Halophyte bio-optical phenotyping: A multivariate photochemical pressure index (Multi-PPI) to classify salt marsh anthropogenic pressures levels

Abstract Salt marsh ecosystems are extremely impacted by human activities whilst also acting as sinks of contaminants such as heavy metals. Halophytic plant species can survive and thrive in estuarine and coastal areas with higher salt conditions that are generally not favourable to most plants. Halophyte distribution and abundance clearly relate to anthropogenic pressure levels and thus the impact of heavy metals needs to be rapidly assessed and monitored in a fast-developing world. To assess and manage this impact the search for suitable and efficient biomarkers is of great importance, aiming to produce a clear picture of environmental quality. The present work aimed to evaluate the application of optical tools, like non-invasive chlorophyll a pulse amplitude modulated (PAM) fluorometry, in four halophytic species (Halimione portulacoides, Sarcocornia fruticosa, Spartina maritima and Spartina patens), from three salt marshes of the Tagus estuary with different degrees of contamination, to investigate biomarkers for inclusion in a Multivariate Photochemical Pressure Index (Multi-PPI) destined for ecological quality assessment. The application of this index allowed to distinguish between less (Alcochete) and more (Rosario and Seixal) heavy metal contaminated salt marshes. This was observed particularly for S. maritima, in which Multi-PPI was lower for the higher contaminated sites, albeit this pattern was similarly observed in S. patens and S. fruticosa. On the other hand, the Multi-PPI index for H. portulacoides, a phytoextractor species, increased with the contamination gradient along the three salt marshes, and therefore this strategy should be considered when applying this index. Therefore, since these species are well represented in the Mediterranean, the use of optical tools to generate an easy fast index will have a great impact on the future of transitional ecosystem pollution impact assessments as well as in ecotoxicology research.