Summary of main results for the 1996 Brome Lake Project By Doctors Yves Prairie and Paul del Giorgio, UQAM (1996)

1) There was general agreement between the patterns of lake nutrients and chlorophyll recorded in 1996 and in the previous two years. The nutrient concentrations of the largest tributaries were still normal. There was a large increase in lake nutrients towards the end of the summer, followed by a bloom a algae. In 1996 these peaks occurred later in the season compared to previous years, and these differences may be linked to changes in the weather patterns and not to changes in the external inputs of nutrients into the lake. The summer of 96 was abnormally cool, windy and wet, and this induced a very different temperature and oxygen regime in the lake.

2) We found that phosphoruS release from the sediments was accompanied by release of nitrogen, particularly ammonia, suggesting that the process may be in part related to biological decomposition of organic matter. Based on the accumulation of ammonia in the hypolimfliOfl~ however, only between 10 to 30% of the total P release from the sediments may be produced from organic matter decomposition. The balance of P release may be related to iron reduction in anoxic sediments.

3) The sudden surge in nitrogen concentration in late summer cannot be completely accounted for by the sudden mixing of N-rich hypolimnetic waters. It is probably associated with an increase in nitrogen concentrations in the inflowing streams, and also with the intense fixation of atmospheric nitrogen by the bloom-forming alga Aphanizomenon flosaquae.

4) Our sediment trap data suggest that rates of sedimentation of organic matter and nutrients in the lake in late summer are extremely high. We estimate an average sedimentatiOn rate between 6 to 10 mg P/m2/d. This rate of P sedimentatiOn is of the same magnitude as the rate of release from deep sediments. All these data suggest a very intense recycling of nutrients within the lake.

5) In 1996 we specifically tested the applicability in Brome Lake of a common management tool, the addition of aluminum sulfate (alum) to lake waters. Alum operates by binding to nutrients in the water and then precipitating to the bottom of the lake carrying these nutrients, which are then trapped in the bottom. This technique does not eliminate nutrients from the lake, but only temporarily eliminates some of the nutrients from the water. In our enclosure experiments we have shown that an average dose of alum applied only once resulted in a dramatic decrease in phosphorus (over 75%) in the water column, and even more dramatic (over 90%) decline in chlorophyll concentration, and the declines in both nutrients and chlorophyll lasted for at least 6 weeks.