**5. Conclusions**

From the statistical analyses, it appears that the first results of the WFD status classes of the Venice Lagoon obtained by macrophyte assemblages produced a satisfactory reliability and provided useful information about estimation of the confidence, which can be useful to managemen<sup>t</sup> organizations to optimize resources for remedial measure adoption. Results also allowed proposing a reduction of the monitoring sampling e ffort, which definitely a ffects river basin managemen<sup>t</sup> costs. For this

purpose, a suitable multi-approach method based on inferential statistics, spatial analyses, and expert judgment was applied in this study, in order to review the sampling effort with the aim at ensuring and keeping on a high reliability of the status classification. Following the approaches described in this study, an increasing role of expert judgment could be observed: from a clear and strictly statistical approach based on *L* values *a priori* defined by ecological class amplitude, to a mixed method where *<sup>L</sup>*op<sup>t</sup> was *a posteriori* defined for each WB, and resulting stations further emendated taking into account the site-specific characteristics. However, this study showed that a strictly statistical approach is obviously objective and standardized, but it could be unachievable with realistic data, since some results could lead to a waste of resource or vice versa to an excessive reduction of information. For these reasons, to optimize the monitoring effort, the inferential statistics were applied as a guideline, with the implementation of criteria arisen from expert knowledge of the area and the problem. Moreover, a robust process of validation was also proposed and adopted which definitively ensure on spatial reductions and reliability of information.

Apart from the complex nature of the Venice Lagoon, the multi-approach proposed in this study could also be applied to any other water body to assess the WFD classification reliability, and to optimize the monitoring effort to any other area.

**Author Contributions:** Conceptualization, R.B.B., P.P., F.C., and A.B.; methodology, F.C., A.B.; formal analysis, F.C., A.B., and E.P.; investigation, A.S.; resources, R.B.B., A.S., P.P. and M.G.; data curation, A.S., F.C., A.B., and M.N.; writing—original draft preparation, F.C.; writing—review and editing, A.B., R.B.B., A.S., and P.P.; visualization, F.C.; supervision, R.B.B.; project administration, R.B.B., M.G., and P.P.; funding acquisition, M.G., P.P., and R.B.B.

**Funding:** This work was supported by ARPAV in the framework of the Mo.V.Eco Project, funded by Veneto Region; contract of 31/01/2013 from the general agreemen<sup>t</sup> between ISPRA and ARPAV of 27/04/2012.

**Conflicts of Interest:** The authors declare no conflict of interest.
