THE BMWP PL METHOD APPLIED FOR EVALUATION OF WATER PURITY IN THE CATCHMENT AREA OF THE MIDDLE AND LOWER DRAWA RIVER

Robert Czerniawski¹, Józef Domagała¹, Małgorzata Pilecka-Rapacz¹, Mirosław Półgęsek^2
1 Department of General Zoology, University of Szczecin, Szczecin, Poland
² Division of Aquaculture, Faculty of Food Technology and Fisheries, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

Quantitative and qualitative composition of the spring and summer macro-zoobenthos was studied at selected sites in the catchment area of the middle and lower Drawa River to evaluate the quality of water in the selected watercourses by the BMWP PL method. The animals found represented 7 phyla of invertebrates including 64 taxa, 62 families and over 70% of Insecta. No statistically significant changes in the abundance of animals between the months of individual seasons were established (P> 0.05), but the differences were statistically significant between spring and summer at all sites studied (P < 0.05). According to statistical analysis, only at two sites the animal abundance was significantly high (P < 0.05). On the basis of the results and using the BMWP PL method the quality of water was evaluated. The quality of water in the Drawa River and its indirect catchment area was found unsatisfactory as at over 50% of the sites studied the quality of water was classified as not good.

Key words: BMWP PL, Drawa drainage, macroinvertebrates, water quality .

INTRODUCTION

Till recently the purity of water in Poland has been determined almost exclusively on the basis of physico-chemical parameters. The recently accepted Water Framework Directives indicate the necessity of taking into account biotic, morphological and chemical factors in water status evaluation. In west-European countries and in the USA, biological indices have been used for a long time for evaluation of water quality [21]. One of important bioindicators is the quantitative and qualitative content of macroinvertebrates, whose changes are determined by long-lasting changes in the environment, not always detected by chemical analysis. The presence of macroinvertebrates in a given watercourse testifies to the long-time unchanged physico-chemical conditions approved by the specific animal species. Biological analysis is a very important element of evaluation of the water environment and the presence of species of specific sensitivity to water pollution indicates the degree of water purity [16]. The first method based on the use of living organisms for evaluation of water quality is the Kolkwitz and Marsson system [12], which despite many attempts at its modification is still difficult and time consuming [8] as needs accurate species determination. In the 1970s, the Biological Monitoring Working Party (BMWP) proposed a biotic index for evaluation of water quality using macroinvertebrates; however this method had some drawbacks [8]. In Poland this method was modified and supplemented with an additional index of biodiversity and it is referred to as the BMWP PL method [16]. It has been proposed to harmonise the methods used in Poland with those used in EU countries and to meet the Framework Water Directives.

The Drawa River has not been the object of many biological studies, although it seems that because of its localisation the state of water in this river should be carefully monitored. It functions as corridor linking two important ecological centres: the Drawski Landscape Park and Drawieński National Park [11]. The papers or reports on the Drawa River concerned mainly the vegetation or ichthyofauna in the area of the national park. A few taxation reports on the Drawa River published a few decades ago concern mainly the management of salmonids in its water. No reports have been published on the macroinvertebrates and evaluation of the water quality based on biological indices. Only Abraszewska [1] reported on the qualitative and quantitative composition of the bivalves of this river. At present the quality of water in Drawa is evaluated on the basis of physico-chemical indices at some selected sites. This paper is the first report on the fauna of macroinvertebrates in the middle and lower Drawa and the quality of water in a considerable area of the Drawa River catchment evaluated on the basis of a biotic index.

The aim of the study was determination of the qualitative and quantitative composition of macro-zoobenthos at selected sites in the middle and lower Drawa and evaluation of the quality of water with the help of the BMWP PL method.

MATERIAL AND METHODS

Samples of zoobenthos to be studied were collected at 19 sites localised in the catchment area of Drawa, between the 20^th and 24^th of each month from March till August in 2007 (Fig. 1).

Fig. 1. Study area. With Roman number a class of the purity of waters was indicated

Site 1. The outflow of Drawa from Lake Lubieszewskie (Lubie) (1440 ha). The bottom of the river was covered with large amounts of gravel and small stones.

Site 2. The Leśny Stream flowing out of Lake Piaseczno (43 ha) in the forest, flowing across the grounds covered mostly with coniferous trees [9]. At this site the bottom of the watercourse was covered with gravel and some stones, near the banks at some places with a thin layer of mud. Above the site the watercourse runs through the marshy land which in periods of high water table can directly influence the water quality.

Site 3. Pokrętna Stream; this watercourse begins in the moor and in its upper course runs across a small lake (10 ha), runs mostly through a mixed forest [23]. The bottom of the watercourse at the site was mostly covered with thin layer of mud and a small number of stones.

Site 4. The outflow of the Korytnica River from Lake Nowa Korytnica (115 ha). This river joins Drawa in the area of the Drawieński National Park. In the middle course of Korytnica there are trout farm ponds. In the upper course the river flows through meadows and arable fields, while in the middle and low course through forested land. At the collection site the bottom was covered with a thick layer of mud.

Site. 5. The mouth of Drawa to Noteć river; the site at the railway bridge at 500 m above the mouth of Drawa to Noteć. At this site the bottom of Drawa was covered with mud at some places with islands of sand.

Site 6. The Drawa River; in the village Prostynia, below the southern border of the Drawski Military Ground. The bottom was covered mostly with mud with some small amount gravel and stones.

Site 7. The inflow of Drawica into Lake Mąkowarskie; Drawica after flowing through a few lakes of which Mąkowarskie is the largest, joins Drawa [23]. The bottom covered with gravel and sand.

Site 8. The Szczuczna River flows out of Lake Szczuczarz and runs through a mixed forest. It is joined on the left side a small watercourse flowing out of a small lake [4]. The site was in the upper course of Szczuczna just after it was joined by this small watercourse, the bottom was covered with mud and sand.

Site 9. The Kamienna River is a right tributary of Korytnica; it originates in the marshy areas [3], flows through two lakes and joins Lake Nowa Korytnica. The bottom is covered with sand and small amount of mud.

Site 10. The Old Pond (Stary Potok); it takes origin in marshes [23], it flows through Lake Trzebuń and joins Prostynia. The site was after the outflow from Lake Trzebuń and the bottom at this site was covered with sand and gravel with some small amounts of mud and large stones.

Site 11. Outflow of the Słopica River from Lake Dominikowo, the river flows through 3 large lakes so the leniticyczna zone of the river has a great area [23]. The river joins Drawa in the Drawieński National Park. At the site the bottom was covered with gravel and sand.       

Site 12. The Sitna River; it originates in the drained arable fields near the former state farm Kraśnik, besides the origin it flows through a mixed forest [20]. The site was localised at 1 km from the inflow to Lake Adamowo and the bottom was covered with gravel and a large amount of small stones.

Site 13. The outflow of Drawica from Lake Mąkowarskie, the bottom covered with gravel and sand.

Site 14. A small watercourse originating in the moor within the limits of the city of Drawno. It is joined at a few places by storm outflows and municipal waste outlets. The bottom is covered with mud and sediments from the waste.

Site 15. The Korytnica River; the site is in the upper course of the river, the bottom is covered with gravel and sand with a small amount of stones.

Site 16. The Płociczna River; it starts from moors, in the middle course it runs through the Drawieński National Park in which it flows through 3 lakes. In its upper course it flows through meadows and arable fields. The site was in the upper course of the river, the bottom was covered with gravel and sand.

Site 17. The outflow of the Słopica River from Lake Szerokie; at 30 m after the outflow, the bottom with sand and gravel with a small amount of mud.

Site 18. Słopica. At 1000 m before the mouth of Słopica to Drawa; the bottom covered with a thick layer of gravel, but near the bank often with a layer of mud.

Site 19. The Człopica River; it is the last left tributary of Drawa; in the upper course it runs through a forest, in the middle and lower course through meadows and arable fields [24]. The site was in the middle course of the river, in the village Huta Szklana, the bottom with a thick layer of mud.

Macrozoobenthos was collected with the help of a scraper of the bottom of the outlet in the rectangular shape of the size 0.20 x 0.35m, along the whole profile of the river bed across the width of 1m, which permitted performance of qualitative and quantitative analyses. Qualitative analyses were made down to the level of family. The quality of water was described by two criteria of the values of the BMWP-PL index and biodiversity index [16]. In this system of biological assessment about 80 taxa of macrofauna are taken into account whose presence is assigned with a score from 1-10 points, according to the taxon sensitivity to pollution. The value of the index is the sum of the points assigned to particular families according to the directives given by Kudelska & Soszka [16]. The value is used to classify the water as representing one of the five classes of water quality proposed by Kudelska & Soszka [16]. The value of biodiversity index was calculated using the formula d = s/logN, where s – is the number of families represented at a given site, N – the number of fauna organisms per a unit area 1 m². The index d is referred to as the class of the water quality  [16].

Statistical analysis was performed by the variance analysis ANOVA with the Duncan's test as post – hoc. To illustrate the differences in selected features of zooplankton between the sites the hierarchic agglomeration method was used, whose measure is the Euclidean distance calculated from the formula (x, y) = [Σ₁ (x_i – y_i)²]^1/2, where: x, y – are the objects whose distance is calculated. The value of this parameter informs about the geometric distance (dissimilarity) in the multidimensional space.

The results were interpreted in the following way: if the class of water quality according to the BMWP-PL index is higher than that indicated by index d, then the final class is by 1 unit lower than that indicated by the BMWP-PL index, whereas a higher value of index d has no effect on the final classification of water quality.

RESULTS

In the samples collected in the catchment area of the Drawa River we distinguished representatives of 7 phyla of invertebrates from 64 taxa of which 62 were families and 45 of these families belonged to the phylum Insecta (Table 1). From among all taxa the most frequently met were Sphaeridae (at 18 sites), Chironomidae (at 17 sites) but the latter were more frequently found in two seasons. Apart from them relatively often met were Lumbicidae (at 14 sites) and Bithinidae (at 13 sites). In July we found many more taxa than in April. At sites no. 3, 17 and 15 the numbers of the taxa were the highest: 21, 20 and 20, respectively. The lowest number of taxa were noted at site 5 (6 taxa), then at each of sites 12, 14 and 19 we found 7 taxa. Relatively often were the representatives of Beraeidae (6 sites) belonging to the group of the highest score, according to Table x. Apart from them the high scored groups of invertebrates were represented by Glossosomatidae, Leptoceridae and Odontoceridae, but their frequency was low. The mean densities of macro-invertebrates at the sites studied are given in Fig. 2 (for both seasons, combined), 3 (spring) and 4 (summer).

Table 1. Qualitative composition and occurrence of macrozoobentos taxa in spring (S), summer (J) and two seasons (E)

Taxon

Stations

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Nematoda

S

J

S

S

E

Lumbicidae

J

E

J

S

E

J

E

J

E

E

J

E

E

E

Lumbriculidae

E

Erpobdellidae

J

Glossiphonidae

J

S

J

J

J

J

J

E

Cladocera

J

Asellidae

J

J

J

J

Gammaridae

E

E

S

S

J

S

E

E

E

E

E

Arachnida

J

J

J

Sialidae

J

J

J

Capniidae

S

E

Chloroperlidae

S

Nemouridae

S

Taeniopterygidae

S

Calopterygidae

S

S

Gomphidae

J

S

E

Coenagrionidae

L

Aeshnidae

S

Ephemerellidae

J

Ephemeridae

E

E

S

J

S

J

E

J

Baetidae

J

Caenidae

E

J

J

J

J

E

Leptophlebiidae

S

J

J

J

J

Sericostomatidae

J

Ryacophilidae

J

Glossosomatidae

J

S

Hydroptilidae

J

J

Hydropsychidae

S

S

J

J

J

Brachycentridae

S

Ecnomidae

S

J

Leptoceridae

S

Odontoceridae

S

Polycentropidae

S

S

J

J

J

J

Philopotamiidae

S

J

S

S

J

J

Phryganeidae

E

S

S

Psychomyidae

S

S

S

J

Beraeidae

S

S

J

J

J

J

Limnephilidae

J

S

Goreidae

J

J

S

Donaciidae

S

Elmidae

J

J

Ditiscidae

J

J

J

J

J

Hydrophilidae

S

Aphelocheiridae

J

J

J

Corixidae

J

J

Pleidae

J

Nepidae

S

Chironomidae

J

E

E

E

E

E

E

E

E

S

E

E

E

E

E

E

E

Culicidae

S

S

J

S

Empididae

S

S

S

Tabanidae

J

J

E

E

J

E

Simulidae

J

J

Limoniidae

E

J

Ceratopogonidae

E

E

S

A

S

J

J

J

Lymnaeidae

S

S

Planorbidae

E

J

E

J

S

J

Neritidae

A

E

J

J

J

J

J

Viviparidae

E

Valvatidae

J

S

S

J

S

J

Bithyniidae

E

J

J

E

E

E

E

E

J

E

J

E

E

Anycylidae

S

Unionidae

E

J

J

J

J

J

Sphaeriidae

E

J

E

E

J

S

E

E

J

E

E

E

E

E

E

E

J

J

Dreissenidae

S

S

E

J

J

Amount of taxa

13

15

20

19

6

12

12

12

12

13

15

7

8

7

20

12

21

18

7

Fig. 2. Mean abundance of macrozoobentos at sites (two seasons together)

Site 1. The highest mean abundance of macro-invertebrates (Figs. 1, 2) was noted; in spring the dominant was Gammarus sp. from Crustacea, whose mean density was 1506.41 ind m^-2 , which made 40.87% of all individuals; in summer the dominant was Sphaeridae with the mean density of 13 445.51 ind m^-2, its mean abundance over the two seasons was the highest of 7019.23 ind m^-2.

Site 2. The dominants were the larvae of Chironomidae, their mean density over the two seasons was 2094.02 ind m^-2; in spring their dominance was insignificant, 17.52 ind. m^-2, close to the density of the families: Hydropsychidae and Brachycentridae from Trichoptera (85.47 and 69.44 ind. m^-2). In total the larvae of Trichoptera made over 40% of all individuals at this site. In summer the dominance of Chironomidae larvae was significant (4070.51 ind. m^-2). Representatives of the high scores families: Glossosomatidae, Leptoceridae and Odontoceridae from Trichoptera were present.

Site 3. In spring the dominants were Psychomyidae (368.59 ind. m^-2) with the co-dominant Gammaridae (336.54 ind. m^-2); in summer the dominants were Gammaridae (5048.08 ind. m^-2); representatives of high score families Bareidae (10 points) and Taeniopterygidae (9 points) were found.

Site 4. The second greatest abundance after site 1; in spring the dominants were Lymnaeidae (80.13 ind. m^-2) and the co-dominants were Lumbriculidae (84.10 ind. m^-2); in summer the dominants were the mollusc from the family Bithyniidae (22 035.26 ind. m^-2).

Site 5. In spring only representatives of Gammaridae (10.68 ind. m^-2) and Chironomidae (5.34 ind. m^-2) were found; in summer representatives of 5 families were noted with the dominance of Chironomidae larvae (368.59 ind. m^-2), while Gammaridae disappeared.

Site 6. In spring the clear dominant were the larvae of Chironomidae, found at the density of 4887.82 ind. m^-2 and making as much as 84.25% of the abundance of all organisms; they were also dominant in summer although their abundance was lower – of 737.18 ind. m^-2.

Site 7. In spring the dominants were Chironomidae (72.44) with co-dominant Lumbriculidae (240.38 ind. m^-2); in summer the larvae of Chironomidae were still dominant (304.49 ind. m^-2) but the co-dominants were Sphaeriidae (288.46 ind. m^-2).

Site 8. In spring the dominants were representatives of Lumbriculidae and Sphaeriidae (993.59 and 929.49 ind. m^-2), making 37.58 and 35.15% of all organisms; in summer the dominants were the larvae of Chironomidae (2051.28 ind. m^-2).

Site 9. In summer representatives of only two families were found: Bithyniidae and Chironomidae (64.10 and 32.05 ind. m^-2); in summer representatives of 13 families with the clear dominance of Bithyniidae (1233.97 ind. m^-2) were noted.

Site 10. In spring the clear dominant was the family Bithyniidae making 52.60% of all organisms found, while in summer the dominant were representatives of Lubriculidae and Unionidae (48.08 ind. m^-2).

Site 11. In spring the dominants were Dreissenidae (96.15 ind. m^-2) and the co-dominants were the larvae of Ephemeridae (80.13 ind. m^-2). In summer the clear dominants were representatives of Bithyniidae, found at the density of 5849.36 ind. m^-2, making almost 60% of all organisms.

Site 12. In spring the clear dominants were Gammaridae at the density of 665.06 ind. m^-2, making 79.81% of all organisms found; in summer the dominants were Gammaridae at the density of 2516.03 ind. m^-2; the presence of the high score family Goreidae was observed.

Site 13. In spring the dominants were representatives of Sphaeriidae (208.33 ind. m^-2) and the co-dominant were Lumbriculidae (144.23 ind. m^-2); in summer the dominants were Lumbriculidae (7403.85 ind. m^-2).

Site 14. In spring the clear dominants were Gammaridae occurring at the density of 208.33 ind. m^-2, making 54.17% of all organisms found; in summer the dominants were the larvae of Chironomidae 1009.62 ind. m^-2.

Site 15. In spring two families Lumbriculidae and Sphaeriidae were dominant, occurring at the densities of 144.23 and 112.18 ind. m^-2 , respectively; in summer the dominants were the larvae of Chironomidae (4887.82 ind. m^-2).

Site 16. In spring the dominants were Ephemeridae occurring at the density of 192.31 ind. m^-2, making 54.55% of all organisms; in summer the dominants were representatives of Lubriculidae (256.41 ind. m^-2).

Site 17. The clear dominants in spring were the larvae of Chironomidae (2355.77 ind. m^-2), making 65.33% of all organisms, in  summer the dominants were representatives of Asellidae (2111.38 ind. m^-2).

Site 18. In spring the co-dominants were Lubriculidae and Chironomidae present at the densities of 288.44 and 256.41 ind.
m^-2; in summer the dominants were Sphaeriidae and Gammaridae reaching the densities of 3974.36 and 3782.05 ind. m^-2.

Site 19. In spring only representatives of the two families Bithyniidae (80.13 ind. m^-2) and Lumbriculidae (64.1 ind. m^-2) were noted, while in summer representatives of 7 families were found, with the dominance of Bithyniidae (144.23 ind. m^-2).

Table 2. Purity class of waters on study sites

Sites	Season	Index BMWP-PL	Biodiversity (d)	Purity class
1	Spring	36	2.8	IV
	Summer	39	2.25	III
	General value	51	3.33	III
2	Spring	91	5.2	II
	Summer	17	1.07	IV
	General value	95	4.3	II
3	Spring	70	4.01	II
	Summer	69	3.85	III
	General value	111	5.98	I
4	Spring	38	4.79	III
	Summer	53	2.74	III
	General value	75	4.66	II
5	Spring	9	1.66	IV
	Summer	21	1.79	IV
	General value	27	2.39	IV
6	Spring	58	2.66	III
	Summer	21	1.36	IV
	General value	69	3.41	III
7	Spring	33	3.51	IV
	Summer	43	2.74	III
	General value	61	4.16	III
8	Spring	46	3.21	III
	Summer	32	1.93	IV
	General value	52	3.4	III
9	Spring	9	4.01	IV
	Summer	62	3.85	III
	General value	62	5.98	II
10	Spring	59	3.27	III
	Summer	25	2.32	IV
	General value	66	3.85	III
11	Spring	49	3.03	III
	Summer	99	4.51	II
	General value	122	5.66	I
12	Spring	45	3.42	III
	Summer	89	4.8	II
	General value	107	5.99	I
13	Spring	26	2.56	IV
	Summer	20	1.25	IV
	General value	31	2.15	IV
14	Spring	18	1.93	IV
	Summer	26	1.92	IV
	General value	31	2.38	IV
15	Spring	39	3.37	III
	Summer	100	4.24	II
	General value	117	5.36	I
16	Spring	20	1.96	IV
	Summer	66	4.59	II
	General value	66	4.88	II
17	Spring	9	1.41	V
	Summer	82	5.26	II
	General value	82	5.68	I
18	Spring	39	3.02	III
	Summer	92	4.33	II
	General value	92	4.63	II
19	Spring	8	0.93	IV
	Summer	37	2.67	IV
	General value	37	2.86	IV

The values of BMWP PL index, d index and the corresponding classes of water quality at particular sites are given in Table 2. Analysis of the data shows that at 9 sites (1, 7, 9, 11, 12, 15, 16, 17, 18) the quality of water was poorer in spring than in summer (Table 2). These sites were localised in the course of upper Drawy and 90% of them at the left tributaries of Drawa, except for site 12. The water quality poorer in summer was established at 5 sites (2, 3, 6, 8, 10) at the outflows of small and shallow lakes, except for site 10. The same water quality in spring and summer was found at 5 sites (4, 5, 13, 14, 19) of which only water at site 4 was not in the poorest class IV.

Fig. 3. Mean abundance of macrozoobentos in spring

Fig. 4. Mean abundance of macrozoobentos in summer

Fig. 5. Agglomeration analysis of macrozoobentos abundance between sites

Table 3. Level of significance of Duncan test and euclidean distance of agglomeration analysis between two sites (1 and 4) and other sites where post-hoc test was showed significant differences (P < 0.05). P < 0.05 is bold

Sites	Level of significance P		Euclidean distance
	1	4	1	4
1	–	0.3169	-	6922
2	0.0067	0.0578	26319	20227
3	0.0162	0.1156	23433	17545
4	0.3169	-	6922	-
5	0.0015	0.0165	32070	25961
6	0.0076	0.0638	30326	25107
7	0.0019	0.0202	31600	25575
8	0.0074	0.0635	27294	21562
9	0.0020	0.0216	30655	24544
10	0.0046	0.0425	31340	25898
11	0.0173	0.1237	21660	15508
12	0.0042	0.0391	28581	22558
13	0.0158	0.1159	21707	15576
14	0.0019	0.0208	31139	25070
15	0.0156	0.1140	21226	15073
16	0.0019	0.0202	31400	25328
17	0.0169	0.1204	24600	19084
18	0.0500	0.2721	16441	10329
19	0.0015	0.0164	32256	26163

Analysis of abundance of the animals at all sites has shown that this parameter did not change significantly between the months of individual seasons. The standard deviation does not indicate a large range of abundance changes in a given season (Figs. 3, 4). Statistically significant difference (P < 0.05) at all sites was found between the abundance of animals observed in spring and in summer. Site 4 was characterised by clearly the highest abundances of animals from among all sites except site 4 (P < 0.05) (Table 3). Site 4 was also characterised by statistically significantly greater abundance than that at sites 5, 7, 9, 10, 12, 14, 16 and 19 (P < 0.05). The results of the agglomeration analysis were very similar as the greatest Euclidean distances testifying to dissimilarity coincided with the results of the differences in the abundance (Fig. 5, Table 3). The euclidean distances between the other sites were not so pronounced.

DISCUSSION

Analysis of the water classes reveals that the water at more than 50% of sites did not represent classes 1 and 2, corresponding to very good and good quality. This observation is rather disturbing as Poland is obliged to restore very good or good quality of water until 2015. Water at four sites was classified to class 1, at five sites – to class 2, at six sites to class 3 and at four sites to class 4. Analysis of the macro-benthos parameters and classes of water purity of the rivers with a few sites of sample collection along their course, it can be concluded that in the lower courses of the rivers the quality of water deteriorated. Water of the Drawa River was of second class at the first two sites and only at site 3 the water was classified as of the third class. This state can follows from the properties of water river continuum, as near the junction or mouth of the river the amount of the organic matter carried increases, which does not have to testify to water pollution [8,26]. The Drawa River in the middle course flows through a few euthropic lakes that can cause an increase of nutrients in the water. The lakes change the character of the river below its outflow (according to the water continuum rules) and disturb its course [10]. In the Drawa River it could be reflected by an increased amount of seston or organic bottom sediments. In the middle course Drawa encountered many sources of pollution: discharge of raw waste or intensely fertilised soils in the vicinity of direct catchment. For the above reasons the structure of zoobenthos in this section indicated water quality of third class. The quality of water of Korytnica also deteriorated with its course although the quality of water of this river deteriorated after the outflow from a large lake in which from May to August an intense blooming of Ceriatium hirundinella was observed. In Słopica the water quality was the lowest in the lower course but at the outflow from the lakes it was class 1. Lakes Szerokie and Dominikowo, through which Słopica flows out are pure reservoirs not subjected to phytoplankton blooming and to degradation. The decreased water quality of Słopica in its lower course could be related to the influence of fish farm ponds near the last site of sample collection at this river and to intense fertilisation of nearby soils. In the lower course the river flows with the regularly-shaped bed through a mixed forest and its bottom is covered with gravel, which are suitable conditions for salmonids [5], so the hydrology of the river could not be responsible for increase in the amount of organic matter or deterioration of water quality. It seems that the only factor that could cause deterioration of its water quality was the anthropogenic pressure.

All watercourses with water of the first class of purity (Site 11, 12, 15 and 17) were to a small degree subjected to anthropopressure. The samples collected from these watercourses were characterised by relatively high value of the diversity index and small intraspecies abundances, which is typical of pure and weakly eutrophicated waters. However, only in Sitna (Site 12) Gammaridae were found in considerable abundances and their presence is typical of pure and weakly polluted water [4,14,16]. In the samples collected at sites 15 and 17, the dominant were Chironomidae, of relatively low score according to BMWP PL scale, but with no influence on reduction of water quality. According to Kownacki [13] the presence of the family Chironomidae is not the best indicator of pollution as the quantitative differences in their abundance in pure and polluted water are insignificant. The presence of representatives of the phylum Trichoptera (high score) is a good bioindicator because of their direct relation to a given type of environment, relatively long life cycle and the possibility of reaching high abundances [19]. Trichoptera occur in abundance in pure or weakly polluted water, rarely in highly polluted environment [6], however, at the sites studied the abundance of high score Trichoptera was relatively small which can be related to the eating by fish whose greatest numbers were observed at the outflows from lakes. Similar problem could occur for  Gammaridae.

The watercourses whose water was classified to the fourth class of purity (Site 5, 13, 14 and 19) run through different types of environment. The juncture of Drawa to Notec was described above. The worst quality of water at the outflow of Drawica from Lake Mąkowarskie (Site 13) could be explained by a long-lasting blooming of Ceriatium hirundinella, lasting for all the time of sample collection. To the watercourses with sites 14 and 19 received raw waste from the storm outlets (14) and from farm building (19). The samples collected at all these sites had low index of diversity and the highest abundances were found of zoobenthos representing the families of low score according to BWMP PL scale.

At many sites the quality of water in spring was worse than in summer and at the majority of these sites water belonged to the first or second class of purity. This observation is difficult to explain as these sites were localised in different types of environment at the watercourses not influenced by lakes or floodings and after the outflows from lakes. These sites were to a small degree subjected to anthropopressure. The worse quality of water in summer than in spring at some other sites can be explained by their localisations. Almost all such sites were localised after the outflows from shallow lakes or marshy area (Site 2). Shallow euthropic lakes show a considerable increase in the amount of organic matter in summer, which could have negative effect on the water quality. The sites with the same water quality in spring and summer were those whose water belonged to the fourth class of purity. The watercourses with these sites were subjected to strong anthropopressure for the whole year, which explained this stability.

The contribution of the high score Ephemeroptera larvae was not particularly high at all sites and the presence of these organisms is considered as the best indicator of water pollution [13]. However, their low density does not have to attest the high pollution because the presence of Ephemeroptera as well as Plecoptera so the orders the most sensitive to pollution, also depends considerably on the character of the substrate [22]. According to Raczyńska et al. [19] the low abundance of Ephemeroptera and Plecoptera can related to unsuitable environment conditions or anthropogenic changes in the river bed and banks of the river. The majority of the watercourses studied had hard or gravel-covered bottom types, which are not preferred by Ephemeroptera and Plecoptera.

At almost all sites the presence of Sphaeridae (low score) was noted. The abundance in species and number of these molluscs was reported from other watercourses in Poland [17,18,25]. Often although less frequently than Sphaeriidae the presence of Bithyniidae was noted. The species from these two families belong to ubiquistic organisms, easily adaptable to new environmental conditions and are met at many sites. Taking into account that the density of Bithiniidae exceeded 20 000 ind. m^-2 at the outflow from a strongly eutrophicated lake, it can be concluded this type of environment is preferred by this family.

CONCLUSIONS

The poorest quality of water (class 4) was found at the sites subjected to strong anthropopressure or below the outflows from strongly eutrophicated lakes. Analysis of the qualitative and quantitative composition of zoobenthos at the sites in the Drawa River has proved that despite the beneficial type of surrounding (Drawieński National Park and Drawska Forest) the water quality was not as good as expected and the water belonged usually to class 3 or 4. The worst water quality was found in lower course of Drawa and its tributaries (class 3 and 4), slightly better was the water quality in the upper and middle Drawa and its tributaries (usually class 3). The water in two tributaries along this section of Drawa, flowing through rather large cities is of class 4, and the water from these tributaries can have negative effect on Drawa water. Moreover, in this section Drawa flows through a few lakes enriching its water in organic matter. In general the water quality of the Drawa River is not satisfactory as the water belongs mostly to class 3. The other left and right tributaries of the middle Drawa show good water quality (class 1 and 2) but they are not subjected to strong anthropopressure.

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Accepted for print: 6.10.2009

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Robert Czerniawski
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24
email: czerniawski@univ.szczecin.pl

Józef Domagała
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24
email: jozef.domagala@univ.szczecin.pl

Małgorzata Pilecka-Rapacz
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24

Mirosław Półgęsek
Division of Aquaculture, Faculty of Food Technology and Fisheries, West Pomeranian University of Technology, Szczecin, Poland
Kazimierza Królewicza 4
71-550 Szczecin
Poland
email: mpolgesek@zut.edu.pl

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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