Experiments utilizing epilimnetic water were conducted to determine the rate of ammonium regeneration, due to zooplankton excretion and microbial mineralization processes, in relation to the rate of inorganic nitrogen assimilation by phytoplankton. The euphotic zone of dimictic, meso-oligotrophic Castle Lake is characterized by a rapid depletion of both nitrate and ammonium soon after spring thaw. External inputs of nitrogen are minimal during this period and levels of inorganic nitrogen remain low until fall overturn. The rate of ammonium assimilation was high relative to nitrate assimilation, and was significantly correlated with the rate of ammonium regeneration. The importance of this rapid turnover of nitrogen to phytoplankton growth is consistent with the results of previous Castle Lake studies, and with the current conceptual model of primary production in the mixed layer of nitrogen-deficient marine ecosystems.

The net effect of zooplankton on phytoplankton productivity was investigated through experiments using natural concentrations of plankton in the epilimnion of Castle Lake, California . Zooplankton enhanced algal productivity during July and August, and nocturnal grazers caused greater proportionate increases than did daytime densities . Consumers had neutral or negative effects in September . Animal biomass was associated significantly with enhanced productivity for the experiment in late July, and with decreased growth rates in the last trial of September . The direct relationship between the activities of zooplankton and primary productivity observed in other lakes is qualified by this seasonal change in net effect. The removal of algae by grazing, increases in the productivity to biomass ratio through nutrient regeneration and temporary photosynthetic inhibition from ammonia excretion appear to have shifted in relative impact during the 3 month experimental period .

Fungal infection of calanoid copepod (Diaptomus novamexicanus) eggs was observed in each of three years in an alpine lake (Castle Lake, California, U.S.A.). Stages in the infection process were examined by light and scanning electron microscopy and evidence was obtained that the Lagenidium-Yikt fungus concerned was a virulent parasite. Fungal destruction of eggs varied in timing and severity from year to year. The maximum impact of the disease was an estimated 48.4% decrease in potential copepod recruitment in 1976 due to the onset of a severe epidemic early in the summer growing season. The minimum impact, a 5.6% decrease in potential recruitment, was recorded in 1975. In this year the proportion of infected eggs was reduced and large numbers of juveniles had been released before the fungal disease began. The 1974 epidemic was intermediate in severity. The effect of these epidemics on Castle Lake calanoid populations is discussed in relation to temperature, food availability and predation.

This paper summarizes concepts underlying the atmospheric input of phosphorus (P) to ecosystems, published rates of P deposition, measurement methods, and approaches to future monitoring and research. P conveyed through the atmosphere can be a significant nutrient source for some freshwater and marine ecosystems. Particle sources and sinks at the land-air interface produce variation in P deposition from the atmosphere across temporal and spatial scales. Natural plant canopies can affect deposition rates by changing the physical environment and surface area for particle deposition. Land-use patterns can alter P deposition rates by changing particle concentrations in the atmosphere. The vast majority of P in dry atmospheric deposition is conveyed by coarse (2.5 to 10 μm) and giant (10 to 100 μm) particles, and yet these size fractions represent a challenge for long-term atmospheric monitoring in the absence of accepted methods for routine sampling. Most information on P deposition is from bulk precipitation collectors and wet/dry bucket sampling, both with questionable precision and accuracy. Most published annual rates of P deposition are gross estimates derived from bulk precipitation sampling in locations around the globe and range from about 5 to well over 100 mg P m year, although most inland ecosystems receive between 20 and 80 mg P m year. Rates below 30 mg P m year are found in remote areas and near coastlines. Intermediate rates of 30 to 50 mg P m year are associated with forests or mixed land use, and rates of 50 to 100 mg P m year or more are often recorded from urban or agricultural settings. Comparison with other methods suggests that these bulk precipitation estimates provide crude boundaries around actual P deposition rates for various land uses. However, data screening cannot remove all positive bias caused by contamination of bucket or bulk collectors. As a consequence, continued sampling with these standard collectors in a region will not reduce the large uncertainty in rates derived from existing data. Calibrated surface accumulation methods hold promise as a primary means to estimate P flux in future monitoring. New methods for long-term P deposition monitoring will require an intercomparison of P flux estimates from surrogate surfaces, impactor sampling of particle concentrations combined with deposition models, and “throughfall” estimates for natural canopies. With better sampling methods and more long-term monitoring data, the importance of atmospheric P deposition in ecosystem dynamics and management can be better understood and predicted.

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