There are several kinds of nets and other equipment that the SEAPLEX scientists will be using. In our relatively few post to this point, several have already been mentioned, so I thought that it would be nice to have a little clarification. These pictures are not from the SEAPLEX cruise, but from CalCOFI cruises to the California Current. Here is a quick description of the major pieces of equipment that will be used during the cruise.
The CTD rosette is one of the most common pieces of oceanographic equipment. It allows us to see the vertical structure of the water and to collect water samples from various depths. It is composed of an array of sensors surrounded by a number of bottles. The sensors include pressure (depth), temperature and salinity (conductivity), which are used to determine density. Depending on the particular machine, there are also sensors to measure chlorophyll fluorescence (a proxy for phytoplankton abundance), nitrate, oxygen, and light levels. The bottles are all open when the instrument is deployed, and they are closed at various depths on the way up. The water collected in these bottles can be used to look at nutrients, bacteria, phytoplankton, and protists from different depths.
The bongo nets are probably the most common way to sample zooplankton. They are paired mesh nets attached to a metal frame, and some think that they resemble bongo drums. There are collection receptacles, called cod ends, at the end of the net where the zooplankton are concentrated. All of the water and particles smaller than the holes in the mesh pass through the net, and everything larger than the mesh is funneled into the cod end. The net is towed obliquely (in a V shape) so that all depths (except the deepest depth) are sampled twice. Most bongo tows go down to 200m depth. There are flow meters in the mouths of the nets so that the volume of water filtered can be calculated accurately.
The manta net is used to sample zooplankton that are at or near the sea surface. It is a single mesh net with two large wings that extend out on either side of the mouth of the net. The wings help the net to stay at the surface. Surface zooplankton are funnelled into a cod end in the same manner as the bongo nets.
The Oozeki trawl is another type of net, and works the same way as the bongo and manta nets. It is used to sample small fishes. The Oozeki is a much larger net (about 6 feet tall) than either the bongo or manta nets. It also has a larger mesh size than either of the other types of nets that will be used on the SEAPLEX cruise. The larger mesh means that most zooplankton pass through the mesh, leaving fish and larger zooplankton such as jellyfish and squid. The Oozeki is towed between about 400m and 2000m deep.