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 (Potamogeton crispus), and a turbid phytoplankton dominated phase with substantial cyanobacterial (Microcystis sp.) 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 Microcystis sp., until the water temperature falls below 15 degrees C. Microcystis sp 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 P. crispus with indigenous species from the seed bank in the sediment.