Optical characterization of coastal lagoons in Tunisia: Ecological assessment to underpin conservation

Abstract

Ghar El Melh is a shallow lagoon (average depth of 0.8 m) that has undergone a eutrophication process due to growing human pressures. To obtain a global frame of the ecosystem functioning, an optical and an ecological classification were used in parallel. Downwelling and upwelling spectral irradiances were measured in situ in 22 sampling stations across the water body; then Apparent Optical Properties (AOPs), namely reflectance R(λ) and vertical attenuation coefficient K d (λ) were calculated for each wavelength of visible spectrum, furnishing typical spectra from turbid waters, rich in dissolved and suspended matter. From water samples of the same stations the concentrations of OASs (Optically Active Substances), i.e. Chromophoric Dissolved Organic Matter (CDOM), Non-Algal Particulate (NAP) and Phytoplankton, were assessed. The use of an optical classification for water bodies rich in TSM and CDOM, integrating AOPs and OASs, highlighted a great spatial heterogeneity, well overlapping with hydrology and human impacts patterns. A modified version of the Ecological Evaluation Index (EEI), considering the macrophyte distribution (based on a visual assessment of macrophyte coverage, without quantitative sampling) was then used, highlighting an intermediate ecological condition, despite high water turbidities. The integrated use of both systems thus furnished a complete characterization, rapidly detecting the most impacted sectors and the possible primary causes. The method might be applied as a monitoring procedure in other Mediterranean coastal lagoons, with the aim to adopt a common conservation strategy for these important transitional water bodies.

Illumina-based analysis the microbial diversity associated with Thalassia hemprichii in Xincun Bay, South China Sea

“…The marine coastal ecosystems were characterized by highlyproductivity and critical ecology and economical functions with seagrasses, mangroves and coral reefs (Feng et al 2009;Nourisson et al 2013). The seagrass meadows with succession of aquatic angiosperms plants stabilize sediments reduce the velocity of flow and provide essential habitats for many estuarine species.…”

Characterisation of a Tunisian coastal lagoon through hyperspectral underwater irradiance

“…As with K d , a marked variability of values and shapes was observed (Figure 4a), but some common features were identified. The values of R(10) in the green band [R(555)] were relatively high (1-19%, average 8.7%), indicating a conspicuous irradiance backscattering (see discussion in Nourisson et al 2013). The blue/green reflectance ratio [R(443) / R(555)] was always smaller than 1 (Nourisson et al 2013; Table 1), i.e.…”

“…The results from laboratory analyses (detailed in Nourisson et al 2013) showed that CDOM concentrations were relatively high (a CDOM (440) mean = 0.389 m −1 ; min. = 0.171, max.…”

“…In addition, the use of optical remote sensing data may be validated by such in situ data, adding a further dimension to the monitoring of the coastal zone (Ferreira et al 2011;Novoa et al 2012;Caroppo et al 2013). Here we assess the suitability of an optical monitoring methodology by examining the relationships between the main LASs and AOPs in the lagoon and by comparing the results with those obtained from the classification of Reinart et al (2003) and from the ecological assessment of this lagoon carried out by Nourisson et al (2013).…”

“…The Reinart et al (2003) optical classification provides reference values for five types of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). The classification has been successfully applied at the Ghar El Melh coastal lagoon on the north-eastern coast of Tunisia (Nourisson et al 2013) to achieve a synthetic and rapid procedure for routine monitoring of coastal waterbodies.…”