Phytoplankton dynamics in relation to physicochemical conditions in large, stratified Lake Charzykowskie (Northern Poland)

Large stratified Lake Charzykowskie has attracted the interest of environmentalists and natural scientists already in the first half of the 20^th century. Hydrobiological studies initiated by Stangenberg & żemoytel (1952) in the 1940s have been continued till today. The research on phytoplankton communities in Lake Charzykowskie has been carried out on and off since the 1940s till the early 21^st century. The results of the first phytoplankton analysis from 1947 and 1948 were published by Cabejszek (1950) and subsequent studies were conducted by Solski (1962), Szulkowska-Wojaczek (1978), Oleksowicz (1988), Wiśniewska (1994, 1996) and Wiśniewska & Luścińska (2012). Long-term observations of the phytoplankton species composition, followed by the observations of biomass and chlorophyll a concentration in relation to habitat conditions (Szulkowska-Wojaczek 1978) made it possible to assess the impact of the catchment area on changes in the trophic status of the lake for the period of almost 70 years. The inflow of wastewater from the town of Chojnice and the tourist village of Charzykowy through Struga Jarcewska (Jarcewska stream) had the greatest impact on the increased trophic status of Lake Charzykowskie in the 1970s. In 1990, a sewage treatment plant was launched in the locality of Igły, which in the future should improve the trophic status of the lake.

Consequently, based on the analysis of physicochemical properties of water and phytoplankton structure, the lake was classified as β-mesotrophic in the 1940s (Cabejszek 1950), and eutrophic (Szulkowska-Wojaczek 1978) or hypertrophic (Wiśniewska 1994) in the period between the 1960s and the 1990s.

In 2008 and 2009, the trophic conditions of the lake significantly improved and its status was defined as meso-eutrophic (Wiśniewska & Luścińska 2012). The results of the research from 2008-2009 prompted the authors to undertake further detailed analysis of the phytoplankton community in relation to environmental conditions. The objective of the study was to confirm the assumption that positive changes occur in the lake.

Description of the lake

Lake Charzykowskie is located in northern Poland, in the mesoregion of Charzykowska Plain, the macroregion of South Pomeranian Lake District. It is a flow-through ribbon lake with a relatively heterogeneous shoreline, located within the north-south axis (Fig. 1). The lake can be divided into three subbasins (also referred to as pools), each of them with a different maximum depth: the southern subbasin (max depth 30.5 m), the central subbasin (max depth 25 m) and the northern subbasin (max depth 10 m). The area of the lake is 1360 ha. Lake Charzykowskie is a dimictic water body; summer thermal stratification does not always occur in the northern subbasin, due to its small depth. For many years, the central subbasin of the lake was under the influence of pollutants delivered by the Jarcewska Stream. The Brda River flows through the northern part of the lake, and a stream called Struga Siedmiu Jezior (the Stream of Seven Lakes) discharges into the lake.

Figure 1

The research was conducted in 2014-2015 and samples were collected once a month from May to September. Phytoplankton samples were collected from the pelagic zone, from the deepest places of the three subbasins. The detailed location and the names of the sites are presented on the map (Fig. 1). Each time the concentration of chlorophyll a, the total count and the total biomass of phytoplankton as well as the structure of their taxonomic groups were determined. At pelagic sites (1, 2 and 3), water was collected from three thermal layers as integrated samples from each thermal layer. The water was collected using a 2 l Limnos sampler, from which 200 ml subsamples were collected and preserved with Lugol’s solution (J in KJ) for microscopic analysis and determination of the phytoplankton count and biomass. Thermal and oxygen profiles were determined at three sites every month of the study, as well as water was collected for physicochemical analysis: transparency (SD visibility), pH, electrolytic conductivity (EC), oxygen (DO), chlorophyll a (Chl a), mineral forms of nitrogen (N-NH₄, N-NO₂, N-NO₃), total nitrogen (TN), orthophosphates (P-PO₄), total phosphorus (TP), magnesium (Mg⁺²), calcium (Ca⁺²), chlorides (Cl^-) and total hardness.

Phytoplankton species were identified using a light microscope at 400× magnification. To identify the diatoms, samples were treated with HCl and ca. 30% H₂O₂. The count of algae was determined under an inverted microscope using the Utermöhl method (1958), and the biomass – using the volumetric method (Hillenbrand et al. 1999; Sun & Liu 2003) and assuming that 1 mm³ of algae is equal to 1 mg. The biomass was expressed as mg l^-1 fresh mass. Only species with at least 5% contribution to the total biomass were considered dominant (Padisák et al. 2003). The dominant species described in this paper were classified into functional groups based on the studies by Reynolds et al. (2002), Reynolds (2006), Mieleitner et al. (2008) and Padisák et al. (2009) and life strategies after Wilk-Woźniak (2009).

The results were statistically analyzed – Pearson’s simple correlation (Past 3) was applied to analyze the relationships between the total biomass and environmental factors. To assess the relationships between the dominant species and environmental variables (CCA), the multivariate statistical package MVSP 3.2 was used (www.kovcomp.com/mvsp/).

The ecological status was based on the Phytoplankton Metric for Polish Lakes – PMPL (Hutorowicz & Pasztaleniec 2014). The PMPL index is an algorithm of the total phytoplankton biomass, cyanobacterial biomass, and chlorophyll a concentration. The method of calculating the index depends on the presence or absence of thermal stratification and on Schindler’s ratio (the ratio of catchment area and lake volume).

The increasing eutrophication and the threat posed by cyanobacterial toxins, dangerous for living organisms, contribute to the growing interest in massive development of cyanobacteria in lakes. Therefore, the mechanisms of algal blooms, both in deep and stratified as well as in shallow and mixed lakes, have been addressed in numerous hydrobiological studies. Prediction and possible prevention of algal blooms is still an important issue in ecological and biotechnological studies (Dokulil & Teubner 2000; Kangro et al. 2005; Hajnal & Padisák 2007; Nõges et al. 2008; Nõges et al. 2010, Dembowska et al. 2015; Grabowska & Mazur-Marzec 2016). Lake Charzykowskie is one of the largest lakes in Poland, and at the same time one of the most extensively exploited for tourism. The lake can be a model water body for monitoring of short- and long-term changes associated with positive activities in the drainage basin.

The research conducted in Lake Charzykowskie during the last ten years indicates changes in the structure of phytoplankton, even though the trophic conditions in the lake are conducive to the development of phytoplankton, while oxygen conditions are very bad. Based on the high concentrations of biophilic elements, mainly phosphorus (on average 0.1 mg TP l^-1), the lake can be classified as eutrophic (Vollenveider 1968; Carlson 1977). Changes in the structure of phytoplankton observed in Lake Charzykowskie during ca. 70 years are presented in Table 2.

Table 2

Structural changes in phytoplankton of Lake Charzykowskie in 1947-2015

The current research has shown that the phytoplankton community structure does not significantly change in the limnological cycle. Diatoms from the functional groups P and C, including diatoms constantly occurring in Lake Charzykowskie, i.e. Aulacoseira granulata (P), Asterionella formosa (C) and Fragilaria crotonenis (P) dominated in spring. The current research confirmed the dominance of the C-strategist Plagioselmis nannoplanctica from functional group X2 in the spring phytoplankton (observed also in 2008 and 2009; Wilk-Woźniak 2009). In late spring, waters of Lake Charzykowskie provide good conditions for the development of green algae Volvox and Eudorina from the functional group G, which prefer waters rich in nutrients and sunlight.

In summer 2014 and 2015, species from the functional group LM: Microcystis aeruginosa and Ceratium hirundinella co-dominated in the plankton. The group is characteristic of summer epilimnion in eutrophic lakes, while species are tolerant to low content of carbon and sensitive to water mixing. Gloeotrichia echinulata (H2) dominated in summer at site 2 (central subbasin). The species is characteristic of large mesotrophic lakes, tolerant of low content of nitrogen, sensitive to water mixing and low availability of light (Reynolds et al. 2002; Padisák et al. 2009). Aphanizomenon flos-aquae from group H1 still significantly contributed to the late autumn phytoplankton. The species is tolerant of low content of nitrogen and carbon, sensitive to the lack of phosphorus. Each autumn, the phytoplankton community is restructured toward the dominance of diatoms and cryptophytes.

The latest studies conducted in 2008-2009 and 2014-2015 showed that cyanobacterial blooms in Lake Charzykowskie caused in the 1980s by Microcystis sp. div. and Aphanizomenon flos-aquae are now short-lived and sporadic. In recent years, however, cyanobacteria were mainly represented by species from the functional group L, such as Aphanothece minutissima (W.West) Komárková-Legnerová. According to Padisák et al. (2009), those species may occur in summer even in oligotrophic waters, and Plagioselmis nannoplanctica (X2) is characteristic of well mixed meso-eutrophic lakes. The main functional groups and their representative species occurring in Lake Charzykowskie are presented in Table 3.

Table 3

Main functional groups in Lake Charzykowskie and representative species in each group (after Reynolds et al. 2002; Padisák et al. 2009) FG Representative species

It has been found in the present study that the biomass of phytoplankton in spring was at a low level of 0.32 mg l^-1, while the maximum value of the biomass was recorded in summer – 37.82 mg l^-1. It should be noted that the maximum value of the biomass was contributed by large dinoflagellates Ceratium hirundinella, and the correlation coefficient between the total biomass of phytoplankton and the biomass of dinoflagellates was high – r = 0.8959, p ≤ 0.001. Ceratium hirundinella was recorded in many lakes all over the world. The extensive development of this species is associated not only with the trophic conditions, but mostly with a high water temperature (Padisák 1985; Mac Donagh et al. 2005; Hart & Wragg 2009; Kozak et al. 2013). The correlation between the biomass values of dinoflagellates and water temperature in Lake Charzykowskie (Table 4) was high and amounted to r = 0.7098 (p ≤0.001).

The concentration of chlorophyll a in 2014-2015 ranged from 2.27 µg l^-1 to a maximum of 78.32 µg l^-1, which indicates eutrophic conditions (Carlson 1977). Statistically significant correlations were determined only between the concentration of chlorophyll a and the biomass of Chlorophyta (Table 4). The content of chlorophyll a in the cells of algae from other taxonomic groups (Cyanobacteria, Dinophyta) is usually lower, hence the lack of statistically significant correlations (Kasprzak et al. 2008; Dembowska et al. 2015).

Table 4

Statistically significant values of Pearson correlation between environmental parameters and phytoplankton biomass (p≤0.05, bold p≤ 0.001)

Cyanobacteria and dinoflagellates dominated in the taxonomic structure of planktonic algae in the water, which is a characteristic phenomenon in lakes with large resources of available phosphorus and increased amounts of dissolved organic matter in the water (Górniak et al. 2002). Canonical correspondence analysis (CCA) demonstrated a clear correlation between the biomass of cyanobacteria and the total content of phosphorus in the water of Lake Charzykowskie (Fig. 4). Cyanobacteria were represented by a large number of potentially toxic species, e.g. Microcystis aeruginosa, Gloeotrichia echinulata, Aphanizomenon flos-aquae and Dolichospermum spiroides (Klébahn) Wacklin, Hoffmann et Komárek. (Błaszczyk et al. 2010; Kobos et al. 2013). However, the biomass of these species is significantly lower compared to that recorded in the 1980s and the 1990s (Wiśniewska 1994; 1996).

Figure 4

The current quantitative and qualitative parameters of phytoplankton determined in the course of the research carried out in 2014 and 2015, and the long-term research indicate the ongoing process of ecosystem regeneration after the period of disturbance. If the main adverse factor was an excessive supply of nutrients from the lake catchment, the reduction in the load of fertilizing compounds does not result in the immediate change of phytoplankton structure (Dunalska et al. 2014; Dunalska & Wiśniewski 2016). Responses of lake ecosystems are usually delayed in time due to the large resources of phosphorus accumulated in the bottom sediments, which represent internal sources of supply for phytoplankton communities. Another factor affecting the processes of regeneration are currently frequent climate changes (Jeppesen et al. 2005). Global warming was recorded in Europe and Poland already at the beginning of the 21^st century (IPCC 2012). According to Olrik et al. (2013), an increase in temperature may result in faster settling of phytoplankton and removal of phytoplankton-bound P from epilimnion. The stratification period in lakes will be extended, while the content of inorganic nitrogen in epilimnion will be reduced (due to denitrification and transformation into atmospheric N₂). On the other hand, oxygen deficits in hypolimnion will contribute to the release of phosphates from sediments into the water. In the end, obligate autotrophs will be eliminated from phytoplankton in favor of mixotrophs (Ceratium) and N₂-fixing cyanobacteria.

The research on phytoplankton in Lake Charzykowskie has been carried out from the mid-20^th century. The trophic conditions in the lake have been monitored for almost 70 years. In the 1940s, the lake was classified as β-mesotrophic (Cabejszek 1950), while from the 1960s to 1990s as eutrophic (Szulkowska-Wojaczek 1978) or even hypertrophic (Wiśniewska 1994). The observations conducted in the last ten years have revealed a small but continuous improvement in the water quality of the studied lake. In 2014-2015, the biomass reduction and significant changes in the structure of phytoplankton were observed. In 2015, the ecological status of the lake was assessed as good. Cyanobacterial blooms are sporadic and less abundant and their duration is shorter. Despite the positive changes observed in the phytoplankton, the high content of nutrients in water is disturbing. Unfortunately, the morphometric conditions, i.e. the great depth of the lake, contribute to the oxygen deficit and release of nutrients from the sediments. Therefore, the sustainability of changes observed may be questionable, which is why the monitoring of Lake Charzykowskie should be continued.