Methods and Evaluation of Data Quality
Zooplankton methods and evaluation of data quality
The zooplankton in our dataset are all captured by vertically towing a conical net, which is usually mounted in a metal ring of diameter about 0.5 m. The net itself has a mesh size of between 200 and 350 μm, small enough to capture most zooplankton but large enough to let water flow through, and most phytoplankton to escape. The contents of the net are then gently washed into a sample jar (as seen above) and preserved by adding a small quantity of formalin.
These samples are then (eventually) examined under a microscrope by a technician (a "zooplankton taxonomist") who has been specially trained to identify the hundreds of species (and their different life stages) that may be present. The technician counts the number of each species in each sample. Once these counts are converted into ocean abundances (i.e. number of individuals per cubic meter of ocean volume) by knowing the volume of water filtered through the net, they can then be converted into biomasses by using a representative "dry weight" for each species and life stage (which comes from a large database of such measurements, made by careful weighing of individuals after drying them in an oven to remove all water).
Replicates are not usually taken, and the variability in biomass from tow to tow can be quite large.
In our figures we use zooplankton data from the most recent decade available in a Fisheries and Oceans Canada database. Samples are obtained from a variety of different programs of which the CitSci program is only a small contributor. However, the map below shows that the frequency of sampling has been much higher since 2015.