Bioavailability of phosphorus transported during storm flow to a eutrophic, polymictic lake

Abstract

The bioavailability of catchment-derived particulate phosphorus (PP) to lentic phytoplankton was investigated. Two stream inflows to Lake Rotorua (Bay of Plenty, New Zealand) were sampled during moderate (March) and large (August) discharge peaks in 2012. Phosphorus (P) fractions in samples were quantified and a bioassay was undertaken using samples from the March event to determine whether P-limited phytoplankton could utilise PP. Phosphorus composition differed markedly between events but was comparable between streams. For the March event, all PP (persulphate extractable) was bioavailable if exposed to anoxia, whereas c. 25% was potentially bioavailable for the August event. Despite this, the laboratory bioassay indicated that, under oxic conditions, suspended sediments acted as a sink rather than a source of bioavailable P to P-limited phytoplankton. This study highlights the importance of considering P speciation and the nature of the receiving environment when assessing the bioavailability of P transported through hydrological landscapes.