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Estuarine - Biology

Results

Stations N and 42 show a far lower diversity compared to the others, both share the same dominant species of C. closterium. The dominance of this species is demonstrated by the amount of specimens in the count, with stations N and 42 having numbers above 500 whereas all the other stations contained less than 100 organisms. The exceptionally high counts of C. closterium may suggest that this species is currently blooming in sites N and 42. This is a species that is often known to bloom in spring and summer (Kraberg et al., 2010). The success of this species could be down to the fact that it produces allelopathic chemicals that suppress the growth of other species, leaving resources free for its own growth and reproduction (Hiromi et al., 1995). No other species appears to be blooming, at least not to the same extent, with the species with the next highest percentage being Coscinodiscus in site Q.

Stations P, Q, 41 and 43 are composed of mostly the same species with similar counts. Notable exceptions include the appearance of R. alata at station P and M. rubrum at station 43. Where there is N. longissima (sites P, Q, 41, 43) there is no C. closterium, which may suggest competition between the two species. Another possible explanation is that the N. longissima could have been misidentified because it is similar in appearance to C. closterium.

Phytoplankton

Collection

Water samples were collected in Niskin bottles and then pipetted into brown glass bottles that contained lugols. The bottles were filled and then taken back to the ashore lab to be analysed.

Analysis in ashore lab

The samples were transferred to glass tubes in the lab. They were vacuum drained, leaving 10ml of sample to be taken to the microscopes. At the microscopes, the sample was pipetted onto a gridded slide, ensuring no air bubbles were between the slides. 100 grids were analysed. In each grid the species present were analysed and the numbers of each species recorded.

Figure 1: : Percentage data for each group seen in the key of zooplankton taken on 10/07/17 at 50°15.2N 5°02.4W taken at 08:43UTC (site N); 50°13.4N 5°01.5W 10:57 (site P); 50°12.9N 5°01.6W (site 41) at 12:34; 50°11.7N 5°02.5W (site 42) at 13:11; 50°10.9N 5°02.6W (site 43) at 13:45. Any phytoplankton with a less than 1% contribution at any site were place in other phytoplankton. All samples taken in the upper 2m.

Zooplankton

The zooplankton has much higher numbers are much higher up the estuary. The farthest point up the estuary (site O) has zooplankton numbers of over 10000 (Fig. 3) which declines to site P to 2074 before it declines to the lowest count at site 44 at the mouth of the Fal. Copepods dominate the sites higher up the river and account for almost 98% of the zooplankton at site O. Despite such large copepod numbers up the estuary their nauplii remain in fairly consistent in numbers across the sites. Polychaete, decapod and gastropod larvae all peak in the middle site showing that the middle estuary is a good breeding ground.  Their number could have been affected by the huge number of copepods at site O.  The profiles could have also been affected by the change of time between the when the samples are taken since copepods, and lots of other zooplankton, show twilight migration (Simard et al., 1985). This would mean the high level of copepods is due to their diel vertical migration and not the location as they move down water column, the sample were taken at the surface, as the day goes on.

Figure 2: Normalised data for each group seen in the key of zooplankton taken on 04/07/17 at 50°14.4N 5°00.9W taken at 0940UTC (site O); 50°14.4N 5°00.9W 12:26 (site P); 14:01 (site 44). Mesh size was 210micrometres (site O and P) and 200micrometres (site 44). Any zooplankton with a less than 1% contribution at any site were place in other zooplankton. All samples taken in the upper 2m.

Figure 3: Percentage data for each group seen in the key of zooplankton taken on 04/07/17 at 50°14.4N 5°00.9W taken at 0940UTC (site O); 50°14.4N 5°00.9W 12:26 (site P); 14:01 (site 44). Mesh size was 210micrometres (site O and P) and 200 micrometres (site 44). Any zooplankton with a less than 1% contribution at any site were place in other zooplankton. All samples taken in the upper 2m.

References

Kraberg, A., Baumann, M., Durselen, C.D., 2010, Coastal Phytoplankton: Photo Guide for Northern Europe Seas, Verlag Dr. Friedrich Pfeil, Munchen, Germany, 204

Hiromi, J., Imanishi, D., Kadota, S., 1995, ‘Effect of Cylindrotheca closterium (Bacillariophyceae) on the groeth of red-tide raphidophycean flagellate Hetersigma akashiwo’, Bulletin of the college of Agriculture and Vetinary Medicine, Nihon University, 52: 122-125

Simard, Y., Lacroix, G. and Legrende, L. 1985, ‘In situ twilight grazing rhythem during diel vertical migrations of a scattering layer of Calanus finmarchius’, ``, 30(3), 598-606

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