Dataset: clarke-bumpus_copes
Deployment: AT113

Samples of plankton and hydrographic data for the present study were obtained from the research vessel, "Atlantis", during eleven cruises to Georges Bank from September 1939 to June 1941 (Table I). On each cruise a net-work of 21 to 52 stations was occupied over the Bank. In all cruises (except that of January, 1940) the stations were ordinarily placed at 15-mile intervals on five or six parallel sections, about 25 miles apart, running SE and NW across the Bank and into the immediately adjacent waters. The location of the stations is indicated in the charts showing the distribution of Sagitta (Figs. 5 and 6). The stations covered the region from South Channel on the southwest to the eastern tip of Georges Bank and from the deep basin of the Gulf of Maine on the northwest to the edge of the continental shelf on the southeast.

Standard hydrographic observations for salinity and temperature were made at every station and Secchi disc measurements of transparency were carried out during daylight stations. Studies of certain chemical characteristics of the water and of the phytoplankton population were undertaken by collaborating investigators (Sears, 1941; and Riley, 1941 and 1942).

The zooplankton was collected at each station by means of two or more hauls with Plankton Samplers (Clarke and Bumpus, 1940) and one haul with a stramin net. Sagittae (Chaetognath) were taken in adequate numbers in both types of equipment and the two sets of hauls served as a check on one another.

The opening of the Plankton Sampler, which is 12.7 cm. in diameter, is provided with a shutter, and each instrument contains a meter which records the amount of water filtered by the net. In the present case, the instruments were equipped with No. 2 silk nets (22 strands/cm.) and "oblique" hauls 1 were made at a speed of about 2 knots for periods of 25 to 40 minutes. Ordinarily between 10 and 20 cubic meters of water were filtered during each tow, but the action of the tide or of clogging was such that values as low as 5 m^3 and higher than 30 m^3 were recorded. This variation makes clear the need for measuring the amount of water which actually passes through the net. The Samplers were arranged vertically so as to divide the total depth of water into two or three strata and, when feasible, were attached to the same cable. The uppermost instrument sampled the "Shallow" Stratum, extending from a depth of 25 m. to the surface. The lower limit of this stratum corresponded roughly with the position of the thermocline in those areas where it existed. At stations where the water was less than 75 m. deep, the "Second-depth" Stratum extended from the bottom to 25 m. In water deeper than 75 m., however, the remaining distance to the bottom (or to a maximum depth of 200 m.) was divided into two equal parts and these comprised the "Second-depth" and the "Deep" Strata respectively. The vertical distribution of the sagittae could therefore be studied on the basis of these strata:

The stramin net (Diameter: 1.5 m., Mesh: 6 strands/cm.) was equipped with rollers at the lower edge of its frame in order that it could be safely lowered until it touched the bottom. One "oblique" haul was made from the bottom (or from a depth of 200 m.) to the surface at each station. When proper allowance was made for the difference in the sizes of the apertures of the stramin net and the Plankton Samplers, a good agreement was found between the numbers of sagittae taken by the former and the sum of the catches of the latter at each station.

References:
CLARKE, G. L., AND D. F. BUMPUS, 1940. The Plankton Sampler-an instrument for quantitative plankton investigations. Linnological Society of America, Special Pub., (No. 5): 1-8.
RILEY, GORDON A., 1941. Plankton studies. IV. Georges Bank. Bull. BingharH Oceanographic Coll., VII: Art. 4, 1-73.
RILEY, GORDON A., 1942. The relationship of vertical turbulence and spring diatom flowerings. Sears Found. Jour. Mar. Res., 5: 67-87.
SEARS, MARY, 1941. Notes on the phytoplankton on Georges Bank in 1940. Sears Found. Jour. Mar. Res., 4: 247-257.

(from: George L. Clarke, E. Lowe Pierce, Dean F. Bumpus. 1943. "The Distribution and Reproduction of Sagitta elegans on Georges Bank in Relation to the Hydrographical Conditions". Biological Bulletin, Vol. 85, No. 3, pp. 201-226
Published by: Marine Biological Laboratory
Stable URL: http://www.jstor.org/stable/1538223)