Falmouth Group 13 - 2017

Temperature

The profiles of temperature show in general that the temperature at the surface exceeds that of the water at depth. The vertical temperature difference between surface and bottom values at each station is largest at the mouth of estuary at station 45 with a range of approximately 1°C. This reduces to only 0.2°C at station 48 located in the middle of the estuary. The decrease in surface and depth temperature difference with distance upstream is likely a result of rapid isopycnal mixing in shallow water columns.


When comparing the stations, a graded increase in temperature with distance up the estuary becomes apparent. This temperature change between station 45 and 51 has a magnitude of roughly 3°C. Another striking comparison between stations is the apparent weakening of water column stratification in the middle of the estuary. This can be seen at stations 47 and 48, which are located in the middle of the estuary, with both having highly homogenous temperature profiles with very little vertical variation in temperature. At station 45, which is situated at the mouth of the estuary, there is the highest stratification in water column containing a large summer thermocline. Towards the head of the estuary the water column temperature stratification returns at stations 49, 50 and 51. This is most likely due to the location of these stations at the confluence of the river Fal and Truro with the estuary. These rivers contribute warmer, low density river water to the surface of the estuary causing the stratification seen at these stations.


Salinity

The salinity decreases with both distance from the mouth of the estuary and vertically at all stations. At stations further up the estuary there is significantly higher stratification in salinity at each station, whilst the profiles towards the mouth are more homogenous. For example, at station 51 located up the estuary, the variability of salinity with depth was 1.2 PSU. Whilst at station 45, at the mouth of the estuary, vertical salinity varied by only 0.2 PSU. This stratification in salinity may be a result of the flooding tidal state working in opposition to the river flow, creating the large halocline between the flooding high salinity water and low salinity surface river water flowing down estuary.


Fluorescence

Fluorescence is used as a proxy for chlorophyll-a and therefore phytoplankton abundance and distribution. As the location of the stations progresses up the estuary there is an evident increase in fluorescence and therefore chlorophyll. This is a result of an increase in nutrient discharge from the river runoff due to weathering of the catchment causing an increase in abundance of phytoplankton. The highest fluorescence is located at station 51 at the confluent of the river Fal and Truro, however this rise in chlorophyll is not uniform but contained most notably to the surface roughly above 5 meters. As seen from the corresponding salinity profile for this location, this spike at the surface is the location of the outflowing river runoff. The anomalous fluorescence is therefore likely to be a direct result of nutrient input or freshwater phytoplankton being discharged into the estuary.


Estuary Physics

CTD

ADCP

All ADCP profiles show general flow velocities of between 0 and 0.6 m/s flow. The flow velocities generally decrease with distance up the estuary with maximum flow rates of around 0.6 m/s located a transects 1 and 2 located closer to the mouth of the estuary. Towards the head of the estuary the velocity generally decrease which is likely due to dissipation of the flooding tidal flow and the expected shelter with distance up the meandering estuary. The transects also show considerable vertical variability in speed suggesting some turbulence in the water columns. This may be a direct result of the weather conditions as during the measurements the wind speed was high which will have caused vertical mixing and turbulence in the water column. The most well structure flow pattern is found towards the open ocean at transect 1. The constant flow in the center of the transect of approximately 0.4m/s can be attributed to the ebbing tidal state during measurement. All flow directions agree with expected tidal flow for Falmouth.  


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