title: Modifying the high rate algal pond light environment and its effects on light absorption and photosynthesis.
abstract: The combined use of high rate algal ponds (HRAPs) for wastewater treatment and commercial algal production is considered to be an economically viable option. However, microalgal photosynthesis and biomass productivity is constrained in HRAPs due to light limitation. This paper investigates how the light climate in the HRAP can be modified through changes in pond depth, hydraulic retention time (HRT) and light/dark turnover rate and how this impacts light absorption and utilisation by the microalgae. Wastewater treatment HRAPs were operated at three different pond depth and HRT during autumn. Light absorption by the microalgae was most affected by HRT, significantly decreasing with increasing HRT, due to increased internal self-shading. Photosynthetic performance (as defined by Pmax, Ek and α), significantly increased with increasing pond depth and decreasing HRT. Despite this, increasing pond depth and/or HRT, resulted in decreased pond light climate and overall integrated water column net oxygen production. However, increased light/dark turnover was able to compensate for this decrease, bringing the net oxygen production in line with shallower ponds operated at shorter HRT. On overcast days, modelled daily net photosynthesis significantly increased with increased light/dark turnover, however, on clear days such increased turnover did not enhance photosynthesis. This study has showed that light absorption and photosynthetic performance of wastewater microalgae can be modified through changes to pond depth, HRT and light/dark turnover.
authors.affiliation: National Institute of Water and Atmospheric Research Ltd. (NIWA), PO Box 8602, Christchurch, New Zealand; School of Biological Sciences, University of Canterbury, Christchurch, New Zealand. Electronic address: Donna.Sutherland@niwa.co.nz.
authors.forename: Donna L
authors.initials: DL
authors.lastname: Sutherland
authors.affiliation: National Institute of Water and Atmospheric Research Ltd. (NIWA), PO Box 11-115, Hamilton 3200, New Zealand. Electronic address: Valerio.Montemezzani@niwa.co.nz.
authors.forename: Valerio
authors.initials: V
authors.lastname: Montemezzani
authors.affiliation: National Institute of Water and Atmospheric Research Ltd. (NIWA), PO Box 8602, Christchurch, New Zealand. Electronic address: Clive.Howard-Williams@niwa.co.nz.
authors.forename: Clive
authors.initials: C
authors.lastname: Howard-Williams
authors.affiliation: Centre for Integrative Ecology, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand. Electronic address: Matthew.Turnbull@canterbury.ac.nz.
authors.forename: Matthew H
authors.initials: MH
authors.lastname: Turnbull
authors.affiliation: School of Biological Sciences, University of Canterbury, Christchurch, New Zealand. Electronic address: Paul.Broady@canterbury.ac.nz.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Broady
authors.affiliation: National Institute of Water and Atmospheric Research Ltd. (NIWA), PO Box 11-115, Hamilton 3200, New Zealand. Electronic address: Rupert.Craggs@niwa.co.nz.
authors.forename: Rupert J
authors.initials: RJ
authors.lastname: Craggs
mesh_terms: D018533:Biomass; D003624:Darkness; D008027:Light; D058086:Microalgae / Q000378:metabolism / Q000502:physiology / Q000528:radiation effects*; D008962:Models, Theoretical; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis
chemical_list: D010100:Oxygen
keywords: High rate algal ponds; Light absorption; Light utilisation; Photosynthesis; Wastewater