Radiation dimming and decreasing water clarity fuel underwater darkening in lakes

Abstract

Long-term decreases in the incident total radiation and water clarity might substantially affect the underwater light environment in aquatic ecosystems. However, the underlying mechanism and relative contributions of radiation dimming and decreasing water clarity to the underwater light environment on a national or global scale remains largely unknown. Here, we present a comprehensive dataset of unprecedented scale in China's lakes to address the combined effects of radiation dimming and decreasing water clarity on underwater darkening. Long-term total radiation and sunshine duration showed 5.8% and 7.9% decreases, respectively, after 2000 compared to 1961-1970, resulting in net radiation dimming. An in situ Secchi disk depth (SDD) dataset in 170 lakes showed that the mean SDD significantly decreased from 1.80 ± 2.19 m before 1995 to 1.28 ± 1.82 m after 2005. SDD remote sensing estimations for 641 lakes with areas ! 10 km 2 showed that SDD markedly decreased from 1.26 ± 0.62 m during 1985-1990 to 1.14 ± 0.66 m during 2005-2010. Radiation dimming and decreasing water clarity jointly caused an approximately 10% decrease in the average available photosynthetically active radiation (PAR) in the euphotic layer. Our results revealed a more important role of decreasing water clarity in underwater darkening than radiation dimming. A meta-analysis of long-term SDD observation data from 61 various waters further elucidated a global extensive underwater darkening. Underwater darkening implies a decrease in water quality for potable water supplies, recession in macrophytes and benthic algae, and decreases in benthic primary production, fishery production, and biodiversity.

How does lake water clarity affect lake thermal processes?

“…The relationship between the SDD and K d varies for lakes in different regions ( Padial and Thomaz, 2008 ; Zhang et al., 2012 ). Such relationships are currently of three general types, including inverse proportional, modified inverse proportional, and power function relationships based on lake-specific data ( Poole and Atkins, 1929 ; Bukata et al., 1998 ; Zhang et al., 2020 ). To express water clarity changes in lake modeling, the relationship between these two parameters can be utilized to investigate thermal responses to lake ecology changes.…”

“…Lake Poyang plays an essential role in supplying water and fishery resources for surrounding cities ( Shankman et al., 2006 ; Zhen et al., 2011 ; Wu et al., 2017 ). Water clarity for this lake has reportedly declined over time, and therefore, aquatic production and biodiversity have likely been affected ( Zhang et al., 2020 ). However, how water clarity changes affect water thermal processes still remains unclear.…”

“…Water clarity data for Lake Poyang were based on in situ observations from national investigations and a literature dataset ( Zhang et al., 2020 ). Water clarity for Lake Poyang has declined over time, with a value of 0.85 m before 1995, and it has decreased to 0.41 m since 2005.…”

“…Two other sets of simulations were carried out with model settings that were the same as the DEFAULT simulation, but with different K d values. For the first five years (2000–2004), these two sets of simulations were conducted with K d of 1.68 m −1 according to the numerical relationship with a water clarity of 0.85 m ( Zhang et al., 2020 ). Then, in 2005, one simulation set maintained the K d at 1.68 (referred to as CTL), but the other simulation set the K d to 3.44 m −1 , corresponding to a lower water clarity of 0.41 m (referred to as DARK).…”

Underwater light attenuation inhibits native submerged plants and facilitates the invasive co‐occurring plant Cabomba caroliniana

“…As a result of global changes, more frequent flooding has occurred in China, resulting in decreasing levels of underwater photosynthetically active radiation (PAR) and increasing levels of water turbidity in freshwater ecosystems (Wu & Ding, 2019). Both the annual average available PAR and the transparency of lakes in China have generally shown a clear decreasing trend in the past few decades, being particularly apparent for lakes in East China, which have experienced more significant declines due mainly to the high levels of water and air pollution (Shen et al, 2022;Zhang et al, 2020). This underwater darkening has many adverse effects: it may significantly reduce species abundances, disrupt food chains and change the energy flows and material circulation paths of freshwater ecosystems (Zhang et al, 2020(Zhang et al, , 2022.…”