Chemistry assignment on: Solutions

Chemistry assignment on: Solutions 

The salinity profile of the Brunswick River estuary shows that the estuary at this time (March 2013) is very slightly stratified (Figure 1). This can be seen by the slight changes of salinity in the water column from the surface of the water at 0m, to the bottom of the estuary. This incurs that there is both river flow and tidal mixing occurring, and a two layer flow with vertical mixing. It can also be seen that there the change in salinity is more prevalent closer to the river mouth with changes of 2ppt until about 4km inland, where the change drops from being 2ppt, to 0.2 ppt until 7km, and then drops to again to changes of only 0.02ppt from 7km onward. The other factor that can be noted from the salinity profile is the varying depth of the estuary, with the deepest measured point at 3m and the shallowest point being 1m.

The Brunswick River Estuary had a total flushing time of about 4.6 days. Various sections of the river, from where site locations were derived did have differing flushing times, with sites closer to the river mouth having faster flushing times, while those further away from the river mouth having longer flushing times. Site one had the quickest flushing time of just .13 days, while the furthest site, site 12 (section 5) displayed a longer flushing time for this section with a flushing time of 0.45 days. The longest flushing time was found at site 6 (section 9) with a total flushing time of 0.63 for the section (Table 1, Figure 2).

 Mixing Plots:

There we remarkable differences in the availability of nutrients and algal biomass at varying salinities along the Brunswick river estuary.  The conservative mixing models illustrate where possible sources and sinks occur along the estuary for nutrients and algal biomass. The observed concentrations of nitrogen dioxide (NO₂) were very similar to the predicted values, with and showed the same pattern as that of the conservative mixing model, with the biggest difference showing a predicted value of 6.5 µmol/L, and an observed value of 5.65 µmol/L.  The observed rise of phosphate (PO₄) were much higher than the predicted values. These ranged from values of 0.45 µmol/L at the saltwater endmember, to 0.89µmol/L at a salinity of 10.42 ppt, closer to the middle of the estuary, and dropped back down to 0.64µmol/L, before rising again to 0.77 µmol/L at the freshwater endmember (Figure 3).

 These variations are similar to those of the dissolved organic nitrogen (DON), with the highest spike in concentration just after PO₄ of 23.59µmol/L at a salinity of 6.49, and dropping back down to 17.26µmol/L at the freshwater location. Ammonium (NH₄) also experiences quite a large spike in concentration, which correlates almost perfectly with the removal of dissolved oxygen (DO). The NH₄  concentration peaks at salinity value of 10.42 ppt, at a concentration of 5.85 µmol/L from the predicted 1.85µmol/L, and the largest drop in DO concentration occurs at the same point with an observed concentration of 191.88µmol/L from a predicted 226.21µmol/L. present returns to the returns to the predicted values at the same point of salinity at 0.27ppt, which may indicate a relationship between the two substances. The pelagic algal biomass also presents a drop in concentration at 10.42ppt, to 0.05 mg/m² from the previous measurement of 0.06mg/m², with the lowest concentration presented at 2.4ppt (0.03 mg/m²). The amount of pelagic algal biomass rises rapidly again at a salinity of 0.27 ppt, the same point at which DO begins to rise, and NH₄ starts to decline (Figure 3).

There appears to be a of sink of dissolved organic phosphate at both the fresh and salt water ends of the estuary, however a source is found closer to the freshwater endmember. This is illustrated by the spike in DOP concentration at a salinity of 6.9ppt, at 1.31µmol/L, which is 0.66µmol/L above the predicted value, while the sinks are depicted by the drop below the predicted values. This occurs at a salinity of 0.27 ppt, where DOP drops to 0.49 µmol/L, as well as closer to the mouth of the estuary, as salinity of 28.04 with and availability of 0.03 µmol/L DOP. The declines in concentration of DOP coincide with the increase in amount of pelagic algal biomass, and vice versa (Figure 3).

The concentration of radon, which is indicative of groundwater input, correlates with saturation of carbon dioxide (CO₂) along the Brunswick river estuary (Figure 4). Both sets of data illustrate low concentrations at the mouth of the estuary (Brunswick Heads), and higher concentrations the further in land (closer to Mullimbimby). These higher concentrations at around sites 8, 9 and 10 (salinities 0.13, 0.1 and 0.9 respectively), just before the spike in DOP and DON, which occurred at sites 5, 6 and 7 (salinities 6.39, 2.4 and 0.27 respectively) (Appendix 1).

Benthic productivity appears to be highest at the fresher part of the estuary, which is further in land from the river mouth, with a maximum dissolved oxygen fluxof 6583.4µmol O₂/m²/hr at 8.1 km from the river mouth. The lowest measured rate of benthic productivity occurs at 2.7km from the river mouth, with a dissolved oxygen flux of 1400.9 µmol O₂/m²/hr. As opposed to the benthic productivity, the carbon to nitrogen ratio (C:N) is at one of the lowest at 8.1 km from the river mouth with a C:N value of 11.8 and its highest at 4.6km from the rivermouth, just after the benthic productivity drops with a C:N value of 22.4 (Figure 5).  The C:N ratio drops after this peak as you move toward the river mouth,  with a final measured C:N value at the river mouth of 9.5. Conversely, after the drop of dissolved oxygen flux (1400.9 µmol O₂/m²/hr at 2.7km), benthic productivity increases toward the rivermouth, with a final value of 3313.3 µmol O₂/m²/hr.The C:N peak and the benthic productivity drop occur downstream in relation to the peak of radon and pCO₂(Figure 4).

Benthic algal biomass fluctuates throughout the measured distance of the Brunswick River Estuary; however appears to follow a similar (not exact) pattern to that of the benthic productivity, with peaks at 7km and 1km, 1234.5 and 1293.2 mg/m2 respectively. The largest dip in benthic algal biomass occurs at 2.7km with a value of 77.63 mg/m², the same location which benthic productivity has its largest dip (Figure 5).

For all flux rates observed, the dark flux is mirrored by the light flux, with a larger flux occurring in light conditions. This is not completely true however for PO4, which shows a similar dark flux rate throughout the estuary, with the largest flux being -11.1µmol and a lowest flux -7.3 µmol, while the largest flux during the light period is 34.63 µmol and the lowest is -5.7 µmol. The N2 flux has a generally decreasing pattern as you get closer to the river mouth, with a starting measurement of 71.5 µmol at 9.5 km from the river  mouth, to a slight peak of 75.7µmol at 8.1km, dropping to 15.5µmol at 0.16 km (Figure 6).

Ammonium flux rates along the river bed appear to be highest further away from the river at a peak of 41.1 µmolat 9.5km. This flux rate drops to a mere -25.4 µmol just after this peak at 8.1km, and peaks again at 7km with a rate of 11 µmol, and then slowly decreases further toward the river mouth, with a final measured flux rate of -23.1µmol at 0.16km from the river mouth. This is a similar pattern to that of NOx, however instead of slowly decreasing toward the river mouth, the flux rates increase. NOx has the highest flux rate furthest from the river mouth with a peak of -12.9 µmol at 9.5km, then drops to -27.0µmol at 8.1km. NOx peaks again at 7km with a flux rate of -15.1 µmol, slightly drops again at 4.6km (-21.0 µmol), then slowly begins to rise reaching a final flux of -16.0µmol at 0.16 km.

Phosphate follows close to an opposite pattern along the river, starting off with a lower flux rate, and peaking closer to the middle of the estuary. PO4 shows a flux rate of -2.8µmol at 9.5 km, before dropping to -8.6 at 8.1 km. It peaks at 7 km with a flux of 12.8 µmol before dropping again to 4.4µmol at 4.6km, slightly increasing toward the river mouth with a final flux rate of -3.4 at 0.16km.

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