The role of zooplankton - phytoplankton community interactions in the nutrient dynamics of Lake Chapala, Mexico.

I investigated ammonia nitrogen (NH^-N) regeneration and grazing by zooplankton and their effects on phytoplankton production and biomass. Two sets of experiments were conducted on samples from Lake Chapala, Mexico from September through December, 1983. One set of experiments was designed to measure zooplankton NH^-N regeneration, and the other set was designed to measure the effects of zooplankton on phytoplankton production, biomass, and production efficiency (P/B). Lake Chapala is a large, shallow, tropical lake with a wet season from late May to September, and a dry season the remainder of the year. I sampled four stations, representing major lake regions, five times each to provide water and organisms for 20 experimental series. Samples were returned to the laboratory for enclosure experiments incubated in a large heated outdoor tank for 3 to 5 days. Polyethylene enclosures retained and/or excluded zooplankton. Six species of cladocerans were found in samples from Lake Chapala, with Ceriodaphnia pulchella as the most abundant. Only two species of copepods occurred, but they were more abundant than the cladocerans. The most frequently encountered species was Diaptomus albuquerquensis at an average density of 31.48 per liter. Zooplankton NH^-N regeneration accounted for 14.5% of ambient NH^-N concentration, for an average turnover time of 7.8 days. Regeneration provided an average of 33.0% of NH^-N uptake by phytoplankton and 93.4% of the estimated nitrogen required for phytoplankton production. Zooplankton NH^-N regeneration significantly contributed to the NH^-N pool but did not supply enough nitrogen to maintain phytoplankton production at all times during the study. Zooplankton NH^-N regeneration was greatest in September when ambient nitrogen concentrations were highest. The effect of zooplankton on phytoplankton production, biomass, and P/B ratio was less clear than the effect of zooplankton in NH^-N regeneration. Few significant differences in phytoplankton production and biomass occurred between experimental and control enclosures. The effect of zooplankton on phytoplankton production and biomass seemed to change seasonally, as NH^-N concentrations declined. The presence of zooplankton improved phytoplankton production efficiency (P/B) in the middle part of the study. The relative change in phytoplankton P/B ratio between experimental and control enclosures increased from September to mid-October and declined thereafter.