The effects of lime addition on aquatic macrophytes in hard water: in situand microcosm experiments

1. Aquatic macrophytes are abundant in ponds and canals that are constructe d in semi-arid regions for water storage and conveyance, as well as in lake s that are culturally enriched. 2. Addition of Ca(OH)(2) to two hardwater ponds at 250 or 275 mg L-1 caused an immediate eradication of submersed aquatic plants. Although these ponds are well-buffered (alkalinity: 2.57-3.94 mequiv L-1; pH: 8.1-9.0), lime ad dition caused an immediate increase in pH of 0.2-3 units. 3. Application of 135 mg L-1 Ca(OH)(2) for 24 h or 210 mg L-1 Ca(OH)(2) for 65 h to two irrigation canals had no effect on macrophyte biomass at the l ower concentration and duration, but resulted in the elimination of aquatic macrophytes 1 month after the higher concentration, longer duration treatm ent. 4. Unlike the macrophyte control achieved following application of 210-275 mg L-1 Ca(OH)(2) to ponds or canals, microcosm experiments in which lime fo rmulation [slaked lime (Ca(OH)(2)), calcite (CaCO3), or a 1 : 1 mixture] an d concentrations (up to 1500 mg L-1) were manipulated failed to elicit a co nsistent change in macrophyte biomass. Macrophytes in microcosms treated fo r the short-term (23-33 days) with greater than or equal to 200 mg L-1 Ca(O H)(2) or a mixed Ca(OH)(2)/CaCO3 formulation always lost pigmentation, but biomass was not consistently reduced. 5. Declines in macrophyte biomass following treatment of ponds and canals m ay have been triggered by a short-term rise in pH which, in these relativel y warm (22-23 degreesC) alkaline (2.28-3.94 mequiv L-1) systems, would have resulted in low concentrations of free CO2 and bicarbonate for photosynthe sis.