*3.3. Validation Process*

The classification reliabilities resulted with the reduced network and comparisons with the MaQI network of 2011 (original network) are shown for each WB in Table 4. The change in probability that the quality status calculated with the reduced network is less than good and differences with the 2011 MaQI network are also reported.

**Figure 2.** MaQI (Macrophyte Quality Index) network of 2011 (MaQI stations 2011) and the new network (new MaQI stations) with reduced number of stations by the multi-approach application. Crossed stations are those deleted.


**Table 4.** Reliability of classification of each water body (WB) of the Venice Lagoon after the optimization process (new network) and comparison with the original monitoring network results.

Figure 3 shows the two maps produced by Kernel interpolation with barrier for the MaQI network of 2011 and the new one, respectively. Prediction errors are reported in Table 5. From Equation (3), an RE of 22.75% resulted.

Comparisons (Student's *t*-test) between status classifications of each WB before and after the reduction of sampling effort are reported in Figure 4. To test the differences, values were extrapolated from each Kernel interpolation map.

**Figure 3.** Kernel interpolation maps for (**a**) the MaQI (Macrophyte Quality Index) network of 2011 and (**b**) the new one with reduced station of 2011 from 114 to 84.


**Figure 4.** Box plots of water body values resulted by the two Kernel interpolation maps of the original (Orig.) and reduced (Reduc.) networks. Results of Student's *t* tests are also given (n.s. = not significance, *p* > 0.05; \* = *p* < 0.05; \*\* = 0.5 < *p* < 0.01; \*\*\* = 0.01 < *p* < 0.001). Note: blue line (first from the top) is the high/good boundary, green line (second from the top) is the good/moderate boundary, orange line (third from the top) is the moderate/poor boundary, red line (fourth from the top) is the poor/bad boundary.
