Evaluating a New Method of Controlling Noxious Algal Blooms in Urban Wetlands

Cyanophyta (blue-green algae, commonly referred to as cyanobacteria) blooms in urban wetlands often indicate nutrient enrichment and ecosystem degradation. Despite this, such blooms are common and well documented in the Perth metropolitan area.  While cyanophyta can be present in healthy wetlands, high abundances of certain taxa are unsightly and can produce toxins (cyanotoxins) that can affect the liver or nervous system, becoming a health hazard.

Several local councils in Perth have recently begun the application of a new product designed to control noxious algal blooms. The product comprises micronutrients, including silica, which promote diatom growth. The premise of the product is that the increase in density and richness of diatoms effectively reduces nutrient availability (particularly nitrate) for nuisance algal growth. While the application of this product has been successfully trialled internationally, no formal evaluation had been conducted in Australia.

Biologic recently undertook phytoplankton and diatom sampling at two local wetlands, before and after the first application, to assess changes in algal assemblages and gauge the success of its use. Promising improvements in water quality were recorded post-application, including a significant reduction in nitrogen nutrients (specifically nitrogen oxides and nitrate) and a substantial increase in dissolved oxygen saturation. Chlorophyll a also underwent a significant reduction between sampling events, suggesting a reduction in photosynthesis post-application.

We also found diatom richness and evenness increased significantly between sampling events. While the potentially toxic Microcystis sp. persisted in high abundances at one of the lakes following application, Nodularia was substantially less abundant, indicating some success of the application in reducing nuisance algae. The presence of diatom species associated with reference sites in Perth, such as Synedra ulna, post-application, was also indicative of improving water quality. While some improvements in ecosystem health were recorded following application in both lakes sampled, further evaluation would be required to completely test the product’s efficacy. The study did highlight the value of monitoring phytoplankton and diatom assemblages, two aspects of aquatic systems that often go unmonitored.

 The potentially toxic Microcystis sp. recorded from one of the lakes pre-application. Photo credit: Syngeon Rodman (Biologic Environmental Survey).