PLYMOUTH FIELD COURSE 2019
OFFSHORE
INTRODUCTION TO OFFSHORE
On Thursday 4th of July 2019, group 2 collected data offshore on the research vessel
Callista. A CTD rosette was used at 7 different stations to collect temperature,
salinity, fluorescence, dissolved organic matter, irradiance and Niskin bottles.
Niskin bottles were used to collect water samples and to later process in the lab
for chlorophyll, nutrients, oxygen and phytoplankton. ADCP transects were also conducted
between the stations to determine current direction with depth and backscatter for
location of organisms, specifically zooplankton. Additionally, plankton nets (diameter=0.5m)
were deployed at some stations, to collect zooplankton samples at a vertical depth
of 5-
The photo collection from the offshore research is available in the slide show to the right.
Please visit the link below for the full video compilation, including the photo collection and additional videos taken during the research:
The video includes more details on the pictures as well as video clips of research procedures.
Temperature and Salinity Profiles
The T/S profiles show a drop in temperature with depth at all stations, with the
biggest change between the surface and depth at Stations 15 (12:39 UTC) and 16 (13:56
UTC) (2.9°C). These two station were the furthest from shore, but were close to
each other. A double thermocline was also observed at these 2 stations at depths
14-
Fluorescence Profiles
In order to identify the deep chlorophyll maximum, fluorescence profiles (Fig ?)
were used as a proxy for chlorophyll. The fluorometer on the CTD was not calibrated
on the day of sampling, therefore, a linear regression was fitted using the CTD data
of fluorescence vs chlorophyll recorded in 2018, which gave a R2 value of -
Oxygen Saturation and Nutrient Concentration:
Water samples were taken at specific depths throughout the water column with increasing
distance from shore based on the physical structure of the water column. In the wet
labs at Plymouth Marine Laboratory concentrations of nitrate, phosphate and silicate
were determined using an auto-
Three stations sampled had 2 or more water samples taken, so these were used to generate oxygen depth profiles. Station 11, the closest station to the shore, shows a marginal decrease in oxygen % saturation with depth, from 93% to 92%. Conversely, Station 12 shows a 4% increase in saturation, from 92% at 10m depth to 96% at 50m depth. Station 15 is the most complete profile and shows greater variability of oxygen % saturation with depth. There is an overall decrease in oxygen saturation from oversaturation at 5m to the lowest observed value of 90% which was also at the deepest measured depth of 63m. A second oversaturated region of the water column was observed at 25m, interrupting the overall declining trend.
Oxygen is required for aerobic respiration by the majority of metazoans, as well as photosynthetic organisms including phytoplankton and higher plants. Consequently regions of the ocean with high chlorophyll or PAR, which are assumed to represent phytoplankton, are expected to have higher dissolved O2 levels due to production from photosynthesis. Similarly, areas of the ocean with higher uptake of O2 and low productivity often have lower levels of dissolved oxygen. High levels of oxygen, exceeding 100% saturation, are observed in the surface waters due to bubble entrainment and high rates of exchange with the atmosphere. Physical parameters also affect oxygen saturation, with gasses being more soluble in colder waters with lower salinity.
Overall, surface levels of all three nutrients measured decreases with increasing distance from the shore, and with the exception of phosphate and nitrate at station 11, all increase in concentration with depth. This is clearly demonstrated by station 15, for which five water samples were taken. At station 15 Silicate has a surface concentration of 0.1 µmol/litre, but is rapidly depleted to below detectable levels within the first 15m of the water column. Phosphate and nitrate were not detectable until 25m depth, with their concentration steadily increasing to 0.3 and 0.9 µmol/litre respectively by 34m depth. Silicate concentration is also observed to start increasing at 25m, but is remineralised at a faster rate, reaching 0.6 µmol/litre by 34m, with a slight subsequent increase in concentration of around 0.5 µmol/litre along the remainder of the profile to the sea bed. Station 12 data shows a similar trend, with phosphate and nitrate being undetectable at the near surface and increasing with depth, despite the maximal concentrations reached at station 15 being more than double that at station 12 for both nitrate and phosphate and eight times greater for silicon.
The two depths sampled at Station 11 show all three nutrients as present at 5m water depth, with Si increasing with depth from 0.38 µmol/litre to 0.5 µmol/litre at 30m. Conversely, phosphate and nitrate both decrease in concentration by 0.1 µmol/litre with increasing depth, with nitrate no longer detectable after 25m.
Zooplankton:
A net of 0.5m diameter and mesh size 200µ was deployed at three stations to sample zooplankton. The specimens were preserved in formalin and 10ml from each sample was placed in Bogorov tray to count the number of organisms present in each category. The volume trawled was then used to calculate the zooplankton count per litre.
Two vertical trawls were completed at stations 11 and 12 to compare the zooplankton communities at different depths, three were done at station 15. At station 15, a substantial shift in the zooplankton community structure with was observed with depth, with the greatest number of zooplankton was present between 35m and 25m, below the DCM. Click HERE to explore the observed zooplankton page.
Phytoplankton:
Samples of the water column were obtained using 10L Niskin bottles mounted on a CTD rosette. 100ml was preserved with Lugol’s iodine solution to preserve the phytoplankton to be quantified via microscope. Another sample was taken and preserved for flow cytometry.
13 different genera of phytoplankton were identified using microscopes, with diatoms being the dominant overall group at all stations.
Microbes (see figure 4):
Microbe samples were also collected from Niskin bottles over three depths at station 15. These were preserved, stained with DAPI, filtered, then images were taken under 60x magnification using Epiflourescence microscopy. The number of microbes in these images were counted and scaled up to find the total microbial biomass gC.
PHYSICS
CHEMISTRY
BIOLOGY
Figure 1: .The taxonomic composition of the vertical zooplankton trawl, with the corresponding CTD temperature profile and the total phytoplankton count.
Figure 3: Flow cytometry data from the analysis of samples from stations 11, 12 and 15. Stations 11 and 12 showed a decline in the total cell concentration with depth, however, at station 15 the greatest cell concentration was found in the sample taken at 24.65m depth, coinciding with the DCM.
Figure 2: The relative proportion of the phytoplankton groups at different depths at station 15 and the corresponding nitrate, chlorophyll and light attenuation. Diatoms were the dominant group at all depths at station 15. The sub chlorophyll maximum is at the top of the nitricline and before the PAR values are too low to sustain net photosynthesis.
Figure 5: The change in the total microbial biomass (gC) and total phytoplankton
cell count (cells µl-
Figure 4: Microbe chlorophyll fluorescence
Figure 1: Offshore profiles of % oxygen saturation with depth for stations 11, 12 and 15.
Figure 2: .Offshore nutrient profiles for stations 11 (a), 12 (b) and 15 (c) showing silicate, phosphate and nitrate concentrations with depth. Note variable axis.
Figure 2: Offshore Fluorescence profiles for stations 11-
Figure 1: Offshore Fluorescence profiles for stations 11-
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All data used to create the results on this page can be found on the University of Southampton FTP Server.