Table 2.
Independent variables selected by the boosted general additive model (boosted GAM) for the full area and polygon area models
| Independent variables | Units | Full area | Polygon area | |
|---|---|---|---|---|
| Fr (%) | Fr(%) | |||
| 1 | bspatial (Lon, Lat) | DD | 1.45 | 18.90 |
| 2 | bbs (Total depth, S) | m | - | 4.80 |
| 3 | bbs (Temperature, W) | °C | 0.09 | 5.40 |
| 4 | bbs (Salinity, W) | UPS | 0.13 | 0.60 |
| 5 | bbs (Oxygen, W) | (mg/L) | 0.17 | 5.00 |
| 6 | bbs (Alkalinity, W) | meq/l | 0.13 | 2.10 |
| 7 | bbs (CO2, W) | mmol/l | 0.13 | 2.10 |
| 8 | bbs (Nitrate, W) | μMolar | 0.21 | 3.00 |
| 9 | bbs (Phosphate, W) | μMolar | 0.09 | 0.70 |
| 10 | bbs (Silicate, W) | μMolar | - | 1.10 |
| 11 | bbs(Sigma T, W) | Kg/m3 | 0.30 | - |
| 12 | bbs(Sand, S) | % | - | 0.50 |
| 13 | bbs (Silt, S) | % | - | 1.10 |
| 14 | bbs (Clay, S) | % | 0.04 | - |
| 15 | bbs (Phosporus, S) | micromol/g | 0.04 | 0.60 |
| 16 | bbs (Nitrogen, S) | micromol/g | - | 1.30 |
| 17 | bbs (PAHL, S) | μg/g | 0.47 | 5.70 |
| 18 | bbs (PAHH, S) | μg/g | 94.70 | 6.90 |
| 19 | bbs (Aliphatic PAHs, S) | μg/g | - | 0.60 |
| 20 | bbs (PCNM2) | DD | 0.30 | 24.00 |
| 21 | bbs (PCNM21) | DD | 0.47 | 3.70 |
| 22 | bbs (PCNM58) | DD | 0.68 | 11.90 |
Fr (%) is the frequency at which each variable is selected during the bootstrapping, and used as proxy of the importance of each of the variables (expressed as percentage) within the model. bbs = base-learner based on B-splines, bspatial = spatial base-learner on geographical coordinates. W = water, S = sediment; PAHL and PAHH are polyaromatic hydrocarbons of low and high molecular weight respectively; PCNM2, PCNM21 and PCNM58 are the spatial variables of the principal coordinates of neighbour matrices (PCNM) analysis, acting at 2, 21 and 58 km respectively