Algal biomass in nearly every lake system studied has been shown to be at least partially controlled by the availability of the three major nutrients, carbon, nitrogen, and phosphorus. However, phosphorus is generally considered to be the limiting and most controllable nutrient affecting algal productivity in most temperate lakes (Schindler 1978), and P also appears to control productivity in most western Washington lakes (Bortleson 1978). For this reason, and because earlier studies on Pine Lake have indicated that control of phosphorus input would achieve the most substantial reduction in algal biomass, the nutrient budget study undertaken by the UW focused predominantly on phosphorus. However, even in Pine Lake where phosphorus supplies are generally low during the growing season, carbon and nitrogen limitation may cause at least short-term limitation of algal growth, and may influence species succession. Silicon limitation appears to influence diatom production in Pine Lake (Bortheson et. al. 1980; Welch et. al. 1981). During bloom conditions pH may rise sufficiently to deplete free—CO₂ concentrations to levels believed to limit algal photosynthesis and growth rate, especially for green algae (King 1970). In addition, summertime epilimnetic concentrations of inorganic nitrogen typically fall to levels believed to limit algal production ( <25 µg N/l), and may also favor blue-green algal dominance (Schindler 1978). But summer soluble reactive phosphorus concentrations are also generally low enough to cause growth limitation ( < 2 µg P/I), and typical available N:P ratios in the epilimnion (11:1 by weight in summer) suggest that P may in fact be the principal growth-limiting nutrient. The presence of utilizable atmospheric sources of carbon and nitrogen for Pine Lake plankton also raises questions concerning the ultimate control of algal biomass by these elements. Phosphorus control should therefore result in algal biomass reduction in Pine Lake.

Weighted-average lake total phosphorus concentrations reached minimal levels in the summer—fall period, followed by a gradual rise during the winter (Figure 10). With the onset of the diatom bloom, sedimentation of phosphorus greatly reduced P concentrations, but shortly after that a large increase occurred in surface concentrations during the blue-green bloom in April and May, 1980. Hypolimnetic P levels began to increase as the bottom waters approached anoxia, and remained high until fall turnover.

The importance of hypolimnetic phosphorus enrichment to the surface waters has been demonstrated for a number of Puget Sound lakes (e.g. Davis et. al. 1978), but its importance to Pine Lake is not well understood. Although hypolimnetic P levels generally exceeded 100 µg/l during summer stratification, the small volume of this water (less than 8% of the total lake, Z<9m) minimizes its overall importance. The surface water increase in total phosphorus during the autumn, 1979 turnover was approximately 6 µg/l, and nearly all of this increase can be attributed to hypolimnetic mixing. Since much of the hypolimnetic phosphorus mixed throughout the water column is likely incorporated into bacterial and algal cells, an unknown fraction of this P may be unavailable and/or subsequently settle out of the water column. Phosphorus supply to the surface waters during summer stratification via diffusion and partial thermocline erosion is minimal.

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Last modified:  April 16, 2004