UV-B as a Photoacclimatory Enhancer of the Hermatypic Coral <i>Stylophora pistillata</i>

Photoacclimation processes are crucial for the survival of all photosynthetic organisms in the photic zone. Changes in photosynthetic active radiation (PAR) are however simultaneous to changes in UV-B radiation. The influence of UV-B levels on bio-optical and physiological parameters of deep (30 m) corals from the species Stylophora pistillata and their symbiotic algae, zooxanthellae, was examined during their gradual, stepwise acclimation to a shallow depth (3 m). Drastic exposure of deeper corals to higher UV-B levels in shallower depths is usually fatal. Hence, the acclimation process lasted 118 days and included 10 intermediate stations with an addition of similar amount of PAR at each depth transfer. Concomitantly, in an on-shore experiment, fragments from the same colonies were acclimated by changing shading nets corresponding in PAR levels to each in situ station. Since UV-B is attenuated more efficiently than PAR in seawater, the PAR: UV-B ratio changes in the depth experiment while remaining constant under the neutral density nets. This provided the opportunity to evaluate the importance of UV-B to photoacclimation. In both experiments all fragments survived, in spite of a four-fold difference in levels of PAR and a 140-fold difference in UV-B flux between the initial and final conditions. Both experimental designs resulted in reduction of zooxanthellae density, photosynthesis rates, and quantum yields of PSII, while cellular chlorophyll content remained unaffected. Zooxanthellae density and maximal photosynthetic rate was found decreased in correlation with UV-B radiation, whether it was elevated logarithmically with reducing depths or linearly with reducing shades. Conversely, quantum yields of PSII were adjusted according to the enhancement of PAR rather than UV-B. We conclude that UV-B enhances the magnitude of photoacclimation to higher PAR. This novel aspect of photoacclimation can provide the basis for our understanding of the underlying mechanisms that result in UV-related bleaching.

Evaporation and CO_(2)fluxes in a coastal reef:an eddy covariance approach

Introduction:We conducted season-long observations of evaporation and carbon flux at the Gulf of Aqaba coast,northern Red Sea.We used the eddy-covariance method with a two-tower setup to measure evaporation rates over land and sea and the advection between them.Using a three-dimensional mass balance approach,we calculated total evaporation as the sum of two main components in our site:horizontal advection and turbulent vertical flux,with half-hourly change of water vapor storage and horizontal flux divergence found to be negligible.Outcomes:Average evaporation rates were 11.4[mm/day]from April through May(early summer)and 10.5[mm/day]from June through August(summer).The coastal reef was a CO_(2)sink over the period of measurements,significantly higher in June through August than in April through May.The main environmental drivers of CO_(2)flux were humidity,water temperature,sensible heat flux,and wind speed.Discussion:The rates of evaporation near the shore were considerably higher than values reported in other studies typically used to represent the mean for the whole Gulf area.We found that evaporation rates computed by common bulk models approximate the mean values of evaporation but have poor representativeness of the intra-daily temporal variation of evaporation.There was a significant correlation between CO_(2)flux and evaporation attributed to common environmental drivers of gas diffusion,turbulent fluxes,and horizontal transport.Conclusion:We conclude that observations of fluxes in coastal waters need to use at least a two-tower system to account for the effect of horizontal advection on the total flux.