Research progress on the effect of light reduction on seagrass

Abstract: Seagrass meadow is one of the most important blue carbon ecosystems and contributes towards climate change mitigation through carbon sequestration and storage. Compared with terrestrial ecosystem, the pronounced capacity for carbon sequestration in blue carbon ecosystem is mainly due to the large range and longterm stable carbon sequestration. In addition, this is enhanced by coastal vegetation which is characterized by its high primary productivity and efficient water column filtration. Although seagrass covers less than 0.2% of the total area of the world’s oceans, the annual carbon storage of seagrass ecosystem is equivalent to 10% to 15% of its total carbon storage. It is estimated that the annual primary productivity of global seagrass ecosystem is about 1 012 g·DW·m-2, and the annual carbon burial amount approximately exceeds 27.4 TgC, which is equivalent to the sum of carbon storage in global tidal salt marshes and mangroves. Therefore, seagrass ecosystem can effectively alleviate the increase of carbon dioxide concentration. Other important ecosystem services include providing habitats, foraging and breeding places for many marine organisms, serving as an attachment base to provide adequate living space and nutrients for epiphytes. Furthermore, seagrass ecosystem can also effectively absorb suspended particulate matter, reduce wave energy, eliminate pollution, improve water quality and maintain the coast.

However, since the 20th century, seagrass ecosystem has been declining at a rate of 7% of its total area annually, and its growth and distribution have been seriously threatened. Climate change and anthropogenic disturbances are considered to be the main factors causing the degradation and disappearance of seagrass ecosystem. To be specific, the reduction of water light transmittance caused by anthropogenic activities is considered one of the main reasons leading to the degradation of seagrass ecosystem. Light is the main environmental factor limiting the growth and distribution of seagrass. Solar energy is affected by many factors before reaching seagrass leaves, resulting in light energy loss. For example, loss of light energy may occur at the interface of air and water; the increase of suspended particulate matter and water depth can also cause the rapid attenuation of solar energy.

Research on seagrass ecosystem in China has started relatively late. Besides, most of the studies on the impact of anthropogenic activities on seagrass ecosystem focus on the aspects of temperature stress, heavy metal pollution and water eutrophication; effects of water light transmission changes are unclear. Studying the effects of light reduction on the growth and physiological characteristics of seagrass ecosystem is helpful for the protection and restoration of seagrass ecosystem. Based on a series of studies on the effects of light reduction on seagrass ecosystems worldwide in recent years, this paper critically summarized and analyzed the minimum light requirements for seagrass growth and the factors of light reduction, and discussed the response mode of seagrass ecosystem to light reduction based on the growth and physiological changes of seagrass under light reduction. The results provided insight for understanding the decrease of water transmittance caused by anthropogenic disturbances and the impact of environmental changes on seagrass ecosystem.

The study showes that the theoretical minimum light requirements for normal growth and development of seagrass is estimated at 11% of surface irradiance. In different light environments, seagrass has certain morphological plasticity, and the morphological characteristics of seagrass will change when the light changes. Seagrass can reduce its respiration requirement under light reduction by reducing leaf area and maintaining low plant density. Simultaneously, the decrease of photosynthetic rate will reduce the biomass of seagrass. Biomass is an important form of synthesis of organic matter by photosynthesis; thus, changes of photosynthetic rate will affect the accumulation of plant biomass. In addition, changes of light will also affect the allocation and utilization of biomass resources of plants. Seagrass can maintain its survival under environmental stress by changing the allocation strategy of biomass. Generally, the aboveground biomass of seagrass is greatly affected by light reduction, leading to the increased proportion of underground biomass. By promoting the growth of rhizome, the survival needs in low light environment are well balanced. However, other studies have shown that seagrass can enhance its light capture in low light environment by increasing the biomass of aboveground. The difference of biomass allocation to different tissues and organs is the effective utilization of limited resources by seagrass, which adapts to different environmental conditions through its own regulation mechanism. Photosynthetic pigment content is a good indicator of the photosynthetic physiology of plants, which can reflect the growth of plants. Generally, light reduction can promote the increase of chlorophyll content of seagrass, so as to effectively absorb limited light energy. In different light environments, the difference of chlorophyll content of seagrass may be related to the time and intensity of light stress, and the interspecific difference of seagrass species may also lead to different responses of chlorophyll content to light change. The longterm reduction of light may lead to the degradation or loss of seagrass ecosystem, resulting in the reduction of carbon storage. It may also cause the carbon stored to be unstable in sediment, because of the loss of seagrass shelter. Finally, it will reduce the carbon sequestration capacity of seagrass ecosystem, and even change it from carbon sink to carbon source.

Currently, a large number of manipulation experiments have been conducted to investigate the effects of light reduction on seagrass ecosystem, but the results from these studies are inconsistent. Such inconsistent results might be affected by multiple factors, including biological, environmental and experimental variables. Then, the paper summarizes the current studies, and points out some aspects that are urgently needed to provide further insights in future studies, including expanding research scope and enriching research objects, improving research methods, strengthening research on the interaction of various factors, broadening research direction, strengthening and deepening mechanism research.

Keywords: seagrass；light reduction；growth characteristic；physiological response