Heat stress of two tropical seagrass species during low tides–net photosynthesis, dark respiration and diel in situ internal aeration

The tropical seagrasses Thalassia hemprichii and Enhalus acoroides in the Sunday Island Group, Kimberley, WA survive and grow on perched intertidal platforms that are daily exposed to a macrotidal regime that varies from 3 m... [ view full abstract ]

The tropical seagrasses Thalassia hemprichii and Enhalus acoroides in the Sunday Island Group, Kimberley, WA survive and grow on perched intertidal platforms that are daily exposed to a macrotidal regime that varies from 3 m in neap tides and 10 m during spring tides. Low water volume and flows over the seagrasses at daytime low tides result in elevated temperatures and at nighttime low tides lead to critically low O2 concentrations. We measured internal tissue aeration in the seagrasses along with key environmental parameters on Jaalan (Talon) Island during neap tides. In the laboratory we tested how temperature influenced underwater net photosynthesis (PN) and dark respiration (RD) of leaves of T. hemprichii and E. acoroides. During the day, the shallow water was characterised by high pO2, pH and temperature affecting PN and RD; PN declined due to heat stress and dissolved CO2 approached zero so that ‘bicarbonate use’ would have predominated, whilst RD increased in the warm water. During the night, in contrast, dissolved O2 declined to hypoxic levels resulting in hours of shoot base anoxia in both T. hemprichii and E. acoroides. However, tissue anoxia was sometimes alleviated by incoming tide carrying O2 rich oceanic water fuelling aerobic RD of the seagrasses. We showed that PN and RD of both species fully recovered during the night after exposure to 40 °C in 4 h the day before, whereas heat stress at 45 °C resulted in more permanent tissue damage. In conclusion, both species displayed heat tolerance, were able to access CO2 from bicarbonate, and tolerated the anoxic periods; being adapted to this dynamic environment.