This paper not to be cited without prior reference to the author

International Council for C.M. 1985/C:47/Ref. L the Exploration of the Sea ICES hydrography Committee Ref. Biological Oceanography Committee

AREAL VARIATION AND TRENDS IN THE NUTRIENT CONCENTRATIONS OF BALTIC HATERS AROUND

by

l 2 l Heikki Pitkänen , Paavo Tulkki and Pentti Kangas

1) National Board of Waters, Water Research Institute, . , ~ Box 250, SF-OOIOl Helsinki, Finland 2) Finnish Institute of t1arine Research, Box 31, SF-00931 Helsinki, Finland

ABSTRACT

Nutrient concentrations and some related properties of Finnish coastal and op~n sea waters were investigated. In general, the total nutrient level was highest in the Gulf of Finland, somewhat lower in the Archipelago Sea and the Bothnian Sea and lowest in the . 4t This gradient coincides with the productivities of the different areas. In the coastal waters local topography, together with river-and waste water loading, greatly determined the distribution pattern of nutrients. In the Gulf of Finland nitrogen appeared to limit primary production in spring according to the wintertime N/P-ratio " whereas phosphorus seemed to be the limiting' factor in the Bothnian Bay. In late summer small amounts of inorganic nitrogen and phosphorus were still present in the loaded waters, but the open sea concentrations were near to zero. In the open Bothnian Bay, however, the concentrations of inorganic N were exceptio­ nally high. In the open sea areas the nutrient concentrations of both surface and near-bottom water have increased since the 1960's, with the exceptions of phosphorus concentrations in the Bothnian Bay and probably also in the deep water of the Gulf of FinZand. In the loaded coastal waters changes in nutrient inputs were reflect~d as trends.

RESUME

Les concentrations en sels nutritifs et quelque autres qualites de l'eau de mer cotiere et au Zarge seront traitees. En generaZ, Ze niveau total en sels nutritifs est le pZus eleve dans Z'eau du GoZfe de FinZande, - 2 -

moins eleve dans les eaux de la Mer de l3Arahipel et du Golfe de Botnie du sud3 et le plus bas dans l3 eau du Golfe de Botnie du nord. Ces variationa-ai aoincident avea la produativite relative de aes regions. Dans les eaux aotieres3 ae sont La topographie loaale et Le ahargement fluaial et industrieL qui determinent la distribution dea sels nutritifs. Dans le Golfe de Finlande a 3est l3 azote qui limite la produation primaire en printemps. Le phosphore parait etre le faateur restrictif dans le Golfe de Botnie du nord et ausii dans les eaux aotieres ahargees. 3 3 Tard en ete3 de petites quantites d azote et de phosphore etaient enaore presents dans les eaux ahargees 3 mais les eaux en haute mer etaient depourvues de aes sels-ai. Dans l3 eau du Golfe de Botnie du nord, aependant3 La aonaentration, an azote inorganique etait exaeptionnellement elevee. En haute mer3 les aonaentrations en sels nutritifs se sont augmentees dans l3 eau de surfaae et dans l3 eau ,du fond depuis les annees 60, exaeptant le phosph~redans le Golfe de Botnie du nord, et probablement aussi dans les eaux du 'fand du' Go lfe de Fin lande. Dans les eaux aotieres ahargees 3 les aorrelations entre les aonaentrations et le temps etaient influenaees par des a~~rg~ments.

1. INTRODUCTION The sea areas surrounding Finland form thc northernmost part of • thc Baltic Sea (Fig. 1). The surface salinity of thc northcrn Baltic Proper is up to 7 0/00 and it decreases to near zero at the very ends of the Gulf of Finland and the Gulf of Bothnia, whcre large amounts of river water are dischargcd. In the near-bottom layers the salinity is higher, up to 9-10 0/00 at the entrance to the Gulf of Finland'and in thenorthcrn Baltic Proper. In the, Gulf of Bothnia thc salinity at the ncar-bottom water is only a little higher than at thc surface. The salinity of the whole Baltic Sca remains relativcly stable from year to ycar. Thc icebound winter scason of the Finnish sea areas lasts 3 months in thc south and 6 months in thc northernmost area (SUHI and Inst. Har. Res. 1982).

Thc hydrographical conditions in thc Gulf of Finland are similar to thosc in the Baltic Proper, since there is no sill betweeri'thcse two se~ areas~ Dueto,the strong density stratification nutrients ~re con­ centrated in thc decp water of these sea areas. From there'the nutrients 4t may reach the sUrface duc to upwelling. '

The sills at the south of the Gulf of Bothnia preventthe northward penetration of the deep waters of thc Baltic Proper. As a'consequence of this fact, dcnsity stratification in the Gulf of Bothnia"is·weak. The hydrography in the two gulfs is therefore very different;particularly with rcgard to salinity, nutrients and oxygcn in the deep water layers.

Conditions in the coastal watcrs depend greatly on geomorphology,' thc freshwater input and loading. liost of the Finnish coast is very broken. Although the length of the coastline is only 1100 km measured rectilincarly, the actual length of the shorcline is 39 000 km, including thc mainland and thc 73 OOO'islands and islets (0. Granö and M. Roto, Dept. of Gcography, University of Turku, Finland. pers. comm.1985).

Rivers are thc main land-bascd sources of nutrients. Of the main Finnish rivers, thc rivers Tornionjoki, Kemijoki, Iijoki and discharge into the northcastcrn Bothnian Bay, Kokemäenjoki to the middle of the Bothnian Sca, and Kymijoki to thc middle of thc Gulf of Finland (Table 1). The total annual load from Finland and Sweden to the Gulf of - 3 -

Bothnia is 8 000 t phosphorus and 150000 tnitrogen, and from.Finland and the USSR to the Gulf of Finland.6 300 and 120 000 t of phosphorus and nitrogen, respectively (Table. 2) • '. ::< '.....' '. . . . "*'''.( t .. . ~ This paper presents'thcaveragö ~~e~i distribution'of phosphorus, nitrogen, chlorophyll and oxygen in (mainly) surface waters around Finland.

2. MATERIAL AND r-ffiTHODS

The material consist~ of data fromJabout 350 sampling stations along the Finnish coast (Fig. 1). It originates from' the results of (1) the monitoring program of the National Board of Waters, (2) local pollution . control programs supervised bythe Board, and (3) the open sea monitoring program conducted by the Finnish Insitute of r1arine Research. Of the different available quality variables, total phosphorus, phosphate phos­ phorus, total nitrogen, inorganic'nitrogen (ammonium and.nitrate), chloro-· phyll-a- and oxygen were chosen for presentation in this work.. ~1ean values were calculated for each of the sampling stations for the summer (July-Sept) and winter (Febr-March) seasons in 1979-1983. ttln general, each of the stations was visited 1 to 2 times during both of the seasons studied. Trends were calculated over the period from . the late 1960's to 1983'or 1984 using linear regression analysis. Winter results are mainly presented because thebiological activity is then at its minimum'and the winter results can reliably be compared with each other. Surface waters and near-bottom water layers were handled separ- ately. . " ::',':

The area:i'~ariation of each variable (except P0ti -P)' i:s presented in , '. ~. the form of isopleths. The station netvlork was densest. in loaded areas, and therefore the location of isopleths is most reliable in such coastal waters.

.' .~ •. ;~ . , 3. DISTRIBUTION OF NUTRIENTS, CHLOROPHYLL AND OXYGEN

3.1 Phosp~orus"

, ' . Clear differences in phosphorus concentrations can be seen between . ' . "the open sea areas. The mean concentration ~~ total phosphorus in the mixed winter surface layer was about 7 mg m in the Bothnian BaYl 12 mg m-3 in the Bothnian Sea, and 30 to 50 mg m-3 in the Gulf of Finland (Fig. 2). In the intermediate area , the Archipelago Sea, the mean winter level was around 2,~mg m-3 • On average, 50-70 % of total phosphorus was in the form of inorganic phosphorus. The distributions of these two phos­ phorus variables were similar.

In the coastal waters the phosphorus concentrations were clearly' elevated. In the loaded areas of the Gulf of Bothnia concentrations were more than double those of the open sea and even higher in the vicinities of estuaries, larger cities and certain industrial plants. In the Gulf of Finland the differences between concentrations in the open sea and coastal waters were relatively smaller. . ~. , ... .

In the coastal waters the phosphorus level was extensively.elevated in the northeastern Bothnian Bay, the archipelago' of ~he Quark and.the inner Archipelago,Sea. Off the River Kymijoki estuary in the Gulf of Finland the effect of river water on phosphorus levels was clear, although the concentrations were lower than in the open sea. - 4 -

During the summer period the level of total phosphorus was clearly lower than in winter, especially in the Archipelago Sea and in the open Gulf of Finland. The summer values of phosphate were near to zero, except in the loaded coastal waters where phosphate was usually present also in summer.

3.2 Nitrogen

In the open parts of the Gulf of Bothnia winter surface concentra­ tions oftotal nitrogen'were about 270 mg m- 3 • The corresponding con­ centrations in the Archipelago Sea were about 320 mg m- 3 and in the Gulf of Finland 350-450 mg m- 3 ' (Fig. 3). About 30 % of total nitrogen was in inorganic form, except in the Bothnian Sea, wherethe corresponding percentage was only 15-20 (Fig. 4).

The effect of coastal loading on total nitrogen concentrations was in all areas more extensive than in the case of phosphorus. Th~ coastal level was usually 100-300 mg m3 higher than the open sea level. The areas with increased concentrations were mainly the same as in the case of phosphorus, but the effect of the larger rivers and cities was more pronounced. In the coastal waters inorganic nitrogen usually accounted tt for,50-aO % of total nitrogen.

In summer the level of total nitrogen in the open sea was slightly lower than in winter in the Gulf of Finland. The level of inorganic nitrogen was near to zero 'in the other sea areas, except in the Bothnian Bay, where mean values of 20-60 mg m- 3 were recorded (Fig~ 5). Inorganic nitr~gen was present in summer in all the loaded coastal waters.

3.3 N:P ratio

In the open Bothnian Bay the wintertime weight ratio of inorganic N and P compounds in surface water was 20-50, in the Bothnian Sea 6-10, in the outer Archipelago Sea 5-6 and in the Gulf of Finland 4-6 (Fig. 6). The ratio clearly increased from the open sea towards the loaded coastal waters in all other areas except in the Bothnian Bay, where it usually decreased. 3.4 Chlorophyll-a • In the open Gulf of Bothnia and the Archipelago Sea the summer values of chlorofhyll-a were generally 1-2 mg m-3 , and in the Gulf of Finland 2-5 mg m- , increasing eastwards (Fig. 7).

From the open sea the concentrations of chlorophyll increased in­ wards. Off the town of Pori by the coast of the Bothnian Sea ~here was an area of 300 km2 with chlorophyll values less than 1 mg m- This areais affected by acidic, iron-rich wastes.

In the loaded coastal areas chlorophyll-a concentrations weregener­ ally 3-5 mg m- 3 • Higher concentrations occurred off the larger cities and off some estuaries. In the easternmost part of the study area' the chloro­ phyll concentrations were rather cvcnly distibutcu. - 5 -

3.5 Oxygen " ~ In the open sea areas and in·,t!W Archipelago Sea the oxygen saturation percentage in the surface water'yas <;llways high (93-99 % in winter and 100-110 % in summer,' ·Fig~· 8) •. In, the deepest waters of the open Gulf of Finland, however, the oxygen'concentration may decrease to zero (e.g. Perttilä et ale 1980), ,especially'at the,entrance into the Gulf. " ",," . In thc loaded coastal waters the le;el of oxygen saturation in the surface water was generally lowered in winter (60-90 %). In' summer the average levels were mainly 90-100 %~

4. TEHPORAL TRENDS

Wintertime nutrient levels have increased in surface waters in nearly all the open sea areas around Finland from the 1960's (Table 3). In the near-bottom water layer all nutrients except phosphorus in the Gulf of Finland showed increasing trends •

In the coastal waters, significantly increasing nutrient trends were recorded in the period 1970-1983 mainly at some outlying stations • (Table 4). Decreasing trends were observed off some larger estuaries '(tbe rivers Oulujoki, Kokemlienjoki and Kymijoki), cities (Turku and Helsinki) and industrial centres (Kemi).

Negative oxygen trends were observed in the inner Archipel~go Sea, off some industrial centres (Rauma, Uusikaupunki) and at some outlying , stations. Positive trends prevailed e.g. off the river.Kymijoki estuary . .,

In summer the trends in the coastal waters were much the same as in winter: in general nutrient concentrations decreased off loaded estuaries and cities, whereas in the open sea concentrations increased.No remar­ kable areal trends were observed in the oxygen concentrations.

5. CONCLUSIONS.,

In general the,total nutrient level is highest in the Gulf of Finland, lower in· .the Archipelago Sea and lowest in the Gulf of Bothnia. This distribution is greatly affected by the hydrographical conditions • of the northern Baltic Sea. The' observations made in this work, are in accordance vlith those of many earlier studies (see Grasshoff and Voipio 1981). In coastal:waters,,~the load from the mainland grcatly dominates the areal distribution of nutrients. The complex topography of the coastal' waters'and' the mosaic of ,land and water slow Ury thc exchange,of water. As,a result,'nutrient concentrationsare clev~t~d in coastal ,-, waters off estuaries, cities, industrial plants and in archipelagoes. By contrast, thc quality of coastal water may resembe that 'of the open sca water in areas whcre thc land-based load is insignificant and the archipelago belt off the c~ast is narrow or absent.

In the eastenmost part of the study arca the situation was so~~~ what exceptional. There, the concentrations of nutrientswere more, evenly distributed than clsewherc, due to upwelling of nutrient~rich deep water off thc coastal zone (cf. e.g. Perttilti et ale 1980) and also to the heavy load of domcstic waste waters and river waters to thc eastcrn Gulf of Finland (Finnish-Soviet Working Group 1984). Thc - 6 - water of the river Kymijoki is relatively poor in nutrients and in winter has even a diluting effect on the high phosphorus concentrations off its estuary.

Due to intensive biological production, the surface concentrations of total arrounts of nutrients are in surnrrer greatly rcduced from the winter levels, especially in the Gulf of Finland. Levels of inorganic nutrients are near to zero except in the Bothnian Bay, where an excess of nitrogen exists. The difference between winter and summer levels is smaller in the coastal waters than in the open sea areas due to continuous nutrient input from the land. In most of the loaded coastal waters both inorganic phosphorus and inorganic nitrogen are left in the water in late summer.

It is probable that because the high vernal peaks of chlorophyll were not included in our material from July to October, the open sea distribution of chloropilyll-a is quite even in the Gulf of Bothnia. Areal differences have been reported in the primary production (cf. Lassig et ale 1978) and nutrient levels of the Gulf of Bothnia. In the Gulf of Finland, however, the levels of productivity, chlorophyll-a and nutrients are clearly higher than in the Gulf of Bothnia. In the loaded coastal waters the chlorophyll level is obviously elevated due • to nutrient inputs. In some areas affected by thc chemical or pulp and paper industries, however, inhibition caused by toxic waste waters decreases the chlorophyll-a level. The wintertime ratio of inorganic nutrients in the open Bothnian Sea and in the outer Archipelago Sea is about optimal for vernal phyto­ plankton production. In the Gulf of Finland nitrogen rather than phosphorus seerns to be the limiting factor, vmereas in the Bothnian Bay phosphorus is limiting (cf. Alasaarela 1979). In late summer both nutrients are almost exhausted in the open sea areas of the Bothnian Sea and the Gulf of Finland and in the outer Archipelago Sea. In the Bothnian Bay, excess inorganic nitrogen exists and phosphorus thuG probably limits primary production in late SumTIer (see Fonselius 1978, Niemi 1979, Alasaarela 1979) .

In the coastal waters phosphorus appeared to be the theoretical limiting factor in both the seasons studied in most of the loaded areas. However, inorganic forms of both nitrogen and phosphorus existed in the water phase of these areas even in summer, which indicates that other factors (e.g. the bioavailability of nutrients) were limiting the pri- .. mary production.

The percentage of oxygen saturation is negatively correlated vlith nutrients in winter. Howcver, in general the oxygen content of the open water is high, except in the deepest parts of theGulf of Finland, where the intensive densitv stratification--restricts the vertical mixing in the same wat as in thc Baltic Proper. In coastal waters oxygcn is rcduced especially in winter; when the river waters mixed with waste waters spread out immediately beneath thc icc cover (Alasaarela and ~lyllymaa 1978) •

In the Baltic Proper, concentrations of phosphorus and nitrogen have increased during rccent dccades (Nehring 1982) and this has also been reflccted in thc Gulf of Finland and in the Gulf of Bothnia. - 7 -

The total load of nutrients and organic matter to the coastal waters has increased during the period 1970-1983, mainly due to high river discharges in the late 1970's and early 1980's (Pitkänen 1985). Non-point loads of nutrients to watercourses probably also increased during the'1970's (Kauppi 1984). By contrast, point source loading due to municipal and industrial waste waters has decreased as a result of water protection measures, especially the.loads of phosphorus and organie matter (Enckel1-Sarkola et al. 1984, Finnish-Soviet Working Group 1984). Thus the nutrient concentrations have decreased in the inner parts of heavily loaded coastal waters, but increased in some outer coastal waters. At most of the coastal sampling stations studied the deviation of con­ centrations was so great that statistically significant trends could not be demonstrated.

6. REFERENCES

• Alasaarela, E. 1979. Phytoplankton and environmenta1 conditions in centra1 and coasta1 areas of the Bothnian Bay. - Ann. Bot. Fenn. 16: 241-274.

Alasaarela, E. & Hy11ymaa, U. 1978. Investigations into the dispersal of river and waste waters in the northeastern part ofthe Bothnian Bay in 1975-1977. - Finn. Mar. Res. 244: 173-182.

Committee for the Gulf of Bothnia 1983. The Gulf of Bothnia - discharges from land and air. 31 pp.

Encke11-Sarkola, E., Pitkänen, 11., Ruoho-Airo1a, T., Eriksson, B., ~'1idel1, A. and Ah1, T. 1983. Be1astningen ps. Bottniska viken (Pollution load to the Gulf of Bothnia). - Kommitteen för Bottniska viken, Arsrapport för ur 1983. Pp. 15-53. (In Swedish).

Finnish-Soviet Working Group on the Protection of the Gulf of Finland 1984. The Gu1f of Fin1and --discharges from land and air. - Rep • • no 3, 15 pp. Fonselius, S.H. 1978. On nutrients and their ro1e as production 1imiting factors in the Ba1tic. - Acta Hydrochim. Hydrobiol. 6: 329-339.

Grasshoff, K. & Voipio, A. 1981. Chemical oceanography. - In: Voipio, A. (ed.) The Baltic Sea. 418 pp. Amsterdam.

Kauppi, L. 1984. Contribution of agricu1tural loading to the deteriora­ tion of surface waters in Fin1and. - Publ. Water Res. Inst. Finland 57: 24-30.

Lassig, J., Leppänen, J.ft., Nierni, A. & Tarnelander, G. 1978. Phyto­ plankton primary production in the Gu1f of Bothnia in 1972-1975 as compared with other parts of thc Baltic Sea. - Finn. f1ar. Res. 244: 101-115. - ß -

Nehring, D. 1982. Langzeittrends des Phosphat- und Nitratbehalts in der Ostsee. - Beitr. Heeresk. 47: 61-86.

Niemi, Ä. 1979. B1ue-green a1ga1 b100ms and N:P ratio in the Ba1tic Sea. - Acta Bot. Fenn. 110: 57-61.

Pertti1ä, M., Tu1kki, P. & Pietikäinen, S. 1980. Mean va1ues and trends of hydrographica1 and chemica1 properties in the Gu1f of Fin1and 1962-1978. - Finn. Mar. Res. 247: 38-50. Pitkänen, H. 1985. Discharges of nutrients and organic matter to the Gu1f of Bothnia by Finnish rivers in 1968-1983. - Puhl. vJater Res. Inst., Fin1and (in press) • SHHI, Swedish Meteoro1ogica1 and Hydro1ogica1 Institute and Institute of !'1arine Research, Finland 1982. Climatological lce Atlas. Sjöfartsverket. Norrköping. 220 pp.

• Table 1 • Mean inputs of phosphorus, nitrogen and COD to Finnish coastal waters from the major rivers in 1968-1983 (I) and 1979- 1983 (lI) • (Material from Pitkänen 1985 and the National Board of ~vaters, Finland) •

River Coastal tot.P(t) tot.N(103 t) COO(103t) area I II I II I II To:rnionjoki Northe:rn 280 310 4.1 4.4 91 101 . Botlmian Bay Kemijoki 11 430 470 6.9 7.6 170 200 Iijoki 11 170 170 2.2 2.4 70 77 OUlujoki 11 230 210 3.9 3.4 95 90 - . ~- ._,._....--.-~_ ...... - ... Siikajoki Central 120 130 1.5 1.5 34 40 Botlmian Bay 11 130 160 1.6 2.1 35 44 ..-_., Iapuanjoki Southe:rn 98 110 1.7 2.2 28 35 Botlmian Bay Kyrönjoki The Quark 160 160 2.5 3.0 35 38 • Karvianjoki Central 89 130 1.1 1.4 19 23 Botlmian Sea Kokerräen.joki 11 530 570 7.9 9.0 130 130 ---_ .. Vantaa Central Gulf 96 78 1.5 1.4 7.6 8.8 of Finland Porvoonjoki 11 83 73 1.3 1.4 4.8 5.9 Kymijoki 11 290 270 5.7 6.4 130 110

• Table 2. Total discharges of nutrients into the Gulf of Botlmia and the Gulf of Finland from Sweden, Finland and the USSR as rreans for the years 1980 and 1981, tormes/year x 1000 (Comnittee for the Gulf of Botlmia 1983 and Finnish­ Soviet Working Group 1984).

Source Gulf of Bothnia, JUand Sea Gulf of Finland and Archipelago Sea tot.P tot.N tot.P tot.N Rivers 5.7 92.4 4.71) 86.01) Municipalities 0.4 4.7 0.6 9.0 Industry 0.5 4.2 0.1, 1.1 AtIrosphere 1.5 - 1.8 43 - 71 0.4 24.0

Sum 8.1 - 8.4 144.3,- 172.3 120.1

1) 'lhe load from Leningrad is included in the discharge of the river Neva Table 3. Trends of nutrients from the 1960's to 1983 or 1984 in the open sea areas around Finland (Material from representative stationsof the Finnish Institute of Marine Research). Increasing (+), decreasing (-), no trend (0).

Sea area Surface layer Nrer-bottom layer P04-P tot-P N03-N tot-N P04-P tot-P N03N tot-N Bothnian Bay 0 + + + 0 0 + + Bothnian sea + + + + + + Aland Sea + + + 0 + + + 0 Gulf of Finland + + + + + + Northern BaItic Proper + + + 0 + + •

Table 4. The wintertime trends (1970-1983) of total phosphorus, total nitrogen and oxygen saturation in the surface layers of coastal waters.

Sea area Coastal area tot.P tot.N °2 (%)

Bothnian Bay Kemi (~)/O 0 - -/0 0 0 Kokkola-Pietarsaari 0 -/0 0/- The Quark 0/ (+) 0 0 Bothnian Sea River Kokerräenjoki 0 0 Southern Archipelago • 0 +/0 -/0 Archipelago Sea -/+ 0/+ -/0 Gulf of Finland Western archipelago 0 0 0 HelsinJd -/0 -/+ 0 Porvoo + + + Kotka archipelago -/0 --/+ ++/-

~~} areal1y eXtensive trend

areally restricted trend '~, '. ( ±) uncertain trend dUE;-tQ missing data "'", r""'l o no significant trends"in the coastal area / inner/euter coastal waters. separate synbols are given if considerable differences exist. •

•

...... , e:;', '. • :~ ~,. , . ,,~, i " , • r , . \ •

Bo~hnian • Sea o o

Fig. ·e ., ;_.

•• l~ •

12 11

12

12 13 •

33

Fig. 2. Total phosphorus (mgp--m-3 ) in surface wat er in winter-.-_. .--- Mean va1ues 1979-1983, .. - •

.-.. t.

'~.

270 2ßO Total nitrogen mg m-3

260

• 270 280

.,'. '

"

350 . ----. -3 --. Fig. 3. Total nitrogen (mgN m ) in surface water in winter. Mean va1ucs 1979-1983. .------

\ \ \ ", • \"', \

.'.:; 46 S4 48 :nitrogen

47

50 S3 •

" .' " ~ ~., ;;:..~:\.~ , .'

. , 103

Fig. 4. Inorganic nitrogen (rngN rn- 3) in surface water in winter. Mcan va1ues 1979-1983. .. ~'.

2

3

• 3 3

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"~, r" r' ,..~ 7 5 "\ ' '-'. > '1 ':':] Fig. 5. Inorganic nitrogen (mgN m ) in surface water in summer. Mean va1ues 1979-1983. • ..

6.8

7.0

8.2 6.3 •

, ' • r •.•••

Fig. 6. Ratio of inorganic nitrogen to phosphorus (w/w) in surface water as rneans for 1979-1983. •

,-... i' 't. '-. \ "~, \ ~

•

1.4

1.3 1.0

• 1.8

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----_._. .. '~j' .. Fig. 7. Chlorophyll-a (mg m ) in surface water in summer. Mean values T979-1983. • ... ~ ..

•

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,.

98

( 2-)0 in surface watcr 1'n winter. Fig. 8. Oxygen satur~979_l983.tion Heun values