*3.1. Reliability Assessment*

The probability that the actual mean value of MaQI EQRs fell within each of the five WFD classes, assessed by the Student's *t*-distributions; MaQI EQRs and status classifications; confidence interval

(*L*j), computed by Equation (1); and the cumulative probability related to the critical boundary G/M are shown for each WB in Table 2.

The Venice Lagoon mean *L* value (*L*mean) used for the following analyses was 0.109.

The WB ENC3 was left out from analyses because of its very small surface, its urban features, and the low number of stations (*N* = 3) that affected the meaning of statistical elaborations.

**Table 2.** MaQI (Macrophyte Quality Index) status class results, confidence, and reliability of classification of each WB (water body) of the Venice Lagoon.


### *3.2. Optimization of Sampling E*ff*ort*

The number of stations (*N*) resulted by the application of the statistical approach (Equation (2)) with the three scenarios (i.e., considering *L* = 0.1, *L* = 0.2, and *L* = *L*mean) changed from 111 (2011 sampling) to 169 (*L* = 0.1), 54 (*L* = 0.2), and 147 (*L* = *L*mean), respectively (Table 3). After emendation by the two steps of expert judgment, following criteria described in Section 2.3.2, 81 stations were proposed for future monitoring.

**Table 3.** Results of the application of the statistical approach and expert judgement criteria to optimize the sampling effort in the MaQI (Macrophyte Quality Index) monitoring program of the Venice Lagoon.


Keys: '*N*original' = original number of stations sampled in 2011, '*NL* = *L*mean' = number of stations resulted by Equation (2) with *L*mean (0.109), '*NL* = *<sup>L</sup>*opt' = number of stations resulted by Equation (2) with a suitable *L* value (*L*opt) deriving from rules (a), (b) and (c), '*N*new' = number of stations considering also elements of Table 1, '3\*' = a default *N* of 3 was given, when the application of Equation (2) resulted as *N* < 2, since standard deviation tends to be zero.

Briefly, no modifications of the number of stations were made to EC and ENC4 as their *L*j values (0.143 and 0.179, respectively) were higher than *L*mean but lower than 0.2 (rule (a)).

Rule (c) was strictly adopted at PC2, PC3, and PC4 since their *L* values (0.066, 0.057, and 0.049, respectively) were lower than *L*mean. Accordingly, the four stations to be maintained at PC2 were

selected on the basis of the presence of salt-marshes and the small canals, which characterize the WB; the three stations deleted at PC3 were chosen from those on the most southern part of the WB, where several restoration activities, such as the construction of artificial salt-marshes, are underway to replace shallow waters available just before; and finally, the three stations maintained at PC4 were those mostly located in the middle of the WB.

The other WBs needed the expert judgment as following. A small or no reduction of monitoring effort was done for ENC1, ENC2, PC1, PNC1, and PNC2 due to their dimensions (Table 1), habitat heterogeneity (Table 1 and Figure 2), and their reliability of the classification (Table 3). Accordingly, five stations with spatially redundant information were deleted at ENC1, the largest WB of the Venice Lagoon, with several canals and two Lagoon inlets (Malamocco and Chioggia) and 97.7% of probability to be good or more. No modifications were done at ENC2, due to its very small dimension and high reliability of classification (84.3% of probability to be less than good). Regarding PC1, three stations were not considered to be sufficient to correctly represent, since it consists of large expanses of salt-marshes, the Dese river mouth, and several small canals that divide it mainly into four areas. No modifications were made for PNC1, which is located between the industrial area of Porto Marghera and the city of Venice; it is quite homogenous (*L*j = 0.045), but it is separated into five sections by the presence of six canals. The morphological characteristics of PNC2 are quite similar to PNC1 with the airport of Venice on the north-west side and Murano and Sant'Erasmo islands on the south-east side. Considering its high reliability of the status classification (100% less than good), one station, located in the middle of the WB, was deleted. Finally, a special approach was adopted for ENC3, which was excluded from the optimization process because no more than three stations could be sampled there, due to its very small dimension, the presence of Chioggia island in the middle of the WB, and the proximity to the Lagoon inlet of Chioggia.

After application of the optimization process, the final number of stations established for the subsequent monitoring programs of the 11 natural WBs (including 3 stations of ENC3) of the Venice Lagoon was 84 (Figure 2).
