Anthropogenic nutrient load of the Baltic Sea

Limnologica 29 (1999)233-241 http://www.urbanfischer.de/journals/limno

LIMNOLOGICA © by Urban & FischerVerlag

1)Baltic Sea Research Institute Warnemtinde, Germany; 2)National Environmental Research Institute Roskilde, Denmark

Anthropogenic Nutrient Load of the Baltic Sea G(INTHER NAUSCH1), DIETWART NEHRING1) ~ GUNNI AERTEBJERG2) With 7 Figures and 2 Tables Key words: Baltic Sea, eutrophication, nutrient discharges and distribution, nutrient trends

Abstract The discharge of nutrients is investigated in relation to their sources and effects in two case studies. The reduction of 47% in the phosphorus load from Denmark to marine areas between 1989 and 1993 has resulted in significantly lower phosphorus concentrations in most Danish coastal waters, and tendency to decrease can be seen in the Belt Sea and Kattegat as well. No general changes in nitrogen concentrations have been observed. This is due to the fact that more than 80% of the nitrogen load in Danish waters originate from diffuse agricultural sources. In the Pomeranian Bight strong nutrient gradients are generated by the mixing of Odra river water and coastal water. The spreading of the river plume could be exactly observed especially in winter, when biological activity is low. In general, different types of distribution, transport and modification patterns can be described. The annual input of nutrients from the catchment area to the Baltic Sea was estimated to be around 1000 kt N and 46 kt R As a result, winter concentrations of phosphate and nitrate are characterized by positive overall trends in the surface layer in all subregions of the Baltic Proper for the period 1969 to 1993. These trends stem mainly from the strong increase in the 1970ies and early 1980ies. Thereafter, the concentrations of both nutrients fluctuate strongly around a high level. The drastic decrease in fertilizer consumption since the late 1980ies mainly caused by the great economic changes in the countries of the former East Bloc is not yet significantly reflected in decreasing winter concentrations, but first signs already have been found in the decrease in averaged phosphate concentrations in winter, especially in the Arkona and Bornholm Seas.

mainly from anthropogenic activities in the catchment area including sewage production, combustion of fossil fuels, traffic and the use of synthetic fertilizers. More recent investigations show that both nitrogen and phosphorus originate mainly through the diffuse leaching of soils (NIXON 1995; BEHRENDT & BACHOR, pers. commun.; BACHOR 1996; LAUN 1993-1996), at least in Germany. From the atmosphere, especially reduced and oxidized nitrogen compounds are deposited on the sea surface, mainly as wet deposition (85% of the total deposition - HELCOM 1991; HELCOM 1996). Nutrient loads from sewage and the atmosphere are relatively evenly distributed over the seasons and from year to year, whereas diffuse phosphorus and, especially nitrogen loads show a pronounced seasonality and vary considerably from year to year in parallel with variations in precipitation and freshwater runnoff (NEHRING & AERTEBJERG 1996; BACHOR 1996; NAUSCH & SCHLUNGBAUM 1995; PASTUSZAK et al. 1996). This presentation uses two examples to describe the fate of nutrients after being discharged into coastal waters as well as some general features of the nutrient conditions in the Baltic Sea.

Regional approaches Introduction The nutrients nitrogen and phosphorus are discharged into the sea from the land via rivers and by direct communal and industrial sewage discharge. The nutrients transported by rivers originate partly from the natural leaching of soil but

The harmonized Danish monitoring programme provides spatial and temporal information on the land-use related distribution of nutrients and their effects on the aquatic environment (KRONVANG et al. 1993). The total nutrient discharge via rivers and direct coastal effluents to the two main marine waters surrounding Denmark, i.e. the North Sea and Skager0075-9511/99/29/03-233 $ 12.00/0

233

rak on the one side and the Kattegat, Belt Sea and Baltic Proper on the other, amounted to 128,400 t nitrogen and 4,490 t phosphorus in 1994, 75-79% of which were discharged into Baltic coastal waters. Riverine nitrogen loading by far exceeded the direct coastal input into the Danish part of the Baltic Sea area, constituting 91% of the total discharge. Phosphorus riverine loading accounted for 61% of the total load. Riverine loads of nitrogen and phosphorus to the coastal waters originated from various point and nonpoint sources. The most significant nitrogen input was derived from nonpoint sources, which accounted for 87% of the total load. Agriculture is the most important source of this nutrient. In contrast, point sources account for 55% of the phosphorus loading, most of it coming from sewage treatment plants in the catchment area of the Danish east coast. The anthropogenic phosphorus load increased until the 1970ies and since that decreased parallel to the development of phosphorus removal from sewage and, more recently owing to the introduction of phosphate-free detergents. Nitrogen loads from both terrestrial and airborne sources have been doubled from the 1950ies to the late 1970ies. A significant annual increase of 3.7% in the export of nitrate was observed in six Danish rivers draining mainly agricultural catchment areas during the period 1967-1978 (KRISTENSEN et al. 1990). In contrast, no significant trends were detected for the period 1978-1989 (KRISTENSEN et al. 1990). Winter nitrogen concentrations increased parallel to the general increase in riverine input until the end of the 1970ies (Fig. 1A). That is in good agreement with observations of nutrient variations in the open Baltic Sea areas (NEHRING & MATTHAUS 1991). As a result, the primary production of the phytoplankton doubled in the Great Belt from the 1950ies to the late 1970ies (Fig. 1B). As a consequence of this eutrophication, oxygen depletion has been observed nearly every year in the bottom water of the southern Kattegat, the Sound and the Belt Sea since 1981. The most serious oxygen deficiences occurred in the years after unusually high winter-spring runoffs: 1981, 1983, 1986, 1988, 1994 and 1995 (SEHESTED HANSEN et al. 1995). From the mid 1970ies to 1994 no general change was observed in the nitrogen load to the transition area (Kattegat, Belt Sea and Sound), but large interannual variations caused by the variations in freshwater runoff were recorded. The runoff is high in winter and low in summer, i.e. the nitrogen load is high during the non-productive season, when the nutrients accumulate in the surface layer. A significant correlation has been found between winter runoff from Denmark (October-January) and winter concentrations (JanuaryFebruary) of nitrate+nitrite in the surface water at various stations in the transition area for the period 1975-1994 (Fig. 2). The winter surface concentrations changed on average by 6% (0.45 pmol/dm 3) per 10% change in runoff. Similar relations can be shown for the Odra river (PASTUSZAK et al. 1996) and several German rivers (BACHOR 1996) as well as in inner coastal waters (NAUSCH & SCHLUNGBAUM 1995) 234

Limnologica 29 (1999) 3

A 10

°

I= 4hi

•

•

5

•

eoe

•

Z

• 0

.

.

.

.

1968

i

.

.

.

.

u

1973

.

.

.

.

i

1978

.

.

.

.

l

1983

,

1988

~ 1200- B

o

•

ell

. ~ ~

E,~ ~1000-

.

80060040013. >, 200E

"~ 13.

O1950

e~ * ~

~ 0

'

o"

g

•

w

1960

i

1970

1980

1990

Fig. 1. (A) Increase in the mean winter concentrations of nitrate (January to February) in the surface layer at Fladen in the western Kattegat during the period 1969-1989. (B) Increase in the mean summer primary production (July to September) at Halsskov Rev in the Great Belt during the period 1953-1989.

but not for the open Baltic Sea (NAUSCH & NEHRING 1996). The results of the Danish monitoring programme show that the 47% reduction in the phosphorus load from Denmark to the marine areas between 1989 and 1993 has resulted in significantly lower phosphorus concentrations in most Danish coastal waters. This tendency seems to exist in the Belt Sea and Kattegat as well. Nitrogen removal techniques have also been implemented at sewage treatment plants but has met only limited success in reducing the total nitrogen load to coastal waters. In the transition area, like in other parts of the Baltic Sea most of the nitrogen load originates from the leaching of arable land. Even the measures implemented to reduce nitrogen losses from agriculture have not resulted in any statistically detectable reduction of nitrogen losses from fields until 1994. This may be due to a delayed response owing to nitrogen pools in the soil or difficulties in normalizing the nitrogen load depending on the large variations in percipitation and freshwater runoff from year to year.

,,-I ,uanol/dms

Nitrate ( + Nitrite)

1986 1983

10

1980 1985 '~ 1 9 •

6 -

* /

1~4

1976

1977~ 4-

Fig. 2. Mean January/February concentrations of nitrate (+nitrite) in the 0-10 m layer in the southern Kattegat and Great Belt as a function of the October to January fresh water runoff from Denmark to the Kattegat and Belt Sea during 1975-1994.

'~'

1981

~1~.19't92 '~ 1987 1979

19~84 19~91

2 -

Runoff (Oct - Jan) 0

1000

I

I

2000

3000

A slightly different approach has been used to study the ecology, biogeochemistry and transport of matter in the Pomeranian Bight in the southern Baltic Sea (voN BODUNGEN et al. 1995). Dense station grids which are sampled as frequently as possible allow the distribution patterns to be described. Around 90% of the drainage to the Pomeranian Bight comes from the catchment area of Szczezin Lagoon. The main supplier of water to the Bight is the Odra river. The total outflow calculated for the years 1951-1960 was 560 m3/s or 17.6 km~/year, respectively. Up to 2/3 of the total annual freshwater discharge enters the coastal waters from December to May. Nutrient discharges from the Odra river vary both seasonally and from year to year. The comparison of the Odra water discharge and the weekly nutrient loads entering Szczezin Lagoon show a very good correlation, particularly for inorganic nitrogen compounds (PASTUSZAK et al. 1996). Inorganic nitrogen species prevail in the outflowing water in winter, spring and autumn, whereas organic compounds predominate in summer. In the case of phosphate, the correlation between water discharge and nutrients seems to be more diversified. The total annual input to Szczezin Lagoon varies between 40,000 and more than 80,000 t N and 4,500 to 8,000 t P (PASTUSZAK et al. 1996; DAHLKE et al. 1995). The buffering capacity of the lagoon itself is not yet completely known, but sediment studies imply that only 500 t N/year and <90 t P/year are deposited in the whole lagoon (DAHLKE et al. 1995; MEYER & LAMPE 1996). Around 70% of the outflow from the Szczezin Lagoon is realized via the Swina. It has been shown that the outflow has a pulselike character with a typical frequency of 3-4 days. The distribution of the nutrient load in the Pomeranian Bight can best be described during winter, when biological activity is low. In this season, the horizontal distribution patterns

t

4000

I

I

5000

6000

are determined exclusively by the hydrographic conditions and thus resemble the distribution of salinity. The outflowing lagoon water also leads to vertical stratification but, regardless of the circulation patterns, the layers are mixed by wind action after around 10 km distance from the mouth of the Swina. Fig. 3 shows the distribution of nitrate (+nitrite) in January 1995. Two pulses of water from the lagoon can be observed. The freshwater masses are transported along the coast of the island of Usedom due to the prevailing winds from south to southeast. Ekman transport widens the plume and deflects it towards the open bight. The water is replaced by denser water masses from the depth of the Arkona Basin. The nitrate concentration at the centre of the plumes reaches values of around 40 pmol/dm 3 (Fig. 3b). Three days earlier, when the outflow of the first plume was observed, it was about 60 ~mol/dm 3 (Fig. 3a). The concentrations in the offshore area of the Arkona Sea varied between 4 and 6 pmol/ dm 3 at that time (NEHRING et al. 1996). The distribution of phosphate (Fig. 4), silicate and ammonia show exactly the same patterns: highest concentrations in the plume, northward transport along the coast and thereafter deflection of the plume towards the centre of the bight accompanied by mixing and decreasing concentrations. Seasonal statistics show that east and south winds like those causing the described distribution pattern, are quite common in spring when freshwater runoff is highest. Westerly winds predominate during most seasons. Under these conditions, the outflowing water masses are transported in a narrow band along the Polish coast again due to Ekman transport. Satellite images (VON BODUNGEN et al. 1995) and results of numerical models show that this band can be traced as far as the Bay of Gdansk, if west winds are sufficiently persistent. The nutrients discharged are immediately transformed into biomass as biological activity is high in summer. During this season, really high nutrient concentraLimnologica 29 (1999) 3

235

Nitrate + Nitrite horizontal distribution 3.0 m depth 58.00 56.00 54.00 52.00 50.00 48.00 46.00 44.00 42.00 40.00 38.00 36.00 34.00 32.00 30.00 28.00 26.00 24.00 22.00 20.00 18.00 16,00 14,00 12,00 10,00 8.00

Z

4

13.00

13.50

14.00

14.50

15.00

15.50

longitude [grd East]

Nitrate + Nitrite horizontal distribution

[gmol/dm3]

3.0 m depth 38.00 38.00 34.00 32.00 30.00 28.00 26.00

Z

24.00 22.00 20.00 ~

18.00 16,00 14.00 1ZOO

2;2 0 6.00 13.00

13.50

14.00

14.50

longitude [grd East]

15.00

15.50

4.00

[gmol/dm3]

Fig. 3. Horizontal distribution of nitrate (+nitrite) in the surface layer of the Pomeranian Bight in January 1995 - observation of two subsequent grids. (IOW 1996, Sektion Chemie.) 236

Limnologica 29 (1999) 3

Phosphate horizontal distribution

3.0 m depth

54.80 t

1.50 ~ A

1.40 1,30 1.20 1,10 Z

1.00 0.90

4

0.80 0.70 0,60 0.50 0.40 13.00

13.50

14.00

14.50

15.00

0,30

15.50

longitude [grd East]

[gmol/dm3]

Fig. 4. Horizontal distribution of phosphate in the surface layer of the Pomeranian Bight in January 1995. (IOW 1996, Sektion Chemie.)

tions can be found only in a narrow band along the Polish coast (VONBODUNGEN et al. 1995). Summing up, two different situations can be described in the Pomeranian Bight. The winter and early spring are characterized by high freshwater runoff. Nutrients are transported mainly in inorganic forms and high concentrations. They are diluted by hydrographic processes. These nutrients finally support the eutrophication process in the Baltic Sea. In summer the freshwater runoff is low. Elevated nutrient concentrations can be detected only in nearshore coastal waters. The nutrients are mainly transported as particulate organic matter and reach the deeper sedimentation basins of the Arkona and Bornholm Sea after repeated biogeochemical transformations.

Long-term variations in nutrients The annual nutrient input from the catchment area of the Baltic Sea has been estimated to be 662 kt N and <45.8 kt P from direct and diffuse sources (HELCOM 1993). For nitrogen, an additional amount of 300 + 30 kt N from atmospheric depositions must be taken into account, whereas airborne phosphate loads are only of minor significance (HELCOM 1991). Large amounts of organic matter, up to 1,400 kt as BOD 7, also enter the Baltic Sea. Table 1 gives an overview of the land-based inputs to various regions of the Baltic Sea. Nutrient trends are studied in the surface layer in winter when biological activity is low and nutrient concentrations

are high (NEHRING & MATTHAUS 1991). This method is based on the assumption that in winter a steady state has developed between microbial mineralization, low biological productivity, and vertical exchange and mixing. The phosphate and nitrate concentrations are characterized by a pronounced seasonality and often decrease near to the detection limit during the period of high biological productivity beginning in spring and ending in late autumn (Fig. 5). The high standard deviations in winter are also caused by positive overall trends as will be shown later. The seasonality of silicate is much less pronounced. In the long-term mean, the maximum of phosphate and nitrate accumulation is reached in the surface layer of the Arkona and Bornholm Sea in February. The winter plateau is best developed in the eastern Gotland Sea between mid-Jan-

Table 1. Input of nutrients and organic matter (kt) in the Baltic Sea (HELCOM 1993). Total-P

Total-N

BOD7

Gulf of Bothnia Gulf of Finland Gulf of Riga B altic Proper Belt Sea Kattegat

5.4 11.8 3.4 > 17.8 4.7 2.7

96 140 85 209 62 69

258 286 142 >609 > 66 > 39

Baltic Sea

>45.8

662

>1400

Limnologica 29 (1999) 3

237

~ ., ""~'',;, -,.-=0 ® o

E~ :

°o

,'.d"

a_

v

-;~,

,.

=

'\

0

I'

i),'

o

4~ JOE

,

/

i1" ~

S' /

.~

~

8_Lup IOLUH

~_uJp IOta#

~_uW iomd

E

e~

ol

N

m

.~

",, ~'~

"o

r

-43

~,

!

.O

-~

t

I

0

e4

,,i.

~

I

(3"-, D-.-

I'

(.~

=

.

I

,,,

¢

.~¢:

& © o t

%

~

IM

,.cZ

O

"0

\

O

O

O

8_Lup Iomrf

o

~"'--\.

.o

~"

r-

O

I

8_uJp IOLUH

~mp IoLur/

O

i

ol

0

"~ =

!

I

I:

o

;/;

E~

o

m,__.

,

'

'

t

o,,~E

-

~ ~ ,7

#I

I ',

/ ,

O

0

-{

/

|

~

r

0

i

o

e.

o

! c_uJp IOUJd

,-

\

,~ c'-

m

'

t

'~

: ~

,),

]/~

,o{

~

°

~

,z

~9

"4t

C~4

0

r,&

I

;)' ~4

f

" ~f

.~

x

L i m n 0 1 o g i c a 29 (1999) 3

c_LUp IoLur/

0

i

°~ o '!..! m= I=

L

!

I~, I/) "

!

(

%"1 I

S)

!

~

N

O O O O ~,~ O O O

8_tup IowH

238

;i I

7'n.F.

I

~)

"~)~

o

Q 0

£jup iocur/

\ x

~ ~-

e_u]p lOW#

u~ cLap IOtUr/'

Arkona 1.2

P04

1,0

Jan-,ar

0.8

Basin S,O '

' '

; : o.~,,, - ,,.,,11

"o

l.. ". i

.

-

NOs (.o,) Jan-F11b

,

: ~

Y - 0,0272X - 49.619

.-

-6 0.0 E

•

, ':'il

0,2.

I

"

•

I

.

2.0

I

•

":ll

:,+

.-'+

E

i

|

",. ,• :j I ,.':il

o.,,

'5 4.0

0,4

,

6.0

I

0.~

5 IgkO 1965 'l'gYO"'l'gYS'"lg~O"'l'9~5"l'g~b"('g~S

Bornholm ,,4

P04

1.2

Jan-Mar

Basin IO.O

NOz (NO,) Jan-Mar

8.0

1.0

'P+ o.e

,,

Y - o.°,+x - ,+.~o,

r

O+4S

.

41 .

I

l

I

.

*

I II . ~ '| . I

i I

-I

I

I

"

•

~.~ | t l.¢.J

I I

0.4

Y - 0.0~52X - 16'~.~t

?E 6 . 0

•

L°t

•

o.%!

•

,

I I

•

2.0

e1

O'O'+'S~+"+i#e~" ",'9~S' ie~+"

Eastern 1.2

1.0 0.8

'E -60.e

~+~s

I+,o ,+'~+ ++,'+~+""~'+'+s

Garland

8.0 +

I

•

. •

i~!~

~.;,

"

Landsort

0.2

"o

4,0

. o+,

• 0,{~

,"

|'

,

III

. : . : I ' , ' ~

E

' 1990

"o o ~0.4.

. -

•

2.0

1::

~E 0.6.

o

6.0 '

.J . ,It.it

0.~ '

0.0

NO~ (.o,) Jan-Apr

• ..: !I

0.4

1.0

•

+k' 'i+'~i)" 'is++" "i+~i)", 1+~+, '+',~i)' "f,+~s' 'f+++' li~'+s

P04

..o,,

11

Basin

F11b-Apr + - o.mT,x -m.4711

l I

"

01

"

t

i!t+

'

t9 '5' ','9'+o" " i+'e+' " i#7+" " i+'TK " ig+b'

",'~8~" ' i¢+o" ,:+g.

Deep

P04

,

Jon-Mar

to.o 8.0

Y - 0.0202x - 30.037 r o.11t ~

• ii

* ;

~ tl.0 "6

NO3 (No,) Feb-Mar ¢ Y " 0,1515X-- 21S,0811 / r ~ 0.67

~

4.0 I

!

2.0

0.0 11

Fig. 6. Trends of phosphate and nitrate (+ nitrite) winter concentrations in tile 0-10 m surface layer of the main sub-regions in the Baltic Proper. L i m n o l o g i c a 29 (1999) 3

239

uary and early April (NAUSCH & NEHRING 1996). Only these periods with the highest nutrient concentrations should be used for trend studies in connection with eutrophication. Significant positive overall phosphate and nitrate trends have been identified in the surface layer of all regions of the Baltic Proper (Fig. 6). They mainly result from the considerable increase between 1969 and 1978 as it could be shown for the Belt Sea and Kattegat area as well. In contrast to phosphate, nitrate continued to increase until 1983. Thereafter, the concentrations of both nutrients fluctuated strongly around a high mean except in the Landsort Deep area, where phosphate and nitrate winter concentrations continued to increase until 1993 (NAUSCH & NEHRING 1996). Fertilizers consumed in the catchment areas are the most important source of eutrophication in shelf seas (NIXON 1995). Large amounts of fertilizers are retained in the soils or lost by denitriflcation. Only a small fraction reaches the coastal zone after transformation by various biogeochemical and technological processes. The amounts of phosphorus and nitrogen fertilizers applied annually in the catchment area of

the Baltic Sea are compared with the phosphate and nitrate winter concentrations in the surface layer of the Baltic Proper averaged over periods of five (eleven) years in Fig. 7. The different curves for fertilizers represent the total catchment area, the catchment area without the former USSR and the new Baltic States and finally without Poland. The strong increase in fertilizer consumption that started in the early 1960ies is followed by increasing phosphate and nitrate winter concentrations after a delay of 5-10 years. If this delay is taken into account, the correlations are obvious. The drastic reduction in fertilizer consumption, mainly caused by the great economical changes in the countries of the former East Bloc, began in the late 1980ies and is thus not yet significantly reflected in decreasing nutrient concentrations. But first signs are already becoming apparent in the averaged nutrient concentrations (Table 2). Partly, they show that nutrient levels are tending to decrease in central Baltic surface waters. This behaviour is more pronounced in the Arkona and Bornholm Basins where stations are closer to the coast, than in the eastern Gotland Basin with its more offshore stations.

2500 [ •

1.6 Catchment area without USSR, New Baltic States, Poland

kt/a ©

'din3 ~,

2000

I'D

1,2. ~m o

t,l

1500

l:r'

0.8 1000

0.G o

0.4

n

0.2

gt ~t.

500

0 1950/5I

1955/56

1960/61

1965/66

1970/71

1975/76

1980/81

1985/86

o

1990/91

4OOO

I 10 . . . . ,idm 3

kt/a 3000

o.

2500 2000 OJ

o

1500

o t'l

1000 g.

500

o

0 1950/51

240

1955/,B6

1960/61

1965/66

Limnologica 29 (1999) 3

1970/71

1975/76

1980/81

1985/86

1990/91

Fig, 7. Consumption of synthetic phosphorus and nitrogen fertilizer in the drainage area of the Baltic Sea, pro-rated by drainage area (lines) and 5 (11) years averaged phosphate and nitrate winter concentrations in the 0-10 m surface layer of the Bornholm Basin.

Table 2. Five (eleven) year averaged winter concentrations of phosphate and nitrate (,umol/dm 3) in the surface layer (0-10 m) of main subregions in the Baltic Proper [exact position of stations cf. NEHRINGet al. (1996); number of data available*) in parenthesis.]. Period

1958/68

1969/73

1974/78

1979/83

1984/88

1989/93

A r k o n a Sea (average of Stat. 069, 109, 113) Phosphate (Jan-Mar) 0.26 + 0.05 (5) Nitrate (Jan-Feb) 2.68 _+0.14 (4)

0.49 _+0.12 (16) 3.93 + 0.78 (14)

0.48-+0.11 (50) 4.61+ 0.83 (20)

0.67+0.09 (52) 4.74-+0.46 (35)

0.61 _ 0.17 (155) 4.17_+ 0.79 (84)

Bornholm Sea (average of Stat. 200, 213,214) Phosphate (Jan-Mar) 0.32 + 0.15 (22) Nitrate (Jan-Mar) 0.69 _+0.37 (3) 2.08 _+0.74 (34)

0.35 + 0.12 (50) 3.16 + 0.56 (46)

0.56__+0.14(46) 3.89+0.94(53)

0.67+0.19 (88) 4.37-+1.16 (90)

0.64__+0.18 3.90__+0.83

Eastern Gotland Sea (average of Stat. 250,. 255,259, 260, 270, 271) Phosphate (Feb-Mar) 0.27 _+0.09 (24) 0.26 + 0.14 (95) 0.54 -+0.09 (61) Nitrate (Jan-Apr) 0.97 + 0.36 (5) 2.44 -+0.40 (75) 3.81 + 0.94.(61)

0.59+0.16(60) 4.15 + 0.93 (60)

0.60+0.15(125) 4.30__+0.99 (132)

0.67 _+0.11 (182)

(89)

(89)

4.23 + 1.28 (200)

*) Mainly from Germany but also from Sweden, former USSR, IBY and HELCOM.

Decreasing nutrient loads may be expected in the Baltic Sea in future due to reductions in the fertilizer consumption and the improvements in the waste water treatment in the catchment area. For this positive development to continue, the measures recommended by H E L C O M for the protection of the Baltic Sea must be consistently implemented by all contracting parties.

References BACHOR, A. (1996): N~ihrstoffeintr~igeaus Mecklenburg-Vorpommere in die Ostsee 1990-1995. Wasser & Boden 48 (8): 33-36. BODUNGEN, B. VON, GRAEVE, M., KUBE, J., LASS, U., MEYER-HARMS, B., MUMM, N., NAGEL, K., POLLEHNE, E, POWILLEIT, M., RECKERMANN, M., SATTLER, C., SIEGEL, H. & WODARG, D. (1995): Stoff-Fltisse am Grenzflu6 - Transport- und Umsatz-Prozesse im Llbergangsbereich zwischen Oder~istuar und Pommerscher Bucht (TRUMP). Geowissenschaften 13 (12): 479-485. DAHLKE, S., LAMPE, R., MEYER, H., MUSIELAK, S. & WESTPHAL, H. (1995): Niihrstoffe im Oder-A.stuar: Zur Funktion des Gewassers als Transformator. GOAP (Greifswalder Bodden und Oderiistuar - Austanschprozesse). Report Statusseminar, Warnemtinde, Dezember 1995. HELCOM (1991): Airborne pollution load to the Baltic Sea 1986-1990. Baltic Sea Environment Proceedings 39. - (1993): Second Baltic Sea pollution load compilation. Baltic Sea Environment Proceedings 45. - (1996): 3. Atmospheric input. In: Third Periodic Assessment of the State of the Marine Environment of the Baltic Se~, 1989-1993. Background Document, Baltic Sea Environmen{Proceedings 64B, pp. 23-26. KRISTENSEN, P., KRONVANG, B., JEPPESEN, E., GRAESBOLL, P., ERLANDSEN, M., REBSDORF, A., BRUHN, A. & SONDERGARD, M. (1990): Freshwater regions - rivers, springs and lakes. The monitoring program of the action plan on the aquatic environment 1989. National Research Institute, Denmark. Technical Report 5, 1-130 (in Danish) [cited in KRONVANG et al. (1993)].

KRONVANG, B., AERTEBJERG, G., GRANT, R., KRISTENSEN, E, HOVMAND, M. & J. KIRKEGAARD (1993): Nationwide monitoring of nutrients and their ecological effects: State of the Danish aquatic environment. Ambio 22 (4): 176-187. MEYER, H. & LAMPE, R. (1999): The restricted buffer capacity of a south Baltic estuary. Limnologica 29: 242-248. LAUN, 1993-1996. Coastal water reports. Landesamt fiir Umwelt und Natur, Mecklenburg-Vorpommern, Stralsund. NAUSCH, G. & NEHRING, D. (1996): 4.4 The state of the marine environment of the Baltic Proper. In: HELCOM (1996), Third Periodic Assessment of the State of the Marine Environment of the Baltic Sea, 1989-1993. Background Document, Baltic Sea Environment Proceedings 64B, pp. 75-104. - & SCHLUNGBAUM, G. (1995): N~ihrstoffdynamik in einem flachen Brackwassersystem (DarB-Zingster Boddengewiisser) unter dem Einflug variierender meteorologischer und hydrographischer Bedingungen. Bodden 2: 153-164. NEHRING, D. & AERTEBJERG, G. (1996): Verteilungsmuster und Bilanzen anorganischer Niihrstoffe sowie Eutrophierung. In: LOZAN, J.L., LAMPE, R., MATTHAUS, W., RACHOR, E., RUMOHR, H. & WESTERNHAGEN, H. YON (Hrsg.), pp. 61-68. Wamsignale aus der Ostsee. Berlin. - & MATTHAUS, W. (1991): Current trends in hydrographic and chemical parameters and eutrophication in the Baltic Sea. Int. Revue ges. Hydrobiol. 76:297-316. - - LASS, H.U., NAUSCH, G. & NAGEL, K. (1996): Hydrographisch-chemische Zustandseinsch~itzung der Ostsee 1995. Meereswiss. Bet., Warnemande 16: 1-43. NIXON, S.W. (1995): Coastal marine eutrophication: A definition, social causes, and future concerns. Ophelia 41: 199-219. PASTUSZAK, M., NAGEL, K. & NAUSCH, G. (1996): Variability in nutrient distribution in the Pomeranian Bay in September 1993. Oceanologia 38 (2): 195-225. SEHESTED HANSEN, I, AERTEBJERG, G. & RICHARDSON, K. (1995): A scenario analysis of effects of reduced nitrogen input on oxygen conditions in the Kattegat and the Belt Sea. Ophelia 42: 75-93. A d d r e s s for correspondence: Dr. GONTHERNAUSCH,Institut ftir Ostseeforschung an der Universit~it Rostock, Meereschemie, P.O. Box 301038, D - 18118 Rostock, Germany; Fax: +49/381/5197440; e-marl: guenther'nansch@i°-wamemuende'de

Limnologica 29 (1999) 3

241