Seasonal Dynamics of Phytoplankton Blooms

Discovery Lake, San Marcos, California

Discovery Lake is located in a residential neighborhood in San Marcos, California. The maximum depth is 2.8 m but may vary depending on rainfall. The lake serves as a recreational area and provides an environment for the local wildlife and waterfowl. However, residential development runoff drains into the lake. Irrigation runoff from surrounding development most likely contains trace amounts of pollutants and nutrients from plant fertilizers that may significantly impact aquatic resources, including algae, fish, insects. The shallow depth of the lake, poor water circulation, and increased nutrient load are contributing factors to recent algal blooms, which leads to oxygen depletion and fish kills. The city of San Marcos launched the water quality improvement program of Discovery Lake in 2020. CSUSM undergraduate students studied the phytoplankton dynamics of Discovery Lake from March to November 2022 as a community engagement project, mastering their new field and lab research skills developed during the Spring 2022 class of Biol 387L Aquatic Ecology Laboratory. Their study, illustrated below, provided the first data on phytoplankton composition and dynamics of Discovery Lake.

1. Field work

We started the field work by recording the aquatic vegetation (Broadleaf cattails, California bulrush, Duckweed) and the riparian native trees (Western sycamore, willow, California coastal live oak, and Pacific poison oak). Local fauna included Ruddy ducks, Mallard ducks, American coots, Red-winged blackbirds, and Great Egrets. Student sampled the lake ten times in 2022 always collecting samples around 10 am in the morning. They measured the water clarity with a Secchi disk and collected whole water samples from Discovery Lake's dock at a 50 cm depth using a Van Dorn Sampler. In addition, they measured water temperature, pH, conductivity, salinity, and total dissolved solids with an Extech EC500 Waterproof ExStik® II pH/Conductivity Meter. Phytoplankton samples were preserved in Lugol’s solution and transported to the CSUSM Algae Lab.

Lake view

Plankton sampling

Water parameters testing

2. Light microscopy

Tiny, photosynthetic algae distributed in the water column are called phytoplankton. Algae are diverse and have different cell pigments and therefore may appear green, blue-green, golden, brown, or rusty. Light microscope analyses of the samples showed that the common phytoplankton in Discovery Lake all year-round consists of single-celled algae which swim actively in the water using flagella. The two most common flagellate algae were the brownish-colored dinoflagellates and cryptophytes responsible for the brownish hues of the water. However, during the hottest summer months of July and August, water became more greenish due to the dense growth (called bloom) of filamentous cyanobacteria (e.g. Aphanizomenon and Dolichospermum). Cyanobacteria do not have motile flagella, but their cells contain gas vesicles that support their flotation on water surfaces. Cyanobacteria strains in Discovery Lake tested negative for toxins, ended up reducing the oxygen availability for other organisms and caused fish kills.

Aquatic explorer

Summer bloom-forming cyanobacteria

Other common phytoplankton

3. Data analysis & student publication

Students analyzed the cell density and biovolumes of each algal species and correlated the phytoplankton data with measured environmental variables to obtain the seasonal dynamics of the lake phytoplankton communities in Discovery Lake. To learn more about their study published in Cougar JGR Journal of Undergraduate Research, click here (coming soon).