The Effects of Low Tide Exposure on the Photosynthetic Health of Saccharina sessilis

Abstract

Intertidal marine seaweeds experience different environmental conditions during high and low tides. Low tide conditions may be stressful for seaweeds because desiccation and extreme light levels may impact their ability to photosynthesize. We examined the effect of low tide exposure on the photosynthetic health of the canopy-forming seaweed Saccharina sessilis in the field and lab. Field studies assessed the health of marked individuals on days with varying environmental conditions, while lab studies separated the effects of those different conditions. We hypothesized that the seaweed’s photosynthetic health depended on weather. We predicted that health would decline over a single stressful low tide (high light, high wind, and high temperature), but not over a benign low tide (low light, low wind, and moderate temperature). We assessed health in multiple ways. First, we measured dark-adapted Maximum Quantum Yield (MQY) via pulse-amplitude-modulated (PAM) fluorometry, which assesses the photosynthetic ability of Photosystem II. Second, because we observed that the seaweed changes color following intense stress, we evaluated tissue damage by quantifying the proportion of blade area in five color categories. A single stressful low tide can reduce MQY readings to 25% of pre-exposure values and damage 60% of a blade’s area. Individuals in benign conditions had MQY values of 97% of pre-exposure values and only 1% of blade area damage. Over the long term, damage from low tide exposure may limit growth and reproductive output. Understanding how individuals and populations respond to daily stresses will aid in understanding their response over longer time scales.