Eutrophication in the Polish coastal zone: the past, present status and future scenarios

www.elsevier.com/locate/marpolbul

Marine Pollution Bulletin 49 (2004) 186–195

Eutrophication in the Polish coastal zone: the past, present statusand future scenarios

E. Łysiak-Pastuszak *, N. Drgas, Z. Piaztkowska

Institute of Meteorology and Water Management, Maritime Branch, Al. Waszyngtona 42, 81-342 Gdynia, Poland

Abstract

In the Baltic Sea eutrophication processes have accelerated in the past 50 years of the 20th century and presently there exists a

major ecological problem for this sea. The Polish coastal zone of the southern Baltic Sea is the recipient of riverine inputs from two

major sources, namely the Odra and Vistula, as well as a number of smaller rivers along the central coast. Hence, the entire coastal

zone remains under severe anthropogenic pressure. The variability of nutrient concentrations, especially the winter nutrient pool in

the euphotic zone, summer level of total nitrogen and total phosphorus, together with such eutrophication indicators as water

oversaturation with oxygen and the summer oxygen minimum, were analysed in the data time series 1959–2001. The temporal trends

were investigated using linear regression and the non-parametric Whirsch test. The future characteristics of the Baltic Sea are

discussed taking into account the development of driving forces.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Eutrophication; Temporal trends; Coastal zone; Baltic Sea

1. Introduction

The Baltic is a semi-enclosed brackish water seasurrounded by several countries, some of them highly

industrialized and with a total human population

amounting to 85 million. The drainage area of the Baltic

is fourfold greater (1.745 · 106 km2) than the area of the

sea itself (0.48 · 106 km2) and 20% of the land area is

used for arable farming (Wulff et al., 1990; Sweitzer

et al., 1996; Jansson, 1997). These facts are directly re-

lated to the degradation of natural environment of theBaltic Sea, and particularly its coastal zones. The deg-

radation is caused by excessive input of phosphorus and

nitrogen compounds, dissolved and suspended organic

matter as well as toxic substances resulting from the

human activities (HELCOM, 1987, 1990, 1993b, 1996,

2002).

The increasing loads of nutrients to seawater have

caused eutrophication; a process bringing about, anapparently positive rise in biological production, and

negative effects, e.g. decreased water transparency, in-

creased oxygen demand on soft sediments due to in-

* Corresponding author. Fax: +48-58-6288-163.

E-mail address: [email protected] (E. Łysiak-

Pastuszak).

0025-326X/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2004.02.007

creased deposition of organic matter and the growth of

macrophytes. In addition, increased blooms of toxic

algae have become a growing concern because of thethreat to both human health and the economy (Rydberg

et al., 1990; Hansson and Rudstam, 1990). The input of

nutrients into marine waters include riverine discharges,

point sources on the coast, atmospheric deposition and

nitrogen fixation. The estimated annual nutrient inputs

into the Baltic Sea are indicated in Table 1 (MARE,

2001). Agriculture and forestry are responsible for the

major part of the input from rivers. Industry and sewageplants constitute major point sources, and road trans-

port and combustion contribute the major part of

atmospheric deposition (HELCOM, 1993a). Nitrogen

fixation by cyanobacteria is stimulated by high phos-

phorus concentrations (MARE, 2001).

The Polish coastal zone (Fig. 1) has strongly differ-

entiated regions regarding its nutrient loading (IMGW,

1987–1999, 2000–2001). Inputs are dominated by inputsfrom the Odra and Vistula as well as a number of

smaller estuaries of the Pomeranian rivers (see Fig. 1).

The coast line also has several lakes with outlet channels

discharging directly to the sea. The mean annual dis-

charge of the river Odra is 15 km3 of water to the

Szczecin Lagoon, where its composition is modified, and

finally it flows into the sea via three outlets: �Swina (75%

mail to: [email protected]

Fig. 1. The network of measurement stations of the Institute of

Meteorology and Water Management in the southern Baltic Sea be-

tween 1959 and 2001.

Table 1

Major estimated annual inputs of nitrogen and phosphorus into the

Baltic Sea (MARE, 2001)

Agency Period N (t a�1) P (t a�1)

Riverine 1980–1993 830 000 41 000

Coastal point

sources

1990 100 000 13 000

Atmospheric

deposition

1985–1989 300 000 5 500

Nitrogen fixation 1980 130 000

Total 1 360 000 59 500

E. Łysiak-Pastuszak et al. / Marine Pollution Bulletin 49 (2004) 186–195 187

of the total outflow), Dziwna (15–25%) and Piana (10%)

(Meyer and Lampe, 1999). The river Vistula, having the

second greatest outflow to the Baltic Sea, discharges into

the Gulf of Gda�nsk by a single artificial channel. Be-

tween 1951 and 1989 the estimated mean annual outflow

from the Vistula was 35.3 km3 (Cyberski, 1992). The

mean annual volume of water discharged by the smaller

Pomeranian rivers falls within the range 0.1–0.9 km3,with a mean value of 0.46 km3 (IMGW, 1987–1999,

2000–2001).

The estimated mean load of dissolved reactive

phosphate (P–PO4) and nitrogen salts (nitrate, nitrite

and ammonia identified as N–Nin) discharged from the

river Vistula was 70,770 tN–Nin a�1 and 10,210 t P–

PO4 a�1 and from the Odra was 48,880 tN–Nin a�1 and

5920 t P–PO4 a�1, and from the Pomeranian rivers 907tN–Nin a�1 and 111 t P–PO4 a�1 (IMGW, 1987–1999,

2000–2001). This amounts to total annual input of

about 120,000 t of inorganic nitrogen and 16,000 t of

phosphorus from riverine sources.

This paper presents an analysis of the variability in

indicators of eutrophication in the Polish coastal zone of

the southern Baltic Sea between 1959 and 2001. We

characterise the most degraded and endangered regions

at present, as well as discuss measures leading to

reduction in nutrient outflow and possible effects.

2. Material and methods

The environmental indicators that receive the

‘‘greatest weight’’ when assessing the level of eutrophi-

cation are (a) winter nutrient concentrations in surface

waters (0–10 m), (b) late summer concentrations of total

phosphorus and total nitrogen in surface waters, (c)water transparency, minimum of oxygen content in near

bottom waters in late summer and (d) biological vari-

ables, e.g. chlorophyll-a, macrovegetation and soft bot-

tom fauna (SEPA, 2000; Łysiak-Pastuszak and Drgas,

2002). In this paper only the winter concentrations of

phosphate, nitrate + nitrite (TOxN) and oxygen mini-

mum values were considered.

The analysis was carried out on data collected in thePolish sector of the southern Baltic Sea between 1959

and 2001, with an average frequency of 7–12 times a

year at the network of stations shown in Fig. 1. Between

1979 and 2001 the data were collected under the

framework of various HELCOM Baltic Monitoring

Programmes and the national oceanographic service of

the Institute of Meteorology and Water Management

(IMGW) while the earlier data were acquired by thenational oceanographic service and other scientific

oceanographic projects (Majewski and Lauer, 1994).

There is an agreed consistency in the analytical tech-

niques for the consecutive stages of the HELCOM

Baltic Monitoring Programme (HELCOM, 1984, 1988,

1997). The analytical determination of nutrients is based

on established methods, including colorimetric deter-

minations (Grasshoff, 1976).The linear regression of temporal trends were evalu-

ated using commercial software (STATISTICA for

Windows) and the WHIRSCH.EXE a non-parametric

test (Hirsch et al., 1982; Trzosi�nska and Łysiak-Pastus-

zak, 1996; Łysiak-Pastuszak, 2000).

3. Results and discussion

A characteristic feature of eutrophication is a rise in

nutrient concentrations in the marine troposphere dur-

ing winter accumulation (Nixon, 1995), hence nutrient

trends are studied in the surface 0–10 m layer in winter

when biological activity is low (HELCOM, 1990). This

approach is based on the assumption that in winter a

steady state develops between microbial mineralisationand vertical exchange and mixing. The five-year mean

phosphate and nitrate + nitrite winter concentrations in

the surface sea layer in individual regions of the Polish

coastal zone: the Pomeranian Bay (Pom), central Polish

Fig. 2. Five-year mean winter concentrations of phosphate (a) and TOxN (b) in the surface layer of the Polish coastal zone and in the SE Gotland

Deep, data series from 1959 to 2001.

188 E. Łysiak-Pastuszak et al. / Marine Pollution Bulletin 49 (2004) 186–195

coast (Stpr) and the Gulf of Gda�nsk (Gda) are presented

in Fig. 2a and b. For comparison the correspondingvalues are shown from the off-shore region of the south-

eastern Gotland Deep (SE Gotl-station P140¼BMP

K1), considered as a relatively undisturbed area of open

Baltic waters (HELCOM, 2002).

Although the time–space coverage in measurements

differed between the regions, the overall trends are

clearly evident. Not surprisingly, there is evidence for a

rapid increase in winter concentrations of phosphate inthe Gulf of Gda�nsk and Pomeranian Bay during the

1960s and 1970s and a less pronounced rise of these

nutrients in the coastal zone along the central Polish

coast as well as in the SE Gotland Deep (Fig. 2a). A

decline in winter phosphate was observed in the Gulf of

Gda�nsk in the late 1980s while the surface water

of Pomeranian Bay still showed an accumulation of

phosphate. Along the central Polish coast stabilisationof winter phosphate concentrations was observed in the

early 1990s. As for the five-year mean values for the SE

Gotland Deep, a considerable inter-annual variabilitymasks any trend.

As regards oxidised nitrogen forms (TOxN), the five-

year mean winter concentrations (Fig. 2b) showed a

continuous increase in the surface sea layer of all the

regions, including the off-shore area of the SE Gotland

Deep, from the end of the 1960s until the turn of

the 1980s and 1990s [Note: the analytical procedure for

nitrate determination by cadmium column reductionand colorimetric measurement of the reduced nitrite

was implemented routinely since 1969/1970 (Majewski

and Lauer, 1994)]. Since then a gradual decrease in

the winter accumulation of nitrogen salts in all the

examined regions of the Polish sector has been ob-

served.

The phasing of eutrophication processes in the Polish

coastal zone is shown in the raw data (Figs. 3–6). Theresults of regression analyses are shown in the figures

Fig. 3. Linear regression trends in phosphate (a) and TOxN (b) concentrations in the surface (0–10 m) layer of the Pomeranian Bay; data series 1969–

2001; full line and bold fonts denote statistically significant trend coefficients (mmolm�3 a�1).


representing the temporal trends in winter and year-

round concentrations, where statistically significant

trend coefficients (t-Student test) are marked bold.

Estuaries are the main conduits of the nutrients to the

sea, hence, the accumulation of phosphate in the surface

water layer in winter proceeded rapidly in the Pomer-

anian Bay prior to 1985 (tgPO4a ¼ 0:12 mmolm�3 a�1)

and in the Gulf of Gda�nsk prior to 1980 (tgPO4a ¼ 0:04mmolm�3 a�1) (Figs. 3a and 5a). The corresponding

values for TOxN were tgTOxNa ¼ 1:39 mmolm�3 a�1 in

the Pomeranian Bay prior to 1988 and tgTOxNa ¼ 0:78mmolm�3 a�1 in the Gulf of Gda�nsk prior to 1984 (Figs.

3b and 4b). Along the central Polish coast neither

phosphate nor nitrate showed statistically significant

trends prior to 1985 (Fig. 4a and b). The effects of

eutrophication in the offshore region of the SE GotlandDeep were manifested by a considerable time delay

and moderate positive trends in phosphate (tgPO4a ¼0:02 mmolm�3 a�1) can be detected in the data series

up to 1988, and in nitrogen salts (tgTOxNa ¼ 0:07mmolm�3 yr�1) up to 1990 (Fig. 6a and b).

The detection of temporal trends in nutrient con-

centrations by linear regression is often controversial

and for this reason a non-parametric test (Sand�en and

Rahm, 1993; Trzosi�nska and Łysiak-Pastuszak, 1996;

Łysiak-Pastuszak, 2000) was carried out on the data

series. The test, applying Kendall’s s, is robust to non-Gaussian distributions, missing measurements and sea-

sonality. The results of trend analysis using this test are

listed in Table 2. The results of trend analysis by both

methods are in good agreement. The data presented in

Figs. 3–6 and in Table 2 point out that by the end of the

1980s, the steep positive trend coefficients in winter

nutrient accumulation decreased and in the early 1990s

even changed direction. After 1988, a negative (statisti-cally significant) trend in winter phosphate concentra-

tions was found in all regions of the Polish coastal

zone and SE Gotland Deep. The nitrogen salts are

Fig. 4. Linear regression trends in phosphate (a) and TOxN (b) concentrations in the surface (0–10 m) layer of the coastal zone along the central

Polish coast; data series 1962–2001; full line and bold fonts denote statistically significant trend coefficients (mmolm�3 a�1).


transported into the sea mainly from non-point (dif-

fused) pollution sources and by the rivers (IMGW,

1987–1999; HELCOM, 1998; IMGW, 2000–2001). They

are, therefore, much more difficult to manage than

phosphate, and started to show decreasing trends in

areas distant from the river mouth (Fig. 4b) only since

1986, and off-shore in the SE Gotland Deep (Fig. 6b)

since 1991. Even in the areas immediately exposed to theriverine outflow, negative trend coefficients were found

since 1994, not statistically significant in the Pomeranian

Bay (Fig. 3b) and highly significant (R ¼ �0:44,p < 0:05) in the Gulf of Gda�nsk (Fig. 5b).

Fertilizers applied in the catchment areas are a

dominant source of eutrophication in shelf seas (Nixon,

1995). Large amounts of fertilizers are retained in the

soils or lost by denitrification but only a small portionreach the coastal zone after transformation by various

biogeochemical and dynamic processes. The amounts of

phosphorus and nitrogen fertilizers applied annually in

the catchment area of the Baltic Sea were compared with

the winter phosphate and nitrate concentrations by

Nausch et al. (1999). The comparison showed that the

strong increase in fertilizer consumption that started in

the early 1960s was followed by an increase in winter

phosphate and nitrate concentrations with a delay of

about 5–10 years, depending on the region. A drastic

reduction in fertilizer consumption, mainly caused by

economic changes in the countries of the former EastBlock, having commenced in the late 1980s, hence the

decreasing trends observed in the sea since mid-1990s.

The overall decreasing trends in phosphate and ni-

trate + nitrite concentrations in surface water along the

central Polish coast (Fig. 4a and b) reflect the effect of

prevailing eastward currents along the Polish coast

which transport nutrients into the area of the SE Got-

land Deep and Gda�nsk Deep (IMGW, 1987–1999;Jankowski, 1996). A new stagnation period in the deep

basins of the Baltic Proper (Nehring et al., 1995;

Matth€aus, 1999) can be considered as another probable

cause for the decreasing trends in nitrate concentrations

Fig. 5. Linear regression trends in phosphate (a) and TOxN (b) concentrations in the surface (0–10 m) layer of the Gulf of Gda�nsk; data series 1959–

2001; full line and bold fonts denote statistically significant trend coefficients (mmolm�3 a�1).


in the surface water because of the excessive denitrifi-

cation in vast areas of oxygen depletion. This causeseems more realistic because the trends in nitrogen and

phosphorus loads in the major Polish rivers are not

equivocal (IMGW, 1987–1999; HELCOM, 2002). A

contribution to the reduction of winter nutrient accu-

mulation is presumably the result of measures under-

taken inland, regarding the waste water treatment and

water management in general according to the Polish

legislation (Decree of the Minister of EnvironmentalProtection, Natural Resources and Forestry on classifi-

cation and conditions of waste water to be discharged

into the surface water or ground from 5 November 1991,

in Polish).

A decreasing trend in winter nutrient accumulation is

the first sign of the slowing of eutrophication in the

Baltic Sea. However, the current status of this process in

the Polish coastal zone shows a highly degraded Gulf ofGda�nsk and Pomeranian Bay and a more stable eco-

logical situation along the central coast. The rich inter-

nal source of nutrients, the amounts supplying primaryproduction after remineralisation of decaying phyto-

plankton from the preceding productive season, to-

gether with the external loading sources sustain the high

level of eutrophication (Wasmund et al., 2001) and its

adverse effects persist. This observation is confirmed by

the frequency of low oxygen concentrations, classified as

oxygen depletion (<2.8 cm3 dm�3) and strong oxygen

depletion (<1.4 cm3 dm�3), which occurred in the nearbottom water of the respective Polish coastal regions in

late summer (August or early September) in five-year

intervals (Table 3).

Analyses of minimal oxygen concentrations using a

ranking scale for the degree of eutrophication in par-

ticular regions indicates that the large number of oxygen

deficit cases in the Gulf of Gda�nsk is alarming (Table 3).

Judging from the frequency analysis, the oxygen contentof near bottom water along the central Polish coast or in

Fig. 6. Linear regression trends in phosphate (a) and TOxN (b) concentrations in the surface (0–10 m) layer of the SE Gotland Deep; data series

1969–2001; full line and bold fonts denote statistically significant trend coefficients (mmolm�3 a�1).


the Pomeranian Bay does not seem poor, although the

trend analysis provides a disturbing picture (Fig. 7a and

b). After 1990 the negative trends in the Pomeranian

Bay and the Gulf of Gda�nsk (Fig. 7a and c) are statis-tically significant, despite the overall (1959–2001) posi-

tive trend values in the year-round and summer oxygen

concentrations, and after 1990 a weak (Rsu ¼ �0:01)negative tendency appeared in late summer oxygen

concentrations near the sea floor the central Polish coast

(Fig. 7b).

Can future reductions in phosphorus and nitrogen

loads into the Baltic Sea be anticipated? What results canbe expected? Model studies of the response of the Baltic

Sea ecosystems to reductions in nutrient loads (Wulff and

Niemi, 1992; Savchuk and Wulff, 1999) indicated that an

overall reduction will be extremely difficult, since the

major share comes from non-point sources and agricul-

tural run-off. A cost effective reduction would have to be

managed regionally and parameter-wise, e.g. for the

Baltic Proper (including the Polish economic zone). The

simulations over 20 years, assuming a 50% phosphorus

load reduction each fifth year, showed that the recovery

would take several decades. Also, an effective strategywould involve a reduction of phosphorus while a

reduction of nitrogen would lead to a dramatic increase

in cyanobacteria blooms. The costs associated with the

nutrient reductions needed for the restoration of the

Baltic Sea is the responsibility of all Baltic countries

(Gren et al., 1997).

The nutrient loads through individual estuaries reflect

the nature of their catchments soils, and also anyanthropogenic inputs (Balls, 1994), hence preventive

measures should be undertaken first of all in the policy

governing agriculture by more effective recycling of

nutrients, including decreased use of mineral fertilizers,

and increasing the residence time of water in river

catchments (Jansson, 1997). Increasing the residence

time of water in the catchment means using capacity of

Table 2

Long-term trends detected in phosphate and nitrate + nitrite concentrations by non-parametric test based on Kendall’s s

Area/parameter/period Year-round Winter

Slope p Slope p

Pomeranian Bay

1959–2001 TOxN 0.25 0.01 0.53 0.06

PO4 – – – –

<1984 TOxN 0.48 0.12 1.95 0.07

PO4 0.03 0.15 0.07 0.13

>1988 TOxN 0.66 0.02 1.09 0.12

PO4 )0.04 0.01 )0.05 0.06

>1994 TOxN – – – –

PO4 – – – –

Central coast

1959–2001 TOxN 0.04 0.09 0.09 0.24

PO4 )0.01 0.01 )0.01 0.16

>1988 TOxN – – – –

PO4 )0.02 0.01 )0.03 0.09

Gulf of Gda�nsk1959–2001 TOxN 0.11 0.02 0.24 0.02

PO4 )0.01 0.1 )0.01 0.23

<1984 TOxN – – 0.87 0.02

PO4 – – 0.02 0.24

<1987 TOxN 0.37 0.12 1.43 0.25

PO4 – – 0.04 0.04

>1988 TOxN 0.12 0.20 – –

PO4 )0.02 0.00 )0.03 0.06

>1994 TOxN – – )0.62 0.23

PO4 )0.01 0.05 – –

SE Gotland Deep

1959–2001 TOxN – – 0.07 0.01

PO4 – – – –

>1988 TOxN – – – –

PO4 )0.01 0.08 – –

>1990 TOxN )0.04 0.21 )0.27 0.03

PO4 )0.01 0.06 – –

Slope–trend coefficient in mmolm�3 a�1; highly significant: p < 0:05, significant: 0:056 p < 0:1; tendency: 0:016 p6 0:25.

Table 3

Frequency of low oxygen concentration in the near bottom water in late summer

Period Pomeranian Bay Central coast Gulf of Gda�nsk

<2.8 cm3 dm�3 <1.4 cm3 dm�3 <2.8 cm3 dm�3 <1.4 cm3 dm�3 <2.8 cm3 dm�3 <1.4 cm3 dm�3

1959–1963 0 0 0 0 2 1

1964–1968 0 0 0 0 4 0

1969–1973 0 0 0 0 8 3

1974–1978 1 0 0 0 13 7

1979–1983 0 0 0 0 7 4

1984–1988 0 0 0 0 7 3

1989–1993 0 0 0 0 15 7

1994–1998 1 0 0 0 9 3

1999–2001 1 0 0 0 8 5


Fig. 7. Linear regression trends in summer oxygen concentrations (cm3 dm�3 a�1) in the near bottom water between 1959 and 2001; (a) Pomeranian

Bay, (b) central Polish coast, (c) Gulf of Gda�nsk.


soil to incorporate and denitrify the water. The loss of

buffering potential of wetlands was drastically demon-strated during the summer flood of the rivers Oder and

Vistula in 1997 in Poland, where about 90% of the

wetlands had been reclaimed. While wetlands have

probably the capacity to reduce 18–24% of the nitrogenloading to the sea, the ecological farming should be able

to recycle most of the phosphorus (Sapek, 1999).


Acknowledgements

This work was supported by the State Committee for

the Scientific Research (KBN Grant 5.1) and theacquisition of historic data into the oceanographic data

base was part of the EU project CHARM.

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