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

Abstract

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 photoprotection 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 quantum 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 response 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 exposure did not provide intertidal seagrasses a 'window' of photosynthetic respite (increase in photosynthesis) 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.

Declines of seagrasses in a tropical harbour, North Queensland, Australia, are not the result of a single event

“…The quality and quantity of light is a critical determinant of seagrass growth and abundance (Ralph et al 2007;Chartrand et al 2012;Petrou et al 2013) and low light levels are thought to be the primary factor limiting the growth of many coastal seagrasses (Waycott et al 2005;Petrou et al 2013). When low light conditions are prolonged, growth rates slow and plants drop leaves and shoots, thus reducing their abundance (Ralph et al 2007;Collier et al 2012).…”

“…The distribution and growth of these important habitats is largely influenced by environmental drivers such as depth, light, physical disturbance, nutrient availability, river flow and tidal exposure (Rasheed and Unsworth 2011;Chartrand et al 2012;Petrou et al 2013;Rasheed et al 2014) and, more recently anthropogenic factors, such as coastal development and degraded water quality Taylor and Rasheed 2011;McKenzie et al 2012). Pollard and Greenway (2013) reported on 'complete and catastrophic loss' of seagrass in Cairns Harbour, tropical northeastern Australia.…”

Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis

“…were recorded by using a custom built spectrophotometer composed of a Tungsten-Halogen light source (LS-1, Ocean Optics), and a fibre optic spectrophotometer (USB2000, Ocean Optics) interfaced with an integrating sphere (FOIS-1, Ocean Optics) (see details in Petrou et al, 2013). Symbiodinium cells were filtered to GF/F filter paper (Whatman) which was placed directly in the light path of the Tungsten-Halogen lamp and across the collection port of the integrating sphere.…”

Estimating the Exposure of Coral Reefs and Seagrass Meadows to Land-Sourced Contaminants in River Flood Plumes of the Great Barrier Reef: Validating a Simple Satellite Risk Framework with Environmental Data

“…Environmental conditions such as cyclonic activity [71], temperature and salinity [18], bathymetry and sediment type [94], tide currents and wave exposure (see references in [46]), or the zonal location of the meadows may have also contributed to the response of seagrass meadows. For example, very shallow intertidal meadows that receive light before, during, and immediately after low tide [95] exhibited the smallest loss in area compared to permanently submersed meadows [19]. Most of the sites considered in this study were occasionally exposed at low tide and this may also have affected how well seagrass responses corresponded to risk from plume water in this study.…”