Photosynthetically Elevated pH as a Factor in Zooplankton Mortality in Nutrient Enriched Ponds

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Abstract
During a nutrient enrichment study of eight small ponds, the mortality of crustacean zooplankton species was investigated using a life table technique. There was close agreement between high pH values and the disappearance of crustacean zooplankton in four of the highly fertilized ponds. Pond water containing phytoplankton was toxic to the dominant Ceriodaphnia reticulata, yet pond phytoplankton isolated from pond water and resuspended in chlorinated tap water supported excellent growth of C. reticulata. Millipore—filtered pond water was not toxic. Analysis of chemical changes occurring in pond water with filtration showed a slight decrease in pH as the only significant alteration that is directly related to zooplankton mortality. The existence of such a pH mortality factor was demonstrated by testing the survivorship of newborn. C. reticulata in artificially adjusted pH treatments. A change from pH 10.8 to 11.2 radically altered the survivorship. Other hypothesis to explain the disappearance of C. reticulata–predation, starvation, allelophathic chemicals–are discounted. It is suggested that the one reported case of allelopathy between a green alga and a cladoceran, that reported by Ryther (1954), may instead have been partly a pH mortality factor. It is further suggested that in culturally enriched waters of low buffering capacity such a pH mortality factor may be common. In such lakes and ponds the quality of the water deteriorate due to the loss of the zooplankton.

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