This webpage presents the results of group eight's hard work, occasional mistakes and large amounts of vomit......

The River's Tamar, Lynher and Tavy all constitute part of the Tamar estuarine system, whilst Plymouth Sound forms the barrier between the estuary and the English Channel. The area covered by this report extends from Calstock, in the upper reaches of the Tamar, to just beyond the Eddystone Lighthouse, roughly 12 nautical miles south of the Plymouth breakwater (click on satellite image to see these locations)

The estuary is a temperate, partially mixed Ria (meaning a drowned river valley). It forms a natural boundary between Devon and Cornwall and is 31km in length, from the upper limit of salinity ingression at Weir Head to 'the Narrows' (Tattersall et al, 2003).

It has a mean tidal range of 3.5m (Tattersall et al, 2003), and thus falls into a meso-tidal classification, although values of more than 5m can be reached during spring tides.

Expectations: The following predictions regarding some of the key environmental variables have been made by studying the literature from previous studies of the area.

HW: 09:48 GMT (Calstock)

Introduction: The RIB practical was carried out on Thursday 24^th June 2004, running from Calstock down river to the Tamar Bridge, sampling at 17 sites. Plymouth-Devonport tide tables showed high water to be at 09:38 GMT. The secondary port calculation for Cotehele added a +10min delay, therefore high water at Calstock was calculated as 09:48 GMT. The required water depth for the RIBs to navigate safely was 1.5m, and the lowest available water depth on that day was 1.8m; therefore we were not constrained by the tide. Two boats (Ocean Adventure RIB and the SRR vessel) were used for the survey. The samples collected from the RIBs were analysed by Group 7. The conditions on the day were variable, ranging from heavy showers to clear skies. The weather over the previous days had been continuously wet and windy. Having cruised up the Tamar to Calstock, observations started at 10.59GMT following the high tide at 09.48GMT at Cotehele.

Objectives:  The investigation aim was to collect biological, chemical and physical parameters of the water column throughout the Tamar Estuary. This should allow us to understand the variation in both abundance and diversity of the phytoplankton and zooplankton taken between the freshwater end member and the salinity found at the Tamar Bridge (ranging from 28-32) as well as the processes that cause this variation.

Equipment: Both vessels were equipped with Secchi disks, Anometers, T/S probes and Niskin bottles, but only the Ocean Adventure carried the Plankton net (mesh size 200µm, diameter 50cm). For the purposes of the physical observations an Anometer took the wind speed and the T-S probe also observed the changes in temperature and salinity within the water column. The probe on the SRR was not equipped with a depth sensor, therefore depth profiles were carried out at every other station, as the two boats “leapfrogged” down the river.  

Procedure: Salinity was the controlling factor in choosing the sample site. The initial decision was to sample at stations every 1.5 salinity units; however between site 9 and 10 it was decided that this would be changed to every 2 units as we wanted to make sure we overlapped with the measurements that Group 7 took on the Bill Conway. Each RIB sampled at every other site. A beaker was dipped into the water for the surface sample at each station which was used for all data analysis. Samples were taken with a 60ml syringe for filtering water 60ml into the glass Nutrient bottle and another 30ml for the plastic Silica bottle. The first two filters per site were kept and stored in test tubes containing 7ml Acetone for Chlorophyll analysis. Zooplankton samples were taken at 3 stations: salinity 16, 20 and 28. Each time the Ocean Adventure was moored to a buoy and the net was deployed for five minutes. Phytoplankton samples were taken at salinity 14 and 22 in glass bottles containing Lugols solution. All data was logged from the T-S probe, which included salinity, depth, percentage dissolved oxygen and pH. The Secchi disk reading was obtained by the same person each time, with cloud cover being noted at the time of lowering. The euphotic zone depth was then calculated by multiplying the Secchi disk depth by 3. Each RIB took one Niskin bottle sample by hand from the surface at salinities 14 and 20 for the oxygen samples.

HW: 12.26 GMT (Devonport)

Introduction:  We decided to sample at four sites; Our first site was sampled just off the breakwater at 50º 20.094N, 4º 08.075W as a practice run.  Three sites were then sampled around Eddystone Rocks; the second sample was east of Eddystone Rocks (50° 11.00N, 4º 14.00W), the third was to the south (50º 10.00N, 4º 16.00W) and the fourth was taken between Eddystone Rocks and Hand Deeps (50º 12.00N, 4º 18.00W) (See Navifish image for more detail of the locations; all sites referred to from this point forward are on the map). The shipping forecast for Portland-Plymouth stated southwest force 4 or 5, occasionally 6, rain then showers. High water that day was at 12.26 GMT (Plymouth-Devonport) at a height of 4.5m and low water was at 18.48 GMT with a height of 1.9m.  The previous day was a neap tide so all current rates were low.

Results:

Site 2 - The silicate concentration  for this site is low, varying between 0.05-0.20 μmol/l. Phosphate concentrations are again low, with values reaching no higher than 0.3 μmol/l. Nitrate concentrations increase with depth from 0.15 to 1.05μmol/l. There is a gradual decrease in temperature from 13.8^oC at the surface to 12.0^oC at 35m, below which it stays constant (link). Salinity, which is at a constant 35.22 above 35m increases to 35.26 below the thermocline. There is a sharp peak in fluorescence at 14m depth.  Once again the highest diatom concentration is recorded at 20m, but this may be an error in the cell counts. The zooplankton trawl between 0-25m shows an abundance of copepods (38.2%), Noctiluca sp (27.1%) and globules of phaeocystis colonies (15.8%). At greater depth the zooplankton composition varies, with almost half the population being dinoflagellates, specifically Noctiluca sp. (33.5%) and Ceratium sp. (13.1%).

Site 3 - The water column for site 3 is stable with increasing density with depth. Silicate levels are very low and show very little variation throughout the water column. The phosphate levels behave in a very similar way maintaining a constant concentration from top to bottom. The peak in nitrate within the euphotic zone (0-25) appears to correlate with peaks in diatom concentrations and fluorescence this might indicate the presence of active phytoplankton. The fact that oxygen levels are higher at 12m also supports this idea. Below the euphotic zone the nitrate levels increase rapidly and the oxygen levels drop, which coincides with an increase in Diatom concentration and fluorescence in the region of 40m. Species which dominate at this site include Noctiluca, Ceratium and other dinoflagellates. ADCP data for this site shows evidence of the effect of the Eddystone rocks, with flow velocity varying considerably along the ADCP track. Such behaviour is suggested in research by Pingree and Maddock, 1985. The ADCP backscatter data shows interesting patterns, with a peak very close to the surface, dropping sharply below 15m.

Site 4 - Silica concentrations are low throughout the column as are  phosphate concentrations. Nitrate concentrations reach a minimum at 12m only to increase with depth. Fluorescence peaks in the region of 20m and then decreases rapidly at 30m reaching a minimum at 45m. There is a noticeable transition zone between warm shallow water (10-0m) and cooler deeper water (from 30m down). The dominating Zooplankton species within this site are Copepods and Ceratium. There appears to be a slight increase in the silicate concentrations, after a small decrease from the surface to15 m. this increase is from 0.05 to 0.35 μmol/l. Phosphate concentrations show a similar trend for site 4 as in site 3. However the nitrate initially increases from the surface to15 m depth by 0.2 μmol/l, then it decreases to 0.05μmol/l at a depth of about 28 m. with further depth, the concentration is seen to increase again to 1.15μmol/l. There is no clear thermocline.

HW: 1644 GMT (Devonport)

Results:

Nutrients: The graph of nitrate vs. salinity shows a fairly linear relationship, with NO₃ decreasing with increasing salinity, and with most points in close proximity to the trendline. Nitrate shows an essentially conservative behaviour in the Tamar estuary.

Surface concentrations of silica show a non-conservative behaviour, with SiO₂ decreasing with increasing salinity. The curvature of the graph is more pronounced at very low salinities, indicating that SiO₂ removal was most effective in the upper reaches of the estuary (Morris, Bale & Howland, 1981).

Phosphate [PO₄] appears to be removed in the upper estuary (~S 1-14), added in the lower estuary (~S 14-30), and extremely low in the seawater salinities. Previous studies have found that phosphate distribution is independent of salinity in the Tamar due to inputs in the lower reaches localized around the two tributaries, the Lynher and the Tavy. The unusual distribution of phosphate may be due to pollutant sources arising from the industrialised and urbanised regions in and around the city of Plymouth (Morris, Bale & Howland, 1981).

CTD Observations: At the point where the Tamar and the Lynher converged a strong front was witnessed as seen in the photograph to the right. The swirling area featured a clear line of surface foam which was investigated by towing the CTD through the front (CTD6). The data revealed some interesting results. Either side of the front temperatures were in the region of 16.55°C, however through the front, temperature decreased rapidly to between 16.3 and 16.36°C when plotted against time shows a rapid increase in salinity from 31.95°C to 32.4°C across the front, which then drops off. Interestingly, the fluorescence drops sharply at the front, where it would have been expected to increase because of the higher nutrient availability.

Plankton: The phytoplankton counts show a clear dominance of diatoms throughout the salinity range with a peak of concentration at salinity 24. The samples collected further up the river show more variation than the samples collected on the Bill Conway where the salinity ranges were more uniform. The zooplankton  diversity increases with salinity and copepods, which were the dominating species at low salinity, slowly give way to the dinoflagellate Noctiluca. 
ADCP: Eddying was found in transects taken across Plymouth Sound on either side of the breakwater. The flow magnitude and direction varied depending on the state of the tide, showing a distinct time lag between low water at the breakwater and low water at Devonport. Transects taken in the Tamar estuary and more importantly within the "Narrows" section of the estuary have highlighted numerous physical processes associated with the topography and dynamic physical properties of the Estuary. The state of the tide has a strong influence over the velocity magnitude, direction and backscatter seen both in the horizontal and vertical profiles of the water body. A detailed analysis of several ADCP profiles can be seen on: 

HW: 07.04 GMT (Devonport)

Results:

West of Breakwater (survey A1): Close to the breakwater are wide channels in the bedrock in which can be found either cohesive very fine sand to mud areas (pale grey homogeneous patches on scan) interspaced with coarse-grained bioclastic sand displaying a ripple bedform. The ripples are sinuous in-phase and have a wavelength of 1.5m and an approximate wave height of 0.5m with a slight evidence of bifurcation, indicating wave influence. They are symmetrical, signifying an equal velocity of flow in both directions. Despite the predominant fetch coming from a southwesterly direction the ripples are orientated parallel to the breakwater; an interpretation could be that the waves refract as they pass over the bedrock. The waves could then reflect off the breakwater, creating these symmetrical ripples. In the centre of the wider bedrock channels there are strips of cohesive mud visible. An explanation could be that the highest current velocity being localised in the middle of the channel, the current scours away the coarser sediment, thus exposing the underlying anoxic mud. A very clearly defined fracture traverses all 8 transects; its orientation is unusual as it is perpendicular to the  main fault lines in the area.

East Breakwater (survey A2): Closest to the breakwater, the scan displays pale grey homogeneous areas indicative of fine sand, interspersed with areas of sinuous in-phase ripples approximately the same size as those on the eastern side. The ripples are orientated at approximately 45^o to the track, or in real terms nearly perpendicular to the breakwater. It is possible that these are caused by ebbing tides flowing around the eastern side and being deflected by the bedrock into this area. The direction of these ripples supports this supposition. As the scan moves nearer to Renney Rocks the image becomes dominated by large areas of bedrock, varying from evenly distributed bedding to sharp rock outcrops considerably higher above the seabed. These are possibly indicative of a harder bedrock that is less easily eroded. In certain areas, gullies between rocks have been in filled with sediment and some show ripple waveforms. Two strong faults running roughly East-West are evident with 4-5m offset, and many fractures are seen perpendicular to these, as strongly defined lines.

Q: Which is more effective, surveying parallel or perpendicularly to a fault?

A: We found that it was much harder to detect the fault when surveying perpendicularly to it, as the sound does not get bounced back. Therefore it is best to plan the survey grid with the main lines of survey running parallel to the fault, with a few perpendicular tie lines to “double check” bedforms.

The data collected from the boat work has allowed us to investigate many of the statements made within the published literature. 

In particular the findings from the data collected from the Bill Conway and RIBs have both been very useful, as clear correlations could be seen in the data. For example, the silicate data indicates classic non-conservative features. In addition to that, the phosphate concentrations reflect the removal of the nutrient at lower salinities, and with addition at higher salinities. It was also observed that nitrate showed conservative behaviour. Similar results have been observed in previous studies (Morris, Bale & Howland, 1981).

For the data collected onboard the Terschelling, there appears to be little indication that the Eddystone Rocks having influenced the biology, in addition to that, all stations appeared to maintain visible thermocline’s.  The chemical and physical structure remains fairly constant too.

Pingree, R.D. and L. Maddock (1985). Rotary currents and residual circulation around banks and islands. Deep-Sea Research. 32 (8), 929-947.