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 FINLAND 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 Bothnian Bay. 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 Oulujoki 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.