Platt’s Bank Journal 2006

By Adam Baukus

The R/V Galatea, a 46-ft. research vessel from the New England Aquarium, anchored above Platts Bank
The R/V Galatea, a 46-ft. research vessel from the New England Aquarium, anchored above Platts Bank

In late July the winds finally subsided enough to allow us to return to Platts Bank. This year we venture to the bank with new instruments aboard the research vessel Galatea, and a hope of addressing new, more specific questions.

After such an amazing show of activity last summer we were filled with anticipation, our conversations composed more of questions than answers. Will there be as many hotspots this year? If there is, what conditions are creating them? If there isn’t, what conditions are different from last year? What other types of information can we gather that will help us figure out what is going on?

The big addition this year is the ADCP, which stands for acoustic doppler current profiler. With this tool we use high frequency sound waves to characterize what the currents are doing at different levels in the water column. Combined with the “Fish”, another acoustic instrument that uses a single frequency to describe what is in the water, we have a pretty good view of what is going on below us.

The ADCP (left), to measure water currents, and the Fish (right) to look for organisms in the water, hang over the side of the boat sending sound waves towards the depths
The ADCP (left), to measure water currents, and the Fish (right) to look for organisms in the water, hang over the side of the boat sending sound waves towards the depths

All these underwater sounds attracted the attention of a curious pod of Atlantic white-sided dolphins, which diverted their path to come directly under the vessel to check things out. Apparently satisfied there was no food to be found, they continued on along the ridge, vaulting into the air, leapfrogging to their next destination. Other residents include a mix of seabirds like shearwaters, gulls, petrels, and sparrows.

Dolphins rush inches below my feet as I stand on the bow     A seabird takes a minute's rest from its foraging journey
Dolphins rush inches below my feet as I stand on the bow A seabird takes a minute’s rest from its foraging journey

Spending a combined total of five days anchored on the bank I got a chance to take in the scene and compare it to what we saw last year. There were some similarities. This time of year the sea state was calm–safe enough for a few brave souls in a small 18-foot aluminum fishing boat to try their luck(we were impressed with the guts it took to travel thirty five miles offshore in such a vessel.) Huge herring boats loomed on the horizon, slowly maneuvering into position for the night’s haul of the nets. A few tuna fishing boats patrolled the area, with no visible signs of excitement, and a headboat filled with anxious anglers apparently found a few groundfish, since they returned to the same spot the next day.

The biggest differences this year were found on the glassy surface of the water; it was never broken by the frenzied rush of predator and prey. We saw almost no krill on the bank this summer. A few whales passed in the distance, but nowhere near the numbers from last year. Herring schools showed up on our acoustic readings but the dramatic bursts to the surface to feed on euphausiids were not observed. One disadvantage this year was that we did not have the plane flying aerial transects over the bank. This meant we lost our bird’s eye view of any activity on the bank, making it harder to pinpoint, if it was present.

A catch of cod and pollock meant a glimpse into what was happening on the sea floor and dinner for the crew
A catch of cod and pollock meant a glimpse into what was happening on the sea floor and dinner for the crew

Although action was not visible from the surface there was certainly still life in these waters. Plankton nets came up full of copepods and other tiny creatures. These prey items are pretty much the bottom of the food chain and attract lots of other organisms. We caught several fish that were feeding near the ocean floor. Groundfish like cod and pollock are important predators on the bank, eating smaller fish, crustaceans and other animals crawling on the bottom. We saw a variety of different species including haddock, cod, pollock, and dogfish sharks. Getting a small sample of which fish were present and what they were eating allowed us to get a glimpse of what was happening on the sea floor. Krill found in the fish stomachs proved that this crucial food source was still around, just not in the surface swarms we saw last year. We became part of the food chain ourselves, as the fish provided a hearty dinner for the crew as we worked through the night.

Without having the distraction of breaching humpbacks and swarms of herring and krill, we were able to concentrate on some of the physical aspects that make the water around Platts Bank so unique. Our main tool for gathering this type of information is the CTD, which measures conductivity (a proxy for salinity), temperature and depth. The instrument takes these readings every fraction of a second, and can either be lowered down through the water column or suspended at a certain depth that you want to investigate.

Nick Wolff waits to deploy the CTD, measuring salinity, temperature and depth of the water
Nick Wolff waits to deploy the CTD, measuring salinity, temperature and depth of the water

One important feature we wanted to learn about was internal waves. These waves are created by tidal forces and they travel below the surface. It starts by having a stratified water column; the surface water is warmer, less salty and therefore less dense than the bottom water which is colder and saltier. These different water types meet at the pycnocline, a narrow band of water where the two layers mix. The depth of this mixed layer varies with the season and location; here it was about 10 meters deep. When tidal forces drag this stratified water column across some abrupt ocean floor topography like a bank or canyon, the two water masses respond with a respective rise or fall, creating a ripple that travels down the pycnocline. Like whipping one end of a jump rope, the bump in the line keeps going until it runs out of energy or runs out of rope. Internal waves are capable of traveling long distances, across the entire Gulf of Maine, but along the way their energy is affected by numerous other factors like tide, topography, and wind. Commonly these waves will pass below you unnoticed, but on calm days slight differences in surface currents altered by the waves can create bands of rough and calm water that correspond with the crests and troughs of the waves.

Alternating rough and calm spots on the water surface created by internal waves
Alternating rough and calm spots on the water surface created by internal waves

Spotting these rough and calm spots gave us a unique opportunity to explore how internal waves affect the organisms that live on the bank. Several questions emerged. Are planktonic organisms redistributed by them? Are animals that prefer the surface water moving upwards to avoid the pycnocline? Are the internal waves a mechanism in creating the surface swarms we saw last summer? Using our variety of tools, the CTD, the ADCP, the “Fish” and plankton nets we could sample in and out of these rough and calm spots to see what the water was doing and who was found in it.

With the summer’s field work done, the Galatea returns to port, soon to be off to other parts of the Gulf of Maine to bring other researchers to their destinations. Much of the analysis is still being done, so answers to all these questions are still ahead of us. The data collected this year holds some exciting clues to figuring out the patterns we see on Platts Bank. True, it was a little disappointing not to see all the surface activity; I would have liked to see it raining krill again, with herring and humpback whales practically leaping into the boat. However, compared to the previous year’s activity, the relaxed scenery shows how dynamic this environment really is, and adds intrigue to learning more about what creates these differences.