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Water clarity in two shallow lake systems of the Central Plateau, Tasmania, Australia

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Uytendaal, AR (2006) Water clarity in two shallow lake systems of the Central Plateau, Tasmania, Australia. PhD thesis, University of Tasmania.

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Abstract

Lake Crescent and Lake Sorell are two adjacent, large, shallow, mesotrophic lakes of
the Central Plateau, Tasmania, Australia. Both lakes are in the same catchment and
have similar geological and morphological characteristics. Early limnological work by
Cheng and Tyler found trophic characteristics to be substantially different, despite
their physical similarities. Historically, each lake exhibited strongly contrasting
'stable-states': Lake Sorell was a macrophyte-dominated clear water system, while
Lake Crescent was turbid and dominated by phytoplankton. Cheng and Tyler dubbed
this a "limnological paradox".
Since the late 1990s, the quality of the trout fishery declined, nutrient and algal
concentrations increased markedly and water clarity declined dramatically. These
changes coincided with unprecedented low water levels due primarily to severe
drought and competition for water by various users. The rapid decline in water clarity
prompted this investigation to determine the underlying processes responsible for the
degradation and to recommend management strategies to improve water quality.
Variables limiting light attenuation, turbidity and water clarity were measured from
April 2000 to August 2002, and modelled using multiple linear regression. Regression
coefficients were used to estimate the relative importance of each water quality
component, and this analysis showed that high levels of inorganic suspensoids were
largely responsible for the decline in water clarity. Although there were increases in
nutrients and suspended sediment in these lakes in the late 1990s, detailed analysis of
inputs to and outputs from these lakes suggested that these increases were derived
from internal sources as inputs from the surrounding catchment were negligible.
The historical record oflake levels was analysed to quantify the areas oflake bed that
would be in contact with the wave base under differing wind conditions and water
levels. This showed that both lakes were more prone to wind effects after 1998 owing
to the lower water levels. Further, prior to 1999 Lake Crescent was, on average, more
turbulent and more prone to wind-driven resuspension events than Lake Sorell, which
suggests a potential mechanism underlying Cheng and Tyler's "limnological
paradox".
The physical disturbance of sediments from wind-driven waves was further
investigated by calculating shear stress from wave theory, and quantifying
relationships between shear stress and suspended sediment concentration. Shear stress
characteristics across the lake basins were modelled under various lake levels and
wind speeds, and the magnitude of shear stress increased dramatically at lower lake
levels.
DYRESM-CAEDYM was used to develop a sediment resuspension model relating
wind, lake-level and sediment flux that was then calibrated and verified against field
observations. (The ecosystem model CAEDYM (ComputAtional Ecosystem
Dynamics Model) is coupled with the hydrodynamic driver DYRESM (DYnamic
REservoir Simulation Model) to accomplish these simulations). The model was used
to ascertain the benefits of managing water levels in the lakes to ameliorate the affects
of sediment resuspension and improve water quality. This modelling suggested that
the "degraded" state of the lakes from the late 1990s was initiated and sustained by
low lake levels leading to increased shear stress acting on the sediments. The
increased nutrient concentrations and algal biomass were also found to result from
low lake levels because external nutrient loading was insignificant. Conversely, the
modelling showed that raising water levels would dramatically improve water clarity.
Alternative, trophically-based explanations of the differences between lakes Crescent
and Sorell were examined by investigating the biotic interactions that influence water
quality and ecosystem function. The aim was to determine if trophic cascades and
stable-state theory would help explain the contrasting phycology of these two lakes.
The historical biological data from lakes Sorell and Crescent was reviewed and
reanalysed, and contemporary data collected to compare the trophic structure of the
"degraded" status at the end of the 1990s with the historical record. The strong
contrasts in phytoplankton productivity and community composition evident between
the lakes in the past were still prominent. The algal community of Lake Crescent has
concentrations up to 100 x those of Lake Sorell, and is still dominated by diatoms
while green algae dominate Lake Sorell. The zooplankton of Lake Crescent is dominated by small cladocerans and copepods, while Lake Sorell has more frequent
occurrences of larger cladocerans such as Daphnia. Lake Crescent also has an order
of magnitude greater biomass of the zooplanktivorous fish, Galaxias auratus, than
Lake Sorell, which leads to a much greater (up to 30 fold) predation pressure on large
zooplankters. By contrast the pattern in biomass of the introduced piscivorous brown
trout (Salmo trutta) between the two lakes is reversed. While some of these patterns
are consistent with differences in the nature of top-down trophic cascades between the
lakes, the accumulated evidence suggests that such relationships break down at the
link between zooplankton and phytoplankton.
The empirical evidence collected suggests that zooplankton grazing had little effect on
limiting phytoplankton productivity in either lake for any significant period of time,
while the greater dominance of meroplanktonic diatoms and the greater susceptibility
of Lake Crescent to wind-driven resuspension suggests a more parsimonious
explanation of the persistent phytoplankton dominance in this lake.
DYRESM-CAEDYM was then employed to investigate plankton and meroplankton
dynamics in lakes Sorell and Crescent, since this technique can be use to test 'N-P-Z'
(nutrients-phytoplankton-zooplankton) models. The hypothesis tested was that
developed above: that differences in resuspension combined with contrasts in the
proportions of meroplanktonic phytoplankton were sufficient to explain the
differences between the two lakes. Modelling of plankton and meroplankton dynamics
in both lakes returned significant contrasts in algal productivity that were driven
largely by contrasting sediment resuspension dynamics between the lakes.
It was concluded that the differing phytoplankton communities of the lakes are a
result of contrasting sediment resuspension dynamics between the lakes, with a
limited influence from contrasting levels of zooplankton grazing pressure.

Item Type: Thesis (PhD)
Keywords: Lake sediments, Limnology, Suspended sediments
Copyright Holders: The Author
Copyright Information:

Copyright 2006 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2006. Includes bibliographical references

Date Deposited: 04 Feb 2015 23:33
Last Modified: 13 Jul 2016 00:06
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