Factors influencing cyanobacteria blooms: review of the historical monitoring data to assess management options for Lake Horowhenua

Factors influencing cyanobacterial blooms in Lake Horowhenua, Aotearoa/New Zealand, have been deduced from over 40 yrs routine monitoring data, detailed studies and high frequency (15-min interval) data from an in-lake monitoring buoy. Lake Horowhenua is currently hypertrophic but flips between a winter clear-water, macrophyte dominated phase (<i>Potamogeton crispus),</i> and a turbid phytoplankton dominated phase with substantial cyanobacterial (<i>Microcystis sp.</i>) blooms from mid-summer to autumn. Nitrogen for growth comes from agriculture and horticulture via groundwater and spring-fed streams. Phosphorus is released from the sediment when the weed beds die back. These nutrients stimulate the growth of <i>Microcystis sp.</i>, until the water temperature falls below 15°C. <i>Microcystis sp</i> can raise the pH to over 10, which releases P from the sediment, sustaining its growth. High pH transforms non-toxic ammonium-N released from the sediment to toxic unionised NH₃, which may contribute to fish kills in summer/autumn. Management strategies cannot use P-inactivation agents for cultural reasons and eradicating the macrophyte beds by spraying would cause the lake to permanently flip to a turbid state. Management of the macrophyte beds with a weed harvester is being trialled to allow a gradual replacement of <i>P. crispus</i> with indigenous species from the seed bank in the sediment.