GROUP
Plymouth Field Course 2019
4
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Estuary Conclusion
Phosphate and silicate both show non-conservative behaviour. According to the mixing diagrams, there was addition and reduction of these nutrients respectively. Nitrate, alternatively, shows conservative behaviour.
Salinity, density and temperature transects were relatively constant with depth shown on the CTD depth profiles. Salinity and density profiles have very similar patterns; both are lower at the surface compared to depth. Salinity of the estuary determines density and temperature. Oxygen concentration was positively correlated with salinity. Station 2 (B) and 6 (G) had the highest oxygen concentrations, indicating an oxygen influx. The variation between oxygen concentration and depth alters between stations. We took counts of phytoplankton for all 9 stations and took zooplankton counts for stations 6 (G), 7 (H) and 8 (I). Station C was dominated by Rhizosolenia, while stations D, E, F, G and H are dominated by Lauderia diatoms. In addition, all three stations in which zooplankton were counted were all dominated by copepods.
Off-shore Conclusion
During the offshore study, 4 stations were analysed using an ADCP and a CTD. Station C25, located near to the research station L4, had some of the greatest chlorophyll concentrations and was enriched in nutrients. A distinct thermocline was observed and oxygen concentration increased at the thermocline. The highest abundance and diversity of phytoplankton was observed at the chlorophyll maximum at 15.5m with Chaetoceros and Ceratium spp. being dominant. Copepods made up over 50% of the zooplankton individuals recorded at C25.
Station C26 was located further offshore than C25. Here, chlorophyll concentration was generally lower and more constant with depth than at C25. Nutrient concentrations fluctuated, but increased with depth overall. The thermocline here was less pronounced and shallower than at other stations. The greatest phytoplankton abundance was observed at 4.3m by a large margin, almost entirely dominated by cryptophytes.
Station C27 saw the largest deep chlorophyll maximum (DCM) at almost 1.8µg/l at 23m. For this reason, flow cytometry and DAPI samples were also collected here. Unfortunately, the bottle at the greatest depth misfired, so all depth profiles are incomplete. Here, nutrient concentrations were noticeably depleted, especially above the DCM. The DCM was located on the thermocline, where nitrate concentration significantly increased. The phytoplankton abundance was greatest at C27, peaking at 4.2m, with Chaetoceros being almost entirely dominant. Copepods dominated zooplankton abundance once again, but Appendicularia and Cladocera were also significant components. Flow cytometry revealed that, whilst total phytoplankton abundance and nanoplankton abundance increased with depth, microplankton abundance decreased with depth. Epifluorescence microscopy of chlorophyll showed that the maximum chlorophyll concentration was actually at 14m, though bias and sampling error could cause this pattern.
Station C28, located near to the research station E1, was the furthest offshore, and had the lowest chlorophyll concentration with a prominent DCM. The nutrient concentrations increase significantly below the thermocline, with nitrate concentration having the highest concentrations recorded out of all the nutrients by a large amount. Phytoplankton was least abundant here, with Pseudo-nitzchia being dominant at 29.4m. Copepods were extremely dominant here, composing over 75% of zooplankton, and Appendicularia was the only other group with over 1000 individuals m-3.
One noteworthy flaw of the data collected is that the oxygen depth profiles from the CTD measurements and the niskin bottle samples are conflicting. The CTD measurements all show an increase in oxygen concentration with depth, with oxygen behaving in almost the exact opposite way to temperature. These data are uncalibrated and this may cause poor measurements. The bottle data, however, generally shows a decrease in oxygen concentration with depth, particularly below the thermocline. There were sampling errors in oxygen sample collection on the Callista and this may be responsible for errors.
The ADCP data was processed using WinRiver II. Due to electronic errors, there were large gaps in the data represented by blank regions on the ADCP profiles. It was still possible, however, to get a general idea of flow direction and speed. The transect between stations C27 and C28 presented evidence of a front, displaying distinct stratified layers. At all stations except C26, there are distinct layers with carrying velocities.
CONCLUSIONS
The ADCP was deployed to survey flow structure and speed throughout the 9 stations. ADCP 10 and 16 have well a mixed water column. ADCP 1, 3 ,5 and 17, had surface flows faster and less dense than at the seabed. ADCP 18 was deeper and more mixed than ADCP 19 was shallower. There were also gaps in the data, which influenced the data readings. The ADCP bottom tracker was switched off on Falcon Spirit, causing incorrect depth measurements.