ArticleRadiation dimming and decreasing water clarity fuel underwater darkening in lakes
Graphical abstract
Introduction
Solar radiation reaching the Earth’s surface is the predominant energy source to support life, exerting many profound effects on the surface temperature, evaporation, the hydrological cycle, and the structure, function and service of terrestrial and aquatic ecosystems [1], [2], [3], [4]. In the past six decades, radiation dimming has been observed worldwide, causing a decrease of 4% to 6% in surface total radiation, due to air pollution and associated increases in aerosols caused by intense human activities [5], [6], [7], [8]. Since the 1990s, a partial recovery has been observed in Europe and North America, known as radiation brightening, due to the increase of atmospheric clear-sky transparency [1], [8]. However, the current surface total radiation worldwide remains markedly lower than in the 1950s, despite 20 years of radiation brightening [9], [10].
In China, a marked dimming in surface total radiation was observed nationwide from 1960 to 1990, while some studies reported a slight recovery in brightening since the 1990s [1], [5]. However, in the early years of the 2000s, renewed dimming occurred, which was partially attributed to a doubling in the consumption of coal in China between 2002 and 2007 [8], [9]. A study in 2018 indicated that the transition from dimming to brightening in China may have only occurred after 2005, based on carefully homogenized data [7]. In China, the high levels of air pollution have counteracted the effects of the radiation brightening since 1990, in contrast to the results obtained from Europe and North America.
Surface total radiation, or more specifically, its photosynthetically active radiation (PAR, 400–700 nm) is the primary driver of primary production. As PAR is proportional to total radiation, the decrease in PAR caused by radiation dimming may decrease carbon uptake of the biosphere and affect the carbon cycle. Furthermore, the gross flux of carbon taken up during photosynthesis is positively and linearly correlated with PAR, and a significant decrease in surface total radiation and PAR may exert a marked effect on the role of vegetation as a carbon sink [3], [11].
Although nutrient limitation is an important paradigm for aquatic plant growth in lakes, the importance of light availability is often underestimated, particularly for the wide distribution of nutrient-poor lakes [12], [13]. In aquatic ecosystems, both phytoplankton and aquatic macrophytes depend on a sufficient amount of underwater radiation for photosynthesis [13], [14], [15], and thus both the biomass and primary production are sensitive to light availability and underwater darkening [16]. Underwater darkening might substantially decrease benthic primary production, destroy food resources and result in a deterioration of the predatory environment for photosensitive aquatic organisms [13], [17], [18]. However, few studies have assessed the effects of underwater darkening on aquatic ecosystems [16]. For aquatic ecosystems, underwater darkening may be caused by a combination of radiation dimming and decreasing water clarity. In summary, water clarity is jointly determined by pure water and three optically active substances: phytoplankton, nonphytoplankton particulate, and chromophoric dissolved organic matter. Therefore, any processes resulting in changes in concentration and composition of three optically active substances will affect water clarity including eutrophication, pollution, browning, land use change and increased soil erosion in the catchment caused by vegetation destruction.
For determining water clarity, Secchi disk depth (SDD) is a quick and useful method to evaluate the underwater light environment and the trophic state of aquatic ecosystems [19], [20]. Traditional in situ SDD measurement using a Secchi disk have been performed for more than 150 years [19]. In the past few decades, remote sensing is considered an important supplement or substitute to obtain an overview of the temporal-spatial patterns of SDD due to its unparalleled advantages of collecting measurements from historical and large-scale perspectives [21]. Many arduous efforts have attempted to develop a series of empirical, semi-empirical, and semi-analytical algorithms to estimate SDD and explore the long-term changes in different inland waters using Landsat, MERIS, MODIS, and other images [20], [22], [23], [24], [25]. A decrease in water clarity is associated with a reduced water quality, the loss of macrophytes and benthic algae, a decrease in primary production, decreases in habitat and fishery production, biodiversity loss, and a decrease in recreational value [13], [18], [26], [27]. Some previous studies have attempted to explore long-term trends in SDD and elucidate the potential effects for many regional lakes [20], [25], [28]. However, China lacks a recent national dataset of changes in lake water clarity, which prevents a comprehensive and systematic assessment of national trends, and limits our understanding of changes in carbon source or sink dynamics and other changes in the ecosystem. More importantly, no published study from any country has examined the combined contributions of radiation dimming and decreases in water clarity on underwater darkening.
We propose a new concept that radiation dimming and decreasing water clarity jointly result in accelerating underwater darkening and decreasing water clarity plays a more important role than radiation dimming. We pursue three specific objectives to: (1) quantify whether a decrease in incident total radiation and sunshine duration has occurred throughout China, (2) quantify whether a nation-wide decrease in lake water clarity has been observed, and (3) elucidate the combined or interactive effects of radiation dimming and decreasing water clarity on the underwater light environment in lakes, and discuss the implications for lake production and carbon dynamics. Our results provide a new perspective on underwater darkening and future studies of carbon dynamics in lakes. Considering the complex and different processes affecting the SDD in a specific lake, our study has not attempted to elucidate the specific factors affecting SDD dynamics.
Section snippets
Surface total radiation and sunshine duration
Surface total radiation data were examined from a 53-year record from 1961 to 2013 at 116 radiation observation stations in China, which was the new and complete total radiation data that we were able to collect. Data were obtained from the archives of the National Meteorological Information Center of the China Meteorological Administration (NMIC/CMA). Detailed information on station names, codes, altitude, starting and ending years of the observation period, and linear fitting of yearly total
Radiation dimming in China
Marked trends in long-term yearly mean total radiation were observed at the 116 individual stations in China from 1961 to 2013 (Fig. S2 online). Total radiation decreased at 91 stations (78.4% of stations), with a statistically significant decrease recorded for 50 stations (43.1%). In contrast, an increase in total radiation was observed at 25 stations (21.6%), with a statistically significantly increase only observed at 4 stations (3.4%). For 14 of the 25 stations with increasing trends, total
Radiation dimming
In China, the yearly mean total radiation was 5.8% lower after 2000 compared to 1961–1970, although a slight brightening was observed in a minority of stations (Fig. S2 and Table S1 online). The sunshine duration decreased through 2013 in China (Fig. S3 online). Severe aerosol and air pollution over China potentially reduce both the incident total radiation and sunshine duration [8], [39]. With the implementation of the “Atmospheric Pollution Prevention and Control Action Plan”, atmospheric
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was jointly supported by the National Natural Science Foundation of China (41621002, 41790423, 41930760), the Key Program of the Chinese Academy of Sciences (ZDRW-ZS-2017-3-4), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDB-SSW-DQC016). We would like to thank our all colleagues attending the second National Lake Investigation, and Hans Paerl, Karl Havens, and Chengfeng Le for their helpful comments and suggestions to improve quality of this
Author contributions
Yunlin Zhang, Boqiang Qin and Kun Shi designed the study. Yunlin Zhang, Kun Shi, Yibo Zhang, Jianming Deng, Yongqiang Zhou, Xiaolong Yao, Miao Liu, Guangwei Zhu, and Lu Zhang conducted the experiments and collected and analyzed the data. Yunlin Zhang wrote the main manuscript text. Boqiang Qin, Martin Wild, Lin Li, Binhe Gu and Justin D. Brookes contributed to writing and editing the manuscript. All authors discussed the results and contributed to editing the manuscript.
Yunlin Zhang is a professor at Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. His study interests cover lake optics and water colour remote sensing, lakes thermodynamics, and chromophoric dissolved organic matter biogeochemistry cycle.
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Yunlin Zhang is a professor at Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. His study interests cover lake optics and water colour remote sensing, lakes thermodynamics, and chromophoric dissolved organic matter biogeochemistry cycle.
Boqiang Qin is a professor at Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, and Director of Taihu Laboratory for Lake Ecosystem Research. His study fields include hydrology, physical limnology, and aquatic ecology. Currently he focuses on dynamics of Lake Eutrophication, nutrient reduction and ecological restoration.