[Advances in Marine Biology] Advances in Marine Biology Volume 29 Volume 29 || Autotrophic and Heterotrophic Picoplankton in the Baltic Sea
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Autotrophic and Heterotrophic Picoplankton in the Baltic Sea
J. Kuparinen and H. Kuosa
Finnish Institute of Marine Research, PO Box 33, SF-00931 Helsinki, Finland
Preface . . . . . . . . . . . . . . . . I. Introduction . , . . . . . . . . .. . .
A. The Baltic Sea . . . . . . . . . . . . B. Picoplanktonic algae .. .. . . . . ..
11. Methods . . . . . . . . . . . . . . A. Autotrophic picoplankton . . . . . . .. . .
V11I. IX. X.
B. Bacterioplankton . . . . . . . . . . . . Phytoplankton Succession in the Baltic Sea . . . . . . Autotrophic Picoplankton in the Baltic Sea . . . . . .
Bacterioplankton in the Baltic Sea . . . . . . . .
B. Distribution of bacterioplankton . . . . . . . . Factors Controlling Autotrophic Picoplankton . . . . A. Nutrients and temperature . . , . . . . . . . B. Grazing . . . . . . . . . . . . . . Factors Controlling Bacterioplankton . . . . . . . .
B. Predation control of bacterioplankton . . . . . . Bacteria in the Pelagic Food Web . . . . . . . . Acknowledgements . . . . . . . . . . . . References . . . . . . . . . . . . . .
A. Areal and vertical distribution . . . . . . . . B. Seasonal variation . . . . . . . . . . . .
A. Annual and seasonal variation of bacterioplankton production
A. Nutrient- and carbon-limited bacterioplankton growth . .
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The introduction of the concept of a size-structured plankton food web (Williams, 1981; Azam et al . , 1983) greatly stimulated studies of aquatic
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74 J . KUPARINEN AND H . KUOSA
microbial ecology in the 1980s, and there was an outburst of publications on the oceans, brackish waters and lakes. In addition, the developing techniques of epifluorescence microscopy and the use of radioactive tracers have provided many new data.
This review summarizes results obtained from various locations in the Baltic Sea, which has been described as one of the most intensively
1 0' 1 4' 18' 22' 26' 30'
FIG. 1 . Map of the Baltic Sea. Study sites from which the majority of the data presented in this paper originate are marked on the map: Station 1 = 63"31'N, 19"48'E; 2 = 63"19'N, ZO"17'E; 3 = 59".50'N, 23"lO'E; 4 = S9"3S'N, 23"18'E; 5 = .59"26'N, 21"30'E; 6 = 59"02'N, 21"OS'E; 7 = S8"4S'N, 17"3S'E; 8 = S7"19'N, 20W2'E; 9 = SYlS'N, lS"S9'E; 10 = 55"00'N, IJOOS'E; I I = 54"36'N, lO"27'E.
BALTIC SEA PICOPLANKTON 75
studied aquatic environments (Jansson, 1980). While Baltic Sea hydrogra- phy and plankton in general are well known, the category of most intensively studied does not yet apply to Baltic Sea picoplankton, on which few publications are yet available. To make good this deficiency we include in this review a substantial amount of new data.
A. The Baltic Sea
The Baltic Sea is a large brackish water basin with limited connection to the North Sea from the southwestern end (Fig. 1). It comprises several more or less distinct basins or subareas (Fig. 1) with pronounced density stratification prevailing throughout the year (Kullenberg, 1981; Malkki and Tamsalu, 1985). The differences in density between the surface and more saline deep waters restrict exchange between the two layers. The salinity of the surface water decreases from more than 20%~ in the opening to the North Sea to below 1%0 in the extreme ends of the Bothnian Bay and the Gulf of Finland. In the Baltic Proper, surface water salinities are between 6 and 8700.
The primary halocline is at a depth of 60 to 70m in the Baltic Proper
Flc;. 2. Typical distributions of temperature (T), salinity (S) and density (D) in the Bothnian Sea (a), Gotland Deep (b) and the southern Baltic Proper (c).
76 J . KUPARINEN AND H . KUOSA
and 40 to 50 m in the Bornhom Basin (Fig. 2) (Kullenberg, 1981; Malkki and Tamsalu, 1985), below which salinities between 10 and 13%0 are common. This layer receives new water irregularly from inflows through the Danish sounds (Grasshoff and Voipio, 1981). A weak secondary halocline, which separates the frequently anoxic bottom water from the overlying layers, can be detected at a depth of c . 110 and 150m. The extent of this area with insufficient oxygen for macofauna has fluctuated, but is approximately 70,000 km2, mainly in the deep parts of the central Baltic Sea (Andersin and Sandler, 1988). The bottom waters of the Baltic Sea are renewed only after exceptionally strong inflows of North Sea water from the Kattegat. Such inflows occurred in 1913, 1921, 1951 and 1976. Due to the lack of major inflows during the past 14 years, the salinity and density of the deep water have decreased continuously.
In most parts of the Baltic Sea a thermocline develops at depths between 15 and 20m in summer (Fig. 2). The layer of cold water from the previous winter can thus be found between the thermocline and the halocline. These two water masses of about the same salinity mix during the autumn turnover. The western Baltic Sea differs from most of the Baltic Sea in its stratification; due to the water exchange from the North Sea, it is salinity rather than temperature dependent (Fig. 2), and this has implications for the picoplankton in the area (Jochem, 1989).
Another key factor influencing the Baltic Sea picoplankton is the fact that the Baltic is a northern sea, with Arctic characteristics, especially in its northern parts. The winter conditions emphasize differences between the subareas and their biology. The mean number of ice days varies from 190 in the northern end of the Bothnian Bay (Lepparanta et al., 1988) and more than 140 in the easternmost part of the Gulf of Finland to less than 10 days in the central Baltic Proper and in the Kattegat. The mean maximum annual ice thickness varies from more than 70 cm in the northern Bothnian Bay to less than 10cm in the southern Baltic Proper (Climatological Ice Atlas, 1982). These winter conditions contribute to the large seasonal temperature differences, from -0.3 to about 20C.
The numerous large rivers that bring fresh water and inorganic and organic compounds into the Gulf of Bothnia and to the Gulf of Finland impart special features to the biota of these areas. In particular the Bothnian Sea receives large quantities of allochthonous organics via the rivers (Fonselius, 1986). Due to the terrestrial origin, allochthonous material is highly refractory. Levels from 3.0 to 4.7g/m3 of dissolved organic carbon (DOC) have been reported from the Baltic Proper (Ehrhardt, 1969). Only a small fraction of this pool is liable for bacterial utilization (Bolter, 1981).
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B. Picoplanktonic Algae
Pic0 is an epithet applied to pelagic organisms with a size less than 2 pm (Sieburth et al . , 1978). The lower limit of pico-sized organisms, either bacteria, algae or protozoa, is 0.2pm. According to the thorough discussion by Raven (1986), the non-scalable properties of algae constrain their theoretical minimum size to just above 0.2 pm. It seems that in the pelagic environment only viruses and a small fraction of bacteria appear in the femtoplanktonic (0.02 to 0.2 pm) size fraction.
Thus according to the scheme of Sieburth et al. (1978) picoplanktonic organisms are those with cell size under 2 p m . However, this scheme is not totally straightforward when we consider algae. If algae were more or less spherical and if the cells of one species consistently showed very limited variability in size there would be few problems. However, as we know that the form of algae varies considerably, and that the size range of a given species is usually large, the precise definition of picoplanktonic algae in natural phytoplankton communities is difficult.
The distinction of picoplanktonic algae as a separate group has clear ecological grounds. One of the most powerful is that it corresponds to a size fraction of pelagic organisms which is probably not effectively grazed by metazooplankton (rotifers, cladocerans and copepods) and, correspon- dingly, is effectively grazed by protozooplankton (see Section VI). The fraction of organic material produced by picoplankton is thus possibly an indication of the structure of the carbon transfer from primary producers to the higher predators (microbial loop vs. grazing food chain) as discussed by Azam et al. (1983); Ducklow et a f . (1986) and Sherr and Sherr (1988). If we confine ourselves to this ecologically based interest in picoplanktonic algae, the actual upper cell size becomes more a matter of choice than a strict definition. Eventually, it may be possible to choose the upper size limit according to the grazing structure in a given water body, and according to our knowledge of the particle capture ability of zooplankton species.
Small algal cells also have other characteristics in common, such as Slow sedimentation rate and high nutrient uptake capacity. These characteristics are probably not as strictly correlated with cell size as grazing, but their existence furthe