Plymouth Field Trip 2019 - Group 1

Tamar Estuary

Introduction and Aims

Tamar-Tavy Estuary is located at the border between Devon and Cornwall. Notably, it includes the confluence of the River Tamar, River Tavy and the River Lynher which is a site of special scientific interest (SSI) (Plymouth-MPA, 2014).  The lower estuary (breakwater to the Saltmarsh) was surveyed by University of Plymouths Falcon Spirit vessel and the upper section (Saltmarsh to Halton Quay) of the estuary was surveyed by the Flying Pig vessel (Fig. 1).

Using data collected from both vessels, the aim of this research was to investigate the entire estuarine system. To understand how the physical, chemical and biological properties vary on a temporal and spatial scale. The influx of freshwater and tides have a large influence of the partially stratified mixing process that occurs within the estuary. Initially, the Flying Pig vessel was set to reach the lowest salinity area located at Calstock but unfortunately due to spring tides the water depth was too low to safely reach this area.

On the Falcon Spirit, 9 stations were targeted due to their varying location within the estuary, with additional notations of habitat type and surrounding features. On the downcast of the CTD at each station, areas of interest were noted and on the upcast niskin bottles were fired at those heights to collect water samples in order to analysis dissolved oxygen, chlorophyll, nutrients and phytoplankton. At a few stations, zooplankton nets were deployed at towed to gather further data.

For the upper estuary, surveyed in the Flying Pig RIB, a multi-parameter salinity monitor was used to create a depth profile for three locations. We measured dissolved oxygen, salinity, total dissolved solids and temperature at each meter as well as at the surface.

Figure 1: A map showing the stations sampled across the estuary system in Plymouth.

Methodology

Results

Table 1: shows the station locations for the Falcon Spirit sampled estuarine stations

Table 2: shows the station locations for the Flying Pig sampled estuarine stations

Pink: A0

Lime: B0

Grey: C-1

Red: C/C0

Cyan:D

Purple: E

Yellow: F

Dark blue: G

Brown: H

Dark green:I

White: J

Biological

Zooplankton

Across stations A, C and H, the most abundant zooplankton genus was Copepoda, which dominated >80% of the zooplankton community. The highest abundance was found at Station A with a total of 421.1 cells/m3. Additionally, this was the only station that recorded the genus Mysidacea, however no Appendicularia spp. were observed, which were present at Stations C and H (Fig. 2)

Phytoplankton

A variation in phytoplankton species and diversity was recorded along the estuary.. Overall, 16 different genera were identified of which the genus Lauderia dominated the whole phytoplankton community at stations I, G, J and C-1, whereas Stations C and F were mostly composed of diatoms of the genus Thalassiosira. Additionally, Thalassiosira was found throughout all stations across the Tamar River (Stations C0 – A0). Furthermore, Chaetoceros was present at most stations.  A very low number of cells was recorded at station H (3 total cells/ml) compared to the samples at the other stations which all had >25 total cells/ml. Station B0 contained the highest abundance and diversity of phytoplankton with 11 genera documented and a total of 141 cells/ml (Fig. 6).

Chemical

The start end member phosphate concentration was 0.67

(m mol/L) at the top of the estuary where salinity was 0 psu. At salinity 20 psu there appears to be an addition of phosphate in to the estuarine system. A potential phosphate source was from agricultural input and fishing practices that occur upstream (Paytan and McLaughlin, 2007). Following this addition, the phosphate concentrations quickly decline from 0.67 (mmol/L) to 0.05 (mmol/L) over a small salinity increase.  0.05 (mmol/L) is the value of the end member. Altogether, at high salinities phosphate concentrations decline. The nitrate start end member value was 121 (mmol/L) at 0 psu. All the nitrate concentration values are clustered together along the TDL with 2 outlying anomalies. They concentrations remain low across the salinity change from the riverine water to Plymouth Sound. The highest recorded nitrate value was 43 (mmol/L) at 21 psu. A source of nitrate could additionally be from agricultural influence and surface runoff (Korda et al., 2008).

Physical

Upper Estuary

Station A0 was the furthest station upstream it has the lowest salinity measurements of all the estuary sites. Salinity showed a large increase from 20.0 at the surface to 22.5 at 3.5m. Temperature decreases from 25.6°C at the surface to 20.2°C at depth.

Station B0 had a surface salinity of around 25.6 which increases to 26.6 at 3.0m. Temperature shows a decrease from 26.7°c at the surface to 25.5°C at 3.0m. Station C0 was the furthest downstream site sampled by the Flying Pig. This site overlapped with the area surveyed by the Falcon Spirit. It was the deepest of the upper estuary sites with a maximum depth of 17m however the salinity monitor only had enough cable to measure 11m. It has the highest salinity but shows the same increase with depth from 30.0 at the surface to 31.4 at 11m. Temperature decreases from 19.5°c at the surface to 18.3°C at 11m.

TDS is an estimate of mass of dissolved solids in the water. It mirrors salinity closely in all three profiles. Dissolved oxygen, measured in percentage saturation, increases slightly below the surface before dropping off dramatically at depth. It also exhibits this behaviour at Station B0. At station C0 dissolved oxygen decreases steadily between 0-4m and then fluctuates to the end of the profile.

The profiles all suggest that the upper estuary is partially mixed, which is consistent with the findings of Uncles et al (1985), with a slight salt wedge structure. This forms because the incoming sea water has a higher salinity and is colder and therefore sinks below the warmer, fresher river water. The further upstream the samples are taken, the lower the overall salinity as there is a greater freshwater influence, however upstream has a greater flux over the tidal cycle (Milne, 1938). If we had made it further up the estuary, we would have expected to find the fresh-salt water interface at Weir Head.

Lower Estuary

Station C-1

Variation in the temperature over a depth of approximately 5m shows a change from around 18.25˚C at the surface to approximately 17.65˚C at depth. Salinity, however, saw an increase in salinity as the depth increases. An initial value of around 31.2 increased to 32.7 as depth increased. Here the denser seawater sits below the less dense freshwater.

Station C

The temperature profile here varied greatly over depth. A peak of 18.6 was seen midway through the water column at around 6.5m. Salinity also varies over the profile. The peak value of approximately 32.47 was collected from the bottom depth, a low value of 32.17 was recorded at around 8.2m. This shows any mixing is dominated by salinity.

Station D

The temperature remains within a 0.2˚C range, a low of 17.68˚C at 18.3m and a high of 17.87˚C at a depth of approximately 3m. Salinity varied greatest at depths above 5m. The peak value of 32.9 came at the surface but decreased to a low of 32.64 by 4m. The location of D being at the mouth of the Lynher River will also have an effect on the temperature and salinity values. However, the clear variation across the surface values could suggest a higher rate of mixing as the water moves toward the seaward endmember.

Station E

A steady decrease in temperature with depth except for a peak at around 3m and again at 18m. Salinity follows a reverse profile with increasing salinity with depth. A peak of around 33PSU is seen at approximately 18m. This shows a well-mixed water column with no clear pycnocline. This could be due to the fresher warmer water being injected by the Tamar and Lynher overlying the cooler more saline water.

Station F

Temperature has a decreasing trend with depth with a peak of around 17.6˚C at the surface. Again, salinity shows an increasing trend with depth up to a peak of around 33.9 at 20m. Station F shows a very similar pattern to E due to the close proximity of the two locations and no dramatic change in the inputs to the system.

Stations G

Both temperature and salinity show a small range. The water column is well mixed and homogenous except for a peak near the surface. The outgoing tide could be bringing warm saline water from the salt marshes out into the body of the estuary. This could cause the peak in both temperature and salinity seen at the surface.

Station H - J

A clear pycnocline could be seen between 5-10m. Below which the water column is relatively homogenous except for a bottom layer of cooler, slightly more saline water which becomes more pronounced further out to sea. This is probably due to the depth of the channel (dredged to allow for larger ships to enter the estuary) meaning the tidal mixing forces are insufficient to penetrate the full depth of the water column leaving the bottom layer relatively undisturbed. The seaward endmember has very little freshwater input meaning that less mixing occurs, any mixing that does occur should be dominated by temperature. Mixing becomes dominated by temperature rather than salinity. Two distinct water layers are seen with a significant pycnocline at station J.

Summary

Figure 6: Phytoplankton abundance (cells/ml) and diversity across all estuarine and river stations sampled.

The upstream site A0 and B0 had the highest nutrient and plankton abundance, suggesting that these are linked. Nutrients such as phosphate and nitrate are required for phytoplankton growth which in turn provides food for zooplankton.

At the seaward side of the estuary, plankton abundance is lower. The increased depth at certain stations means that tidal forcing  is weaker and have reduced ability to filly mix the water column, causing nutrient depletion. With lower nutrients, phytoplankton and zooplankton numbers are lower and they cannot grow as efficiently.

For  figure legends please scroll over the figure.

For  figure legends please scroll over the figure.