Characterisation of a Tunisian coastal lagoon through hyperspectral underwater irradiance

North African coastal lagoons are unique ecosystems that often suffer degradation due to human activities. Therefore, monitoring methods are required to identify stressors and assist with the management of these valuable and often understudied ecosystems. A synthetic indicator of water ecological quality would be desirable for regular monitoring of these ecosystems under pressure. In 2008 an optical procedure was developed and applied in Ghar El Melh, a Tunisian lagoon which has been increasingly impacted by pollutant loading, especially from agriculture. In situ hyperspectral irradiance was measured at several stations, from which the apparent optical properties (AOPs), namely the irradiance attenuation coefficient K(λ) and the reflectance ratio R(λ), were obtained in order to relate them to water composition, in terms of light-attenuating substances (LASs). The significant relationships observed between R and LAS values enabled the application of a hyperspectral optical classification, which effectively highlighted threatened sectors of the lagoon. The pattern of differing water quality across the lagoon system that was derived from the hyperspectral classification agreed well with that obtained from a conventional optical classification that included AOPs and LASs. We suggest that hyperspectral analysis and classification is a useful monitoring tool for the assessment of change in coastal lagoons, and perhaps also in other shallow-water ecosystems.     

Oscillatory Transpiration and Water Uptake of Avena Plants IV. Transpiratory Response to Sine Shaped Light Cycles

The transpiration rate of individual 6-day-old oat plants was forced to oscillate by cyclic sine-shaped changes in the leaf irradiance (frequency 2 cycles h^?1, amplitude and average value 1.4 mW cm^?2, red light 620–800 nm). By means of a specially designed cuvette with three chambers the transpiration rate from three different segments of the leaf could be measured simultaneously. The leaf segments were illuminated individually and the illumination on each leaf segment could be modulated independently. The experiments showed that there was a strong correlation between the transpiration rates from the different leaf segments, dependent on a coupling mechanism in the plant. The coupling phenomenon disappeared when the root system was eliminated or when the water potential of the root medium was lowered. It was experimentally shown that CO₂ diffusion in the leaf could not be the primary cause for the coupling. Therefore the stomatal dependence on the leaf water potential was considered the most probable reason for the coupling. The frequency of the forcing light cycles could be linearly changed during an experiment and this swept-frequency technique was used to obtain a frequency response of one single oat plant. The technique made it also possible to study the strength of the coupling between different leaf segments.     

Effects of environmental salinity on gill endothelin receptor expression in the killifish, Fundulus heteroclitus

We recently determined that rapid changes in environmental salinity alter endothelin-1 (EDN1) mRNA levels in the euryhaline killifish, Fundulus heteroclitus, so we hypothesized that EDN1 may be a local regulator of gill ion transport in teleost fishes. The purpose of the present study was to examine the effects of changes in environmental salinity on the gill endothelin receptors: EDNRA, EDNRB, and EDNRC. Using quantitative real-time PCR, we determined that after a fresh water (FW) to seawater (SW) transfer, there is a two to threefold increase in gill EDNRA and EDNRB mRNA levels. Likewise, we found a two to three fold increase in gill EDNRA and EDNRB protein concentration. In addition, killifish that have acclimated to FW for 30 days had significantly lower EDNRA mRNA and protein levels than SW killifish. ENDRA were immunolocalized to the mitochondrion-rich cells of the killifish gill, suggesting that EDN1 signaling cascades may affect MRC function. EDNRB were found throughout the gill vasculature and on lamellar pillar cells. We previously immunolocalized EDN1 to the pillar cell suggesting that EDN1 acts as an autocrine signaling molecule and potentially regulates pillar cell tone and lamellar perfusion. We conclude that EDN1 is physiologically active in the teleost gill, and regulated by environmental salinity. Future functional studies examining the physiological role of this system are necessary to completely understand EDN1 in the fish gill.