ON TWO-PEAKED WAVE SPECTRA
BLUE-GREEN ALGAE AND NITROGEN FIXATION
The nitrogen-fixing blooms of blue-green algae in the Baltic Sea consist mainly of three heterocystic species (Rinne et al. 1978): Aphanizomenon flos-aquae and Nodularia spumigena and to a lesser degree Anabaena lemmermannii (sometimes A. baltica, cf. Niemi
& Hällfors 1974). In 1978 Aphanizomenon dominated in the Bothnian Sea. Both Aphanizomenon and Nodularia occurred in the Bothnian Bay but only in small abundances. In 1979 most blue-green algae occurred in the central part of the Bothnian Sea (st. F-26). Aphanizomenon and Nodularia were found in equal abundances. Anabaena was very scarce in the Gulf of Bothnia.
In the central and northern part of the Bothnian Sea and in the Bothnian Bay the vertical maximum of Aphanizomenon occurred at 10-20 m. The highest vertical abundances of Nodularia, however, were found near the surface. The results of the acetylene reduction
TABLE 2. 1. Name of station. 2. Acetylene reduction yM C2 H2 • m-3 : (2h)-'. 3. Aphanizomenon biomass mg . m-3. 4. Number of heterocysts : 106 : m-3. 5. Nodularia biomass mg : m-3. 6. Number of heterocysts • 106 . m-3. 7. Anabaena biomass mg : m-3. 8. Number of heterocysts • 106 : m-3. 9.
Total biomass mg : m-3. 10. Total number of heterocysts : 106 : m-3. 11. Heterocystic activity pMC2H2 (het)-' (2h)-t.
measurements and the total number of heterocysts showed the same geographical distribu-tion as did the total biomass of the algae mendistribu-tioned (Table 2). Nitrogen fixadistribu-tion values between 0.5 (st. US-5) and 3.4 (st. F-26) µMC2H2m-3 (2h)–i (dimension expressed according to the incubation time used) were measured. The same level was found in the Åland Sea.
The studies carried out in this area in 1975 (Rinne et al. 1978) showed the same level of nitrogen fixation (10 AMC2H2m-3 (2h)–l).
In the Quark and in the Bothnian Bay no marked nitrogen fixation was observed or it was negligible (0-0.2 tMC2H2 • m-3 (2h)–l). The nitrogen fixation in the surface layer of the archipelago waters outside the town of Uusikaupunki, which are loaded with phosphorus-rich industrial discharges, was measured at 6 stations in 1975 (Table 3). At that time the three heterocystic species mentioned occurred in the area. Their biomass varied between 150 and 33 mg m-3. The highest acetylene reduction (10µMC2H2 m-3 (2h)–') was obtained at the innermost station, where the biomass was highest and where the influence of the phosphorus-rich discharge was strongest. The lowest acetylene reduction and biomass values were obtained at the outermost stations. The heterocystic activity in the Uusikaupunki archipelago varied from ca. 0.3 to 0.6 pM C2H2 (2h heterocyst) (Table 3). In this area the low N:P ratio near the industrial discharges seems to promote nitrogen-fixing blooms (see Häkkilä 1980).
DISCUSSION
In coastal waters, nitrogen fixation by planktonic, heterocystic blue-green algae has been studied by Rinne (1976), Vuorio (1977) and Vuorio et al. (1978) in eutrophicated waters off Helsinki in the Gulf of Finland. Brattberg (1975, 1977) has conducted similar studies in polluted waters off Stockholm (northern Baltic Proper). Lindahl et al. (1978, 1980) have studied nitrogen fixation in the Askö area, in the outer Stockholm archipelago and in the Oregrund archipelago (Åland Sea).
In the Baltic open sea areas pelagic nitrogen fixation has been studied by Hubel & Hubel (1976a, b) in the southern Baltic Sea, and in the northern Baltic Proper, Gulf of Finland, Åland Sea and Gulf of Bothnia by the MERININNI group (Rinne et al. 1976, 1978, 1979, 1980).
Blooms of heterocystic blue-green algae are natural phenomena in the open Baltic Sea.
Observations of blooms were made back in the last century in the Baltic Sea (Pouchet &
deGuerne 1885). The dominant algae, Aphanizomenon and Nodularia, are favoured by phosphorus but they are more or less independent of the amount of available nitrogen (e.g.
Melin & Lindahl 1973, Horstmann 1975, Rinne & Tarkiainen 1975). Blooms of heterocystic blue-green algae fixing molecular nitrogen are also typical of natural lake waters with a phosphorus excess (Whitton 1973, Fogg 1975: 127). Thus upwelling phosphorus-rich water promotes the development of blooms of such algae (Jansson 1978).
The Baltic surface water has a relatively low ratio of inorganic nitrogen to phosphorus (W ern & Pekkari 1973, Fonselius 1976, Niemi 1979, Perttilä et al. 1980). The ratio in upwelling Baltic deep water is even lower (Tarkiainen et al. 1974, Fonselius 1976, Nehring 1980). Phytoplankton generally use nitrogen and phosphorus in the ratio 7-8:1 (on weight basis, cf. Redfield et al. 1963, Sen Gupta & Koroleff 1973). In winter there is a surplus of phosphorus in the surface layer of the Baltic Proper and the Gulf of Finland (Nehring
124
oQ 4 5 krn
W I~
c c:?c
UUSIKAUPUNKIo0 o ~d
14 11
9 8 3 2
4.8 3.4 6.6 3.5 8.0 10.0
8.4 6.1 18.0 6.3 16.0 30.0
0.57 0.56 0.37 0.56 0.50 0.33
45 38 130 33 83 150
61 68 81 80 81 75
36 21 12 10 8 14
3 11 7 10 11 11
28.6 15.6 7.1 8.2 5.4
Station
N-fixation iMC2H2m-224h—t Heterocysts
106/m3
Heterocystic activity pMC2H2/2h/het.
Heterocystic blue-green algae Biomass
mg m-3
Aphanizomenon % Nodularia %o
Anabaema % N:P ratio (total W/ W)
TABLE 3. Sampling stations and surface values of nitrogen fixation, number of heterocysts, heterocystic activity, biomass of heterocystic blue-green algae, percental share of Aphanizomenon, Nodularia and Anabaena of total heterocystic blue-green algae and N:P ratio on August 12, 1975 in Uusikaupunki archipelago.
1980). The low inorganic N:P ratio of nutrients available for phytoplankton production probably remains at a low level (during the summer stratification).
The nitrogen and phosphorus discharge from the land probably has some influence on the nutrient level in the open sea areas. The estimated discharge of total nitrogen and phosphorus to the Gulf of Finland and the Gulf of Bothnia has a N:P ratio of 10:1 and 15:1, respectively (Finnish-Soviet Working Group on the Protection of the Gulf of Finland 1979, National Environment Protection Board, Sweden and National Board of Waters, Finland 1979). Input from the atmosphere is not included in the estimates of the Gulf of Finland, but if it were, it would further increase the high N:P ratio (cf. Nehring & Wilde 1979). These inputs must increase the N:P ratio, i.e. they will not promote blooms causing nitrogen fixation. The essential contribution of phosphorus must come from the upwelling Baltic deep water rich in phosphorus (Niemi 1979).
The absence of blooms of heterocystic species and nitrogen fixation from the northern Gulf of Bothnia is apparently connected with the high inorganic N:P ratio in that area.
Even in summer there is an excess of nitrate nitrogen in the euphotic layer of the Bothnian Bay (Voipio 1976, Pietikäinen et al. 1978, Alasaarela 1979a). The scarce phosphate in that area is rapidly consumed by the vernal phytoplankton immediately after the break-up of the ice in late May. However, low temperature may prevent development of blooms. According to Hubel et Hubel (1976 a, b), Nodularia cannot grow effectively without high temperatures.
In eutrophicated coastal areas in the Bothnian Bay blue-green algal blooms have been caused off Oulu by the non-heterocystic Oscillatoria agardhii (Alasaarela 1979b) and off Haparanda by the heterocystic Anabaena (Nauwerck 1978). Such blooms in polluted areas may be promoted by other factors, e.g. higher temperatures. Nevertheless as is shown in the Uusikaupunki area where eutrophication is caused by phosphorus-rich industrial wastes, the most important factor still seems to be the N:P ratio. This is corroborated by studies off the city of Helsinki (Rinne & Tarkiainen 1975).
Nitrogen fixation in the Gulf of Bothnia plays a conspicuously less important role in the nitrogen budget of the sea, than that noted in the earlier data collected during the MERININNI project, which concentrated mainly on the northern Baltic Proper and the Gulf of Finland. We roughly estimate that the annual nitrogen fixation in the Bothnian Bay is only 22 tons (<0.1 % of the total nitrogen input to the Bothnian Bay), and in the Bothnian Sea 1500-5000 tons (5-15 % of the nitrogen input). These values are smaller than the fixed values for the northern and central Baltic Proper; these were estimated at some 105 tons in 1974, which is the same order of magnitude as the land-based input of nitrogen in the area (cf. Rinne et al. 1978, 1979).
Blue-green algae fixing molecular nitrogen also occur in the littoral ecosystem of the Baltic Sea. Using the acetylene reduction method, Hubel & Hubel (1974 a, b, 1976a, b) have shown marked nitrogen fixation in littoral blue-green algal communities (Rivularia atra, Calothrix scopulorum, Anabaena torulosa) in eutrophicated coastal waters in the southern Baltic Sea. These species are also common in the northern Baltic archipelago waters. The epilithic Calothrix scopulorum inhabits the large areas covering splash zones in the archipelagos; hence, quantitatively, it is probably of considerable importance in the littoral nitrogen budget (cf. Niemi 1976, 1979). The occurrence of these species and their effect on the nitrogen budget have, however, not been studied in the Gulf of Bothnia.
126
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