Further improvement, validation, and application of CymoSkew biotic index for the ecological status assessment of the Greek coastal and transitional waters

The CymoSkew index, a quantitative expression of Cymodocea's photosynthetic leaf length (PLL) asymmetry, has been used as an early warning response indicator of coastal ecosystem status and trends. Besides its strengths, the number (N) of PLL used and the fact that they are chosen randomly, as well as the difficulty of its application in low-density meadows, were CymoSkew's main weaknesses. A new version of the index (CymoSkew^m) was developed using previous and new monitoring studies in eight meadows of the North Aegean Sea. It was also validated on another dataset carried out at six coastal and two transitional meadows and tested against MA-LUSI anthropogenic stress index. To avoid the effect of randomness on the index's accuracy, a random subsampling resampling technique in all PLL values of a meadow has been developed, producing a large number of different subsamples of N=20-300PLL drawn without replacement from all PLL values of a meadow, in order to reconstruct the underlying theoretical distribution of the CymoSkew index and infer about the unknown population mean. The iterated process constitutes the basis for evaluating the empirical distribution of CymoSkew mean values, while the mean value of this new distribution represents the CymoSkew^m meadow value, accompanied by empirical confidence intervals. In this way, low-density Cymodocea meadows can be accurately assessed, as PLL's from the whole meadow are utilized and not from one quadrate. Both versions of the index (CymoSkew, CymoSkew^m) were significantly linearly correlated together and with the MA-LUSI index (for both analyses p < 0.001). When applying the same procedure to maximum total leaf length (MTLL = PLL + sheath) values per shoot instead of PLL, results were comparable, opening new perspectives of the index's (CymoSkew^mMAX) usage, such as its rapid application (1) non-destructively under the seawater, or (2) on carefully cut at the base sheaths to be measured in the field or the lab later on. For routine biomonitoring within the European Union Directives (WFD, 2000/60/EC; Habitat Directive, 92/43/EEC), an open-source web application tool (http://index.cymoskew.gr/) for the computation of Cymoskew^m or Cymoskew^mMAX has been developed.