Field Updates December 2023: A Tale of Two Eelgrass Beds

Winter Eelgrass Monitoring

With the return of the winter king tides, the Morro Bay estuary experiences some of the highest high tides and lowest low tides of the year. These extreme low tides leave our eelgrass beds exposed, making it the perfect time for monitoring. In partnership with Cal Poly, we are conducting drone flights to create a detailed bay-wide eelgrass map. Our staff are also heading out for ground surveys to measure changes in eelgrass density and algae abundance. When combined, the two monitoring efforts provide a snapshot of eelgrass health and acreage.

The Benefits of Ground Surveys

Each type of monitoring has its benefits and limitations. Drone surveys are an incredibly efficient way to measure eelgrass acreage throughout the bay, allowing us to track the eelgrass decline and recent recovery. On the other hand, ground surveys focus on specific eelgrass beds to track smaller scale changes to better understand the drivers behind bay-wide acreage changes. Even though eelgrass acreage has increased in the estuary as a whole, each eelgrass bed has behaved differently during the recovery period. To highlight some of these differences, we’re going to take a look at two eelgrass beds that are exhibiting very distinct trends.

This map displays the location of two eelgrass beds that we visit each winter during our Permanent Transect ground surveys. The Embarcadero bed is located just north of Tidelands Park, and the Marina bed is south of the entrance to the State Park Marina.

Why Measure Eelgrass Density?

Eelgrass density is the number of eelgrass plants within a defined area. High eelgrass density is typically an indication of a healthier, more robust bed, while low density tends to represent a bed in a weakened state. Historically, the Embarcadero bed had a high density of eelgrass shoots, while the Marina bed had no eelgrass present. Since 2017, eelgrass density has steadily risen at the Marina bed, consistent with the overall increase in acreage observed throughout the estuary.

*Photos of the Marina eelgrass bed in 2017 and 2023. Eelgrass was completely absent in 2017 but has since been steadily increasing in density.*

But the situation is different at the Embarcadero site. Since 2020, this bed has experienced a declining trend in shoot density.

Since eelgrass began its estuary-wide recovery in 2017, eelgrass density at Embarcadero (red) and Marina (blue) have been on two different trajectories. During our most recent surveys, Marina surpassed Embarcadero’s density for the first time since monitoring began at these sites.

Declining or On the Move?

There are many factors that can influence eelgrass density, including elevation change, competition with algae, and the makeup of microbial communities that live on eelgrass blades. Since eelgrass prefers to grow within a certain range of water depths, changes to elevation can alter the area available for eelgrass growth. Excess sediment deposition can smother eelgrass beds and raise the elevation above the ideal depth range, while high levels of erosion can uproot and wash away patches of eelgrass.

A sprawling eelgrass bed is prevented from moving northward due to the steep slope along the sandspit. Eelgrass thrives at an optimal elevation. If the elevation is too high, exposed eelgrass can dry out at low tides. If the elevation is too low, the plants may not receive enough sunlight for photosynthesis.

While the Morro Bay estuary has experienced phases of large-scale erosion and sediment deposition in the past, many sites exhibited localized elevation changes that don’t fit the overall trends observed in the bay. Our ground surveys are conducted at the exact same locations each year. If the elevation changes at a site, eelgrass may shift toward nearby areas with more suitable depths. So, while eelgrass may no longer be present at our monitoring location, it may have merely shifted its location and the change does not necessarily indicate overall eelgrass loss.

Algae Abundance on the Rise

At first glance, these photos appear to indicate an increase in eelgrass at the Embarcadero bed from 2017 to 2023. In reality, most of the vegetation seen in the photo from 2023 is Ulva, a type of green algae known as “sea lettuce.”

The accumulation of algae can be another stressor for eelgrass beds. Thick mats of algae can build up on top of eelgrass beds and shade them from the sunlight they need for photosynthesis. The green algae Ulva has increased in abundance since 2020 at multiple monitoring sites, with our fall 2023 data indicating some of the highest measured amounts at the Embarcadero site. These Ulva mats could be contributing to the decline observed at Embarcadero, although further research is needed. The recent rise in algae has spurred the development of a project dedicated to tracking algae abundance, which was covered in a previous blog post.

*The percent cover of Ulva within the area sampled at the Embarcadero and Marina eelgrass beds has risen since 2020, with the highest coverages recorded at the Embarcadero bed.*

Microbes Make a Difference

Zooming in to the microscopic level, studies have shown that the bacterial communities living within eelgrass beds can either positively or negatively influence their overall health depending on the types of microbes present (Beatty et al. 2022). To study the eelgrass microbe community in Morro Bay, our partners at Cuesta College have been collecting eelgrass tissue samples since 2019 for molecular analysis, with results expected later this year.

Cuesta students collect eelgrass tissue samples from a bed at Coleman beach. The samples have since been analyzed for the presence of the pathogen that causes eelgrass wasting disease and are awaiting more detailed analysis to characterize the entire microbe population.

It is important to understand the influence of small-scale changes to eelgrass beds, as the factors contributing to the change may not stay small for long. Even with the rapid expansion of eelgrass, the Estuary Program is committed to continuing our monitoring efforts so we can make informed management and restoration decisions.

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Jan 12, 2024