Massachusetts Oceanography and the Impacts of Drought

In the deserts of Arizona, I always talked about oceans the same way that we might talk about unicorns: It’d be nice to see one, but good luck with that.

Going to grad school in New England means I finally have an ocean within walking distance of my office.  So, between SCUBA and oceanography courses, I’m taking every chance to learn more about the Atlantic.

Boston University has partnered with the Stellwagen Bank Marine Sanctuary to use the Research Vessel Auk to collect seawater and plankton data around the Sanctuary (a rectangular area stretching roughly from Cape Cod to Cape Ann).  I know the phrase “collect data” can sound inherently boring, so here’s a whale picture to convince you that it’s actually fascinating:

We had a "whale" of a time watching these humpbacks feeding!
We had a “whale” of a time watching humpbacks feeding!  There are actually three whales in this picture.  Together, they dive, blow a wide ring of bubbles to corral fish, then emerge from the water with their mouths agape, netting as many fish as possible.  I’ve heard that this behavior, called “bubble netting”, wasn’t seen in Stellwagen Bank until a few years ago.  This implies that bubble netting is a behavior these whales picked up from other whales, like a kind of cultural exchange.  Basically, whales are smart.

Humpbacks migrate to bountiful New England waters during summer months to feed on sand lance and other small fish.  I’ve seen humpbacks in Hawai’i before, where the Pacific whale population spends its winters.  In clear tropical waters, food is scarce, but predators are rare, so whales will raise their calves in the tropics before migrating north in the summers to feed.  I’d never seen feeding whales before, and it was quite a spectacle!

Fun fact: the scientific name for humpback whales, Megaptera novaeangliae, means “big-winged New Englander.” Maybe because they jump out of the water so much that they might as well be birds.

This year, whale sightings in Stellwagen Bank have been unusually common, and the waters have been more productive than usual.  At the same time, Massachusetts has been in an “extreme drought” for most of 2016.  So, I was curious:

How has the 2016 drought impacted the waters of Stellwagen Bank?

To answer this question, I went looking for long-term records of ocean temperature and salinity in Stellwagen Bank.  Fortunately, one buoy, NERACOOS Buoy A01 (42°N, 70°W), is right near one of our oceanography study sites.  It’s bristling with sensors that record weather data, as well as water conditions from a range of depths (1-50 meters below the surface).  Lucky for me, it’s been recording since 2001 (with a few gaps, because stuff breaks in the ocean!), which gives me lots of data to work with.

NERACOOS Buoy A01

I also scrounged up rainfall data (GPCC, 42-43°N, 71-72°W) for comparison.  Looking just at the summers (June-August averages), here’s what I found:

figure_1e
Figure 1: Average Stellwagen Bank seawater salinity in summer (June-August) for 2001-2016.  The size and color of the markers correspond to average summer rainfall.  In other words: higher marker = saltier water, and big, dark markers = rainy summers.  In 2016, the ocean is saltier and received less rain than usual.

In Stellwagen Bank, the summer of 2016 had both the least rainfall and the highest salinity (saltiest water) since at least 2001.

That makes sense: rain is freshwater.  When it rains a lot, rivers carry that freshwater to the ocean, which becomes less salty.  This summer, the drought lowered water levels in rivers and reservoirs across Massachusetts, so less freshwater entered the ocean.

Ocean salinity is incredibly important, since it changes the density of seawater: adding salt to water will make it denser, so it sinks more easily when mixed with freshwater:

When freshwater (the yellow layer) meets an ocean (the blue layer), the freshwater forms a layer over the denser ocean beneath.  Oceanographers call this “ocean stratification” (“strata” means “layers”).  The stronger the stratification, the harder it is for the water to mix.  We saw that the 2016 drought made the surface waters saltier than normal (Figure 1), which generally makes them denser and easier to mix.  This led to my next question:

Did the drought weaken the stratification of Stellwagen Bank?

To test this idea, I calculated ocean stratification for every month that the Stellwagen Bank buoy data was available:

Figure 2: Stellwagen Bank ocean stratification, here measured as the difference between densities at 50m and 1m depth.  Light colors correspond to high surface salinity.  In other words: higher markers=stronger stratification (easier mixing between water layers), and light colors=saltier waters.

You can see that stratification varies naturally throughout the year.  In the winter, surface waters are salty and dense, mixing completely with the water beneath (that’s when stratification =~0).  During spring, surface waters become fresher, probably because of snow melt.  This makes the surface water less dense than the water beneath, so it floats above the deep water.  During summer and part of fall, the water stays stratified, with little mixing between the surface water and deeper water.

Since deeper water usually has more nutrients than surface water, mixing is important: it brings deep-water nutrients to the surface, where sun-loving organisms, like phytoplankton, can grow.  In general, weaker stratification (more mixing) could lead to more phytoplankton in the months afterward.

The Stellwagen Bank buoy measures chlorophyll (which is produced by phytoplankton), but the data has a lot of gaps, since I’m guessing the sensor broke for a few years.  Here’s the best I can do, with the amount of chlorophyll on the horizontal axis and stratification on the vertical axis:

Figure 3: The relationship between autumn phytoplankton (chlorophyll concentration) and summer stratification in Stellwagen Bank.  Colors correspond to years (darker=closer to 2016).  The line of best fit shows that phytoplankton increases as stratification decreases, though this trend is only significant at p<0.10 (r = -0.65, p = 0.08).  In other words: the line shows that more plankton generally grow after summers with low stratification (more water mixing), but we need more chlorophyll data to be sure.

Though there are a lot of caveats (for example: not enough data, only one buoy, not a long time span), this figure implies that there’s more phytoplankton after summers with lots of water mixing.  Taken together, these data show that, in Stellwagen Bank:

  1. Summer 2016 had the least rain since at least 2001
  2. Summer 2016 also had the saltiest surface water and the weakest ocean stratification, which likely made it easier to bring nutrients from deep waters to the surface
  3. That weak stratification may have increased phytoplankton growth in Stellwagen Bank.

In short, this means that the 2016 drought is probably one reason we’ve been seeing so many whales lately: phytoplankton forms the base of a food chain that ripples all the way up to sand lance and the whales that eat them.  It’s evidence that to understand the behavior of “charismatic megafauna” (like whales and dolphins), we first need to understand the climate and oceanography of the place they live.

 

P.S.: “Hey Emma, what’s with all these graphs?”

After getting sick of MATLAB license keys that I needed my advisor’s password to install, I said, “nope! I’m out!” and switched to Python as my primary programming language.  Among other things, I discovered plot.ly, an online graph generator that integrates really well with Python.  The two interactive graphs above were made in Python + plot.ly.  For me, it’s sparked a new interest in data visualization!

One thought on “Massachusetts Oceanography and the Impacts of Drought

  1. I am really impressed with the work you did.
    Maybe the ocean stratification is also formed by ocean dynamics (internal tides).
    If we could use 3-D mathematical modelling (what I am doing now) to model how the amount of fresh water and tide influence ocean stratification, coupled with plankton production equations…
    It would be a very great research and paper. Hope can cooperate with you in the further~

    PS: for the Matlab license, the IT center can solve the problem. It is also cool to use Python which can also be used in GIS and remote sensing…

    Cheers,
    Sean

Leave a Reply

Your email address will not be published. Required fields are marked *