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Offshore Plankton Distributions

To observe the changes in planktonic populations through-out the water column zooplankton nets were taken at the surface. Plankton counts and oxygen were taken from the NISKIN bottles at the surface, chlorophyll maxima and bottom. Flow cytometry was also performed on all bottles to give cell counts and chlorophyll counts which were also obtained by individual sampling of each bottle, this was used to calibrate the CTD’s voltage which was used to observe fluorescence in situ. Finally, fluorescence microscopy was used to allow a count of chlorophyll and micro-bacteria that are stimulated to fluoresce. The methods used for the above will be elaborated on below.

Methods

The zooplankton and phytoplankton were detected using fluorescence under 1 volt, on the CTD rosette and backscatter from the ADCP. Physical samples were taken using NISKIN bottles allowing water and chlorophyll samples to be bottled and preserved on deck. Zooplankton nets of 50cm diameter and 210μm mesh these were then towed for 5 minutes each at 12.50 and at 14.39 to sample the zooplankton at the surface in a 1L bottle 10ml of which was then placed in a bogoslof chamber and zooplankton identified using a microscope and then zooplankton per m3 calculated. Phytoplankton were concentrated into 10% of the volume and then placed on a slide and examined with a microscope counting the number and species in 100µm. Flow cytometry was used utilising cytoflow passing the samples one by one through a laser and analysed the backscatter of red fluorescence as this is indicative of chlorophyll and is used to give a cell count and images of the cells. Chlorophyll samples of 7ml were also calculated in the lab by analysis with a fluorometer which was then multiplied by 7/50 to give the amount of chlorophyll (µg/L) in 50ml. Oxygen on the CTD was calibrated from the bottle samples whose values were found by adding 1ml of sulphuric acid with a magnetic stirrer to mix the solution, the solution is then titrated to find an end point using sodium thiosulphate, using this endpoint the volume of oxygen can be calculated. Finally, fluorescent microscopy was used here a laser is fired at the fluorophore followed by lower energy light. This causes longer wavelength light to be emitted which is used for cell imaging with a Mercury lantern microscope using 5 photons per litres of sample and taking averages giving 500um x 400um images of the cells in the samples taken.

Results

Fluorescence

This graph shows the un-calibrated voltage readings for all the CTDs taken at L4. This is indicative of fluorescence and therefore phytoplankton, the calibrated data is presented below. The overall trend shows an in increase in fluorescence with depth, with a peak at 19m (Figure 1). After the peak there is a general decrease with depth. However, there are many spikes of fluorescence at different depths through-out the time series. Profiles at midday and early afternoon have a more prominent spike than those taken earlier and later in the day. This could be due to the lower light intensity, which could indicate a decrease in phytoplankton activity. There are exceptions to this such as the fourth cast, whose CTD profile shows a second peak at 40 meters. Most of the profiles have a peak between 18-27m, this is expected to be the area of most productivity. It coincides with a weak thermocline closely followed by a second deeper area of productivity, due to the increase in nutrients concentration at depth.

Figure 1: Graph of Voltage 1 (v) against Depth (m). Each CTD taken has time displayed in (UTC +1) with CTD 5 removed due to an error in the data collected.

Chlorophyll

The graph shows the concentration of chlorophyll in µg/L taken at different times in the day with multiple casts at the same location (L4 buoy). This resulted in a time series of chlorophyll concentration throughout the water column during the day. The graph, however, only shows 4 measurements out of 11 as only 6 casts had NISKIN bottles fired and chlorophyll samples taken. Two of these samples were several orders of magnitude too high due to errors in the computer system and a mistake in the lab and were therefore not used in the analysis.

The profiles do not show much variation throughout the water column, except for some spikes of chlorophyll registered at different depths (25, 30 and 40 meters) (Figure 2). The sample taken at 13:01 UTC(+1) stands out as it registered much higher concentrations at depths below 10m.

Uniformity in the chlorophyll profiles with occasional peaks during the month of July have already been registered in previous studies (Holligan and Harbour, 1977).

Chlorophyll variability throughout the water column can be due to differences in temperature, nutrient availability and physical processes like fronts, upwelling, tidal states, eddies and winds that affect the distribution of light and nutrients across the water column. This consequently has an effect on the concentration of chlorophyll with an increase in depth (Pingree et al., 1978).

Figure 2: Calibrated chlorophyll concentrations (µg/L) plotted with depth and time UTC(+1) displayed for each sample taken.

Oxygen

The processed data from 11 CTD depth profiles taken during the offshore survey at L4 show an increase in oxygen concentration (µmol/L) with depth. Oxygen concentration was found to be lowest at the surface (213.9µmol/L) and increased linearly over the first 25m of the profile (Figure 3). At depths greater than 25m the rate of oxygen concentration began to decrease with a sudden peak past 45m (214.6µmol/L).  

In order to remove the effect of temperature and salinity on the samples, oxygen saturation (%)was plotted against depth. Supersaturated water near the surface (102.4%) was detected at 12m (Figure 4). Oxygen saturation decreased with depth to a minimum of 88.6% at 46m depth. This could be a result of the phytoplankton concentration being highest at the surface and then decreasing with depth, due to an attenuation of light, until a peak at the deep chlorophyll maximum. Decrease in phytoplankton concentration results in less oxygen production and therefore in a decrease in the oxygen saturation profile.

Figure 3: Corrected oxygen concentration (µmol/L) against depth for each cast displaying the time UTC(+1).

Figure 4: Oxygen saturation (%) plotted against depth with results processed from the NISKIN bottle samples.

Zooplankton

The abundance of zooplankton in a m³ of surface seawater decreased approximately by 22,000 number/m3 from the first deployment to the second deployment, this is most evident in Copepoda spp. with a 21,000number/m3 decrease in abundance (Figure 5). There is a significant decrease in Copepoda spp. abundance within a two-hour time difference. Since no significant errors were detected this could be explained by spatial variability. A notable decrease in abundance was also apparent in Decapoda spp. larvae. Copepoda spp. nauplii, Gastropod larvae and Chaetognatha were present in deployment 1 but not in deployment 2 and fish eggs were only observed in the second deployment.

Figure 5: Zooplankton concentration (m3) plotted with time. Two zooplankton trawls were taken at the surface at L4 using a plankton net aboard the Falcon Spirit. Figure 6a. was the first deployment taken at 12:45 UTC(+1). Figure 6b. was the second deployment taken at 14:34 UTC(+1). The two graphs show the abundance of different zooplankton groups in m³.

Phytoplankton

Surface

A variability of phytoplankton was found throughout the surface time series at L4 buoy. The 10:03 UTC(+1) sample was mainly dominated by Leptocylindrus spp. and showed a high species richness (Figure 6). Further throughout the day, the 12:00 UTC(+1) sample showed the greatest diversity of phytoplankton, the majority being Ceratium spp. (Figure 8). The last sample taken at 14:58 UTC(+1) showed the lowest species diversity and out of the two recorded species Chaetoceros spp. was the most abundant (Figure 11).

Throughout the day there is a fluctuation in phytoplankton species abundance (cell/ml) at the surface. The 13:58 UTC(+1) sample showed the most, while the 14:58 UTC(+1) sample showed the least abundance (Figure 18).

Mid-depth

Phytoplankton samples were also taken at mid-depth or at chosen depths where the water column structure on the CTD live stream showed interesting features. The 10:03 UTC(+1) sample was taken at 25m depth and it showed a high diversity of species, Guinardia spp. being the most abundant (Figure.12). The 12:00 UTC(+1) sample was taken at 28m depth and it was mainly dominated by Eucampia spp., showing the highest species richness out of all the mid depth samples (Figure 14). Finally, the 14:58 UTC(+1) sample, taken at 28m depth, showed a similar species richness to that of the first figure, it however is dominated by Chaetoceros spp. (Figure 17).

Throughout the day there is a fluctuation in phytoplankton species abundance (cell/mL) at mid-depth. The 10:03 UTC(+1) sample showed the lowest abundance, while the 14:58 UTC(+1) sample showed the most (Figure 18).

Flow cytometer

The figures show a general decrease in trend of the time series taken at the L4 buoy (Figure 19). There is a peak cell count recorded at 10:03 UTC(+1). Concentration decreases until to 13:58 UTC(+1) with the values decreasing from 17.07 µ/L to 14.15 µ/L (Figure 20). At mid-depth there is more variability in the concentration. Unusually low concentration of cells was observed in the samples taken at 10:03 UTC(+1) and 12:00 UTC(+1), with 8.35 µ/L and 9.26 µ/L respectively. Relative to the other three samples taken, samples at 10:03 UTC(+1) and 12:00 UTC(+1) was taken at deeper depth, both below 20m. This deeper depth could have therefore, impacted the concentration of cells observed. There is a higher concentration of cells in the surface samples compared to at mid-depth, this could be due to a greater irradiance found at the surface.

Throughout the day, the total red fluorescence (RFL) at the surface remained around 2x107 apart from the samples taken at 10:03 UTC(+1) and 12:00 UTC(+1) (Figure 21). This coincides with the low cell concentration at the mid-depth (Figure 20). Total RFL at mid depth shows similar concentrations throughout the time series. However, the total RFL at samples 10:03 UTC(+1) to 12:00 UTC(+1) show a decrease, until a sudden increase and plateauing at 2x107 at 13:01 UTC(+1) (Figure 22).

Figure 18: Phytoplankton total abundance in cell/mL. Time series UTC(+1) location is L4 buoy, samples were collected both at the surface and at mid-depth.

Comparing this data to that of E1 there is a lot of similarity, E1 also has a dominant zooplankton population of copepods but has a higher percentage of Nauplii, but a lower diversity of species than L4. There was also a similar diversity in phytoplankton between the two data points. E1 had a more consistent depth for the chlorophyll bloom but still had much variability as the water column was more stratified than at L4.

References

Holligan, P. M., & Harbour, D. S. (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.

Pingree, R. D., Holligan, P. M., & Mardell, G. T. (1978). The effects of vertical stability on phytoplankton distributions in the summer on the northwest European Shelf. Deep Sea Research, 25(11), 1011-1028.