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

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\ud 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