For stations N, O, P and Q, a simple T/S probe was used to determine temperature and salinity. The reading was taken from a YSI reader (Monitor YSI 556 handheld multiparameter instrument).

For stations 33, 34, 35 and 35, a CTD was deployed using an A frame winch off the rear of the vessel. The instrument transmitted the data for depth (m), Temperature (°C), Salinity, Transmission and Fluorescence directly to the software. ADCP transects were recorded across the estuary for each of the four stations. For each transect the distance from shore was recorded at the start and end of each transect along with start and end positions and times. The depth, speed and direction of flow were recorded on to software for transfer to WinRiver.  A Secchi disc was also used as a method to measure light attenuation of the upper part of the water column. The disc was deployed over the starboard side of the vessel at station 36 and lowered through the water column until it was no longer visible from directly above. A tape measure was used to measure the length of the rope when the disc was recovered to deck.  This length was dependent on the turbidity of the water column. The depth of the euphotic zone is calculated as being 3X the Secchi depth (Wright & Colling, 1995). The relationship between Zs and Kd is between 1.4 and 1.7 and an average of 1.55 was used.

CTD rosette with Niskin bottles attached

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Richardson Number

The figure shows how the estuary experiences a large amount of mixing with depth. This is seen by many points which have a Ri below 0.25. Station 33 to 35 have shown a halocline between 8 to 11m depth this is shown by laminar flow with a Ri above 1. Ri numbers between 0.25 and 1 show an area in transition indicating a partially mixed depth. The estuary as a whole shows a well-mixed system with depth. Falmouth experiences a large tidal range which can cause mixing at depth, which is further increased by wind blowing across the surface water causing mechanical mixing. Mixing is further enhanced by instabilities in the water column such as a change in salinity and temperature which has an effect on density.

Figure 19. Ri numbers for stations 33 to 36 on board Rv Bill Conway

Temperature Salinity Depth Profiles along the Estuary

Figure 17. Salinity against depth collected from Rv Bill Conway using a CTD

Figure 18. Temperature against depth collected from Rv Bill Conway using a CTD

Station 33 was collected in the upper estuary. Figures 15 and 16 show how station 33 has highest temperatures and lowest salinities compared to station 36 which has the inverse results. This is because station 33 has larger amounts of fresh water which is less saline and warmer. Salinities gradually increase with stations towards the mouth of the estuary. Station 33 and 34 show little mixing compared to station 35 and 36. This is because these stations have a large increase in salinity with depth compared to station 35 and 36 where salinity remains constant with depth. This is further backed up by temperature which shows similar results. Mixing in the estuary is caused by tides and wind. The tide was rising as samples were being taken which would show an amplified effect on salinity as more salt water would be entering the estuarine system.