GROUP 9 FALMOUTH 2017

OFFSHORE

Introduction

The RV (research vessel) Callista is a catamaran with an overall length of 19.75m and a max speed of 14kts, she is a valuable work platform for ocean and estuarine research.

On the 6th July 2017, the Callista travelled 20nmiles offshore from Falmouth Harbour collecting samples at 5 different stations (C14-C18). Water samples and zooplankton net tows were analysed in the lab to identify the concentration of nutrients and phyto/zooplankton in the water column. The aim of this research is to create a comparable data set to observe any major differences between estuary and offshore parameters.

Methodology

Before leaving the dock the Acoustic Doppler Current Profiler (ADCP) started to record measurements until we reached the first station (C14). The ADCP measured the flow velocity and backscatter, between and at every station.

From the deck, the CTD and Niskin bottles, which were attached to the rosette, were lowered into the water by a winch. During descent, the CTD took measurements of the salinity, fluorescence, temperature and depth. Once the rosette reached a maximum depth (just above the seafloor), depths of interest were determined from the CTD measurements and the Niskin bottles were fired at the according depths.  

Water samples were taken from the Niskin bottles to be prepared and stored for lab analysis. We measured oxygen, chlorophyll, silicon, nitrate, phosphate and phytoplankton.

After recovering the CTD rosette, the plankton net was lowered into the water to a target range of depths collecting zooplankton at every station excluding C18. They were then identified in the lab.

This process was carried out at 3 more stations; C15, C16, C17 and a repeat station of C15 called C18. At station C16, Minibat was deployed which collected data between two oscillating depths (5m and 27.5m) showing the depth profile in more precision. The Minibat was towed very slowly until site C18 where it was recovered.

References

: http://www.southampton.ac.uk/assets/imported/transforms/content-block/UsefulDownloads_Download/219D055E06EE40038DE85C0542AD7E75/R.V.Callista.pdf

ADCP Transect map and plots

Temperature Results:

On the depth profile, the overall trend shows temperature further inland to be cooler than temperatures further out to sea. However, later in the day at the repeat station (C18), temperature increased further than previously recorded at C15. Another trend shown on the depth profile is that beyond 20m depth, there is a dramatic decrease in temperature by approximately 5°C at all the stations. Temperatures from 20m downwards are relatively stable in temperature.

Chlorophyll Results:

All trendlines show a peak in chlorophyll concentration around 20-25m depth where we hypothesised the Deep Chlorophyll Maximum (DCM) to be observed. The site located furthest offshore, C17, has the lowest chlorophyll concentration compared to the other stations. Also, the smallest range of 0.8 to 4umol-1 in chlorophyll concentration can be seen at station C17 at approximately 23m, where the DCM would be expected to be found. Only at station C16, a decrease in chlorophyll concentration is experienced in the surface waters (5-15m). Station C16 and C18 start to increase in chlorophyll after 40m which doesn’t occur in any other stations.

Results

Fluorescence Results:

As hypothesised, fluorescence peaked at 20-25m with a rapid increase and reaching a maximum value. On the other hand, there was an anomaly experienced at C15, where fluorescence peaked at a similar depth, however the maximum fluorescence was a lot higher than expected and peaked below the thermocline.

.

PAR Results:

All stations dramatically decrease in Photosynthetic Active Radiation (PAR) in the surface waters (0-5m) from 2000-500Wm¯². Nevertheless, C17 has a particularly high amount of PAR in surface waters, but below 20m depth, all the stations show a stable value.

Density Results:

Figure 6 demonstrates less dense water lying above denser saline water. All stations illustrate a peak density around 20m, however deeper waters have an increased density. The density gradient is very steep from surface waters to 20m depth, but the gradient decreases with depths beyond 20m. Finally, the general trend with density shows that further from shore, denser water is observed.

Nutrient Results:

Across stations C15 to C18, there is a general trend within the concentrations of phosphate and silicon to increase with depth. This trend is continued in nitrate for stations C14, C16 and C18 but C15 and C17 show a peak at a shallower depth before decreasing in concentration again. Most of the stations show a relatively constant concentration in the shallower waters for nitrate except for C14 and C16. Station C14 also shows a higher concentration of phosphate in surface waters than any other station. Stations C17 and C18 also show a marginally higher concentration of phosphate and silicon at depths than the other stations

Phytoplankton Analysis

C15 Depth
Compared to the chlorophyll collected at the same depths the quantity of phytoplankton is unexpectedly low. In the chlorophyll concentrations (Figure) there is a sharp peak in the chlorophyll concentration at about 20m to above 13µmol-1, however at the equivalent depth sample of phytoplankton the cell count per ml is just 200. Low phytoplankton diversity was found in this sample.

Compared to the Chlorophyll depth profiles there were unusually low counts of phytoplankton at mid-depth, samples either side of this anomalous value contained high numbers of Nitzschia spp.

C16 Depth
Cell count per ml followed similar trends to the chlorophyll concentrations found in the samples with a peak in cell number at 23.5m. The water at this depth was dominated by Nitzschia spp. and Cilliates, with increased counts of Rhizosolenia spp. found in deeper waters at 31.4m, just below the peak in chlorophyll content.

Counts followed chlorophyll depth trends closely with peak count at 23.5m with a majority of Nitzschia spp. At 31.4m Cilliates were found in high counts, just below peak chlorophyll concentration.

C17 Depth
Samples from 30.6m at this station yielded high counts of Chaetoceros spp. and Ceratium spp. Very little phytoplankton was recovered from the near surface trawl at 6.6m, with the sample comprised of small counts of a great variety of plankton.

C18 Depth
C18 was as close a repeat of C15 as we could obtain on the day. So the chlorophyll profiles are very similar to C15, with a peak in chlorophyll at 23.9m. There is also a smaller secondary peak at 49.9m. Unlike in C15 this is shown in the phytoplankton counts per ml. Counts per ml rose until the peak at 23.9m and swiftly decreased after that before rising slightly at the deepest depth of 49.9m. Like in C16 the mid-depths were dominated by Nitzschia spp. and a sample from 29.9m produced high counts of chaetoceros spp. like in C17.

Repeat data for C15 yielded similar results with mid-depths dominated by Nitzschia spp. and a sample at 29.9m producing a high count of Chaetoceros spp., which was also seen at the same depth in C17.

C14- C17 follows in the sequence of increasing distance from the shore sailing south of the Fal Estuary. All of the phytoplankton were common marine species like Nitzschia spp. and  Rhizosolenia spp.  with no obvious trends in the changing species across the transect of stations observed. This lack of trend in station was also observed in West scottish continental shelves by Fehling et al. (2012). Also highlighted in this paper is the difference between the quantity of phytoplankton, far higher counts were found in the estuarine samples than the Offshore samples. Understandably, phytoplankton like Nitzschia spp. and Guinardia spp. were found in the upper photic levels of seawater where the efficiency of photosynthesis would be greater. Mixotrophic ciliates like Mesodinium were found in deeper waters. Diatoms like Rhizosolenia can tolerate low light conditions, allowing them to live at the deep chlorophyll maximum (Kemp and Villareal, 2013)

The most dominant zooplankton organisms recovered from the offshore sampling stations (C14-C17) belong to the phylum Arthropoda, subclass Copepoda. Numerically they are the most dominant organisms within zooplankton communities around the British Isles (Todd et al. 1996). Ranging in size from 500μm - 5000μm (Larink, 2006), our mesh size of 210μm could retain large numbers whilst permitting smaller phytoplankton species to pass through. Identifiable features of these organisms include: prominent fused head and thorax (also known as the cephalothorax), generally supporting two pairs of antennae, and a noticeably shorter and narrower abdomen.

Slightly less prevalent than the adult copepod morphotypes came the initial free-living stage seen in most crustaceans – the Nauplius (Dahms et al. 2006). Characteristics of Copepoda nauplii are as follows: a median, Nauplius eye as well as 3 or more head appendages. The noticeable presence of Cladocerans (another Crustacean subclass) indicates that the section of the offshore water column sampled (between 10-35m – site dependent) was dominated by crustaceans (during early Summer). Aside from the persisting crustacean presence, there are fluctuations in the composition of the zooplankton communities between stations, however this can be attributed to natural variability seeing as the contributions of the non-crustacean groups to the population are minimal.

Falmouth Offshore Zooplankton Community Composition

Minibat

The Minibat is a valuable piece of equipment for any oceanographer, by towing this equipment behind a boat at approximately 5 knots and remotely operating the wing angle, the depth of the Minibat can be manipulated.

Using this equipment, the Minibat can oscillate within the water column giving a detailed image of the water column by collecting data of multiple parameters including salinity, temperature, fluorescence and turbidity.  

At 14:08 on 6/07/17 the Minibat was deployed off RV Callista. It was towed for two hours travelling on a transect heading towards Falmouth coastline starting at a Latitude of 49°46.917 N and Longitude of 004°51.397 W and finishing at a Latitude of 50°05.659 N and Longitude of 004°51.295 W.

The results show in Figure 12 a band of higher fluorescence between 15 – 30 dbar with the surface waters having the lowest fluorescence and increasing with depth creating a band of higher fluorescence at depth. This band of higher fluorescence is deeper at the start of the tow between 20 – 30 dbar and rises to 15 – 20 dbar between 6000 -1200 metres along tow, and returning to the same depth at the start of the tow transect.

Figure 2 looks at temperature and shows the summer thermocline depth throughout the tow transect. Warmer surface waters were present peaking at 16.5 °C and decreasing with depth, the lowest temperature 12.5 °C being at 25 – 30 dbar. Initially the thermocline is deeper at 17 dbar however after 6000 metres the warm surface layer gets thinner and the thermocline is 10 dbar deep.

Comparing the two graphs it shows that the fluorescent layer between 15 – 30 dbar is below the thermocline in the water column. Fluorescence can be used as a proxy for chlorophyll and phytoplankton. This shows that phytoplankton are mainly between 15 – 30 dbar and not in the surface layers. Being offshore light will penetrate deeper than in estuarine conditions therefore light in deeper water is not a limiting factor for the phytoplankton. In the surface waters there are fewer nutrients as the thermocline limits mixing with the nutrient rich deeper waters. The phytoplankton therefore cannot grow if any major nutrient is not present. At 20 – 30 dbar neither nutrients nor irradiance are limiting factors causing the phytoplankton to thrive at this depth (Holligan and Harbour, 1977).

References

Holligan, P., & Harbour, D. (1977). The vertical distribution and succession of phytoplankton in the western English Channel in 1975 and 1976. Journal of the Marine Biological Association of the United Kingdom, 57(4), 1075-1093. doi:10.1017/S002531540002614X

Salinity Results:

Very small variations in salinity (approximately 1) were observed at all the stations, however highest variations were seen in the surface waters.

Discussion:

The majority of the stations show nutrients tending towards lower concentrations in shallower waters. These shallower waters are the areas with most light available, highest temperatures and lowest density, therefore it is likely that these lower concentrations are due to the nutrients being taken up by phytoplankton (Pasciak and Gavis, 1974). The most noticeable result related to this is station C14. For nitrate and phosphate particularly, C14 retains a higher concentration in comparison with the other stations. As C14 was the closest offshore station to the estuary this could be due to a couple of reasons: one hypothesis is that the riverine input of nutrients mean that the concentration of nutrients is higher despite a similar level of uptake by phytoplankton (Caddy and Bakun, 1995). Alternatively, due to the larger sediment input, the water may be cloudier which would result in light being less able to penetrate to deeper depths (Hessen et al., 2010). This would limit the growth of phytoplankton which would in turn mean they are not taking up nutrients as quickly as they would be in clearer waters.

Chlorophyll-a concentrations are higher in coastal waters than in offshore waters (Carder Ltd, 2017). This is supported by our results, as C17 (our furthest offshore station) had the lowest chlorophyll concentration. This is likely to be due to high amounts riverine and terrestrial runoff in coastal areas, which input large amounts of chlorophyll and nutrient concentrations.

As we expected to find, nutrient concentrations are higher in offshore waters compared to coastal waters. This is due to phytoplankton (chlorophyll) taking up and depleting nutrient levels, for photosynthesis, increasing phytoplankton biomass. Offshore waters experience lower chlorophyll concentrations and higher nutrient concentrations compared to coastal waters, therefore fewer phytoplankton up taking nutrients for growth. This is supported by High Nutrient, Low Chlorophyll (HNLC) hypothesis (Edwards et al., 2004). At stations C16 and C18, chlorophyll concentrations begin to increase from around 32m, opposing the general trend. This is due to remineralisation of sinking particles at depths, increasing nutrient levels. Therefore, sinking fast diatoms and other phytoplankton present at these depths, take up the readily available nutrients that have been remineralised, increasing phytoplankton biomass (Dugdale and Wilkerson, 1991).

Salinity increases further offshore due to less freshwater inputs from rivers and terrestrial runoff. Saline water has a higher density than freshwater, hence why our results show density increasing with distance from the Fal Estuary. Additionally, density increases with depth due to freshwater lying above dense saline water, causing saline water to sink.

References

Caddy, J. and Bakun, A. (1995). Effects of riverine inputs on coastal ecosystems and fisheries resources. Rome: FAO.

Carder Ltd, M. (2017). Concentrations of Chlorophyll-a in the Greater North Sea and Celtic Seas. [online] Oap.ospar.org. Available at: https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/pressures-human-activities-v2/eutrophication/chlorophyll-concentrations/ [Accessed 13 Jul. 2017].

Dugdale, R. and Wilkerson, F. (1991). Low specific nitrate uptake rate: A common feature of high-nutrient, low-chlorophyll marine ecosystems. Limnology and Oceanography, 36(8), pp.1678-1688.

ADCP Transect 046

Transect 046 was taken offshore whilst performing operations at station C18. Whilst a region of slightly higher velocity can be observed closer to the seafloor, the most noticeable feature is the large area of missing data. This corresponds with an increase in the boat speed and this is the most likely cause. Due to the ADCP’s method of ‘pinging’ it is likely that whilst the boat was moving at a higher speed the deeper pings were bouncing back after the boat had moved on and were therefore not being picked up by the ADCP.

ADCP Transect 045

Transect 045 was taken during transit between stations C17 and C18. Without averaging the data the velocity seems to be relatively uniform with a slightly lower velocity closer to C17 (further offshore). After averaging to 5 ensembles per average this area becomes more defined and a layer of higher velocity becomes apparent at the seafloor as well as a possible layer around 5m deep of very marginally higher flow rate.

Richardson number

Richardson number results

Station C14 and 30

The Richardson number at station C14 is well below the critical value of 0.25 at all depths. A small peak at around 10m depth is observed in figure x. Similarly, the Richardson number in the estuary at station 30 is largely below the critical value of 0.25 at depths and a maximum value occurs at 10m depth, as illustrated in figure x. Most data points lie between Ri = 0 and 0.2 in the estuary at station 30, whereas they lie between 0 and 0.01 offshore at station C14. Consequently, the shear was stronger, creating a greater imbalance between shear and buoyancy forces.

Richardsons number discussion

The Richardson number is the ratio of the balance between the buoyancy and shear forces. Values 0.25 and below indicates instability in the water column, resulting in turbulent flow (Knauss, 1997). This is therefore typical in coastal and estuarine waters during changing tidal states, which were at the time of data collection at stations C14 and 30. Turbulent mixing leads to a vertically well-mixed water column in terms of temperature, salinity and nutrients (Pond and Pickard, 1993) - therefore the Richardson number is a vital parameter to monitor due to its implications on estuarine biota. The differences between the average Richardson numbers in the estuary at station 30 and offshore at station C14 can likely be attributed to offshore data being collected closer to high tide. Richardson number at 10m depth, observed at stations C14 and 30 are due to stratification processes.

References

Knauss, J.A. (1997). Introduction to Physical Oceanography. 2nd ed. New Jersery: Prentice Hall, p. 250.

Pond, S., Pickard, G.L. (1993). Introductory Dynamical Oceanography. 3rd ed. Oxford: Butterworth-Heinemann, p. 60.

Meta Data

Callista

Date: 06/07/17

Time: 09:31 GMT

Location: Offshore

Latitude: 50'04.860 N

Longitude: 004'51.751 W

Low Tide: 09:41 (1.26m)

High Tide: 15:36 GMT (4.58m)

Cloud Cover: 3/8

Introduction

ADCP

Minibat

Zooplankton

Phytoplankton

Results CTD

Method

Richardsons number

Figure 1: Temperature depth profiles for stations C14 - C18 (Station 1=C14, Station 2=C15, etc.)

Figure 2: Chlorophyll depth profiles for offshore Stations C14-18

Figure 3: Salinity depth profiles for offshore Stations C14-18

Figure 4: Flourescence depth profiles for offshore Stations C14-C18

Figure 5: Photosynthetic Active Radiation vs. Depth for all offshore stations (C14-C18)

Figure 6: Seawater density depth profiles for offshore stations C14-C18

Figure 7: Nitrate depth profiles for offshore stations (Callista=C14-C18, Terramare=13-17)

Figure 8: Phosphate depth profiles for offshore stations (Callista=C14-C18, Terramare=13-17)

Figure 9: Silicon depth profiles for offshore stations (Callista=C14-C18, Terramare=13-17)

Figure 10: Phytoplankton community structure at each offshore sampling station (Callista only), phytoplankton were counted per millilitre of seawater.

Figure 11: the composition and relative proportions of the zooplankton communities sampled at 4 different stations in offshore Falmouth. Plankton were identified to group/order level.

Figure 12: Fluorescence and temperature data collected from the 2hr minibat tow/deployment.

Figure 13: ADCP plot showing flow velocity against depth and distance, colours represent flow velocities in the key above the plot.

Figure 14: ADCP plot showing flow velocity against depth and distance, colours represent flow velocities in the key above the plot.

Figure 15:

Station C14 Richardson number and critical value of Ri=0 .25 plotted as function of depth at station C14.

Figure 16:

Station 30 Richardson number and critical value of Ri=0 .25 plotted as a function of depth at station 30.

Figure 17: Nutrients plotted against salinity with a Theoretical Dilution Line (TDL) for (A). Nitrate, (B). Silicon and (C). Phosphate.

The views and opinions expressed re those of the individual and not neccesarily those of the university