Using Chemicals in Fish Tissue to Develop an Accurate Food Web
When we think about conserving estuaries, our attention often turns to salmon, shorebirds, or the endangered Delta Smelt. Researchers recently found humans have been modifying these estuarine ecosystems for at least the last 7,000 years. Yet some of the most important clues about ecosystem health come from organisms no larger than a grain of rice. Zooplankton—tiny drifting animals that feed on algae and are, in turn, consumed by fish—form the critical bridge between primary producers and higher levels of the food web. Understanding what these organisms eat and how their diets change over time can reveal whether an ecosystem is functioning as it should.
During my research at the Romberg Tiburon Center for Environmental Studies, I explored how stable isotope analysis can be used to better understand the trophic ecology of zooplankton in the San Francisco Estuary. This work aimed to improve our understanding of the estuary's food web while developing methods that allow scientists to compare modern samples with decades of archived specimens. Together, these approaches provide valuable tools for conservation and habitat restoration.
Reading Nature's Chemical Fingerprints
Every organism carries a unique chemical signature that reflects what it has been eating. Stable isotopes—naturally occurring forms of elements such as carbon and nitrogen—serve as ecological tracers because they move through food webs in predictable ways.
Carbon isotopes (δ¹³C) help identify the original sources of organic matter supporting an organism, distinguishing among food sources such as phytoplankton, terrestrial plants, or detritus. Nitrogen isotopes (δ¹⁵N), meanwhile, generally become enriched with each step up the food chain, allowing researchers to estimate an organism's trophic position.
Rather than relying solely on direct observations of feeding, which can be difficult in complex aquatic ecosystems, stable isotope analysis provides an integrated picture of what organisms have been consuming over weeks or months. This makes it especially valuable in dynamic estuaries where food availability changes with tides, river flows, seasons, and human activities.
Why the San Francisco Estuary Matters
The San Francisco Estuary is one of the largest and most ecologically important estuaries on the west coast of North America. It supports diverse fish and wildlife while providing drinking water, recreation, and economic benefits to millions of people.
Over the past several decades, however, the estuary has undergone dramatic ecological change. Nutrient inputs, altered freshwater flows, invasive species, and habitat loss have reshaped its food web. These changes have coincided with declines in several native pelagic fish species, including the endangered Delta Smelt, prompting researchers to ask an important question:
Has the base of the food web changed in ways that reduce the energy available to native fish?
Building Reliable Tools for Long-Term Ecology
One challenge in answering this question is that many historical zooplankton samples were preserved using formalin rather than frozen. Because preservation methods can potentially alter isotope measurements, researchers first need to determine whether preserved specimens still provide reliable information.
A major objective of this project examined whether preservation method and storage duration influenced stable isotope values. The results were encouraging. Carbon isotope values showed only small differences between formalin-preserved and frozen samples, while preservation effects remained relatively consistent over time. Although nitrogen isotopes appeared more sensitive to preservation, the differences were systematic enough that species-specific correction factors may allow archived collections to be used confidently in long-term ecological studies.
This finding is more important than it might initially seem. Museums and monitoring programs have preserved biological samples for decades. Demonstrating that these collections remain scientifically useful means researchers can reconstruct historical food webs without having to travel back in time. Long-term ecological records are invaluable for understanding how ecosystems have responded to environmental change and for establishing realistic restoration targets.
Looking Back to Move Forward
Historical collections offer a rare opportunity to compare today's estuary with conditions several decades ago.
Preliminary analyses suggest that zooplankton isotope signatures have shifted through time, with more recent samples tending toward lower carbon isotope values and higher nitrogen isotope values. These patterns may indicate increasing reliance on terrestrial or freshwater-derived organic matter and changes in trophic interactions, although additional analyses are needed to fully interpret these trends.
Such shifts suggest that the energy pathways supporting today's food web may differ from those that historically sustained native fish populations. Understanding these changes helps scientists identify not only what has been lost, but also what ecological processes restoration efforts should seek to recover.
Connecting Science to Habitat Restoration
Habitat restoration often focuses on rebuilding marshes, reconnecting floodplains, or improving water quality. While these actions are essential, successful restoration ultimately depends on rebuilding ecological processes—not just physical habitats.
Stable isotope analysis helps answer questions that traditional monitoring cannot:
Are restored habitats producing food that moves efficiently through the food web?
Which primary producers support zooplankton and, ultimately, fish?
Are restoration projects increasing reliance on productive phytoplankton-based food webs or less efficient detrital pathways?
How have these relationships changed over time?
Because zooplankton occupy such a central position in aquatic food webs, they serve as sensitive indicators of whether restoration efforts are rebuilding the ecological connections needed to support higher trophic levels. Modern restoration increasingly recognizes that success is measured not simply by acres restored, but by the recovery of ecosystem functions and food-web resilience. Stable isotope techniques provide one of the most powerful ways to evaluate that recovery.
Tiny Organisms, Big Implications
One of the most rewarding aspects of this research was seeing how chemistry, ecology, and conservation intersect. A tiny copepod drifting through the estuary may seem insignificant on its own, but collectively these organisms determine how efficiently energy moves from microscopic algae to economically and ecologically important fish species.
By improving methods for analyzing preserved samples and applying stable isotope techniques to modern food webs, this research contributes to a broader effort to understand how estuarine ecosystems function—and how they can be restored.
Conservation is often about looking ahead, but sometimes the best way to plan for the future is to learn from the past. Archived zooplankton samples represent decades of ecological history waiting to be decoded. Stable isotope analysis allows us to read that history, helping scientists identify long-term changes in ecosystem function and providing guidance for restoring one of California's most valuable and vulnerable estuaries.
In the end, protecting species like the Delta Smelt depends on far more than conserving the fish themselves. It requires understanding the invisible connections that sustain them—from microscopic algae to drifting zooplankton and every link in between. By uncovering those hidden pathways, stable isotope ecology offers an important tool for building healthier, more resilient aquatic ecosystems for generations to come.