Written April 2024, by Gene Welch - Professor Emeritus, Civil and Environmental Engineering, University of Washington
Several reports indicate that HABs (harmful algal blooms) are increasing in lakes throughout the world. A recent investigation showed that 68% of large lakes globally had increased blooms, while blooms decreased in only 8%. The HABs are due to blue-green algae or cyanobacteria, some of which can be toxic. Of increasing interest is how much of the observed increase in HABs is due to climate change? That is, to increased lake temperature and processes related to temperature?
A report on 245 large world lakes showed that summer surface temperature in most (219) had increased an average of 3.2 degrees F between 1985 and 2015. Lake Washington was in the group with a temperature increase of 1.5F. Temperature decreased in only 26 lakes. Extrapolating the decadal rate of increasing temperature in warming world lakes to 2025 would amount to a probable increase of 4.3F. The report had 65 authors who were noted lake scientists (limnologists).
Has temperature increased in Moses Lake? Surface temperature in Parker Horn and South Lake during July-August averaged 76.2F during 2017-2023. That is an increase of 2F since the 1970s-1980s. The May-September average also increased 2F, but was 3F higher the past three years. The increased lake temperature was probably due to higher air temperature, which increased 4.4F during May-September between 1949-1965 and 2014-2021. The question is would that increase in surface water temperature of 2F have caused an increase in HABs?
Blue-green algae usually dominate during mid to late summer as temperature increases. However, as lakes become more enriched with phosphorus, blue-greens start earlier in the spring when temperature is lower. Blue-greens usually started in spring in Moses Lake when water temperature reached about 68F, and the same occurred in Upper Klamath Lake, Oregon. Massive blooms of Aphanizomenon, the same species as in Moses L, bloomed in early spring in Pine Lake, King County, at a water temperature of 46F. Also, blue-greens form blooms under ice cover. They can still accumulate into a bloom even though they grow slower at low temperature. Nevertheless, as temperature increases, blooms can accumulate rather quickly; blue greens grew at 100% per day in Moses Lake at an average temperature of 72F. Thus, an increase of near 5F in world lakes may encourage blue-greens to bloom earlier and grow faster, but will the magnitude of blooms be expected to increase if more phosphorus is not available?
The cause for eutrophication, with increased HABs, is nutrient enrichment. That connection was established long before lake temperatures increased and phosphorus is the nutrient usually least available in freshwater. Algae also need high amounts of carbon and nitrogen, but they can get those major nutrients from the atmosphere through diffusion of carbon dioxide and fixation of abundant atmospheric nitrogen by some blue-greens, such as Aphanizomenon and Anabaena, which are common in Moses Lake. Fixation of atmospheric nitrogen was adequate to form blue-green blooms in a lake fertilized with only phosphorus. Algae cannot grow biomass without nutrients, even with increased temperature, and can bloom at low temperature. For example, Pine Lake looked like pea soup in April at low temperature due to an input of wetland water with a very high soluble phosphorus at 150-300 ppb (parts per billion). The spring blooms ceased when that source of high phosphorus was diverted to the lake outlet.
Data from the past seven years in Moses Lake show that phosphorus determined the amount of algae (as chlorophyll), as well as the fraction of algae that was blue-greens. The average blue-green fraction was very low during May-September at only 20% in 2021-2023 when CRW inputs were nearly 200,000 AF and TPs averaged 25 ppb at five sites representing 74% of the lake.
The bottom line is that increased phosphorus will usually increase algal biomass, as well as the fraction that is blue-greens. If phosphorus is decreased the reverse will occur. That fact has been demonstrated with hundreds of alum treatments that reduced internal loading of phosphorus, total wastewater diversions of both phosphorus and nitrogen, and continued wastewater inputs with phosphorus removal. The evidence from Moses Lake shows that total algal biomass and the fraction composed of blue greens will be relatively low if average lake total phosphorus is reduced to around 25 ppb. Those conditions of low phosphorus, chlorophyll and blue-greens occurred in the lake the last three years even though average surface temperature had increased 3.0F since the 1970s-1980s.
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