Analysis of Samples

The volume of plankton in each sample was recorded in a graduated measuring cylinder after settling for ten minutes. Where the duration of a tow was greater or less than the standard of 20 minutes the total volume of plankton was adjusted by simple proportion. From the number of samples in each month the mean monthly volume was estimated.

For comparison of samples with wide variations in volume the following sub-sampling technique was employed to obtain a constant representative volume (l0cc) of each sample.

Samples were diluted to approximately two litres with fresh water and well stirred. Sub-samples of 50cc were then removed and allowed to settle in a 50cc measuring cylinder. The excess water was siphoned off and successive sub-samples were added to the measuring cylinder to give exactly 10cc of settled plankton. This 10cc sub-sample was diluted to approximately 100cc with water and poured into a very large, shallow Petri dish which was placed on top of a circular piece of black paper divided into ten equal sectors with white ink. Three randomly selected sectors were counted for each of the species considered in the analysis. If the results of these three counts were erratic, further sectors were counted. When the total volume of a sample greatly exceeded 10cc, a further 10cc sub-sample was taken and treated in the above manner.

The number of individuals of each species in a 10cc sub-sample was thus obtained by simple proportion from the number counted in three or more of the sectors, and the result was adjusted to the total volume of the sample by simple proportion.

The counted and estimated animals were then divided into the following five major elements: Pleurobrachia pileus (O. F. Müller); Obelia geniculata (L.); Octophialucium funerarium (Quoy and Gaimard) together with Phialella quadrata (Forbes); Salpidae; and Crustacea. These five elements representing either individual species or groups of species were found to have considerable seasonal variation in numbers. This is reflected in their respective contributions to the total volume of the plankton samples (Text-fig. 8).

The species constituting the bulk of the volume together with the species selected as seasonal indicators were used to compile the annual plankton calendar (Text-fig. 7).