Dec 09, 2015

Clean Water, Great Life – Bay Water Quality Update Part II

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The Morro Bay estuary is a special place that is central to many of our lives, providing a beautiful place to live, work, and visit. We play in these waters and enjoy the food they provide. These waters are also home to countless species of plants, fish, and invertebrates.

A great blue heron keeps an eye out for fish that live in the estuary’s waters.
A great blue heron keeps an eye out for fish that live in the estuary’s waters.

The monitoring efforts of the Estuary Program and its volunteers help to determine if Morro Bay provides clean waters that can support sensitive marine life, as well as activities such as swimming, boating and fishing.

Clean water is central to a healthy estuary that supports a diversity of life. Pictured here is a Slender Crab, one of the many crab species found in our bay.
Clean water is central to a healthy estuary that supports a diversity of life. Pictured here is a Slender Crab, one of the many crab species found in our bay.

Last week, we looked at what the Estuary Program’s monitoring efforts can tell us about how safe the bay is for recreational uses like swimming. This week, we discuss how well Morro Bay’s waters can support wildlife.

 

Can Morro Bay support wildlife?

Fish and other aquatic wildlife need well-oxygenated waters. The amount of oxygen present in water is called dissolved oxygen (DO). The most sensitive wildlife need dissolved oxygen levels between 5 and 7 mg/L.

Without enough oxygen in the water, fish and other animals can asphyxiate and die.

The consequences of low DO concentrations can be dire for wildlife. In King Harbor Marina located in Redondo Beach, a storm pushed large numbers of sardines into this shallow harbor. Oxygen levels dropped to 0.72 mg/L, resulting in 170 tons of dead fish and over $400,000 in clean-up costs. Learn more about this 2011 fish kill. Photograph by Bruce Evans.
The consequences of low DO concentrations can be dire for wildlife. In King Harbor Marina located in Redondo Beach, a storm pushed large numbers of sardines into this shallow harbor. Oxygen levels dropped to 0.72 mg/L, resulting in 170 tons of dead fish and over $400,000 in clean-up costs. Learn more about this 2011 fish kill. Photograph by Bruce Evans.

 

In the early morning hours, Estuary Program volunteers paddle to seven locations around the bay to collect DO measurements.

Volunteers monitor DO levels in the early morning hours because that is typically when we find the lowest levels of the day.
Volunteers monitor DO levels in the early morning hours because that is typically when we find the lowest levels of the day.

The following map shows the averaged DO concentrations from monthly volunteer monitoring between 2008 and 2014. Sites closer to the bay mouth and the channels typically have higher DO levels. Sites further back in the bay, where the waters are shallow and undergo less tidal mixing, are more likely to experience DO levels low enough to negatively impact aquatic life.

At both Sharks Inlet and Cuesta Inlet, nearly 95% of DO measurements were less than 7 mg/L, which is the level determined to be protective of aquatic life.
At both Sharks Inlet and Cuesta Inlet, nearly 95% of DO measurements were less than 7 mg/L, which is the level determined to be protective of aquatic life.

 

Human activities can play a role in bay oxygen levels. Rain that falls on our yards, driveways and streets ends up untreated in the estuary. This stormwater carries with it fertilizers, pet waste, car fluid leaks and other pollutants into our estuary. This pollution can introduce extra nutrients into the aquatic system, which in turn can lead to algal blooms.

This excess algae, like other aquatic plants, undergoes photosynthesis during the day, putting oxygen into the water. But at night, these same plants switch into a respiration mode, which consumes oxygen in the water. Thus, the lowest DO levels of the day—the most dangerous for plants and animals—are typically measured in the early morning hours before the sun gets high in the sky and photosynthesis resumes.

Algae is more efficient at utilizing excess nutrients, which can result in a bloom. Algae can crowd out more valuable habitat types such as eelgrass. Pictured here is an eelgrass bed (the long blades) interspersed with red algae and green algae, also known as sea lettuce.
Algae is more efficient at utilizing excess nutrients, which can result in a bloom. Algae can crowd out more valuable habitat types such as eelgrass. Pictured here is an eelgrass bed (the long blades) interspersed with red algae and green algae, also known as sea lettuce.

While human activities may be part of the reason for these low DO levels, the natural circulation and hydrology of the back bay plays a large role, too. The waters of the back bay naturally undergo less tidal mixing because they are farthest from the harbor mouth. They are also shallow, which means that they are quickly warmed by the sun, and warm waters cannot retain as much oxygen as cold waters can.

The bottom line

Although Morro Bay has lower than ideal DO levels in the back bay, meaning that it may have trouble supporting sensitive species at certain times during the year, we don’t experience dire phenomena such as fish kills.

 

Make a Difference

Each of us can make a difference in keeping our estuary clean and healthy.

What can YOU do?

  • Be a bay-friendly gardener. Over-fertilizing your yard is a waste of money and results in harmful nutrients ending up in the bay. See our Bayside Guide for more tips.
  • Get those leaks fixed. Car leaks, even small ones, result in oil and other automotive fluids being washed into the bay during rainstorms and causing problems for plants and animals.