Introduction

On the 24th June 2016 the Bill Conway was taken into the Fal estuary with the aim of investigating the distribution of nutrients in relation to the physical and biological characteristics of the area. High water was at 07:34 UTC and low water was at 14:00 UTC, with 8/8 cloud cover and mist. The surveying began at 07:10 UTC and finished at 13:00 UTC, the majority of the data was collected on the ebb tide. The survey began at Black Rock and progressed up river, with both horizontal and vertical transects being taken (using the ADCP). Temporal and spatial transects were taken with the ADCP between and at sites.

The sampling was focused at four locations, each was visited twice at different times during the day, see figure- below for clarification of the locations. The exception to this was site A, which had 4 CTD profiles created over the space of an hour.

Methodology

A secchi disk was used to approximate the attenuation coefficient at each of the sites. The corrected secchi value was converted into light attenuation using the secchi calculation: 1.44 ÷ secchi depth.

A rosette mounted CTD was used to determine the structure of the water column, in addition a fluorometer was mounted on the rosette to measure fluorescence. There were also six Niskin bottles attached to the rosette that were used to take samples from 3 different depths- with 2 bottles being fired at each depth. The CTD was lowered through the water column and the depths at which the Niskin bottles were to be triggered at were determined by looking at interesting features in the temperature and salinity profiles. The Niskin bottles were triggered on the upcast of the CTD remotely from the computer. Water samples were used to assess silicon, phosphate, nitrate, oxygen, chlorophyll and phytoplankton concentrations.

An ADCP was used to assess the speed and direction of the flow between and at each station as well as across the river channel. This was done using the on board ADCP.

A zooplankton net was used to collect three zooplankton samples from different sites. The sites at which the samples were taken were determined by looking at the CTD profile for each site to find any interesting areas. The zooplankton net was 0.5m in diameter and had a 200µm mesh size and was towed behind the boat for five minutes. A flowmeter was attached to the net so that the amount of water sampled could be worked out in the lab later.  

For procedures used in the lab, please see here.

Results & Discussion

Temperature and Salinity

The figure below shows that the entrance to the estuary is well mixed- the depth profiles for temp and salinity at site G are vertical. However it shows that site A is stratified, this could be due to the fact that the Fal is a tidally dominant estuary and so the freshwater and saline water mix further up the channel than in a river dominated estuary3.

Temperature increases with distance upriver, this could be because shallower water heats up faster and there is proportionally less seawater dilution. Salinity shows an inverse relationship with temperature- it decreases with distance upriver.   

Fluorescence

The figures below both show a clear trend of increasing Fluorescence as CTD cast location moves up from the estuary mouth to the upper river. As fluorescence may be considered a rough indicator of chlorophyll this might imply an increase in phytoplankton upriver. This is supported by the lowest numbers of phytoplankton being found at station G31 and the highest being found at A37, although there was a lot of variability in phytoplankton taken at the same stations. Although most of the stations fluoremetry show no defined peaks at a specific depth, both G31 and A35 show peaks at around 14 m, possibly indicative of phytoplankton grouped at this depth.

Transmission

For each station the beam transmission, recorded in volts, varied little with depth which is unsurprising as the area is well mixed. However, the figure below shows a decrease in beam transmission upriver with an average of 3.9 volts across the A stations compared to 4.6 at the estuary mouth. Lower transmission at the river end results from the high levels of phytoplankton and suspended solids produced by weathering of carbonates, silicates and evaporates and anthropogenic inputs such as agricultural runoff. These are kept suspended by the higher velocity flow of the river. As the river water becomes more diluted towards the estuary mouth and some material flocculates and sinks out of the water column, the concentration of suspended matter decreases. It should be noted that five other rivers apart from the Fal feed into Carrick Roads each contributing their own suspended load.

ADCP

The ADCP spatial transects provide a range of information about different points in the estuary. Firstly, as seen in the figure below, they illustrate the shape and depth of the the seabed from the estuary mouth to the top of the river. The G31 and G40 transects at the estuary mouth clearly show the central deep channel (maximum depth of average 30.5 m from at the transect location), indicative of a ‘Ria’ or drowned river valley. Moving up the river Fal and then into the river Truro the depth gradually shallows to a final depth of 6.9 m at stations A34 and A37 while the deep channel disappears into a normal river cross section.

The ADCP data also gives an indication of flow speed and direction. There is a slight increase in average current speed (as indicated by the colour scale on the the transect profiles) toward the top of the river. This is expected as the channel widens out the water flowing through the channel slows. Station D32 and D39 in particular show a faster fresh surface layer on top of a slower salty layer.

Silicon

The figure below indicates that silicon is conservative in nature in the Fal estuary as the samples are in line with the theoretical dilution line (TDL). As the samples were taken on the ebb tide it may be expected that silicon would show non-conservative behaviour due to tidal removal. However the tidal removal is likely balanced by riverine input of silicon and hence shows conservative behaviour.

Nitrate

Nitrate shows non-conservative behaviour in relation to the TDL, see the figure below. The points indicate that nitrate is removed from the system. This could be from phytoplankton activity, or it may be due to the fact that samples were taken on the ebb tide and so the nutrients were taken out to sea and not replenished by the river. The low concentration in the upper parts of the river may be accounted for by dilution from the tributaries that feed into the main channel.

This is interesting because the Fal estuary was designated a nitrate vulnerable area3,4 so it unusual to see such low amounts of nitrate.

Phosphate

Phosphate also shows non-conservative behaviour in relation to the TDL, see the figure below. However the plot indicates that phosphate is being added to the system- rather than removed. This may be a result of fertilizer runoff from the land replacing what was lost from the system on the ebb tide, as most of the surrounding land near the river is used for farming and high rates of precipitation had occurred in the previous days. The high values associated with the lower salinities may be due to input from the Newham sewage treatment works3, which is adjacent to the river Truro.  

Oxygen saturation

The figure below shows the oxygen saturation values taken from the surface water at each site. Site G31 and G40 show low oxygen saturation in comparison to the other sites. This may be due to little photosynthesis occurring because of the low numbers of phytoplankton present (see the fluorescence graph & phytoplankton graph) or the high numbers of zooplankton undertaking respiration at G31 (see the zooplankton graph). This may also be the case for D32 which shows a high number of zooplankton and low numbers of phytoplankton. Conversely the high oxygen saturations at site B33 and A37 may be a result of the high phytoplankton numbers at these sites (see the phytoplankton and fluorescence graphs). Some of the variations in oxygen between sites located in the same place may be due to the differing depths the samples were taken at. For example B33 and B38 show a 10% difference in saturation, while this may be a result of tidal differences it is likely because B33 was taken closer to the surface and so more atmospheric exchange will have occurred.

Phytoplankton

Phytoplankton samples were taken from the surface fired Niskin bottles, twice at stations G and A and once at stations B and D. The figure below shows that total numbers of phytoplankton per ml vary largely from 70 to 1420. The diatom genus Guinardia, commonly distributed in coastal regions1, was the most abundant overall and the most abundant at each station with the exception of stations B33 and G40. It should be stressed that the calculations involved in estimating phytoplankton numbers assume a lot about phytoplankton distribution in the surface, and therefore these numbers may not be truly representative. At station G, an increase of 1110 phytoplankton per ml was observed over a 6 hour, 20 minute period. Given that the tide was ebbing over the same period, this might suggest that phytoplankton were upriver at high tide during the first measurement and then moved into the estuary with the ebb tide. This is supported by increasing phytoplankton from station D32 to station and then B32. However, at station A, instead of decreasing in numbers as might be expected with the ebbing tide, there is a 1280 increase. It might be postulated that a second bloom or group of phytoplankton had moved to this location from further upriver, but without a higher resolution time series for each station it is not certain whether this is a trend or simply error in calculation.

Zooplankton

Zooplankton tows were taken after certain estuarine stations, sample Z1 was towed after station G31, Z2 after station D32 and Z3 after station G40. Unsurprisingly, given their status as the most abundant metazoan group in the world’s oceans1, copepods are the most abundant group overall (446 m-3) (see figure below) as was the case in the offshore transect. At the time of sampling, the tow at the at the river mouth recorded a greater abundance of zooplankton than the tow at the estuary mouth an hour and 13 minutes before. However, we see the opposite trend with Cladocera and Hydromedusae. Comparing between station G31 taken at 08:04 UTC and station G41 taken at 13:30 UTC there is a dramatic reduction in total zooplankton numbers. With the understanding that the tide was ebbing during this period this might infer zooplankton being swept out to sea by the tide or that diel vertical migration has taken place in order avoid the swiftest tidal currents2.

References

[1] Kraberg, A., Baumann, M. and Dürselen C. D. 2010, Coastal Phytoplankton, 1st edn, Dr. Friedrich Pfeil, Munich.

[2] Lampert, W. 1989, “The Adaptive Significance of Diel Vertical Migration of Zooplankton”, Functional Ecology, 3 (1): 21-27.

[3] Langston, W.J., Chesman, B.S., Burt, G.R., Hawkins, S.J., Readman, J. and Worsfold, P. 2003, ‘The Fal and Helford (candidate) Special Area of Conservation’, Marine Biological Association Occasional Publication, 8.

[4] Maier, G., Nimmo-Smith, R. J., Glegg, G. A., Tappin, A. D. and Worsfold, P. J. 2009, ‘Estuarine eutrophication in the UK: current incidence and future trends’, Aquatic Conservation: Marine and Freshwater Ecosystems, 19, 43-56.

Secchi Disk

The attenuation coefficient or the rate of irradiance loss with depth follows a clear trend across the stations. A lower attenuation coefficient is observed at the estuary mouth this then increases towards the upper river. This is unsurprising given that the river delivers a large load of suspended matter which is diluted towards the estuary mouth and scatters light as observed in the beam transmission data. There is a higher concentration of phytoplankton at site A- phytoplankton absorb light for use in photosynthesis and so influence how far light penetrates in the water column. The depth at which phytoplankton are able to photosynthesise will also be affected by how far light can penetrate, which is much further at the estuary end.