Physical and Chemical

Pontoon Data

Irradiance

Irradiance shows an exponential decrease with depth throughout the time series. There is no clear increase or decrease of irradiance throughtout the day. The highest surface irradiance was at 12:30 UTC with a reading of 0.508, the lowest was at 10:05 UTC with a reading of 0.288. Irradiance values at depth are very close together; ranging from 0.012 to 0.008. By taking the natural log of Ez/Eo and taking the gradient of the line as the Attenuation Coefficient (K), using the equation ln(100)-ln(1)/k we calculated the 1%light depth and therefore the base of the euphotic zone. The Attenuation coefficient and the 1% light depth decrease in the morning as the tide is coming in and the clearer seawater pushes the sediment loaded river water further up the estuary. The highest 1% light depth is 3.7m at 10:30 UTC, this was approximately high water and therfore fits with our expectations that clearer seawater will have a higher 1% light depth than the sediment laoded rivers. As the tide changes to the ebb tide 1% light depth shows a slight increase to 3.39m at 12:05. There is no linear pattern of a decrease of 1% light depth as the tide flows seaward; This could be due to the numerous boats passing and mixing processes around the pontoon could have affected the data.  

Winnie the Pooh Data

Chlorophyll

There is a general decrease in chlorophyll concentration at the surface of the water with an increase in salinity. For this graph an increase in salinity corresponds to samples moving further down the estuary towards the mouth. The highest chlorophyll concentration was at 4.2 PSU at 67.1 µg/L and the lowest chlorophyll concentration was at 31.1 PSU at 11.8 µg/L. At salinities of approximately 20-24 PSU, the chlorophyll concentration shows a slight increase up to 20.8 µg/L. This increase could be driven by the tidal rhythm changing from the flood tide to the ebb tide, which occurred at 10:45 UTC, approximately mid-way through sampling. Yin and Harrison, 2000 found chlorophyll concentrations to be higher during flood tides than ebb tides, which matches our data, and found it could be due to feeding by benthic organisms.

Temperature and Salinity Salinity

Falcon Spirit Data

ADCP

Station H

Station H’s ADCP reading was taking during the flood tide. Flow on the ships track is heading in a North-West direction, towards the river. Strongest flow is shown towards the sea bed, mainly West of the transect [0.6m/s].

Flow reduces towards the surface layer [0.19 m/s] which could be due to the influence of the less-dense surface river water in relation to the denser, deeper waters strong flood tide. The weather a week previous was predominantly dry, so a reduced river discharge flow against a strong flood tide could be causing a lower velocity surface layer flow.

Station F

At Station F, flow is strongest towards the East of the transect and towards the sea bed [0.2 m/s – 0.3 m/s], this is most likely due to the incoming flood tide lying below the fresh surface flow of the river. Again, due to the strong flood tide pushing against the river flow, surface flow appears much more reduced at this Station.

Velocity marks on the ship track show that flow is heading North up the river, coinciding with the flood tide.

Station D

Flow direction is North-West as seen from the ship track. Strongest flow is at the surface [0.2 m/s] and flow is weakest at depth towards the sea bed [0.05m/s - 0.1 m/s]. This may be because Station D is located further up the estuary and river, and therefore tidal forcing may be much weaker.

Station B

The ADCP transect was taken at the opening of the ‘Tavy River’ towards the right of Station B. At the time the transect was taken, the tide was beginning to approach slack tide, which could be a reason as to why we see reduced flow and why flow is mainly heading towards the West of the ships track. Stronger surface flows [0.11 m/s] are located at the centre of the transect and could be due to the river discharge and weakening flood tide. Overall, all flow is low in comparison to other Stations.

Station C

Surface flow along this Station is strongest at the surface [0.1m/s – 0.2 m/s] with maximum flows of 0.3 m/s. Weakest flows are towards the sea bed [0.05m/s – 1m/s]. This could be because slack tide is reached at 11:44 UTC, therefore there is reduced saltwater flow.

The direction of flow as seen on the ships track is South and South-West, and could be influenced the most by river flow heading downstream [South] being stronger than the turning tide.

Station E

As seen from the ship track, flow is heading in a South-East direction which could be influenced by the way the river meanders when looking on the map, focusing the flow in this direction. Strongest flow is located at the surface [0.3 – 0.39 m/s] and decreases with depth towards the seabed.

As this Station’s ADCP was taken on the turn of the tide when looking at the tide tables, the tide is just beginning to ebb. This may be why there is a strong surface river flow heading South [downstream] and low velocity saltwater flow [0.09m/s - 0.1m/s].

Station G

At this Station, the tide is just beginning to ebb, and overall flow is heading South from the ships track. Strongest flows are located at the surface layer [0.4m/s – 0.6m/s] and reduce with depth towards the sea bed [0.008m/s – 0.164m/s]. This may be because the ebb tide has only just begun and is not strong. Therefore, in comparison to the ebb flow, the river flow still dominates.

Station I

Overall flow is heading South-South-West as seen from the ship track. At this Station, the tide is as the beginning of it’s ebb, but still shows reduced flow with depth [0.01m/s – 0.13m/s]. Surface flow averages around [0.25m/s – 0.47m/s].

Station I is located right behind the barrier, which may affect the readings on the ADCP, and could explain why the ebb tide is not showing stronger flows.

Station J

At Station J, flow is heading in a South- East direction. Flow velocity is fairly uniform throughout the water column [0.25m/s], however is strongest towards the east of the transect. This may be because part of the Station [West side when looking at the map] is more sheltered by the barrier compared with the East.

The water flow speed varied from 0 – 0.28 m/s throughout the water column from 8:30 to 11:00 UTC. At 9:00,10:30 and 11:00 UTC, there was higher variabilty in the flow speed in the surface waters than at depth. However at times 8:30, 9:30 and 10:00 UTC the current flow varied greatly throughout the water column. Direct of flow shows no pattern. The reasons for these observation could be because of the many boats passing the pontoon throughout the time series and causing current shifts. Also the pontoon acts as a large barrier to flow which may have affect our results.

Current Speed

Temperature shows a slight decrease of 1-1.5 °C with depth throughout the time series due to a higher attenuation of light at the surface warming the surface waters. However, at 12:00 UTC the temperature at depth was measured higher than the surface at 22.2°C which is also the highest temperature recorded. It must therefore be considered that this may be an anomalous result and could be due to the probe hitting the sediment or the noted lag of the probe depth reading compared to the actual depth. There is a general trend of an increase in temperature from the morning into the afternoon. At 8:30 UTC the surface water temperature was lowest at 19.24°C, this then increased to 20.14°C at 10:00 UTC and further increased to 22.7°C at 13:00.

Salinity shows a general decrease throughout the time series. The first reading at 8:30 UTC has a salinity of 32.45 psu at the surface. The salinity decreases to 31.65 at 11:00 UTC, which is approximately high water. The lowest surface salinity reading taken was at 13:00 UTC at 28.2 psu, by which point the tide was ebbing, reducing the salinity. Overall, there is an increase in salinity with depth. This relates to the temperature graph as it shows the more saline, cooler and therefore denser sea water flowing below the warmer, less saline river water as temperature has a greater effect on density than salinity.

Dissolved oxygen saturation percentage in approximately depths of 1-3m shows a general increase from high water at 11:00 UTC to 13:00 UTC as the tide ebbs. This is expected as freshwater holds more oxygen than seawater. There is no clear, consistent pattern of increase or decrease of dissolved oxygen with depth. The surface reading for 13:00 UTC may be considered an anomalous result due to an error in readings taken.

At each time the chlorophyll concentration reading is lower in the surface waters up to 1m, it then increases between approximately 1-3m and is depleted in the deeper waters. Chlorophyll concentration is related to the attenuation of light through the waters (McMahon et al. 1992). For our irradiance graph we can see that there is higher irradiance in the surface waters than at depth, which explains why there are greater chlorophyll concentrations in the surface waters. There is a general increase in chlorophyll concentration with time. The lowest chlorophyll concentration was 1.94 µg/L at 4m and 8:30 UTC. The highest chlorophyll concentration was 7 µg/L at 1.8m, 13:00 UTC. As the tide is flooding (around 1.5 µg/L), the chlorophyll concentration changes relatively little compared to the change when the tide turns (around 3 µg/L).

Figure shows the chlorophyll concentration agaisnt salinity, sampled on Winnie the Pooh and Falcon Spirit along the stations down the Tamar estuary. Unfortunately some of our samples from Falcon Spirit have been lost. Therefore we do not have data for chlorophyll at all stations.

Salinity in psu plotted against average chlorophyll concentration in µg/L along the Tamar estuary. Letters correspond to station: A – Calstock, B- Above R. Tavy Confluence, C – Saltash pontoon, D – Lynher River, E – Looking Glass Point, H – Cremyll, I – Breakwater Western Channel.

There is a general decrease in chlorophyll concentration at the surface of the water with an increase in salinity. For this graph an increase in salinity corresponds to samples moving further down the estuary towards the mouth. The highest chlorophyll concentration was at 4.2 psu at 67.1 µg/L and the lowest chlorophyll concentration was at 34.6psu at 2.408 µg/L. At salinities of approximately 20-24psu, the chlorophyll concentration shows a slight increase up to 20.8 µg/L. This increase could be driven by the tidal rhythm changing from the flood tide to the ebb tide, which occurred at 10:45 UTC, approximately mid-way through sampling. Between sites B and C there is a sewage outlet into the Tamar, which increases phosphate levels and could also be a trigger for the increase in chlorophyll in this area. Yin and Harrison, 2000 found chlorophyll concentrations to be higher during flood tides than ebb tides, and found it could be due to feeding by benthic organisms.

Combined Data

Graphs show the change in water speed in m/s with depth in m, with arrows corresponding to the direction of the flow at each measured data point. Measurements were taken every 30mins. From left to right down the times of measurement are 8:30, 9:00, 9:30, 10:00, 10:30, 11:00 UTC.

Irradiance, measured as the ratio of irradiance at the surface (E0) with irradiance at depth (Ez), change with depth in m at each time measurement. Times are: 9:20, 10:05, 10:30, 11:20, 12:05, 12:30, 13:00, 13:40, 14:00 UTC.

Graph shows the change in attenuation coefficients and 1% light depth in m with time (UTC). Red line corresponds to Attenuation Coefficient and black line refers to 1% Light depth.

Change in temperature in °C with depth in meters. Different lines correspond to the different times the measurements were taken. Times are 8:30, 9:00, 9:30, 10:00, 10:30, 11:00, 11:30, 12:00, 12:30 13:00 UTC.

Graph shows the change in salinity (psu) with depth (m) across the time series. Times are 8:30, 9:00, 9:30, 10:00, 10:30, 11:00, 11:30, 12:00, 12:30, 13:00 UTC.

Graph shows the change in dissolved oxygen percentage with depth (m) across the time series. Times are 8:30, 9:00, 9:30, 10:00, 10:30, 11:00, 11:30, 12:00, 12:30, 13:00 UTC.

Graph shows the change in chlorophyll a concentration (µg/L) with depth (m) across the time series. Times are 8:30, 9:00, 9:30, 10:00, 10:30, 11:00, 11:30, 12:00, 12:30, 13:00 UTC.

This mixing diagram shows that there is a lot of addition of nitrite into the River Tamar at the lower salinities with all of the samples taken from the Winnie the Pooh transect having a higher concentration of nitrite then the River Tamar endmember with sample 3 having a concentration of 1.174 µmol/L which is considerably higher than the River Tamar endmember which had a concentration of 0.208 µmol/L. This could be explained as nitrite may be being produced as a by-product of oxidation of ammonium (NH4+)  by bacteria which may be explained by the release of ammonium fertilizer runoff  into the estuary leading to a spike of nitrite being produced but the nitrite could be also be produced as a by-product in the sediment.( Know et al. 1986)As we go from upstream on the River Tamar to downstream we see a marked change with some fluctuating data points but the majority of data follows that with increasing salinity the concertation of nitrite decreases.

This mixing diagram shows that at the highest salinities the concentration of nitrate is conservative as it follows the theoretical dilution line closely as the salinity decreases the concentration of nitrate starts to deviate from the theoretical dilution line this behaviour is known as removal. At the top of the River Tamar there was removal this correlates with the chlorophyll data which was collected by the Winnie the Pooh at Site 1 with a salinity of 4.2 had the highest chlorophyll readings with 69.86 and 64.4 which shows that there is a high amount of primary productivity which would explain why there is greater removal seen at the higher parts of the River Tamar. The increase in nitrate at lower salinities may be because as stated above there may be nitrifying bacteria which are oxidizing additional ammonium in the water and going from nitrite from nitrate so this may cancel out some of the removal seen caused by phytoplankton (Know et al. 1986; Butler & Tibbitts, 1972)

This mixing diagram shows that the at the higher salinities the concentration of both nitrite and nitrate follow the theoretical dilution closely indicating conservative behaviour. As the salinity decreases upriver again the concentration of the combination of nitrite and nitrate deviates from the theoretical dilution line.

Mixing Diagrams

Silicon Data

Data
Station A shows a highly varied silicon concentration, however due to the location of station the depth was very shallow and a depth profile cannot be made the highest concentration was 49.39 μg/l and the lowest was at 0.610 μg/l. The rest of the stations show a very low silicon concentration all throughout the water column. When the high riverine values (from site A) are removed from the dataset we can see that at the first 3 sites there is increased silicon with depth, sites E, F and I seem to show a decrease of silicon at first and then an increase after a depth. The rest of the sites (B, C, D, G, H, J) show an increase in silicon with depth.

Figures – 1, plot with riverine data. 2, plot with riverine data removed.

Discussion – The concentration of 46.39, was also from the riverine endmember, this may be due to the lack of phytoplankton being able to tolerate these conditions, many phytoplankton photosynthesis rates drop to close to 0 at this point (Qasim et al, 1972) and so population levels may be so low they’re undetectable. When we look at the mixing diagram and phytoplankton stacked bar plot we can see that there is a high amount of diatoms which are siliceous, remineralisation of this silicon by both zooplankton (Yool and Tyrrell, 2003) and other fauna could lead to the increases at depth. However, the rest of the plots show the removal of silicon which is also shown by dilution plot, which shows removal throughout the estuary. There are also many siliceous diatoms such as Cryptomonas spp which are siliceous (Mommaerts, 1969) which also aren’t present at the very low  salinities.

Estuary Oxygen Figure One- Showing in Northings and Eastings the locations of our Sites down the Estuary

Having relabelled our Stations, we plotted them against the Oxygen Saturation. As we took multiple depth readings at each of the sites we plotted three lines showing deep, mid and surface water data however, it was hard to see a pattern. On average, our largest Oxygen readings were at the Surface water. Our highest readings seemed to be at Site 8. This may be because there are large amounts of production at the surface i.e. photosynthesis releasing oxygen, this is due to there being a larger supply of nutrients in the surface layer.

This temperature salinity profile demonstrates that as the Station numbers are increasing the Salinity falls and the temperature increases. This shows that as the estuary becomes more saline temperature falls, and in fresher water temperatures are higher. This is what we would expect as more saline water is cooler and denser than freshwater.

Estuary Oxygen Figure Three- A temperature salinity profile showing the location sites made in figure showing changes down the Estuary

When Oxygen was plotted against depth at the different sites we saw many different patterns. For Sites 4,5,6 oxygen saturation increased with depth however all the others decreased. This may be due to locations 4,5 and 6 being located where there is the most mixing of fresh and saline water so the oxygen profile may be disrupted by this. Site 3, appears to have a very low Oxygen Saturation in comparison to the others, this may be due to an error in our data. The highest Oxygen Saturation was at depth at Site 6, at 116.0536% this is where mixing would be high.

Estuary Oxygen Figure Seven- Showing how the oxygen saturation percentage varies with varying depth profile at different Sites down the Estuary

Estuary Oxygen Figure Eight- Showing how the Chlorophyll concentration varies with the oxygen saturation percentage at different depths in the Estuary

Comparing oxygen to chlorophyll we found that for the Mid ocean profile oxygen increases with chlorophyll concentration. This could be due to more productivity occurring in higher concentrations of chlorophyll so more photosynthesis is taking place, so more oxygen is present. For the surface waters with changing concentrations of chlorophyll the oxygen saturation was mildly influenced. This may be due to the surface waters using up lots of available nutrients quickly, so little are available. In the deep ocean, oxygen is less stable regarding changing chlorophyll concentrations suggesting other factors in the deep ocean may also take effect here.

Estuary Oxygen Figure Two- Showing the Station numbers made in figure one against the oxygen saturation percentage

Due to our sites not being in a linear pattern we relabelled the plots 1-9 so that we could linearly compare the locations of our sites against our Oxygen Saturation Percentages.

Phosphate Data writeup Eastuary

Data – Station A shows a very high variance in phosphate, varying from 0.285 μg/l to 1μg/l ,  stations B and G show an increase with depth in the estuary. Stations E, F and H have initial decreases and then increases. Finally, the rest of the stations (C, D, I & J) just decrease with depth.

Figure 1 – All stations concentration plotted against depth.

Discussion – Figure A shows a wide range of phosphates relative to other measurements but still low, this is likely to pollution from the city of Plymouth entering the river. The next phenomenon of an initial decrease then an increase could be due to stratification of the river layering the phosphate (Mommaerts, 1969). Phosphate is low throughout the estuary because it is the limiting nutrient (Gotham and Rhee, 1981), concentrations of total nitrogen and silicon are higher at every station. Finally, for the last set of sites the nutrient levels decline throughout the water column because phosphate is constantly being mixed and used up by phytoplankton and zooplankton throughout the estuary. The mixing plot shows that there is removal in the upper estuary and addition in the lower estuary however this could just be remineralisation in the lower estuary by zooplankton and microbial breakdown.

Oxygen saturation data Analysis

Estuary Oxygen Figure Four- Illustrating how salinity correlates to the changing oxygen saturation percentage down the Estuary

When plotting Oxygen Saturation Percentages against Salinity we again plotted them at different depth layers. No obvious pattern was shown. The surface water remained most constant with little effect regarding salinity. In general, the oxygen percentage dipped the most at around 34.5PSU and peaked at 34PSU.

Figure : A plotted theoretical dilution line with the known riverine endmember for Phosphate and a calculated seawater endmember calculated by using the highest salinity recorded. The individual data points of salinity against phosphate concentration µmol/L are plotted against this theoretical dilution line.

Phosphate concentration shows non-conservative behaviour, with addition and removal occurring with increasing salinity down the estuary. Removal occurs at lower salinities, e.g. at a salinity of 10 phosphate concentration is ~0.3 µmol/L. Addition occurs at higher salinities, e.g. at a salinity of 32 phosphate concentration is ~0.48 µmol/L.

Silicon concentration shows non-conservative behaviour, with heavy removal occurring at low salinities. There is a big difference in concentration between the river endmember concentration, of ~46 µmol/L, and silicon concentration of ~5 µmol/L measured at the lowest salinity. The silicon concentration only fluctuates a small amount with increasing salinity, remaining mostly uniform across the estuary. A small amount of addition occurs at high salinities of ~33, this could be anomalous.

Figure : A plotted theoretical dilution line with the known riverine endmember for Silicon and a calculated seawater endmember calculated by using the highest salinity recorded. The individual data points of salinity against Silicon concentration µmol/L  are plotted against this theoretical dilution line.

Addition

Removal

Removal

Figure : A plotted theoretical dilution line with the known riverine endmember for nitrite and a calculated seawater endmember calculated by using the highest salinity recorded. The individual data points of salinity vs nitrite concentration µmol/L  are plotted against this theoretical dilution line.

Figure : A plotted theoretical dilution line with the known riverine endmember for nitrate and a calculated seawater endmember calculated by using the highest salinity recorded. The individual data points of salinity vs nitrate concentration µmol/L  are plotted against this theoretical dilution line.

Figure : A plotted theoretical dilution line with the known riverine endmember for total nitrate and nitrite and a calculated seawater endmember calculated by using the highest salinity recorded. The individual data points of salinity against total nitrate  and nitrite concentration µmol/L  are plotted against this theoretical dilution line.