Abundance, Distribution Patterns, and the Contribution of Transparent Exopolymer Particles and Dissolved Acidic Polysaccharides to Organic Carbon in Lake Taihu, China

Abstract

Transparent exopolymer particles (TEP) are essential for the carbon cycle in aquatic environments. However, the distribution of TEP, its precursor, and the contribution to the organic carbon pool in the large freshwater lake remain inadequately understood. Here, we focused on the spatial distribution of TEP and dissolved acidic polysaccharides (dAPS) in four typical seasons in Lake Taihu, the third-largest lake in China. TEP concentrations in Lake Taihu ranged from 0.05 to 5.19 mg Xeq/L, with a mean value of 1.31 ± 1.08 mg Xeq/L. The concentration of dAPS in the lake averaged 3.7 ± 2.19 mg Xeq/L (range, 0.19–13.12 mg Xeq/L). Higher content of TEP and dAPS was found in summer, and their distributions appeared to be influenced by the cyanobacterial blooms, as they showed significant correlations with chlorophyll-a content in Lake Taihu in summer. In addition, TEP accounted for an average of 24.3% of particulate organic carbon, and dAPS represented 25.9% of dissolved organic carbon. These results provide valuable insights into the contribution of TEP to organic carbon pools in inland waters and highlight the role of TEP in the carbon cycling of large freshwater lakes.

1. Introduction

Transparent exopolymer particles (TEP) are clear, gel-like particles mainly comprising acidic polysaccharides and are heavily implicated in the biogeochemical cycling of carbon and other elements in the aquatic system [1,2,3]. TEP is a ubiquitous component of the carbon flux and contributes significantly to the carbon pool for water ecosystems [2]. Recent studies on water systems have shown TEP concentrations in oligotrophic seawaters and the importance of TEP contribution to particulate organic carbon (POC). The TEP concentration on the coastal site of the oligotrophic North Western Mediterranean (Blanes Bay) reached an average of 81.7 µg Xeq/L and contributed to 77% of POC in early summer [4]. In the NE Aegean Sea, the TEP concentration ranged from 15.4 to 188 µg Xeq/L and contributed to about 70% of POC [5]. TEP affects the carbon pump because it is carbon-rich with high flexibility and can also promote particle aggregation with strong viscosity [6,7]. So far, two potential pathways have been recognized in the process: (1) TEP aggregates can sink to the deep water and result in carbon sink [8], and (2) TEP can accumulate at the water surface and then influence air-water gas exchanges, as carbon source, like that of carbon dioxide (CO₂) [9]. Thus, studying TEP is fundamental to understanding the carbon cycle in water bodies.

The origin of TEPs and exopolysaccharide precursors in waters are manifold. Generally, phytoplankton is the major source of TEP and precursors [1,10,11]. Other organisms, such as bacteria [12], seagrass [13], and zooplankton [14], can also affect TEP abundance in waters. Another factor that influences TEP concentration and distribution is turbulence, which could accelerate TEP production [15]. TEP concentrations often increase during the algal blooms caused by eutrophication [16], especially when algal cells become nutrient-stressed [17]; this complicates the TEP dynamics. Although much work has been done since the early 1990s to investigate the distribution features of TEP in oceans, the existing knowledge about the abundance and distribution of TEP and its precursor and their contribution to carbon pools in the large shallow lake is still rather poor. Limited studies on TEP concentrations in freshwaters determined by spectrophotometry are listed in

Table 1. TEP concentrations in freshwaters based on spectrophotometry.

Freshwater Systems	TEP Concentrations (μg Xeq/L) Determined by Spectrophotometry (μg/L) (Values in Parenthesis Correspond to Mean Values)	References
Quentar Reservoir (Spain)	1.9–335.2 (48.0)	De Vicente et al. (2009) [18]
Kinneret Lake (Israel)	48–1160 (219)	Berman et al.(2001) [19]
	759–2385 (1605)	Berman et al. (2010) [20]
Pearl River Estuary (China)	521.5–1727.4 (988.6)	Sun CC et al.(2010) [21]
North Temperate Lakes (USA)	36–1462	De Vicente et al. (2010) [22]
Mediterranean Lakes (Spain)	66–9038	De Vicente et al. (2010) [22]
Neuse River Estuary (USA)	805–1801	Wetz et al. (2009) [23]
the Indian Sundarbans (India)	110–206	Chowdhury et al. (2016) [24]
the Changjiang River Estuary (China)	40–1423.33	Guo et al. (2020) [25]

.

Lake Taihu, with an area of 2338 km², an average depth of 1.9 m, and a maximum depth of 2.6 m [26,27], located in the lower reaches of the Yangtze River basin, is the third-largest freshwater lake in China (Figure 1). It is characterized by the presence of cyanobacterial blooms in the northern region and macrophytes in the eastern zone [26,28]. Since TEP may account for an important fraction of POC mass, the study of the lake is important for better prediction of the magnitude of the biological carbon pump and the future dynamics of atmospheric CO₂. Thus, the primary aim of this study was to acquire new data on the spatial and temporal distribution of TEP and estimate the contribution of TEP and TEP precursor to particulate and dissolved organic carbon (DOC) in Lake Taihu.

2. Materials and Methods

2.1. Study Sites and Sampling

Surface water samples (0–0.5 m) from 32 locations in Lake Taihu were collected at mid-month of August, October, February, and May during 2012 and 2013, which are assumed to be the representative months of summer, autumn, winter, and spring, respectively. The 32 sites were monitored quarterly by the Taihu Laboratory for Lake Ecosystem Research (TLLER), Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (Figure 1). 1 L of surface water at each station was collected using a 1 L plastic bottle, stored in an incubator containing ice in the boat, and then sent to the laboratory for further analyses. Data for 32 sites in the lake were divided into eight regions to best capture spatial variation in TEP and dAPS in Taihu. The eight regions included Meiliang Bay, Zhushan Bay, Gonghu Bay, Northwest Lake, Southwest Lake, Central Lake, East Taihu Bay, and East Lake (Figure 1). As shown in Figure 1, Meiliang Bay and Zhushan Bay belong to typical algae-type regions that accumulate large amounts of algae blooms. East Lake and East Taihu Bay were considered typical macrophytes-type regions. Littoral zones included Northwest Lake, Southwest Lake, East Lake, East Taihu Bay, Zhushan Bay, Meiliang Bay, and Gonghu Bay. Central Lake belongs to the open water region [28,29,30].

2.2. Parameters Analyses

TEP was measured according to the colorimetric method proposed by Passow and Alldredge [31]. Samples from Lake Taihu were filtered through polycarbonate filters of 25 mm diameter and 0.4 μm pore size. The filters were stained with 500 μL of pre-calibrated (with a xanthan gum solution) Alcian blue (0.02%, pH 2.5) for 5 s and rinsed with Milli-Q water. Subsequently, they were soaked in 80% sulfuric acid for 3 h, and the absorbance of the extract was determined at 787 nm on a spectrophotometer. Standard materials were calibrated using xanthan gum and expressed in mg Xeq per liter. To compare TEP with POC stocks, we converted TEP into carbon units using the conservative conversion factor of 0.51 g C g/Xeq [32].

Dissolved acidic polysaccharides (dAPS) content was also determined by Alcian blue staining [33]. As described by Thornton et al. (2007), dAPS was filtrated through a 0.4 µm polycarbonate membrane, acidified to pH 2.5, and the absorbance was measured at 610 nm. Standard materials were calibrated using xanthan gum and expressed in mg Xeq per liter. To compare dAPS with DOC stocks, we converted dAPS into carbon units using the conservative conversion factor of 0.32 g C g/Xeq [34].

For measuring POC, 500 mL of lake water was filtered on pre-combusted Whatman GF/F filters (450 °C, 4 h). The filters were frozen at −20 °C until further analysis. Before the analysis, the filters were thawed at room temperature and then put in an HCl-saturated atmosphere for 24 h to remove inorganic compounds. Then the filters were dried at 60 °C again, ground to a powder, and analyzed using an EA3000 Elemental Analyzer (EuroVector, Italy).

DOC samples were filtered through 0.45 µm pre-combusted 47 mm Whatman GF/F membranes under a gentle vacuum. The filtrates were measured for DOC analysis using a total organic carbon analyzer (Model 1020, OI-Analytical, College Station, TX, USA).

Chlorophyll (Chla) concentration was determined according to the hot-ethanol extraction method after filtering 500 mL of seawater on Whatman GF/F filters [35].

2.3. Statistical Analyses

A parametric one-way analysis of variance (ANOVA) coupled with a multiple comparison post hoc test was used to test for significant differences among seasons and regions. An independent sample t-test was applied to test for significant differences between typical regions (including algae-type and macrophytes-type regions, littoral zones, and central area). The partial correlation analysis to assess the correlation between TEP and Chla concentrations and that between dAPS and Chla in the study area was explored using 2-tailed Pearson’s correlation analyses. The spatial distributions of TEP, dAPS, Chla, POC, and DOC in Lake Taihu during the four seasons were plotted using ArcGIS 10.0 software (ESRI, Redlands, CA, USA) after Kriging interpolation analysis. Geostatistical analysis, including the global Moran’s index and the coefficient of variation (CV), was used to analyze the distribution trend of TEP and dAPS in Lake Taihu.

3. Results and Discussion

3.1. Concentration of Transparent Exopolymer Particles and Dissolved Acidic Polysaccharides Concentrations in Lake Taihu

TEP concentrations in Lake Taihu ranged from 0.05 to 5.19 mg Xeq/L, with an average of 1.31 mg Xeq/L and a coefficient of variation of 0.83 (

Table 2. Concentration range and mean values of transparent exopolymer particles (TEP), dissolved acidic polysaccharides (dAPS), particulate organic carbon (POC), and dissolved organic carbon (DOC) in Lake Taihu.

	Summer			Autumn			Winter			Spring
	Range	Mean	CV	Range	Mean	CV	Range	Mean	CV	Range	Mean	CV
TEP	0.32–5.19	2.18	0.58	0.15–4.37	1.09	0.93	0.21–3.45	1.27	0.61	0.05–2.20	0.67	0.82
dAPS	0.53–8.73	4.85	0.39	1.24–13.12	4.9	0.45	0.22–7.82	2.71	0.55	0.19–9.62	2.31	0.79
TEP/dAPS	0.08–5.42	0.71	1.51	0.02–0.85	0.25	0.91	0.10–5.86	0.65	1.51	0.01–11.58	0.74	2.74
POC	0.93–46.19	5.12	1.51	1.39–23.25	4.53	0.93	1.02–6.32	3.33	0.35	1.31–7.63	3.45	0.43
DOC	4.04–6.35	5.12	0.12	3.06–7.06	4.52	0.19	3.26–6.37	5.0	0.16	2.40–5.06	3.47	0.18
TEP/POC (%)	5.10–206.50	41.0	1.18	1.50–125.90	18.4	1.31	2.30–71.20	24.6	0.81	0.40–63.40	13.1	1.09
dAPS/DOC (%)	3.30–53.90	30.50	0.39	13.10–106.30	35.1	0.46	2.10–45.00	17.3	0.53	1.90–78.20	20.6	0.69

); this was comparable with those reported for other areas, as listed in Table 1. Investigation of TEP concentrations mainly focused on the deep-water lakes and estuary districts [19,20,23,36], where the TPE values can range from 1.9 to 9038 µg Xeq/L, which is relatively higher than most reported results in marine systems, with the mean values of TEP ranging from 80 to 3000 µg Xeq/L [1,4]. Lake Taihu had high TEP concentrations, and the reasons may be as follows: (1) Lake Taihu is a eutrophic lake with frequent cyanobacterial blooms. Extracellular polymeric substances (EPS) secreted by high-density algal cells can be the main source of TEP. Previous studies showed that algal blooms, such as diatom blooms, generally have high TEP concentrations [37,38,39,40]. This observation is also supported by the similar distribution of TEP (Figure 2a) and Chla concentrations (Figure 2d) in summer in the northern region of Lake Taihu, where the cyanobacterial bloom is frequent. (2) Lake Taihu is a large shallow lake with frequent turbulence that may promote the formation of TEP in some regions [41]. In addition, strong turbulence may also promote the secretion of algal cells and the shedding of the bound extracellular polymers on the cell surface in the algae-type regions. Appropriate turbulence may benefit the TEP precursors to form TEP [1].

High TEP concentrations in the waters of Lake Taihu indicate the important role of TEP in the freshwater ecosystem. With a maximum depth of only 3 m [26], frequent resuspension in Lake Taihu could keep TEP in the water column, not sinking into the sediments, which provide it more residence time in the water to have a more active role in the whole lake ecosystem. The higher the concentrations of TEP, the more important its role in waters, which can increase the relative viscosity of particulate matter, promote its aggregate formation, and affect the circulation of particulate matter and nutrients in the aquatic ecosystem [1,42]. In addition, because of the high viscosity, TEP may promote the formation of the cyanobacterial colony in Lake Taihu during the outbreak season of cyanobacterial blooms. During the decline period of the cyanobacteria bloom, abundant TEP can aggregate cyanobacterial cells and sink to the sediments, finally affecting the fate of the blooms.

The dAPS concentrations in Lake Taihu ranged from 0.19 to 13.12 mg Xeq/L and averaged 3.7 mg Xeq/L, with a 0.59% coefficient of variation (Table 2), which was similar to that reported previously [33]. Since dAPS was also measured by Alcian blue staining and soluble polysaccharides may form gelatinous particles through spontaneous condensation [43], in a dissolved state, dAPS can be considered the TEP precursor. Thus, the proportion of TEP to dAPS can reflect different forms of extracellular polysaccharides in waters, which is helpful in studying the mutual transformation of particulate TEP and dissolved dAPS in pools of extracellular polymeric substances. As shown in Figure 2, summer has the highest ratio of TEP to dAPS, followed by winter, spring, and autumn. A significant positive relationship between TEP and dAPS in spring was found in the algae-type region, and a negative correlation was in the macrophytes-type region (Figure 3), which indicated the different sources of TEP and dAPS between different ecological environments. This also caused the different TEP/dAPS ratios among the other regions. In addition, significantly higher ratios of TEP to dAPS in summer were observed in the macrophytes-type regions than those in the algae-type regions and the central lake zone (ANOVA, p < 0.05), consistent with a higher size distribution of TEP in the macrophyte-dominated zone [44].

3.2. Spatial Distribution of Transparent Exopolymer Particles

The distribution of TEP among seasons varied. Based on the geostatistical analysis, the global Moran’s index of TEP in three seasons (summer, winter, and spring) was significantly positive (p < 0.1) (Figure 4), indicating a significant spatial aggregated trend of TEP in these three seasons in Lake Taihu. Among the three seasons, the lowest coefficient of variation (0.58) for TEP was observed in summer. The average value of TEP in the central open waters in summer was significantly lower than the typical algae-type and macrophytes-type waters (ANOVA, p < 0.05). On the other hand, TEP in the littoral zones was significantly higher than that in the central waters (p < 0.05), which was consistent with the previous report that TEP content in open areas in the ocean is much lower than in coastal areas [6]. This is due to higher levels of phytoplankton in coastal zones or bays caused by anthropogenic emissions of high-nutrient pollutants. Phytoplankton secretes a large amount of EPS so that TEP concentration in the coastal area is greater than in the open area. Previous studies have shown that the blooms always prevail in this region [26,27], and the tendency of cyanobacterial bloom increases from southeast to northwest in summer. In addition, a significant partial correlation was observed between TEP and Chla in summer after controlling the factor of dAPS (r = 0.413, p < 0.05). These findings prove the influence of the blooms on the TEP distribution.

It was noted that TEP concentrations in the eastern regions, including the East Lake region and East Taihu Bay, were relatively higher, as shown in Figure 1 and Figure 2a. The eastern regions have abundant macrophytes that can support the growth of macroorganisms. TEP can also be produced by macroorganisms such as macroalgae, oysters, and benthic organisms [45,46,47]. Seagrass can also be a major source of TEP [13]; thus, abundant aquatic plants in the eastern region may also be an important source of TEP.

3.3. Temporal Changes in Transparent Exopolymer Particles Concentrations

TEP contents in the whole of Lake Taihu showed obvious seasonal differences (Figure 2). In the summer and autumn, TEP concentrations in the whole lake were relatively high; TEP concentrations during the summer and autumn were significantly higher than those during the winter and spring (t-test; p < 0.01). Among the four seasons, the mean TEP concentration in summer was significantly higher than in the other seasons (ANOVA, p < 0.01). The highest TEP concentration (5.19 mg Xeq/L), as well as the highest mean value (2.18 mg Xeq/L) in lake water, were both detected in summer and followed by autumn, at 4.37 mg Xeq/L and 1.07 mg Xeq/L, respectively.

In summer and autumn, especially in the northern region of the lake, the distribution of TEP was similar to that of Chla concentration, further verifying that cyanobacterial blooms were the major source of TEP. According to earlier reports, EPS produced by phytoplankton photosynthesis is the main source of TEP [38,48,49]. In addition, although cyanobacteria still prevail in autumn, it is also accompanied by the decline in the algal population, the decomposition of the cyanobacteria population, and the shedding of bound EPS, which can directly form TEP. High TEP concentrations have been detected at the end of diatom blooms [38,48]. The dead cyanobacterial cells can rapidly release a large amount of soluble N and P into the water body, resulting in the explosive growth of bacteria in a short time [49]; however, some studies have shown that TEP is relatively resistant to microbial degradation, and the increase in bacterial population during the decline of algal bloom does not affect TEP concentration. This may be because, at this time, to save energy, bacteria may prefer to use low molecular weight organic matter, such as dissolved organic matter, rather than granular TEP [50]. In addition, TEP mainly comprises deoxysugars and galactose [51,52], which are rich in sulfate covalent bonds, and make bacterial degradation of TEP more difficult; besides, the hydrolysis rate of sulfated polysaccharides is slower than that of other polysaccharides [53].

3.4. Temporal and Spatial Changes of Dissolved Acidic Polysaccharides Concentrations

Like TEP, variation coefficients of dAPS during all seasons were less than 1 (Figure 4), indicating a medium variability of the dAPS for all the seasons (i.e., 0.1  <  CV  <  1). The minimum variation coefficient of dAPS was 0.39, also detected in summer, followed by autumn, winter, and spring. The global Moran’s index of dAPS was significantly positive among four seasons (p < 0.05), showing significant positive spatial auto-correlation and strong global spatial agglomeration characteristics of dAPS. This observation suggested the clustering distribution pattern of dAPS in summer. According to Figure 2, the seasonal variation of dAPS was consistent with that of Chl a, and a significant correlation between dAPS concentrations and Chla was observed in summer (R² = 0.22, p < 0.01) and autumn (R² = 0.27, p < 0.01) (Figure 5). Cyanobacterial bloom was high during the blooming periods (summer and autumn); therefore, dAPS are also affected by cyanobacterial blooms, and the main source of dAPS is cyanobacterial cells. The average concentration of dAPS in the water of Lake Taihu was 3.7 mg Xeq/L. Slightly different from TEP, the highest concentration of dAPS (13.12 mg Xeq/L) in the whole of Lake Taihu occurred in autumn. The average dAPS concentration in summer and autumn was 4.9 mg Xeq/L. During the cyanobacterial bloom, a high concentration of TEP and dAPS was detected, which was bound to have an important impact on the aquatic ecosystem. These matters would play a highly significant role in the carbon and nutrients cycling pool, and the transformation mechanism among different states in the EPS pool needs further study.

3.5. Contribution of Transparent Exopolymer Particles and Dissolved Acidic Polysaccharides to Organic Carbon

As shown in Figure 6a, there was a decreasing trend in the changes in POC concentrations during the four seasons from the northwest to the southeast, except for winter. The DOC concentrations also decreased gradually from the northwest to the southeast region of the lake (Figure 6b). Hence, cyanobacterial blooms play an important role in organic carbon cycling. However, cyanobacterial blooms decay in winter and the sources of particulate organic matter become manifold, so the distribution of POC concentrations in winter was different from that in other seasons. To explore the importance of TEP to organic carbon in water, we used the lowest conversion factor (0.51) to estimate the carbon content of TEP. This is the first study to report the estimated contribution of TEP to POC and the contribution of dAPS to DOC in Lake Taihu simultaneously. The average contribution of TEP to POC in Lake Taihu was 24.3% (±31.2%) and accounted for the highest percentage of POC in summer (41%), followed by winter (24.5%), autumn (18.4%), and spring (13.1%) (Table 2). We noted that TEP-C in summer and autumn appeared to exceed 100% of the POC in sites 23#, 25#, 27#, and 16#, indicating a substantial uncertainty associated with estimating the carbon content of TEP. One uncertainty was the difference in aquatic ecosystems; Engel’s formula [32] for TEP-C is based on samples from seawater, not from freshwater, as discussed by Chateauvert (2012) [54]. In addition, these outliers in the present study mostly appeared in the macrophyte-dominated zone, indicating the difference between phytoplankton-dominated and macrophyte-dominated zones. The actual carbon content in TEP in the macrophyte-dominated region may be lower than TEP-C values estimated by us, and the conversion factor of carbon in TEP in this kind of region may be re-estimated in future work.

The rates of the contribution of TEP to POC in Lake Taihu are comparable to those reported in previous studies on organic carbon contributions from marine systems, which corresponds to diatom-dominated phytoplankton ecosystems. The average contribution rate of TEP to the organic carbon pool in summer and autumn, respectively, in the Northeast Atlantic of the United States, was 17% and 18% [6]. In the NW Mediterranean Sea, TEP was estimated to account for up to 22% of total organic carbon [55]. In cyanobacteria-dominated seawaters, for example, TEP in the Baltic Sea can account for up to 40% of POC [56]. Studies have reported that TEP in freshwater lakes, for instance, the Mackenzie River Delta lakes, can account for up to 84% of POC [54]. These results suggest an extremely important role of TEP in the carbon cycle in lakes as well as marine water bodies.

The average contribution of dAPS to DOC in Lake Taihu was 25.9%, in the range of 1.9% to 106.3% (Table 2). The highest mean value of the dAPS to DOC ratio in autumn was 35.1%, followed by summer (30.5%), spring (20.6%), and winter (17.3%). The existing estimate has only been reported by Thornton et al. (2009a) [34], who estimated a 16.0% contribution rate of dAPS to DOC in the tidal estuary area in summer. In contrast, in this study, the contribution rate of dAPS to DOC in the entire Lake Taihu was greater than that in the tidal estuary area, suggesting that dAPS in Lake Taihu represents a non-negligible carbon pool.

4. Conclusions

The transparent extracellular aggregates of TEP and dissolved polymeric substances, dAPS, were investigated in Lake Taihu during four seasons. The mean concentration of TEP and dAPS was 1.31 mg Xeq/L and 3.7 mg Xeq/L, respectively. The distribution pattern of TEP in summer and autumn was similar to that of dAPS, showing a decreasing trend from the northwest to the southeast, which generally correlated with the cyanobacterial bloom. High TEP content was also found in the eastern region of the lake, which harbors aquatic plants. TEP accounted for 24.3% of the total POC, and dAPS accounted for 25.2% of the total DOC in Lake Taihu, indicating that both particulate and dissolved extracellular polymeric substances play an extremely important role in the carbon cycle in lake waters.