CONCLUSIONES

The role of DOC in the river ecosystem is here investigated. To this aim, we analyzed the behavior of systems with different stream DOC concentrations, implementing (i) a first set of simulations where stream DOC concentration was the only changing parameter (Cases 1-5, see Table 4.5) and (ii) a second set of simulations where the ratio between stream DOC and nitrogen concentrations was kept constant (i.e., cDOCincreases similarly to the previous set of simulations while the ratios cN O⁻₃/c_DOC and cN H₄⁺/c_DOC are constant; Cases 6-10, see Table 4.6). Stream DOC concentration varies from 21 to 40 mg/l in both sets of simulations, while NO3⁻ and NH4⁺ vary from 1.0 to 1.9 mg/l and from 0.05 to 0.095 mg/l, respectively (see Table 4.5 and Table 4.6). Notice that the system analyzed in Section 4.3.1 corresponds to Case 5.

Figure 4.10 illustrates the trend of heterotrophic and autotrophic biomass concentrations as a function of stream DOC concentration. For increasing values of stream DOC concentration, the heterotrophic bacteria show a similar growing behavior up to a certain value of DOC concentration, followed by either a decay behavior (only DOC variation) or a saturation behavior (DOC and nitrogen variation). In spite of an increasing stream DOC concentration, therefore, biomass concentration undergoes a reduction or reaches a limit value. This behavior can be explained considering the bioclogging phenomenon: a greater amount of dissolved organic carbon entails a rise in heterotrophic metabolism since heterotrophic microorganisms have a higher quantity of substrate to feed and grow. However, this process is not monotonic because an excessive growth of biomass can induce an occlusion of the upper sediments, preventing the bacteria in the deeper part of the riverbed to receive the nutriment necessary to survive and, consequently, causing their death.

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Fig. 4.10 Spatial integration values over the whole domain of (a) heterotrophic and (b) autotrophic biomass as a function of DOC concentration in the stream. The blue line is referred to only in-stream DOC variation, while the red one is referred to both in-stream DOC and nitrogen variation.

Autotrophic biomass shows a different behavior when only stream DOC concentration and both stream DOC and nitrogen concentration vary (see Figure 4.10). The concentration of autotrophic bacteria decreases for higher values of stream DOC concentration when the stream nitrogen concentration is constant and increases for higher value of stream DOC concentration when the stream nitrogen concentration is increased. In the first case, an increase in stream DOC concentration entails that the heterotrophic bacteria have available a greater amount of nutriment respect to the autotrophic bacteria and, therefore, win the competition for the occupancy of the pore space. In the second case, both heterotrophic and autotrophic biomass grow since a higher stream concentration of c_{N H}⁺

4 improves the efficiency of the nitrification reaction, with a consequent increase of autotrophic biomass.

Table 4.5 and Table 4.6 display the values of reaction rates and biomass concentrations – integrated over the whole domain – for each value of stream DOC and nitrogen concentration at the final time (steady state). The reactions rates show the same non-monotonic behavior of the biomass concentrations, indicating more reactive bacteria for increasing stream solute concentrations until a limit value is reached. In addition, the value of front depth of het-erotrophic biomass (i.e., the maximum depth of the riverbed sediments at which

Table 4.5 Areal integrated values of biomass concentrations and reactions rates per unit stream width at the steady state for systems with different DOC concentration in the stream (K₀ = 10⁻³ m/s). Front depth indicates the maximum depth at which the heterotrophic biomass survives within the domain.

Case cDOC X_h R_DOC X_a R_{N H}⁺ Table 4.6 Areal integrated values of biomass concentrations and reactions rates per unit stream width at the steady state for systems with different DOC and nitrogen concentrations in the stream (K₀ = 10⁻³ m/s). Front depth indicates the maximum depth at which the heterotrophic biomass survives within the domain.

Case cDOC c_{N O}⁻

heterotrophic biomass receives nutriment and can survive) is reported in the tables for each case. A decreasing value of the front depth for increasing values of DOC concentration is observable, when the values of biomass concentra-tion and reacconcentra-tion rates start to drop. This can be explained considering the

bioclogging phenomenon, that entails a filling of the pore space due to the biomass growth with consequent reduction of the front depth. In fact, in Case 5 a reduction of heterotrophic biomass – respect to the other cases – occurs despite an increase of stream DOC concentration since the occlusion induced by bacteria in the upper part of the sediments causes the death of the biomass in the deeper sediments.

An interesting parameter to analyze the hydrodynamic aspect of bioclogging process is the total flux across the river-bed interface (Eq. 4.15), that is represented in Figure 4.11 as a function of stream DOC concentration for both sets of simulation (Cases 1-5, panel on the left, and Cases 6-10, panel on the right). In the same figure, we show the concentration, integrated over the whole domain, of heterotrophic biomass – that mainly induce the bioclogging process – in order to better investigate the dependence of hyporheic flow on the distribution of microorganisms. Both variables are made dimensionless by their initial value (t = 0 d). It can be observed that the exchange flux decreases when the DOC concentration increases, showing an opposite behavior respect to the biomass concentration. This can be explained considering that a greater concentration of biomass induces a greater reduction of hydraulic conductivity and, consequently, of pore water flux.

Figure 4.12 represents the comparison among the hydraulic conductivity profile along the y-direction for Cases 1-5 (panel on the left) and Cases 6-10 (panel on the right). The values of hydraulic conductivity are averaged along the x-direction and are referred to the steady state. The results show that for higher DOC concentrations the gradients of hydraulic conductivity are stronger since the biomass is confined to a thinner layer near the sediment-water interface (the front depth decreases, see Table 4.5 and Table 4.6) and, therefore, the hydraulic conductivity rapidly varies in the upper part of the domain where the microorganisms manage to survive. The value of hydraulic conductivity in conditions of absence of biomass (K0 = 10⁻³ m/s) is, therefore, reached at decreasing depths for increasing values of stream DOC concentration.

The occlusion process of the upper sediments due to microbial growth, which prevents the survival of the bacteria in the deeper part of the riverbed, is evident from the strong gradients of hydraulic conductivity profiles. Results show that this effect increases for higher values of stream DOC concentration, since more heterotrophic biomass is present in a smaller portion of sediments

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Fig. 4.11 Flux across the river-bed interface and heterotrophic biomass over the whole domain for increasing DOC concentration in the stream at the steady state.

Both variables are made dimensionless by the value at the time t = 0 d. Panel on the left is referred to Cases 1-5 (only in-stream DOC variation) while panel on the right is referred to Cases 6-10 (in-stream DOC and nitrogen variation).

4 6 8 10

Fig. 4.12 Vertical profiles of hydraulic conductivity averaged along the horizontal direction for increasing stream DOC concentration at the steady state for only stream DOC variation (Cases 1-5, panel on the left), and for both stream DOC and nitrogen variation (Cases 6-10, panel on the right).

(higher values of heterotrophic biomass concentration and shallower portions of sediments in which the biomass can survive, see Table 4.5 and Table 4.6).