Abstract
Tropical seagrasses, experience considerable spatio-temporal changes in light and other biogeochemical conditions and adopt species-specific acclimatization strategies. Variation in photo-acclimatory responses of three tropical seagrass species (Cymodocea serrulata, Thalassia hemprichii and Enhalus acoroides) were studied by measuring photosynthetic electron transport rates (ETR) using Pulse Amplitude Modulated (PAM) fluorometry technique. Quantitative values of ETR and rates of photosynthetic O2 evolution (net O2 exchange corrected for dark respiration) were compared to establish species-specific relationships in these shallow water conditions. The apparent average molar ratio of O2 evolution to ETR for Cymodocea serrulata, Thalassia hemprichii and Enhalus acoroides were estimated as 0.23, 0.19 and 0.20, respectively. The highest photosynthetic activity (ETRmax) varied significantly among the three studied species in the decreasing order as: E. acoroides (59.27); T. hemprichii (54.06) and C. serrulata (46.72). The effective quantum yield (Y) of PS II (Photosystem II), one of the most useful indicators of stress conditions for seagrass, was observed to be significantly higher for E. acoroides compared to the other two species. Variability of Y in the ambient conditions are explained by the difference in water temperature and pCO2 for the three seagrass species. This study is useful in predicting the photo-acclimation strategies to any change in light availability and subsequent biogeochemical changes at the bottom surface, by these shallow water tropical seagrass species.
Highlights
Seagrass meadows are best known for their efficiency in capturing and converting light energy into organic matter, and its supply to other trophic levels (Hemminga and Duarte, 2000; Ow et al, 2015)
The results suggested that among the three studied species, T. hemprichii is highly adapted in the low light conditions followed by C. serrulata
Using the measured light attenuation co-efficient (Kd) measured from the shallow waters of Palk bay, in the equations given by Gallegos and Moore (2000), it is estimated that an increase of 50 cm depth, could reduce available light availability by 15, 10 and 10% in the existing E. acoroides, C. serrulata, T. hemprichii beds, respectively. These results further indicated that future increases in attenuation of light (Kd), caused by the greater phytoplankton abundance, higher riverine inputs of suspended particulate matter and colored dissolved organic matter could substantially alter seagrass growth and distribution in the shallow coastal waters
Summary
Seagrass meadows are best known for their efficiency in capturing and converting light energy into organic matter, and its supply to other trophic levels (Hemminga and Duarte, 2000; Ow et al, 2015). Alexandre et al (2012) suggested that the global effects of CO2 increase on the DIC utilization of seagrass may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Since tropical seagrass meadows exhibit a greater photosynthetic affinity for CO2 than HCO3−, increased availability of CO2 under ocean acidification can further boost the gross productivity (Durako, 1993). This DIC utilization is largely regulated by light availability as other physico-chemical parameters are often altered through varying land runoff (Ow et al, 2015) in addition to the availability of water. Durako (1993) observed a significant increase in photosynthesis and subsequent growth in response to enhanced DIC concentrations in both tropical and temperate seagrass species
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