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Assessment of the Trophic Impact on the Lake Environment in Poland: A Proposal and Case Study ANNA HILLBRICHT-ILKOWSKA Institute of Ecology Polish Academy of Sciences AIMS AND CONSTRAINTS OF IMPACT ASSESSMENT The proper assessment of anthropogenic impacts on the environment is the initial and fundamental step in any effective program of environ- mental protection and management. In general, an assessment system is based on research data on the responses of the population, community, ecosystem, and landscape components to differing intensities of impact fac- tors (James and Evison, 1978; Best and Haeck 1982; Gower, 1980; Myers and Shelton, 1980~. These assessments are based on field observation, controlled experimentation, and laboratory simulation, and are designed to be used in management practices. Compromises must be made between the ecological complexity of the given phenomena and the need to simplify the information as is usually done in any classification system. The specific aims of environmental assessment are: to predict impacts over short and long time periods, with special emphasis on situations in which disturbances are irreversible under real technical and economical conditions; and to assess accurately the stimulus/response relationship and to indi- cate the kind of protective measures and/or treatment to be undertaken. The proposed assessment system discussed below concerns trophic impacts on a lake ecosystem and the process of eutrophication as the response of its biota to nutrient loading. Eutrophication the uncontrolled increase in the fertility and produc- tivity of aquatic ecosystems still remains the most common and widespread process of anthropogenic impact on lake environments. In fact, it is the only 283

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284 ECOLOGICAL RISKS process of significant disturbance in the belt of Baltic lakelands in north- ern and Eastern Europe which are managed chiefly for touristic purposes. This post-glacial landscape consists of numerous small- and medium-sized lakes which are surrounded by arable and/or forested land. According to Wuhrman (1984), the further eutrophication of most European lakes is in- evitable, even after the elimination of point sources of nutrient input (e.g., sewage discharge and industrial effluents), since inputs from surface runoff and atmospheric depositions are sufficient to stimulate the "algal bowl." There are many systems which can be used to quantify lake eutroph- ication and the water quality resulting from this process (Kudelska et al., 1981, 1983~. Most of these systems are based on a few easily-measured physicochemical properties of water (e.g., oxygen, BOD, nutrients, and chlorophyll concentrations), or they utilize one of the saprogenic systems (Sladecek in James and Evison, 1978~. The proposed assessment system is based on a more holistic approach to the lake ecosystem as a component of the landh~ater/atmosphere com- plex. Specifically, it assesses: . trophic impact in terms of phosphorous nutrient loading from external sources; impact of the watershed on the sources and transport of nutrients; natural resistance of lakes to trophic and pollution impacts; and - responses of biological communities and basic ecological processes in the lake ecosystem across increasing intensities of trophic impacts. Each of these four assessment components is based on a range of values of selected parameters converted into a three- to four-point scale to reflect the intensity of the process involved or the change in the response. The final result gives the position of the lake in each of the four scales. This combination provides a basis for the choice of proper management and protection treatments for a particular lake and its watershed. It also indicates the highest risk of further degradation. RATE OF TROPHIC IMPACT AND THE ROLE OF THE WATERSHED IN SUPPLYING AND TRANSPORTING NUTRIENTS Vollenveider's (1976) concept of permissible and dangerous phospho- rous (P) loads as related to the mean depth and residence time of water in a lake and determining the amount of total phosphorus (TP) in the spring appears to be useful for management of most P-limited lakes. It is still used in many assessment systems and models. There is a relationship between the annual loading rate of TP from external sources Hop mg yearn) and the average concentration of TP in the water column (ma m~3) during the spring overturn. Furthermore, TP is

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IMPACTS ON AQUATIC ECOSYSTEMS 285 related to the transparency (SD m) and chlorophyll concentration (Chl mg m~3) in the summer season. For a group of 35 Polish lakes located mainly in the Masurian Lake District, the relevant equations were determined by Uchmanski and Szeligiewicz (1989) as follows: (~1) TPsp~ing = ~ t1 - -~+0.68741 (2) log SD = - 1.09 log TP ~ 2.015 (3) log Chl = 0.792 log TP0.172 where V = lake volume, in cubic meters, and ~ = annual exchange rate These equations are valid for P-limited lakes those with positive re- tention of P - with an annual exchange rate higher than zero, and without significant internal loadings. These include lakes that are moderately eu- trophic in which TP in spring is below 250 fig 1-i and 0.100 ,ug 1-i in equations (2) and (3), respectively. The annual total loading to a lake includes at least the sum of the phos- phorus load from point sources (e.g., sewage, polluted effluents), nonpoint sources (e.g., surface runoff from direct watershed), and bulk precipitation. The behavior of these phosphorous inputs were studied in special exper- iments (Hillbricht-Ilkowska et al., 1981; Hillbricht-Ilkowska and Kawacz, 1983, 1985~. For the initial assessment, calculations were made relying on such average regional- or site-specific parameters as export rates from different land-use areas, concentrations of TP in precipitation or in sewage with known treatment, and total bunk loadings. However, these calculated values do not indicate the seasonality of the inputs, and they ignore short- term events like storm effects in runoff, variations in sewage input, and the bioavailability of phosphorous. This last factor often requires bioassay studies but, in general, the contribution of available phosphorous (dissolved and/or phosphate phosphorous) in different inputs can be estimated. The three categories of endangerment (CatEND) in terms of phospho- rous loadings are proposed as a scale of trophic impact (Hillbricht-Ilkowska, 1984, 1985~. When the actual annual TP load (i.e., total input from sewage, runoff, precipitation, and tributaries) is below permissable levels according to Vollenvieder's (1976) criteria, the lake is in CatEND 1. CatEND 2 means that the actual TP load is equal to or higher than the permissible level, and CatEMD ~ means that the TP load is equal to or exceeds dangerous levels (Table 1~. The percentage of TP due to sewage inputs is considered important supplemental information, since this part of TP is ultimately controllable. The distribution of CatEND 1-3 in the representative sample of Polish lakes is presented by Cooper (Chapter 18, this volume).

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286 ECOLOGICAL RISKS TABLE 1 Four-point classification of watershed impacts on lakes (ClaWI). NUMBER OF POINT; PARAMETER O 2 3 Ohle's index <10 Water balance -- Drainaze density 10 40 40-150 >150 with outflow without outflow flowthrough (km - km~;~) 1.5 Average slope (%) 20 Share of depression (%) >60 45-60 20-45 <20 Geological substratum loam sand-loam loam-sand sand Land use forest-swa~np forest-arable arable arable win urban areas ClaWI Mean Point Value Watershed Impact O <1 very weak 1 1.1 - 1.4 weak 2 1.5 - 1.9 moderate 3 >2.0 strong SOURCE: Bajkiewicz-Grabowska, 1987. The physiographic properties of the watershed influence its transport function and have been assessed by a four-part watershed impact classifi- cation system (CIa WI) as shown in Table 1. The system is based on the range of values for selected properties that impact the processes of source and transport of nutrients (Gower, 1980; Myers and Shelton, 1980~. The drainage density gradient (i.e., the ratio of the length of watercourses in the watershed to its surface), average sloping, and the ratio of watershed to lake area (Ohle's index) are factors that are positively correlated with the rate of surface transport and accumulation of matter. In contrast, the contributions of marshy areas (without surface outflow) and forested areas (as opposed to arable and urban land) are factors that negatively influence input to the lake. In addition, the dominance of clay as opposed to sand or gravel in the geological substratum is negatively correlated with the rate of underground transport to the lake. Circumstances of strong impact (i.e., ClaWI 4) would be watersheds with very high values for at least three or four parameters, such as density gradient, ratio of watershed to lake surface, mean slope, and an area covered by sandy substratum without clay intrusions and at least 10% arable and/or urban areas in the lake watershed.

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IMPACTS ON AQUATIC ECOSYSTEMS TABLE 2 Four categories of natural lake resistance (CatLR). 2~7 NUMBER OF POPS PARAbDi'l~ 0 1 2 3 Mean depth (m) >10 5-10 3-5 <3 Ratio: lake volume >5 3-5 1-3 <1 (103 m3) to shoreline length (m) Fraction of unmixed >35 20-35 10-20 <10 layer in lake volume (%) Ratio: bottom area in 0.30 epili~runion (m2) to its volume (m3) Annual water >10 5-10 1-5 <1 exchange rate Schindler's index 100 CatLR Mean Point Value Lake Resistance 0 <0.8 high 1 0.9-1.6 moderate 2 1.7-2.4 weak 3 >2.4 very weak SOURCE: Kudelska et al., 1983; Bajkiewicz-Grabowska, 1987. ASSESSMENT OF NATURAL RESISTANCE OF IAKES TO TROPHIC IMPACI Nutrient loading of comparable intensity will produce different effects in lakes with different morphometry and turnover rates. ~ assess these conditions, four categories of lake resistance (CatLR) were proposed by Kudelska et al. (1981,1983) and modified by Bajkiewicz-Grabowska (1987) (Table 2~. The morphometry and flow-through properties of a lake are the factors involved in the accumulation of the external TP load, the release of internal loading, and the recirculation of nutrients. Important factors are mean lake depth, the ratio of lake volume to length of shoreline, the percentage of the unmixed layer in lake volume, the ratio of sediment area covered by epilimnion to the epilimnion volume, and the ratio of the sum of watershed and lake areas to lake volume (known as Schindler's ratio). Decreasing or increasing values for these indices indicate four categories of lake resistance. CatLR 1 means that the lake should be relatively insensitive to the impact of non-point sources (e.g., watershed and precipitation). It usually includes deep, large lakes and lakes with high flushing rates. CatLR 4 means lower resistance, and usually includes shallow, well-mixed lakes with small outflow and well-developed shoreline.

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288 ECOLOGICAL RISKS TABLE 3 Trophie elassifieanon of lowland temperate lakes. Concentrahon in summer, surface layer Phytopl. TP SD Chl a biomass Gel- (m) Gel- mg-l~~ Dimietie lakes: 1. mesotrophie <50 23 <10 <5 2. moderately eutrophic <100 <3 <30 <20 3. strongly eutrophic >100 30 >20 Polymietie lakes: - 1. mesotrophie and/or <100 <2 <30 <30 mc~derat~lv P,~trr~nhi~ a ~ r^~~ 2. strongly eutrophic <300 <2 <100 <100 3. hypertrophie >300 100 >100 SOURCE: Kajak, 1983a, b; Hillbneht-IUcowska, 1984. THE ECOLOGICAL RESPONSE OF LAKES TO TROPHIC IMPACT Research was carried out on about 50 Masurian lakes (including the Great Masurian Lakes) to evaluate trends and magnitudes of changes in lake ecosystems along the trophic continuum and under known conditions of P- loading (Kajak, 1983; Zdanowski et al., 1984~. The relationship between the principal discriminant of trophic state (i.e., summer concentration of TP in surface layer) and numerous qualitative and quantitative indices of ecosystem processes and communities were determined (Kajak, 1983) Noble 3~. This system was proposed for the quantitative assessment of ecological responses of lowland, temperate lakes experiencing differing levels of eutrophication (Hillbricht-Ilkowska, 1984, 1985~. Three trophic classes of lakes (~CIaLT) are defined according to TP concentration in summer, with different ranges for dimictic lakes (D-lakes), which are of sufficient depth to be permanently stratified in summer, and for polymictic lakes (P-lakes), which are shallow and permanently or frequently mixed to the bottom. The generally very high correlation between internal and external [P loading in polymictic lakes is the main reason for dividing the whole system into two subsystems, with different ranges of values for the same set of parameters. In polymictic lakes, the relation "Lop - TP in spring" does not exist because of unpredictable inputs of TP from sediment during the entire vegetation season. The boundary values for successive classes (1-3) were set as follows: 150 fig 1-i for D-1, D-2, and D-3 lakes, respectively; and < 100, < 300, ~ 300 fig 1-i for P-1, P-2, and P-3 lakes, respectively. The two other parameters considered as secondary discriminants of trophic state are water transparency (SD) and chlorophyll concentration

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IMPACTS ON AQUATIC ECOSYSTEMS 289 (Chla), and are closely related to TP in surface waters during the summer. The respective correlations (r) between TP vs. SD and TP vs. Chla are 0.768 (0.05) and 0.743 (0.01) for dimictic lakes, and 0.484 (0.05) and 0.730 (0.01) for polymictic lakes (Kajak, 1983b). The indices and parameters related to nutrient availability, abundance and composition of producers and consumers, and transfer efficiencies in the plankton food chain were found to be strongly correlated with ClaLT and, therefore, are useful indices for both biotic and functional responses to trophic impacts (Hillbricht-ILkowska, 1985~. These indices of nutrient availability are: TN: TP mass ratio in summer-surface layers as an indicator of the nutrient deficiency to algae demands; the fresh weight of phytoplankton biomass in summer; the blue-green algae as a percentage of the total biomass and as the ratio to chlorophyll; and the percentage of small algae (nanoplankton-size, less than 30 microns) as a rough indicator of the amount of edible items vs. nonedible producers. The biomass of rotifers, daphnids, and all invertebrate plankton preda- tors, as well as biomass ratios of zooplankton to phytoplankton, Cyclopida to Cladocera, daphnids to blue-green algae, and predatory to nonpredatory species were found to demonstrate both decreases in the efficiency of the grazing food chain with eutrophication and increases in the efficiency of consumer links. Some of these values could also indicate the influence of fish predation on the zooplankton community. The clogging effect of ex- cess biomass of blue-green algae on daphnid biomass is also apparent. The extreme values of these parameters and indices usually occur in D-3 lakes and/or in P-2 and P-3 lakes which are the most eutrophic and hypertrophic deep and shallow lakes. The above three trophic classes of lakes are compatible with the four classes of lake water quality (ClaWQ) developed by Judelska et al. (1981, 1983) and Cyzdik and Soska (1988), and are based on spring and summer values of 18 parameters which include TP, TN, SD, Chla, and oxygen (Table 4~. Some events, such as massive fish kills or high numbers of E. cold titre, will determine the classification irrespective of other parameters. This classification system is widely used in Poland by state environmental agencies to control the quality of lake water. Relevant statistics are given in Chapters 18 and 19 (this volume). The combination of these five evaluation systems provides the basis for the proper choice of correction measures in lake management practices. For instance, D-1 lakes with very good water quality (CIaWQ 1 or 2) are rather insensitive to watershed inputs. L~kes in CatLR 1 or 2 are

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290 ECOLOGICAL RISKS TABLE 4 Lake quality evaluation system in Poland using water purity indices for three classes of lake water (n = stratified; ns = non-stratified lakes). CLASS OF WATER PURITY N PARAbD3TER PERIOD LAYER 1 2 3 (s) Hypolimnetic oxygen summer 240 >20 25 saturation (mean %) (ns) Oxygen content summa, over >4.0 >2.0 >1.0 (ma O2 1 ) bottom (s,ns) C.O.D. Bichromate summer, surface 520 <30 ~50 method (ma O2 1-~) (s,ns) B.O.D.5 (ma O2 1~) summer, surface c2 <4 ~8 (s,ns) B.O.D. (ma O2 1-~) summer, over bottom <2 ~5 <10 (s,ns) P-PO (ma 1-~) spring, surface ~0.02 ~0.04 ~0.08 (s) p pO (ma 1-~) summer, over bottom <0.02 S0.04 <0.08 (s) Total P (ma 1-~) summer, over bottom S0.06 ~0.15 ~0.60 (s,ns) Total P (ma 1-~) spring and summer ~0.050 S0.100 ~0.200 (mean value), surface (s,ns) Inorganic N (NNH4 + spring, surface 50.20 ~0.40 ~0.80 NNO3)(mg 1-~) (s) N-NH4 (ma 1-~) summer, over bonom ~Q20 ~1.00 ~5.00 (s,ns) Total N (mgl~~) spring and summer ~1.0 <1.5 ~2.0 (mean values), surface (s,ns) Specific conductance spring, surface <750 ~300 5350 (us cm~~) (s,ns) Chlorophyll (ma my) spring and summer <8 ~15 <5 (mean value) surface (s,ns) Dry mass of (ma 1-~) spring and summer <4 <8 512 (mean value), surface (s,ns) Secchi Disc (m) spring and summer 24 28 212 (mean value) (s,ns) Fecal cold title spring and summer sur- 21.0 20.1 20.01 face and above bottom (the worst result) (s,ns) Biological field all year, whole occurrence of fish kills or mass observations lake mortality of other aquatic organisms (both in littoral and pelagial); puts the lake "out of the class" irrespective of the other parameters SOURCE: Kudelska et al., 1981; Cydzik et al., 1986; cited in Hillbricht-Ukowska, 1984.

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IMPACTS ON AQUATIC ECOSYSTEMS 291 usually large or deep and are situated in forested areas. These are the most valuable bodies of water and require special protection in order to maintain their quality. Protective measures are needed immediately if the lake appears to be CatEND 2 or 3, since the actual external TP load is already dangerously close to threshold level and its present state could deteriorate rapidly. Some change in land use in the watershed might also be needed if ClaWI is close to 3 or 4, e.g., as a result of deforestation. Another extreme example is that of a rather shallow P-2 lake with very bad water qualibr (ClaWQ 4, or "out of classification"), a very dangerous TP loading (CatEND 3), a naturally moderate resistance to degradation (CatLR 2) associated with a high flow-through regime, and a mostly forested watershed (ClaWI 1 or 2~. This situation is the clear result of long-term discharge of sewage, and not the effect of improper watershed management. Probably the sewage diversion, together with the restoration of the lake by the inactivation of internal loading, will be sufficient to restore water quality. ASSESSMENT OF TROPHIC IMPACT ON LAKES IN THE MASURIAN I~NDSCAPE PROTECTED AREA AND SOME PROPOSALS FOR THEIR PROTECTION The assessment systems cited above are intended for use in scientific management and were applied to lakes situated in a protected area of about 700 km2 in the Lake District in Poland known as the Masurian Landscape Protected Area (Figure 1~. This area was established in 1977 to protect the land forms, forest complexes, marsh and bog habitats, rivers, and lakes of the postglacial Masurian landscape. The largest lakes in Poland are located in this area: Lake Sniardwy, a biosphere reserve of 110 km2, and Lake Luknajno, with a population of mute swans. Other small lakes and marshes in the area form reserves for the protection of rare plant communities and water fowl colonies. There are 24 lakes larger than 50 hectares, ranging in depth from 0.5 to 30 m, and most are mesoeutrophic in character. Each year as many as 80,000 tourists visit the area. It is protected from industry but is open to limited farming and forestry activities in addition to tourism. Because of the good natural connection between the lakes and the river Krutynia, the area is one of the most popular in Poland for canoes and sailboats. The area was divided into principal watersheds associated with the larger lakes, and data was collected in order to evaluate watershed impact, natural lake resistance, water quality, and trophic state (Figures 1 and 2) (Hillbricht-Ilkowska, 1988~. For most of the lakes, including the deepest and largest ones, conditions of Cat END 3 were assessed, i.e., the actual TP

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292 ECOLOGICAL RISKS I_ . ~ . \ cowl ~ l QD-1 ~P-1 ~P~3 120% i, J FIGURE la Masunan Landscape Protected Area and its lakes. Lake Sniardwy (A=110 km2; z=5.8 m; Zma==234 m); Lake Mikokajskie (A=5 km2; z=11.2 m; zm`~=25.9~; Intake Beldany (A=9.4 km2; z=10.0 m; Zm~ =46~0 m), Lake Mokre (A=8.5 km2, z=12.7 m; Zmax51.0 m); Lake Luknajno (A=6.8 km2; z=0.6 m; Zma==3~0 m). D-1, P-1 and D-2, and P-3 and D-3=trophic groups of lakes according to TP content in summer (surface layer), i.e., 150 fig 1-1 for D-lakes, and 300 fig 1-1 for P-lakes in above three groups. The percent (~) contribution of TP in sewage is indicated next to each lake (vertical black bars). load exceeded danger levels many times over. Only one deep lake (Lake Mokre) receives an annual TP load within permissible limits (CatEND 2). The TP input from tributaries, as in the case of Lake Sniarduy, or from rivers, like Lake Beldany, predominates in annual TP loadings. In only a few lakes does sewage contribute from 30-70% of total inputs; Lake Mikolajskie, Lake Beldany, and some smaller lakes experience these inputs. This means that for the rest of the lakes, dangerous levels of inputs are attained only from nonpoint sources of TP and from tributaries which are uncontrollable sources. In this situation, high water quality in these lakes should be a temporary condition. Eutrophication rates appear to decrease due to relatively high natural resistance to watershed impact (mostly CatLR 1 and 2) resulting from the depth, shape, and flow-through regime, as well as by moderate watershed impacts (ClaWI 2 and 3) resulting from intermediate or lower value density gradient, prevalent forest landscape, and the clay substratum (Figure 2~. The most threatening conditions are

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IMPACTS ON AQUATIC ECOSYSTEMS 293 \\\\ ~ or E]Cat END Cat END FIGURE lb CatEND=category of endangerment when annual TP loading is at permissible levels (2) or above dangerous levels (3) according to Vollenweider's criteria (1976~. the narrow and deep lakes (Mikolajskie and Beldany) and several smaller lakes which have a higher position in all five ranking systems, including the category of watershed impact (Figures 1 and 2~. Based on these findings, three principal methods of protection and management were proposed to maintain the actual status of some of the lakes and to control the further eutrophication and hypertrophication of others (Figure 3~. For the central part of the area around Lake Mikolajskie and Lake Beldany, a program of lake restoration was proposed utilizing deep water aeration, inactivation of TP internal loading, and diversion of principal point sources of pollutants (Figure 3~. The intensive protection of the watershed and shore zone includes the recultivation of near-lake slopes and shore zones around whole lakes, including anti-erosion installations and reforestation. An immediate ban was placed on the further develop- ment of year-round and seasonal tourist centers, and touristic paths were redistributed in order to disperse them over the area. Full protection was provided for small, marshy areas and forest fragments, which included re- forestation of sections of plowed land in contact with the lake shore, and control of fertilization and cattle breeding in nearby arable land. For the western part of the Masurian Landscape Protected Area in

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294 ECOLOGICAL RISKS Cat Wi- C]~ 1st 2nd 3rd 4t Cla LR 1S2rld 3r 4th FIGURE 2 Categones of watershed impact on lakes CatWI) Tom 1 to 4 (lowest to highest) and classes of natural lake resistance to eutrophication (CIaLR) from 1 and 2 (high and moderate resistance) to 4 (very low resistance) for lakes and watersheds in Masunan Landscape Protected Area (Bajkiewicz-Grabowska, 1987~. which the deep mesotrophic Lake Mokre and other mesotrophic shallow lakes are located special protection of watersheds and lakes was estab- lished to preserve their actual state, including control of fishing operations and a ban on the introduction of planktivorous fishes. Full protection of riparian forests included a ban on wood harvesting. Deconcentration of tourist activity and a ban on its further development was required, as was a ban on the liming and fertilizing of small lakes for fishery purposes. For the eastern part of the area, including Lake Siniardw~y (Figure 3), some of the same suggestions were made concerning fishery, forestry, and agricultural operations, and somewhat more relaxed suggestions were made concerning tourism. In two cases, proposed watershed and lake protection measures were to be fully subordinated to the needs of waterfowl protection, including a ban on all tourist activity in the vicinity of the lake.

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IMPACTS ON AQUATIC ECOSYSTEMS 295 ~_` ,._~r~` for ~ S1 _' 1 2 3 4 5 6 FIGURE 3 Recommended trends in watershed and lake protection for Masunan Landscape Protected Area. 1=intensive protection with recultivation of land around the lake and lake restoration; 2=spemal protection in order to maintain the status quo of mesotropic lakes and their watersheds; 3=partial protection of watersheds in order to control further eutrophication of lakes; 4=protection of lake and shore area with regard to requirements of waterfowl protection; S=diversion of point source of pollution; 6=localization of deep water aeration installation in lakes. CONCLUSION Eutrophication remains the primary cause of anthropogenic distur- bances of natural lakes and their environments, even after the elimination of point sources of nutrient inputs. It is also the primely cause of changes in the numerous small- and medium-sized natural lakes situated in arable regions (e.g., the North European postglacial lakelands) which function as important tourism centers. An assessment system of trophic impacts based on the role of the watersheds in the supply and transport of nutrients, the natural resistance of lakes to eutrophication, the response of the lake biota and its internal processes along the trophic continuum was proposed as the operational system for management purposes. This system, which is composed of relevant classes and categories, was applied to the case of the 24 large lakes of the Masurian Landscape Protected Area, and methods for different forms of protection of watersheds and lakes were recommended.

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296 ECOLOGICAL RISKS REFERENCES Bajkiewicz-Grabowska, E. 1987. Natural degradation ability of lakes and the role of drainage area in the process. Ekol. pot. 33:280-289 (PolishJEnglish Summary). Best, E.P., and J. Haeck, eds. 1982. Ecological indicators for the assessment of the quality of air, water, soil, and ecosystems. Dordrecht (Boston): D. Riedel Publishing Company, pp. 407. Cydzik, D., D. Kudelska, and H. Soszka. 1986. Ike system of evaluation of lake quality and its application by the field agencies of the environmental protection in Monitoring of the Lake Ecosystems, A. Hillbricht-Ilkowska, ed. Ossolineum, Wroclaw, 11-20. Gower, NM., ed. 1980. Water quality in catchment ecosystems. Chichester John Wiley and Sons, pp. 335. Hillbricht-Ilkowska, A. 1984. The indices and parameters useful in the evaluation of water quality and the ecological state of temperate, lowland lakes connected with their eutrophication. Pp. 55-69 in the Proceedings of the International Conference, "Conservation and Management of World Lake Environment." Otsu Shiga, Japan, August 24-30, 1984. Hillbricht-Ilkowska, ~ 1985. The eutrophication process and its control in Poland and the perspectives of lake restoration. Pp. 362-372 in Lake Pollution and Recovery, R. Vismare and R. Marforio, eds. Hillbricht-Ilkowska, A., ed. 1988 The lakes of Masurian Lakeland Park: Eutrophication, protection, and management. Ossolineum, Warszawa-Workaw. (in press) (in Polish with English summary). Hillbricht-Ilkowska, As, W. Grozczynska, and M. Planter. 1981. Nonpoint sources of nutrients to the lake watershed of river Jorka, Masurian Lakeland, Poland. Pp. 152-183 in Impact of Non-Point Sources on Water Quality in Watersheds and Lakes, J.H.A.M. Steenvoordenand and W. Rast, eds. Amsterdam, The Netherlands, May 11-14, 1981. Hillbricht-Ilkowska, A., and ~ Kawacz. 1983. Biotic structure and processes in lake system of river Jorka watershed, Masurian Lakeland, Poland. I. Land impact loading and dynamics of nutrients. Ekol. Poll 31:539-585. Hillbricht-Ilkowska, A., and W. Kawacz, eds. Factors affecting nutrient budget in lakes of the river Jorka watershed, Masurian Lakeland, Poland. Ekol. Pol.33~2~:171-318. James, A., and L: Evison, eds. 1978. Biological indicatom of water quality. Kajak, Z. ed. 1983a. Ecological characteristics of lakes in northeastern Poland versus their trophic gradient. Ekol. Poll 31:239-530. Kajak, Z. 1983b. Dependence of chosen indices of structure and functioning of ecosystems on trophic status and mictic type of 42 lakes. Ekol. Poll 31:495-53(). Kudelska, D., D. Cydzik, and H. Soszka. 1981. A proposal of lake water quality classification. Wiad. Ekol. 27:149-173 (in Polish with English summary). Kudelska, D., D. Cydzki, and H. Soska. 1983. Lake water quality classification system in Polish. Publ. Inst. Environment Management, Warsaw, pp. 44. Myers, W., and R.L Shelton, eds. 1980. Survey methods of ecosystem management. New York: John Wiley and Sons, pp 403. Uchmanski, J., and W. Szligiewicz. 1989. Models for predicting the water quality in lakes. Ekol. Poll (in press). Vollenweider, R.A. 1976. Advance in defining critical level for phosphorous in lake eutrophication. Mem. Inst. Ital. Idrobiol. 33:53~3. Wuhrman, K. 1984. Lake eutrophication and its control. Pp. 26-37 in the Proceedings of the International Conference, "Conservation and Management of World Lake Environment," Otsu Shiga, Japan, August 24-30, 1984. Zdanowski, B., A. Ko~ycka, and J. Zachwieja. 1984. Thermal and oxygen conditions and the chemical composition of the water in the Great Masurian Lakes. Ekol. Poll 32:651-678.