Icy waters: What does the future hold for Bay critters?
by NOAA Fisheries 26 Jan 2018 22:41 GMT
You're right—it sure has been cold. In fact, it was the coldest first week of January for Chesapeake Bay water temperatures (at Thomas Point Lighthouse off Annapolis) in 30 years. All that cold water led to significant icing in large areas of the Chesapeake Bay.
If definitely felt colder than usual, and it makes sense that the water temperature was colder than usual, too. But how much? The graph below shows water temperature anomaly at Thomas Point Light from 1985 through today—that's how different the water temperature on a given date was from the average water temperature on that date. From January 1-7, 2018, the water temperature was 6 degrees C—almost 11 degrees F—below average. The only time the difference from average was greater was in December 1989.
Not only did the water get really cold—it did so very quickly. The following graph shows how water temperatures at Thomas Point Light generally behave. The black line is average water temperature over the course of a year, based on data from 1985 through today. Shaded areas above and below the black line illustrate the probability of the water temperature straying from average. In early December 2017, the water temperature was quite a bit above average—but by early January, had dipped down below average into the very lightest shade of blue. To move so rapidly from well above to well below average is a quick turnaround indeed.
The quick drop in water temperature—nearly 20 degrees F, plummeting from above 50 degrees F in early December to below 32 degrees F in early January—is the focus of the following graph. Water in the Chesapeake Bay is brackish, including a range of salinity from nearly fresh water at the top of the Bay at the mouth of the Susquehanna River to salt water at the mouth of the Bay. That means its freezing point varies based on the salinity level at any given point—but is generally a little less than 32 degrees F. But with such low water temperatures, many areas of the Bay did freeze up—and quickly.
Many effects of the ice were immediate and tangible, as icebreaking boats maintained shipping channels to Bay ports and access to places including Smith and Tangier Islands. At the NOAA Chesapeake Bay Office, buoy technicians responded to ensure the Chesapeake Bay Interpretive Buoy System buoys and sensors stayed safe from ice.
Other effects, including what an ice-covered Bay could mean for living resources, will take longer to emerge.
While many blue crabs stay in the Chesapeake over the winter, they protect themselves from cold water by burying themselves into the muddy bottom, mostly in the southern part of the Bay. Every winter, a portion of the crab population succumbs to what scientists call "overwintering mortality." Basically, if the water gets very cold early in the season and lasts for a long time, crabs may not have enough energy reserves built up to make it through to spring. On average, roughly 10% of the crab population doesn't live to see spring.
The water temperatures this winter dropped quickly. While in early and mid-December water temperatures in much of the Bay were close to 50 degrees, in early January, ice was frequent. Did that precipitous drop in temperature give the crabs enough time to get south and into the mud, or will there be a large overwintering mortality event?
Every winter, Maryland and Virginia conduct the "winter dredge survey," a scientific count of the crab population. How that survey will be affected by remaining ice—and what that survey finds regarding crab numbers—remains to be seen. More information on blue crabs, how they are managed, and how they have fared over past winters is available in the annual Blue Crab Advisory Report that is issued by the Chesapeake Bay Stock Assessment Committee.
Oysters can be affected by cold weather, too. While oysters submerged under ice will generally fare okay, they will die if exposed to subfreezing air temperatures for an extended time. The weather system that brought the extreme low temperatures to the Chesapeake Bay region at the turn of the year also brought strong winds from the northwest for an extended period that essentially blew a large amount of water both out of the Bay and over to the Eastern Shore. These "blowout tides," which persisted for several days, left some oysters high and dry...and frozen. Particularly hard hit were some shellfish aquaculture farms that focus on farming their oysters in what are usually shallower waters—but were exposed by the blowout tides.
And while finfish can swim in attempts to find warmer water, the speed at which the water temperature drops can affect their survivability—if it drops too quickly, they are not able to adapt or move to warmer water. This year, fish kills of speckled trout have been observed around the Bay. 2017 was the first year speckled trout had been seen in significant numbers in the Chesapeake following the frigid winters in 2014 and 2015. It remains to be seen whether their numbers will bounce back after this cold spell. Scientists have also noted that while temperature drops affect speckled trout, they are also affected by when there is a quick thaw and large amounts of cold, dirty fresh water flow into creeks, rivers, and the Bay.
For all of the Bay's living resources, the effects of the winter of 2017-18 are still playing out. A large amount of winter weather is still ahead—just how cold it is and how much snow and ice the Bay sees could have a big impact on the Bay and its living resources.