A volunteer collects seawater from the deep on the Seward Line, Blog 2 of 4
October 10, 2017
This is the second of a four-blog journal. See Blog 1.
What a difference a day makes. At dawn we had light west wind, a clear sky, seas down to four feet, and to the north a ragged row of snowy peaks is just visible above the horizon. This morning, after one “cast,” the term for lowering and retrieving the CTD, I helped Ana Aguilar Islas test her new system for collecting surface water samples while the vessel is underway.
She has fashioned the whole contraption out of a pneumatic water pump and a pair of plastic bells lashed to the railing up on the boat deck, connected by a long hose that runs down to a 4-foot-long brass and stainless steel “fish” that looks like a torpedo. The fish is lowered by a cable from a boom swung outboard from the boat deck, with the hose trailing over the side to the fish. It’s a lot more complicated than it sounds, but basically the boat tows the fish off to the side, the fish has a water pickup, the pump on deck sucks the water up the hose about 12 feet to the pump which forces the water into either of the two plastic bells, one of which has a fine pore filter.
Ana has three different fish devices and over the course of the voyage we will be testing each, as well as making adjustments to the hoses, the pump, the filters and valves. Numerous things about each of the setups don’t suit her—we keep making modifications and eventually she’s satisfied. We’re not actually even collecting water with her new system on this voyage, we’re just proving out the collection setup.
Ana is collecting water for nutrients, trace minerals and rare earths. Though the minerals are dissolved in seawater, she uses fine pore filters to remove the phytoplankton (microscopic plants that consume dissolved minerals during photosynthesis) which would alter the nutrient content of the water.
The devices in the lab back in Fairbanks used to detect trace minerals are so sensitive that we have to adhere to strict protocols for washing bottles, syringes for filling bottles, and anything else the water may contact to prevent contamination. We wear nitrile medical examination gloves, discarded after each cast, to ensure that not a single skin cell off our hands gets into the sample bottles, and we cease transferring water if the wind shifts so that engine exhaust from the vessel’s stacks, 30 feet above us and separated by a covered deck, threatens to contaminate our samples.
Ana’s particular focus is iron, which is essential for phytoplankton growth. Phytoplankton, of course, are essential for zooplankton, which in turn feed forage fish which then feed commercially important fish. Waters over the inner shelf (closer to shore), from glaciers and rivers, are relatively rich in iron. But there is an iron gradient and waters on the outer shelf and slope are increasingly iron-poor, resulting in progressively less primary productivity from photosynthesis.
There is a temperature gradient, too, with warmer water closer to the surface and closer to shore, despite the cooling effect of glacial runoff.
By the end of the day we’ve nearly completed the Seward Line stations, having zigzagged back and forth among them since each has to be sampled both day and night and we don’t have time to wait on each station between the day and night activities. If all goes well tonight, we should be able to head for Prince William Sound tomorrow.
— By Terry Johnson
See Blog 3
Alaska Sea Grant is a statewide marine research, education, and outreach program, and is a partnership between the University of Alaska Fairbanks and the National Oceanic and Atmospheric Administration. Alaska Sea Grant Marine Advisory Program agents provide assistance that helps Alaskans wisely use, conserve, and enjoy marine and coastal resources.