Home Offshore Time Series Geophysical Estuary

University of Southampton Falmouth Field Course 2013    26th June - 6th July 2013    Group 9

4th July 2013 – Estuary Sampling

Falmouth Tides (UTC): HW 02:09 4.20m

LW 09:31 1.50m

HW 15:39 4.30m

LW 22.03 1.5m

Physical Discussion

Temperature

Observed water temperatures (Figure EP.1) of the upper water column (1.5 m) at Stations 1, 4 and 5 (15.7°C, 15.4°C and 15.2°C respectively) are higher than temperature values observed at Stations 6, 7 and 8 (14.6°C, 14.4°C and 13.6°C respectively). This is due to shallower water depths at Stations 1, 4 and 5 (closest to the head of the estuary), where solar radiation input would be absorbed and heat content contained throughout the water column would promote the development of weak daily thermoclines (Figure EP.1). Lower upper water temperatures at Stations 6, 7 and 8 would be due to tidal and wind mixing of cold deeper waters with upper water layers, thus reducing surface temperatures. Daily thermoclines at Stations 6, 7 and 8 are not observed, which would also be due to the influence of wind and tidal mixing, with highest current velocity of 0.51 m/s. Variability in current velocities (Table 1) would be due to estuary morphology, where estuary width ranges throughout the length of Fal estuary.

Salinity

Salinity vertical profiles at all stations at the time of sampling (Figure EP. 2) reveal that the sampled section of Fal estuary exhibits salt water edge type estuary near the head of the estuary (Station 1), where the influence of fresh water input from river Fal and low tide create lateral variations in salinty, opposing to vertical variations in salinity at Stations 6, 7 and 8 (Figure EP. 2), where the water column becomes well mixed as the influence of tide becomes more pronounced. As the position of the salt wedge depends on the river discharge and the presence and influence of tide (Brown, 1989), it is expected that the position and or occurrence of the observed halocline at Station 1 to show variability. The middle section of the sampled estuarine transect shows partially mixed type (Station 5), while well mixed type is shown at Station 8 (Figure EP. 2).

Fluorescence

Fluorescence values at all stations (Figure EP. 4) reflect chlorophyll a (Chl-a) concentrations (µg/ L), which in turn indicate phytoplankton species presence and distribution within the water column (Viljanen, et al, 1999). Fluorescence readings from estuarine data (0.22 µg/L to 0.42 µg/L) are much higher than fluorescence readings from offshore data (0.05 µg/L to 0.25 µg/L). This is to be expected as estuaries are nutrient rich environments due to river discharges (mainly dissolved silicon (H4SiO4)) and anthropogenic sources (effluent discharges, agricultural run-off).

Transmission

Highest transmission readings are  observed at Station 8 (4.22 v), where water column depth is larger (Table EP.1), thus tidal influence, i.e. amount of resuspended material within the water column is much less than at Station 1 (average reading of 3.82 v) and Station 4 (average reading of 4.3 v), where the effect of tidal stirring increase with decrease of the water depth. It is also possible that lower transmission readings at Stations 1 and 4 are due to river debris. Lateral variation in transmission readings at Station 1 (from 3.72 v to 3.92 v) between 1 m and 4 m depths corresponds to the strong lateral halocline (Figure EP. 2) due to salt wedge development at that location.

Current velocity changes along sampled area

Eddies observed in current velocity contour plot at Station 1 (Figure EP. 6) would be due to shear stress occurring at the interface between the fresh and salt water layers, where the generated turbulence produces instabilities and eddying (Brown, 1989).

References

Brown, J., 1989, Waves, Tides and Shallow Water Processes, BPC Wheatons Ltd., Exeter, UK

Viljanen, M. Holopainen, A-L. Bilvenuoinen, R. 1999, Fluorometer measurements and transmission of light in different parts of Lake Ladoga, Boreal Environment Research, Vol 4, pp239-244

Estuary