Improving weather, different sampling methods

As the weather improved we made steady progress working our way east towards the northeast peak of Georges Bank. The weather was so good that on Friday we made a second deployment of our In Situ Ichthyoplankton Imaging System (ISIIS) device on the southern flank of Georges Bank, running a transect from the bank itself off into the slope water for a comparison of the hydrographic conditions and zooplankton in both areas. This was undertaken successfully, along with a series of comparison bongo net plankton tows, to supply ground-truth samples for comparison with the many images taken by the ISIIS video camera.

On Friday evening one of the crew members was taken ill, and the Delaware II steamed back towards Cape Cod to rendezvous with a Coast Guard vessel. The person was transferred to that vessel on Saturday morning and taken ashore for medical attention.   By Sunday we had returned to our work area on the northeast peak of Georges Bank, continuing our sampling operations there and in the Northeast Channel, a particularly interesting area that marks an entry point where the Labrador Current brings cooler, less saline water from the northern ice shelf into the Gulf of Maine. With continued excellent weather, and less swell from Hurricane Danielle than had been anticipated, a repeat deployment of ISIIS was undertaken on Sunday evening, on the exact same transect made two days earlier, to replicate that sampling effort under nocturnal conditions to determine if we could observe any faunal changes.

It is interesting to compare plankton sampling by these two different methods. The bongo sampler, so named because of its side by side aluminum plankton net frames, is lightweight (about 50 lbs.), extremely rugged , and coupled together with a Conductivity, Temperature and Depth (CTD) device  provides two plankton samples and simultaneous salinity, temperature and depth information. It can also be deployed with only three people on deck; one to run the winch, one to run the A-frame, and one to handle the gear, which consists of the aluminum bongo frame with nets and flow-meters, the CTD and a 45 kilogram (about 100 pounds)  lead weight to act as a depressor. This array can be fished in seas of up to ten feet with winds of up to 30 knots before it becomes too dangerous to deploy safely.

Scott Sperber revoers the bongo net aboard ship.

Science teacher Scott Sperber retrieves the bongo sampler. Visible are the attached lead weight, which he is pulling on, and the white CTD unit above the bongo frame. (Photo by Jerry Prezioso, NOAA)

The ISIIS requires much more care and manpower for safe deployment. The 900-pound device is lowered and retrieved from the stern of the vessel through the trawl-way. It requires a larger number of people to deploy safely, with one person running the winch, one person running the hydraulic gantry, and four people manning tag lines to prevent the instrument from sliding sideways against the trawl-way and damaging its diving wings. On the Delaware II it can only be safely deployed in winds of 10 knots or less, and seas of five feet or less.  Once in the water,  it too will measure temperature and salinity at different depths, and with its camera provide plankton identification and abundance information by depth, something that the bongo sampler, with its double oblique tow through the entire water column does not.  Additional sensors on ISIIS also provide dissolved oxygen, light and chlorophyll levels. ISIIS can be “flown” up and down through the water column by a “pilot” on board the vessel and towed for a distance of approximately 30 miles at five knots before overwhelming its computers with data from its camera which is snapping images at 17 frames per second. This ability to be towed at speed for a considerable distance makes it a particularly useful tool for mapping changes across large frontal areas, such as between shelf and slope water.

Recovery of the ISIIS on the ship's stern.

It takes quite a few hands to retrieve the ISIIS system and get it aboard ship. (Photo by Delaware II Commanding Officer Steve Wagner, NOAA).

Perhaps oneday, these diverse systems will be integrated into something that combines the best of both: the simplicity and durability of a net sampler with the depth discrete imagery and suite of hydrographic data that the ISIIS can provide over a large area. Until that time comes, we’ll be often manning cruises much like this one, using mostly nets but relying increasingly on electronic instrumentation to gather data about long term trends in our changing marine ecosystem.

Jerry Prezioso
Chief Scientist

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