Estuarine: Chemical

Background

The Fal estuary receives a high volume of freshwater input due to relatively intense rainfall and an impermeable underlying bedrock in the surrounding area. Alongside the intense farming in the surrounding area this results in high nutrient concentrations throughout the estuary, with hypernutrification being a regular occurrence (Langston et al., 2003). Investigating how key chemical concentrations change along an estuarine gradient and with depth allows the chemical behaviour to be analysed, which has significant implications for the local biota.

Pontoon: Chlorophyll

Surface chlorophyll remained low (below 0.1 μg/L) from 10:30 to 13:00, then began to increase around high tide, 14:21 UST. This could have been due to the lack of flushing associated with slack tide, causing a more stable water column and resulting in phytoplankton biomass remaining high in surface waters (e.g. Lauria et al., 1999). The extreme high chlorophyll concentration of 5.88 μg/L recorded at 15:00 UST could be an anomalous result caused by sampling an aggregation of mucus-bound phytoplankton (e.g. Kiorboe & Hansen, 1993).

Figure 1: Chlorophyll concentration against time with tidal curve shown. Click on figure to enlarge.

Estuarine Mixing Diagrams

Findings

The strong correlation observed between silicate and salinity showed that it was behaving conservatively, i.e. not being removed or added in the estuary. However phosphate and nitrate concentrations varied more more with increasing salinity, possibly being removed at the head of the estuary and added at the mouth. This is due to phosphate and nitrate’s integral role as macronutrients and are therefore taken up by all phytoplankton, whereas silicate is only used by a few families. This was corroborated by the decrease in chlorophyll down the estuary, suggesting that it was low nutrient concentrations limiting the growth of phytoplankton rather than low light levels.

Silicate

Silicate decreased proportionally with increasing salinity, with an initial riverine concentration of 63.83 decreasing to 0.67 at the highest salinity of 34.8. The proximity of all the points to the Theoretical Dilution Line in the zoom gave an r2 value of 0.9375 indicating that silicate was conservative. However we do not know what was occurring in the intermediate salinities. Silicate usually behaves semi-conservatively, often acting non-conservative and conservative depending on the balance of removal and addition. Silicate enters rivers through weathering of rocks and pore water, and as Cornwall’s geology is predominantly silicate-rich granite (Ghosh, 1927) there is a high steady source with little other addition. Silicate is mainly removed by diatoms and some flagellates in the form of opal (SiO2) for their tests (Burton et al., 1970), but as diatoms bloom mainly in spring they were not depleting silicate from the system. Any nuances observed at the higher salinities may therefore have been due to localised blooms, remineralistion, input from the ocean or the precipitation onto particulate matter in the electrolyte-rich seawater (Bien et al., 1958).

Figure 2: Silicon estuarine mixing diagram. Click on the higher salinities for greater resolution.

Phosphate also decreased with increasing salinity, from 1.0 µmol/L at the riverine end member to 0.14 µmol/L at a salinity of 34.8. Whilst the points appear close to the TDL, when viewed in greater resolution there appeared to be significant variation from the TDL. Prior to a salinity of 33.5 the phosphate levels were lower than those predicted by the TDL, which suggests that it is being removed. This non-conservative behaviour is to be expected as phosphate is a macronutrient essential to growth. However at salinities higher than 33.5 the concentrations are highly varied, appearing above and below the TDL. A potential reason for concentrations lying below the line is the adsorption onto particulate matter like silica (Balls, 1994).It was observed in the laboratory analysis that the riverine member phosphate concentration was unexpectedly high, possibly due to sewage contamination (Langston et al., 2003). If this is correct, the TDL may be less steep than currently deduced, and could therefore be aligned to the lower concentrations recorded at higher salinities. This would confidently conclude that phosphate is being added from the ocean at the highest salinities.

Phosphate

Figure 3: Phosphate estuarine mixing diagram. Click on the higher salinities for greater resolution.

Nitrate

Nitrate also decreased approximately linearly with the increasing salinity. Apparent addition of nitrate often occurred between salinities of 32 and 34, possibly due to nitrate pollution from industrial waste water such as Newham STW as observed by Langston et al., 2003. However below salinities of 32 nitrate concentration was lower than predicted, and there is also a notable presence of 0 nitrate concentrations at salinities higher than 34. One cause for this could be increased phytoplankton production, however chlorophyll has not increased in a bloom-like manner. These two features disagree as to whether the increase in saline water is causing the removal or addition of nitrate. The lack of pattern shows that the variation in nitrate concentration is due to an unpredictable factor other than input of salt water, new nitrate (such as sewage or agriculture) or a nitrocline. Our ability to infer the reason is limited as there are no data points from 8 – 29 salinity. It was noted during laboratory analysis that the riverine end member had a lower nitrate concentration than expected. This could mean that the TDL should be at a steeper angle, similar to the higher concentrations observed above the current TDL. As these high concentrations would then be expected we could then deduce that nitrate is being predominantly removed rather than added, which would be as expected as nitrate is an essential macronutrient that is often depleted during phytoplankton growth.

Figure 3: Nitrate estuarine mixing diagram. Click on the higher salinities for greater resolution.

Chemical Contour Plots

Estuarine Mixing Diagrams

Figures 4-6 demonstrate how concentrations of nitrate, phosphate and silicon changed from the riverine to seawater end of the estuary. The three major nutrients all exhibited their greatest concentrations at station 1, with nitrate at 74 μmol/L, phosphate 0.36 μmol/L and silicon 10.9 µmol/L. Surface water nutrient concentrations fell towards the seawater end, with all three nutrients being nearly depleted by station 7. These observations were as expected given the dependence on terrigenous nutrient input. Phosphate and silicon concentrations (Fig. 5 and 6) were relatively high in the deep samples at stations 6 and 7, reaching 3.25 and 0.20 μmol/L respectively at between 20 and 31 m deep, whereas nitrate (Fig. 4) was low (0.20 µmol/L), potentially indicating that nitrate was limiting phytoplankton growth, allowing the other nutrients to remain high (Ryther & Dunstan, 1971; Tyrrell, 1999). This is also supported by the low chlorophyll concentrations measured at this depth. Rapid depletion of silicon in the shallow waters towards the seawater end is indicative of the dense populations of diatoms which were observed in the phytoplankton samples (hyperlink to phytoplankton graph).

Figure 4-6: Nitrate, phosphate and silicon contour depth profiles respectively.

4

5

6

Chlorophyll concentration (Fig. 7) followed the same pattern as the major nutrients horizontally, with concentrations of 9.83 μg/L at 4 m deep at station 1, and decreasing to 0.81µg/l by the deepest sample from station 7. This is likely to be due to the high nutrient concentrations stimulating some phytoplankton growth, but not to the extent that nutrients were depleted. Whilst chlorophyll concentrations were generally greatest in the surface samples, a maximum also occurred at around 7 m depth, with concentrations reaching 4 – 6 μg/L at stations 3 and 4, which could be evidence for enhanced phytoplankton growth occurring around the thermocline (hyperlink to CTD temperature data). The relative homogeneity in chlorophyll concentration at stations 6 and 7 implied that light was not limiting phytoplankton growth.

Oxygen (Fig. 8) exhibited greatest concentrations within the surface samples of stations 1 and 3 with saturations of 125.4% and 128.0% respectively. One explanation for this could be the increased productivity associated with the riverine end, as demonstrated by higher chlorophyll concentrations. Oxygen appeared to be the most vertically heterogeneous of the chemicals measured, for example station 7 which had a saturation difference between surface and depth samples of 6%. This could be due to enhanced heterotrophic respiration and remineralisation in the deeper waters resulting in increased oxygen depletion. The lowest concentrations, 101 - 102% saturation, observed below the thermocline (hyperlink to CTD data) at station 7, could also be as a result of respiration exceeding photosynthesis, as supported by the extreme low chlorophyll concentrations (0.81 µg/L) measured in this location (Fig. 7).  

The close-up of the stations’ sample was added as the large gap in salinities measured between he stations and the riverine end member (8 – 25) greatly increased the regression coefficient and did not reflect the variation observed between the stations. Due to having no single sea water end member and a very small range of salinities sampled we cannot make any deductions about the chemicals’ behaviour in the mid-salinity range of the estuary.

Note that a maximum of three samples were taken at each depth due to time and equipment limitations, which means that caution must be taken when drawing any conclusions about the vertical profiles.

Falmouth, 2014

Limitations

Figure 7: Chorophyll depth profile, obtained using (a) Niskin bottle samples, and (b) a CTD-mounted fluorometer. Click to enlarge.

Figure 8: Oxygen saturation depth profile.

References

Grasshoff K.; Kremling K.; Ehrhardt M., 1999, Methods of seawater analysis. 3rd edition. Wiley-VCH.

Johnson K.; Petty R.L., 1983, Determination of nitrate and nitrite in seawater by flow injection analysis, Limnology and Oceanography, 28, 1260-1266.

Kiorboe T.; Hansen J.L.S., 1993, Phytoplankton aggregate formation – observations of patterns and mechanisms of cell sticking and the significance of exopolymeric material, Journal of Plankton Research, 15(9), 993-1018.

Langston W.J.; Chesman B.S.; Burt G.R.; Hawkins S.J.; Readman J.; Worsfold P., 2003, Characterisation of the European Marine Sites in South West England: the Fal and Helford candidate Special Area of Conservation (cSAC), Marine Biodiversity, 186, 321-333.

Lauria M.L.; Purdie D.A.; Sharples J., 1999, Contrasting phytoplankton distribtutions controlled by tidal turbulence in an estuary, Journal of Marine Systems, 21(1-4), 189-197.

Parsons T.R.; Maita Y.; Lalli C., 1984) A manual of chemical and biological methods for seawater analysis” , Pergamon p.173.

Ryther J.H.; Dunstan W.M., 1971, Nitrogen, phosphorous and eutrophication in the coastal marine environment, Science, 171(3975), p.1008-1013.

Tyrell T., 1999, The relative influences of nitrogen and phosphorous on oceanic primary production, Nature, 400(6744), p.525-531.

a

b

This page looks at:

• Background information
• Pontoon: Chlorophyll
• Estuarine Mixing Diagrams:
  Silicate
  Phosphate
  Nitrate
• Chemical Contour Plots
• Limitations