Life of a Bloom

Chlorophyll displayed the most dramatic short-term changes, especially in Little Dixie Lake. Let’s look at this short-term variability and the possible causes for these changes.

During mid-July chlorophyll levels in Little Dixie Lake bounced between 50 and 60 ug/L. This period was dry and solar radiation (a measure of available solar energy) was above the overall summertime average; meaning plenty of sunlight for photosynthesis. Chlorophyll concentrations started to increase on July 17 th, reaching 88 ug/L on July 20 th (Figure 2). By the next day, chlorophyll was up to119 ug/L; a 35% increase in 24 hours!

Levels continued to climb and maxed out on morning of the 23 rd at 142 ug/L. The afternoon of the 23 rd and the morning of the 24 th turned out to be very cloudy, with solar radiation values that were ~50% and ~10% of those from the week before, respectively (Figure 2). Algal chlorophyll began to drop as quickly as it had risen, with a decrease of 44 ug/L on the 24 th. By the 26 th, three days after the peak, chlorophyll levels were back around 50 ug/L.

During this period there were slight fluctuations in the phosphorus concentrations (range 61-78 ug/L), but not enough to account for the large fluctuations in algal chlorophyll. Nitrogen levels also displayed changes, going from around 1100 ug/L to a peak of 1430 ug/L, and then back down to 1100 ug/L (Figure 2). It could be that the increase in nitrogen fueled the growth of algae, but more likely the growth of algae led to the increase in nitrogen.

As lake conditions change, so do the types of algae present. Some blue-green algae (also known as cyanobacteria) are capable of fixing atmospheric nitrogen (just like legumes such as peanuts and soy beans). If blue-green algae were responsible for the increase in algal chlorophyll, it is very likely that the nitrogen increase came about because algae were adding it into the system. This case study demonstrates that lakes can be extremely dynamic, with algal blooms building quickly and dissipating just as fast!

 

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