Species-specific phenotypic plasticity of two tropical seagrass species in response to in situ fertilisation under different trophic conditions

Abstract

Seagrasses are considered one of the most rapidly declining ecosystems in the world. One of the main reasons for this decline is eutrophication during which anthropogenic impacts, including light deprivation and nutrient enrichment often result in cumulative stress for seagrasses. This study aims to investigate the physiological and morphological responses of seagrasses to nutrient enrichment and eutrophication in Hainan, China, to understand the processes leading to seagrass losses from eutrophication. Therefore, we tested how a four-week in situ fertilisation affected traits of a multi-species seagrass community in two seasons with different trophic conditions. The species investigated in this study are Cymodocea serrulata, Cymodocea rotundata, Halodule uninervis and Thalassia Hemprichii. The dry season was characterised by high light availability and low nutrient availability. Enhanced seagrass leaf growth rates and leaf area of all species as a response to fertilisation indicate that seagrass growth is nutrient-limited in the dry season. In contrast, under eutrophic conditions in the wet season, growth and leaf area of all species were lower than in the dry season and fertilisation had no effect. Thalassia hemprichii showed signs of morphological adaption under eutrophic conditions, suggesting a higher species-level phenotypic plasticity and resilience to the stressors involved. As expressed in the fast response under eutrophic conditions, our experiment indicates synergetic adverse effects of light reduction and nutrient enrichment on seagrasses. An improved understanding of species-specific physiological and morphological response mechanisms of seagrasses to eutrophication may help to identify thresholds of exposure to stressors early on, and hence to be able to predict and possibly avoid an upcoming large-scale loss. It also has implications for aquaculture and coastal zone management.