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Falmouth 2015 - Group 1

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Chemical

Samples acquired from the RV Bill Conway and processed in the lab.

Nitrate

The nitrate depth profile shows the same progression as the silicon depth profile in that it gives an example of how mixing along the estuary occurs at the stations closest to the marine end member. Stations 36, 37 and 39 exhibit less mixing due to their location furthest being up the estuary and experiencing less tidal force. Stations 36-39 experience nitrate concentrations from 26.8 ug/L to 1.7 ug/L while the stations 40/41-45 experience larger tidal mixing and their nitrate concentrations range from 2.1 ug/L to 0.7 ug/L.

Phosphate

A relatively small range in phosphate concentrations were recorded within all 8 stations (0.10-0.30 umol/L). Although less evident, a similar trend in stratification was still observed with distance along the estuary. For example, station 39 had the largest stratification with a 0.2 umol/L increase in 11.7m depth. After station 42, all stations at all depths recorded a phosphate concentration of 0.10 umol/L, further suggesting the presence of a well-mixed water column in this region. With that said, the first initial depth profile demonstrated no stratification, although this may be attributed to the fact that both readings were shallow samples within close proximity to each other.


Estuarine Mixing Diagrams

Silicate, nitrate and phosphate are all essential nutrients for biological processes in estuarine waters. For example, silicate is needed as a component of diatom frustules, nitrogen is needed for all amino acids and phosphate forms strong bonds in biology’s own energy currency ATP. Riverine sources provide the dominant inputs of nutrients into the marine system. However, these inorganic constituents behave differently depending on the interactions between physical and biological factors ie planktonic uptake  The degree of mixing of freshwater nutrient inputs and seawater in the estuary, determines how the concentration of nutrients varies with salinity. (Liss and Pointon, 1973).


From figures 1,2 and 3 the TDL (theoretical dilution line) shows that nitrate, phosphate and silicate follow the expected trend of having a significantly higher riverine end member concentration then the seaward end member at the mouth of the estuary. This is due to the different chemical composition of seawater and river water, due to the high level of riverine input as mentioned above. For example rivers have two orders of magnitude more dissolved silica than the surface ocean (Demaster, 1981).


Silicate, phosphate and nitrate concentrations at all stations sampled fall below the TDL, suggesting removal of these nutrients. This could be due to flocculation, biological removal and temporal variation in the end members, riverine inputs and pore waters. The weather conditions being dry and hot leading up to sampling could have potentially caused increase in the salinity of the seawater end members. This could have potentially skewed the TDL, causing sampled stations to falsely appear to be experiencing removal.

Oxygen

There is a general decreasing trend in oxygen concentration with depth. The depth profile at station 39 shows a decrease in oxygen concentration from 326.9 µmols/L at the surface depth of 1.1m, to 236.0 µmols/L at a depth of 12.8m. The decrease may be due to the water at increasing depths being further from interaction with the atmosphere at the air-sea interface where oxygen concentrations are highest. Station 45 contradicts this by having an oxygen concentration at 0.9m depth of 236.0 µmols/L which increases slightly to 268.98 µmols/L at 7.3m depth before decreasingly again to 266.67 µmols/L at 19.6m depth; therefore opposing the trend of station 39 further up the estuary. The depth profile trend at station 45 may have deoxygenation at the surface as a result of the introduction of a contaminant or unnatural pollutant such as an oil slick removing oxygen from the water. Furthermore there may be a very low abundance of phytoplankton at this station causing less oxygen to be released into the surrounding waters via respiration; this is only speculation and would need to be further investigated.




Chlorophyll

Stations 40/41 and 39 share similarly high chlorophyll values at the surface of ~5 ug/L. At station 40/41 there is a subsurface chlorophyll maxima of 6.112 at 4.4m, chlorophyll then decreases until the maximum sampling depth of that station, 3.92 ug/L at 10.4m. Conversely station 39 has a decrease in chlorophyll concentration with depth until 4.2m with a concentration of 2.7 ug/L, after which it increases again to 3.46 ug/L at 12.8m. All other stations show a slight variation with depth but generally remain near constant with depth and have relatively low concentration of between 0-~1 ug/L except for station 37/38 which has a higher surface concentration of 2.1 ug/L at 1.1m.


In conclusion as expected chlorophyll concentrations are lower at the seaward end of the estuary and are greater in the mid-estuary. The reasons for variation within chlorophyll concentrations in the estuary may be due to a difference in phytoplankton abundance.

Figure 23: Nitrate concentration against depth (Click to enlarge)

Figure 24: Phosphate concentration against depth (Click to enlarge)

Figure 25: Nitrate Estuarine Mixing Diagram (Click to enlarge)

Figure 26: Phosphate Estuarine Mixing Diagram (Click to enlarge)

Figure 27: Silicon Estuarine Mixing Diagram (Click to enlarge)

Figure 28: Chlorophyll against depth (Click to enlarge)

Figure 29: Oxygen against depth (Click to enlarge)

References

DeMaster, D. (1981). The supply and accumulation of silica in the marine environment. Geochimica et Cosmochimica Acta, 45(10), pp.1715-1732.


Kamatani, A. and Takano, M. (1984). The behaviour of dissolved silica during the mixing of river and sea waters in Tokyo Bay. Estuarine, Coastal and Shelf Science, 19(5), pp.505-512.


Liss, P. and Pointon, M. (1973). Removal of dissolved boron and silicon during estuarine mixing of sea and river waters. Geochimica et Cosmochimica Acta, 37(6), pp.1493-1498.

Disclaimer: The views expressed here are not associated with those of the National Oceanography Centre Southampton or the University of Southampton.

Silicon

The silicon depth profiles from stations 36 – 45 illustrate an example of how mixing progresses along an estuary, with most stratification occurring in the first 2 stations furthest along the estuary (With a 3.6 umol/L drop in 2.4m at station 36 and a 4.6 umol/L drop in 5.1m at station 37/38). Because of its distance upstream, these stations are where the least amount of tidal influence is expected. Mixing increases with each station until almost complete conservative behaviour is observed in stations 44 & 45 (with no more of a 1 umol/L change throughout the entirety of both vertical profiles). These stations are the lowest down the estuary and closest to tidal influence (and therefore where the most mixing would be expected to occur).

Figure 22: Si concentration against depth (Click to enlarge)