Chris Guo, Jesse Ross, and Michael Stocker fishing for zooplankton
As we were already monitoring a seismic survey in the area, what better opportunity than to sample the vitality of zooplankton directly in front of, and directly behind the survey vessel? But before diving too deeply into describing the methodologies, it is important to know a bit about zooplankton, their habitat, and especially about the Cook Inlet environment.
Plankton are the living organisms suspended in the water column that are unable to propel themselves against currents, and thus are subject to turbulence and tidal flows. Phytoplankton are the photosynthesizing eukaryotes (multi-celled organisms) which include diatoms, dinoflagellates, and coccolithophores – which provide some 50-80% of the oxygen we breathe!
Zooplankton are the animals that dwell in the same habitat. And while many of the plankton can move voluntarily within their small surrounding areas (including some of the phytoplankton!), they are collectively subject to the water motion in their larger surroundings. And in most places we get to visit in the ocean, they comprise a significant feature of the water. Most of green or brown/red tinge of ocean water (and the haze that limits underwater visibility) is plankton.
Plankton is the foundation of the aquatic food chain – providing food from the smallest zooplankton up to the largest whales. And being that it is pretty ubiquitous in coastal waters out to the edge of the continental shelf, it’s pretty easy to net. “Bongo nets” are used in plankton research because they provide a known architecture, and a routine collecting method.
The Cook Inlet is an exciting body of water. Sometimes a little too exciting – with 23 ft. tidal swings, and howling northerly winds it can get pretty rough. This is exacerbated by the tidal mixing. On the ebb tides, the ocean water flowing out into the Gulf of Alaska is being “chased” by freshwater from the glacial melt flowing down through the many rivers feeding the Inlet. But on the flood tide, this seaward flowing fresh water is being met by huge volumes of incoming salt water.
Recalling from high school physics that salt water is more dense than fresh water, the glacial meltwater wants to float on top the seawater, but as the tides come in, the fresh water tends to be underneath the flood tide seawater. This turns the sea surface into a washing machine.
In the end I believe that this very mixing was what yielded ambiguous data that did not substantiate our hypothesis that seismic surveys damaged marine zooplankton.
We went out to sample during the flood tide due to the survey vessel having to run against the tide – to keep their airguns and streamers lined up behind the ship. While we were able to fish up zooplankton from directly in front of the boat, the flood tide water was turbulent, so zooplankton distribution was erratic. And then when we took the “post exposure” samples, we needed to be behind the streamers – and more than 3km behind the airgun arrays, also in messy water. This is detailed a bit more in our report.
I spoke with Rob McCauley about this. He was pretty circumspect, saying “if it was easy, it would have been done already.”
