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Rivers – Nutrient retention metrics
 

INHABIT project

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Rivers – Nutrient retention metrics

Following nutrient retention metrics have been calculated:

  1. 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
  2. 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
  3. 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.

Curone    San Sudario