Phytoplankton
Highlights:
Although the above average precipitation in the spring of 2023 resulted in high inflows and increased phosphorus concentrations in the Reservoir, the algae populations were lower than expected due to cooler water temperatures and decreased turnover rate. However, when the rain slowed and temperatures warmed the productivity and associated chlorophyll - a concentrations increased.
Only one cyanobacteria bloom occurred that required closure of the Reservoir to contact based on toxin production in affected areas for a few days. However, the bloom dissipated over two weeks and the Reservoir was reopened to recreation.
Phytoplankton are photosynthetic organisms that are the primary producers in aquatic systems. They form the base of aquatic food chains and are grazed upon by zooplankton and herbivorous fish. A healthy lake should support a diverse assemblage of phytoplankton, in which many algal groups are represented.
Phytoplankton and zooplankton data continued to exhibit characteristics of an over-productive and nutrient-rich Reservoir, as indicated by WY 2023 planktonic communities. The most abundant phytoplankton taxa present in Cherry Creek are Cyanophyta, commonly referred to as “blue-green algae” (or cyanobacteria, depicted in red), which are of concern, but also many Chlorophyta (“green algae”, depicted in green), and Bacillariophyta (diatoms, shown in blue) are present which are both considered to be good or beneficial algae.
Cyanophytes may be responsible for the majority of algal blooms that occur in freshwater ecosystems based on competitive advantage over other groups of phytoplankton.
Due to unforeseen delays in late season plankton sample analysis, thee detailed phytoplankton evaluation summary was delayed. Although cyanobacteria (blue-green algae) represented the highest populations during WY2023, they normally represent a small percentage of the total biovolume, except during bloom conditions. Although there was not a monitoring event that aligned with the exact timing, the Reservoir was closed to contact by Colorado Parks and Wildlife (CPW) during the bloom in late July due to cyanotoxin detection above the recreational threshold.
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Phytoplankton Concentrations and Biovolumes
- These are typically the first algae to bloom in early spring. When nutrients and light conditions in upper mixed layer are right, their competitive edge and rapid growth rate allows them to dominate phytoplankton.
- These are important contributors to the food chain and are a food resource for zooplankton.
- Freshwater diatoms commonly observed in reservoirs are indicators of eutrophic (over-enriched) conditions. Their degradation contributes to increased oxygen demand and phosphorus recycling.
- These appear during periods of high nutrient concentrations and indicate both nitrogen and phosphorus are in excess supply.
- Colonial and single-celled greens are a good food source for zooplankton.
- Filamentous and large colonial greens do not add to food web and create water quality problems.
- Appear during periods of over-abundant enrichment and with very high nutrient levels, especially phosphorus.
- Do not contribute greatly to the food web; generally not considered beneficial organisms in a lake.
- Create water quality problems; e.g., oxygen depletion when excessive growth produces blooms.
- Some species may produce cyanotoxins under certain conditions.
- Cryptophytes can live through the winter under ice-cover and with little solar radiation for photosynthesis.
- Serve as important food for zooplankton, which are food for fish and other organisms in food web.