# Effects of Shading on the Growth and Carbon Storage of _Enhalus acoroides_
## 1 Introduction  
As one of the typical blue carbon ecosystems, seagrass meadows contain an annual carbon stock of at least 27.4 Tg C worldwide, which is equivalent to 10-15% of the total global ocean carbon sequestration, and thus play an important role as the global carbon sinks [1]. In addition, seagrass meadows can provide habitat, breeding and feeding places for many marine organisms and have the functions of adsorbing suspended particulate matter, improving water quality, reducing wave energy and maintaining the coast [2]. However, large-scale degradation and the disappearance of seagrass meadows have occurred globally due to human activities and climate change [3]. Among them, the reduction in water transmittance caused by human activities is considered one of the main reasons for seagrass ecosystem degradation [4]. Light intensity plays a key role in the growth, survival and distribution of seagrasses [5]. In contrast to effects on terrestrial plants, solar energy is affected by many factors before reaching seagrass leaves, causing light energy loss, among which the air-water interface leads to a certain amount of light energy loss; meanwhile, the suspended particulate matter in the water and the increase in water depth cause the rapid attenuation of solar energy. Studies have shown that the seabed environment in which seagrasses live requires an average of 11% surface irradiance in order to maintain the normal growth and development of seagrasses [6].  
The weakening of the light environment of water Is mainly caused directly or indirectly by human activities, mainly including dredging projects, the construction of coastal dwarures, and the discharge of aquaculture sewage and domestic wastewater [7; 8; 9]. Light reduction decreases the photosynthetic rate of seagrasses, which in turn affects theirgrowth and physiological properties, mainly in terms of a reduced growth rate, altered leaf morphology, reduced biomass [10; 11; 12], and increased consumption of non-structural carbohydrates [13]. However, there is some variation in the response of different species to changes in the light environment due to factors such as the growth, habitat and life history of seagrass [14].  
_Enhalus acoroides_ is a large, long-lived seagrass with the ability to reproduce both sexually and asexually. It is widely distributed in the Indo-West Pacific, and in China, it is found only on Hainan Island, mainly in Xincun Bay and Li'an Lagoon in Lingshui [15]. The fishery is well developed in Li'an Lagoon, and the discharge of aquaculture sewage and domestic wastewater, among other pollutants, has a significant impact on the health status of seagrass beds [16]. In the past 10-20 years, the distribution area and plant density of _E. acoroides_ have decreased dramatically, and few studies have been reported regarding the effects of external environmental changes on _E. acoroides_[17]. In this study, we examined _E. acoroides_, simulated an environment reflecting the reduced light transmission of water by building a shade shelter in situ, explored differences in the response of _E. acoroides_ to changes in the light environment, analyzed the degradation mechanism of _E. acoroides_, and provided a theoretical basis for determining a reasonable conservation strategy.