determine changes in receiving bay water quality over time and to establish prepower
station baseline conditions, a sediment core was obtained from Wyee Bay, sub
sectioned, 210Pb dated and analysed for fossil benthic diatoms. Diatom-inference models were
then developed based upon a localised reference dataset obtained from sample sites across
Lake Macquarie and NSW central coast estuaries. The assemblage profile displayed periods
(several years to decades) of relative homogeneity and heterogeneity (years to a decade), coincident
with the major phases of power station operation. A change in assemblages was also
observed around 1925-30 and prior to power station commissioning, indicating a change to
receiving bay ecology at an earlier time. Although the salinity and temperature data were both adequate for modelling, reconstruction errors meant that only longer-term trends were
discernable for temperature. When compared to real-time data (monitoring) variability in
salinity within the core was relatively well represented by the model and appeared to respond
to climatic factors such as SOI and rainfall. Longer-term salinity trends, however, were not as
reliable as those for temperature.
In an effort to improve the temperature-inference model for the Wyee Bay core an
alternative reference dataset was developed by sampling across a natural temperature
gradient. Triplicate samples from 13 estuaries from Noosa (southern Queensland) to Eden
(southern NSW) (26 °S – 37 °S) were analysed for benthic diatoms and combined with
environmental variables to establish a Temperature-Latitude (TL) dataset. Sensitivity testing
was also conducted to examine the roles of eveness/unevenness (structure) within the sample
design, numbers of species and taxonomic resolution on the multi-variate analyses and model
output. While latitude, salinity and phosphate were the variables that consistently explained
the greatest proportion of variability across the different datasets, salinity, temperature and
nutrients were dominant when latitude was removed. Most models using TL datasets
improved reconstruction errors; however, the nested (nTL1) dataset provided the best
inferred-temperature history for Wyee Bay when validated by long-term monitoring data.
Generally, the environmental variables attributable to gradients between lakes were the same
regardless of the number of species, sample design or taxonomic resolution.
To increase our understanding of the changes in diatoms and lake ecology over a
greater pre-power station and pre-industrial (heavy metals) period two additional cores were
obtained for the final phase of the study. At both a plume-effected and a control site cored
sediments were analysed for heavy metals and 210Pb to establish and cross validate
geochemical chronology and identify pre-industrial boundaries. While only sediments of the
Crangan Bay core were preserved adequately for dating it provided key information relative
to the original Wyee Bay core. Prior to heavy metal enrichment in the southern lake (1925-
1942), Crangan Bay (control) (20-42 cm) and Wyee Bay (pre-1935) supported a similar
diatom flora. Thus, it was inferred that the environmental condition of the lake’s southern embayments were likely to be similar at that time and relatively stable for Crangan Bay to at
least ~1790.
The work described here demonstrated the applicability of benthic diatoms as tools
for understanding spatial and temporal changes in a coastal lake associated with a power
station thermal plume. Diatoms and the multi-taxa approach could be utilised as bioindicatortype
tools for regular broadscale assessments of south-east coast estuarine health.

Item Type: Thesis - PhD
Authors/Creators:Ingleton, TC
Keywords: benthic diatoms, pollution, point source temperature
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