Seasonal heterogeneity in the photophysiological response to air exposure in two tropical intertidal seagrass species

Photosynthesis, chlorophyll a fluorescence, leaf bio-optical properties and pigments were measured in 2 tropical intertidal seagrass species, Zostera muelleri ssp. capricorni and Halophila ovalis before, during and after air-exposure over a tidal cycle. Data were collected across 4 seasons (October and January — growing seasons; May and July — senescent seasons) to determine seasonal dynamics in physiological responses to air exposure. Both species showed clear light-dependent responses with a decline in photosynthetic efficiency and increased photo- protection during periods of combined maximum daily irradiance and air exposure for all seasons. In Z. muelleri ssp. capricorni there was a negative correlation between air-exposed effective quan - tum yield and light intensity, suggesting exposure was driving this decline. Conversely, sensitivity (decline in effective quantum yield of photosystem II) to increased irradiance dominated the re- sponse in H. ovalis, with no change in the magnitude of this response between air-exposed and submerged blades. The response to air exposure observed in Z. muelleri ssp. capricorni showed seasonal variation, with a greater decline in photosynthesis during the spring (October). Tidal ex- posure did not provide intertidal seagrasses a 'window' of photosynthetic respite (increase in pho- tosynthesis) from high natural or anthropogenic turbidity. However, the periods immediately prior to and after exposure were important for providing an optimum period for net photosynthetic gain.

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