Nutrients and phytoplankton in six lakes of southwestern Manitoba with particular reference to seasonal anoxic conditions

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Srisuwantach, Vijai
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Physical and chemical characteristics together with phytoplankton parameters in six lakes of southwestern Manitoba were investigated from February 1976 to February 1977 with a special emphasis on relationships between dissolved oxygen, nutrients, and phytoplankton. These lakes varied in mean depth (1.5-3.4 m), average Secchi disc transparency during ice-free period (0.8-1.5 m), total ions in mid-summer (659-1691 mg/1), maximum winter ammonia nitrogen (331-669 ug/1), maximum winter soluble reactive phosphorus (8-159 ug/1) maximum summer chlorophyll-a content (12-260 ug/1), and maximum summer gross primary production (1.1-6.2 gC/m2/day). They were classified as non-stratified, shallow, eutrophic, moderately saline to saline lakes with Mg++, SO4=, and HCO3-, as predominant ions. Oxygen depletion in winter (winterkill) developed between February and March in lakes where the mean depth was 2.8 m or less. No winterkill was observed in a lake with the mean depth of 3.4 m. Oxygen depletion in summer (summerkill) occurred in a winterkill lake that contained maximum winter concentrations of 669 ug/1 ammonia nitrogen (NH3-N) and 159 ug/1 soluable reactive phosphorus (SRP), and consequently developed a noxious bloom of Aphanizomenon flosaqueae with a maximum of 260 ug/1 chlorophyll-a. The collapse of the bloom caused the dissolved oxygen to drop down to near zero (0.1 mg/1). The high phosphorus content of the lake appeared to be the cause of this bloom. A maximum winter concentration of 150 ug/1 SRP or more was found to be the critical level for a high probability of the summerkill occurrence. A high concentration of nutrients during the summerkill period was recorded. These nutrients did not lead to further Aphanizomenon blooms since weather conditions in early fall became less favorable for the growth of this alga. High winter concentrations of nutrients were observed in both winterkill and non-winterkill lakes. a direct relationship between winter nutrient concentration and summer algal standing crop was found in three lakes where the water was well mixed by wind action. This relationship was obscured in three other lakes by submerged macrophytes or nutrient accumulation in the bottom water during the summer months. Higher nutrient concentrations in the following winter in these lakes also appeared to be related with higher chlorophyll-a concentrations in the previous summer.