Page 199 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 187

bongo net, and a trongo net, the main difference being increasing numbers of
nets (and hence concurrent samples) from 1 to 2 to 3, respectively. A vessel tows
a plankton net for several minutes (or a shorter-time period when concentrations
of plankton are high), after which the sample is concentrated and preserved
in formalin for subsequent microscopy work in the laboratory to determine
plankton densities, based on the volume of sea water that has passed through the
net opening. Clearly, an important consideration is the mesh size, which will be
determined by the size of the smallest organisms that are to be sampled. Typical
(cruise-averaged) results for a zooplankton survey are given in Table 7.5.
Finally, another simple point sampling instrument is the secchi disk. A
secchi disk is simply a 30-cm diameter white disk that is weighted and lowered
into the water. The depth at which the secchi disk is no longer visible relates
(inversely) to the turbidity of the water, and so is a simple measure of near-
surface sediment concentration. However, the secchi disk is subject to operator
error, for example, the effects of sunlight and waves on the water surface make
it difficult to accurately and consistently record a secchi depth.

TABLE 7.5 Mean Near-Surface Zooplankton Concentrations
at the Northeast Approach to the Menai Strait (UK),
November 8–13, 2012
3
Zooplankton Density (Individuals per m )
Diatoms 3082.5
Copepods 369.4
Chaetognaths 44.4
Tunicates 9.2
Echinodrm larvae 7.6
Polycheate larvae 3.8
Caprellids 3.5
Crab larvae 3.2
Barnacle larvae 2.4
Ctenophores 2.4
Fish eggs 1.0
Bivalve larvae 0.5

Notes: These values represent the averages over five stations that were visited
each day for 6 days.