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.
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?
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?
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
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).
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?!
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.