Earlier this spring, Our City Forest partnered with the Watershed Stewards Program to create a bioswale outside of our Urban Forestry Education Center. A bioswale is a form of green infrastructure that uses vegetation in sloped channels to capture water run-off. This storm water capture allows the vegetation to slow the rate of the water’s flow, meaning it will be able to slowly percolate down into the soil and reach the groundwater. The vegetation is also able to biologically filter pollutants, heavy metals, and excess nutrients that have collected from the urban environment.
So what does this have to do with the ocean?
To answer this question, we need to delve into the concept of watersheds and how they connect urban environments to marine ecosystems. A watershed, also referred to as a drainage basin, is a geographic area where all surface water collects to a single point such as creeks and rivers until it eventually outflows into the ocean (NOAA).
Source: The Watershed Project
From Urban Forests to the Ocean
Santa Clara County has six watersheds, meaning there are six distinct geographic regions that water is channeled through in order to reach the bay.
Let's take a closer look at one of these watersheds and its significance. The bioswale installed at our Education Center is located in the Guadalupe River Watershed. All parts of the urban forest are important in protecting all parts of the watershed, from stream to ocean. As discussed in our 2024 World Wetlands Day post, when the tree canopy is removed (and understory plants, presumably) and replaced by impermeable surface such as roads, parking lots, concrete, etc., there is an immediate impact on the watershed (PSU). Water runs off of the many impermeable surfaces found in urban landscapes rather than being slowed by the tree canopy and absorbed into the soil. These large amounts of water are often carried directly to streams, lakes, or rivers (and eventually to the bay and the ocean) without first being naturally filtered by trees and other plant species. This large amount of water increases erosion and deposits large amounts of sediment, both of which can destroy important habitats and breeding grounds. The canopy is first to slow incoming rainfall which then drips down into the soil where it is absorbed and gradually released into the watershed, including important groundwater stores. According to Penn State University,
“Average interception of rainfall by a forest canopy ranges from 10-40% depending on species, time of year, and precipitation rates per storm event. In urban and suburban settings a single deciduous tree can intercept from 500 to 760 gallons per year; and a mature evergreen can intercept more than 4,000 gallons per year”.
This water is then filtered by trees and other plants before entering the watershed. Excess nutrients, such as nitrates and phosphates, come from agricultural sources as well as urban fertilizer use (such as for lawns). While abundant nutrients may sound like a good thing, high levels cause a detrimental effect called eutrophication. Excessive nutrients result in unbridled growth of plants and microorganisms such as algae. When these plants and microorganisms decay, the amount of bacteria needed to break them down consumes the dissolved oxygen in the water, creating what is often referred to as “dead zones'. These dead zones, stripped of oxygen, suffocate animals such as fish which have a key role in the food web . Furthermore, other contaminants such as metals and pesticides are also removed from the water by plants, particularly woody plants like trees and shrubs. While wetlands can filter pollutants and nutrients, the accumulation of too many nutrients can outweigh the ecosystem’s filtering capacity.
Eutrophication has been a particular issue across California’s coast line. It is suspected that red tides, or harmful algae blooms (HABs) are caused (in part) by fertilizer use, especially in the agricultural industry. Not too far from the San Francisco Bay, the Elkhorn Slough has been experiencing an increase in water eutrophication. The slough enters the ocean in Moss Landing, a small town in the middle of agricultural plots that stretch for miles and miles. These HABs do more than just strip water of its oxygen, but they can also produce harmful substances.
One of these substances is called domoic acid. Domoic acid in marine environments is produced by the phytoplankton Psuedonitzschia australis (Ocean Connections). This acid is the cause of a serious condition called domoic acid toxicosis. This condition affects many marine mammals, especially sea lions, and is a major cause of many stranded sick animals. Domoic acid is also toxic to humans when large quantities are ingested through sea food.
To fully grasp the interconnectedness of these elements, we can use the concept of systems thinking. This image here demonstrates the movement of water from our Education Center, through the Guadalupe river, into the Bay, and eventually out into the Ocean. This discussion of watersheds, urban environments, pollution, and marine illness is a great example of systems thinking which sees different problems as parts of a greater whole that works as a living system, just like your body is a system. There are plenty of other factors in this system that we could explore that demonstrate the various connections between urban environments and the ocean.
Let me start with a question: what is a possible connecting link between an orca and a tree? You may be surprised at how many answers you could find, but what I had in mind are fish. More specifically, chinook salmon.
In December 2022, CBS News reported that chinook salmon have been spawning in an urban watershed. That watershed is in fact the same watershed that our Education Center is located in: the Guadalupe river watershed. In the article, the founder of the South Bay Clean Creeks Coalition stated that:
“This waterway, the Guadalupe watershed, is the furthest south on the North American continent where you have Chinook salmon coming into an urban setting to have a re-occurring spawning event each year.”
This means that the health of our urban watershed directly impacts a keystone species that is federally listed as endangered (specific populations). Can you guess what makes up approximately 80% of the Southern Resident orca’s diet? It’s chinook salmon.
Spring Chinook Salmon. Credit: Michael Humling, U.S. Fish & Wildlife Service. Source: NOAA
There are three types of orcas that can be found along different parts of California’s coast: Southern Resident rcas, Bigg’s Orcas (also known as Transients), and Offshore Orcas. Each of these “eco-types” has distinct behavioral and genetic characteristics that differentiate each other. While Bigg’s Orcas are mammal-eating, Southern Resident Orcas eat salmon almost exclusively, specifically Chinook salmon, even in the presence of other food sources such as seals. They could eat those seals but they do not. Why? Like humans, orcas have cultural norms and practices that guide their behavior.
Differences in the characteristics of the Pacific orca ecotypes. Source: Southern Resident Orca Recovery
Believe it or not, you can take a 45-minute drive from San Jose to Santa Cruz or farther down to Monterey and see these orcas in the wild. In the fall of 2023, I was lucky enough to encounter a small pod of orcas just 8 minutes out of the moss landing harbor. I first saw them as their dorsal fins cut like black blades through the water. They were on the move, hunting sea lions (a good indicator they were not Southern resident orcas) in a dramatic chase. What made it even more suspenseful were the two ginormous and very upset humpback whales that were hunting alongside the great masses of sea lions that made the surface of the water boil with fish.
We have talked about the orca and the salmon, but what about the tree? Well, let’s think back to the discussion on how urban trees affect watersheds. Trees and other urban plants, along with forms of green infrastructure like bioswales, help capture stormwater and filter it so that it can be directed into the watershed with less pollution. As you now know, pollution in the form of excess nutrients can have devastating effects on a watershed by creating zones stripped of the dissolved oxygen that animals like salmon need to breathe. Other pollutants that enter the watershed can negatively affect the salmon as well. Trees and vegetation along riparian corridors are also key to stabilizing salmon spawning grounds because they prevent erosion.
In reciprocity, trees also need salmon. The Alaska Department of Fish and Game states that:
“According to Robert Naiman of the University of Washington, streamside vegetation gets just under 25 percent of its nitrogen from salmon. Other researchers report up to 70 percent of the nitrogen found in riparian zone foliage comes from salmon… Scientists have long known that nitrogen content in a tree can be measured from its growth rings, and several research projects explore the link between nitrogen, tree rings and the size of past salmon runs.”
The growth indicators of the tree rings and the known numbers of salmon spawning the year earlier have a positive correlation, according to the department.
We need trees and all the other plant life of the urban forest to protect not just the environments we inhabit in our daily lives but also the greater systems we are a part of. While our new bioswale is a small action in the grand scheme, it is no less a part of the new system our community is striving to create to protect our urban and wild environments so that they may thrive long after our lifetimes.