Sediment dredging and Phoslock® addition were applied individually and in combination in an enclosure experiment in a Dutch hypertrophic urban pond. These measures were applied to control eutrophication and reduce the risk of exposure to cyanobacterial toxins. Over the 58 days course of the experiment, cyanobacteria (predominantly Microcystis aeruginosa) gradually decreased until they dropped below the level of detection in the combined treated enclosures, they were reduced in dredged enclosures, but remained flourishing in controls and Phoslock® treated enclosures. Cyanobacteria were, however, less abundant in the enclosures (medians chlorophyll-a 30–87 μg l−1) than in the pond (median chlorophyll-a 162 μg l−1), where also a thick surface scum covered one-third of the pond for many weeks.
Soluble reactive phosphorus (SRP), total phosphorus and total nitrogen concentrations were significantly lower in the combined dredged and Phoslock® treated enclosures than in controls. Median SRP concentrations were 24 μg P l−1 in the combined treatment, 58 μg P l−1 in dredged enclosures, and 90 μg P l−1 in controls and 95 μg P l−1 in Phoslock® treated enclosures. Hence, the combined treatment was most effective in decreasing SRP and TP, and in lowering cyanobacterial biomass.
Microcystin (MC) concentrations were analyzed by LC–MS/MS. MC concentrations and cyanobacterial biomass were positively correlated in all treatments. Mean MC concentrations in controls (71 μg l−1), Phoslock® treated enclosures (37 μg l−1) and dredged enclosures (25 μg l−1) exceeded the provisional guideline of 20 μg l−1, whereas mean MC concentrations were 13 μg l−1 in the combined treated enclosures. All samples contained the MC variants dmMC-RR, MC-RR, MC-YR, dmMC-LR and MC-LR; traces of MC-LY and nodularin were detected in few samples. The different treatments did not change the relative contribution of the variants to the MC pool; MC profiles in all treatments and the pond showed dominance of MC-RR followed by MC-LR. In the surface scum of the pond, total MC concentration was extremely high (64000 μg l−1 or 1300 μg g−1 DW), which poses a serious health hazard to children playing on the banks of the pond. Based on our results and pond characteristics, we propose combined sediment dredging and Phoslock® addition, fish removal and strong reduction of duck feeding by the neighborhood as most promising measures controlling cyanobacterial hazards in this pond.