How Salmon Feed Wildflowers And Transform Entire Landscapes
This is the first study to demonstrate a connection between salmon and coastal plant growth and reproduction
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I grew up in salmon country, and quickly developed a tremendous respect for these amazing fishes.
Salmon are anadromous — they hatch in freshwater before migrating to saltwater where they live and grow for two to seven years, depending upon species, before returning to freshwater as adults to spawn. Why do they migrate to the ocean? Freshwater lakes, streams and rivers are nutrient poor. To grow big and to produce masses of eggs, salmon must have the abundance of nutrients that the sea provides.
When adult wild salmon return to their natal streams to spawn, they carry energy and a variety of nutrients in their bodies and after they die, these resources are transferred to animals and plants living alongside the stream. One of the most important of these nutrients is nitrogen. Because the particular form of nitrogen in air — nitrogen gas — cannot be assimilated by most organisms, it is usually a limiting nutrient for the growth of living beings. Nitrogen is a critically important component of amino acids, proteins and even DNA. It’s also a vital component of chlorophyll, which is used in photosynthesis by plants to make food.
To better understand how nitrogen is transferred from dead salmon to plants living streamside, a team of researchers conducted a three-year field experiment on a small river in Haíɫzaqv (Heiltsuk) territory on British Columbia’s central coast. Led by salmon ecologist Allison Dennert, a PhD Candidate in Biology at Simon Fraser University whose doctoral research focuses on the relationship between nutrients from the sea and terrestrial ecosystems. Ms Dennert also works as a quantitative salmon ecologist with the Raincoast Conservation Foundation.
To do this work, Ms Dennert divided the study area into equally-sized plots and selected the four most common wildflower species growing there (Figure 1) as her study organisms: silverweed, yarrow, Douglas’ aster and common red paintbrush.
The river that flows into this estuary receives an average of less than 200 spawning salmon annually, and the stream geomorphology prevents deposition of large amounts of drift seaweed onto the banks of the study area. The study area is accessible only by boat, so therefore, human disturbance is minimal. The study watershed is small, with a catchment area of 4.5 km2, but features a large estuary meadow populated with grasses, sedges and salt-tolerant wildflowers.
Ms Dennert experimentally added pink salmon, Oncorhynchus gorbuscha, carcasses to some experimental plots. Ms Dennert also conducted the same experiment on other study plots but instead of dead salmon, she added rockweed, a seaweed that provides a different set of nutrients from salmon. She also added a combination of rockweed and salmon carcasses to other plots and, as an experimental control, added nothing to the last group of plots.
Later, Ms Dennert returned and randomly plucked leaves from the study plants for analysis. Ms Dennert and her collaborators found that the addition of salmon carcasses led to larger wildflower leaves, particularly in yarrow and common red paintbrush, and a larger seed set in yarrow in the third year.
“Following our experiments, we found that some species of wildflower grew larger leaves where a salmon carcass was deposited, and in some years, some species grew larger flowers or produced more seeds”, Ms Dennert summarized.
This study helps underscore that the importance of wild salmon to the plants and trees growing alongside their spawning streams. It also is consistent with previously published research. For example, previous work found that wildflowers can coordinate blooming with the arrival of salmon, which is also helpful to their pollinators (ref). Another study found a strong correlation between tree girth and streams with more salmon (ref).
“Understanding the interconnection between ecosystems is incredibly important to our knowledge of how to protect them”, Ms Dennert stated.
Currently, many salmon populations along the Central Coast of British Columbia are declining or are of conservation concern due to the effects of climate change, pollution, overfishing, habitat degradation and habitat loss.
“In some areas on our coast, we’re rapidly losing salmon biomass and the ocean’s connection to life on land”, Ms Dennert pointed out.
Last year, a study conducted by another SFU researcher and colleague within her supervisor’s research area found that chum salmon, Oncorhynchus keta, had declined by almost 50% within just the last 15 years, and by over 70% within the last 50 years (ref).
Ms Dennert’s study also provides more information about the impact of climate change on the rivers and streams travelled by salmon and this, in turn, could help inform more integrated ecosystem planning and management.
“Currently, lands and waters are managed under separate provincial and federal jurisdictions. Scientifically and management-wise, we think of the land and sea as separate and unconnected entities”, Ms Dennert said. “This work furthers the idea that ecosystems don’t exist in isolation, and that what happens in one can influence the other.”
Allison M. Dennert, E. Elle and John D. Reynolds (2023). Experimental addition of marine-derived nutrients affects wildflower traits in a coastal meta-ecosystem, Royal Society Open Science 10:221008 | doi:10.1098/rsos.221008
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