Group 2 Plymouth Field Course

THE FALCON SPIRIT

On the Falcon Spirit 10 stations were sampled in the River Tamar estuary, from the edges of the breakwater to Cargreen. At each station a rosette was deployed which held a CTD to get data for a depth profile and to determine which points in the water column were of interest to take water samples at with Niskin bottles. The water samples were taken and smaller samples were taken and prepared for later analysis of dissolved oxygen, chlorophyll and phytoplankton, as well as nutrients. Zooplankton nets were also deployed at a couple of stations, as well as a Secchi disk

Richardson Numbers

THEORETICAL DISSOLUTION LINES

The AutoAnalyser was used to find concentrations for Si, PO4, NO2 and NO3. Additionally, concentrations were manually calculated for Si and PO4 by hand.

When the AutoAnalyser values were plotted against the manual ones for PO4, it was found that despite value differences, the relationship was linear. Therefore the conclusions drawn from the TDLs would be unaffected by the method of measurement. We used AutoAnaylser values for all nutrients for consistency and ease of comparison.

The Theoretical Dilution Line for nitrite shows heavy addition along the river, before rapid removal at about 35 PSU when the river water meets the oceanic water. THis is an example of non-conservative behaviour

The TDL for nitrate shows slight removal along the river

The phosphate concentration is fairly consistent along the river (between about 10 and 30 PSU, where the concentration remains in the region of 0.3-0.5 micromols/L). There is then rapid removal upon reaching the oceanic water where concentration drops. This implies removal in the upper estuary and addition in the lower.  The phosphate distribution in the Tamar has been said to be independent of salinity, due to inputs in the Lynher and Tavy tributary rivers, possibly from the city of Plymouth (Morris, Bale & Howland, 1981).

Silicon shows clear removal in the mid-lower estuary.

Fig 1 - Richardson number with depth for each station from the Falcon Spirit, showing where the water column is turbulent and laminar.

AutoAnalyser vs Manual Nutrient Analysis

Nitrite Mixing Diagram

Nitrate Mixing Diagram

Phosphate Mixing Diagram

Silicate Mixing Diagram

Fig 2 - Correlation between the manual nutrient analysis and the AutoAnalyser nutrient analysis.

Fig 3 - Estuarine mixing diagram for nitrite through the Tamar Estuary with the Theoretical Dissolution Line (TDL) shown

Fig 4 - Estuarine mixing diagram for nitrate through the Tamar Estuary with the Theoretical Dissolution Line (TDL) shown

Fig 5 - Estuarine mixing diagram for phosphate through the Tamar Estuary with the Theoretical Dissolution Line (TDL) shown

Fig 6 - Estuarine mixing diagram for silicate through the Tamar Estuary with the Theoretical Dissolution Line (TDL) shown

The views and opinions expressed are those of the individual and not representative of the University of Southampton or the National Oceanography Centre.

Fig 2 - ADCP Backscatter and velocity transects for Falcon Spirit stations B, D and I.

Station I38 shows high back scatter at the surface down to 8.5m. This could potentially be a phytoplankton bloom. Since this station is positioned near the break water, it is expected the water column to be stratified. The phytoplankton samples taken showed that there were high cell densities at this location, confirming the back scatter to be caused by the presence of phytoplankton high up in the water column.

Station D32 showed low water velocity, at 12.75m. This highlights that perhaps there is little movement of water at this depth to re-suspend material due to tidal activity. The measurement was taken around high tide thus little tidal movement occur at this time. It also suggests that there were little phytoplankton in the water column, which was supported by the phytoplankton sample taken at this station, with a count as low as 24 cells per ml.

Station B33 back scatter data indicated areas in the surface with high suspended material, between 1.5-2m. The phytoplankton samples had a higher count of 58 cells per ml, confirming that it is most likely phytoplankton. It would be expected that this station would be turbulent as it was positioned further up the estuary, with an average velocity of 0.250 m/s, which was faster than stations positioned out near the break water. However, this velocity was homogenous in the water column, due to it being very shallow and measurements taken at high tide.