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All issues Volume 3 / No 5 (2024) Natl Sci Open, 3 5 (2024) 20230061 Full HTML
Open Access
Issue
Natl Sci Open
Volume 3, Number 5, 2024
Article Number 20230061
Number of page(s) 16
Section Earth and Environmental Sciences
DOI https://doi.org/10.1360/nso/20230061
Published online 29 February 2024

© The Author(s) 2024. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

Seasonal ice covers greater than half of the world’s lakes [1,2]. Lake ice can be seen as a direct physical characteristic of climate change, whose changes are affected by external environmental factors, such as temperature. Influenced by internal factor differences (e.g., area, depth, and salinity), lakes differ in their freeze-thaw duration and ice thickness [3,4]. In recent years, under rising global temperatures, a reduction in lake ice duration and lake ice thickness has been observed in the northern hemisphere [5]. Lake surface temperatures have increased by an average rate of 0.34°C year−1. Moreover, an approximate five day reduction in the lake ice-covered period has been observed for every 1°C increase in average temperature [6]. Changes in lake ice duration and ice thickness will lead to dynamic seasonal lake alterations that will affect ecosystem function of lakes [7].

In aquatic ecosystems, gross primary productivity (GPP) is the ability of primary producers, such as phytoplankton, epiphytic algae, and aquatic vascular plants, convert inorganic carbon to organic carbon through photosynthesis per unit time [8]. Phytoplankton are the most important primary producers in lacustrine ecosystems, while their biomass and community structure control the GPP of waterbodies. Many environmental factors, such as salinity [9], pH [10], water level [11], air temperature [12], and solar radiation [13], will directly or indirectly affect phytoplankton biomass and community structure. At present, research on lake GPP is mainly concentrated in early autumn and summer. This is because these seasons represent the rapid growth period of phytoplankton. In winter, due to the limited sampling conditions and its indeterminate open water season, lake GPP during the freezing period has been typically considered less important. Therefore, impact factors (i.e., lake ice) have often been ignored during this period [14]. Subglacial habitats are less dormant than once assumed. For instance, phytoplankton life activities continue during ice cover periods. Moreover, carbon dioxide (CO2) always accumulates under ice, for which most is externally emitted during periods of ice melt. Biologically, winter is harsh, but key life activities continue under ice [15,16].

In low temperature environments, phytoplankton communities mainly composed of diatoms and green algae are formed. For example, in Noth America, Lake Erie’s diatom community remains strongly active in winter [17,18]. At the same time, phytoplankton blooms have been shown to occur under ice in Russia’s Lake Baikal, Sweden’s Lake Erken, and lakes in central Japan [19–21]. Moreover, sunlight continues to drive primary production in many snow-covered lakes. When lakes are free of snow cover, photosynthetically active radiation (PAR) on the ice surface can reach up to 95%, which ensures the continuance of normal photosynthetic algal processes [22,23]. Low dissolved oxygen (DO), or hypoxia, is common in snow-covered lakes in winter. Moreover, ice and snow cover prevent lake water oxidation through diffusion and aeration. Some organisms may even die from hypoxia during extended periods of ice cover [24]. However, some studies have suggested that the potential of wind-blown snow is greater at altitudes above 3900 m, where wind transports snow from areas above 3900 m to lower altitudes [25]. Therefore, at higher altitudes, wind speed is strong in winter. Accordingly, light transmittance is higher at higher altitudes, helping to maintain ongoing photosynthetic phytoplankton processes and oxygen release in lake water, which subsequently maintain life activities under the ice. Furthermore, the ice-free period is also an important phytoplankton growth stage.

The Qinghai-Tibetan Plateau (QTP), the world’s largest and highest plateau, is commonly known as the “Asian Water Tower” (AWT). It has been shown to be extremely sensitive to global climate change [9]. Under climate warming, the lake ice-covered period on the QTP is generally delayed, the melting period is advanced, and the duration of ice cover is reduced [18]. This reduction in ice cover duration will have an enormous impact on plateau lake GPP. To date, research on inland lake GPP has mainly focused on the open water season or in low altitude areas [15]. Additionally, only limited research exists on the regulation of phytoplankton growth respective to plateau lake ice cover duration, which seriously limits our ability to predict prospective lake ecology and the production capacity in plateau regions. Here, we hypothesized that dynamic changes in lake ice are key regulatory factors associated with plateau lake GPP. At the same time, GPP is also affected by meteorological factors. Accordingly, this study aimed (1) to analyze the dynamic changes in lake ice on the QTP during the first two decades of the twenty-first century; (2) to determine spatial lake GPP patterns on the QTP during the same period; (3) to explore the regulatory mechanism of dynamic changes in lake ice on lake GPP and carbon source and sink status under a background of climate change.

RESULTS AND DISCUSSION

Changes in lake ice phenology and thickness on the QTP between 2002–2019

Between 2002–2019, the trend in lake ice duration on the QTP can be defined as periodic fluctuations. During this period, lake ice duration ranged from 106.27 to 139.52 d (123.36±2.43 d average). The shortest average lake ice duration was observed in 2010 (106.28 d) and the longest was observed in 2012 (139.52 d) (Figure 1a, Table S1). The average change rate in lake ice was −2.20 d year−1 between 2002–2010 and −0.93 d year−1 between 2011–2019. Overall, lake ice on the QTP exhibited a periodic shortening trend. However, lake ice duration in high-altitude lakes was longer (i.e., 125.65±5.99 d), followed by mid-altitude lakes (i.e., 76.38±6.27 d). Although Figure 1b shows that lake ice duration of low-altitude lakes was 133 d, this value is not representative due to the small sample size. It is generally believed that lake ice duration increases with increased altitude. Lake size also influences lake ice duration. Lake ice duration was longest for mid-sized lakes (126.05±7.77 d), followed by small-sized lakes (124.42±10.91 d) and large-sized lakes (112.71±14.40 d) (Figure 1c). Moreover, lake ice duration was longer for shallow lakes (131.13±11.19 d) compared to deep lakes (122.18±6.42 d) (Figure 1d).

thumbnail Figure 1

Trends in lake ice duration on the QTP between 2002–2019.

Figure 2a shows changes in annual average and maximum and minimum thickness of lake ice on the QTP between 2002–2019. This figure clearly shows that average, maximum, and minimum lake ice thicknesses on the QTP has fluctuated over the past 20 years. The average ice thickness was between 55–66 cm; the maximum ice thickness was between 90–102 cm; the minimum ice thickness was between 4–24 cm. Figure 2b shows the spatial distribution of lake ice thickness on the QTP. As shown in these figures, lake ice thickness on the QTP exhibits clear spatial characteristics. Lake ice layers in certain smaller areas in the northern, northwestern, and southwestern regions of the QTP were thicker (i.e., approximately 0.55–0.94 cm) while those in its eastern, southern, and northeastern regions were thinner (i.e., approximately 0.12–0.54 cm). Distribution characteristics of lake ice thickness may be related to the topographical characteristics of the QTP, namely, high in the west and low in the east (Figure 2b).

thumbnail Figure 2

Changes in lake ice thickness on the QTP between 2002–2019.

Changes in GPP characteristics on the QTP between 2002–2019

Between 2002–2019, lake GPP on the QTP ranged from 0.17 to 3.35 g C m−2 d−1 (Figure 3). For lake basins located in the northeastern, southern, and southwestern regions, the spatial distribution of GPP on the QTP was relatively high, while lake GPP differed at different altitudes. For low-altitude lakes, freshwater lake GPP was 4.04 g C m−2 d−1. For mid-altitude lakes, freshwater and saline lake GPP was 0.83 and 0.61 ± 0.22 g C m−2 d−1, respectively. For high-altitude lakes, freshwater and saline lake GPP was 0.92 ± 0.23 g C m−2 d−1 and 0.61 ± 0.10 g C m−2 d−1, respectively. Lake GPP was highest in low-altitude lakes, while freshwater lake GPP was generally higher than saltwater lakes (Figure 4a). Lake GPP also differed for different sized lakes. For example, the GPP of small-sized, mid-sized, and large-sized freshwater lakes was 1.38 ± 0.94, 0.68 ± 0.11, and 2.74 g C m−2 d−1, respectively. Correspondingly, saline lake GPP was 0.69 ± 0.29, 0.74 ± 0.14, and 0.43 ± 0.07 g C m−2 d−1, respectively. In general, the GPP of larger-sized freshwater lakes was higher, while that of saltwater lakes increased as lake size decreased (Figure 4b). Additionally, lake depth and the lake recharge type can also affect lake GPP. Figure 4c shows that the GPP of deep freshwater lakes was 0.92 ± 0.22 g C m−2 d−1, while the GPP of shallow saltwater lakes (0.90 ± 0.46 g C m−2 d−1) was higher compared to deep saltwater lakes (0.61 ± 0.09 g C m−2 d−1). Figure 4d shows the GPP of non-glacial recharge lakes was generally lower than that of glacial recharge lakes. The GPP of glacial recharge freshwater and saltwater lakes was 0.64 ± 0.12 and 0.59 ± 0.13 g C m−2 d−1, respectively, while the GPP of non-glacial recharge freshwater and saltwater lake GPP was 1.34 ± 0.50 and 0.90 ± 0.16 g C m−2 d−1, respectively.

thumbnail Figure 3

Spatial distribution of lake GPP on the QTP between 2002–2019.
thumbnail Figure 4

Changes in lake GPP on the QTP under different environmental gradients.

Changes in environmental variables

Between 2002–2019, the average temperature on the QTP was between 17°C–21°C. Temperatures in lower altitude northeastern, eastern, and southwestern regions of the QTP were higher, while temperatures in central, northwestern, and western regions were relatively low (Figure 5a). Between 2002–2019, the average precipitation rate of the QTP ranged between 0–5200 mm. From a spatial distribution perspective, precipitation rates in the northern and northwestern regions of the QTP were generally higher compared to the central and southern regions (Figure 5b). Due to the relatively high altitude of the central QTP region, spatial wind speed characteristics were generally high in the middle section of this region and low in its surrounding areas, with an annual average wind speed between 0–4.7 m s−1 (Figure 5c). On the QTP, the annual average solar radiation value is 4159–8544 MJ m−2. Solar radiation is highest in the southwestern region of the QTP and gradually decreases in an eastward direction (Figure 5d).

thumbnail Figure 5

Distribution of average temperature, precipitation, wind speed, solar radiation, and snow depth on the QTP between 2002–2019.

Lake depth on the QTP ranges between 1.3–120 m, and deeper lakes are generally larger. Water levels generally range between 2795–5386 m, which is mostly due to the QTP’s high altitude (Figure 6a). Lake salinity levels on the QTP vary greatly, most being saline. Lake salinity levels are between 0‰–115.2‰. Additionally, lakes on the QTP are basically alkaline, with a pH range between 7–10.4. Generally, lake pH values gradually increase as altitude increases. DO concentrations ranged between 4.5–12.8 mg L−1, with an average value of 6.93 ± 0.31 mg L−1. As altitude increases, DO concentrations in lakes decrease (Figure 6b).

thumbnail Figure 6

QTP lake salinity, water level, depth, area, pH, DO, and altitude values.

Influencing factors related to lake ice phenology and ice thickness change

Lake ice conditions are affected by both the climate status and physical and chemical characteristics. On the one hand, climatic conditions are determined by the geographical location of lakes, which directly correlate to the duration and thickness of lake ice [26,27]. Air and water temperatures of certain high altitudinal or latitudinal lakes are warmer, while the duration of lake ice cover may be longer. On the other hand, more mineralized lakes have a lower freezing point, while larger and deeper lakes require more heat during freezing/fracturing processes. Results confirmed that lake ice duration and thickness on the QTP significantly and positively correlated with wind speed, altitude, water level, and salinity. However, lake ice duration significantly and negatively correlated with temperature, precipitation, solar radiation, and water depth, while ice thickness significantly and negatively correlated with temperature, precipitation, and pH (Figure 7a).

thumbnail Figure 7

The interaction model of lake ice, GPP and environmental factors on the QTP. (a) represents for spearman correlation analysis of lake ice, GPP, meteorological factors, and physical and chemical factors; (b)–(d) represent lake GPP and CO2 exchange patterns on the QTP under different lake ice cover statuses ((b) where lakes are completely covered by ice; (c) where lake ice calving appears prior to melting; (d) where lake ice has completely melted; red arrows represent gas emissions; black arrows represent gas consumption).

In recent decades, the QTP and most of its surrounding area have experienced a significant warming trend. Between 1980 to 2018, its average annual temperature (0.42°C 10 year−1) is more than twice the global warming rate over the same period (0.19°C 10 year−1) [28,29]. This continuous temperature increase has reduced lake ice duration on the QTP. As the temperature rises, precipitation also shows an increasing trend. Data shows that from 1980 to 2018, the annual precipitation on the QTP increased by 11 ± 9.7 mm 10 year−1 [30]. Rainfall carries atmospheric particulates that affix to lake ice, reducing the capacity of lake ice to reflect light while simultaneously increasing its melting rate [31,32]. At the same time, researchers have found that in recent decades precipitation has become the main reason behind lake expansion [33], and this has been coupled with increasing temperatures during the same period. Also, an increase in glacial meltwater will effectively recharge rivers and lakes [34], which will subsequently increase the lake area of the QTP. In this study, that the duration of larger lake ice cover was found to be shortest (Figure 1c). Therefore, influenced by precipitation and glacial meltwater, lake area on the QTP is continuously increasing, and the larger the lake area is, the greater its water storage capacity will be. The enhanced dynamic mixing of lakes is conducive to vertical heat transfer processes. This will effectively reduce water surface evaporation and heat consumption, increasing the overall heat capacity while slowing down lake freezing processes [35]. This also explains the significant negative correlation observed between lake ice duration and ice thickness and precipitation.

Wind speed is also an important factor affecting the formation and dissipation of lake ice. Wind accelerates lake convection processes, especially in high altitude areas during the winter, and strong wind speeds may move cold air to areas above lakes, accelerating the freezing process and delaying the calving process [36]. In recent years, wind speeds on the QTP have gradually weakened. This has delayed lake ice freezing periods, which may be involved in reducing ice cover duration. Additionally, shortwave radiation significantly affects lake ice freezing and melting processes [37]. The amount of solar radiation on the QTP shows a fluctuating weakening trend. Although the duration of the ice cover is generally shortened, in some years, such as 2005, 2007, and 2008 (Figure 1a), the duration of the ice cover increased slightly. Therefore, there was a negatve correlation between radiation and ice sheet duration (P < 0.05).

In addition to the meteorological factors mentioned above, the ice condition of the lake is also closely related to the physical and chemical properties of the lake. Being influenced by meteorological factors (i.e., temperature and precipitation), lake water storage capacities on the QTP have risen, lake area has continued to expand, and lake depth has increased. Consequentially, lakes on the QTP freeze slowly in winter. Lake salinity is also a key factor affecting lake ice conditions [38]. For freshwater lakes, lake size expansion makes it more difficult for ice cover to form during colder seasons. Under the background of climate warming, the salinity levels of the widely distributed saltwater lakes on the QTP have decreased due to increased precipitation and glacier meltwater. As salinity levels decrease, the freezing point of lakes increase, and lake water will more easily freeze. Therefore, at least for some QTP lakes, ice cover duration is longer and ice thickness is greater.

Lake carbon sequestration capacity response mechanisms to lake ice under environmental change

Inland water contributes significantly to the C budget, and GPP is key to how lake C sequestration functions [39,40]. Phytoplankton biomass and community structure control the GPP of aquatic bodies. Sunlight, together with temperature, dissolved oxygen, and other relevant environmental factors, limit the photosynthetic rate, which affects phytoplankton growth and biomass [41]. Intense photosynthesis can increase the primary production of phytoplankton, reduce CO2 saturation within waterbodies, and benefit the C sink capacity of lakes [40]. At the same time, results from this study showed a significant negative correlation between ice sheet duration and ice thickness and GPP (P < 0.05). Ice cover greatly reduces the aeration potential of lake water while also weakening light conditions in the euphotic zone. Additionally, winter is extremely cold on the QTP, which negatively impacts the primary production that occurs beneath ice cover (Figure 7a) [15].

Generally, light and temperature could be the main limiting factors for subglacial photosynthesis in winter. Ice conditions and snow cover cause inhomogeneous spatial and temporal light transmission conditions while altering spectral distributions. When not covered by snow, unobstructed surface lake ice can transmit up to 95% of PAR. However, phytoplankton biomass accumulation at the ice-water interface may be limited by light conditions under a >13.5 cm snow depth [42]. In recent years, under climate change, there has been a reduction in ice cover duration and a thinning in ice layers in at least some lakes on the QTP, leading to higher overall light transmittance. Moreover, results from this study showed that snow thickness on lake ice mostly ranged between 0–10 cm on the QTP. Only a limited number of lakes had a snow thickness above 10 cm. In addition, strong wind conditions can blow snow away from ice cover, increasing light utilization beneath lake ice [43]. Moreover, the high wind speeds that the QTP can also help clear some snow from lake ice surfaces. Therefore, although the lakes of the QTP are covered with ice and snow in winter, the underwater environment of most lakes is not completely dark. At the same time, phytoplankton can also optimize light availability by changing their pigment composition, the number and size of their photosynthetic units, and the ratio of their photoprotection to photosynthetic pigment color. This helps phytoplankton to better adapt to changes during lake ice freeze/thaw periods and under intense solar radiation [18,44].

In addition to these limiting light and temperature factors, anoxic conditions may also be a major challenge for organisms that subsist beneath ice. Oxygen consumption by lake sediments is an important cause of anoxic in winter [45,46]. In early winter, before ice cover form, oxygen is evenly distributed throughout the water column through mixing. Due to the high solubility of oxygen at lower temperatures, oxygen concentrations remain high. By mid-winter, as ice caps affix, available oxygen is used up within a few weeks of freezing when the oxygen supply is cutoff, especially in some small lakes with organic sediments [47,48]. In deep lakes, only the shallow layer near the bottom becomes anoxic [49]. At the same time, aquatic organisms mainly reside within the euphotic zone beneath ice during winter and are therefore less affected by the anoxic conditions at the bottom of lakes compared to shallow lakes. Moreover, Lei et al. [50] found that lakes may be recharged by groundwater in the western region of the QTP. During ice cover periods, water levels in most endogenous lakes in the western region of the QTP increase. Although ice cover acts as a barrier against wind-driven mixing processes during winter, convective mixing still occurs beneath ice. This can be due to differences in thermal gradients, which can transport some DO into the euphotic zone [46]. At the same time, during photosynthesis, phytoplankton will also release a certain amount of oxygen to maintain the oxygen supply. Also, temperatures are lower during winter, especially in high-altitude regions such as the QTP. At low temperatures, diatoms and green algae dominate, and this has been found to be the case for the QTP [51]. Beneath ice, the zooplankton feeding rate is generally low [52], which may lead to phytoplankton blooms. Therefore, accounting for the influence of these various factors, QTP lakes may also maintain a certain amount of primary production throughout the duration of the ice sheet period (Figure 7b).

As previously mentioned, life activities beneath ice continue under certain conditions during winter. However, under low winter temperatures, phytoplankton respiration rates may become extremely high, and the CO2 produced by respiration cannot be completely consumed through photosynthesis [53]. Ice cover effectively stops any gas exchanges from occurring between waterbodies and the atmosphere. Therefore, CO2 continuously accumulates under the ice during winter. This will subsequently lead to the release of a large amount of CO2 in spring when ice melts and the water column circulation processes (overturning) commence [54] (Figure 7c). When ice cover duration is reduced, phytoplankton photosynthetic processes continue unobstructed in the euphotic zone under relatively sufficient oxic conditions, decreasing overall pCO2 levels. When lake ice melts in spring, the amount of CO2 emitted into the atmosphere will decrease. At present, most studies have shown that QTP lakes mainly act as a “C source”, continuously emitting CO2 into the atmosphere [53,55,56]. However, under the influence of climate change, ice cover duration for greater than half of QTP lakes is continuously decreasing. Aquatic primary production processes will increase under a reduction in the duration and thinning of ice cover, while also reducing CO2 emissions from subglacial organisms in winter. Studies have shown that CO2 emissions during the melting of thermokarst lakes on the QTP account for 19.2% of the annual CO2 emissions [57]. Additionally, combined with other published data, the CO2 emissions during the melting of boreal lake account for approximately 17% of the annual CO2 emissions. Although such an explosive emission is difficult to capture using conventional sampling techniques, it could have a profound impact on annual lake CO2 emission budgets [54,58]. The CO2 emission on the QTP lakes in 2020s is approximately 1.16 Tg C a−1 [53]. If CO2 lake emissions account for approximately 17% of all annual lake CO2 emissions during the ice melt period, it would then account for approximately 0.20 Tg C a−1 on the QTP alone. In the future, that number is likely to be even smaller.

Therefore, in the context of global climate change, influenced by meteorological and physical factors such as temperature, light and DO, the duration of lake ice in most lakes on the QTP has been continuously shortened and the thickness of ice cover has become thinner, which will increase the carbon sequestration rate of underwater organisms in winter and reduce the outflow of CO2 during ice melting. In addition, with the shortening of the duration of the ice sheet, the ice-free period of lakes on the QTP is extended, and phytoplankton have a longer time and more suitable conditions for photosynthesis (Figure 7d). Phytoplankton continuously converts inorganic carbon into organic carbon through photosynthesis, which is buried in the sediment and finally achieves carbon fixation. In the future, the lakes of the QTP have great potential of C sink, and the shortening of lake ice cover in winter may contribute to the improvement of the carbon sequestration capacity of the lakes of the QTP.

CONCLUSIONS

This study found a decreasing trend in lake ice cover duration on the QTP over the past twenty years (i.e., the beginning of the 21st century). Moreover, the duration and thickness of ice cover negatively affected lake GPP. Under the influence of lake ice duration and ice thickness, euphotic light conditions and lake temperature and DO concentrations made it increasingly suitable for phytoplankton to survive winter ice-covered periods. This will increase lake primary production while reducing the amount of CO2 that is emitted to the atmosphere. Additionally, under a reduction in lake ice cover duration and an extension in the lake ice-free period on the QTP, lakes will increasingly absorb CO2. Although lakes on the QTP presently act as “C sources”, their future potential to transform into “C sinks” is great.

MATERIALS AND METHODS

Study area

The study area is located on the QTP (26°00′ N–39°47′ N, 73°19′ E–104°47′ E). The length of the QTP is approximately 2800 km from east to west and the width is 1000 km from north to south, covering a total area of approximately 2.5×106 km2. With an average altitude greater than 4000 m, it is commonly known as the “Roof of the World”. Its terrain is complex, which includes alpine glaciers, grassland canyons, rivers, and lakes. The climate of the QTP is typical of China’s high-altitude regions. Compared to similar latitudinal regions, temperatures on the QTP are relatively low while wind speeds are strong. The annual average temperature on the QTP is approximately −6°C–20°C, while spatial differences in water and thermal conditions significantly differ between the east and west. There is a decreasing trend in annual average precipitation from the southeast to the northwest, namely, from approximately 2000 mm in the former to 50 mm in the latter [59].

The QTP marks the birthplace of many of Asia’s large rivers. Its total lake area is 4 × 104 km2, accounting for approximately 57% of China’s total lake area. The QTP’s unique alpine climate has led to the formation of its relatively primitive and diverse lakes, mainly comprising of freshwater lakes and saltwater lakes [51] (Figure 8). The lakes of the Tibetan Plateau are covered with ice for 4–5 months of the year, and the ice cover and winter dynamics are very sensitive to small changes in the global heat budget. The interaction of alpine lakes on the QTP with monsoon circulation and global water cycle makes the lakes more responsive to global changes [60].

thumbnail Figure 8

Location of the study area and sample site distribution.

Data source

For this study, lake ice phenology data were obtained from China’s National QTP Science Data Center ( https://data.tpdc.ac.cn/zhhans/data/c4480050-ece0-4623-867d-80236bccd885). Lake ice thickness data derived from the Climate Data Store ( https://cds.climate.copernicus.eu/cdsapp#!/search?type=dataset). GPP data derived from a study by Deng et al. [27,61]. Precipitation, temperature, solar radiation, wind speed, and snow depth data derived from the Climate Data Store ( https://cds.climate.copernicus.eu/cdsapp#!/search?type=dataset).

Data calculation and statistical analysis

Ice phenology

For this study, lake ice duration was calculated on the calendar year. Therefore, lake ice duration is defined as the number of days during which lake ice-covered between the first day of the year and the last day of the year. In other words, it is the number of days from January 1 to spring and summer when the lake ice is completely melted plus the number of days in the fall and winter when the lake ice is completely frozen until the last day of December.

GPP

P P eu = 0.66125 P opt B ⋅ E 0 E 0 + 4.1 ⋅ Z eu ⋅ C opt ⋅ Dirr,

where P opt B is the maximum C fixation rate within a water column (mg C/[mg Chl h]); E0 is the PAR of surface photosynthetic effective radiation of a lake system; Zeu is the euphotic depth (m); Copt is the chlorophyll a (Chl-a) content at location P opt B, which can be substituted for remotely sensed surface Chl-a data; Dirr is day length in decimal hours.

Statistical analysis

According to the lake classification criteria established by Dou et al. [62] and Wang and Dou [63], as well as other relevant literature [64], lakes are classified based on their elevation, salinity levels, and surface area size. In accordance with altitude gradients, the lakes selected for this study were subdivided into low-altitude lakes (with an altitude <2800 m), mid-altitude lakes (with an altitude between 2800 and 3500 m), and high-altitude lakes (with an altitude >3500 m). For area gradients, lakes were subdivided into large-sized lakes (with an area >500 km2), mid-sized lakes (with an area between 100 and 500 km2), and small-sized lakes (with an area <100 km2). For depth gradients, lakes were subdivided into shallow lakes (depth level <6 m) and deep lakes (depth level >6 m).

This study used Spearman’s rank correlation coefficients to test correlations between lake ice duration and lake water GPP. Linear regressions were performed on each lake to calculate the average rate of change.

Data availability

The original data are available from corresponding authors upon reasonable request.

Acknowledgments

The authors of this study would like to thank all anonymous reviewers for their helpful remarks. We thank Brian Doonan (McGill University, Canada) for his help in writing this paper and providing useful suggestions.

Funding

This work was supported by the National Natural Science Foundation of China (42225103 and 31988102), and the Chinese Academy of Sciences (CAS) Project for Young Scientists in Basic Research (YSBR-037).

Author contributions

Y.G. designed and conceptualized this paper. D.S. and Y.W. wrote the original draft. J.J., S.W. and K.S. reviewed and edited the paper.

Conflict of interest

The authors declare no conflict of interest.

Supplementary information

Supplementary file provided by the authors. Access here

The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

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All Figures

thumbnail Figure 1

Trends in lake ice duration on the QTP between 2002–2019.
In the text
thumbnail Figure 2

Changes in lake ice thickness on the QTP between 2002–2019.
In the text
thumbnail Figure 3

Spatial distribution of lake GPP on the QTP between 2002–2019.
In the text
thumbnail Figure 4

Changes in lake GPP on the QTP under different environmental gradients.
In the text
thumbnail Figure 5

Distribution of average temperature, precipitation, wind speed, solar radiation, and snow depth on the QTP between 2002–2019.
In the text
thumbnail Figure 6

QTP lake salinity, water level, depth, area, pH, DO, and altitude values.
In the text
thumbnail Figure 7

The interaction model of lake ice, GPP and environmental factors on the QTP. (a) represents for spearman correlation analysis of lake ice, GPP, meteorological factors, and physical and chemical factors; (b)–(d) represent lake GPP and CO2 exchange patterns on the QTP under different lake ice cover statuses ((b) where lakes are completely covered by ice; (c) where lake ice calving appears prior to melting; (d) where lake ice has completely melted; red arrows represent gas emissions; black arrows represent gas consumption).
In the text
thumbnail Figure 8

Location of the study area and sample site distribution.
In the text

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