We determined the contribution of calcite to the total particle-size spectrum of Lake Michigan seston during different seasons in 1978–83 and 1985, employing a novel Coulter counter method, to examine the intensity and ecological effects of calcite whitings that result from autogenic precipitation of calcite. The whitings were most intense during September, when 12–56% of the total particle volume was calcite. Overall, food web dynamics were more controlling of than controlled by whitings. Year-to-year variation in calcite concentration was probably caused by predation-controlled variation in primary production, which drives calcite precipitation through CO2 uptake. Feeding rate of cladocerans in Lake Michigan is slightly reduced (16%) by calcite, but that of copepods is not. Light extinction is only slightly increased. Coprecipitation and sinking of P with calcite is probably a minor factor in regulating P concentration in the epilimnion of Lake Michigan. Removal of nutrients is enhanced by increased sinking rates caused by inclusion of calcite in fecal pellets and matter. Although all of the effects of calcite on plankton dynamics appear to be subtle in Lake Michigan, large effects can be expected for lakes of greater calcium hardness and eutrophy.

A model predicting the summer concentration of cholorophyll a in a phosphorus-limited lake is derived from simple empirical and semitheoretical relationships. The model is rearranged and expressed as a phosphorus loading plot which agrees closely with the predictions of Vollenweider's model. The model can be used to gain insight into the phosphorus loading concept. The primary conclusion is that a lake's tolerance to phosphorus loading is a function of two processes: sedimentation and flushing rate. At low areal water loads, in-lake forces which remove phosphorus to the sediments predominate. At high areal water loads, flushing of phosphorus through the lake's outlet is the factor governing eutrophication. The importance of the steady state assumption is also demonstrated by using data for Lake Washington.Res., 17, 135-149, 1974. /68, pp. 1-182, Organ. Econ. Coop. Develop., Paris, 1968. Vollenweider, R. A., MiSglichkeiten und Grenzen elementarer Modelie der Stoffbilanz von Seen, Arch. Hydrobiol., 66, 1-36, 1969. Vollenweider, R. A., Input-output models with special reference to the phosphorus loading concept in limnology, Schweiz. Z. Hydrol., 37, 53-84, 1975. Vollenweider, R. A., and P. J. Dillon, The application of phosphorus loading concept to eutrophication research, Tech. Rep. 13690, 42 pp., Nat. Res. Counc. of Can., Ottawa, Ont., 1974. ( Vollenweider, R. A., The scientific basis of lake and stream eutrophication, with particular reference to phosphorus and nitrogen as eutrophication factors, Tech. Rep. DAS/DSI

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Measurements of ortho‐phosphate (PO4‐P) concentrations in natural waters obtained by molybdenum blue methodology are based on the assumption that molybdate (Mo) complexes only with PO4‐P (in the absence of arsenate and silicate interference) to form the reductant‐sensitive complex 12‐molybdophosphoric acid (12‐MPA). The hypothesis that Mo causes or accelerates PO4‐P release from bound sources or forms reductant‐sensitive complexes with organic compounds (organic‐PO4‐Mo) was tested by exposing filtered Lake Michigan water to acid, acid plus Mo simultaneously, and Mo, respectively, before measuring PO4‐P by the Chamberlain‐Shapiro extraction method. Not only does Mo accelerate hydrolysis in the presence of acid, but it also either causes hydrolysis or forms organic‐PO4‐Mo before samples are acidified. Although the relative amounts of 12‐MPA and organic‐PO4‐Mo formed in lake water cannot be assessed quantitatively, a major fraction of the blue color formed during routine analysis by this or by similar methods apparently is not the product of acid hydrolysis, but may also be due to reduction of organic‐PO4‐Mo. Future developments in molybdenum blue methodology aimed at minimizing hydrolysis must consider the sequence of reagent additions and Mo, as well as acid, contact times.

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