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

Rushingisha George,Mats Björk,Mwita M Mangora,Matern S P Mtolera,Martin Gullström

doi:10.1002/ece3.3952

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.

Highlights

Tidal regimes strongly influence the productivity of coastal plant systems (Bridges & McMillan, 1986; Burdick, Dionne, Boumans, & Short, 1996; Koch & Beer, 1996)
This study showed that tropical seagrasses are tolerant to repeatedly occurring high temperature stress, and revealed clear differences in how seagrass plants respond depending on if measured as photosynthetic capacity or percent plant-tissue reduction
Significant effects of recurrent high midday temperature stress were generally found at lower temperatures for biomass than for photosynthetic activity, demonstrating that tropical seagrass plants could maintain an apparently unaffected photosynthetic capacity during high temperature stress, while at the same time suffering major losses of biomass

Summary

| INTRODUCTION

Seagrasses have been found to be capable of a certain physiological adaptation to high temperatures (Drew, 1979; Evans, Webb, & Penhale, 1986; Zimmerman, Smith, & Alberte, 1989), but periods of high temperature have been seen to cause rapid and large losses in plant biomass (Lee, Park, & Kim, 2005). Such a future scenario could threaten the survival of intertidal seagrasses in the WIO region and other tropical shallow-water environments. We explicitly tested the hypotheses that: (1) photosynthetic performance is influenced at similar temperature stress levels as above-and belowground biomass loss, (2) there are species-specific threshold levels where photosynthetic performance and biomass are reduced, and (3) the effect of midday temperature stress on photosynthetic performance will increase with days of repeated stress

| MATERIALS AND METHODS

| Experimental setup

Findings

| DISCUSSION