Production of Potamothrix hammoniensis in the Profundal of Eutrophic Lake Esrom

Abstract

Production, biomass and PI/ ratio was calculated for a single species population of Potamothrix hammoniensis (Michaelsen) in the profundal of Lake Esrom. Two methods were compared. The first was the Allen curve method in which numbers were plotted against mean weight for a cohort hatching in 1956. The second involved using estimates of instantaneous growth rates on a daily basis extrapolated to annual rates by calculation of the number of degree-days available for growth. Annual rates could then be calculated using the formula P = g B. The two methods gave almost identical results. Yearly total production (P) during periods of growth was on an average 998 mg, mean biomass (B) 1222 mg dry wt m-2 yr-1 and PI/ 0.82. Between-year changes were small. Production of cocoons amounted to 4-30 % (average 15 %) of total annual production. Negative production during the stratification period was about 16 % of total annual production during periods of growth, and P/B was also lowered by 160% to 0.68. P and A increased, but PI/ decreased as the 1956 population grew older during their four year life span. This very abundant population made up the majority of total production when three years old.