High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species

Abstract

The effect of repeated midday temperature stress on the photosynthetic performance and biomass production of seagrass was studied in a mesocosm setup with four common tropical species, including Thalassia hemprichii, Cymodocea serrulata, Enhalus acoroides, and Thalassodendron ciliatum. To mimic natural conditions during low tides, the plants were exposed to temperature spikes of different maximal temperatures, that is, ambient (29–33°C), 34, 36, 40, and 45°C, during three midday hours for seven consecutive days. At temperatures of up to 36°C, all species could maintain full photosynthetic rates (measured as the electron transport rate, ETR) throughout the experiment without displaying any obvious photosynthetic stress responses (measured as declining maximal quantum yield, Fv/Fm). All species except T. ciliatum could also withstand 40°C, and only at 45°C did all species display significantly lower photosynthetic rates and declining Fv/Fm. Biomass estimation, however, revealed a different pattern, where significant losses of both above‐ and belowground seagrass biomass occurred in all species at both 40 and 45°C (except for C. serrulata in the 40°C treatment). Biomass losses were clearly higher in the shoots than in the belowground root–rhizome complex. The findings indicate that, although tropical seagrasses presently can cope with high midday temperature stress, a few degrees increase in maximum daily temperature could cause significant losses in seagrass biomass and productivity.

Methane emission and sulfide levels increase in tropical seagrass sediments during temperature stress: A mesocosm experiment

“…The stress responses recorded upon the plants were similar in magnitude to a previous study on the same species (George et al, ), but with a slightly stronger influence at 40°C, as indicated in the ETR and Fv/Fm levels. The negative effects of high midday temperature stress on the photosynthetic performance were intensified by the number of repeated days of exposure, indicating chronic damage in the photosynthetic machinery as the maximum quantum yield (Fv/Fm) could not recover even after several hours of darkness and ambient temperatures (Beer, Björk, & Beardall, ; Hanelt, ; Maxwell & Johnson, ).…”

“…The loss of photosynthetic capacity, and a subsequent decrease in the internal O 2 transport to below‐ground tissues, has been shown to cause anoxia in seagrass sediments (Greve, Borum, & Pedersen, ; Nagel, ), and a decrease in photosynthetic capacity (by shading) of tropical seagrasses has been suggested to have a negative effect on the part of the microbial community of seagrass sediment using photosynthetically produced exudates from the seagrass (Barber & Carlson, ; Schrameyer et al, ). Such effects might have shifted the microbial activity to cause an additional increase in sulfate reduction and methanogenesis, causing the increase in sulfide levels and methane emissions observed in this study, possibly also strengthened by the decay of below‐ground tissues, which has been observed under events of elevated midday temperature stress (George et al, ) and suggested to be a source of the organic matter supporting sediment CH 4 production in seagrass meadows (Barber & Carlson, ). As expected, any changes in the microbial activity and composition did, however, not result in any significant change in the organic matter content of the sediment.…”

“…After the heat stress, the small containers were gradually drained until reaching 75% of the original water level, and subsequently refilled with new seawater of ambient temperature in order to lower the experimental temperatures to ambient levels (to simulate a returning high tide). The temperature treatment levels were chosen based on available field data recorded during the northeast monsoon (George et al, ), previous experimental works on tropical seagrasses (Campbell et al, ; Collier & Waycott, ) and the expected increase in sea surface temperature under a projected global warming scenario by the year 2100 (Bernstein et al, ; Pachauri et al, ). The ambient containers were also partially drained, with 75% of the water being removed and exchanged with new seawater once per experimental run (on the third day) to avoid effects of evaporation and nutrient limitation.…”

“…In tropical intertidal seagrass habitats, the water temperature is highly influenced by contemporary tidal regimes, where high diurnal temperature changes are common (Bridges & McMillan, 1986;Burdick, Dionne, Boumans, & Short, 1996;Koch & Erskine, 2001;Pedersen, Colmer, Borum, Zavala-Perez, & Kendrick, 2016). During daytime, and especially in spring low tide, seagrasses within the intertidal areas of Zanzibar, Tanzania, frequently experience elevated water temperatures of 40-44°C for periods of 3-4 hr, which have clear and immediate negative effects on the photosynthetic performance of the seagrass plants (George, Gullström, Mangora, Mtolera, & Björk, 2018). The frequency and intensity of temperature stress events are anticipated to increase in the future under human-driven climate change (Bernstein et al, 2008;Pachauri et al, 2014) and are thus likely to exacerbate the negative impacts of warming on tropical seagrass meadows.…”

Global environmental changes negatively impact temperate seagrass ecosystems

“…, George et al. ). The shading caused by the storm event in the experiment would have direct negative effects on the amount of oxygen produced in the leaves of the seagrass, thus reducing the amount of oxygen transported from the leaves to the roots.…”