A blessing or a curse: Responses of eelgrass (Zostera marina) seedlings to combined stressors of nutrients, hypoxia and sulfide

Abstract

Eelgass (Zostera marina) beds have many functions; however, eelgrass populations have declined in recent decades. Thus, it is important to recover eelgrass populations. Multiple stressors coexist in eelgrass habitats and it is considered that stressors reduce the eelgrass population. Nevertheless, many seedlings are established under combined stressors. It is speculated that there are antagonistic and synergistic effects among these stressors, and the interaction between them determines the seedling fate. To verify this hypothesis, 840 eelgrass seedlings were exposed to combined stressors of nutrients, hypoxia and sulfide for 7 d. All of the seedlings survived. The pH of the experimental system decreased from the 0th to 24th h, but it increased after the 72nd h. The root activity was 119.87±5.48 μg.g−1.h−1 and was inhibited by the interaction of dissolved oxygen (DO) and ammonia, and increased with nitrate and the interaction of phosphate and sulfide. Both the proline content and the Chl a content were not affected by the stressors. The Chl b content was 0.14±0.16 mg.g−1 (calculated by wet weight) and was reduced by nitrate. The ratio of Chl a content to Chl b content was 2.53±0.07 and was affected by the stressors. There is antagonism and synergy among these stressors and the interaction of them determines seedling survival. Nitrate at a high concentration leads to seedling death. It is not accidental that seedlings survive or die under combined stressors, and the intensity and the spatiotemporal distribution of environmental stressors determine the recruitment of seedlings. Targeted habitat improvement contributes to the survival of eelgrass seedlings.