Science Stories: Adventures in Bay-Delta Data

Phantastic Phytoplankton
  • April 7, 2023

Blog by Rosemary Hartman, Data by Tiffany Brown, Sarah Perry, and Vivian Klotz. Photos from BSA submitted to DWR.

The base of the estuarine food web is phytoplankton – microscopic, floating, single-celled organisms drifting on the currents (“phyto” meaning “plant” and “plankton” meaning “drifter"). Most people know that trees produce oxygen, but phytoplankton put them to shame. Phytoplankton in rivers, lakes, and oceans worldwide produce an estimated 80% of the world’s supply of oxygen. Phytoplankton are also the base of the aquatic food web – providing food for zooplankton, other invertebrates, and fish. They are also the source of the omega-3 fatty acids that make seafood so good for you!

The Environmental Monitoring Program, a collaborative team of scientists and technicians from DWR, USBR, and CDFW, have been collecting phytoplankton samples to monitor the status and trends of phytoplankton in the San Francisco Estuary for the past forty years, and just recently made their data from 2008-2021 available online!

How are the data collected?

  • The monitoring program crew go out on DWR’s premier research vessel, the Sentinel, once per month and visit 24 fixed stations and 2-4 ‘floating’ stations across San Pablo Bay, Suisun Bay, and the Delta.
  • At each station, scientists collect a 60 mL water sample from 1 meter depth and stain it with Lugol’s iodine solution to make the phytoplankton easier to see.
  • The samples are shipped to a lab where highly-trained taxonomists identify and count the phytoplankton under high-powered microscopes.

What do the data look like?

For each sample, we see the count (number of cells) for each type of plankton as well as the size (biovolume) of these cells.

  • Each phytoplankter is identified to genus or species, but we often lump them into larger taxonomic groups to make it easier to see trends in the data. These groups are based on genetic and morphologic similarity, so they have similar shapes, pigments, motility, etc. Our understanding of the microbial tree of life is constantly evolving, so it is vital that we keep our entire data set up to date on the latest science as we categorize these groups.
  • The phytoplankton samples are collected at the same time as water quality, nutrients, and zooplankton samples, so you can put all the data together if you want to see the bigger picture.

What trends do we see?

  • There was a big change in average biovolume and relative abundance of cyanobacteria in 2014 when we switched contracting laboratories. Differences in methods made a big difference in the data, and we’re still trying to work out the consequences (See Figure 1 and Figure 2).
Bar plot showing annual average phytoplankton biovolume by taxonomic group.
Figure 1. Biovolume of each algal taxonomic group by year. Centric diatoms and pennate diatoms make up most of biovolume in every year, but differences in contractors in 2014 means we can't compare to earlier years. Click to enlarge.
Relative abundance of each algal group by year. 2012 and 2018 had a lot of centric diatoms, 2016-2018 had the most chrysophytes, and 2021 had particularly high percentage of centric diatoms.
Figure 2. Relative abundance of biovolume of each algal taxonomic group by year. Click to enlarge.
  • We always catch more critters in the spring and summer, when days are long and temperatures are warm (Figure 3, Figure 4).
Bar graph with average biovolume per month color-coded by algal group. The highest biovolume is Feb-April.
Figure 3. Average Biovolume of major algal taxonomic groups by month (data from 2014-2021 only, to control for changes in contractors). Click to enlarge.
Bar graph of organisms per mL by month, color-coded by algal group. All months are totally dominated by cyanobacteria (>80%).
Figure 4. Average concentration (organisms per mL) of major algal taxonomic groups by month (data from 2014-2021 only, to control for changes in contractors). Click to enlarge.
  • Cyanobacteria are very small in comparison to other phytoplankton, so even when we catch a lot of them we don’t get much biovolume. Looking at the graphs, Figure 3 shows the biovolume of each phytoplankton group in each sample while Figure 4 shows the number of organisms in each group in each sample. The cyanobacteria are hard to see in the biovolume graph, but they totally dominate the number-of-organisms graph!
  • Environmental factors frequently impact abundance of phytoplankton. For example, if we plot the abundance of each group versus net freshwater flow coming through the Delta, we find higher concentrations of most groups during months with higher flow (Figure 5).
Set of nine scatter plots showing relationship between algal concentration and Delta Outflow for each major algal group. All taxa have position relationships, but some are steeper than others.
Figure 5. Relationship between monthly mean concentration (organisms per mL) of each algal taxonomic group and net Delta outflow. Click to enlarge.

There are lots more questions we could ask with this dataset. Are certain taxa more common in wet years or dry years? Do certain taxa occur more frequently in salty water or fresh water? How have abundances of certain taxa changed over time? With a dataset like this, the sky is the limit! If you see anything interesting in the data, we encourage you to join the Water Quality and Phytoplankton Project Work Team to share what you see!

What’s your favorite phytoplankton? These are some of the most common taxa in our samples:

  • Cyanobacteria - Bacteria that photosynthesize! Some can even fix nitrogen out of the atmosphere. Others can produce toxins harmful to fish and wildlife, but most are harmless.
    Anabaena - a cyanobacterium that looks like a string of beads with one large bead on it.
  • Centric Diatoms - Big critters that look like wagon wheels and have a case (also called a ‘test’) made of glass-like silica. Considered very tasty and nutritious for zooplankton.
    Diatom in the genus Stephanodiscus. It looks like a small cylinder.
  • Pennate Diatoms - Closely related to centric diatoms, these guys also have a silica shell and are highly nutritious. Unlike centric diatoms, they are shaped like canoes and frequently live on surfaces instead of being part of the plankton.
    Diatom Asterionella sp. It looks like rods connected at one end.
  • Green Algae - Green cells that can be single or colonial and also have some flagellated species. They are also the distant ancestors of land plants.
    Green algae in the genus Cosmarium. It is round with some symmetric blobs inside.
  • Cryptophytes - Single-celled algae with a pocket in one end with two flagella sticking out of it.
    Algae in the genus Cryptomonas. They look like ovals with bits of dirt coming out of one end.
  • Euglenoids - Single cells with a flagellum that are frequently heterotrophic – they can eat other cells or photosynthesize to produce energy.
    Phytoplankton in the genus Euglena. It looks like an oval that is pinched on either end.
  • Crysophytes - Also known as “golden algae”, these guys have two flagella and many are encased in a silica cyst.
    Algae in the genus Dinobryon. They look like blobs with a lot of tails hanging off them.
  • Dinoflagellates - These single-celled algae have two flagella, one that circles their “waist” and one streaming off the side. They are more common in marine waters than freshwater, and can cause “red tides” which are harmful to fish.
    Dinoflagellate in the genus Peridinium. It is roundish with some grooves in it.

Further Reading

Categories: Underappreciated data

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