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.

本研究基于新西兰奥特亚罗瓦地区霍罗温湖（Lake Horowhenua）逾40年的常规监测数据、专项研究成果，以及湖内监测浮标采集的高频（15分钟间隔）监测数据，推导得出影响该湖蓝藻水华的相关因素。霍罗温湖目前处于重度富营养状态，其生态状态在两种模式间切换：冬季为清水态，以大型水生植物菹草（*Potamogeton crispus*）占优；仲夏至秋季则转为浊水态，浮游植物占主导，伴随大规模蓝藻（微囊藻属，*Microcystis sp.*）水华暴发。微囊藻生长所需的氮素来源于农业与园艺活动产生的面源污染，经地下水及泉水补给溪流输入湖体；当菹草群落衰亡时，沉积物会释放磷营养盐。上述营养盐会促进微囊藻属物种增殖，直至水体水温降至15℃以下。微囊藻属物种可将水体pH提升至10以上，进而诱导沉积物再次释放磷素，维持自身种群生长。高pH环境会将沉积物释放的无毒氨氮（ammonium-N）转化为有毒非离子氨（unionised NH₃），这可能引发夏秋季鱼类大规模死亡。受文化因素限制，当地治理方案无法采用磷钝化剂（P-inactivation agents）；且通过喷洒药剂清除大型水生植物床群，会导致霍罗温湖永久转为浊水态。当前正试点采用水草收割机治理大型水生植物床群，以期借助沉积物种子库中的本土物种逐步替代菹草（*P. crispus*）。