Phytoplankton biomass and potential nutrient limitation of phytoplankton development in the southeastern North Sea in spring 1985 and 1986

131

Netherlands Journal of Sea Research 25 (1/2): 131-142 (1990)

PHYTOPLANKTON BIOMASS AND POTENTIAL NUTRIENT LIMITATION OF PHYTOPLANKTON DEVELOPMENT IN THE SOUTHEASTERN NORTH SEA IN SPRING 1985 AND 1986 EDUARD BAUERFEIND 1, WOLFGANG HICKEL, ULRICH NIERMANN ~ HEIN V. WESTERNHAGEN Biologische Anstalt Helgoland, Notkestrasse 31, D-2000 Hamburg 52, FRG 1present address: Universit~t Kiel, SFB 313, Olshausenstrasse 40-60. 2300 Kiel, FRG

ABSTRACT The vernal phytoplankton bloom was observed during cruises to the southeastern part of the North Sea (east of 6020 ' E, south of 56050 ' N) in 1985 and 1986. Maximum phytoplankton biomass expressed as phytoplankton carbon was similar in both years (14.5 and 17 g PPCm -2 respectively). In 1985 the bloom was located in the less saline coastal water close to the North Frisian coast. Phytoplankton was dominated by Coscinodiscus concinnus and Thalassiosira nordenskiSIdii. In 1986, highest phytoplankton biomass was observed northwest of the island of Sylt, where Thalassiosira nordenski61dii was the dominant phytoplankton species. Within the areas of high phytoplankton standing stock, concentrations of the inorganic dissolved nutrients phosphate and silicate had dropped to nearly undetectable concentrations, whereas both in 1985 and 1986 the water was still rich (10 - 20 p M . d m - 3 ) in inorganic nitrogen (DIN). This, as well as the high ratios of DIN:PO 4 and DIN:Si(OH) 4 ( > 50) point to phosphate and silicate as potential limiting nutrients during the spring phytoplankton bloom. The ratios of total nitrogen (TN) to total phosphorous (TP) ( > 30) indicate also that phosphorus might then be in short supply. Phosphate and silicate have to be considered as potentially limiting nutrients during the vernal plankton bloom in the coastal waters of the southeastern North Sea, with nitrogen being present in surplus at that time of the year. However, in the more offshore areas nitrogen may be considered the potentially limiting element at the same time.

during recent years. An increase in the plant nutrients has become evident from the long-term measurements of the Biologische Anstalt Helgoland, carried out at Helgoland Roads every workday since 1962. From these data, an increase in nitrogen and phosphorus concentrations has become obvious, particularly in the less saline waters (GILLBRICHT, , 1986; RADACH 8- BERG, 1986). Measurements of nutrient concentrations in the German Bight in 1978 and 1985 showed a two to threefold increase in the phosphate winter concentrations when compared to measurements made in 1936 (WEICHART, 1985, 1986). Similar increases in nutrients are also reported for the area of the southern North Sea (FOLKARD ~ JONES, 1974; POSTMA, 1978). The main sources of nutrient input into the German Bight and south- eastern North Sea are the major rivers Rhine, Elbe and Weser. The input of total nitrogen and total phosphorus by the rivers Weser and Elbe amounts to 292,000 and 22,570 tons/year for N and P respectively (BROCKMANN ~ EBERLEIN,1986; Carlson, 1986; Fransz, 1986). For nitrogen, the input via the atmosphere has also become important (GERLACH, 1984). Parallel to the elavated nutrient concentrations, a 2-3 fold increase of phytoplankton biomass, due to a 10-15 times increase of flagellate biomass, and a seemingly increased occurrence of nuisance algal blooms have been reported (HAGMEIER, 1978; B~,TJE ~f MICHAELIS, 1986; FRANSZ, 1986; RADACH et al., 1986; WEISSE et al., 1986; VELDHUlS, 1987). By some authors these changes are attributed to the increased nutrient input; on the other hand no correlation with nutrient concentrations was found during a Ceratium summer bloom near the Island of Helgoland in 1981, (GILLBRICHT, 1983). In this contribution we deal with spring phytoplankton blooms in 1985 and 1986, with emphasis on the potentially limiting nutrients for the vernal blooms.

1. INTRODUCTION Eutrophication and its consequences on the coastal waters of the North Sea have become a matter of public interest and have been the subject of intensive debates

Acknowledgements: We thank Mrs A. Reiners for carrying out CHN- analyses and Ms G, Dymek for the analysis of nutrients and chlorophyll concentrations. We also acknowledge the assistance of the crew of R.V.

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PHYTOPLANKTON BIOMASS AND NUTRIENT LIMITATION

Friedrich Heincke during the cruises. This study was financed by the Bundesministerium fL~r Forschung und Technologie under contract MFU 05384. 2. MATERIAL AND METHODS 2.1. AREA OF INVESTIGATION The region studied covers the area of the German Bight and the neighbouring Danish waters (east of 6°20'E and south of 56°50'N) (Fig. 1). The German Bight is a transition zone between the well mixed low saline coastal waters and the deeper waters of the southeastern North Sea, stratified during summer. Between these, a convergence zone exists and various kinds of frontal systems can be observed (GOEDECKE, 1968;KRAUSE et al., 1986;HESSE, 1988). The plume of the Elbe river stretches out into this area; its extension and position is very variable depending on the prevailing meteorological conditions. Another factor that influences the hydrography and currents of the area studied is Horns Rev, a shallow zone with a depth of 3-5 m, that extends out to the West at 55°30'N (Fig. 1). As the residual currents within the southeastern North Sea are directed towards the north, substances brought in by the rivers into the German Bight are transported northward within the near coastal regions. Therefore increased concentrations of these substances can be expected in the same regions, which was also shown by the results of recent model studies (ANONYMOUS, 1987). 2.2. METHODS During the years 1984-1987, three to four cruises to the southeastern North Sea were carried out every year with R.V. 'Friedrich Heincke' in order to study the causes and consequences of the oxygen deficiencies that had occurred in this area of the North Sea in 1982 and 1983 (DETHLEFSEN 8- V. WESTERNHAGEN, 1983; V. WESTERNHAGEN et al., 1986). During each cruise hydrographic parameters, seston and plankton parameters were studied. However, in this paper we only do refer to data gathered in spring 1985 (18-29 April) and 1986 (12-23 April) when the investigations coincided with the spring phytoplankton bloom. In 1985, only the southern part of the investigation area could be studied, due to bad weather conditions. Cruises were started in the mouth of the Elbe River and sampling was done on the way to the north at the stations indicated in Figure la-ld. At all stations visited, vertical profiles of the hydrographic parameters, temperature and salinity, were made along with measurements of the oxygen concentration with an 'OTS-probe' (ME - Meerestechnik Elektronik) that was connected to a rosette water sampler. Water samples were taken at 3-5 different

133

depths with 5 I Niskin bottles. From the water bottles, subsamples were drawn for the analysis of inorganic dissolved nutrients (NO3, NO2, NH4, PO4, and Si(OH)4), determination of total nitrogen (TN) and total phosphorous (TP), measurement of seston, particulate organic carbon (POC) and particulate organic nitrogen (PN), chlorophyll a concentration, analysis of phytoplankton composition and phytoplankton biomass. Measurements of inorganic nutrients were performed on board immediately after sampling, using the standard methods of seawater analysis, as outlined in GRASSHOFF et al. (1983). Samples for nutrient analysis were screened through 100/lm plankton gauze prior to analysis. For the analysis of TN and TP 50 cm 3 subsamples were taken. These samples were quickly deep frozen and stored at -20°C until the analysis in the laboratory. Analysis of TN and TP were done according to the wet oxidation method (Koroteff), as described in GRASSHOFF et al. (1983). Since this method is suspected of not working satisfactorily at high concentrations, the quality of the oxidation was checked at regular intervals by means of dilution series. Analysis of seston, POC and PN were carried out on material filtered (100-500 cm 3) on precombusted (490°C for 2 h) and preweighted glass fibre filters (Whatman GF/C, diameter 25 mm). The filters were immediately deep frozen after the filtration and stored at -20°C. The POC and PN content were determined by high temperature combustion (1050°C) by using a Hewlett Packard model 185 CHN-analyzer. Subsamples for the evaluation of phytoplankton composition and phytoplankton biomass (100 cm 3) were fixed with borate buffered formalin (final concentration 0.4%). Counting was done on 10 or 25 cm 3 subsamples, with an inverted microscope. The volume of the organisms was calculated with the help of the stereometric shapes and formulae as given in EDLER (1979): from these the carbon content of the organisms (PPC) was calculated by using the formulae given by STRATHMANN (1967) and SMETACEK (1975). 3. RESULTS 3.1. HYDROGRAPHICPARAMETERS The sea surface temperatures ranged from 4-6°C in 1985 and 3-4°C in 1986 in the area investigated. These temperatures were 1-2°C (1985) and 2-3 °C (1986) lower than the average sea-surface temperatures for this region in the years 1971-1980 (BECKER et al., 1986). Salinity distribution in the surface waters exhibited the same general pattern during both cruises, with the isohalines paralleling the North Frisian coast. The salinity was higher in 1986 (S = 31 - 32) in the region close

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to the North Frisian coast than in the preceding spring when in the same area a salinity of S < 30 - 32 was measured (Fig. 2). In 1985 a steep horizontal gradient in salinity (S = 4 within 12 nautical miles) was observed close to the coast. This indicates, that the convergence zone which marks the borderline between the low saline coastal water and the surrounding water masses of the Southern North Sea was driven to the near coastal regions by the southwesterly winds that prevailed at this time. No such clear distinction between these water masses could be observed in April 1986 when the cruise was carried out during a calm period that followed a period with strong winds at the beginning of April 1986. The S = 34 isohaline that marks the border between the water of the southern and central North Sea showed more or less the same geographical distribution in April 1985 and 1986 (Fig. 2a, 2b). 3.2. PHYTOPLANKTON, SPECIES COMPOSITION AND BIOMASS Diatoms dominated the phytoplankton composition in both investigations during the spring development. In 1985, Coscinodiscus concinnus (W. Smith), a large single celled diatom, was the dominating species; to a lesser extent also Thalassiosira nordenski61dii (Cleve) and Thalassiosira rotula (Meunier) were present. The highest cell numbers were observed in the less saline waters close to the North Frisian coast, south of the island of Sylt ( > 2500 cells.dm-3). In April 1986, two regional centres with a high standing stock of phytoplankton were found. One with high phytoplankton numbers was located in the North Frisian coastal water again, where the small chain - forming diatom Skeletonema costatum (Grev., Cleve) dominated, with a maximum up to 2 x 106 cells.dm -3 (Fig. lc). Another region with high phytoplankton content was situated northwest of the island of Sylt, with the small chain-forming species Thalassiosira nordenski61dii (Cleve) dominating ( > 4 x 106 cells.dm -3) at its maximum (Fig. ld). Porosira glacialis (Grun, J¢rgensen) and various Chaetoceros species were found in lower numbers. In the northwestern part of the area Coscinodiscus concinnus formed a considerable portion of the plankton in 1986. Microflagellates and dinoflagellates were present only in low numbers during both investigations. Figure lc and ld shows the distribution of phytoplankton cell numbers and biomass (PPC) in the surface waters for April 1985 and 1986. Maximal phytoplankton biomass was similar in both years, with 700-800 pg PPC.dm -3 in the surface waters. At some stations, the phytoplankton consisted almost exclusively of Thalassiosira nordenski61dii with its biomass exceeding more than 90% of total PPC in

April 1986. Even though in 1985 phytoplankton biomass was not dominated to such an extent as in 1986 by one single species, Coscinodiscus concinnus contributed > 60% of total PPC in the area with maximum phytoplankton standing stock. Not only the surface concentrations of PPC were similar in both years but also the depth integrated phytoplankton biomass exhibited similar values with 1517 g PPC.m -2 at its maximum. Outside these regions of maximum biomass the PPC ranged from 10-15 g PPC.m -2. In the more offshore waters (S > 34) PPC was lower with < 5 -10 g PPC. m-2. In the areas with the largest standing stock of phytoplankton the share of PPC to POC was exceptionally high. Here > 80% of the POC was due to phytoplankton carbon. Outside these regions the PPC content usually ranged between 10-30% of the POC. The large biomass as well as the POC:PN ratio that ranged from 5 - 6 in 1985 and 1986 in the areas with high phytoptankton content are indicative for a growing bloom, with the phytoplankton still in good condition. 3.3. NUTRIENTS The concentration and the distribution of the dissolved inorganic nutrients were similar in April 1985 and 1986. Figure 2b shows the surface concentration of the nutrients NO3-, PO43- and Si(OH)4 in April 1986. Very high concentrations were found in the mouths of the Elbe and Weser river and in the less saline areas close to the North Frisian coast (nitrate 50 - > 100 pM.dm -3, phosphate 2 - > 5 MM.dm -3 and silicate 15 - > 20 pM.dm -3. Nitrate dominated the dissolved inorganic nitrogen pool (DIN = ~. NO3- -t- NO22 + NH:). More than 80% of DIN was due to nitrate in the less saline waters but also in the other areas nitrate dominated and was usually more than 60% of DIN. Nitrite and ammonia were of minor importance with concentrations < 2 /~M dm -3 and < 1.5 pM.dm -3, respectively. During both investigations the concentrations of phosphate and silicate were very low in the regions with high phytoplankton biomass (PO4 <0.2 pM.dm -3 and Si(OH)4 < 0.5 pM.dm-3). In April 1986 at several stations these nutrients were below the detection limit. Low concentrations of silicate and phosphate ( ~ 0.1 /~M.dm-3) were also measured off the coast of Jutland in April 1986, whereas nitrate concentrations were still high (5-10 pM.dm- 3). In areas influenced by water from the central North Sea (S > 34) all nutrients were generally lower in 1986. This differed from the situation in April 1985, when higher concentrations of silicate and phosphate, > 1 MM.dm -3 and > 0.3 pM.dm -3 where found in the western part of the region investigated (North of 55°N and West of 7°E). Nitrate, on the other hand showed

PHYTOPLANKTON BIOMASS AND NUTRIENT LIMITATION

approximately the same concentration as the surrounding less saline waters ( ~ 5-10 /~M. dm-3). 3.4. TOTAL NITROGEN (TN) AND TOTAL PHOSPHORUS (TP)

Measurements of TP and TN should include most forms of nitrogen and phosphorus present in the water, whether dissolved or particulate. Therefore, these values are a better measure of nutrients that are potentially available for phytoplankton growth than the inorganic dissolved fractions. During the cruises dealt with here concentrations (/IM.dm -3) of TN and TP of ~ 100 > 200 (N) and 5 - 10 (P) were measured in the estuarine waters. In the coastal waters (S < 31), the concentrations were 50 - 100 (N) and 1 - 2 (P). In the areas showing large phytoplankton biomass, concentrations of 25 - 60 (N) and 0.9 - 1.5 (P) were found. Lowest concentrations were measured in the water with salinities S > 34 with concentrations of 5 - 20 and 0.5 - 1 /LM.dm -3 for TN and TP, respectively. The concentrations for TP are comparable with phosphorous concentrations measured in the German Bight in January 1978, when most phosphorous is present in the inorganic form (WEICHART, 1986). The share of the inorganic dissolved P and N of TP and TN was high with > 50% DIN and 50-70% PO4 in the waters influenced by freshwater inflow. In the more offshore waters (S > 34) the values were much lower (10-20% (DIN) and 30-40% (PO4) in 1986). In April 1985 in the same areas percentages were slightly higher with 30-50% and 40-50% for DIN and PO4, respectively. In the areas with phytoplankton blooms DIN still contributed 20-50% to TN, whereas the share of PO4 was < 10% in both years. This shows that the greatest part of phosphorus had already been transformed from the dissolved inorganic fraction to other forms of the phosphorus pool due to phytoplankton activity. DIN, on the other hand, was still present at high concentrations in the same regions. The high percentages of DIN even in the areas of phytoplankton blooms as well as in the waters with low salinities are indicative of the high nitrogen input into the coastal areas. The surplus DIN concentrations in the bloom areas might be used as a measure of eutrophication of the southeastern North Sea. The long-term time series carried out at Helgoland roads that exhibits a change in the annual cycle of nitrogen concentrations, with a shortening of the period with reduced nitrate concentrations, support this fact (RADACHet al., 1989).

phosphate and silicate stocks is evident. This fact becomes more obvious when considering the ratios of the different elements. Figs 3a and 3b show the distribution of the DIN:PO4 and DIN:SI(OH) 4 ratios as calculated from the data obtained at the survey in April 1986. In Table 1 the mean ratios of these nutrients are listed for waters with different salinities for the period April 1986 and 1985. Taking the 'Redfield - ratios' (N:P = 16, N:Si ~ 1 as a guiding figure in seawater (REDFIELDet al., 1963), the relative shortage of phosphate and silicate compared to nitrogen becomes obvious, despite the high concentrations of these nutrients in the estuarine waters entering the German Bight (DIN:PO4 = 30 - > 40 and DIN:SI(OH) 4 = 20 - > 30, Fig. 3a, 3b). The high natural background concentrations of nitrogen in waters from terrestrial sources are well known (GERLACH, 1987; BROCKMANNet al., 1988), but the increase of nitrogen and phosphate concentration during the last decades in the rivers entering the southern North Sea is also well documented. The dominance of nitrogen becomes obvious also if one considers the ratios of total nitrogen (TN) to total phosphorus (TP). These values also demonstrate the dominance of nitrogen in the estuarine and less saline coastal areas (Fig. 3c). A ratio of 20 may be taken as a value indicative of a system in biological and chemical balance (BUTLER et al., 1979). In the area influenced by waters of the central North Sea (S > 34), TN:TP ratios close to these values were found. This again was similar during the investigations in April 1986 and 1985. 4. DISCUSSION Phytoplankton production in spring is mainly based on the nutrients accumulated in the water due to river input TABLE I Average values of DIN:PO 4 and DIN:Si(OH)4 in the n e a r - s u r f a c e waters obtained on cruises to the southeastern North Sea in A p r i l 1985, A p r i l 1986 and August 1986. Mean r a t i o s are given f o r a l l s t a t i o n s v i s i t e d and f o r areas w i t h s a l i n i t i e s S < 31 and S > 31. ( i n brackets = number of observations). DIN: PO4-ratios cruise

A p r i l 85 3.5, NUTRIENT RATIOS

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PHYTOPLANKTON BIOMASS AND NUTRIENT LIMITATION

and remineralisation during winter; and the yield in biomass produced during the course of the spring bloom is 'new production' sensu DUGDALE It GOERING (1967). Organisms growing under these conditions belong to the 'r- strategists' that are adapted to high ambient nutrient concentrations, show high growth rates and exploit the initially high pool of dissolved nutrients (HARRIS, 1986). The final standing stock of the spring phytoplankton bloom thus should be roughly equivalent to the nutrient pool at the end of the winter. Measurements of nutrient concentrations during the non- productive winter season covering the whole area of the southeastern North Sea are very rare. The data available (Nov. 1977, March 1978 and Febr./March 1984/85) give a range of 10-40 /iM NO3.dm 3, 1-2 /zM.PO4 dm 3 and 6-20/~M Si(OH)4).dm-3. In the less saline coastal areas, the corresponding concentrations were > 30- >40, > 2 and 15- > 20 respectively (WEICHART, 1985, 1986; Hesse, 1988; Hickel, unpublished). The latter values correspond well with the longterm means from the time series of the Biologische Anstalt Helgoland. Concentrations within that range were also measured in February 1986 by Danish scientists during a cruise covering the German Bight (G. Aertebjerg, pers. communication). In the neighbouring more offshore waters, influenced by water from the central North Sea, nutrient concentrations were lower with 5-6/~M NO3, 0.5-0.7/~M PO4 and 4-5/~M Si(OH)4 as reported for the year 1984 by BROCKMANN ~ WEGNER (1985). During April 1985 and 1986 large phytoplankton standing stocks were found in the southeastern North Sea during the present study. Phytoplankton composition was governed by only a few diatom species, and the initially high inorganic nutrient concentrations had already been reduced considerably due to the rapid uptake by the dominating diatoms. Silicate and phosphate were almost depleted in the bloom areas. Nitrate concentrations, on the other hand, were reduced to a lesser extent. The remaining nitrogen concentration of 5-15 /~M.dm -3 had a potential capacity to produce approximately the same biomass as already present. Thus it is evident that nitrogen cannot be considered the limiting nutrient in the springs of 1985 and 1986. The lack of ambient nutrients on the other hand does not necessarily indicate nutrient limitation of phytoplankton growth. This especially holds true for phosphate which is recycled very fast and can also be present as dissolved organic phosphorus (CEMBELLAet al., 1983, 1984). But there is also evidence in the literature that during vigorously growth of phytoplankton nutrient depletion occurs and growth rate decreases through the exhaustion of inorganic nutrients (SAKSHAUG et al., 1983; HARRIS, 1986). In the case of silicate, limitation may already start at concentrations which are far from exhaustion. For in-

139

stance culture experiments and field studies yielded half saturation constants (K s) to vary between 0.8 and -4/~M Si(OH)4.dm -3, and Si limitation was found to start at concentrations < 1 /~M.dm -3 in the Hudson river plume (PAASCHE, 1973; AZAM 8- CHISHOLM, 1976; MALONE et al., 1980). From these evidences it can be concluded that the very low phosphate and silicate concentrations found in April 1985 and 1986 may well have limited the diatom spring bloom. Indications of potential P-limitation were also found in the area of the Dutch Wadden Sea, the Wadden Sea of Sylt and in Norwegian waters that are influenced by freshwater runoff (SAKSHAUG ~- OLSEN, 1986; WEISSE et al., 1986; VELDHUIS, 1987). Another indicator of nutrient deficiency can be derived from the ratios at which the nutrients are present in the water. As a guiding figure the 'Redfield ratio' is taken (REDFIELD et al., 1963), which is similar to the molar ratio of N:P:Si of ~ 15:7:1 reported by SCHOTT EHRHARDT (1969) for the North Sea. Phytoplankton at maximum growth rates takes up N and P in the same ratios as they occur in the ambient water ( ~ 16:1), while Si is taken up by diatoms in approximately the same quantities as N (RICHARDS, 1958; REDFIELDet al., 1963; LEVASSEUR ~t THERRIAULT, 1987). These values, however, are very variable; they differ from species to species and during the life cycle of the organisms and they certainly do not represent the 'real world'. But great deviations from the 'Redfield ratios' can be taken as indicators of potential nutrient limitations. Thus, the high ratios of DIN:PO 4 and DIN:SI(OH) 4 measured strongly point towards P and Si potentially limiting phytoplankton production in spring 1985 and 1986 (Table 1 and Fig. 3). Both ratios were > 100 in the areas with highest phytoplankton biomass (Fig. 1 and Fig. 3). The ratios in the less saline (S < 31) coastal waters point out the large supply of N by the river water compared with P and Si and compared with the requirements of phytoplankton. The deficiency in phosphorus is also supported by the high TN:TP ratios (Fig. 3) which were > 30 in the bloom areas and in the regions influenced by the Elbe and Weser river. The high TN:TP ratio and low phosphate and silicate concentrations off the coast of Jutland can be taken as an indicator of a preceding bloom. In the more offshore waters with salinities S > 34, on the other hand, indications of a potential nitrogen limitation were found in both years (Fig. 3). In this waters the ratios were: DIN:PO 4 < 10, DIN:SI(OH) 4 > 1 -10 and TN:TP < 20. These ratios were similar to those measured during the summer cruise in 1986 when we found indications for N-shortage in the whole area of investigation except the areas close to the river estuaries (Table 1, Fig. 3). The results of potential P and Si limitation obtained

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E. BAUERFEIND, W. NICKEL, U. NIERMANN 8- H.V. WESTERNHAGEN

during spring phytoplankton blooms in the two subsequent years are contrary to the observations reported for the Swedish west coast, the east coast of Scotland, the English Channel and the east coast of the United States. For all these areas a potential nitrogen limitation is reported in the literature (RHYTHER Et DUNSTAN, 1971; PINGREE et al., 1977; GRAN£LI, 1987; JONES 6" HENDERSON, 1987). Indications of Si and (or) P limitation on the other hand are reported for the Dutch and German Wadden Sea and Dutch and Danish coastal areas (v. BENNEKOM et al., 1974; GIESKES AND KRAAY, 1975; SOMER, 1985; WEISSE et al., 1986). In the southeastern North Sea excess quantities of N and indications of a P and Si shortage were observed during the spring phytoplankton bloom. Si is needed in greater quantities by diatoms only and N is present in still sufficient quantities and assuming a quick turnover of phosphorus nutrient conditions become favourable for the growth of non-diatomous phytoplankton after the spring diatom bloom. The regular occurrence of Phaeocystis blooms in the coastal areas of the North Sea after the preceding diatom bloom (B~,TJE 6" MICHAELIS , 1986; WEISSE et al., 1986; VELDHUIS, 1987), as well as the general increase of flagellate biomass within the last decades may well be an effect of eutrophication, a process that presumably already has started in some regions off the US coast (OFFICER ~ RHYTER, 1980). Therefore, in the actual discussion about eutrophication of the coastal zones of the North Sea, not only the concentrations of the nutrients should be considered, but emphasis should be placed on the ratios at which nutrients are introduced into these areas. Present nitrogen input is far in excess with respect to the needs of phytoplankton. The latter fact may have major consequences, for it may result not only in an increased production, but may also be responsible for the observed shift towards a flagellate-governed phytoplankton community and its possible impacts on the foodweb in the southeastern North Sea. These theoretical assumptions are confirmed by the long-term time series of the Biologische Anstalt Helgoland which show an increasing biomass of flagellates at Helgoland roads (RADACH Et BERG, 1986). From our studies on spring phytoplankton blooms in the southeastern North Sea, we conclude that: 1. Nitrogen is present in the coastal areas in excess quantities. 2. Nutrient ratios as well as nutrient concentrations point towards P and Si potentially limiting the spring phytoplankton blooms. 3. In more offshore areas (S > 34), however, nitrogen is the potentially limiting element. 4. The relative shortage of P and Si favours the growth of none diatomous phytoplankton after the spring bloom.

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WESTERNHAGEN H.V., W. HICKEL, E. BAUERFEIND,U. NIERMANN 8" I. KRONCKE,1986. Sources and effects of oxygen deficiencies in the south-eastern North Sea.--Ophelia. 26: 457-473.