Journal of Plankton Research Volume 4 Number 4 1982 Spatio-temporal dynamics of chlorophyll in the open Baltic Sea Mati Kahru, Ain Aitsam and Juri Elken Department of the Baltic Sea, Institute of Thermophysics and Electrophysics, Paldiski St. 1, Tallinn 200031, USSR (Received February 1981; accepted April 1982) Abstract. Five charts of the chlorophyll and hydrographic fields by vertical profiles of in situ fluorescence and CTD were made on a stationary grid of 20 x 25 nautical miles with a 5-mile spacing in the open Baltic Sea. Both chlorophyll levels and variability were maximal close to the spring bloom. High chlorophyll levels in summer are sustained by recurrent nutrient injections from the deep saline layer. Two of the surveys showed close coupling between the coarse-scale ( - 10 km) chlorophyll distri- bution and the hydrographic structure determining the intensity of nutrient transfer. Vigorous advec- tion, stirring and current shear, associated with a strong mesoscale eddy, probably dominated the chlorophyll pattern on three surveys. The upward velocities in the cyclonic eddy resulted in accumula- tion of phytoplankton in the aphotic zone. Intensive heat input from the surface caused a sudden sink- ing of the phytoplankton and the formation of a pronounced sub-surface chlorophyll maximum. Introduction Several investigations (Steele, 1978; Steeleand Henderson, 1979; Pingree et al., 1978, among others) have exemplified that the study of phytoplankton spatial heterogeneity can provide valuable information on the control and functioning of the whole pelagic ecosystem. Due to the plankton spatial variability and its inter- action with a broad spectrum of motions in the sea, temporal sequences of point data are inherently difficult to interpret. Whilst it is generally acknowledged (e.g., Cushing, 1975) that factors controlling the temperate pelagic ecosystem are temporally different, there are only a few studies concerning the temporal succes- sion of plankton spatial patterns (Horwood, 1978; Steele and Henderson, 1979). The in vivo fluorescence of chlorophyll a, in spite of some methodological prob- lems, has widely been used as an index of phytoplankton biomass. Even using the method of in vivo fluorescence, it is inherently difficult to obtain sufficient areal coverage while retaining adequate resolution of the smaller horizontal and verti- cal scales. Hence, most investigations are restricted to mapping only the surface distribution of chlorophyll a along a transect. However, surface distributions need not be representative of the whole chlorophyll layer, since, as a common feature, a significant portion of the phytoplankton standing crop and its highest variability may be associated with the thermocline (Holligan and Harbour, 1977). As part of an interdisciplinary study, spatial surveys of the chlorophyll and hydrographic fields were made on a stationary area in the Central Baltic Sea in 1979 and 1980. Vertical profiles on a rectangular grid with a spacing of 5 nautical miles sacrificed the horizontal fine structure but provided a three-dimensional picture of the coarse-scale (~ 10 km) distribution. ©IRL Press Ltd., Oxford, England. 0142-7873/82/0404-0779S2.00/0 779