Falmouth Field Course 2017

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Biology

Introduction

The total of 11 phytoplankton samples and 6 zooplankton samples were taken to measure biological parameters offshore. These were performed on the 11.07.17 between 07:05-14:15 UTC and aimed to develop an understanding of the offshore plankton communities at each of our stations.

Methodology

Phytoplankton: Using a 50ml measuring cylinder, 100ml of surface water was stored in a sample bottle containing Fugol’s Iodine to preserve the plankton in situ for analysing later in the lab. During the lab, water was placed on a slide with Fugol’s Iodine and analysed under a light microscope. The organisms were identified using books and pictures. The number of species found in 100 squares (ml) was written and the number of individual members of that species was tallied.

Zooplankton: We performed 6 zooplankton tows using a 200micron mesh, 60cm diameter net. A CTD profile taken first at each site meant that it was possible to perform subsurface tows at specific depths (eg. The chlorophyll maximum) and therefore sample the more biologically active parts of the water column. The plankton tow 10ml of sample water collected from the zooplankton nets was placed in a clear dish and analysed under a microscope. Using pictures and books, we noted every species seen and tallied the individual organisms of that species.

Limitations: During the surface plankton trawls the net often wasn’t fully submerged the whole time, due to the rough sea conditions. The sea conditions may have also lead to minor fluctuations at depth trawls as well. Furthermore, the mesh collection end of the plankton net, when it was poured into the sample bottle, always left some residual plankton behind. All of these limitations may have led to a minor reduction in the representation of the plankton population.

References:

Hecky, R. & Kilham, P., 1988. Nutrient limitation of phytoplankton in freshwater and marine envrionments: A reveiw of recent evidence on the effects of enrichment. Limnology and Oceanography, 33(4part2), pp. 796-822.

Kleppel, G., 1993. On the diets of calanoid coepods. Marine Ecology, Volume 99, pp. 183-195.

S.W, K., Onbe, T. & Yoon, Y. H., 1989. Feeding habits of marine cladocerans in the Inland Sea of Japan. Marine Biology, Volume 100, pp. 313-318.

Sommer, U., 1994. The impact of light intensity and daylength on silicate and nitrate competition among marine phytoplankton. Limnology and Oceanography, 39(7), pp. 1680-1688.

Results: Phytoplankton

The greatest species abundance was observed in station C39’s 23.24 m trawl with 138 cells per ml, whilst the lowest abundance was seen in station C40’s 23.97 m trawl with only 22 cells per ml. Offshore stations generally showed greater species abundances than inshore stations; stations C38 and C39 showed a total species abundance of 182 and 173 cells per ml respectively, whilst stations C40, C42 and C43 showed a total species abundance of 102, 25 and 148 cell per ml respectively. However, inshore stations showed a greater species richness than offshore stations; samples taken from station C42 displayed a richness of 19 species, the highest of any sample. Abundances of phytoplankton were generally unmatched by abundances of zooplankton; for example, there was a peak in phytoplankton abundance at station C39, but the same station at a similar depth recorded the second lowest abundance of zooplankton.  

Discussion

Copepods were present in all our zooplankton samples, due to their ability to freely adapt their feeding mechanisms based on the available food sources. Kleppel (1993) explains this behaviour as a means to enhance the probability of obtaining a nutritionally complete diet. As seen in figure 2a, approximately 600 per m3 Cladocera were estimated from surface samples at C38. This corresponds to a community of three different diatom species seen at the same station in figure 1a: Rhizosolenia imbricata, Rhizosolenia setigera and Rhizosolenia alata. Studies by Kim et al. (1989) found that the diet of Cladocera favours a diet of diatoms which explains the positive correlation between Cladocera and diatom species richness.

Figure 1a shows greater species abundance in samples taken from mid and deep water than samples taken from the surface. This could be because nutrients such as silicate, phosphate and nitrate are depleted in the surface waters (Hecky & Kilham, 1988) as seen in the figures on the “Offshore- Chemistry” page of this website. This pattern is most evident in station C39 where there is an approximate 27% increase in individual numbers of phytoplankton from surface water samples and samples taken at 23.24m. As well as a change in abundance, phytoplankton community is also altered potentially by the availability of nutrients (Sommer, 1994). The surface of C39 is dominated by Psuedo-nitzchia seriata whilst Chylindrotheca closterium dominates the mid water.

The division of the large task of processing plankton samples in the lab across the entire group created some uncertainty in the results. Each sample was processed under a light microscope by a different team member. The subjective nature of the identification process created a lack of consistency across the results. As different people identified cells and organisms differently, some plankton taxa could be incorrectly included in results and others could be incorrectly omitted.

Results: Zooplankton

The greatest species abundance was detected in station C38’s surface trawl with 1,160 organisms per m3, whilst the lowest abundance was seen in station C42’s surface trawl with 301.84 organisms per m3. For all stations, surface trawls showed greater abundances than trawls at depth; for example, station C39’s surface trawl showed 476.8 organisms per m3 and trawl at 20 m showed 388.51 organisms per m3. The greatest species richness was seen at stations positioned inshore; samples taken at both surface and depth for station C40 displayed 12 different species, the highest number for any trawl. A large abundance of Cladocera observed in samples from station C38 was positively correlated with the presence of Rhizosolenia imbricata, Rhizosolenia setigera and Rhizosolenia alata.

Figure 2a-b: A stacked vertical bar chart of the main zooplankton species numbers (per m-3) at each of the samples. Figure 2a. consists of all of the identified zooplankton species that had all of the values >10 for all the sites, the remaining species are compiled as ‘other’ in Figure 2a. Figure 2b. contains the quantities of the ‘other’ species shown in figure 2a. All samples were taken from the R.V. Callista on 11/07/17 from 07:05-14:15 UTC. High water: 06:31 and 18:42 UTC, low water: 00:41 and 12:55 UTC.

Figure 1a: A stacked vertical bar chart showing the quantities of the main taxa of phytoplankton identified and enumerated in the lab on 12/07/17. Most taxa were identified to genus level, some to species level. Figure 1b: The same chart derived from the same samples, but showing the minor or less common taxa as ‘other’. All samples were taken from the R.V. Callista on 11/07/17 from 07:05-14:15 UTC. High water: 06:31 and 18:42 UTC, low water: 00:41 and 12:55 UTC.