SELECTIVE SUPPRESSION OF ALGAL BLOOM IN AQUACULTURE PONDS WITH SODIUM CARBONATE PEROXYHYDRATE

Massive algal bloom formation in aquaculture ponds is a major water quality issue worldwide. The majority of these blooms are formed by cyanobacteria (blue-green algae) which oftentimes produce toxins, and are also accountable for the off-flavor in pond-raised fish species. Nutrient reduction is arguably the best strategy to control harmful algal bloom; however this is a lengthy process. Consequently, there is an urgent need for effective intervention techniques that can rapidly as well as selectively restrain the proliferation of cyanobacterial blooms without degrading overall water chemistry.

We examined the effectiveness of Sodium carbonate peroxyhydrate (SCP, trade name Pak 27^®) to selectively suppress noxious cyanobacteria in the aquaculture ponds. SCP is a relatively new hydrogen peroxide based active compound (27% H₂O₂). Overall, the aim of present study was to- (i) determine the efficacy and most optimal dose of SCP to selectively control the target noxious algal bloom (cyanobacteria) and (ii) monitor the dynamics of toxins (microcystin) and H₂O₂ residue after SCP application. To determine the appropriate dose of SCP (quantified as H₂O₂) for full-scale aquaculture ponds (0.1 acre each, bloom was raised using fertilizer), a mesocosm experiment was conducted wherein a series of circular enclosure (75L) were installed in the experimental  ponds,  and treated with a concentration of 0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mg/L H₂O₂. Changes in the strongly dominated cyanobacterium Planktothrix sp.  abundance, chlorophyll a content and eukaryotes green algae density were monitored daily till 10 days. The results suggest that H₂O₂ concentration ranging from 2.5- 3.5 mg/L can selectively suppress the cyanobacteria, therefore, we used 2.5 and 3.5 mg/L for full scale pond study.

Following pond treatments, cyanobacterial population declined by 80% in 3-5 days in a dose-dependent fashion (Fig.1). Our results also revealed that microcystin concentration decreased at a similar pattern as the cyanobacterial density, suggesting that H₂O₂ could rapidly oxidize microcystin soon after the cell lysis. However, non-targeted biota including the green algae and zooplankton community were adversely affected by 3.5 mg/L H₂O₂. The added SCP (2.5mg/L H₂O₂) actively degraded in 3 days leaving no measurable traces of H₂O₂, while persisted till 5 days for higher dose (3.5 mg/L).  Our results suggests that SCP at a concentration of 2.5 mg/L H₂O₂ can selectively mitigate harmful cyanobacterial bloom from the aquaculture ponds and leaves no long term traces of H₂O₂ in the water.