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Lab Protocol

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GEOPHYSICS

TAMAR SAMPLING

OFFSHORE

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Home Geophysics Tamar Sampling Offshore

INTRODUCTION

On the 5th of July, part of the group was deployed on the Falcon Spirit in the lower part of the Tamar estuary; where experiments into temperature, salinity, chlorophyll, and various nutrients were conducted. These were carried out at 7 different sites at different parts of the lower estuary, and so together with the RIB data can allow inferences about overall processes in the Tamar estuary.

AIMS

Armed with a Rosette CTD and Niskin bottles, the aim of the Falcon Spirit study was: to gather data regarding variations in the vertical profiles of chemical properties such as temperature and salinity; to study the nutrient concentrations of the river and their spatial variation along the estuary; and to gather information about plankton species, specifically abundances and diversities of the lower Tamar.

METHODOLOGY

Due to a battery failure of the CTD and firing failures, only 7 horizontal transects were conducted sampled, via horizontal transects, across the river mouth (Lynher), St John’s Lake SSSI and the Sound. As a consequence, dissolved oxygen samples and phytoplankton samples were unable to be obtained, other than at station 1 (C-1). At each station; the T/S probe on the CTD was functioning and used to sample vertical profiles of temperature, salinity and depth throughout the lower regions of the estuary. Surface chlorophyll samples were taken via the vessel’s underway pumping system and processed using the same method as on the RIB’s (see RIB methodology).

TEMPERATURE & SALINITY PROFILES:

CTD T/S profiles overall illustrate that the Tamar was well mixed in terms of temperature and salinity on the 05/07/2019. The temperature of the surface waters slightly decreases as one heads down the river; starting at 18.3°c and finishing at 17.0°c. This is due to greater mixing of colder sea water towards the mouth of the estuary. The vertical profile of temperature also changes between the stations as you head seawards, with high variability in the shape of these vertical profiles although in general greater stratification in the upper stations. For example; the first station shows a shallow thermocline at approximately 2m, whereas station 6 has a thermocline at 10m and station 7 shows a near linear decrease in temperature with depth.With regards to salinity, this increases as one travels downstream as the water column mixes with saline ocean water. However, the upper estuary stations have a more consistent water column in terms of salinity at around 32.8 PSU; only slightly increasing with depth. Whereas the last station features a halocline around 4m and also a greater range from 34.5-35.0 PSU.

ADCP

At 15 different sites (7 stations, 7 transects and the harbour area), an ADCP was run which gives a clear indication of any stratification and the flow velocity of the estuaries water column. These show a well-mixed column towards the harbour (Fig.30) and generally more variation as you head up the estuary. Some stations such as stations 28 (Fig.33), show very clear stratification between a fast flowing upper layer and slower bottom; which in this case is likely caused by the nearby confluence of the rivers Tamar and Tavy increasing turbulence and resulting in increased velocity. Other stations show inputs to the surveyed area, shown by a band of increased flow in one vertical section of the track; clearly seen in the right of station 29 (Fig.35). Whilst some feature possible fronts of cold water moving much slower, such as transect 5 (Fig.40); these could be due to a close proximately to shore and possible slowed-flow/backflow.