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Offshore- Biology Terramare

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.

References:

Forward, R. 1988, 'Diel vertical migration: Zooplankton photobiology and behaviour', Oceanography and Marine Biology Annual Review, 26, 361- 393

Hutchinson, G. 1967, 'A treatise on limnology. Volume II. Introduction to lake biology and the limnoplankton. John Wiley and Sons, New York, London and Sydney

Huntley, M., 1985. Experimental approaches to the study of vertical migration of zooplankton. Contributions in Marine Science, 68.

Turner, J. 1991, 'Zooplankton feeding ecology- do cooccurring Copepods compete for the same food', Reviews in Aquatic Sciences, 5(2), 101-195

The changes in the zooplankton concentrations for each species (Fig 1 and 2) shows that each group is undergoing diel vertical migration (Forward, 1988). Simplistically vertical migrations of zooplankton can be classed into 3 categories (Hutchinson, 1967):

• Nocturnal migration where minimum depth is reached at night and conversely the maximum depth is at midday
• Twilight migration in which minimum depths occur after a rise at both sunset and sunrise before normal night or day depths are resumed after
• Reverse migration where the maximum depth is at night and minimum depth during the day. The vast majority of zooplankton exhibit the nocturnal or twilight migration (Huntley, 1985). 

Dominant Species

Copepods are the dominant zooplankton and their nauplii are therefore the most abundant in the smaller mesh size. Copepods which were mainly found in the 200 micrometres net were fairly consistent in their abundance before showing a large increase at the final site.

Copepod nauplii show huge increases, to over 350 counts per litre (Fig 3), at site, 6 before completely disappearing an hour later.  The decrease also coincides with an increase in mature copepods and the large increase of nauplii may have caused the decrease in Copepods at site 6 (Turner, 1991) since copepods feed on the same groups throughout their life stages.

Other Species

Most of the zooplankton apart from echinoderm larvae showed the same pattern across the two nets. The reverse trend between the two nets for echinoderm larvae is an anomaly which would require more data to explain or show as an irregularity.

Most of the smaller cohort of zooplankton increase in numbers during the day, moving from site 4-8. Since they peak later in the day they are most probably rising back up the shallow depths for twilight. This can be seen the majority of the zooplankton having lower densities at the 5m sample compared the deeper standard samples (Fig. 5). The exceptions to the rule are the echinoderm and decapod larvae which have highest values at the shallow sample and therefore must be displaying a reverse migration pattern. Although we can conclude diel migration is occurring it's hard to infer any further migration patterns without data for a 24hr period. 

Zooplankton

Fig 1: Normalised data for each group seen in the key of zooplankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5); 14:01 (site 6) and 15:01 (site 7). Sites 4 and 5 were taken at 25m and 6 and 7 at 22m. Mesh size was 100 micrometres. Any zooplankton with a less than 1% contribution at any site were place in other zooplankton.

Fig 2: Normalised data for each group seen in the key of zooplankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5); 14:01 (site 6) and 15:01 (site 7). Sites 4 and 5 were taken at 25m and 6 and 7 at 22m. Mesh size was 200 micrometres. Any zooplankton with a less than 1% contribution at any site were place in other zooplankton.

Fig 3: Count data for each group seen in the key of zooplankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5); 14:01 (site 6) and 15:01 (site 7). Sites 4 and 5 were taken at 25m and 6 and 7 at 22m. Mesh size was 100 micrometres. Any zooplankton with a less than 1% contribution at any site were place in other zooplankton.

Fig 4: Count data for each group seen in the key of zooplankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5); 14:01 (site 6) and 15:01 (site 7). Sites 4 and 5 were taken at 25m and 6 and 7 at 22m. Mesh size was 200 micrometres. Any zooplankton with a less than 1% contribution at any site were place in other zooplankton.

Fig 5: Count data for each group seen in the key of zooplankton taken on 04/07/17 around 50°05N 4°52W taken at 12:26UTC (site 5). The counts of the two nets (100 micrometres and 200micrometres) was added together.

Fig 2: Count data for each group seen in the key of phytoplankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5) and 15:01 (site 7). All samples were taken at 2m depth. Site 5 was repeated.

Fig 1: Count data for each group seen in the key of phytpolankton taken on 04/07/17 around 50°05N 4°52W taken at 1033UTC (site 4); 12:26 (site 5) and 14:01 (site 6). Sites 4 and 5 were taken at 25m and site 7 at 22m depth. Site 5 was repeated.

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The views and opinions expressed on this website are of the individuals and not necessarily of the University.

Data collected on-board Terramare generally shows that phytoplankton numbers are higher at the chlorophyll maximum than in surface waters. This is demonstrated in Figure 1 and Figure 2.

The figures show that species composition within the water column can vary greatly between different depths. For example, at site 4 the surface waters are not dominated by one particular phytoplankton group however in the chlorophyll maximum 7 main species are dominating (R. flaccida, C. fusus, Chaetoceras sp, R. alata, K. mikimotoi and R. stolterforthii). At the chlorophyll maximum at site 5a a large proportion of the phytoplankton population is made up of one single species (R. setigera). This is a species which was present in large numbers across the area sampled which indicates that it has a particularly dominant role in the ecosystem.

In general, surface populations tend to have fewer dominant species but more individuals in each population. For example, at site 6 there are fewer species present than at site 7 at the chlorophyll maximum but each species has more individuals than at site 7. This is also true when comparing site 5b at the surface and at depth. This suggests that there is a higher diversity in phytoplankton populations at depth than at the surface.

Figure 2 shows that as time increases, overall numbers of individuals present in surface waters increases. For example, population levels peak at site 6 at 14:01 UTC. This could be due to migration by individuals to the surface, to increase production by ensuring reactions can occur at maximum speeds.

Two data sets are present for site 5, (5a and 5b) as a repeat was done on this data. The repeat data recorded for this site varies greatly in number of phytoplankton present as well as species present. This makes it difficult to infer results and make reliable and accurate predictions on population structure. The difference seen is likely to be due to human error as the counting and identifying procedure was extremely subjective and could vary between individuals due to a lack of experience within the field.