Roughly 60 years ago, the pioneering conservationist Aldo Leopold wrote a short piece called “Odyssey,” collected in his Sand County Almanac. The odyssey in question is that of a nitrogen atom. Leopold imagined its travels from a starting point in a limestone ledge through a multiplicity of prairie food webs, and finally to the sea.
There was as much poetry as science in “Odyssey.” No one at that time was able to track an atom from rock to root to flower to grazer to predator. These days, thanks to a technique called stable isotope analysis, scientists can at least identify way stations on such journeys.
Stable isotopes—variant forms of hydrogen, carbon, and nitrogen—have become an indispensable part of the biologist’s tool kit. Ratios of deuterium to hydrogen in a warbler’s feathers indicate how far north it was when it grew those feathers before heading south for the winter. Carbon isotopes show where a road-killed mountain lion ate its last deer. And nitrogen isotopes help measure what riparian ecosystems owe to salmon.
That last area has seen a florescence of studies in the last decade. It’s now recognized that the salmon that swim up their natal streams, spawn, and die represent a huge pulse of nutrients. The corpses of spent salmon nourish the streambed, supporting microorganisms and benthic invertebrates. Alaskan brown bears eat salmon, then go and fertilize the woods. For each scavenger, there’s a pathway.
A recent study by Katie Christie and Thomas Reimchen of the University of Victoria underscores the importance of the salmon subsidy. Christie and Reimchen compared songbird diversity on a sample of rivers in coastal British Columbia. Some of the rivers had salmon-friendly gradients; others had waterfalls near their mouths that barred the passage of the fish, or falls farther upstream with salmon-free reaches beyond them. The biologists did point-counts of common riparian birds like winter wrens, Swainson’s and varied thrushes, and Pacific-slope flycatchers, and analyzed the birds’ abundance in terms of the presence or absence of salmon, distance from the stream, and vegetation structure.
Christie and Reimchen found salmon to be an important predictor of songbird abundance, with sharp differences above and below salmon-blocking waterfalls. They speculated that the linkage involved aquatic insects like chironomid midges that feed on salmon carcasses as larvae and emerge as flying adults, prey for the wrens and flycatchers. Christie, in separate research, found salmon-derived isotopes in the bodies of winter wrens. Nutrients from the salmon may also benefit the huckleberries and salmonberries whose fruit the thrushes feed on.
Closer to home, UC Davis fish biologist Peter Moyle and EBMUD scientist Joseph Merz used nitrogen isotopes to compare salmon contributions to two California rivers: the Mokelumne, which still has a consistent chinook run, and the Calaveras, which has pretty much lost its salmon. Nitrogen comes in two forms, the heavier of which is more abundant in seawater. Higher ratios of the heavier isotope to the lighter create a marine signature.
Moyle and Merz calculated salmon biomass in the Mokelumne by counting migrant fish at the Woodbridge Dam, then multiplying by average weight to get an annual input of nearly 180,000 pounds. They surveyed salmon carcass distribution by scavengers, and sampled the leaves of native riparian vegetation and riverside vineyards to determine nitrogen ratios.
They found that the leaves of wine grapes, Fremont cottonwoods, and sandbar willows near salmon spawning sites on the Mokelumne had significantly higher levels of the marine form of nitrogen than other sample sites on the river, or any sites on the Calaveras, consistent with receiving 18 to 25 percent of their nitrogen from a marine source. Vultures and other scavengers appeared to be transporting salmon remnants into the vineyards. (Fourteen bird and mammals species were detected feeding on salmon carcasses, including blacktail deer and western gray squirrels.) Since the Mokelumne seldom floods during the spawning season, direct river transport of salmon nutrients was considered a less likely pathway than scavenger distribution.
Extrapolating from the Mokelumne, Moyle and Mertz figured that salmon runs, diminished though they may be, contribute 6,000 metric tons of biomass, 1,800 metric tons of carbon, and 337 metric tons of nitrogen to the Central Valley every year. The nitrogen alone would be an important input. Nitrogen feeds the yeasts that ferment wine, increasing sugar and ethanol formation, and nitrogen levels can directly affect wine quality. The salmon subsidy could also reduce the need for fertilizer in riparian restoration projects.
The more we learn about the workings of the natural world, the more support there is for John Muir’s dictum about everything being hitched together. It’s fascinating to consider the nutrient flows from sea to river to forest and vineyard. With salmon populations crashing, it’s also sobering to think of the connections that have been broken.