Phytoplankton
Highlights:
The phytoplankton populations (algae) in Cherry Creek Reservoir represent the “primary productivity” and are responsible for chl α production. Early WY2023 had lower productivity and diversity due to the cooler spring and early summer with above-average precipitation, but as the season warmed algae populations including cyanobacteria increased resulting in increased chl α concentrations.
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, the 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.
The graph to the right illustrates the phytoplankton populations by group (y-axis) and biovolume by the size of the bubble for each of the monitoring events during W Y2023. In addition the total populations and biovolume for the year are shown in the pie charts.
As you can see as usual the cyanobacteria represented a higher percentage of the total algae on almost all dates but, the diatom blooms in December 2022 and July 2023 through the end of the year represented the highest biovolume.
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Phytoplankton Concentrations and Biovolumes
info_outlineEach point on this graph represents a sampling event. The y-axis represents the concentration (number) and the size of the bubble represents the biovolume (size). The colors represent different plankton groups. Use the filters to target a specific group or date. To learn more about a specific data point, hover over it with your mouse.
- 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.