Why what happens on the land is critical to the health of our oceans

On October 18th, the city of Vancouver braced itself as an atmospheric river made landfall, an increasingly familiar event in recent years. While residents of the city are used to a wet Fall and Winter, the sheer volume of rain during these events is unprecedented. Rain poured down on the city, flowing off the streets and into Burrard Inlet. Images of chum salmon following the stormwater flow into suburbs briefly reminded people of the more than 30 streams that used to bisect the land that Vancouver is built on, and which have mostly been lost during the city’s development.

A concerted effort to review and rethink understanding of the role of the land in our oceans

Vancouver lies at the heart of the North Pacific Temperate Coastal Rainforest, a region that stretches from northern California to Alaska. Rain feeds thousands of small rivers and streams that drain through forest, soils, wetlands and bedrock, soaking up nutrients that are delivered to the coast. While the biggest rivers, the Fraser and Columbia, produce huge summer plumes of turbid waters over the ocean surface, easily visible evidence of their connection to the coast, small stream outflows go largely unnoticed. What is the significance of these myriad small streams to the surrounding ocean, so important to people’s livelihoods, culture, and well-being in British Columbia?

This was the question that the Coastal Rainforest Margins Research Network set out to answer. Western science has mostly compartmentalized the land and the ocean into two distinct and separate realms, a result of which is that the connections between the two have often been ignored. Funded by the National Science Foundation, the Coastal Rainforest Margins Research Network was a collaborative initiative that aimed to bridge this gap. The network brought together US and Canadian scientists, from forest ecologist to oceanographers, in a concerted effort to review and rethink understanding of the role of the land in our oceans, particularly the importance of the largely ignored smaller rivers and streams that flow from the coastal rainforest zone. These efforts have recently been synthesized in a paper published in the journal Limnology and Oceanography.

“Working with this diverse group of scientists took me far out of my comfort zone, and completely changed the way I think about British Columbia’s oceans”, said lead author and Institute for the Oceans and Fisheries (IOF) associate professor Dr. Brian Hunt. “I come from an oceanography background, and it seemed a big leap to connect with my land-based colleagues. But when you learn about how soil microbes cause nutrients to leach into lakes and rivers, you have to start wondering what the effect of these nutrients is when they reach coastal waters.”

“When you realise how much freshwater actually runs off the land, and the quantity of nutrients it transports, it becomes clear that it must be important to what is happening in the ocean. If you calculate the collective volume of water coming from the small coastal rainforest rivers, it is equivalent to discharge of the Mississippi River! To put that in context, the Mississippi is North America’s biggest river, and tenth biggest in the world. Add this to the discharge of the big continental rivers like the Fraser and you realise that the oceans along our coast are infused by the land. The rivers literally give it flavour,” said Hunt.

The Riverine Coastal Domain

All that freshwater sets up a surface current, coined the Riverine Coastal Domain, up to 20 km wide and flowing northwards from Washington State to the Arctic. This is the highway that salmon migrate along from the rivers of their birth to the ocean and back again. Many other species likely use it in a similar way. The nutrients that these freshwater delivers shape the growth of the tiny microscopic plants, phytoplankton, and other microbes that sustain most of the marine life that we know. The influence of the land goes well beyond the coast. Eddies that periodically spin away from the coast, like those emanating from Haida Gwaii, into the Gulf of Alaska support more than half of the phytoplankton growth in that offshore region. So, these eddies and the materials from land that they transport are crucial to the health of the offshore feeding grounds where most of our salmon grow and mature before coming home.

“You can think about the land-ocean system of the northeast Pacific as this huge interconnected meta-ecosystem,” says Hunt. “We often think about salmon as leaving freshwater for the ocean, but in fact, they never leave its influence, even when thousands of kilometres offshore in the Gulf of Alaska. And when they swim back to their rivers to spawn, salmon close the loop, delivering all the nutrients they gained while growing up in the ocean back to the land and its lakes and rivers.”

How the land connects with the ocean is changing

Logging, dams, urban and industrial development, fire and extreme weather events that are causing both flooding and drought, are all acting to change how and when freshwater reaches the ocean. These stressors also impact the quantity and quality of nutrients, now also including pollutants, that freshwater delivers to the coast. In reviewing the existing western science knowledge, the members of the Coastal Rainforest Margins Research Network realized that there are still many gaps in our understanding of how the land-ocean meta-ecosystem works. There is an urgent need to change how we approach research in and management of the ocean, to one that takes into account the influence of the land. Restoring or maintaining healthy oceans depends on healthy land ecosystems too.

Advancing an integrated understanding of land–ocean connections in shaping the marine ecosystems of coastal temperate rainforest ecoregions was published in Limnology and Oceanography.

Tags: Brian Hunt, British Columbia, Coastal Rainforest Margins Research Network, extreme weather, land-ocean system, Pelagic Ecosystems Lab, plankton, pollution, Washington