Microbial dynamics and phosphorus turnover in Lake Erken

Our aim was to couple microbial dynamics with measures of nutrient stress during the period of summer stratification (June through August, 1988) in the epilimnion in mesotrophic Lake Erken, Sweden. We made twice weekly measurements of bacterial abundance and production (thymidine incorporation), chi a, primacy production, nanoflagellate and ciliate abundances, alkaline phosphates activity (APA), phosphate turnover time, and the phosphorus deficiency index (PDI; rate of light saturated photosynthesis divided by the maximum phosphate uptake veracity). The lake was characterized by three periods with different phytoplankton strategies for acquiring P. In early June when flagellates were >90% of phytoplankton biomass and approximate to 67% of the flagellates were potential bacteriovores, flagellate grazing roughly match bacterial mortality, and we calculated that approximate to 50% of the flagellate P needs were potentially met by ingesting bacteria. During July, when bacterial production was greatest, bacterial abundance remained constant at approximate to 1x10(9)/l. At this time potential flagellate bacteriovory constituted >25% of bacterial mortality. Densities of the small cladocerans, Diphanosoma brachyrum and Daphnia cucullata, reached >100/l in late July and were likely the dominant bacterivores. The PDI increased from indicating severe P deficiency in June to moderate P deficiency in July. By August, the phytoplankton community was approximate to 50% composed of the two N(2-)fixing cynobacteria, Anabaena sp. and Gloeotrichia echinulata. These cyanobacteria have internal P stores, and the PDI showed low P deficiency with the dominance of cyanobacteria, heterotrophic bacterial production decreased by an order of magnitude, but the exact cause for the decline was not evident. During late July, approximate to 20% of the bacteria were in "aggregate" that were a refuge from grazing. The large bacteria in the aggregates were probably a "sink" for nutrients conomita with the rapid turnover of the free planktonic bacteria. Thus bacteria as do algae have growth forms that escape grazing.