Vertical Migration of Zooplankton in the Gulf of Finland

Abstract

Vertical migrations of zooplankton in the Gulf of Finland were measured over a 12-hr period (1800-0600 hr). Most species migrated upward as the surface light intensity decreased and downward with an increase in light intensity. Salinity and temperature gradients did not affect this pattern. There was no midnight sinking of the zooplankton. INTRODUCTION The diel vertical migration of zooplankton has been observed in many locales for many different species (Russell, 1927; Clarke, 1934a,b; Moore, 1949; Cushing, 1951; Raymont, 1963; McNaught and Hasler, 1964). Few of these studies, though, have been conducted at high latitudes. Bogorov (1946) found in the summer in the Barents Sea that the high light intensities at night at this high latitude seemed to prevent vertical migration by the zooplankton. He also found in the White Sea, located at a lower latitude than the Barents Sea, that some of the zooplankton migrated vertically, while others did not migrate. In the summer at 60?N in the Gulf of Finland, vertical migration of zooplankton was measured under a light regime in which the sun set just below the horizon, creating light conditions intermediate between the total darkness seen at midnight in lower latitudes and the summer midnight sun conditions of higher altitudes such as the Barents Sea. All of the zooplankton sampled in the water column were counted, and their vertical migration patterns were determined relative to light intensity, temperature and salinity. These results were then compared to those of Bogorov (1946) from the Barents Sea and White Sea, and to the most common vertical migration pattern found in lower latitudes. In the most common pattern exhibited in lower latitudes, zooplankton move to the surface as the sun sets, then sink during the darkest part of the night (so-called midnight sinking) and finally return to the surface before the sun rises, only to sink once again as the sun rises (Russell, 1927; Cushing, 1951; Bainbridge, 1961; Raymont, 1963). MATERIALS AND METHODS Vertical sampling was conducted at Station 12 (depth 34 m) near Tviirminne Zoological Station in the Gulf of Finland (Halme, 1944). Samples were taken every hour from 1800 on 23 June 1970 to 0600 on 24 June at 0, 5, 10, 15, 20, 25 and 30 m, with samples first collected from the surface followed by collections at progressively greater depths. Collection of the seven samples took between 15 and 20 min. The sampling device, designed by K. J. Purasjoki, was a 20-liter stainless steel box which could be closed from the surface by a messenger weight. Mounted at the base of the sampler was a 110 trm mesh net which allowed filtration of the sample. After filtration, the plankton concentrate was preserved in a dilute formaldehyde solution. In the laboratory the plankton were further concentrated by filtration through a piece of 110 ,um mesh net. All of the zooplankton were then placed on several 1-mm ruled glass slides and identified and counted under a phase contrast microscope at lOox. A Ruttner bottle was used to obtain water for salinity and temperature determinations. Temperatures were taken every 2 hr, while water for salinity tests was collected only at 1800 and 0600 hr. Salinities were measured in the laboratory using a Metrohm Herisau Dosimat. Approximate relative light intensities at the water