PLYMOUTH FIELD COURSE 2019

INSHORE

THE TAMAR ESTUARY

The Tamar estuary is part of the Plymouth Sound and Estuaries Special Area of Conservation (cSAC) and Special Protected Area (SPA). Processes in the lower estuary is mainly influenced by naval dockyards and the urban area of Plymouth, whereas the upper estuary is surrounded by a more rural and agricultural landscape, with salt marshes and tidal mudflats supporting a rich diversity of wading birds and other wildlife. The River Tamar is approximately 80 km long with an average flow rate of 30m/s, this fluctuates seasonally between 5m/s and 38m/s depending on rainfall, forming the dominant freshwater supply to Plymouth Sound (W.J. Langston et. al., 2003). Its catchment area covers 1700m2, with predominantly sandstone and shale geology (the ‘Culm Measures’). A long history of mining and smelting for metal ores still strongly influences the chemistry of the estuary due to draining from spoil heaps.

OUR METHODS

Lower Estuary:

On a spring tide on 02/07/2019, the lower estuary was sampled on RV Falcon Spirit from breakwater to Station C-1 (shown in red on Map ##). At each station, ADCP transects were conducted from west to east across the estuary, with an additional transect across the mouth of the River Lynher at Station D. Seawater samples were collected using Niskin bottles on a CTD rosette from just above the bottom, just below the surface and at an area of interest in mid-water. Nutrient samples were passed through 4μm filter paper and kept in labelled bottles for subsequent analysis. For chlorophyll samples, 50mL of seawater was filtered and the filter paper was transferred to 7mL acetone. Phytoplankton samples were also collected from each station and preserved

Dissolved Oxygen:

Dissolved oxygen varies along the course of the estuary as riverine freshwater mixes with seawater. Oxygen is used up in the biological degradation of organic matter by bacteria attached to resuspended particulate material in the turbid waters of the Saltwater-Freshwater Interface (SFI). Oxygen levels are often monitored in order to determine the impacts of sources of organic enrichment in an estuary, high levels of which may cause eutrophication and subsequent oxygen depletion (when removal exceeds supply from the atmosphere and photosynthesis).

In the present study, the highest values of oxygen saturation were measured at the highest and lowest salinities, decreasing to a minimum around intermediate salinities.

Nitrate:

Nitrogen is a key constituent of amino acids and nucleotides, the building blocks of proteins and nucleic acids, thus is essential for life. Dissolved nitrogen gas is the most abundant form of nitrogen in the ocean, some microbes are able to fix this into ammonium however most biologically available (fixed) nitrogen is in the form of nitrate and nitrite. Possible sources of fixed nitrogen in the estuary include atmospheric deposition and terrestrial runoff (especially in agricultural areas) as well as bacterial cycling in surface waters.

Despite an apparent slight addition in the upper estuary, where the data points lie slightly above the Theoretical Dilution Line (TDL), and slight removal in the lower estuary, where the data points lie slightly below the TDL, nitrate showed predominantly conservative behaviour.

Phosphate

Phosphate is another essential nutrient for life, as it is required for key macromolecules such as nucleic acids, amino acids, ATP and phospholipids. Phosphate from mineral deposits is released during the weathering of catchment rocks, entering river water via terrestrial runoff. Possible point sources include wastewater treatment plant, sewage effluent and industrial discharges, these become more common in closer proximity to urban areas such as Plymouth.

According to the mixing diagram, phosphate exhibited non-conservative behaviour in the Tamar Estuary during the study period. The plotted data lies above the TDL, indicating addition processes are increasing the concentration of phosphate in the estuary above that which would be observed if mixing processes alone were influencing the distribution of this nutrient. In terms of data points to highlight, samples collected at salinities of 14.2 and 27.4 (stations A6 and A13) stand out as having an unexpectedly high silicate concentration (1.71 and 0.796μmol/L respectively) given the trend exhibited by the rest of the data.

Silicate

Silicate ions are released following the weathering of silicate rocks and enter river water via terrestrial runoff. Silicon is a major element within riverine systems, making up approximately 10% of the total dissolved load. On a global scale, anthropogenic perturbations such as dam construction are currently causing significant changes to the supply of silicate to rivers. Diatoms require silicon for their siliceous tests, this biological removal may cause silicate to exhibit non-conservative behaviour along the estuary at times of high production.

The concentrations plotted in the mixing diagram lie below the TDL, indicating that silicate is not behaving conservatively during the present study and was removed throughout the Tamar Estuary. The river was shown to be the source of silicate as highest concentrations were measured in the freshwater endmember, falling to only 0.199μmol/L in the seawater endmember.

CHEMISTRY

in Lugols solution.

Upper Estuary:

The upper estuary was sampled on a smaller vessel (Dolphin) by 3 members of our group. Seawater samples were collected and processed in the same way at intervals of 2 salinity units. Zooplankton samples were also collected at Stations A, B and C using a 0.5m diameter net with a 200μm mesh dragged at low speed for 30 seconds. Depth profiles for temperature and salinity were constructed for Stations A, B, and C using measurements recorded from a T-S probe.

Sample Analysis:

The nutrient concentrations of our samples determined using a QuAAtro39 Autoanalyser (SEAL Analytical) at Plymouth Marine Laboratory. Titrations were used to measure dissolved oxygen, chlorophyll a concentrations were determined using a fluorometer and plankton was counted under a light microscope.

Chlorophyll:

Measurements of chlorophyll concentration are used to monitor the distribution of algal populations. High levels of chlorophyll indicate a high level of photosynthesis in that region of the estuary.

In the present study, chlorophyll concentrations remained low until a salinity of around 30 where a sharp peak was observed towards a maximum concentration in the seawater endmember.  

Phytoplankton:

The relative abundance of different phytoplankton groups (Ciliates, Diatoms and Dinoflagellates) is indicative of the relative physical and chemical conditions in that part of the estuary, as each group thrives under a different set of parameters.

Data for phytoplankton relative abundance was collected for the lower estuary where chlorophyll concentrations were highest. Highest abundance of dinoflagellates and ciliates were recorded near breakwater at station I, whereas highest diatom abundances were recorded slightly further up the estuary at station C. Summaries of the peaks in abundance of these three groups are identified in the table below along their associated salinity.

Zooplankton:

An idea of how the biology changes along the length of the estuary can be obtained by analysing the relative abundances of the different taxonomic groups present in zooplankton samples. Click HERE for more information about the zooplankton groups identified in this study.

Quantity and relative composition of the zooplankton was found to vary across stations A, B and C. Greatest diversity of taxonomic groups was observed at station C, whereas highest zooplankton counts were observed at station B. This mostly consisted of Cirripedia larvae, which were by far the most abundant taxonomic group recorded at this station, whilst Copepoda was the most common group at stations A and C. This group was found to increase in abundance along the length of the estuary from stations A to C.

BIOLOGY

TEMPERATURE AND SALINITY PROFILES

Station A:

In the upper estuary, the T-S profile shows an exponential drop in temperature  and exponential increase in salinity with depth. This difference in temperature between surface and bottom waters is indicative of a fairly stratified and stable water column structure.

Station B

Salinity and temperature profiles at station B show a similar respective exponential increase and decrease from surface to bottom. In comparison to station A, a smaller difference in temperature was observed between the surface and the bottom waters. This indicates that the water column had become less strongly stratified with distance down the estuary.

Station C

Temperature and salinity showed a similar trend with depth at station C, however an even smaller difference in values between surface and bottom waters suggests that the water column here was less stable due to higher levels of mixing lower down the estuary.

Station I

At station I, higher temperatures and lower salinities in surface waters were again found to overlie a cooler, more saline bottom water mass. The smallest difference in values between surface and bottom waters was recorded here, following the observed trend whereby water column stability was found to gradually decrease with distance down the estuary.

PHYSICS

The photo collection from the Tamar Estuary and Inshore research is available in the slide show to the right.

Please visit the youtube link below for the full video compilation, including the photo collection and additional videos taken during the research.

Inshore YouTube Video

The video includes more details on the pictures as well as video clips of research procedures.

Table 1 : Most notable points of data from the Percentage Oxygen Saturation measured along the Tamar River and Estuary salinity gradient.

Figure 1 (right) : Percentage Oxygen Saturation measured along the Tamar Estuary salinity gradient.

Figure 6 (right) : Chlorophyll concentration measured along the Tamar Estuary salinity gradient.

Table 5 : Chlorophyll concentration at the lowest measured salinity and the seawater endmember along the Tamar river and estuary.

Table 2 : Freshwater and Saltwater endmember for the nitrate concentration in the Tamar river and estuary

Figure 2 (right) : Mixing diagram with the Theoretical Dilution Line of the Total Nitrogen sampled in the Tamar river and estuary

Table 3 : Freshwater and Saltwater endmember for the phosphate concentration in the Tamar river and estuary

Figure 3 (right) : Mixing diagram with the Theoretical Dilution Line of the Total Phosphate sampled in the Tamar river and estuary

Table 4 : Freshwater and Saltwater endmember for the silicate concentration in the Tamar river and estuary

Figure 4 (right) : Mixing diagram with the Theoretical Dilution Line of the Total Silicate sampled in the Tamar river and estuary

Table 6: Data points of maxima abundance of Diatoms, Dinoflagellates and Ciliates recorded at lower estuary stations.

Table 6 : Abundance of Diatoms, Dinoflagellates and Ciliates recorded at lower estuary stations

Table 7: Zooplankton abundance in three upper estuary sites, A, B and C. Temperature and Salinity for reference. Full zooplankton counts for the 3 sites.

Figure 7: Relative abundances of zooplankton taxonomic groups recorded at stations A, B and C

Figure 8 (right): Temperature, salinity diagram for Station A with depth

Table 8: Surface and bottom data for depth, temperature and salinity at Station A

Table 9: Surface and bottom data for depth, temperature and salinity at Station B

Figure 9 (right): Temperature, salinity diagram for Station B with depth

Table 10: Surface and bottom data for depth, temperature and salinity at Station C

Figure 10 (right): Temperature, salinity diagram for Station C with depth

Table 11: Surface and bottom data for depth, temperature and salinity at Station I

Figure 11 (right): Temperature, salinity diagram for Station I with depth

All data used to create the results on this page can be found on the University of Southampton FTP Server.