Key Takeaways & Executive Findings
- •• • The mean light attenuation coefficient (Kd) was 10.31±3.76 m⁻¹, indicating severe light limitation; to support submerged macrophyte colonization, Kd must be reduced to ≤4.95 m⁻¹, a 52% reduction, which is essential for achieving the required euphotic depth. • • The mean euphotic depth (Zeu) was 0.53±0.24 m, significantly lower than the average water depth of 0.94±0.29 m; thus, Zeu must be increased to ≥0.94 m to allow light penetration to the bottom, a critical prerequisite for macrophyte establishment. • • Water transparency (SD) must be ≥0.41 m, derived from the Zeu–SD relationship; this corresponds to a Secchi depth increase of approximately 77% from current levels, which is a practical target for water clarity management. • • Chlorophyll-a (Chl-a) must be ≤3.8 μg/L, as it was identified as the most influential direct factor on light attenuation; controlling algal biomass to this threshold is necessary to reduce Kd and improve underwater light availability.
Abstract
The underwater light environment is a critical limiting factor for the colonization of submerged macrophytes and the ecological restoration of shallow lakes. Previous studies rarely quantified the contribution of aquatic environmental factors to the water quality–underwater light–macrophyte relationship, nor did they comprehensively consider factor correlations or establish thresholds for macrophyte colonization. This study, conducted in a typical national wetland nature reserve (Hongze Lake), measured photosynthetically active radiation, light attenuation coefficient (Kd), euphotic depth (Zeu), water transparency (SD), total suspended solids (TSS), chlorophyll-a (Chl-a), total nitrogen (TN), and total phosphorus (TP). A simulation model for Kd was developed, spatial distributions of environmental factors were analyzed, and contribution rates to light attenuation were quantified. Results showed that the mean Kd was 10.31±3.76 m⁻¹, and the mean Zeu (0.53±0.24 m) was lower than the mean water depth (0.94±0.29 m), with a spatial pattern of shallower Zeu in the west and deeper in the east. TSS and Chl-a were the primary direct influencing factors, while TN acted mainly indirectly. To achieve effective macrophyte colonization under average water depth conditions, thresholds were determined: Zeu ≥ 0.94 m, SD ≥ 0.41 m, Kd ≤ 4.95 m⁻¹, and Chl-a ≤ 3.8 μg/L. These findings provide quantitative guidance for lake restoration and water quality management.
1. Introduction
Shallow lake ecosystems worldwide face degradation due to eutrophication, leading to the loss of submerged macrophytes and a shift from clear-water to turbid, algae-dominated states. Underwater light availability is the primary constraint on macrophyte growth and colonization, yet existing restoration efforts often lack quantitative targets for light-related water quality parameters. Previous studies have not adequately disentangled the direct and indirect contributions of suspended solids, chlorophyll-a, and nutrients to light attenuation, nor have they established clear thresholds for macrophyte colonization under realistic depth conditions.
This study addresses these gaps by conducting a comprehensive field survey in Hongze Lake National Wetland Nature Reserve during the summer, when light conditions are most challenging due to high algal biomass. By measuring key optical and water quality parameters and modeling light attenuation, we quantify the contribution of each factor and derive specific thresholds for transparency, light attenuation, and chlorophyll-a that are necessary for successful macrophyte colonization. These findings provide a scientific basis for setting water quality targets and guiding ecological restoration in shallow lakes.
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WU Zijing, QIU Xintian, XU Qiao, YIN Xin'an, ZHANG Yi, SUN Yue, YANG Baiheng, ZHANG Borui, SUN Chuqi (2026). Colonization Requirements of Submerged Macrophytes Based on Underwater Light Environment. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202605010
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Frequently Asked Questions
What are the specific thresholds for water quality parameters to ensure submerged macrophyte colonization in shallow lakes?
To achieve effective colonization under average water depth (0.94 m), the study determined that euphotic depth must be ≥0.94 m, water transparency ≥0.41 m, light attenuation coefficient ≤4.95 m⁻¹, and chlorophyll-a ≤3.8 μg/L. These thresholds are based on summer conditions, representing a conservative scenario for year-round management.
How do total suspended solids and chlorophyll-a directly affect light attenuation, and what is the role of total nitrogen?
Total suspended solids (TSS) and chlorophyll-a (Chl-a) are the primary direct factors influencing light attenuation, with Chl-a having the greatest direct impact. Total nitrogen (TN) acts mainly indirectly by promoting algal growth, which increases Chl-a and consequently light attenuation. This highlights the need to control both particulate matter and nutrient loads.
What is the spatial heterogeneity of underwater light environment in the study area, and why is it important?
The study area exhibited a heterogeneous pattern with lower euphotic depth in the west and higher in the east, consistent with distributions of TSS, Chl-a, and TN. This spatial variability means that restoration efforts must be site-specific, as uniform water quality targets may not be sufficient to ensure light availability across the entire lake.
Why were summer data used, and how do the derived thresholds apply to other seasons?
Summer represents the period of highest algal biomass and most complex optical conditions, making it the critical season for macrophyte colonization. Thresholds derived from these relatively unfavorable conditions provide a conservative and safe reference for year-round ecological restoration, ensuring that light conditions remain adequate even during peak stress.
What is the relationship between euphotic depth and water transparency, and how was the transparency threshold derived?
The study established a fitting curve between euphotic depth (Zeu) and water transparency (SD). Using this relationship, the required Zeu of 0.94 m corresponds to a transparency of ≥0.41 m. This conversion allows managers to use easily measurable Secchi depth as a proxy for light availability.
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