GROUP 9 FALMOUTH 2017
ESTUARY
08/07/17
The Bill Conway is a 11.74m Lochin 38, purpose built for teaching and research (Southampton,
2017). Setting off from Falmouth Marina at 9am on the 8th July 2017, we travelled
up the river Fal to collect samples of nutrients and chlorophyll at more dilute salinities.
Starting from Black Rock we took samples from 3 stations (Station 30-
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CTD Conway
Fluorescence
All three graphs in figure 1 show a gradual increase in fluorescence with depth and
a slight peak in fluorescence at around 2-
CTD profiles showing changes in fluorescence with increasing depth, top shows changes at Station 30, middle shows changes at station 31 and bottom shows changes at station 32.
the upper layer will have freshwater of lower density from tributary contributions and the lower layer experiences greater tidal influence, introducing more saline waters from offshore settings.
ADCP
An ADCP is an Acoustic Doppler Current Profiler. It is used to measure the rate of water flow across a water column. It works by sending sounds at a constant frequency and measuring the frequency of the sound being returned. If the sounds bounces back off an object that is moving away, then the frequency returned is lower and if the object is moving towards the instrument the frequency is higher. By measuring this difference, the instrument can calculate how fast the object is moving. It can tell how far away the object is by timing the difference between the sending of the original ‘ping’ and the time it returns.
Conway 30
About 75% of sample 30 is made up of Nitzschia spp. and Rhizosolenia spp.,
two phytoplankton common in cold brackish water.
Conway 31
Again Nitzschia spp. and Rhizosolenia spp. numbers make up over half of
the total sample. However, there are also large numbers of the ciliate genus Mesodinium,
a genus often found in estuaries.
Conway 32
Whilst Nitzschia spp. is common in the counts for this sample, Mesodinium
spp. are beginning to count for larger numbers in the sample.
Across the three lower estuarine stations; 30, 31 and 32, Mesodinium spp. gradually becomes more populous until it comprises over 60% in station 32. This station was sampled downriver of the King Harry car ferry crossing, as a polluted estuary species this could contribute to the species increasing in population with proximity to the crossing. Mesodinium spp. are predators of Cryptophytes (under which cryptomonas spp. and cryptophyceae fall) (Peterson et al., 2013). So where populations of Mesodinium spp increase so there is a decrease in the population of either of these two species
Nutrients with depth
For the estuarine nutrients, some general trends can be observed across all stations (figure 5). Silicon has a higher concentration at the surface than at depth for every measured station, however station 33 has a considerably higher silicon concentration in the surface with a decrease of 4.8 to 1.6µmol/L.
Phosphate concentration remains relatively constant with increasing depth except for station 34, where concentrations increase from 0.25 to 2.5µmol/L.
Nitrate is much more varied: stations 33 and 35 show a similar trend to that of silicon with concentrations of around 3.6 and 5.8µmol/L near the surface that decrease to 0.7 and 1.7µmol/L with depth. Stations 32 and 36 show a constant concentration with depth of around 0.8µmol/L whilst station 31 also shows a constant concentration of around 1.9µmol/L. Station 30 and 34 look obscure compared to the other stations because they increase with depth below about 2m.
Salinity
Salinity depth profiles produced from the three estuarine stations showing decreased salinity at the surface of the water column and salinity increasing with depth. Trends are particularly clear in Station 32 where surface salinity is about 33.2 at 2m below the surface but 34 near the bottom at 12m.
CTD profiles showing changes in Salinity with depth, top shows salinity in the water column at station 30, middle shows changes at station 31 and bottom shows changes at station 32.
The trend in salinity with depth can be explained by the degree of stratification of the water column in the Fal estuary. Due to recent warm weather, the surface layer of the estuary remains separated from the more dense, colder, lower layers which prevents vertical mixing. This causes a gulf in salinity between the two layers since the upper layer will have freshwater of lower density from tributary contributions and the lower layer experiences greater tidal influence, introducing more saline waters from offshore settings.
Temperature
Trends in temperature produced by the three CTD profiles (figure 3)are much clearer than any trends in the other CTD profiles. All three stations have warmer surface water with cool water lying underneath. In all cases the surface water is more than a degree celcius cooler and profiles appear to decrease in temperature in a stepwise manner. Every station increases in temperature at surface water with surface water at station 32 1C warmer than surface water at station 30.
Temperature depth profile showing changes in temperature with depth at every station. Each station has been represented with a different profile colour.
Transmission
These CTD profiles show light transmission in the water column at each station. Both stations 31 and 32 have transmission values that increase very lightly with depth. Station 30 shows an s shaped curve of transmission values that peak both at the surface and at 20m depth with least transmission at 5m.
Transmission profiles showing light transmission through the water column, top is station 30, middle is station 31 and bottom is station 32
Meta Data
Bill Conway
Date: 08/07/17
Time: 09:15 until 12:00 GMT
Location: Estuary
Latitude: 50'08.6880 N
Longitude: 005'01.416 W
Low Tide: 11:08 (1.01m)
High Tide: 16:57
GMT (4.88m)
Cloud Cover: 6/8
Discussion.
Fluorescence is a widely used proxy for Chlorophyll a concentrations due to Chlorophyll a fluorescing at a single particular wavelength of light (Lorenzen, 1966). Therefore, the peaks in the graphs show spikes in chlorophyll a and so phytoplankton, showing larger concentrations of phytoplankton in the near surface layers of water.
Transmission CTD profiles (figure 4) are measurements of light transmission through the water in the water column and so is used as a proxy for the volume of particulate matter in the water column. (Bricaud et al., 1998). Figures (Transmission figure) show the volume of particulate matter in the water column, so the minimum transmission in station 30 shows high particulate matter in the water column. Which may be phytoplankton linked due to a corresponding increase in fluorescence. Similar corresponding changes are seen in station 32 where the decreased phytoplankton at depth is leading to the increase in transmission
The temperature profiles give a visual representation of the water column and its stratification. Recent warm weather has created an upper layer of less dense, warmer water. However, with depth, less heat is retained and water temperature decreases slowly until the thermocline is reached and the stepwise reduction in temperature occurs. This largest decline in temperature represents where the warmer surface layer meets the cold lower layers but reduced mixing is occurring (Geyer et al. 2000).
The trend in salinity with depth can also be explained by the degree of stratification (NOAA, 2008) of the water column caused by the recent conditions. The surface layer of the estuary remains separated from the cold and dense bottom water which prevents vertical mixing. This causes a gulf in salinity between the two layers (Dyer, 1973) since the upper layer will have freshwater of lower density from tributary contributions and the lower layer experiences greater tidal influence, introducing more saline waters from offshore settings.
References
Bricaud, A., Morel, A., Babin, M., Allali, K. and Claustre, H. (1998). Variations
of light absorption by suspended particles with chlorophyllaconcentration in oceanic
(case 1) waters: Analysis and implications for bio-
Lorenzen, C. (1966). A method for the continuous measurement of in vivo chlorophyll
concentration. Deep Sea Research and Oceanographic Abstracts, 13(2), pp.223-
Dyer, K. R. (1973). ‘Estuaries, a physical introduction’. John Wiley and Sons. London
NOAA. (2008). ‘Slightly Stratified Estuaries’. National Ocean Service. [Available Online]: http://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar05c_stratified.html Date Accessed: 16/04/2017
Geyer, W. R., Trowbridge, J. H., Bowen, M. M. (2000). ‘The Dynamics of a Partially Mixed Estuary’, Journal of Physical Oceanography
08/07/17
On the 8/07/2017, between 12:15-
This location was used in order to see how the flooding tidal state affected the chemical physical and biological properties of the estuary, due to negligible effects of other influences, i.e. precipitation and riverine inputs.
BACK TO TOP
08/07/17
On Setting off from the Pontoon at King Henry Crossing at 12pm four individuals of
the group headed up the river Fal to Malpas. The aim was to find an area of low salinity
high up the river so that we could construct data series representing the changes
in zooplankton, silicon and other nutrients along a salinity gradient. In the Fal
Estuary low salinity values are typically found in the waters near Truro, however
at the time of sampling awkward tidal times prevented access to the upper reaches
of the Fal due to the river nearly drying at low tide.
The same techniques for collecting
Silicon, Nitrogen/Phospherous, Chlorophyll and Plankton were used on the smaller
boat at 4 stations with salinities ranging from 28-
At two forks in the river
zooplankton nets were deployed against the tide and towed behind the boat for 5 minutes.
Samples taken here were treated with Formalin to kill any living organisms to prevent
predation of the sample and prevent decay.
Results
Using contour graphs (figure 7) to display the time series data, it shows the changes that occurs in the estuary during a tidal flood. All four figures show significant changes in the conditions of the estuary over the three hours of sampling, surface waters increased in salinity from 33 – 33.54. Salinity increased with depth as more saline waters are denser so expected to be at lower depths (Talley, 2002). Over the three hours the surface salinity did not change much before 13:30 where it starts to get denser.
The tidal flood also brought in warmer surface waters and colder waters at depth, as expected the temperature decreases with depth as the surface waters have greater irradiance heating the water and the colder waters are denser so would stay at the estuary floor. The graph shows a small amount of stratification where the contour lines are very close together especially at 2.5 metres at 13:48.
There was other significant changes in chlorophyll and oxygen saturation. As the tide floods the estuary it has a much higher chlorophyll concentration, doubling at some depths, chlorophyll was lowest at the floor of the estuary with the highest concentrations being between 1 – 2 metres deep. Oxygen saturation shows a similar trend, the floor of the estuary had the lowest measurements however between 11:15 and 12:00 the surface layer had the lowest oxygen saturation. As the tide came in the oxygen saturation of the top 3 metres of water increased, with water deeper than 3.5 metres having a much lower oxygen saturation percentage.
Figure 8 shows the irradiance at depth from the LICOR par sensor, both measurements at 11:29 and 12:21 have very similar percentages of surface irradiance, however the reading at 13:29 shows less irradiance throughout the water column. This data is supported by figure 6 from the flow velocity probe. Both 11:29 and 12:21 velocities do not increase past 0.1 m/s and 13:29 velocities peaking at 2.8 m/s at 2 metres. 12:21 and 13:29 have their highest velocities at 2 metres with surface velocities being much smaller.
Discussion
These changing conditions mean that the fauna living in the estuary have to change their behaviour to survive.
As salinity increases some faunal species such as Peringia ulvae , who are osmoconformers,
bury and re-
As the velocity of the flooding tide increases it creates greater sheer stress on the estuary floor, this higher velocity creates turbulence causing sediment to be suspended in the water column, because of this the irradiance will decrease throughout the water column as the sediment reflects light.
The higher chlorophyll concentrations as the tide floods in around 12:30 are likely due to large quantities of photosynthetic phytoplankton being carried up the estuary in the flooding tide. Phytoplankton that are not mobile enough to fight the tide are drawn into the estuary where the higher turbidity of the estuary causes them to regulate their buoyancy decreasing their depth in the water column (Gemmell et al., 2016). Irradiance near the estuary floor is lower than at the surface and turbidity is higher during the flood making photosynthesis less productive resulting in low concentrations of chlorophyll at the estuary floor. During the photosynthetic reaction oxygen is produced and will increase the oxygen saturation percentage of the water, this is the reason for both chlorophyll and oxygen saturation following similar trends.
References
Talley, LD. (2002). Salinity patterns in the ocean. In Encyclopedia of global change. Volume: the earth system: physical and chemical dimensions of global environmental change (eds
MacCracken MC, Perry JS), pp.629–640. Chichester, UK: John Wiley & Sons0
Gemmell, B., Oh, G., Buskey, E. and Villareal, T. (2016). Dynamic sinking behaviour in marine phytoplankton: rapid changes in buoyancy may aid in nutrient uptake. Proceedings of the Royal Society B: Biological Sciences, 283(1840), p.20161126.
NEWELL, R. (2009). BEHAVIOURAL ASPECTS OF THE ECOLOGY OF PERINGIA (=HYDROBIA) ULVAE
(PENNANT) (GASTEROPODA, PROSOBRANCHIA). Proceedings of the Zoological Society of
London, 138(1), pp.49-
Meta Data
Winnie the Pooh
Date: 08/07/17
Time: 12:00 until 15:00 GMT
Location: Malpas
Latitude: 50'14.518 N
Longitude: 005'01.670 W
Low Tide: 11:08 (1.01m)
High Tide: 16:59 GMT (4.88m)
Cloud Cover: 6/8
Meta Data
Pontoon
Date: 08/07/17
Time: 12:00 until 15:00 GMT
Location: Fal River
Latitude: 50'12.580 N
Longitude: 005'01.400 W
Low Tide: 11:08 (1.01m)
High Tide: 16:59 GMT (4.88m)
Cloud Cover: 6/8
Zooplankton Community Structure
The zooplankton communities sampled by Bill Conway and Winnie the Pooh in the river Fal (figure 10), revealed that Calanoid copepods are the most dominant zooplankton group within the estuarine system. The highest abundances of zooplankton were recorded on Winnie the Pooh Site A which was the furthest up the river, this may be due to proximity to several sewage treatment facilities (South West Water/Environment Agency, 2011) present in the Fal river which can be responsible for nutrient loading leading to enhanced phytoplankton productivity on a seasonal and annual scale. This creates a larger food supply for the abundant copepod communities, thus raising the overall biomass of these organisms (Zervoudaki, 2009).
Compared to the offshore zooplankton communities, the estuarine population has much lower diversity, this could be due to the difference in salinities between the offshore and estuarine systems and the limited larval dispersal into the estuary compared to the open ocean. The lower estuarine salinities limit the persistence of more marine zooplankton groups (Gao et al. 2008). The differences in salinity between WP (A&B) and Bill Conway are too slight to have Estuarine coastal systems, due to their high productivity, experience lower exploitation of the phytoplankton (lower grazing impact) resource by zooplankton compared with the offshore ecosystem (Zervoudaki, 2009). The high carbon demand of copepods is not satisfied by the comparatively weaker phytoplankton.
References:
Larink, O. and Westheide, W. (2011). Coastal plankton. München: Verlag Dr. Friedrich Pfeil.
Todd, C., Laverack, M. and Boxshall, G. (2006). Coastal marine zooplankton. Cambridge: Cambridge University Press.
Zervoudaki, S., Nielsen, T. and Carstensen, J. (2009). Seasonal succession and composition
of the zooplankton community along an eutrophication and salinity gradient exemplified
by Danish waters. Journal of Plankton Research, 31(12), pp.1475-
Dahms, H., Fornshell, J. and Fornshell, B. (2006). Key for the identification of
crustacean nauplii. Organisms Diversity & Evolution, 6(1), pp.47-
Gao, Q, Xu, Z. and Zhuang, P. (2008). The relation between distribution of zooplankton
and salinity in the Changjiang Estuary. Chinese Journal of Oceanology and Limnology,
26(2), pp. 178-
Figure 1:CTD profiles showing changes in fluorescence with increasing depth, top shows changes at Station 30, middle shows changes at station 31 and bottom shows changes at station 32.
Figure 2: CTD profiles showing changes in Salinity with depth, top shows salinity in the water column at station 30, middle shows changes at station 31 and bottom shows changes at station 32.
Figure 3:Temperature depth profile showing changes in temperature with depth at every station. Each station has been represented with a different profile colour. Legend)
Figure 4: Transmission profiles showing light transmission through the water column, top is station 30, middle is station 31 and bottom is station 32
Figure 5: Profiles of the changes in nutrient concentrations with depth at each estuarine station
Figure 6: Pie charts showing percentage of different phytoplankton species at each station sampled on the Conway
Figure 7: Contour graphs created from data for chlorophyll, oxygen saturation, salinity and temperature collected in a time series at the pontoon
Figure 8: Plots produced from a LICOR par sensor showing Velocity of flow on the left and irradiance on the right
Figure 10: Zooplankton counts assembled from a zooplankton trawl collected by the RV Bill Conway
Figure 9: Pie charts showing percentage of zooplankton groups present in two samples collected on Winnie The Pooh. Trawl A was collected upshore of trawl B.
The views and opinions expressed re those of the individual and not neccesarily those of the university