Introduction

Offshore of Falmouth, in the Western English Channel, primary productivity fluctuates over time.  Light availability and vertical mixing of nutrients in the water column limit production and directly impact plankton abundance. An increase in phytoplankton production and a decline in nutrients results from thermally stratified water, which is expected to occur in the summer months due to increased solar radiation; this affects the plankton community structure. The objective of this investigation is to understand how the rate of vertical mixing affects phytoplankton and zooplankton populations and productivity offshore.

Methods

Four sites were sampled in the English Channel progressing in a line from Falmouth, with the first station occurring at Black Rock. At each site Niskin bottles and plankton nets were deployed, CTD and ADCP profiles were recorded and Secchi disc depths were measured. A further site was sampled on the estimated position of the front where only the CTD and ADCP profiles were taken. Water samples were obtained for each of the stations which were then prepared for later use in the lab, along with the zooplankton samples obtained from the plankton nets.

The lab was split into two sections: biology and chemistry. In the biology section, the zooplankton and phytoplankton were observed and counted under microscopes to find the concentration per ml. In the chemistry section, the samples were analysed for chlorophyll, phosphate and silicon using the methods shown in Parsons et al.1 the dissolved oxygen tested for using the method from Grasshoff et al.2  and the nitrate was analysed by flow injection 3.

References

1 Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon.

2 Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH.

3 Johnson K. and Petty R.L.(1983)  “Determination of nitrate and nitrite in seawater by flow injection analysis”.  Limnology and Oceanography 28 1260-1266.

4 http://www.westernchannelobservatory.org.uk/l4_phytoplankton

5 Ban, S. and Burns, C. The paradox of diatom-copepod interactions. Available at :http://www.int-res.com/articles/meps/157/m157p287.pdf. [Accessed 1 July 2013].

The views expressed above are those of the authors and not those of the University of Southampton or the National Oceanography Centre Southampton.

Biological Findings

Light is one of the fundamental controlling factors on phytoplankton abundance and primary production. The attenuation coefficient, k, is uniform throughout all the stations so there is little variation in light received (Figure 7).

The preliminary findings for phytoplankton show a clear diatom dominanted community structure (Figure 11.a) especially of Chaetoceros (Figures 11.b-11.e). This concurs with the usual trend found at the nearby Plymouth stations4 with a clear dominance of diatoms during the months of June, July and August, followed by a bloom of dinoflagellates. Dinoflagellates are also present in the samples but only have a small proportion in comparison to diatoms. Chlorophyll and fluorometry data (Figure 9 and 10) for each station show peak values that coincide approximately with the bottom of the stratified layer where phytoplankton can obtain both plenty of nutrients from the well mixed bottom waters and enough sunlight for photosynthesis. Station 5, which was located on a tidal front, showed a large concentration of phytoplankton throughout the stratified layer as nutrients here can be obtained in a number of ways other from the deep water. Therefore the phytoplankton are not limited to the stratified layer boundary at this station.

Date: 24/06/13

Vessel: RV Callista

Wind: North Westerly between 4 and 5, dropping later to between 2 and 3.      

Tides: High tide of 5.1m at 06:31(all times UTC) and low tide of 0.3m at 13:51

Station 1(Blackrock):

Location: 50°08.832N 005°01.551W

Time(UTC): 08:24

Cloud cover: 6/8

Depth: 31.0m

Station 2:

Location:50°07.478N 004°58.789W

Time: 10:17

Cloud cover: 7/8

Depth: 31.2m

Station 3:

Location: 50°05.675N 004°58.276W Time: 11:38

Cloud cover: 5/8

Depth: 63.6m

Station 4:

Location: 49°59.899N 004°40.413W Time: 13:27

Cloud cover: 7/8

Depth: 71.5m

Station 5(Front):

Location: 50°07.512N 004°57.807W Time: 15:24

Cloud cover: 5/8

Depth 49.25 m

Offshore station locations

Offshore

Figure 11.a Concentration of phytoplankton genera found at each station.

Table 1. Absence/presence table for Zooplankton taxa for each station.

Physical Structure

The sampled stations showed varying levels of stratification dependant on their distance offshore. Station 1, off Blackrock, showed a well-mixed water column with little to no stratification in temperature (Figure 1). This mixing was likely caused by tidal flow as the tidal range on the 24th was 5.1m indicative of a macro tidal estuary, which are typically well mixed. Station 2 shows much more stratification than 1 but less than Stations 3 or 4, which showed strongly stratified, water columns.  All stations show a less pronounced degree of stratification in salinity than temperature (Figure 2) but still show an increase in salinity with depth. However, the increase at station 1 is twice as steep as any of the other stations, due to it still being part of the estuarine system of the Fal. Both the halocline and thermocline are at approximately 25m for Stations 3 and 4 and shlightly shallower for Stations 2 and 5 . This water column structure was verified by calculation of Richardson numbers for each meter of depth. Station 1 showed values far below 0.25 suggesting that shear velocity outweighed buoyancy and created a well-mixed water column. Station 3 and 4 showed much higher Richardson numbers, which verifies the above mentioned stratification present.

Nutrients and Chemistry

All Station showed depleted nitrate levels in the surface water, which is what would be expected in the stratified conditions found with higher nutrient levels found in the deeper water below the stratification. In Stations 3 and 4 however, Nitrate does not follow this structure with minimum values seemingly occurring in the deepest samples (Figure 3).  All stations show the expected relative nutrient concentrations for Silicon and Phosphate concentrations (Figure 4 and 5), with the bottom samples having high concentrations and depleted values coinciding with chlorophyll maxima at each station. Surface values are not depleted in the way nitrate values are suggesting nitrate was the limiting factor in the surface waters.

Dissolved oxygen is fairly uniform with depth for all Stations (Figure 6) excluding Station 4 which has much higher percentage Oxygen saturation in the surface water Most probably due to the increased photosynthesis indicated by the high chlorophyll levels at Station 4.

Figure 1. Temperature profile for all stations from CTD

The backscatter recorded by the ADCP can be used to locate the position of the plankton within the water column, shown by the backscatter recorded at station 4 (Figure 12). The main zooplankton biomass is usually found below the phytoplankton maximum. Phytoplankton blooms provide food for zooplankton, so naturally they were also found in abundance where the phytoplankton peaks occurred. All sampled stations showed a high concentration of copepods (graph 13.a-c) which were the dominant zooplankton taxa; Table 1 shows the taxa of zooplankton which were present at each station. It is also important to note that there were several ctenophores sampled that were larger than the majority of zooplankton and could not be included in the count due to sampling methods. These would have had a major impact on the food web dynamics.

Copepods feed largely on diatoms5 and this marries nicely with the phytoplankton data. However, it has been shown that their interactions are not always straightforward so the food web is probably more complex than first appears.

Figure 12. ADCP Backscatter plot

Tidal Front

A tidal front was found on the underway data whilst heading offshore (Figure 14) and was then sampled with a CTD as Station 5. It had a broader chlorophyll peak than the other stations and if sampled for phytoplankton would have been likely to have been dominated by more dinoflagellates. This station is interesting for the comparison of the well mixed inshore water column and the stratified offshore water.

Figure 14. Screenshot of underway data onboard RV Callista across tidal front