Documentation prepared by Yiwu Zhu

An OPC (Optical Plankton Counter) detects and sizes objects using a light blocking scheme. An OPC returns a digital size value when a target passes through its light beam. The digital size is further converted into an equivalent spherical diameter (ESD) by OPS-DAS software.

In the Ross Sea Site-Survey, Process 1, Process 2 and Process 3 cruises, an OPC was mounted on a MOCNESS. The OPC has a sampling beam area of 0.005 m2. If the MOCNESS went down to a depth of 400 m, the OPC would sample only 2 m3 of water volume. To be able to have a statistically meaningful measurement of size structured zooplankton, OPC data was integrated into two depth layers: 0 to 150 m and everything below 150 m.

The valid maximum and minimum ESD values of our OPC are 14 mm and 0.29 mm, respectively. An ESD value can be converted to a volume or a carbon value. Thus, we have the equivalent maximum and minimum value for volume and carbon.

The OPC valid data range was divided into 40 classes, column 1 (opc_class), based on the carbon, the ESD and the volume separately. The intervals of all the classes are equal on a log10 basis. For example, for ESD based classes, the increment of a class is dESD = (log10(14) - log10(0.29)) / 40. ESD-class k covers the range from (log10(0.29) + (k-1)*dESD) to (log10(0.29) + (k)*dESD). The middle value of each class is taken to represent the class and is called the class value. Column 2 (opc_wt_mid), column 5 (opc_size_mid) and column 8 (opc_vol_mid) are the class values for carbon based classes, ESD based classes and volume based classes separately. Taking an example, the class value of ESD based class 2 is calculated as (log10(0.29) + (1+0.5)*dESD).

For carbon based classes, there are two sets of data. Column 3 (opc_wt_biomass) is the biomass spectra vs. carbon classes. It is, for a carbon class, the total carbon that the OPC counted in this class divided by the carbon range of this class, and divided by the water volume filtered by the OPC in the whole integrated depth range. The unit of column 3 is ugC/ugC*m3, and so is 1/m3. Column 4 (opc_wt_abund) is the abundance spectra vs. carbon classes. It's value for a carbon class is the total number of plankton that the OPC counted in this class divided by the carbon range of this class, and divided by the water volume filtered by the OPC in the whole integrated depth range. The unit of column 4 is 1/ugC*m3.

For ESD size based classes, column 6 (opc_size_abund) is abundance spectra vs. ESD class, and column 7 (opc_size_biovol) is bio-volume spectra vs. ESD class. Column 6 means, for a ESD class, the total number of plankton that the OPC counted in this class divided by the ESD range of this class, and divided by the water volume filtered by the OPC in the whole integrated depth range. The unit of column 6 is 1/mm*m3. Column 7 is the same as column 6 except that, instead of the total number of plankton, it is the total volume of the plankton.

And the last category is the volume based classes. Column 9 (opc_vol_biovol) vs. column 8 (opc_size_mid), is bio-volume spectrum vs. volume class. The calculation of column 9 is, for a volume class, the total volume of the plankton that the OPC counted in this class divided by the volume range of this class, divided by the water volume filtered by the OPC in the whole integrated depth range.

In order to make x-y figures, the following combinations can be plotted: col.2-col.3; col.2-col.4; col.5-col.6; col.5-col.7; or, col.8-col.9.