Using our data to investigate eelgrass die-offs

An eelgrass bed. You can read more about the importance of eel grass here.

An eelgrass bed. You can read more about the importance of eelgrass here.

Eelgrass, Zostera marina, is an important species of submerged underwater vegetation in Chesapeake Bay. This sea grass is not only a pretty neat organism, but it a vital habitat for many iconic organisms, such as the blue crab. However, eelgrass beds have been declining since the 1930’s due to many pressures including disease, nutrient loading, and even Hurricane Agnes in 1972 (Golden et al., 2010).

Another pressure that critically affects eelgrass is extreme temperatures. In 2005, a massive eelgrass die-off was observed in Chesapeake Bay as a result of an extended period of daily high temperatures between 33-35°C (Najjar et al., 2010). Generally speaking, when shallow water temperatures exceed 30°C, eelgrass becomes stressed since the rate of respiration begins to surpass the rate of photosynthesis.

In other words, these eelgrass beds are running a marathon and not able to stop for food to refuel.

Figure 1:

Figure 1: The summertime %days when the daily maximum temperature was hotter than the 90th percentile (aka, was a pretty hot day).

This thermal threshold is particularly important since eelgrass in Chesapeake Bay is already at its most southern extent, meaning that any increases in water temperature (like those attributed to climate change and variability), could increase the probability of these 2005 die-off events. Our work on extreme climate indices, then, can be used to give insight to eelgrass die-off events associated with extreme heat.

For example, the percentage of summertime days which exceed the 90th percentile of the daily maximum temperature had a significantly different probability density distribution and mean from 1951-1980 to 1981-2010 (see Figure 1 for example). This infers that the past 30 years have observed more days above the 90th percentile (approximately 31°C or 87.8°F), which proposes that eelgrass are feeling pretty stressed out.

When did >30°C water temperatures occur?

Table 1: Summary of when the water temperature at Goodwin Islands was warmer than 30C .

Table 1: Summary of when the water temperature at Goodwin Islands was warmer than 30C .

For this simple “glimpse” at eelgrass thermal thresholds, I used water temperature data from the Goodwin Islands station located in the Virginia CBNERRS. Then I used R to ask the question: when did the water temperature exceed 30°C?

Not surprisingly, we can see in Table 1 that July and August are the months when water temperatures most often exceeded 30°C. (That makes sense!).

We can also see that the year to year occurrences of >30°C water are very variable. At this specific site, 2005 represented the most “>30°C events” in this 17 year time series. This table says that 2005 contained 20.7% of the occurrences when the water temperature was hotter than 30°C.

What air temperature does this correspond to?

Figure 2:

Figure 2: Linear regression of air temperature at West Point, VA versus water temperature at Goodwin Islands.

Water temperature is significantly correlated to air temperature (Figure 2), allowing us to derive a linear equation.

For anyone rusty, an equation of a line is

Y=mx+b

Where m is the slope (rise/run or Δy/Δx) and b is the y-intercept.

From the linear regression of the air temperature versus water temperature plot, we can define the slope (0.960010) and y-intercept (4.505453). Now, by inserting 30°C in for x, we can calculate the approximate air temperature (y) needed to achieve a water temperature that could hit this thermal threshold for eelgrass.

This air temperature is 33.31°C (92°F).

The water temperature for the summer of 2005 at Goodwin Islands, when an eelgrass die-off occurred.

The water temperature for the summer of 2005 at Goodwin Islands, when an eelgrass die-off occurred.

Quick disclaimer: This simple linear model does not take into account the duration of a warm event or other factors which could influence water temperature, such as wind and lag! (But we can address those!). I also used the daily maximum temperature; using the daily mean or daily minimum temperature would tell us other helpful information.

This linear model gives us a rough indicator: if the daily high temperature reaches 92°F, then there is a potential for eelgrass stress!

More to come, such as what does that TX90p index mean for these eelgrass stressing events?!

Citations

Golden, Rebecca R., Kathryn E. Busch, Lee P. Karrh, Thomas A. Parham, Mark J. Lewandowski, and Michael D. Naylor. “Large‐Scale Zostera marina (eelgrass) Restoration in Chesapeake Bay, Maryland, USA. Part II: A Comparison of Restoration Methods in the Patuxent and Potomac Rivers.”Restoration Ecology 18, no. 4 (2010): 501-513.

Najjar, Raymond G., Christopher R. Pyke, Mary Beth Adams, Denise Breitburg, Carl Hershner, Michael Kemp, Robert Howarth et al. “Potential climate-change impacts on the Chesapeake Bay.” Estuarine, Coastal and Shelf Science 86, no. 1 (2010): 1-20.

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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