The Great Flush

Rainforests in coastal British Columbia contribute a lot more to the ocean than we thought.

Sheets of rain pour down on the Central Coast of British Columbia on a chilly day in November. A pulse of water—the color of chai tea—rushes over the boggy landscape, down a steep slope, and empties into the ocean. That brown color signifies carbon and other organic material dissolved in the water. These temporarily swollen streams may be small, but their size belies their importance.

In the past few decades, we’ve learned that the ocean has a big influence on the coastal landscape through subsidies, inputs of nutrients like salmon providing nitrogen to aid tree growth. But the sharing goes both ways, and these storm-swollen streams are one big way that carbon from the land is injected into the ocean.

Understanding these land-to-sea exchanges of nutrients is critical to calculations of carbon budgets over a larger geographic area. But these calculations had never been measured locally on the BC Central Coast, says Allison Oliver, a Hakai postdoctoral researcher at the University of Alberta and lead author on the new study based at Hakai’s Calvert Island Ecological Observatory that was published last week in the journal Biogeosciences.

“What sets this area apart is that we have tons of small [streams] that drain directly into the ocean. I was pretty confident the [dissolved organic carbon] numbers coming off this landscape would be high, but you never know until you go out there and actually collect the data.” It turns out those numbers were on the upper end of what they imagined—roughly 33,300 kilograms of carbon entered the sea per square kilometer of land. That’s as much carbon as four and a half elephants.

In this part of the coast, land plants, mosses, and soils provide most of the dissolved carbon. As the fresh water moves toward the coast via these streams, microbes in the soil, as Oliver puts it, “chew on the carbon,” breaking it down. These streams are critically different from larger, mountain-born watersheds like the Fraser or Skeena. In those mighty rivers, most of the carbon is usually broken down or taken up by plants and animals along the way. The dissolved carbon from small streams that directly enter into the ocean is less degraded. It’s “fresher,” and there’s more of it to feed ocean creatures. Hence, why scientists are so interested to gather data on these streams.

“It is really critical to have good stream flow data including big storm events in the winter,” says Oliver. “The challenge is to be able to get to these watersheds and access them during these storm events when a lot of carbon is entering the system.”

Find out how scientists measure stream flow during the worst winter storms when they aren’t even there.

To estimate the contribution of smaller streams in the region, scientists measured seven creek catchments entering Kwakshua Channel from Calvert and Hecate Islands. The additive effect of all of those small streams is enormous. When you extrapolate the amount of carbon run-off across the comparable areas of the coastal temperate rainforest, stretching from northern Vancouver Island to Southeast Alaska, you get a yield of nearly one billion kilograms of carbon. That number is extraordinarily high, Oliver says. “The amount [per square kilometer] even rivals small streams that come out of mangroves in Southeast Asia, which are known to be very high in carbon.”

The majority of carbon was transported into the ocean between September and April, when rainfall is highest. And the tiny plankton that are the primary users of this carbon must react quickly. These nutrient pulses don’t last long, so the potential food source is ephemeral.

The next step is identifying how this carbon is being used in the coastal ocean. “We’re trying to trace the different avenues [the carbon] might take,” says Oliver, who adds that we still have a lot to learn when it comes to the carbon cycle in this part of the coast.

It’s clear that the ocean shares its nutrients with plants and animals on land, but the land does its part too. Now, scientists can set out to discover how much of the plankton starts their day with a boggy chai tea. One thing is for certain. Never underestimate the potential power of a small stream.

Acknowledgements

The paper was a collaborative effort of scientists from the Hakai Institute, University of Alberta, Simon Fraser University, Vancouver Island University, the University of Northern BC, and the BC Ministry of Forests, Lands, Natural Resource Operations, and Rural Development.