Identification of total phosphate, submerged vegetation cover and zooplankton size thresholds for success of biomanipulation in peri-urban eutrophic ponds

In small shallow lakes and ponds, the clear-water state can generally be maintained at higher nutrient concentrations compared to larger shallow lakes. The main objective of this study was to identify thresholds for total phosphorus (TP), submerged vegetation cover and zooplankton size that determine biomanipulation success in peri-urban eutrophic ponds. Additionally, the relationship between transparency and TP is discussed with regard to similar relationships and thresholds reported for shallow lakes. Using classification trees, a threshold TP concentration of 0.300 mg P L-1 was determined below which a clear-water state was generally maintained after biomanipulation. When the average TP concentration was >0.300 mg P L-1, the stability of the clear-water state largely depended on the presence of sufficiently large zooplankton (>0.87 mm) or a submerged vegetation cover of >82% at some point during the year. This threshold TP concentration is considerably higher than the threshold of 0.1 mg L-1 which is generally suggested for longer-term success of biomanipulation in shallow lakes. Such threshold nutrient concentration is important when restoring ecological quality in eutrophic small lakes and ponds. Extended follow-up of biomanipulation success in eutrophic ponds could provide more insight into the feasibility of these thresholds on the longer term.