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Meet the Group! From left to right: Emma Collins, Lucinda Bufton, Jonathan 'Skwirrel' Yirrell, Dominic Cooke , Mark Evans, Michael Thompson, Lydia Gibson, Beverly Oh, Emily Kearns.
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Introduction to the Fal Estuary
Southampton University Survey From 4th to the 17th July 2006 students from the University of Southampton carried out the first in depth oceanographic survey of the Fal Estuary in Falmouth, England. The aim of the study was to determine the variation in biological, chemical, physical and geological parameters and the interactions between these parameters within the upper and lower reaches of the Fal estuarine system and offshore from the estuary. To accomplish this five research vessels were used, each adapted for particular sampling requirements to facilitate a survey of the entire estuarine area from the head of the Fal to offshore in the Western English Channel. The following results are based on the data collected by group 3. |
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The RV Bill Conway along with the small boats has been an essential part of looking at both the estuarine chemistry, biological processes and physical mixing of the Fal Estuary this would allowing the group to ascertain the dynamic nature of the estuary. Aims Methods Nutrients: Calibrated against standard solutions and colorimetry used to
determine concentration. For nitrate flow injection colorimetry used.
CTD Analysis The main thermocline for all the stations and sharpest increase in temperature occurred within the first 3m which is expected due to the upward movement of the thermocline in response to summer stratification. The largest difference in temperature, on average, was 1oC below 5m for stations 2 and 5. These two stations also showed the largest difference in salinity of just over 1 below 5m. This was expected as station 2 was our station furthest up the estuary so would have been fresher water which holds heat less efficiently than sea water which has a larger surface area. There was also the highest oxygen concentration at this station due to the lower temperature; it was the only station where oxygen increased with depth which may be due to the sun heating the top layer of water reducing the concentration of oxygen held in solution. This may also be explained by the corresponding increase in chlorophyll biomass at this site. The high fluorescence at this station can in turn be explained by a higher nutrient flux, as seen from the nutrient mixing diagrams. Within the first meter station 5, situated furthest seaward, had the sharpest increase in salinity from 34.5 to 35.3 which may be due to dilution from rainwater. Station 9 also had a similar profile to station 2 which may be due to an inflow of fresh water from Saint Just Creek. Station 11 was situated midway between the estuary mouth and the top of the estuary. This was reflected in the midway temperature and salinity profiles in contrast to the other profiles which shows a degree of freshwater and seawater mixing. There also appears to be a salt wedge of varying strengths at all stations bar 5 and 9. Fluorescence shows a sharp decrease at stations 2, 3 and 9 with depth which may be in response to high riverine sediment load which reduces light availability. The sharpest decrease in oxygen concentration at station 3, situated adjacent to the mussel farm, is most likely explained by increased heterotrophic respiration. Towards the mouth of the estuary mixing velocity decreases enabling sediment to settle decreasing turbidity. This is also seen from the shallowing euphotic zone further up the estuary towards the river, as the secchi depth decreases up the estuary. All of the oxygen saturations were above 100% therefore indicating super saturation showing that the estuary is very productive with a large nutrient supply. Chemical Analysis Silicate At the head of the estuary the points fall mostly very close to the line indicating conservative behaviour of silicate with salinity. There are a few points that are above the line at ~7 salinity which may indicate a small addition of silicate. This sometimes occurs when there are areas of sewage effluent input as the freshwater entering the system ay have higher silicate content. At around salinity 14 the points start to fall below the TDL, indicating non-conservative behaviour of silicate within the lower regions of the estuary. Although we cannot say for sure, it is likely that silicate is being biologically removed by diatoms as they use dissolved silicate to form tests or frustules. It is also worth noting that station 3 has higher silicate concentrations than expected which maybe due to the presence of a mussel farm found close to this station.Nitrate In the lower reaches of the Fal, nitrate shows non-conservative behaviour as points fall below the TDL. In the upper reaches there is possible addition of nitrate, however more sampling stations are needed as the points are scattered quite far from the line. The reasons for addition of nitrate can be found by examining the land area around the upper regions of the estuary. The area has a large number of farms, in particular livestock farms which create high volumes of manure, very rich in nitrate. This is a major input of nitrate to the upper estuary through direct run-off into the Fal, Truro and Tressilian Rivers and via leeching through soil. Also, upon examination of phytoplankton samples, lower numbers of phytoplankton were counted in the upper estuary (most probably due to the fact that most species are marine and cannot cope with freshwater conditions). This is an indication that phytoplankton are utilising nitrate in the lower regions of the Fal, therefore showing removal of nitrate on the mixing diagram. Another reason for lower nitrate concentrations may be due to the increasing distance from sources of nitrates. Phosphate Analysis of the phosphate estuarine mixing diagram shows non-conservative behaviour with removal of phosphate from the water column. There is a large proportion of uptake in the lower salinity region, although the actual concentration being removed is small in comparison to nitrate, showing agreement with the Redfield Ratio. Inputs of phosphate originate from sewage, agriculture and urban run-off from the extensive farmland and towns around the Fal estuary. Phytoplankton Phytoplankton Growth of phytoplankton is affected by the available nutrients in the water column. This availability varies throughout the estuary depending on inputs and uptake by phytoplankton. At mid-salinities there is the highest proportion of removal of phosphate and nitrate, which corresponds with the high chlorophyll concentration at around 15-20 salinity. Phytoplankton abundance is also high at low salinities, near the mouth of the estuary and ciliates and dinoflagellates were found only at this point. At all other stations where samples were taken, diatoms are the dominant species. It is important to note that the phytoplankton samples were taken over a small salinity range, with station 6, the most northerly station, having salinity 31.39 and station 5/6 having salinity 35.4. The reasons for the low phytoplankton numbers at station 5/6 may be due to the tidal state which was on an ebb tide. The ADCP data shows a reasonably strong flow out of the estuary which may have given the low phytoplankton numbers and corresponding low chlorophyll concentration. This is an anomaly however, and cannot only be explained by the tidal flow. It is possible that due to the nature of patchiness of phytoplankton that the sample was taken from an area of low biological importance. Zooplankton Net 2 and net S1 were collected in the King Harry Ferry area and show similar structure of zooplankton with the main difference being the large number of eggs that were found. The reason for the high abundance of eggs in this trawl may be due to a number of reasons. Firstly the mussel farm in the area may act as a source of eggs which have a degree of buoyancy and therefore enter the plankton net. Secondly the estuary was being sampled at spring tides which is often the period when fish choose to release their eggs as it gives the best chance eggs reaching the lower estuary. Net 1 was collected from around Malpas point at salinity 32.7 and was the most northern trawl. Estuarine species must be physiologically flexible and pressures in the upper estuary are often greatest as these areas experience large salinity fluxes. Therefore as expected, zooplankton numbers and diversity are very low as most species cannot adapt to the stressful environment. In the southern most station diversity is high and the greatest range of species can be found. This also fits the expected theory that where conditions are easier to cope with, a higher number of species can co-exist. There is a lower numbers of species however which may be due to competition. Further up the estuary where net 2 and net S1 were trawled the species that are able to thrive do not suffer such great competition and are therefore found in higher numbers.
Transect 6 - Mylor Harbour to Messack Point
Start – 50 10.555N 005 02.569 W 13.59.30 GMT This transect was taken in an easterly direction across the middle of the estuary across the deep channel. From the velocity magnitude transect it was determined that water at the surface is flowing slightly faster than at depth. The velocity direction profile showed that all of the water was flowing in a southerly direction.
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Aims The main aim of this investigation is to perform a benthic habitat survey of a selected area of the Fal estuary and characterize the sediment structure as well as biological composition. Sidescan sonar was used to survey the seafloor along the Inner Falmouth Bay aboard the RV Grey Bear. 6 transect lines 100m apart and 1km long, were taken parallel to Gyllyngvase Beach starting at 50008.7N 05’03.1W. This is an interesting site as it is a known dumping ground for dredged material. Whilst surveying, particular attention was paid to identifying bedforms and different sediment types, and spotting important anthropogenic features such as wrecks, artificial reefs, pipelines, buoys, sewage outfalls and dredged channels. 5 grab sites were then selected based on the information obtained from the side-scan sonar output. A geological map of the seafloor was then produced ( see Figure x). Results Sidescan Sonar The main types of bedform in Gyllyngvase Bay (inner Falmouth Bay) are megaripples. These are non-cohesive, flow-transverse bedforms and are characteristic of medium to coarse sediment which was found over the entire survey area. There were distinct differences in the ripples, although all ripples were classified as megaripples (0.06-1.5m height, 0.6-20m length). The data suggests that the crests are not sharp as there is a subtle contrast between the dark and light areas, which may mean that these are 3-D megaripples, however this is difficult to infer from the data as this may have more to do with data quality than to do with the actual shape of the ripples. The megaripples occur in one large ripple field which suggests an abundant supply of sediment. Ripples closest to the shore in the northern part of the first transect were at the lowest end of the scale, whilst ripples over the rest of the survey area were significantly larger (table 1). There was also variation within the larger ripples at the survey site. Near the centre of the site the megaripples were larger than to the eastern and western edges of the area. The small megaripples are likely to be storm generated due to evidence of bifurcation of the ripple beds.The larger megaripples may be generated by current and are therefore indicative of the deeper water flow in the area.
Grab Sites
Conclusion References
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Aims Our aims were to use the small boats (ocean adventure and coastal research) to access the shallower parts of the estuary up the Truro river to measure salinity, temperature, pH and dissolved oxygen using a YSI multiprobe. Secchi disc depth was also measured and water samples were taken to analyse for oxygen and nutrients later on in the lab. The data can be used in connection with data from the Bill Conway on the same day to give an overview of the biological, physical and chemical interaction within the Fal estuary. Method In order to sample the upper reaches of the Fal estuary (i.e. Fal and Truro rivers) most effectively, we employed a “leap-frog” method of sampling. The RV Coastal Research and RV Ocean adventure sampled at alternating stations up the river to just North of Malpus point. At each station, a YSI multiprobe was deployed to give vertical profile measurements for temperature, salinity, pH and dissolved oxygen concentration. Turbidity measurements were made from the bow of the ribs using a secchi disk. In addition, surface water samples were collected and filtered at every station for nutrient analysis back in the lab. Water samples for silicate analysis were stored in plastic bottles, while samples for nitrate/phosphate analysis were stored in brown glass bottles, and bottle numbers recorded. The glass-fibre filters used for filtrating water samples were extracted and stored in test-tubes filled with acetone. These were in turn preserved in a cooling box for chlorophyll analysis in the lab. Oxygen samples and replicates were collected using Niskin bottles at only 2 stations (start and end station). On collection, the concentration of dissolved oxygen was fixed using 1ml each of Maganese Chloride and Alkaline Iodide. The glass bottles were stored under water to maintain air-tight conditions until the time of lab analysis. 100ml of seawater was collected at 4 stations (station 1, 8, 9 & 16) and preserved in bottles containing Lugols Iodine for phytoplankton lab analysis. Two zooplankton net trawls were carried out over 3 minutes Results Analysis of estuarine mixing diagrams. The nutrients sampled include nitrate, phosphate and silicate and are found in high concentrations in riverine water compared to seawater. To analyse the abundance of these nutrients the Theoretical Dilution Line (TDL) was plotted using riverine and marine end-members. Very low salinities were unable to be collected as the RIBs were unable to reach zero salinity as the river became too shallow. The assumptions upon which the TDL is based are: -that the estuarine system is in a steady state (not true as estuarine environments are dynamic and as such are continually changing) -that end-member concentrations remain constant -that there are no additional sources to the water column e.g. pore water from sediments. Nitrate Nitrate was found to be lower in concentration in the lower reaches of the Fal estuary. This was shown to increase as the salinity of the estuary decreased. The behavior of the nitrate in the estuary was shown to act in a non conservative manner as the majority of the points fell below the TDL. To gain a better understanding of the nitrate distribution within the estuary, it would be necessary to collect more samples as there were none collected at salinities lower than 20 psu. A possible reason for the decrease in the nitrate concentration with in the Fal may be due to the increasing distance from the sources of nitrates. The removal of the nitrate in the estuary is also likely to be caused by the increase in phytoplankton numbers as the estuary becomes more saline (due to most of them being marine species). This would indicate that the phytoplankton are utilizing the nitrate in the lower reaches of the Fal and would explain the non conservative behavior shown on the mixing diagram. Phosphate The behavior of phosphate within the Fal estuary and Truro River show that it acts in a non conservative manner and both addition and removal occur throughout the estuary. The additions of phosphate are likely to originate from inputs from sewage, agriculture and urban run-off from the extensive farmland and towns around the Fal estuary. These additions are shown to occur throughout the estuary. The phosphate is likely to have been utilized by the phytoplankton in the water column and so is likely to be the explanation for the removal shown on the graph. It would also explain why the removal occurs in greater concentrations in the lower reaches of the estuary. Silicate The concentration of silicate in the estuary is a demonstration of non conservative mixing with both addition and removal occurring throughout the estuary. The addition of silicate to the water column is likely to have been caused by areas of sewage effluent input. The removal of the silicate within the estuary is likely to be caused by the biological removal of silicate by diatoms, used to form tests and frustules. This is a likely cause as the water samples collected showed abundance in diatoms throughout the estuary. The presence of the mussel farm may also have an effect on the concentrations of nutrients found within the estuary.
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Aims To assess how the physical and chemical properties in offshore waters off Falmouth affect the structure and functional properties of plankton communities, paying particular attention to the structure of the water column and the development of the thermocline and front in the Western English Channel. Methods The position of the furthest station was pre-planned. Other stations were sampled using the ADCP, CTD with attached flurometer, light sensor and transmissometer, as well as zooplankton nets. Water samples were collected in Niskin bottles attached to the CTD rosette. Calibration of the CTD was carried out at Mylor dock before departure and station 1 was taken at Black rock at the mouth of the estuary for an equipment check and for sampling continuity.
CTD Analysis
Station 1 at the mouth of the
estuary (black rock), showed a gentle thermocline between 6-8m with
temperature constant at 15.5°C above and 14.3°C below this depth.
Flurometer readings reveal a chlorophyll maximum of about 2.52 volts
just below the thermocline. As light is not a limiting factor due to the
relatively shallow water column, phytoplankton species are able to
establish themselves lower down where there is greater availability of
nutrients. This is supported by the similar trend in fluorescence at
station 2 and 3. Salinity is fairly constant throughout the shallow
water column as there are few freshwater inputs at this point.
Richardson Numbers
Station 1 – Black Rock At station 1 the Richardson number (Ri) indicates that above and below the thermocline the water column is well mixed and over the thermocline we see several stratified and stable areas where there is likely to be a large amount of biological activity due to the density change giving ideal conditions.
Station 3 – Gerrans Bay This station shows a sharp increase Ri at just over 6m metres despite there being no clear thermocline. This means that there is still a density change where Ri increases but is likely due to a combination of factors such as salinity and surface and deep water flowing in opposing directions. Opposing flow leads to Kelvin-Helmholtz disturbances at the interface which are directly related to the Ri values. Course Conclusion ESTUARINE
OFFSHORE
Over the 14 days we have learnt and improved upon many new skills and scientific techniques, such as team building skills, scientific write ups and using new software, for example WinRiver. Fortunately we found we got on well together as a group, and managed to overcome many obstacles, such as losing data, hangovers and staying on board the boats! We liked exploring the setting of the Fal estuary, and we recorded new findings in a previously undiscovered area. Although the course was hard work and with early starts we enjoyed ourselves. We would like to thank Brian Dickie, our tutor, for his guidance and assistance in labs, Simon Boxall for coordinating the course, the boat crew for all their help with boat practicals, and the rest of the staff for all their hard work at making our work possible.
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All views and opinions that are expressed on this website are entirely that of group not necessarily representative of the views and opinions of the university. Any anomalous data is a representation of the dom factor.