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Stalk forming fouling diatoms : a problem for the hydro- electricity industry

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Ravizza, M (2015) Stalk forming fouling diatoms : a problem for the hydro- electricity industry. PhD thesis, University of Tasmania.

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Abstract

The Tarraleah power scheme (102.2 MW) is a significant component of Tasmania’s
Hydro-Electric generation system operated by Hydro Tasmania, which contributes
60% of Australia’s renewable energy. The efficiency of this scheme is negatively
impacted by the presence of biofouling, consisting mainly of diatoms and bacterial
slimes, which can grow up to 5 mm thick on the canal walls and cause up to 10%
reduction in flow carrying capacity, therefore reducing power generation capability.
In Tarraleah No. 1 Canal biofouling is constantly present, although the amount
varies seasonally, from location to location, and dependent on canal cleaning
regimes. Engineering studies have demonstrated that the effective roughness of the
biofilm is much greater than its physical roughness, which has implications for the
frictional drag induced by the biofilm. Observations of the canal wall during
outages indicate that fouling is less prevalent on the southern wall of the canal,
which is less shaded by vegetation, thus indicating that the fouling diatoms prefer
low light conditions. The succession of fouling diatoms begins with Tabellaria
flocculosa which dominates under low flow regimes (< 2 m s-1) and progresses to
the stalk-forming Gomphonema tarraleahae which is the dominant species in high
velocity areas.
In this study, the target species G. tarraleahae was compared with other stalk
forming diatoms, both marine and freshwater species (Licmophora flabellata,
Didymosphenia geminata and Gomphonema cf. manubrium), to elucidate the
environmental factors promoting their stalk formation and to determine the best
strategies to mitigate their impact. Both laboratory culture studies and field
experiments were conducted.
Simulated laboratory trials were conducted on the marine stalk-forming diatom
Licmophora flabellata, to define environmental factors influencing its growth and
stalk formation. Growth rates in multiwell plates were estimated using in vivo
fluorescence in a plate reader. Low to moderate light intensities (50, 100 and 150
μmol photons m-2 s-1) produced no or poor growth, while growth rates of 0.24 -0.42
div/day were achieved at the highest light intensities of 233 μmol photons m-2 s-1.
High growth rates coincided with long stationary growth periods (21-36 days) and
long (800-3500μm) branching stalks. With this eutrophic estuarine diatom N, P and Si nutrients had no effect on growth nor stalks, while high turbulence reduced stalk
length but not growth.
Pulse Amplitude Modulated (PAM) fluorometry was applied to define light and
nutrient responses of the diatom biofouling on both walls of Tarraleah No. 1 Canal
at three different depths (top, medium and bottom). Rapid light curves (RLC) and
Fv/Fm (indicators of physiological health) confirmed that biofouling was suppressed
by high light (on the south wall at a depth less than 1 m) and inhibited by silica
whilst nitrogen or phosphate addition had no effect.
To further confirm the critical role of light for Tarraleah hydrocanal fouling an
experimental rig was designed and built on the banks beside the no.1 Canal. It
comprised four pipes through which natural Tarraleah water was allowed to flow at
1.22 m s-1 but under varying substratum conditions (metal, opaque PVC, frosted
PVC and clear PVC) and light levels (0 in the metal pipe and maximum of 6, 1937
and 2957 μmol photons m-2 s-1 in the opaque PVC, frosted PVC and clear PVC
pipes, respectively). Nutrient bioassays using PAM fluorometry on fouling samples
harvested on a monthly basis during winter and spring showed significant effects
of both pipe material and nutrients on photosynthetic performance. Optimal fouling
dominated by T. flocculosa (81.4 – 96.4% of cells) occurred at medium light (in
opaque and frosted PVC), and Fv/Fm responded positively to silica in April 2013
and to nitrogen and phosphate addition in July 2013. Unexpectedly,
photosynthetically viable diatom fouling communities (Fv/Fm 0.36-0.59) were
observed in the metal pipe even after 2 months in complete darkness.
The New Zealand stalk-forming diatom Didymosphenia geminata represents a
potentially invasive freshwater pest, of considerable concern if it were to become
established in Tasmania. Comparative PAM nutrient responses were studied for
New Zealand Buller River D. geminata, Lake Rotoiti G. cf. manubrium and
Tasmanian G. tarraleahae. Although these freshwater stalk-forming diatoms had
different requirements in terms of light, flow rate and nutrients, all three species
were inhibited by silica addition while D. geminata was also stimulated by iron.
Like G. tarraleahae, D. geminata blooms preferentially occur in hydrologically
stable oligotrophic waters, but are remarkably tolerant towards light and occur over
a wide range of flow velocities. Comparative PAM nutrient assays on L. flabellata cultures showed no prohibitive effect from silica addition with that eutrophic
diatom.
The present results call for species-specific approaches towards mitigating the
impact of fouling diatoms. In L. flabellata and D. geminata high light intensities
stimulated stalk formation and length. While these species were adapted to stable
low flow waters, G. tarraleahae showed preferences for high velocity flow and low
light intensities. L. flabellata showed more tolerance towards nutrient addition and
could grow under a wide range of temperatures, but D. geminata and G. tarraleahae
instead were adapted to oligotrophic waters and cold temperatures. Currently
mitigation methods in Tarraleah No. 1 Canal involve scrubbing of the canal walls
on a regular basis, but this is expensive and benefits are short-lived with rapid
fouling regrowth occurring, notably in spring. Alternative mitigation solutions
including clearing vegetation along canals and the application of white paint on the
canal walls have shown better promise to increase light-intensity to mitigate lowlight
adapted G. tarraleahae fouling. However, this clearly is not a universal
strategy that can be applied to all fouling diatoms. Undoubtedly, the best approach
for all fouling species would be to stop their attachment to the substratum, even
though this remains a challenging task that requires continued research.

Item Type: Thesis (PhD)
Keywords: diatoms, stalk formation, biofouling, Gomphonema tomoleahae
Copyright Holders: The Author
Copyright Information:

Copyright 2015 the author

Additional Information:

Chapter 4 appears to be the equivalent of a post print of an article published as: Matilde Ravizza and Gustaaf M. Hallegraeff, 2015, Environmental conditions influencing growth rate and stalk formation in the estuarine diatom Licmophora flabellata, Diatom Research, 30(2), 197-208. The Version of Record of this manuscript has been published and is available in Diatom Research 15/4/2015 http://www.tandfonline.com/10.1080/0269249X.2015.1020071

Date Deposited: 16 Aug 2016 03:05
Last Modified: 16 Aug 2016 03:06
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