Metadata Physical Chemical Biological

On 03/07/14 a survey of the Fal estuary was conducted to investigate physical, chemical and biological properties of the water. The survey began at Station 1, at the mouth of the estuary, and proceeded with 7 more stations at points progressively further up the estuary.

At each station, a CTD profile was taken using a rosette sampler fitted with a multiprobe and Niskin bottles to simultaneously collect bottle samples. The CTD profiles show the temperature, salinity and fluorescence of the water while plankton amounts, nutrient levels, chlorophyll and dissolved oxygen were analysed from bottle samples. Further, at each station, the current across a transect perpendicular to the flow of the estuary was also investigated using an ADCP .

The water reached high tide at 0835 UTC, near the beginning of the survey and ebbed as the survey moved upstream. This provided an exaggerated change in water properties with location than if the stations had all been surveyed simultaneously.

Metadata

Date:

Weather:

High Tide:

Low Tide:

03/07/14

Dry, calm weather.

0935 UTC

1615 UTC

Chemical

Nutrients

The vertical profile of Station 4 took place at Feock the bottle-neck from Carrick Road to the upper estuary. The salinity increases discreetly with depth suggesting a well-mixed water column. Silicon, nitrate and chlorophyll concentration peak together at the 5m water sample, just before the small halocline. It is important to note this station was sampled just after high water and so it may be experiencing an injection of nutrients from further up the estuary. Chlorophyll, using the proxy of fluorometry,  remains high in the upper water column, and drops by 3.5-4 µg/L at 17.5m, however due to the small number of water samples for this station the whole picture is not shown. Nonetheless, lack of light penetration and high turbidity within the estuary may explain this.

Figure 3.1

Silicon (µmol/L), phosphate (µmol/L), nitrate (µmol/L), and fluorometry (V)  concentrations at Station 4 in the Fal estuary.

Dissolved Oxygen

At Station 6, a comprehensive depth profile was taken, allowing study of oxygen saturation changes throughout the water column. Oxygen saturation is high initially at the surface, with 117% saturation at 1m depth. The general trend shows a decrease in concentration with depth, to 102% saturation at 13m depth. The saturation peaks at 4m depth at 120% saturation, but it is expected it to be highest at the surface2 due to the immediate contact with atmosphere allowing O2 to dissolve into the surface waters. The by-product of photosynthesis is oxygen and therefore where there is high phytoplankton abundance, the oxygen concentration is expected to increase. Chlorophyll a concentration can be used as a proxy of phytoplankton abundance, however species’ chlorophyll a content per cell will vary it cannot be used conclusively as a phytoplankton indicator. Chlorophyll for this example however does show a decrease in concentration with increased depth which is as would be expected.

Figure 3.5

Dissolved oxygen (%Sat) and chlorophyll (mg/L) concentrations at Station 6 in the Fal estuary.

Biological

Phytoplankton

There was a clearly dominant group – the diatom taxa Chaetoceros – that was present at all Stations. Their abundance is lowest (630 cells/mL) at Station 1, and highest (33020 cells/mL) in the mid estuary at Station 5, where, further, the largest proportion of other species were present, including the dinoflagellate taxa Alexandrium and Dinophysis. This is due to the presence of essential macronutrients from the riverine source, particularly dissolved silicon, which limits diatom and dinoflagellate growth and is required for the construction of a silica test, a ‘frustule’ 3. At the seaward Station 1, nutrients are well mixed throughout the water column and, hence, less available for phytoplankton species to exploit in the surface eutrophic zone.4

Total zooplankton numbers were found to be at their highest at Station 1 (520.4/m3 ), which was located at the mouth of the estuary. Echinoderm larva dominated in species abundance at this station (235/m3 ), followed by copepods (137/m3  ). Echinoderm larva dominated, possibly due to the exploitation of a biological niche amongst the other species.

Species diversity was also at its highest at Station 1, with 9 total species compared to 4 (Station 2) and 5 (Station 3). A larger species diversity at Station 1 could be explained by the horizontal migration of sea originated zooplankton upstream to more phytoplankton rich waters.

Total zooplankton numbers then substantially decrease at Stations 2 (64.7/m3 ) and 3 (51.60/m3 ) which are further up the estuary. This could be evidence of environmental conditions affecting species distribution at this station e.g. decreased salinity, increased temperature and decreased 02. The expected result was the increase in zooplankton correlating with the increase in phytoplankton5.

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Figure 4.2

Zooplankton (cells/m3) for Stations 1, 4 and 8 in the Fal estuary.

Zooplankton

Figure 4.1

Phytoplankton (cells/mL) concentrations at successive stations in the Fal estuary.

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ADCP

Successive transects along the estuary of River Fal illustrate changes in flow speed that reflect both tidal movements and the local geography of the estuary channel. For the early Transects 1 and 2 the respective slack tide and beginning of the ebb tide reflects the small average flow speeds of 0.046 ms-1 and 0.053 ms-1. Increasing average seaward flow speeds of 0.089ms-1 and 0.137ms-1 respectively are found at Transect 3, due to stronger tidal ebbing, and Transect 4, in addition likely due to the narrower channel width. Further, the transects taken subsequent to high tide, Transects 5 and 7, contains the highest flow speeds of at 0.199ms-1 and 0.214ms-1 respectively, likely due to position within the tidal cycle and the narrow regions of the river channel. Though Transect 6 does not fit this pattern and instead shows a slower flow speed of 0.193ms-1, this likely reflects the high width resulting from the confluence of the Rivers Fal and Truro.

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Physical

CTD

The surface temperature increases from 17.4°C to 19.4°C between the first and last stations due to the times at which each station’s data was recorded; each was measured at a later point in the morning and thus a warmer time of day. Temperature decreases fairly uniformly with depth, except for at Stations 6 and 8 where a relative thermocline is apparent at mid depths. This is likely due diurnal solar warming as the successive stations were taken progressively throughout the day, in addition to tidal influence affecting the stratification.

As expected, the overall salinity between Stations 1 and 8 decreases from over 35 to 34.0 at depth and 34.8 to 3.3 at the surface. This is due to increasing proportions of freshwater further up the estuary which is enhanced by the direction of travel between stations against the ebbing tide. Salinity changes with depth at most early stations were only a tenth of a unit, and therefore assumed insignificant, though still showing a general increase. At Stations 5, 6 and 8 salinity changed to a greater extent with depth (up to 1 unit) and showed stratification of the water with areas of rapid change, for example, by 0.8 in 3m at Station 6.

Chlorophyll generally increases further up the estuary as more nutrients from river water are available alongside that of increased surface runoff, with the possible additional influence of direct chlorophyll riverine input in detrital matter. The chlorophyll maximum also rises from below 5m at early stations to approximately 3m at Stations 6-8, again due to increased nutrient concentration in surface riverine input.

Station 1

Temperature decreases from surface to the base of the profile, with a reduction in temperature of 0.2°C, changing uniformly with depth. Salinity increases from 33.3 to 34.7, with a clear shift at the surface. Chlorophyll initially increases with depth before becoming constant.

Station 2

Temperature decreases uniformly with depth from 17.7°C at the surface to 17.4°C at the base of the profile. Salinity shows some increase with depth. Chlorophyll appears to peak around between 5-10m before declining with depth.

Figure 2.1

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 1.

Station 3

Temperature decreases from surface to the base of the profile, with a reduction in temperature of 0.5°C. Although appearing to fluctuate dramatically, the overall change in salinity is negligible here. Chlorophyll peaks around 5m before declining and becoming constant with depth.

Station 4

Temperature decreases by 0.9°C from surface to the base of profile, becoming approximately constant with depth at 13m. Salinity increases by only 0.3 and shows relatively large fluctuations. Chlorophyll peaks just above 5m before declining with depth.

Station 5

Temperature decreases uniformly with depth by 1.1°C. Meanwhile, salinity increases by 0.4, at a rate fastest between 6-13m. Chlorophyll peaks around 5m before declining with depth.

Station 6

Temperature decreases by about a degree from top to bottom, most rapidly between 4-7m where it changes from 18.8 to 18.0°C. Salinity shows an opposite pattern, increasing most quickly between 4-7m from 34.1 to 34.9 before showing a slight decrease below 8m. Chlorophyll peaks between 2-4m before declining and becoming constant with depth.

Station 7

Temperature decreases relatively consistently with depth, from 18.9°C at the surface to 18.0°C at depth. Salinity also shows a lack of stratification, declining uniformly with depth from 34.1 to 34.6. Chlorophyll peaks between 2-4m before declining with depth.

Station 8

The temperature decreases from 19.4°C to 18.5°C with depth, with a faint thermocline between 3-4m. Salinity also shows stratification, increasing by 0.6 between 3 and 4m. Chlorophyll increases from the surface, before plateauing below 2m.

Figure 2.2

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 2.

Figure 2.3

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 3.

Figure 2.4

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 4.

Figure 2.5

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 5.

Figure 2.6

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 6.

Figure 2.7

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 7.

Figure 2.8

Salinity, temperature (°C) and chlorophyll concentration (µg/L) with depth (m) at Station 8.

Figure 1.0

Google Earth image of positions of sampling Stations 1-8 within the Fal estuary on 03/07/2014.

Figure 3.1

Google Earth image of positions of Stations 1-8 and ADCP Transects 1-7. Transects can be clicked to display current velocity (m/s) and direction relative to ship track (Figure 3.2 - 3.8).

Station   Time     Latitude     Longitude    Water Depth    Wind Velocity   Wind direction   Secchi Disc Depth     Octaves of Cloud                  (UTC)                                          (m)              (ms-2)                                          (m)                      Cover                   

1          0740      50°08.695   005°01.413      36.9              4.4                254.6°                  7.40                       1/8

2          0833      50°10.287   005°02.087      27.0              4.9                242.3°                  6.13                       1/8

3          0852      50°04.406   005°02.623      21.2              5.0                257.6°   

4          0917      50°12.204   005°02.416      20.6              3.5                233.8°                  4.15                       1/4

5          0942      50°12.466   005°01.694      20.4              5.4                230.4°                  4.44                       1/4

6          1017      50°13.326   005°01.604      15.8              3.0                182.7°                  2.11                       1/2

7          1045      50°13.500   005°00.971      18.3              3.2                229.1°                  2.38                       1/8

8          1112     50°14.392    005°00.882      7.2                2.3                210.6°                 1.60                        3/4

Station 6 was positioned further up estuary around halfway between Station 4 and Station 8. Similar to Station 4 salinity changes are discreet with depth. Overall salinity is increasing with depth. Chlorophyll remains highest within the upper surface of the water column at ≈11 g/L and decreases with depth. Nutrients appear to be highest within the least saline part of the water column. Nitrate is most depleted at the surface, likely due to it being utilised by primary producers; whereas silicon continues to increase, suggesting a possible absence of siliceous phytoplankton.

Figure 3.2

Silicon (µmol/L), phosphate (µmol/L), nitrate (µmol/L), and fluorometry (V)  concentrations at Station 6 in the Fal estuary.

Station 8 was the top-most station in the estuary and thus was the one most influenced by freshwater, albeit only a little. Similar to the other stations salinity changed only discreetly with depth. However, chlorophyll was highest at this station at ≈17 µg/L which peaked at the small halocline seen at 4m. Nutrients were overall low and was most likely utilised by nearby primary producers.

Figure 3.3

Silicon (µmol/L), phosphate (µmol/L) nitrate (µmol/L), and fluorometry (V) concentrations at Station 8 in the Fal estuary.

Surface silicon concentrations decreased with increasing salinity. This is as expected, as the river water with a higher concentration of silicon is diluted by seawater with a lower concentration. Concentration of nitrate showed an overall decrease with increasing salinity, apart from at one station (Station 5) where relatively high nitrate concentrations were present at high salinities. This may be due to increased surface run off from the nearby creek but may also be anomalous. The overall decrease is likely caused by biological removal as plankton from the seawater mixes with the riverine freshwater. Phosphate concentration in the surface water was negligible (less than 0.01 µg/L) indicating that it may be limiting primary production, as is often the case in the estuarine environment1. Chlorophyll concentration decreased with increasing salinity, indicating favourable conditions for primary production further upstream. The fall in chlorophyll is likely associated with decreasing nutrients.

Figure 3.4

Silicon (µmol/L), phosphate (µmol/L) nitrate (µmol/L), and fluorometry (V) concentrations at Station 8 in the Fal estuary.