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Article

Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland

by
Włodzimierz Meissner
Ornithology Unit, Department of Vertebrate Ecology and Zoology, Faculty of Biology, University of Gdańsk, Wita Stwosza 59, 80-308 Gdańsk, Poland
Water 2025, 17(8), 1209; https://doi.org/10.3390/w17081209
Submission received: 15 February 2025 / Revised: 12 April 2025 / Accepted: 16 April 2025 / Published: 18 April 2025
(This article belongs to the Section Biodiversity and Functionality of Aquatic Ecosystems)
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Abstract

:
Lake Żarnowieckie, one of the largest lakes in northern Poland, hosts a high number of waterfowl during the non-breeding season. This study presents data on bird abundance from two periods. From 1986 to 1990, coinciding with the early colonization of the lake by zebra mussels Dreissena polymorpha, counts were conducted monthly between October and April. From 2011 to 2025, only one count was performed in January. Between 1986 and 1990, the highest numbers of waterfowl were observed in November and December, with maximum counts exceeding 20,000 birds. Three most abundant species—Eurasian coot Fulica atra, tufted duck Aythya fuligula, and mallard Anas platyrhynchos—accounted for 95–98% of all waterfowl. In January, their numbers significantly decreased, remaining low in the following months. This sharp decline mainly affected diving benthivores (tufted duck and Eurasian coot), which exerted strong pressure on prey populations, leading to a rapid decrease in their numbers and forcing a shift in foraging sites. The greatest decrease occurred in seasons with the highest number of waterfowl in December, when diving benthivores were most abundant and applied the greatest pressure on benthic organisms. These seasonal changes suggest that a single January count may not fully represent the significance of the lake for waterfowl. Over the study period, both ichthyophages, the great cormorant Phalacrocorax carbo and great crested grebe Podiceps cristatus, showed substantial increases in number. For cormorants, this growth coincided with the increase in the European population, while the rise in great crested grebes likely exemplifies the establishment of a new wintering site for this species in a location rich in food and free from intense human disturbance.

1. Introduction

The wintering grounds of waterfowl cover vast areas. Their distribution within these areas is not uniform, with the highest concentrations found on large waterbodies that offer abundant food resources [1,2,3,4,5]. Among waterfowl, there are several morpho-ecological groups that differ in diet and foraging methods. Consequently, their distribution and changes in numbers, influenced by local-scale factors, may vary significantly even within a single water body [2,4,6]. Progressive climate warming [7] is causing waterfowl to shorten their migration distances by wintering closer to their breeding grounds. This leads, on the one hand, to an increase in their numbers in areas where they previously did not winter in large numbers and, on the other hand, to a decrease in numbers in the most remote parts of their wintering range [8]. The appearance of new, abundant food resources is also contributing to the increased abundance of waterfowl on certain water bodies. For example, the distribution and number of wintering zoobenthos-feeding waterfowl (diving benthivores) have been strongly influenced by the spread of the zebra mussel Dreissena polymorpha in Europe. This invasive species began spreading from the Caspian and Black Seas in the early 19th century and has rapidly expanded across European inland waters [9]. During the last seventy years, there has been a steady expansion to isolated European lakes and lagoons, facilitated by the growth of aquatic leisure activities [10]. The zebra mussel has become a key component of the diet of diving benthivores at many wintering sites [1,11,12]. The extremely abundant presence of this bivalve species has significantly increased the carrying capacity of many waterbodies and provided abundant food resources for various species of diving benthivores across Europe [1,2,12].
Counts of wintering waterfowl are typically conducted through a single survey of a waterbody in January to ensure data comparability between years and to minimize the impact of bird movements on the results [13]. The results obtained in this way are used to estimate the abundance of individual species and their biogeographic populations on a continental scale [13]. These data are also used for the designation of Important Bird Areas (IBAs) and subsequent assessments of the effectiveness of area-based conservation efforts [14,15]. However, waterfowl often exhibit movements in response to deteriorating winter weather [16] or a decrease in the abundance of their food resources [17,18]. Therefore, only multiple counts on dates covering the entire wintering period can provide a complete picture of changes in the number and distribution of waterfowl on a given waterbody [5,19,20,21,22].
One of the lakes included in the long-term monitoring of wintering waterbird numbers is Lake Żarnowieckie in northern Poland. The only previously published data on waterfowl wintering there are reports of large winter concentrations of certain species [23,24]. This study aims to compare the species composition and number of waterfowl wintering on Lake Żarnowieckie between 1986–1990 and 2011–2025, and to describe changes in the abundance of the most numerous species during the early phase of the lake’s colonization by the invasive zebra mussel. It has also been hypothesized that during the early stages of zebra mussel colonization in the lake, food resources for diving benthivores were insufficient to sustain many thousands of individuals throughout the wintering period. When present in large numbers, these birds exert strong predation pressure on their prey populations, leading to a rapid decline in carrying capacity [12,25,26]. Consequently, in areas where diving benthivores are abundant, the reduction in zebra mussel biomass after winter can exceed 90% [12]. Therefore, it was suggested that the decline in bird numbers during winter was greater when more birds were present at the beginning of the wintering period, as a higher initial population led to a faster depletion of food resources.

2. Materials and Methods

2.1. Study Area

Lake Żarnowieckie is located in northern Poland, in the Pomerania region, along the course of the Piaśnica River (Figure 1). It is one of the larger lakes in northern Poland, with a surface area of 1425 ha and a volume of approximately 121 million cubic metres (121 km3). The lake’s maximum depth is 19.4 m, while its average depth is 8.4 m [27]. In the southern part of the lake is the largest pumped storage power plant in Poland, which was completed in 1983. Due to its operation, water level fluctuations in the lake reach about 1 m within a daily cycle [28,29]. The diurnal water movements prevent the area near the discharge channel from freezing completely, even during the harshest winters. The shores of the lake have been significantly altered by recreational development, but anthropogenic pressure remains very low in winter (own observations).
The first comprehensive study of the Lake Żarnowieckie environment was carried out in 1973–1975 for the power plant [30] and was repeated in 1994 [31]. In the 1970s, the dominant component of the zoobenthos was the zebra mussel, but it occurred in low density, lower than 100 individuals/m2, while in other Polish waterbodies, this species reached density above 28,000 individuals/m2 [32]. The zoobenthos community at that time reached maximum densities of 500 g/m2 at a depth of 6 m and 200 g/m2 at a depth of 3 m [33]. Surveys conducted in the 1960s did not indicate the presence of zebra mussels, and the maximum densities of zoobenthos did not exceed 50 g/m2 [34]. In 1994, densities of zebra mussels and the whole zoobenthos community were much higher, reaching 12,600 g/m2 at a depth of 6–8 m [35]. Additionally, a very high abundance (1500–1800 g/m2) of benthic communities was found in the form of periphyton on concrete structures in the area of the power plant discharge [35]. Therefore, it can be assumed that the zebra mussel population started to increase around the late 1960s to early 1970s, and the first period of bird counts (1986–1990) presented in this paper corresponds to the early stage of the lake’s colonization by this invasive mussel species. The local breeding population of waterfowl was small and consisted mainly of several pairs of great crested grebe, Eurasian coot, and mallard (author’s unpublished data).
In 1994, based on hydroacoustic surveys up to the 5 m isobath, fish abundance was estimated at more than 2.8 million individuals, with high density (ranging from 1000 to 3000 individuals per hectare) found over 61% of the lake area [36]. Lake Żarnowieckie is also recognized as one of the best fishing locations, attracting a high number of anglers [37]. Additionally, populations of selected fish species are supported through intensive stocking of the reservoir.

2.2. Field Study

Data collected between 1986 and 1990 came from counts organized by the Waterbird Research Group KULING. Birds during this period were counted once a month from October to April, around the middle of the mouth. In 2011–2015, bird counts were conducted only in January as part of the National Monitoring of Wintering Waterbirds and were carried out by the author of this paper. Surveys were conducted from the shores of the lake following the standard International Waterbird Census methodology [13]. Observations were made from locations such as shorelines without rushes, elevated ground above rushes, and piers, ensuring optimal visibility for counting birds. During both periods (1986–1990 and 2011–2015), each survey covered the entire lake surface, including the discharge channel from the pumped storage power station.

2.3. Weather Conditions

The weather conditions in the lake region are similar to those recorded at the climatological station in Łeba, located 30 km away [29]. Therefore, data from this site were used in this study. The severity of winter was determined based on the average daily temperature from the 15 days preceding the survey date and the degree of lake ice coverage, which was assessed during field inspections. Temperature data for the years 1987–1990 and 2011–2020 were obtained from the European Climate Assessment & Dataset (ECA&D) [38], while data for 2021–2025 were sourced from the Tutiempo.net portal (accessed on 14 February 2025). Ice cover affecting approximately 90% of the lake’s surface was recorded in January on three occasions: in 1987, 2011, and 2016. In 1988, it covered 30%, while in 2013 and 2017, it extended over about 10% of the surface. In the remaining years, no ice cover was observed on the survey day in January, or ice was present only in small bays in the northern part of the lake, covering less than 1% of its total surface area.

2.4. Data Analysis

Seasonal changes in bird abundance between 1986 and 1990 are presented as mean and maximum numbers for consecutive months. To confirm the hypothesis that the size of the bird grouping at the beginning of the wintering period influences the magnitude of the decline in bird numbers during winter, bird numbers in December and January were compared. December is a transitional month between the autumn migration period and wintering for many waterbirds, including those most numerous in Lake Żarnowieckie [39,40,41,42], whereas by January, they are assumed to be already on their wintering grounds. For this reason, January has been chosen worldwide as the standard month for international censuses of their numbers [13].
Pearson’s correlation coefficient was used to assess the potential relationship between the decline in abundance of the most abundant species between December and January, as well as the relationship between the average temperature 15 days before the bird count and the number of the most abundant wintering bird species over the entire study period. Since the number of great cormorants Phalacrocorax carbo and great crested grebes Podiceps cristatus was extremely low during the first study period (1986–1990), with some years showing no records of these species, the analysis of their relationship with temperature was limited to periods when their abundance was higher. For great cormorant, this included the entire second study period (2011–2025), while for great crested grebe, it covered 2019–2025, when the species began to regularly appear on the lake in winter.
The statistical analyses were performed using Statistica 13.3 software (TIBCO Software Inc., Palo Alto, CA, USA).

3. Results

3.1. Intra-Seasonal Changes in Bird Numbers During the Period 1986–1990

From 1986 to 1990, the highest numbers of waterfowl on Lake Żarnowieckie were observed in November and December, with maximum counts of 20,207 individuals in November 1989, and 19,783 individuals in December 1988. In January, a significant decrease in their numbers was noted, and in the following months, the numbers remained at a low level (Figure 2). The three most abundant species—Eurasian coot Fulica atra, tufted duck Aythya fuligula, and mallard Anas platyrhynchos—accounted for between 95% (in March and April) and 98% (in October) of all waterfowl. However, seasonal changes in their abundance differed. The mallard reached its highest average number in December, and its numbers remained high throughout January and February (Figure 2). In contrast, the tufted duck and Eurasian coot showed high average numbers starting in October, with their numbers peaking in November and December, exceeding 10,000 and 8000 individuals, respectively. From January onwards, both species experienced a significant decline (Figure 2). The magnitude of the decrease in diving benthivore numbers between December and January was strongly and positively correlated with their numbers in December (Pearson’s correlation coefficient, r = 0.997, p = 0.003) (Figure 3).

3.2. Long-Term Mid-Winter Changes in Bird Numbers According to January Counts

The number of all waterbird species on Lake Żarnowieckie in January was highly variable. The maximum counts, exceeding 12,000 individuals, were recorded in 2013 (12,240 ind.), 2014 (12,190 ind.), and 2025 (12,047 ind.), with 10,587 individuals recorded in 2020 (Figure 4). In the years 1987, 2011, and 2016, when heavy ice cover covered approximately 90% of the lake, bird numbers in January were slightly lower. However, in 2016, their number was higher than in some years without ice cover (Figure 4).
The highest number of mallards on Lake Żarnowieckie was recorded in January 2020, with 3257 individuals. However, in some years with little or no ice cover, the abundance of this species was very low, not exceeding 700 individuals (Figure 4). The tufted duck appeared in January in high numbers, exceeding 3000 individuals every few years, interspersed with seasons of low abundance. The highest January count of tufted ducks was recorded in 2020, with 4717 individuals. However, it should be noted that the highest numbers of tufted ducks during the entire study period were observed not in January but in November and December, exceeding 10,000 individuals in both 1988 and 1989. In January, the number of this species significantly decreased (Figure 3). The Eurasian coot reached very high abundance in January only in two years. In 2012, there were 9562 individuals, and in 2013, there were 8300 individuals. In other years, its numbers were much lower, but once again, they exceeded 4000 (4017) individuals in 2025 (Figure 4). A strong increase in the number of great cormorants was noted during the study period. Between 1987 and 1990, only a single individual of this species was recorded here twice. Until 2021, great cormorants appeared in low numbers in January, with a maximum of 414 birds recorded in 2017. In the following years, their abundance increased greatly, reaching 2015 individuals in 2025 (Figure 3). The numbers of great crested grebes were very low until 2020 and have also increased significantly in recent years, with maximum counts of 1010 and 1036 individuals recorded in 2022 and 2025, respectively (Figure 4).
A significant positive correlation between the mean temperature 15 days before the January bird count date and bird numbers was found only for the mallard and tufted duck (Table 1). The highest concentrations of both species were recorded during the warmest winters. In contrast, when approximately 90% of the lake’s surface was covered with ice, their numbers were low (Figure 5). However, similarly low numbers were also observed in some seasons with significantly higher temperatures (Figure 4), indicating that the size of the ice cover is not the only factor limiting the presence of these species on the lake.

4. Discussion

Lake Żarnowieckie is one of the most important sites for waterfowl in autumn and winter in the Pomerania region, and in some years, the abundance of some species recorded here in January is among the highest in Poland [23,24]. However, data from 1986 to 1990 show that the maximum number of the three most abundant species did not occur in January, but in November or December. The timing of peak mallard abundance in Poland, where the species appears in large numbers, is variable and occurs between November and February, e.g., [19,20,22,40,41,42]. The same variable pattern of peak mallard numbers is observed in other European countries further west and south, e.g., [2,21,43,44]. Mallards, as omnivores, forage not only in the areas where they stay during the day but also move at night to agricultural fields [45,46]. Therefore, contrary to diving benthivores, they are not as strongly dependent on the food resources available in the water bodies they inhabit. Interseasonal differences in the timing of their peak numbers may also be related to the variable timing of migration to wintering grounds, which in this species is highly prolonged [47], and to a shortening of migration distance during mild winters [48].
Diving benthivores, such as the tufted duck and the Eurasian coot, concentrate in large numbers during the non-breeding season in areas with suitable and abundant food resources, which in winter consist mainly of bivalves [1,11,49,50]. In Lake Żarnowieckie, as in many other inland water bodies in Europe, the dominant bivalve species is the zebra mussel. The very high densities of zebra mussels and snails is most likely the main reason for the exceptionally high numbers of diving benthivores in autumn and December. The zebra mussel was absent in Lake Żarnowieckie in the 1960s [34], but its density was high in the 1970s and very high by 1994 [33,35]. The increase in the abundance of this mussel species remains rapid until the environmental carrying capacity is reached [51,52,53]. We therefore expected that zebra mussel densities increased between 1986 and 1990, when monthly bird counts were conducted, but were not as high as in the second bird count period, 20 years later. In areas where diving benthivores stay in high numbers, they put strong pressure on prey populations, leading to a rapid decline in their numbers [12,25,26]. The decrease in zoobenthos density forces these birds to change their foraging sites [54]. Hence, the sharp decline in the number of tufted duck and Eurasian coot observed in January was likely caused by the depletion of food resources. This is further confirmed by the fact that on Lake Żarnowieckie, the greatest decrease in their abundance between December and January occurred during seasons with the highest number of diving benthivores in December—i.e., the seasons in which they exerted the greatest pressure on benthic organisms. Food resources may have been quickly depleted and insufficient to support such a large population of Eurasian coots and tufted ducks, especially during the period when the zebra mussel was in the early stage of the colonization of the lake. Similarly, in some water bodies, a decrease in the abundance of overwintering diving benthivores was observed following a reduction in the number of available bivalves [17,18]. Such a decline in their number was not recorded on large water bodies, where it is likely that food resources were sufficient to support numerous benthivorous birds throughout the overwintering period, e.g., [2,5,21,55].
Other factors may also influence the observed long-term fluctuations in the numbers of mallards, tufted ducks, and Eurasian coots, including variations in breeding success [56] or temporary shifts in the location of wintering grounds in response to milder or harsher winters [14,15]. Since individuals of these species wintering in northern Poland originate from vast areas of northeastern Europe and northwestern Asia [13], and because it is not possible to distinguish juvenile individuals during winter observations [39], we were unable to assess changes in their breeding success over the years. The long-term trend in the number of northwestern wintering mallard population has been assessed as declining [13], which may partly explain the decrease in their numbers on Lake Żarnowieckie observed after 2014. However, this trend appears to have reversed over the past two years. In contrast, the populations of tufted duck and Eurasian coot wintering in this part of Europe show no clear increasing or decreasing trends [13], suggesting that the long-term changes observed in their numbers on Lake Żarnowieckie are likely driven by other, more local factors. The increase in the breeding population of the great cormorant began in Europe in the 1980s, intensified in the 1990s [57], and slowed at the turn of the 20th and 21st centuries [58]. These population changes were also observed in many European wintering sites [3,5,59,60] as well as in Lake Żarnowieckie (this study). The number of great crested grebes wintering in Europe showed an increase between 1974 and 1993, then decreased and became stable after 2009 [13]. The number of individuals of this species wintering in Lake Żarnowieckie showed a marked increase in 2020. This rapid and strong increase in the number of great crested grebes exemplifies the establishment of a new wintering site for this species in a location rich in food and free from intense human disturbance. This may result from a northeastward shift in wintering grounds due to climate warming, as has already been documented in Europe for other waterfowl species [15,61].
Even when the ice cover, which covered about 90% of the lake’s surface, was present, some thousand birds congregated in the southwestern part of the lake, near the discharge point of the upper reservoir of the pumped storage power plant. Due to diurnal changes in water levels caused by the power station’s operation, this area remained partially ice-free even during severe frosts. Even at very low temperatures, the absence of complete ice cover is crucial for access to food resources, particularly for benthivores and piscivore birds [4]. As a result, the temperature dependence of bird numbers was significant in only two species. However, their highest numbers, recorded during the warmest seasons and driving the significant correlation, may have been due to a shortening of migration distances and wintering closer to breeding grounds. During mild winters, such migration shortening is a common phenomenon in waterfowl. It not only reduces the energy expenditure associated with long flights but also allows birds to respond more quickly to changing weather conditions and return to breeding sites earlier [48].
Counts of wintering waterfowl, conducted as part of extensive international cooperation (International Waterbird Census [13]), help identify the most important water bodies for different species, e.g., [6,24,62]. The seasonal changes in the number of two common species of diving benthivores on Lake Żarnowieckie demonstrate that a standard count conducted in January may not fully reflect the importance of a given water body for waterfowl. This is particularly true for small lakes, where the rapid depletion of food resources may lead to a significant decline in bird numbers. Therefore, to accurately assess the significance of such sites for wintering waterfowl, counts should be conducted not only in January, but also during other months of the non-breeding season.

Funding

The research conducted between 2011 and 2025 was supported by the Polish Society for the Protection of Birds (OTOP) as part of the National Bird Monitoring of Poland, funded by the Chief Environmental Inspectorate and the National Fund for Environmental Protection and Water Management.

Data Availability Statement

The data presented in this study are available on https://doi.org/10.18150/455OHH.

Acknowledgments

Thanks to Z. Klawikowski, W. Ruk, and Z. Giebień for the help in bird counts during the 1986/87 to 1989/90 seasons.

Conflicts of Interest

The author declares no conflicts of interest.

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Water 17 01209 g001
Figure 1. Location of Lake Żarnowieckie.
Figure 1. Location of Lake Żarnowieckie.
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Figure 2. Mean (black bar) and maximum (black bar plus grey bar) number of total waterbird population and three most abundant species on Lake Żarnowieckie during 1986/87–1989/90 seasons, by month.
Figure 2. Mean (black bar) and maximum (black bar plus grey bar) number of total waterbird population and three most abundant species on Lake Żarnowieckie during 1986/87–1989/90 seasons, by month.
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Figure 3. Decrease in number of diving benthivores between December and January relative to their number in December. Year for January is shown.
Figure 3. Decrease in number of diving benthivores between December and January relative to their number in December. Year for January is shown.
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Figure 4. Changes in abundance of all waterfowl and five most numerous species in subsequent years, in January. Years with ice cover covering 90% of lake’s surface are marked with asterisk.
Figure 4. Changes in abundance of all waterfowl and five most numerous species in subsequent years, in January. Years with ice cover covering 90% of lake’s surface are marked with asterisk.
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Figure 5. The relationship between the mean temperature from 15 days before the bird count date and the January number of mallards and tufted ducks over the entire study period. The grey points indicate data from the years 1987, 2011, and 2016, when approximately 90% of the lake’s surface was frozen.
Figure 5. The relationship between the mean temperature from 15 days before the bird count date and the January number of mallards and tufted ducks over the entire study period. The grey points indicate data from the years 1987, 2011, and 2016, when approximately 90% of the lake’s surface was frozen.
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Table 1. Pearson’s correlation coefficients for the relationship between the mean temperature 15 days before the bird count date and the number of the most abundant wintering bird species. The period included in the analysis is given.
Table 1. Pearson’s correlation coefficients for the relationship between the mean temperature 15 days before the bird count date and the number of the most abundant wintering bird species. The period included in the analysis is given.
SpeciesPeriodrp
mallard1986–1990, 2011–20250.570.011
tufted duck1986–1990, 2011–20250.460.047
Eurasian coot1986–1990, 2011–20250.100.698
great cormorant2011–20250.150.588
great crested grebe2019–2025−0.080.859
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Meissner, W. Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland. Water 2025, 17, 1209. https://doi.org/10.3390/w17081209

AMA Style

Meissner W. Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland. Water. 2025; 17(8):1209. https://doi.org/10.3390/w17081209

Chicago/Turabian Style

Meissner, Włodzimierz. 2025. "Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland" Water 17, no. 8: 1209. https://doi.org/10.3390/w17081209

APA Style

Meissner, W. (2025). Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland. Water, 17(8), 1209. https://doi.org/10.3390/w17081209

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