Low irradiance disrupts the internal O dynamics of seagrass (Thalassia testudinum) leading to shoot meristem HS intrusion

Hypoxia and hydrogen sulfide (HS) intrusion at night contribute to large-scale seagrass mortality events world-wide. Declining water quality has lowered irradiance and enhanced hypoxia in seagrass ecosystems, but linkages between low irradiance and seagrass internal pO in situ are not well understood. We examined low irradiance effects on leaf and meristem pO dynamics of a dominant tropical seagrass, Thalassia testudinum, using microsensors over multiple diurnal cycles. Further, we determined how O dynamics affect HS intrusion into shoot meristems under low irradiance. Sequential days of low irradiance disrupted internal leaf O status in three ways: i) causing a longer lag in morning O pressurization, ii) depressing maximum pO during the day, and iii) shortening the time where maximum pO was sustained into the afternoon. There was a close relationship between leaf and water column pO during the day (R = 0.93 ± 0.09); thus, internal O dynamics appear more dependent on water column pO under low irradiance. The nighttime minimum of leaf and meristem pO (1.2 and 0.4 kPa, respectively) were very low and the length of time the meristem sustained hypoxia (< 1.5 kPa pO) at night was high (9:40 h:min). HS intrusion into the meristem at night following 24–48 h of shading was persistent, likely the combined effect of limited internal O flow without saturating irradiances for photosynthesis, and a breakdown of the sediment oxic microshield. Under low irradiance, more frequent intrusions of HS to the meristems increases the chances of large-scale seagrass mortality events at night.