Warm water events at the Goodwin Islands CBNERR

One of our “vignettes” investigates the connection between submerged aquatic vegetation diebacks events and warm water temperatures.


Eelgrass, an important SAV species in Chesapeake Bay. credit

Submerged aquatic vegetation, also known as SAV to reduce your word count, is a foundation species in Chesapeake Bay. Like the foundation in your house, a foundation species is key in structuring an ecosystem! To estuarine critters, SAV is a food source, a water purifier, a shelter, a hunting ground, and more!

Like most organisms, SAV has a thermal threshold, or a critical temperature that is just too hot for the plant to survive. For eelgrass (Zostera marina), an important SAV species in Chesapeake Bay, that water temperature threshold is around 30°C (86°F). So, if you have a period of time when the water temperature stays above 30°C, you have the potential to observe an SAV dieback event.

In recent memory, the years of 2002, 2005, and 2010 all had  observed SAV dieback events occur for various, and likely compounding, factors. Of course, the environment is never simple and there are many factors that could contribute to a dieback event (such as water quality, disease, predation, or even a huge storm) in addition to hot temperatures.

But, for the purpose of this post (and vignette), we wanted to look at the times when water temperatures at Goodwin Islands was above that 30°C mark!.

Table 1:

Table 1: The percentage of 30 degC exceedances by year and month for the time series. For example, 20.7% of the >30 degC events between 1998 to 2014 occurred in the year 2005.

A table, revisited!

Table 1 shows you when water temperature was above 30°C from 1998 to 2014 by month and by year. (You’ve seen this plot before!)

What you should learn from this table is that water temperatures at Goodwin Islands only exceeded 30°C in the summer months of June, July, and August. This is not Earth-shattering, since warm water should happen when it is warm out, but allows us to narrow our window.

In other words, over the past ~15 years, these warm excursions did not occur at the end of May or beginning of September. Additionally, we can see that July (46.4%) and August (44.9%) are equally the most probably months when a heat-caused SAV dieback could occur at this CBNERR site.

The other thing we can see from this table: the years of 2002, 2005, and 2010 had the most events >30°C in this time series. If you remember from above, these years are also noted as years with observed, bay-wide, SAV dieback events.

Plot of the week!

We now know that June, July, and August of the years 2002, 2005, and 2010 had the most occurrences of water temperatures greater that 30°C/86°F in this time series. So let’s plot temperature for the summer months of those years!


Figure 1: The instantaneous temperature (taken at 15 minute intervals) at Goodwin Islands during the summer months of the years 2002, 2005, and 2010. A line is drawn at 30 degC since that is the approximate temperature when eelgrass begins to get thermally stressed.

For your convenience, a solid black line is placed at 30°C, so anything above that line is in a “danger zone” for causing heat stress to SAV. Interestingly, each year was different!

In 2002, we can see that warm temperature events happen throughout the summer pretty frequently! This agree with our work that suggests the summer of 2002 was really warm!

In 2005, we observe something fairly unique. The temperature range (those temperature wiggles up and down between day and night) are very small! In other words, this summer may not have been as hot as 2002, but it stayed pretty warm, even at night! Temperature increased slowly  throughout the summer, the teetered between 30°C in mid-July to mid-August.

In 2010, we can see it was warm, but in 3 episodic events. It got warm at the end of June, the cooled down, and got warm again by the end of July!

Final Thoughts

It is always helpful to plot data to see what was going on! 2002, 2005, and 2010 all had the same outcome of a high frequency of warm water events, but when and how that warm water appeared was unique to each event! Luckily, we have found that using the percent of time above the 90th percentile worked pretty well to model SAV thermal stress (Check our older posts here and here).

More to come!

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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One Response to Warm water events at the Goodwin Islands CBNERR

  1. Sarah Nuss says:

    Thanks Kari! This was a great one and something that I plan to use at our climate teacher workshop in June! 🙂

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