Phytoplankton and primary productivity (fact sheet)

In many parts of Puget Sound, hypoxic waters are thought to be at least in part due to overgrowth of microscopic algae, which is triggered by excess nitrogen. That means it’s important to understand the dynamics of primary productivity – the rate at which those microscopic algae, known as phytoplankton, produce organic matter through photosynthesis and in this way provide the base of the food web. Researchers are investigating different types of phytoplankton and rates of primary productivity throughout the Salish Sea, and seeking to understand how primary productivity is likely to change as climate change alters patterns of coastal upwelling and freshwater flow into the Sound.

View of bright green, segmented, phytoplankton with spines under microscopic magnification.
Phytoplankton is the base of several aquatic food webs. In a balanced ecosystem, they provide food for a wide range of sea creatures. Photo: NOAA

What is the level of primary productivity in the Salish Sea?

Total primary productivity in the Strait of Georgia “was quite controversial over time,” says Sophia Johannessen, a geochemical oceanographer with Fisheries and Oceans Canada (DFO) Institute of Ocean Sciences. Johannessen spoke at a workshop on phytoplankton and primary productivity in the Salish Sea.

A 1970 study estimated total primary productivity at 120 grams of carbon fixed by photosynthesis per square meter of water surface per year. A 1979 estimate was nearly three times greater, at 345 grams carbon per square meter per year, an increase attributed to eutrophication, probably due to sewage.

Finally, a 1983 study was able to reconcile the two numbers: “It turned out really patchiness had been the problem,” Johannesen says. The low earlier estimate came from sampling low productivity areas, while the later high estimate was based on samples taken in high-productivity spots. A broader sampling methodology in the 1983 study yielded an intermediate estimate of  280 grams carbon per square meter per year.

The Salish Sea is a relatively high-productivity area of the global ocean due to coastal upwelling of nutrients; global average ocean productivity is about 50 grams carbon per square meter per year.

Has primary productivity in the Salish Sea changed over time?

This question, too, has been a controversial one. In the early- to mid-2000s, fisheries models assumed that declines of fish at higher trophic levels such as salmon were due to decreasing productivity at the bottom of the food chain. These models suggested there had been a 30% decrease in primary production in the Salish Sea since the 1970s.

Meanwhile, though, measurements of the declining oxygen concentration in the deep water of the Strait of Georgia suggested, if this change was due to eutrophication, an increase in primary production of a whopping 250%.

But an analysis of the nitrogen budget of the Strait of Georgia – based on a variety of monitoring data on sea water and river water nutrient concentrations, suspended particles, sinking particles, sediment traps, atmospheric deposition, and various municipal inputs – yielded exactly the same estimate for primary productivity as had been found based on water sampling in 1983: 280 ± 20 grams carbon per square meter per year.

Similarly, an analysis of carbon and nitrogen isotopes in sediment cores collected throughout Puget Sound and the Strait of Georgia suggested that the flux of marine-derived organic carbon has neither increased nor decreased over the last 100 years in the Salish Sea.

“There's a surprising amount that you can find out about primary productivity using geochemical tools,” Johannessen says of these studies.

Has the type of primary productivity in the Salish Sea remained constant or changed over time?

To gain insight into this question, Johannessen analyzed data from a Washington State Department of Ecology program that has been monitoring 16 water quality indicators at 37 stations throughout Puget Sound on a monthly basis since 1973. Patterns of nitrogen and silica content in the water suggest that primary productivity may have changed from a system dominated by diatoms to one dominated by small phytoplankton species.

However, water column data suggest that there are different trends in Puget Sound versus the Strait of Georgia. And analysis of organic carbon flux in sediment cores suggests that changes are different in Hood Canal than in the Main Basin of Puget Sound.

Certain measures of nitrogen and silica content also show multi-year patterns, such as an increasing trend for a few years followed by a decrease over the next several years. “So there's some kind of multiannual control on these water properties that we definitely need to investigate,” Johannessen says. “And we have yet to link those changes to climate stressors.”

How does the phytoplankton community vary across different parts of the Salish Sea and throughout the year?

Remote sensing data can help answer these questions about large-scale temporal and spatial dynamics, says Maycira Costa, head of the Spectral Remote Sensing Laboratory at the University of Victoria in Victoria, British Columbia, who also spoke at the workshop.

The European Space Agency Sentinel-3 satellites capture data on sea surface height, sea and land surface temperature, and sea and land surface color. Based on Sentinel-3 images, a program known as Algae Explorer maps daily chlorophyll patterns for the British Columbia coast and parts of the Salish Sea. Subtle differences in the sea surface color enable researchers to track the blooms of different phytoplankton functional groups across space and time.

The researchers also fed about 8,000 images of chlorophyll generated by Sentinel-3 from 2016 to 2021 into a neural network to define “bioregions” with similar phytoplankton community characteristics and bloom timing. Leveraging data from other satellites going back to 1997 they can track these patterns over time.

“What we're doing right now is looking at within each bioregion on a yearly basis: What is changing?” Costa reports. “And what is the importance of those changes?”

Costa and her colleagues have ground-truthed the satellite images via sensors installed aboard two B.C. ferries and other in-situ data. Using the satellite products inside Puget Sound would require similar validation, she says.

What is the relationship between phytoplankton and zooplankton?

Researchers classify phytoplankton into three groups based on size: picophytoplankton have a diameter <2 micrometers, nanophytoplankton 2 to 20 micrometers, and microphytoplankton >20 micrometers. (Those are generally much smaller than the diameter of a human hair, which can be between 17 and 181 micrometers.) “Phytoplankton composition affects nutrients available to zooplankton,” says Brian Hunt, an ecosystem oceanographer at the University of British Columbia, who also spoke at the workshop.

Hunt and other researchers are using measures of fatty acids and stable isotopes to track the availability of food sources for zooplankton throughout the year. “One of the important metrics that we use is the DHA to EPA ratio,” Hunt says, referring to two highly nutritious fatty acids. “Generally it's accepted that the higher the DHA to EPA ratio, the better quality the food is for the zooplankton.”

Diatoms and flagellates are the most important source of fatty acids for the zooplankton, Hunt and his colleagues have found. The DHA to EPA ratio peaks in the summertime, likely due to an abundance of fatty acids in picophytoplankton that small zooplankton can convert into DHA and EPA. “So the picophytoplankton can support a varied and nutritious prey field,” Hunt says – leading to more nutritious prey for salmon and other parts of the food web.

Funding for this fact sheet was provided by King County as part of a series of online workshops addressing scientific uncertainities around nutrient pollution and hypoxia. 

About the Author: 
Sarah DeWeerdt is a Seattle-based freelance science writer specializing in biology, medicine, and the environment. Her work has appeared in publications including Nature, Conservation, and Nautilus.