Jimmy Willcox

Timothy Hore

Kerrith Barrington-Cook

Hannah Young

Jennifer Fookes

Sacha Neill

Julie Knott

Edward Westwood

Danny Devereaux

Anna Cunnington

Introduction & Aims

Background

Falmouth Estuary, located on the SW coast of the UK, has a significant biological and socio-economic importance which has resulted in its recent European designation as a Special Area of Conservation (SAC). Made up of large shallow inlets, bays, saltmarshes, intertidal mudflats and sub-tidal sandbanks, the Fal’s physical diversity supports a myriad of marine plants and animals such as the rare Shore Dock, Rumex rupestris and maerl beds (http://www.jncc.gov.uk/protectedsites/sacselection/sac.asp?EUCode=UK0013112).

Human pressures however, particularly from tin and copper mining within the Fal catchment, has put huge environmental pressures on the area since the 18^th Century and has resulted in significant changes in the chemical and biological make-up of the region (Langston et al., 2003). Increased partnerships between industry and environmental organisation such as Natural England have allowed more comprehensive management of the region however, despite this Falmouth Estuary remains the most polluted estuary in the UK.

Aims

The aim of this research is to create a multidisciplinary snapshot of the Falmouth Estuary extending from the creeks which feed the system to the offshore marine environment.  Biological, chemical, physical, and geological data will be collected over the two weeks to generate an inclusive overview of the system allowing investigations to be carried out on the interactions within the system. This data can be used in conjunction with previous year’s research carried out by Southampton University as well as long-term data collected by Plymouth Marine Laboratories to enhance our understanding of coastal processes with the Western English Channel.

Research Vessels

Equipment

CTD + CTD Rosette

Secchi Disc

A CTD- Conductivity, Temperature, Depth profiler is used to investigate the vertical structure of the water column. It works by taking continuous recordings of salinity, temperature and depth as it is lowered through the water. Salinity is measured through the conductivity of the water passing between two charged plates. The CTD is mounted onto a Rosette together with a fluorometer and Niskin bottles which could be closed electronically using a messenger at selected depths.

The metal disk is lowered through the water by hand, the depth at which the black and white colours can no longer be distinguished is known as the secchi depth. The attenuation coefficient and subsequently the depth of the Euphotic zone can then be estimated at almost 3 times the secchi depth.

This net is used to collect Phytoplankton samples from predetermined depths through the water column using a 200micron mesh. Sample containers attached to the bottom of the nets can be closed on ascent using a messenger.

The ADCP (acoustic Doppler Current Profiler)  is used to measure how fast water is moving across an entire water column.  Current speeds are measured using the Doppler effect which operates through the transmission of pings of sound at a constant frequency. As sound waves travel they ricochet off particles in the water column and reflect back to the transducer which records the change in frequency of the returning soundwave.  The change in frequency is called the Doppler shift and it is used to calculate how fast the particle and the water surrounding it are moving. 

This net is predominately used for surface trawls. Two different mesh sizes are used enabling collection of both zooplankton and phytoplankton samples. Zooplankton samples are collected using a 200micron net and 100 microns is used to collect phytoplankton samples. Specimens are collected into two plastic containers attached to the bottom of the nets, ready for analysis once back onboard.

Niskin Bottles

The T/S probe contains as electrical component called a thermistor, which is sensitive to changes in the resistance of an electrical current. It is this part of the instrument which measures temperature.

Salinity is a measure of the conductivity of salt particles and is carried out by two metal plates (electrodes) within the T/S probe. When an electrical current is passed between these plates within a water sample, the ease in which the current flows is an indication of conductivity.

Sidescan sonars, commonly known as a ‘towfish’ can be used to generate large images of the sea floor known as a sidescan trace. Towed behind the research vessel at an approximate depth of 1m, a sound pulse is emitted simultaneously from two transducers either side of the towfish. When this pulse reaches the seafloor, it is then reflected and the degree of reflection is dependant on the density of the material reflecting the pulse as well as the slope of the seafloor. The sound pulse is transmitted at a frequency of 100-500 kHz indicating a long wavelength but a relatively low resolution.

This is a metal grabbing device which can collect sediment samples from the seabed. Deployed from the back of the research vessel at designated points along the sidescan transects, the samples provide evidence of seabed composition which can support that observed on the sidescan traces. They also provide samples of benthic fauna which can be indicators of particle size, organic content and pollution.

A waterproof video camera was deployed prior to the collection of sediment grab samples to observe the seabed and ensure the site was suitable to take a sediment sample i.e. the seabed was not just made up of hard rock or there was no Seagrass present which we are not permitted to grab. 

The plastic bottles are deployed through the water column using a hydroline to a pre-determined depth. They can then be closed using a messenger sent down the line from on board. The water sample is then brought back on board and can be analysed for dissolved oxygen and nutrient content. These bottles can also be attached to a CTD Rosette and closed electronically.

Chemical Analysis

Determination of Dissolved Silicon.
The samples collected were chilled and stored in plastic bottles to minimise bacterial activity. Dilute standards were prepared using stock silicon solution and Milli-Q (MQ) water and contain silicon at 1.4, 2.8, 7.1, 14.2 and 21.4 µmol/ L. Molybdate Reagent was used and a mixed reducing reagent was made up using 20 ml metol sulphite, 12 ml oxalic acid, 12 ml sulphuric acid (50% v/v) and 16 ml MQ water. Once added, these solutions were left to stand for colour development and analysed using a spectrophotometer. A calibration curve was used to determine silicon concentration. The methods followed are as per Parsons et al (1984).

Determination of Phosphate.
 The working standard was prepared so that it contained 15µmol phosphate/L using the stock standard solution and MQ water. The standards were set up in triplicate in the following volumes 0(blank), 50, 100, 200, 500, 1000 µl. The samples were made up to 10 ml using MQ water and the concentrations were as follows: 0.07, 0.15, 0.3, 0.75, 1.5 µmol. The mixed reagent was prepared with the following volumes: 20 ml ammonium molybdate, 50 ml sulphuric acid, 20 ml ascorbic acid and 10 ml potassium antimoyl titrate. 1 ml of the mixed reagent was added to each of the samples and standards and blanks. The absorbance of each of the samples, standards and blanks was measured using a spectrophotometer.  The methods followed are as per Parsons et al (1984).

Determination of Chlorophyll.
The samples were refrigerated overnight in acetone, there was no requirement to sonicate or centrifuge the samples. The contents were transferred to a glass tube and placed in the fluorometer. This takes as average reading of the chlorophyll in the sample.

Nitrate Analysis.
The Nitrate analysis of the water samples was conducted through the use of the flow injection analysis method.  This method typically involves the use of a pump, an injection valve, connecting tubing, and flow through detector. Eleven samples were measured by injecting 1 ml of the sample into the injection valve and then waiting approximately 2.5 minutes for the concentrations to appear as a peak on the plotter.  (Johnson and Petty 1983).

Oxygen analysis.
Oxygen samples were taken from Niskin bottles at varying depths at stations 1 through 4.  Samples were handled with precaution to ensure no bubbles contaminated the sample. The samples were then stored in water until oxygen content analysis. Oxygen content analysis was determined using the Winkler method (Grasshoff et al 1999).

Offshore

A total of three stations were fully sampled (1, 2 & 3), then a CTD drop and ADCP profile were carried out at a further 3 stations (4, 5 & 6). The first station was carried out near Black Rock to check all the equipment was working correctly. We then proceeded 45 minutes out on a bearing of 120° to station 2. We then continued a further 45 minutes on the same bearing to station 3. Time constraints and a temperature change of 1°C during sampling at station 2 and a 0.2°C temperature increase indicated by the ADCP suggested that the front may be between station 1 & 2 where the water depth decreases. Therefore station 4 was carried out approximately half way between these stations. CTD and ADCP data indicated we were not quite on the front therefore station 5 was carried out 2nm towards station 1 on a bearing of 240°. Computing errors resulted in a loss of the CTD data from station 4 therefore we returned to station 4 to repeat the CTD profile (4b).

Station Location
Station 1:             050°08.542’N 005°01.450’W    CTD, ADCP, Zooplankton & Water samples
Station 2:             050°03.945’N 004° 50.471’W   CTD, ADCP, Zooplankton & Water samples
Station 3:             049°59. 871’N 004°38°641’W   CTD, ADCP, Zooplankton & Water samples
Station 4:             050° 05.418’N 004°54.341’W    CTD, ADCP & Water samples
Station 5:             050° 06.723’N 004°58.187’W    CTD & ADCP
Station 6 (4b):   050° 06.232’N 004° 56.952’W   CTD & ADCP

                                           CTD & Nutrients

Station 1 (fig 4:1):
Surface waters down to 2m are well mixed with a temperature of 16.8°C. There, is a slight thermocline from 2-4m where the temperature decreased to 16.3°C and from a depth of 4m, the water column becomes relatively well mixed. The fluorometer indicates a chlorophyll maximum below the thermocline at a depth of 8m. Surface chlorophyll levels from the discrete water samples had a concentration of 1.51mg/L however levels just below the chlorophyll maximum were 0.86mg/L. Nitrate levels at the surface were completely depleted whilst phosphate levels were higher at 1.55m (0.09mg/L) than at 11m (0.07mg/L). Silicon levels however increased below the chlorophyll maxima from 0.92mg/L to 1.0mg/L. Oxygen levels were highest at 1.55m with concentrations of 108% and decreased to 103% at 11m.

Station 2 (fig 4:2):
Station 2 had a very strong thermocline with a temperature difference of 4.0°C from 20-25m. The fluorometer indicates a chlorophyll maximum just below the thermocline at 30m. Chlorophyll levels from the discrete water samples also showed chlorophyll maximum at 33.1m (11.32mg/L) and levels were lowest at 66m with a value of 0.89mg/L. At a depth of 33.1m, nitrate and phosphate levels were low with values of 0.43 & 0.14mg/L however silicon concentrations were relatively high (2.7µg/L).  Surface waters had the lowest nutrient concentrations with values of 0.35, 0.6 & 0.07mg/L whilst nutrient concentrations below the thermocline at 66m where were nutrient concentrations were highest.

Station 2 Repeat (fig 4:3):
The temperature within the top 3m decreased by 0.83°C indicating the presence of a surface thermocline which was thought to be caused by Langmuir circulation which creates areas of surface convergence due to wind forcing (Langmuir, 1938). The fluorometer also showed a double chlorophyll maximum with a small peak at 15m and then a larger one at 32m. The discreet water samples also showed a maximum chlorophyll value of 11.23µg/L at 32m with chlorophyll values of only 1.46µg/L at 15m. However oxygen concentrations were low at 32m (83.3%) and decreased to 75.7% at 66m. An oxygen maximum was observed at 15m with a value of 123%.

Station 3 (fig 4:4):
Station 3 has a very interesting temperature profile with a double thermocline made up of a surface and seasonal thermocline. Temperature decreased by…..There are also two very distinct chlorophyll maximums, one at 15m and the second at 34.5m. Nitrate concentrations were lowest at 21.75m (0.48µg/L), just below the first chlorophyll maximum and very similar concentration were observed at 4.99m and 61m (0.61µg/L & 0.65µg/L). Silicon and phosphate concentrations increased with depth from 0.1µg/L & 0.01µg/L at the surface to 2.5µg/L & 0.2µg/L. Oxygen concentrations however are highest at 21.75m with a value of 126.6%

Station 4 (fig 4:5):
The temperature profile at station 4 shows surface waters to be relatively well mixed with slight stratification at a depth of 5m. From 5m there is evidence of a relatively weak seasonal thermocline with a temperature change of 4.7°C from 15m to 30m. The fluorometer indicates a chlorophyll maximum at 20m and the discrete water samples indicate a very good degree of accuracy with a chlorophyll maximum of 5.66µg/L at 15.5m whilst there is a marked decrease at 21.75m. Chlorophyll levels remained high down to a depth of 35m. Nitrate and phosphate levels are severely depleted at 15.5m with values of 0.61µg/L & 0.02µg/L, whilst oxygen levels are highest with at 131.6% at 15.5m and lowest at 35m (89%).

Station 5 (4:6):
The temperature profile for station 5 shows well mixed surface waters down to a depth of 5m. A fairly weak thermocline is present between the depths of 5m and 25m with a temperature difference of 2.5˚C. Waters become well mixed again between the depths of 26m and 34m and again from 35m. The fluorometer showed a chlorophyll maximum at a depth of 20m. Chlorophyll levels remain high until 26m where there is a rapid decrease of 0.19 volts within 0.1m.

Station 1: gr1000r

High levels of backscatter were recorded at the surface in the top 5m and from a depth of 6m the degree of backscatter decreased to <73dB. Station 1 is located in a channel within the mouth of the estuary therefore is bounded by regions of shallower water (<10m). Chlorophyll levels are relatively low within the surface waters with values less than 1.51µg/L (fig 4:1) however within 8-4m zooplankton number have values up to 6x10⁷ (fig 6:4). Therefore zooplankton abundance is the likely cause of high levels of backscatter and low phytoplankton abundance suggests that grazing rates are high. Although spring tides do not occur until July 7^th (Featherstone & Du Port, 2009), low nitrate levels (fig 4:1) indicate that we have surveyed post a phytoplankton bloom with the mouth of the Estuary.

Flow velocities are relatively low at station 1 although flow is slightly enhanced at the surface with values reaching >0.25m/s.

During sampling at station 1 the boat drifted therefore we repositioned ourselves before carrying out the zooplankton samples. This can be observed on the track plot.

Station 2-gr1003r

Station 2 shows two shows two distinct bands of backscatter at 23.6m and 32.11m. This shows a relatively good correlation to the CTD data taken from this station (fig 4:2) and particularly to the second CTD profile taken here (fig 4:3). Phytoplankton concentrations here were significantly greater than station 1 and predominately dominated by diatoms however zooplankton numbers were also high at the surface and down to a depth of 30m with surface values reaching 9.5x10⁷ m^-3 (fig. 6:4). The high degree of backscatter is therefore likely to be caused by both diatoms which posses hard silicon skeletons (Miller, 2004) and zooplankton such as hydromedusae which were the most dominant zooplankton at 0m.

The velocity magnitude of flow indicates a band of high velocities up 0.37m/s from a depth of 20m to 50m. Flows at the surface and the seafloor are lower with minimum flow values of 0.1m³/s. This region of faster flow corresponds to the depth of both the chlorophyll maxima and the thermocline.

Station 3-gr1006r

At station 3, the two bands of backscatter become more distinct and occur at depths of 9m and 25m. Although the fluorometer readings from the CTD data demonstrates a similar pattern (fig 4:4), the depths of the two chlorophyll maximums are slightly different with the upper maximum occurring at 15m and the lower maximum occurring at 35m. A zooplankton trawl was carried out to try to determine whether the low chlorophyll values between these two maximums were a result of grazing however zooplankton abundance from 20-20m had values of only 1.2x10⁷ m^-3 (fig. 6.4). This is the second lowest zooplankton abundance value from all four stations. Chlorophyll concentrations from the discrete water sample taken at 21.75m, just below the first chlorophyll maximum record by the fluorometer indicates phytoplankton abundance is still high therefore an error may have occurred with the fluorometer during sampling.

During station 3, the velocity magnitudes appeared to change considerably with relatively high values of 0.3m/s at the beginning throughout the entire water column.  The average velocities then decreased to 0.27m³/s. Surface values to a depth of 7.6m are consistently low however, at the seabed velocities of flow reach 0.96m³/s.

Station 4-gr1008r

There is a very strong band of backscatter from 30-35m which is slightly deeper than the chlorophyll maxima observed on the CTD profile. However the discrete water samples taken at a 34m depth showed consistently high chlorophyll values of 5.3µg/L (fig.4:4). Additionally, the phytoplankton counts taken at 35m recorded the highest total abundances of all the samples taken with concentrations of 21204 per 100ml (fig. 6:3). The dominant phytoplankton at this depth were dinoflagellates however, high abundance here are surprising as the depth of the euphotic zone calculated using the secchi disk was only 23.1m. This indicates that mixing must be occurring pushing phytoplankton below the euphotic zone. Unfortunately no zooplankton samples were taken at station 4.

In contrast to station 2 where velocities where higher mid water column, the velocities at station 4 appear to be highest at the surface and seafloor with velocity values of up to 0.32m³/s. In contrast the lowest velocity values were recorded at a depth of 33m with a value of 0.03m/s. This data does not support above explanation of high phytoplankton concentrations below the euphotic zone and weak thermocline.

The Richardson number is a measure of the flow stability of a body of water. It is calculated by comparing the densities and velocities of different ‘sections’ within the water column, using the following equation: 

  At all stations, the Richardson numbers were calculated every half metre throughout the depth of the water column. With the exception of a few anomalies, the Richardson numbers (Ri) along the thermoclines were greater than 0.25 indicating that there is little or no mixing occurring and stratification within these regions is strong. Ri values below 0.25 are indicators of mixing and occur in regions where the temperature profile is vertical or near vertical and the temperature is uniform with depth. This pattern is complicated at station 3 where there are internal processes occurring creating a double thermocline.  Additionally at station 4 (fig 4:5) within a well mixed region down to a depth of 15m, the Richardson numbers indicate that there is both areas of stability and instability. Whilst it was initially thought to be caused by very low velocity values, analysis of the ADCP data indicates that this was not the case. Sampling at station 4 was carried out at 1439 (GMT), three hours before high water therefore flooding waters of speeds up to 0.4m/s were observed at the station. In conclusion a more precise comparison of ADCP data and Richardson values is required for a more comprehensive analysis of vertical stability profiles.

Samples for phytoplankton were collected at stations 2, 3 and 4. Station 2 was a surface trawl sample and at stations 3 and 4 various depths were sampled. The cell count of diatoms may appear smaller than the actual amount in the water column, as only chains of diatoms were counted rather than individual cells. 

Diatoms dominated surface waters at station 3. Out of the 5850 diatom cell counted in the sample, 4700 were Chaetoceros. Both dinoflagellates and flagellates were also recorded in smaller numbers. At station 3, dinoflagellates dominated, the most abundant species being Karenia mikimotoi. Karenia mikimotoi is usually found in calm stratified waters and are relatively small in size so do not easily sink through the water column. Two thermoclines, recorded at around 20m and 30m indicate highly stratified waters. There were also two chlorophyll maximums recorded for this station at depths of 15m and 40m, with a significant depletion at around 20m. Diatom numbers increased slightly with depth, with highest numbers recorded at 61m. As diatoms are fairly heavy they do fall out of the water column, particularly in stratified waters. Although higher numbers of diatoms were found at 61m, species diversity at 22m was highest with 7 genus groups counted. Station 4 was dominated by dinoflagellates, highest numbers were recorded at a depth of 35m, this abundance is predominately Karenia mikimotoi. Although diatoms often dominate coastal waters, dinoflagellates will become more predominant with distance from land (Pinet 2006).

Flagellates were present in shallow depths and again at depth, but none were recorded at 22m. At this depth there was an overall lack of phytoplankton compared to other stations and depths with only 1850 cells counted. Generally Karenia mikimotoi was the most abundant phytoplankton group at each depth for stations 3 and 4.

Zooplankton

Zooplankton samples were collected from stations 1, 2 and 3.  Samples were collected using niskin bottles deployed at various depths. From figure 6:4 it is evident that the organism abundance is highest in the samples collected from the surface. The maximum total zooplankton abundance was observed at the surface at station 2, this showed a value of 9.5x10⁷ m^-3.  At each station zooplankton abundance reduces with depth, the lowest total zooplankton abundance was found at station 1 from the 8-4m sample at a value of 6.8x10⁶ m^-3.   

Hydromedusae are the most abundant group at each depth, at each station, with greatest abundance at the surface at station 2 of 4.78x10⁷ m^-3. Copepoda and copepod nauplii grouped together make up the second largest section in samples from stations 2 and 3.  In the 0m sample from station 1 Echinoderms were recorded as the second largest zooplankton group.  The surface waters from station 2 also showed Echinoderm to be prevalent.

The high levels of zooplankton were found in surface waters as this is where food resources such as plankton and other zooplankton are most abundant. However this is not reflective of the phytoplankton data which shows relatively low abundance at 0m for station 2 and 5m for station 3.

The data shows that the water column is dominated by hydromedusae. This may be put down to patchiness as a result of zooplankton behaviour such as vertical migration and swarming.  A wide range of stirring and mixing methods can interact with biological processes to produce plankton patchiness (Abraham, 1997). However, the high numbers of hydromedusae witnessed may be a reflection of the seasonal variation.  All hydromedusae larvae are released at the same time of year, it is likely that this is what is being observed.

Prior to surveying on 29/06/2009, the SW coast of England had been experiencing anticyclonic conditions (http://www.metcheck.com/V40/UK/FREE/synoptic.asp). Low wind speeds and high temperatures associated with these weather conditions can result in reduced mixing leading to a shallowing of the mixed layer as stratification within the water column increases and the seasonal thermocline decreases in depth. This was particularly observed at station 1 where a slight thermocline was recorded at a depth of 2-4m (fig 4:1). With a euphotic depth of 27m and a water column depth of 18m, phytoplankton should be able to photosynthesise within the full depth of the water column however chlorophyll levels were relatively low throughout. Nutrient concentrations, particularly nitrogen were very low within the surface waters indicating that primary production at the mouth of the estuary was nutrient limited. 

Further offshore, the thermocline became more pronounced and at station 2 had increased from 4m to 20m (fig 4:2). The temperature profile at station 2 indicates very stratified surface waters and by station 3, a double thermocline has developed. A TS diagram was constructed to determine whether this physical feature was a result of mixing of two different water masses however, Figure 7:1 concluded that this was not the cause of the observed temperature ‘step’. An alternative explanation could be that alternating regions of stratification and mixing within the water column could force a temperature profile like that observed in figure 4:4. Group 9 who carried out the same survey also observed an unusual temperature profile at station 3 which could represent the onset of a double thermocline. Our data was recorded during very warm, calm weather when small internal density differences were the dominant mixing process. Group 9 however, carried out the exact same station 4 days after a storm event when wind mixing had completely overturned the water column and the temperature profile was only just re-establishing itself. There could be a number of reasons for such an unusual temperature profile and would a very interesting topic to investigate further. 

Station 3 also had a very interesting double peak in the fluorescence data indicating two bands of phytoplankton. This would be most easily explained by a band of zooplankton between the two phytoplankton peaks where grazing had reduced the number of phytoplankton however, our data shows that the zooplankton levels at this depth are very low (fig 6:4). If the temperature profile is in fact a result of two water masses coming together this double peak could be a result of one water mass containing high numbers of phytoplankton and the other containing low levels. 

Station 4 was carried out in between station 1 & 3 to attempt to identify the location of the front. We were unsuccessful in this aim as we would have expected to see an outcrop of the thermocline at the surface yet the surface waters were still relatively well mixed down to a depth of 5m. We did nevertheless observe a very good transition profile from station 1 to station 2.

Geophysics

Introduction
Aim: To investigate the bathymetric features and benthic habitats between Pendennis Point and Black Rock within the Fal Estuary using side scan sonar and grab samples.

Method: Four transects were followed from NW to SE spread 100m a part. Sidescan sonar was used to create a bathymetric image of the seafloor allowing observations of any geophysical features. A Van da Veen grab was then used to collect samples at four chosen sites to investigate further any features and analyse benthic ecology identified along the transects. An underwater video camera was also used to view the bottom composition in situ.

Samples were collected on 03/07/09 on the Survey Support Vessel Xplorer between 07:00 and 13:00 hours GMT.

Weather: Fine, Sunny, 6 Oktas
Sea State: Slight to Moderate
Wind Speed: 8 knots – 24 knots

                Tides: (GMT) Low Tide: 01:47 4.326m
                                                       14:23    4.420m
                                       High Tide: 08:17    1.914
                                                       20:51    1.952m

                                                   Side Scan Sonar

Transect 1
Depth: 6.4m
Start Time: (GMT) 08:46           End Time: (GMT) 08:57
Start Position: 50°08.653’N       End Position: 50°09.505’N
                          05°02.281’W                               05°02.663’W

Transect 2
Depth: 6.8m
Start Time: (GMT) 09:01             End Time: (GMT) 09.14
Start Position: 50°09.147’N        End Position: 50°08.646’N
                          05°02.658’W                                05°02.182’

Transect 3
Depth: 7.4m
Start Time: (GMT) 09:17                End Time: (GMT) 09:28
Start Position: 50°08.696’N           End Position: 50°08.562’N
                          05°02.125’W                                   05°02.608’W

Transect 4
Depth: 7.7m
Start Time: (GMT) 09:31                End Time: (GMT) 09:43
Start Position: 50°09.534’N           End Position: 50°09.755’N
                          05°02.301’W                                   05°02.084’

                                                    Van Veen Grabs

GRAB 1

Transect: 1
Time: 1032 GMT
Location: 50˚ 08.701’N, 005˚ 02.294’W
Depth: 7.2m

Sediment: Poorly sorted. Predominately gravel            (2-4mm) & shell fragments with some large stones      (64-256mm) (Wentworth, 1922).

Benthic Fauna:

• Encrusting-
  • Barnacles
  • Mollusc
  • Algae
  • Membraniporidae (Bryzoan)
• Chiton (Mollusk)
• Brown Macroalgae

GRAB 2

Transect: 3
Time: 1103 GMT
Location: 50˚ 08.879’N, 005˚ 02. 073’W
Depth: 10.3m

Sediment: No sediment collected in grab indicating         a hard rocky substratum.  

Benthic Fauna:

• Brown Macroalgae

GRAB 3a

Transect: 2
Time: 1112 GMT
Location: 50˚ 09.154’N, 005˚ 02. 504’W
Depth: 8.2m

Sediment: Poorly sorted. Predominately small gravel particles and shell fragments.

Benthic Fauna:

• Nucella
• Maerl- 90% living, 10% dead
• Shrimp
• Ampluexius
• Unidentified Amphipod
• Topshell

Estuary

                                           CTD & Nutrients

Station 1 (fig: 10:1) 
The temperature and salinity readings from station 1 were taken furthest up the estuary by RV Xplorer, West of Turnaware Point.  A thermocline was apparent from depths of about 2m to 6m where the temperature decreased from 15.7° C to 14.2° C.  From this depth the temperature continued to decrease to 13.5°C at near the seafloor at around 17m.  The salinity collected from station 1 was 34.2 at the surface which increased with depth to approximately 35.3.  This indicates that a layer of riverine water was overlying a deeper, denser and more saline body of water.  Fluorescence remains near constant, decreasing slightly with depth.  The surface of the water mass appears to be well mixed to a depth of 5m.

Station 2 (fig: 10:2)
Station 2 was located south of Messack navigation buoy, off Messack Point.  The data from this station shows that the water column is well mixed to a depth of 16m. Chlorophyll concentrations from the discrete water samples appear to decrease with depth, indicating that there are higher numbers of phytoplankton at the surface, utilising higher light levels for primary production. Oxygen levels at the surface are lower than expected (96.5%). At 10m, oxygen is much higher (103.1%) and decrease again at 20m depth (96.9%). This could be interpreted as anomalies as high chlorophyll levels would indicate high levels of primary production and therefore high levels of oxygen.   

Station 3 (fig: 10:3)
Station 3 was the located to the west of Carclase Point.  The data shows a strongly stratified water column. There is a well defined thermocline at 6m with a decrease in temperature from 16°C to 13.8°C.  The salinity plot shows a sharp halocline at 6m, indicating riverine water overlying saline water. Chlorophyll data indicates higher levels at the surface of the water in comparison with depth, again, indicating high numbers of phytoplankton within the euphotic zone.

Station 4 (fig: 10.4)
 Station 4 was located North East of Falmouth Docks, the salinity is almost homogenous down the water column however vertical mixing is inhibited by a weak thermocline of only 0.5°C between 10m and 20m. Despite the fact that phosphate, chlorophyll and oxygen are higher with depth the euphotic zone is only 17.25m indicating that these nutrients are not being utilised by phytoplankton.

Station 5 (fig: 10:5)
Station 5 was located due East of Black Rock, at the mouth of the estuary. Mixing occurs within the water column to a depth of 20m. It would appear as though there is a weak thermocline at 21m with the temperature changing by 0.3°C, however, there is also a very weak halocline at the same depth. This will prevent mixing below 21m. There is a small increase in fluorescence at 20m which may indicate a bloom in phytoplankton activity. Oxygen levels are higher at the surface than at mid or deep depths, however, chlorophyll is not particularly high at the surface and continues to increase with depth. There is not an obvious explanation for this, although it is possibly related to a lack of mixing bringing up nutrients from depth, it could also be explained by diel vertical migration.  

The TS contour plots indicate that the highest temperatures are observed within the upper reaches of the estuary were water depth is low. The deep central channel within the estuary can therefore be observed on temperature contour. Conversely, salinity values are lowest in the upper reaches and according to the salinity contour plot, there appears to be two regions of high salinity. It is important to note that the resolution of the TS probe is only 0.1. On closer examination, these regions occur where a large number of samples were taken during the CTD station however, there looks to be a plume of fresh water coming down the estuary. Although the data was collected during a flooding tide, sampling was carried out extremely close to the entrance of Truro River therefore there maybe a surface flow of freshwater although this can not be identified within the ADCP data for transect 1.

Residence Times: 

                  Where:
                   S_mean= Mean salinity
                   S_sea= Salinity of Seawater
                   V_total= Total volume of Estuary (m³)
                   R= River flow rate (m³s^-1) 

                                                   = 13.04 days

Tidal Flushing:

Where:
V_estuary= volume of estuary (m³)
 T_tidal= Time of tidal cycle (Time difference of 1 tidal cycle)             

           = 17.7 hours

Tidal Prism:

 Where:
 A_estuary= Area of the estuary (m)
 h= tidal range (HW-LW)

V_prism= 24,820,000 x 3.5

          = 86870000m2

Assumptions & Considerations:

• Mean salinity is the average of all the salinity values collected onboard Xplorer, the ribs and those taken from the pontoon.
• Sea salinity is the highest salinity value recorded within the estuary.
• No riverine flow rates were sampled during this investigation therefore the river flow rate from 2001 has been used as the conditions in which this value was taken are most similar to the weather conditions experienced over the past couple of weeks.
• The river flow rate is only known for the Fal River therefore this value (0.723m³s^-1) was multiplied by three to include the River Perculi and the River Carnon.
• We are assuming conservative behaviour.

These calculations are based on a number of assumptions and are therefore only rough indications of residence and flushing times. This is evident from the disparity between the two values and therefore can only be considered as approximate values for the Fal estuary. The residence time calculated using river flow rates is expected to be a larger value than that calculated using the tidal prism method because the Fal estuary is predominately a tidal dominated estuary.

Transect 1 & 2

                Transects 1 & 2 were both carried out at the bottom of the Truro River which feeds into the Fal estuarine basin however transect 1 included the output from Pill creek. The maximum depth along this transect was 13.61m and the flow throughout the water column was unidirectional in a northwestly direction. This is indicative of a flooding tide and therefore water flow along transect 2 was also in a NW direction. Along transect 1 however there is a slight reversal of flow along the western boundary of the estuary where riverine inputs with a maximum flow of 0.031m/s from Pill Creek are disrupting the flow. The highest velocity recorded along transect 1 was 0.513m/s whereas the maximum flow along transect 2 was only 0.493m/s. The highest velocities were observed in the deeper regions and decreased towards the shallower areas at the edges of the estuary. Transects 1 was carried out on slight bend in the channel therefore higher velocities were observed on the outside of the bend. It is important to note that the ADCP transect carried out at station 1 was recorded the wrong way round and proceeds right to left instead of left to right (West to East). 

Transect 3

                Transect 3 was also completed during the incoming tide however the maximum velocity along this transect was greater than transect 1 & 2 with a value of 0.568m/s. An ebbing flow was also observed along the western side of the estuary due to the riverine inputs from the Carnon River which flows through Restronguet Creek. Flows speeds here were found to be approximately 0.187m/s. Additionally the velocity maximum occurred within the water column at a depth of 13.61m and surface velocities were around 0.286m/s.

Transect 4

                The ADCP data shows that along transect 4, the main tidal flow is within the deep channel with a maximum of 4.94m/s. This region of high velocity, unlike transect 3, extends from the surface down to the seabed and although velocities were greatest at a depth of approximately 19.6m, the velocities at the surface were still comparatively high with values of 0.492m/s. Either side of the deep central channel there are shallower regions with an average depth of 5.6m and along the western boundary of the estuary a reversal of flow is observed and at a depth of 5.6m reached a velocity of 0.254m/s.

Transect 5

                The data for transect 5 was collected the following day (08/07/09) by another group because the ADCP data file collected on the 07/07/09 became corrupted during transect 5. This repeat transect was carried out at 1425 (GMT), during a flooding tide therefore a similar profile was expected. The ADCP profile showed two distinct regions were velocities were higher. The first was within the deep estuarine channel; surface velocity values here reach 0.566m/s. A second region was found just north of Black Rock with velocities of 0.6m/s. This region which lies to the west of the main estuarine channel could be a result of eddying within the estuary and the presence of black rock at the mouth of the estuary could enhance this. Additionally high riverine inputs of fresh water from Percuil River could also result in instabilities and mixing at the mouth of the estuary. Although velocities decrease towards the edge of the estuary where the water depth decreases, higher velocities were still observed along the western margin of the estuary. Flow direction also reversed below a depth of 6.60m with water flowing out of the estuary at the margin. Again this could be a result of lagging tides or the influence of Black Rock.

Bottle samples 67, 81 and 104 were taken from the upper end of the Falmouth Estuary within the River Truro. Bottle samples 21 and 23 were taken from the lower and of the Falmouth Estuary with salinities decreasing respectively. Generally diatoms are the dominant phytoplankton group within the sampled area, dinoflagellates are present at all sampling points but often numbers were significantly smaller. 

Bottle sample 67 was taken from the lowest salinity point; here diatom cells dominate the phytoplankton population within the surface water. Cell count for diatoms was recorded at 19818 calls per 100ml, compared to only 10 cells per 100ml for dinoflagellates. 

Sample 81 was taken from a stationary pontoon downstream of the first sampling point. Diatom cell numbers increased from the previous station to 24228 cells per 100ml, dinoflagellate numbers also increased to 263 cells per 100ml. This is the maximum cell count for diatoms, subsequently cell count decreased with rising salinity values from this sampling point. This maximum could be linked to the high values of silica recorded within this area, as silica is utilised in the production of frustules in diatoms. Dinoflagellate numbers increased slightly at sampling point 21; here values reached a maximum of 657 cells per 100ml. This could be explained due to changes is phytoplankton dominance. Diatoms can inhabit different environmental niches (for example different salinities, lower light levels, different levels of nutrients) and it is possible that unfavourable conditions have lead to diatom decrease, enabling other phytoplankton groups to dominate, in this case dinoflagellates.   

Significantly lower values of diatoms were found in sample 23 where cell counts were recorded at 554 cells per 100ml.  

The major phytoplankton groups found were Chaetoceros, a form of diatom chain, and Alexandrium and Karenia, both dinoflagellates.

Zooplankton samples were collected aboard the R.V. Xplorer by trawling a 200 µm plankton net at approximately 2 knots for 5 minutes .  The samples were obtained from the top of the water column at station 6 and station 10.  Station 6 is the furthest north station from the Xplorer, where the estuary narrows into the River Fal,  station 10 is at the mouth of the estuary (fig 12.2).

Figure 12.2 shows that at the mouth of the estuary the total number of zooplankton nearly 6 times greater than that at the head of the estuary.  The proportion of each group of zooplankton at each station is similar however there are notable differences.  The four largest groups of zooplankton are the same at each station, these are Hydromedusae, Decapoda larvae, Copepoda and Copepoda nauplii. Although at station 6 Copepoda and Copepoda nauplii together make the largest group totalling 57 organisms m^-3, whereas at station 10 the largest group is hydromedusae at 197 organisms m^-3.  Another difference is the much smaller proportion of Gastropoda larvae and Polychaeta larvae at station 6 than at station 10. 

The structure of the zooplankton community at station 10 resembles that of the zooplankton sample collected from offshore (fig 6:4).  The high proportion of hydromedusae at station 10 is therefore also considered to be a combination of patchiness and seasonal variation.  The zooplankton community of station 1 (mouth of the estuary) from the offshore data shows large proportion of Echinoderm larvae, however at station 10 from the estuary data the Echinoderm larvae proportion is much smaller.  This difference is likely to be identification error.

Physical structure of the Fal Estuary:

The high salinities recorded at station 1 on the rib and the relatively low residence time calculated indicates that the Fal estuary is a predominately saline estuary where the effects of tidal flows are the major controlling factor of physical structure within the estuary. The reversal of flow observed along the entire western boundary of estuary implies that small scale internal instabilities are occurring which can result from interactions with the sea floor. No layered flow was observed with depth indicating a uniform flow. This would suggest that the Fal is a well mixed estuary and homogeneous salinity profiles were indeed observed at station 4 & 5. Further up the estuary figure 10.1 indicates that a layer of saltier water is flowing over a layer of well mixed water. This could be a result of the strong flooding tide and it is likely that if we were to sample at the same locations on the subsequent ebbing tide, we would see a more homogenous salinity profile further up the estuary.

The temperature profiles from the CTD stations indicate that from station 3 to station 5 which is located at the entrance of the estuary, thermal stratification is occurring within the water column. Salinities here are relatively homogenous irrespective of tidal state, therefore temperature variations are the most important factor leading to density instabilities and thermal mixing becomes the most dominant mixing process. This situation is reversed in the upper reaches of the estuary were large salinity variations, which have a greater effect on density per salinity unit compared to a 1^oC temperature change.

Turbidity at all stations remained very similar (between 4.15 and 4.5 volts on the transmissometer) however, the profiles show a decrease in turbidity down the estuary, possibly due to the loss of particulate matter from the water column as the river water is slowed down on contact with seawater. This also results in an increase in the depth of the euphotic zone. The offshore stations displayed a greater variation in turbidity at and between the stations, but there was a general trend towards slightly higher turbidities than recorded in the estuaries. Peaks in the turbidity were found at depths equating to the thermocline of each station, which is consistent with the large number of phytoplankton found at these depths. At station one, the turbidity is low and the euphotic zone extends to the seabed. The depth of the euphotic zone decreases as the stations move further offshore.

Chemical:

The nitrate estuarine mixing diagram provides a textbook example of non-conservative behaviour where removal from the water column at the mouth of the estuary has resulted in a decrease in concentrations below the TDL. The estuarine mixing diagrams for silicate and phosphate however portray a more complicated picture where there appears to be non-conservative behaviour yet an injection of both nutrients at salinities of 27. This salinity value was observed on the rib at station 1, right at the very head of the (fig.10:1) and lower salinity values were only recorded in the samples taken from streams further inland. Due to very low concentrations within the lower part of the estuary, the estuarine mixing diagrams represent a distorted view of the estuary because no samples were taken between salinities 4.5 and 27 yet there is some sort of large input of nutrients between these two stations and beyond this point phosphate and silicate appear to act conservatively. However silicon concentrations are completely depleted in the lower part of the estuary. Whilst this input could be from runoff due to the large rainfall that has occurred in the last two weeks, elevated values would have also been observed in the lower part of the estuary. It is unlikely that high silicon concentrations are a result of a large diatom mortality event because the abundance of diatoms throughout the estuary was very high with abundances reaching a maximum at station 2 where salinities were 30.2. Our data is therefore inconclusive.

Biological:

The phytoplankton abundance within the estuarine system was considerably different to that observed offshore. Diatoms dominated from the head down to the mouth of the estuary however Dinoflagellate abundance did increase from station 4. Station 1 offshore was carried out in almost the same location as Station 4 of the estuary however the phytoplankton sample taken here was lost therefore we were unable to make a comparison at this station. Diatom abundance was still high at station 2 which is an indication that diatoms may also have been the most abundant phytoplankton at station 1. This is surprising because silicon concentrations where not depleted on 30/06/09 and diatoms were abundant but by the 07/07/09 silicon concentrations near Black Rock were completely depleted yet diatoms remained abundant. It would be interesting to investigate the silicon concentrations measured offshore on 07/07/09 see if a there was a comparative decrease in silicon offshore. At station 3 & 4 offshore, dinoflagellates remain the dominant which is indicative of the calm, stratified conditions observed.

The zooplankton abundance was considerably greater offshore than within the estuary however the community structure within both systems were relatively similar. Hydromedusae remains the most dominant group however numbers become considerable reduced at station 6, the furthest most estuarine sample. Here copepod nauplius is the most dominant however this group is one of the most dominant at station 3 offshore. One notable difference is the increase in larvae, particularly Decapod larvae within the estuary where water depth is lower and shoreline habitats create ideal nursery grounds.

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