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GEOPHYSICS

TAMAR SAMPLING

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Home Geophysics Tamar Sampling Offshore

INTRODUCTION

The Tamar-Tavy estuary incorporates the tidal estuaries of the River Tamar, Tavy and Lynher which collectively drain an extensive part of Devon and Cornwall (Prichard, 1967 and JNCC, 2010). Prichard (1952) defined estuaries as ‘a semi-enclosed coastal body of water with a free connection to the open sea and within which seawater is measurably diluted by freshwater.’ (Callaway and Specht, 1982). Being a meso-tidal, partially mixed estuary a distinct thermocline would be expected between less dense surface waters and denser deeper waters in which tidal energy is likely to be sufficient to create shear along the halocline. The River Tamar and its tributaries provide the dominant freshwater input to the Plymouth Sound and estuaries system with an average annual flow of 30m3s-1. 

 

AIM

To sample from Calstock down to Breakwater in order to investigate the chemical, biological and physical mechanisms occurring within the estuary and the degree of mixing from freshwater and seawater interactions. 

Tamar Estuary Sampling

RESULTS

In an attempt to achieve this, the estuary was sampled using three vessels, Porky Pig (sampling upper Tamar), Flying Pig (sampling mid Tamar) and Falcon Spirit (sampling lower Tamar) and the data obtained was used in conjunction to achieve results representative of the estuarine system as a whole.

 TEMPERATURE/SALINITY

The temperature and salinity graph displays how both temperature and salinity vary with respect to distance down the estuary, beginning at the sample site furthest up the estuary (50°30.182'N, W004°10.826') and sampling downstream. It can be seen that from sites 1-12 salinity increased linearly from 0.42 to 22.5 PSU, followed by a sharp increase and then a gradual shallowing of the line. There’s a subsequent sharp decrease of 7.22 which is attributed to switch in sampling from group 1 to group 2 and it can be seen from the recorded coordinates of sample sites that samples from the two groups overlapped slightly (by 0°3.146 latitude and 0°1.288 longitude). From this site salinity increased, albeit at an inconsistent rate, from 23.18 to 34.6 PSU. 

The variation in temperature between sites is much less prevalent, thus, demonstrating that the estuary is well mixed. Evidence for this is also portrayed via the temperature and salinity profiles collected via the CTD on Falcon Spirit. Slight variations can be seen between sites 1-16, however once again a large difference (increase of 3.2°C) is seen at the point which sampling swapped from group 1 to 2. However, it could also be due to inter-group differences in both methods and the probes. A sharp decrease is also seen when sampling swapped from group 2 to the Falcon spirit, which again could be due to slight inaccuracies in the equipment and differences in method.

NUTRIENTS

Silicate (Fig. 6&7)

The estuarine mixing diagram for dissolved silicate concentrations suggests non-conservative behaviour as a result of removal with the majority of individual sample points lying below the TDL. This correlates with high chlorophyll concentrations at corresponding stations with a high number of diatoms. This would be expected for July (05/07/2019), due to the dynamics of temperate phytoplankton blooms suggesting removal of dissolved silicate by diatoms - utilised for their opal tests (Western Channel Observatory, 2011).

NO3+NO2 (Fig. 8)

The relationship between salinity and total nitrogen concentrations for the Tamar evince conservative behaviour of total nitrogen concentrations in the Tamar Estuary due to the close association of the individual data points to the theoretical dilution lines. There are some discrepancies in which three data points stray noticeably from the TDL [(0.42, 2.50), (23.18, 13.28), (27.5, 56.46)]. Further analysis is required to decipher the nature behind these points and whether they are potential anomalies. Consequently, variation in the distribution of total nitrogen concentrations on the 05/07/2019 is likely to predominantly be due to the mixing and the physical dilution of river water by sea water (Western Channel Observatory, 2011).

NO3 (Fig. 9)

Similarly nitrate concentrations demonstrate conservative behaviour despite one noticeable deviation from the TDL (32.08, 45.433). This is likely to be due to the fact that the majority of nitrogen in the estuary is in the form of NO3 as opposed to NO2. There is slight evidence of removal however, further analysis/ research is needed to understand the potential reasons behind this.

Nitrite (NO2) and Phosphate (PO4) (Fig . 10 & 11)

IIn contrast nitrite concentrations portray an evident positive deviation from the TDL providing evidence of non-conservative behaviour by addition However, this should be investigated further to decipher the exact cause.

Due to technical difficulties with the spectrophotometer in the lab, phosphate samples were not taken in the field, consequently, the estuarine mixing diagram is constructed on the results of the auto-analyser. Thus, its representativeness can be queried. Phosphate also demonstrates non-conservative addition, like Nitrate, which could potentially be a consequence of additional point sources, perhaps due to sewage discharge along the river in the densely populated region, especially from station B to C (Langston et al., 2003).

PHYTOPLANKTON ABUNDANCE

The majority of species found in samples were diatoms, this is to be expected due to their observed seasonal succession in these waters (Widdicombe et al., 2010). It can be seen that different species of phytoplankton dominate at different points along the Tamar estuary, for example at a salinity of 26.9 PSU (Fig. 16) 53% of the present speacies were Cilliate spp. Whereas at 0.42 PSU (Fig. 20) both Alexandrium spp. and C. asteromphalus had 38% dominance.

For the phytoplankton samples, the phytoplankton with the highest overall abundance along the Tamar Estuary appear to be Skeletonema, Pluerosigma spp., Mesoporos perforates and Chaetoceros spp. High abundances of Pleurosigma spp would be expected in the upper reaches of the Tamar, due to the fact it is a benthic species that is often re-suspended via estuarine mixing.  The highest numbers of Skeletonema occurs toward the top of the estuary, with a total of 44 organisms being counted in the samples below a salinity of 26.9 PSU (Fig, 16). The Chaetoceros spp.  is more abundant in the middle course of the estuary, where between the salinities of 22.5 (Fig. 18)and 26.9 PSU (Fig. 16), the total count was 25.

ZOOPLANKTON

For each sample taken, the highest number of Zooplankton counted was Copepoda, although this varied from 378.59m-3 at site B0 down to just 37 m-3 at G. This relates to the large difference in overall Zooplankton numbers seen; the overall total of organisms seen at sites A1 (Fig.22) , B0 (Fig.23) and C0 (Fig. 24) are 463.43 m-3, 561m-3 and 519.03m-3 respectively, while the respective values at sites G (Fig. 25) and I  (Fig. 26) are 52.6m-3 and 256.5m-3. Past the constant higher Copepoda numbers, the numbers of other zooplankton seen show little trend, other than the Decapoda larvae numbers counted were second highest at sites B0 (Fig.23) (56m-3), C0 (Fig.24) (103.81m-3) and I  (Fig.26) (33.3m-3). The Zooplankton numbers are a lot higher further up the estuary that towards the mouth, which can be seen especially at site I.

The above graph displays the inter-site variation in abundance of different zooplankton species. It is evident that copepod species heavily dominate at all sites. Their dominance is likely due to their torpedo-shaped body, sensory armed antennules and the ‘gearing’ of the muscle motor allow them to be very efficient in detecting and evading predators. It could also be due to their capability to remotely detect prey and either capture them in a current or as they swim within the copepods perceptive range. Another potential reason for their dominance is their efficiency in finding a mate, especially when populations are sparse (Kiorboe, 2010).

CHLOROPHYLL

Figure 27 demonstrates a noticeable peak in chlorophyll concentration to 23μg/L at approximately 27.1 PSU. This is potentially due to the fact that this is where the River Tamar and Tavy meet resulting in increased mixing and nutrient re-suspension resulting in an abundance of phytoplankton represented by this increase in chlorophyll (Mommaerts, 1969). From observing estuarine mixing diagrams, there is, perhaps, an associated increase in phosphate concentrations to 0.68μM/L at a salinity of 27.1 PSU. This is potentially a result of point sources located further downstream, consequence of the dense population and resultant sewage discharges into the Tamar (Langston et al., 2003). Dissolved silicate concentrations evince removal at a salinity of 27.1 PSU which is therefore, likely to be due to uptake by diatoms for their opal tests (Mare, 1940).