Chemical Findings

Estuarine mixing diagrams reveal the extent to which a solute’s concentration is determined by mixing between a riverine and a marine end-member, joined by the Theoretical Dilution Line. Solutes plotting off the line are affected by biological, chemical or physical processes throughout the estuary and are said to be behaving “non-conservatively”.

Surface salinities from each station sampled were used to plot mixing diagrams for each of silicon, nitrate and phosphate. Surface salinities from Black Rock were used as a marine end-member as it was the most offshore sample and Truro River was sampled separately on a different day to obtain a riverine end-member with much lower salinity.

Phosphate plotted off the TDL; with large amounts being added at the mouth of the estuary (Figure 5.). This can be explained by the fact that Fal estuary is affected by sewage treatment works, mine drainage sites and extensive farming, all major sources of the nutrient1.

On the other hand, nitrate and silicon can be considered as conservative as they both plotted along the TDL (Figures. 6 and 7). These nutrients are usually depleted in estuaries through biological uptake by phytoplankton unless the estuarine flushing rate is unusually high

Residence time of the estuary was calculated using the following equation (with total volume found using Admiralty charts and flow volume taken from the ADCP transect from Black Rock):

Residence time= Total estuarine volume/

Flow volume

Residence time found was a very short 1 day 3 hours 3 minutes which means that nutrients flush out of the estuary at a relatively high rate. They do not stay around long enough for biological, chemical and physical processes to have any profound effect which would explain the apparent conservative behaviour of silicon and nitrate.

Biological Findings

The initial findings related to phytoplankton, Figures 10a-f, show that there was a net decrease in diatoms but phytoplankton quantity in general increases up the estuary. This correlates with previous research that indicates the overall preference of higher salinity waters by most diatoms2. The diatom bloom is typical of this period of the year in the western English Channel3 . The dinoflagellate numbers are greatest at Station 4 in the middle of the estuary. Their numbers here are more unusual for this time of year as they should be blooming during the months of September and October. This may be due to a number of environmentally controlled factors which would need deeper analysis to understand. Figures x to y showed the main species found; the most interesting finding is the dominance of Chaetoceros sp. at station 2 and its presence at every other station. This shows that the conditions in the estuary are favourable to development of this particular species, which correlates with the well mixed nature of the lower estuary and the uniform chlorophyll with depth.

Throughout transect one, Figure 11a, copepods dominated the zooplankton population, followed by their nauplii. The “other” category, which is comprised of eleven more species found in the sample that were not prominent enough to be considered, also makes up a large proportion of the zooplankton. Larval forms of gastropods and polychaetes were present in smaller amounts, but not significant enough to be considered as a dominant species.     

In transect two, shown in Figure 11b, the range on dominant species increased with the inclusion of decapod larvae, and cirripedia larvae. The most common zooplankton found was gastropod larvae, accounting for 27% of the sample. Copepods decreased dramatically in population, falling to 7% of the zooplankton community. Diatoms are known to inhibit the hatching of nauplii larvae, explaining the large drop within this transect4.

Transect three, Figure 11c, showed a return of the adult copepod population. The station corresponding to this transect was not sampled for phytoplankton and therefore conclusions of their return based on the diatom population cannot be made. However we infer that the reoccurrence of the copepods is due to a lower diatom abundance. This transect was the only location sampled where ctenophores were found.

References

1Langston, W.J., Chesman, B.S., Burt, G.R., Hawkins, S.J., Readman, J. and Worsfold, P., 2003, ‘Site characterisation of the south west European marine sites: Fal and Helford cSAC’, Marine Biological Association of the United Kingdom Occasional Publication, 8. 

2(http://ir.library.oregonstate.edu/xmlui/handle/1957/33145)

3 (http://westernchannelobservatory.org.uk/l4_phytoplankton)

4Ban, S. and Burns, C. The paradox of diatom-copepod interactions. Available at :http://www.int-res.com/articles/meps/157/m157p287.pdf. [Accessed 1 July 2013].

5 (http://www.science.oregonstate.edu/01E05115-F794-433D-843A-8F646E969F73/FinalDownload/DownloadId-837B6330F51B508865BE89DCCBB7DE21/01E05115-F794-433D-843A-8F646E969F73/ocean.productivity/references/JMR%202006.pdf)

The views expressed above are those of the authors and not those of the University of Southampton or the National Oceanography Centre Southampton.

Introduction

The Fal estuary has freshwater inputs from the River Carnon, the River Kennal, the River Penryn, and the River Truro.  Salinity and temperature differences between water sources promote stratification within the water column, resulting in warm, less dense freshwater overlaying cooler, denser seawater.  Stratification within the estuary is dependent on several variables; the flux of freshwater inputs, wind induced mixing within the water column, and the strength of the tidal current.

Natural and anthropogenic nutrient availability within the estuarine system originate from freshwater inputs and vary throughout the estuary due to chemical properties.  Conservative and non-conservative mixing of constituents in the Fal estuary affect nutrient cycling in addition to the system’s phytoplankton and zooplankton abundance.  The investigation aimed to understand how the Fal estuary changed biologically, physically, and chemically as water samples were obtained and measurements were recorded at stations in the estuary.

Methods

Fieldwork was carried out aboard Bill Conway and measurements were taken at four different stations along Fal estuary, ranging upstream from Black Rock, Station 1 (see Station map) to the turning point, station 4.

At each station an ADCP transect and a vertical CTD profile were taken. At station 2, a second vertical CTD profile was taken because the Niskin bottles failed to fire on the first try. The ADCP transects measured salinity and temperature whereas the CTD profiles also measured fluorometry and transmission. The CTD also carried Niskin bottles that were fired at different depths which were chosen based on the CTD profiles and presumed water column structure; at each station bottles were fired at the surface. From these bottles, nutrient (nitrate, phosphate and silicon) and chlorophyll samples were taken for every depth.

Dissolved oxygen and phytoplankton samples were also taken but not at every sampled depth due to a limited number of storage bottles. Dissolved oxygen samples were thus only taken from surface and deepest samples. Only three phytoplankton samples were taken overall from surface bottles at Stations 1, 2 and 4, whilst zooplankton trawls were only taken from stations 1 and 4.

The processing of nutrient, chlorophyll, plankton and dissolved oxygen samples in the lab followed the same method as the offshore samples (see offshore methods).  

Physical findings

Figures 1 and 2 showed an increase in the degree of stratification of the water column inshore. Station 1, Blackrock, showed a well-mixed water column for both temperature and salinity with a temperature of approximately 12.4°C and salinity of 35.85. The surface to bottom variation in water body structure increased through Stations 2 to 8. However, whilst the relative difference between the stations showed some level of stratification there is very little loss in salinity of the water between stations; with the salinity never below 33. This is because the Fal estuary is a Ria with a low fluvial input and as such is strongly tidally dominated. The degree of stratification only became significant in the upper stations of the estuary and even there it was only strong for temperature, whilst salinity was relatively uniform. Therefore the Fal is probably a well-mixed estuary. The Transmission and Fluorometry profiles produced from the CTD data (Figures 3 and 4) allowed the particulate nature of the water column at each station to be assessed. The fluorometry values were fairly uniform throughout all the stations with varying levels of noise; this suggests that there were few phytoplankton within the estuary at this time of year and that there distribution was relatively uniform within the water column. As such, any change in transmission is likely due to suspended sediment rather than plankton. The transmission readings were similar for Stations 2 and 3 and only a little lower than Station 1. However, Stations 4 to 8 had a progressively lower transmission reading suggesting that there was a gradient of relatively greater level of suspended particulate matter in the water column up the estuary. This was probably due to the shallower water depths and strong tidal forcing resuspending particles from the floor of the estuary.

Date: 29/06/13

Vessel: RV Bill Conway

Cloud cover: 7/8

Wind: westerly/north westerly 4 or 5

Tides: High tide of 4.6m at 09:31 (all times UTC) and low tide of 0.9m at 16:02.

Stations 1-4: Upper estuary

Station 1(Blackrock) :

Location: 50°09.051N 005°01.862W

Time: 07:23, Depth:34.0m

Temperature: 13.7°C, Salinity 35.3

Station 2:

Location: 50°10.242 005°02.127W

Time: 08:36, Depth: 28.0m

Temperature: 13.3°C, Salinity 34.7

Station 3:

Location: 50°11.451N 005°02.704W

Time: 09:19, Depth: 19.5m

Temperature: 13.4°C, Salinity 34.6

Station 4:

Location: 50°12.201N 005°02.432W

Time: 10:00, Depth: 21.5m

Temperature: 13.8°C, Salinity 34.5

Stations 5-8: Upper Estuary (data collected by group 13)

Station 5:

Location: 50˚12.485N, 005˚01.677W

Time: 13:20

Station 6:

Location: 50˚12.950N, 005˚01.590W

Time: 14:00

Station 7:

Location: 50˚13.637N, 005˚00.938W

Time: 14:20

Station 8:

Location: 50˚14.303N, 005˚00.980W

Time: 14:42

Estuary

Estuary station locations

Figure 8 shows that the general trend in chlorophyll was a decrease in concentration with depth, apart from stations 3 and 7 which showed a chlorophyll maximum about 6m and station 2 which had a minimum at 9m but then  increased with depth. This suggests the phytoplankton are in the upper part of the water column to photosynthesise where there are higher light levels; the deeper chlorophyll maximums may be due to the phytoplankton utilising nutrients at a greater depth or a possible anomaly. In the surface waters chlorophyll was lowest at Station 2 (2.044ug/L) and highest at Station 6 (4.186ug/L). Anthropogenic inputs could be causing the higher photosynthetic rates further up the estuary, which is supported by the percentage oxygen saturation, which was also higher at the head of the river.

The oxygen saturation, Figure 9, decreased with depth at each station, apart from station 1, due to the decreasing photosynthetic rates with depth, suggested by the chlorophyll concentrations. Oxygen exchange in surface waters could have contributed to this distribution. Heterotrophic activity at depth could also be higher which would consume more oxygen. Oxygen saturation can increase by approximately 4% with high tide, as a result of more well mixed water with higher percentage oxygen saturation replenishing the low oxygen estuarine water.

The secchi disk measurements taken from black rock (station 1) towards the riverine end of the estuary are shown in Table 1. K values vary from 0.23 to 1.2. The beam attenuation coefficient is associated to the number of particles in the water, organic and inorganic, and so turbidity5. As one goes from the estuarine environment in station 1 to the riverine environment in station 8, there is an increase in the k value. This indicates a greater number of particles in the water and so a shallower light penetration. The greater particles are brought in to the estuary by the riverine flux where they are then dispersed and mixed towards the coastal front.

Table 1. Secchi depths measured for each station and attenuation coefficient, k, calculated from those values.

Figure 1. Depth profile of temperature for each station

Figure 5. Mixing diagram for phosphate with TDL

Figure 8. Depth profile for chlorophyll at each station

Figure 10a. Concentration of phytoplankton taxa at each station

Figure 11b. Proportion of dominant zooplankton taxa along transect 2