This is the third year for Ecosystem Modeling and Biological Oceanography labs' zooplankton sampling cruise, and this is the first time that I've managed to get on the boat. I was there to practice whale surveys and to test the hydrophone. Since there were no whales (well, one whale) the most interesting part of the trip was watching the team do its thing. After three years, Becky, Nick, Pete, and Cameron have developed into an oceanographic sampling force.
As Nick documented, the cruises is divided into a series of transects that run perpendicular to shore. Along each transect, we've specified several stations where the actual samples are taken. The first step at any station is to lower the CTD instrument to near the bottom. The instrument has three sensors. The "C" stands for conductivity and this sensor measures how easily an electrical current can travel through the water. The more dissolved in the water, the easier the current can flow, so the conductivity sensor measures the salinity of the water. The "T" stands for temperature, and you all know what that measures. Together, salinity and temperature determine the density of the water. "D" stands for depth, and by measuring all three together, we can learn how the temperature and salinity change from the surface to the bottom. This year, the Bio-Oce lab has added a PAR sensor to the CTD. This sensor measures that amount of Photosynthetically-Active Radiation at each depth--basically, how far light reaches into the ocean. In addition to being important for photosynthesis, visual predators like fish need light to see their prey,
After the CTD comes the main event, the ring net tows. The ring net is actually two nets bolted together. These are also lowered to the bottom and then pulled back up. Water and plankton enter the mouth of the net. The water goes out, but the plankton are collected. We (meaning Becky and Cameron) will then identify and count the plankton. In order to determine the density of plankton (number per cubic meter of water), we need to know how much water passes through the net. Each net is equipped with a flow meter. The tow begins with two people--usually Pete and Cameron, calling out the numbers on each flow meter.
The net is then lowered into the water. The crew member running the winch monitors the depth of the net by the amount of wire payed out. When the net comes up, two people guide the nets on to the deck. They read out the numbers on the flow meters and then hose them down to make sure all the plankton are washed down to the cod-end. Then, the cod ends are unclipped and the contents are dumped onto a sieve.
The cod ends are rinsed and dumped a few more times. Then the stuff on each sieve is dumped into a jar containing formalin (a preservative).
The last activity at the station is the Laser Optical Plankton Counter (LOPC) that Nick's been writing about. This uses lasers to detect plankton and estimate their size and shape. The LOPC allows us to get an idea of whether the plankton collected in the nets were packed into a few dense layers or spread out over a large range of depths.