Plankton

Phytoplankton (right) and Zooplankton (left)

Foundation of Aquatic Foodways

Plankton is where the rich estuarine soup of life begins. Plankton simply refers to tiny, microscopic, organisms that float in the water column going wherever the currents and tides take them. “Life moves with the water” states Matthew Morse Booker who then proceeds to write a lyrical description of the fecundity of life at the margin of San Francisco Bay, starting with plankton swirling in the Bay’s water. A drop of water entering the Bay from the Delta might travel hundreds of miles up and back, pushed by the tide and the current before it exits through the Golden Gate. Plankton go with these drops swirling through the water column and deeper Bay waters, but they tend to collect at the margin of the Bay, the edges of 
the estuary.

diatoms

Assorted diatoms, Assorted diatoms as seen through a microscope. These specimens were living between crystals of annual sea ice in McMurdo Sound, Antarctica.

Photo by Prof. Gordon T. Taylor, Stony Brook University

Plant or Animal?

Phytoplankton are single-celled organisms which, like plants, use the energy of the sun and nutrients in the water to create their own food. They are also called single-celled algae. In algae, chlorophyll harnesses sunlight’s energy, and photosynthesis converts CO2 and water into oxygen as a byproduct. The evolution of phytoplankton in the primordial ocean created the oxygenated atmosphere that the rest of life needed to begin evolving, and phytoplankton in oceans, streams, lakes, and estuaries create about half of the oxygen in our atmosphere today. Like a plant fueled by nutrients from the soil, phytoplankton pull nutrients out of the water. These are the minerals washed down from the mountains through the Delta to swirl around in the Bay. Nitrogen and phosphorus are the most important, but trace metals such as iron, manganese, and zinc are also needed. Diatoms are a kind of phytoplankton that live in microscopic glass houses—they need silica to build their complex cell walls.

Phytoplankton, including diatoms, are the basic food source for oysters, other bivalves, and the small juvenile stages of many organisms from invertebrates to fish. An Olympia oyster, glued to its rock or another oyster at the edge of the bay, brings water in through the mantle and over its gills where a mucus layer collects phytoplankton and other particles and funnels these nutrient packets into the mouth and stomach where it is transformed into proteins and carbon, i.e. biomass. The talents of the oyster’s gills are such that it funnels the needed particles of food towards its mouth and stomach while waving the mucus packets of larger, non-digestible particles the other direction and back out into the water.

copepod

Unidentified species of copepod in the order Calanoida.

Photo by Uwe Kils

Zooplankton are the animal counterpart to phytoplankton. Zooplankton can be a range of sizes from microscopic to almost seen. Olys also filter and eat small zooplankton, including fish eggs and other small organisms. Common zooplankton are copepods (a tiny crustacean) or the larval stage of fish, oysters, barnacles, or seastars. The shallower South Bay has a different composition of zooplankton than the cooler, fresher North Bay.  As animals, they depend on the phytoplankton as their food source and are in turn eaten by those just larger than themselves. They are the energy pathways from primary producers—phytoplankton and bacteria, their food—up to fish and other top predators. The larvae of native oysters are an important part of this zooplankton assemblage, broadcast and spawned in numbers large enough to sustain settlement and recruitment despite many becoming lunch for others.

Zooplankton waste, filtered oyster waste, sloughed-off eelgrass, and other organic matter sink to the bottom making the mud within these habitats a bonanza home for worms and other mud dwellers as well as becoming a significant carbon sink. The muddy bays and shorelines that support the most biodiversity have a surprising hero in the fight against global warming and sea level rise: the mud itself. It can sequester vast amounts of carbon….so long as it remains undisturbed.

Power of the Sun

Phytoplankton (which includes diatoms) use the energy of the sun and base nutrients such as nitrogen, phosphorus, iron, and magnesium derived from ocean water to grow and reproduce. They are considered primary producers, generating oxygen. This diagram shows the zooplankton and oysters and other mollusks that feed on phytoplankton. The zooplankton in turn become food for larger fish and organisms in the Bay. Oyster will also feed on small zooplankton.

Corbula amurensis

Corbula amurensis from San Francisco Bay

Photo by Andrew N. Cohen, Center for Research on Aquatic Bioinvasions

An Invasive Phytoplankton Hog

The delicate balance between phytoplankton, zooplankton and fish populations can be easily disrupted. In 2019 reports came out that the invasive overbite clam (Corbula amurensis) was partly responsible for a dramatic decline in phytoplankton in the Bay Delta. The clams were filtering so many phytoplankton that the zooplankton had nothing to eat. Without the base, fish populations that depend on zooplankton, plummeted. One can only wonder how the Olympia oysters were doing anywhere near the clams. They too might have starved.

Algal Bloom

Photo by California Department of Water Resources

Harmful Algal Blooms or HABs

A healthy Bay supports a balanced amount of phytoplankton (algae) and oysters have always played a key role in maintaining this balance by processing vast quantities of primary production and transforming it into biomass. But as industry and agriculture pump more and more nitrogen and phosphorus into waterways, and water temperatures rise in the summer, the effects of over-nutrification can mean an enormous overgrowth of a particular algae, creating green mats on the surface of the water, blocking sunlight. Many of these microorganisms produce toxins that can build up in shellfish (often known as a red-tide) and have acute health effects on people and other organisms. The bacteria that process all this phytoplankton as it dies is equally devastating. The enormous quantity of bacteria uses up all the oxygen in the water they are in, literally suffocating the fish and other breathing animals in the vicinity. A July 2022 algal bloom in San Pablo Bay devastated the sturgeon population, leaving 700 carcasses on the shores. A Bay-wide HAB in 2023-2024 reached all the way into Lake Merrit and the far reaches of the Central and South Bay.

Constant work is being done by Bay Keeper and other organizations to treat and recycle wastewater to decrease the nutrient load and pollutants flowing into the Bay. Increased water flow through the Delta is also a big help.