Following nutrient retention metrics have been calculated:
- the uptake length (Sw) is the average distance travelled by a nutrient molecule before being removed from the water column; it is expressed in meter and shorter distances suggest greater nutrient retention efficiency than longer distances
- the mass transfer coefficient (Vf) that is independent from the physical properties and it is more appropriate to compare stream with different size, it describes the velocity by which a nutrient molecule moves through the water column to the sediment
- the uptake rate (U) is the mass of a nutrient taken up from the water column per unit stream bed area and time.
Uptake length values varied between 83 and 1120 m and 145 and 837 for NH4 and PO4 respectively in Sardinian sites. In Piedmont we found length intervals of 105 -667 m for NH4 and 162 e 1000 m for PO4.
At some sites, both Sardinian and Piedmont, we could not measure any retention of PO4 and/or NH4. In most cases it was possible to identify a cause to explain this behaviour. For example in two sites in Sardinia the high river discharge occurring during the nutrient addition excessively reduced the travel time of solutes creating not suitable conditions for the method application. In other cases, an excessive increase of the nutrient concentrations during the experiment in the plateau phase, may have produced a saturation condition.
Calculated retention metrics – Sardinia
Station | SwNH4 | SwPO4 | U_NH4 | U_PO4 | Vf_NH4 | Vf_PO4 |
m | m | mg m2 min-1 | mg m2 min-1 | mm s-1 | mm s-1 | |
Campu 'e Spina_V | 722 | 643 | 0.001 | 0.004 | 0.024 | 0.027 |
Campu 'e Spina_M | 756 | 837 | 0.002 | 0.002 | 0.023 | 0.021 |
Cialdeniddu | 797 | 439 | 0.063 | 0.014 | 0.062 | 0.112 |
Monte Pecore | 455 | 0.036 | 0.107 | |||
tricarai Ref | 909 | 714 | 0.030 | 0.035 | 0.070 | 0.089 |
tricarai_VP | 244 | 455 | 0.051 | 0.035 | 0.231 | 0.124 |
San Pantaleo_V | 303 | 323 | 0.109 | 0.028 | 0.184 | 0.173 |
San Pantaleo_M | 385 | 385 | 0.082 | 0.014 | 0.099 | 0.099 |
Sa Faa | 204 | 231 | 0.013 | 0.018 | 0.032 | 0.028 |
Tirso | 313 | 145 | 0.005 | 0.003 | 0.034 | 0.073 |
Canale M. D. | 83 | 294 | 1.278 | 0.002 | 0.106 | 0.031 |
Barrastoni | 429 | 769 | 0.173 | 0.026 | 0.041 | 0.023 |
Lorana_ M | 258 | 0.113 | 0.094 | |||
Lorana_V | 1120 | 0.010 | 0.017 | |||
Porceddu | 588 | |||||
Posada Affluente | 1111 | 833 | 0.034 | 0.001 | 0.057 | 0.025 |
Baldu_V | ||||||
Baldu_M | 389 | 435 | 0.167 | 0.177 | 0.128 | 0.114 |
Corr'e Pruna | 649 | 0.248 | 0.092 | |||
Corr'e Pruna_M | 526 | 0.040 | 0.094 | 0.164 | 0.041 |
Calculated retention metrics – Piedmont
Station | SwNH4 | SwPO4 | U_NH4 | U_PO4 | Vf_NH4 | Vf_PO4 |
m | m | mg m2 min-1 | mg m2 min-1 | mm s-1 | mm s-1 | |
CURONE M. 3/2011 | 1667 | 939 | 0.0078 | 0.0185 | 0.0067 | 0.0119 |
CURONE M. 11/2011 | 190 | 162 | 0.0125 | 0.0039 | 0.0199 | 0.0233 |
CURONE V. 10/2011 | 215 | 0.0070 | 0.0202 | |||
SAN SUDARIO 3/2011 | 363 | 0.0380 | 0.1568 | |||
SAN SUDARIO 6/2011 | 164 | 303 | 0.0231 | 0.0329 | 0.0694 | 0.0376 |
SIZZONE 3/2011 | 143 | 0.1322 | 0.3497 | |||
SIZZONE 7/2011 | 263 | 204 | 0.0334 | 0.0108 | 0.0861 | 0.1110 |
SIZZONE 10/2012 | 278 | 0.0160 | 0.0756 | |||
StrEGO 3/2011 | 1000 | 0.1830 | 0.0343 | |||
StrEGO 5/2012 | 733 | 0.0095 | 0.0518 | |||
StrEGO 11/2012 | 381 | 0.0933 | 0.0058 | |||
OLOBBIA 2/2011 | 213 | 189 | 0.1070 | 0.2261 | 0.2689 | 0.3030 |
OLOBBIA 10/2012 | 105 | 0.0582 | 0.1439 | |||
GUARABIONE | 178 | 0.0371 | 0.2000 |
The following figures show a comparison between the values obtained in different dates, for river reaches investigated in different seasons. The Curone stream exhibited very different uptake lengths for NH4, indicating a low efficiency of retention in March and a good retention in November. This site, classified as a reference, was affected by a flood occurred few months before that has altered the structure of the habitat and caused, in particular, the disappearance of peryphiton. In November, when the habitats were partially recovered, the presence of a large leaves cover has probably enhanced favorable conditions for nutrient retention.
The relation among the three retention metrics is usually not apparent, since each of them provides unique information about the processes of retention. The variability of the uptake length is due to both biogeochemical and hydraulic and morphology effects, while the mass transfer coefficient and the uptake rate are more linked to biological uptake processes.
A more detailed description of the activity is presented in Deliverable I2d2.