By Nicholas Wright

This blog is the first in a three part series on ecological subsidies that will appear throughout summer and fall ’23.

It’s easy to think of aquatic and terrestrial organisms inhabiting entirely separate worlds–they experience distinct biophysical conditions, interact with different ecological communities, and are imperiled by divergent environmental threats. But there are far more ecological connections between land and water than meets the eye. Organisms and organic matter move back and forth between terrestrial and freshwater ecosystems and are consumed or die in one or the other, transferring energy and nutrients in a phenomenon known as a trophic subsidy. Subsidies primarily flow from more productive food webs to less productive ones, at times allowing abundant life to exist in otherwise unproductive habitats. Some of these linkages are small and surprising, and others are more profound. Understanding cross-system flows of energy and nutrients is essential for conservation during a time of breakneck environmental change.

We start our story with a small lake, clear as glass and cold as ice, nestled among a dark conifer forest and surrounded by towering granite peaks. This lake, like thousands of others in the Sierra Nevadas, is fairly small–around the size of two football fields–and was carved by glaciers. The water is so clear because little algae can grow due to the cold temperatures and lack of nutrients. Without much algal production, there’s almost no food for consumers being produced in the lake–but that doesn’t mean there’s no food.

In spring, as the sun hits the mountain slopes, the thick snowpack starts to melt. The water percolates down into the mulchy top layers of the forest soil, which is rich with decomposed organic matter from pine needles and dead grass, fallen trees, and animal carcasses. As the water flows it picks up and dissolves organic matter and then carries it deeper into the soil and the upper layers of crumbling bedrock, flowing downhill until it meets a headwater stream that eventually flows into an alpine lake. The dissolved organic matter provides a terrestrial trophic subsidy, a flow of energy from forest to lake that feeds aquatic consumers. This subsidy offers the most significant food source in the lake at this time of year, supporting a food web of microbes, zooplankton, aquatic insects, frogs, and fish.

One study on cross-system subsidies in the Sierra’s showed that, during spring, lakes are so reliant on this terrestrial trophic subsidy that they are actually heterotrophic, meaning more organic matter is consumed than produced, which causes lakes to release CO₂ into the atmosphere (Piovia-Scott et al 2016). In late spring and early summer, as days get longer and warmer, algal growth increases. In these warm days, as the snowpack melts away and stream flows decline, less terrestrial organic matter is carried into the streams and lakes. During this period the algal growth makes Sierra lakes become autotrophic, which means they produce more organic matter than they consume and they begin absorbing CO₂ from the atmosphere. Over the course of an entire year, mid-elevation lakes that receive lots of organic matter subsidies from their forested watersheds tend to be net heterotrophic, while lakes above the treeline, where watersheds are predominantly bare rock, receive a much smaller flux of terrestrial organic matter and are typically slightly net autotrophic. 

The greatest fluxes of terrestrial organic matter in the Sierras come from wet meadows (Piovia-Scott et al. 2016). These meadows, located in mountain valleys, are often created by beavers damming streams and they sequester lots of carbon in their reach peaty soils (Yarnell et al 2019). Water percolates slowly through these heavy soils, turning the riparian meadows into vital wetlands, even during periods of drought, and leaching dissolved organic matter into alpine streams and lakes. 

Organic matter from decomposing vegetation that feeds the bottom of the food chain is not the only terrestrial trophic subsidy to the lake. Swarms of flying insects fall into the water or hover just above it, where they make easy prey for fish. In Castle Lake, in far northern California, terrestrial insects were found to provide about a third of the food items some fish species consumed (Vander Zanden et al. 2006). In fall, as the algae grow slower and little food is produced within the lakes, insects continue to provide a stable food source that allows larger populations of fish to survive than would otherwise be possible.

Once the most abundant run in California, Spring-run Chinook Salmon historically depended on terrestrial insect subsidies, but has now declined throughout streams of the Sierras. The cold water, gravel substrate, and swarms of insects made Sierra streams an ideal habitat for salmon parr, which have a longer freshwater residence time than other California salmon, to rear in great numbers before migrating downstream out of the mountains and into the Sacramento River. However, throughout the last two centuries mining operations degraded many of the salmon’s spawning grounds and the construction of dams made many of the Sierra streams inaccessible to salmon. Now the remaining Central Valley spring-run salmon must scrape out a living in a few lower-elevation reaches of streams in the Sierra foothills and Sutter Buttes and the population has been federally listed as ‘threatened’.

Nicholas Wright is junior specialist in the Johnson-Jeffres research group.

Further Reading:

Piovia-Scott, J., Sadro, S., Knapp, R.A., Sickman, J., Pope, K.L., and Chandra, S. 2016. Variation in reciprocal subsidies between lakes and land: perspectives from the mountains of California.Canadian Journal of Fisheries and Aquatic Sciences, 73, 1691–1701.

Pope K.L., Piovia-Scott J., and Lawler S.P. 2009. Changes in aquatic insect emergence in response to whole-lake experimental manipulations of introduced trout. Freshwater Biology, 54(5): 982–993.

Rypel, A.L. 2022. Nature has solutions…What are they? And why do they matter? California Water Blog https://californiawaterblog.com/2022/03/27/nature-has-solutions-what-are-they-and-why-do-they-matter/

Vander Zanden M.J., Chandra S., Park S.-K., Vadeboncoeur Y., and Goldman C.R. 2006. Efficiencies of benthic and pelagic trophic pathways in a subalpine lake.Canadian Journal of Fisheries, 63(12): 2608–2620.

Vredenburg V.T. 2004. Reversing introduced species effects: experimental removal of introduced fish leads to rapid recovery of a declining frog. Proceedings of the National Academy of Sciences, 101(20): 7646–7650.

Yarnell, S. M., Pope, K., Burnett, R., Wolf, E., Wilson, K. (2019) An experimental study of the ecohydrologic and carbon sequestration benefits of beaver dam analogue restoration techniques in Childs Meadow, CA, USA. AGU Fall Meeting 2019.