Falmouth Field Trip 2014- Group 3

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Produced by: Alice Duff, Philippa Fitch, Joanna Gordon, William Harris, Thomas Jefferson, Eirian Kettle, Jesse Marshall, Dominique Mole, Emma-Jo Pereira, Joshua Walton

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Phytoplankton

Results

Station 45

13 different genera of phytoplankton were identified in the samples from Station 45. Chaetoceros were dominant, showing a slight decline in abundance at 40.70m followed by an increase again at 60.50 m. These fluctuations in abundance are likely to be due to counting error rather than natural patterns as abundance is expected to decrease with depth. Counting errors may have been introduced as six people analysed the phytoplankton samples. The greatest diversity of phytoplankton was seen at 33.17 m . This was expected as it was the depth of the chlorophyll maximum indicated on the CTD depth profile. The surface reading at 5.75m showed the lowest diversity and generally had the lowest count.

Station 47

There was less diversity at Station 47 than Station 45 with only 8 genera of phytoplankton identified. Chaetoceros was dominate at both depths sampled, but was slightly more abundant at 20m. Leptocylindrius danicus also follows this pattern. Ceratum furca was present at both depths (8000 cells L-1 at 2.21 m and 7000 cells L-1 at 20m). Dinophysis, Karinia mikimotoi and Rhizosolenia sp were all only identified at 20m, whereas Guinardinia was only identified at 2.21m.

Station 48

A greater diversity of phytoplankton were identified at Station 48 than were presents at Station 47. However, from the 11 genera identified, only 2 were found in surface water at 3.12 m depth; Chaetoceros and Rhizosolenia sp. Chaetoceros were found in their highest numbers in the surface waters (32000 cellsL-1 at 3.12 m and 16000 cellsL-1 at 12.37m) which could indicate a bloom at this station. Dinophysis and Guinardinia delicatula were only identified at 46.00m whereas Thallassiosira were only found at 12.37 m at this station. Phytoplankton tend not to reside in shallow surface water (3.12m) as nutrients are limiting at the surface and predators are more prevalent. Ceratum furca, Coscinodiscus, Guinardinia, Mesodinium rubrum and Nitschia reversa showed the same number of cells L-1 (approximately) for samples collected at 12.37m and 46m.

Station 50

10 phytoplankton genera were identified at Station 50. From the CTD depth profile, the chlorophyll maximum was shown to be approximately 12m. The highest counts of phytoplankton tend to be in the samples collected at 11.48m, with the exception of Guinarfinia and Eucampia spp which were only present in the 63.90m samples and Nitschia reversa which was only identified at 5.95m samples. There was a higher diversity of phytoplankton in the 63.90m sample, but the number of phytoplankton tended to be lower. Surface numbers were also generally lower except for Chaetoceros, Guinardinia delicatula and Thallassiosira which were all present in fairly high numbers (<5000 cells L-1). This count indicate a bloom in these particular groups of phytoplankton.

Discussion

Phytoplankton tend to be lower at the surface samples than the deeper samples. This pattern is probably due to the phytoplankton remaining at the nutricline where nutrients are high rather than at the surface where nutrients are limited. The phytoplankton that were present at the surface may be more efficient at utilising the small amount of nutrients or may have already absorbed the nutrients they require so can remain shallow water to photosynthesis. Chaetoceros is considered one of the largest genus of marine plankton with approximately 400 species (von Quillfeldt, 2001). This could explain why they dominated phytoplankton numbers at each station as it is hard to distinguish between species so they were grouped together.

Zooplankton

Overall, 15 phyla of zooplankton were identified. The most abundant phyla were Cladocera (621 individuals; 8899m3), Hydromedusae (285 individuals; 4671m3), Echinodermata (235 individuals; 3571m3) and Copepoda (224 individuals; 3348m3). All but Echinodermata was found at every station and depth sampled. Station 50 was a front, indicating notable differences in water densities, temperature, salinity, nutrients and chlorophyll. This correlated with the high abundance and diversity of zooplankton found at the site compared to the other stations sampled (Figure 5).

Station 45: 11 Phyla of zooplankton were identified, 3 of which were found at both depths. The results show a consistent abundance of hydromedusae and copepoda from 45m to the surface. 9 phyla were found between 25-40m while 5 were found above 25m. The CTD measurements presented a high fluorescence level (0.2523v at 30m) at the location of sampling. This correlates with the higher abundance and diversity of zooplankton phyla found. Overall the inference made is that the majority of zooplankton abundance is found below 25m at this station. This could be largely due to the thermocline found between 10m (17.7259 oC) and 20m (14.322OC). (Figure 6)

Figure 7 and Figure 8 show the results for zooplankton type and abundances at Stations 47 and 48 (respectively). These stations were not located on the frontal system so zooplankton abundance reflect typical patterns for conditions found on the landward side of fronts.

Station 50: 15 phyla of zooplankton were found at this location. All 15 were found below 25m and 11 were found above 25m. However, a clear prevalence of zooplankton biomass can be seen below 25m. This was the most abundant station with a significant increase in phyla and individuals recorded. This is most likely due to time constraints on other stations’ sample analysis. The data collected indicates a front at the area of sampling. The notable difference in velocity within the surface 10m ( 0.859 m/s at 15m – 0.519 m/s at 26m) indicates the differences in density of two water bodies associated with a front. The reduction of nutrient availability (Si 4.36, PO4 0.37, N 2.33) compared to other stations (Station 45: Si 4.6, PO4 0.37, N  2.28) and the increase in chl (1.0408 v) compared to Station 45 (0.304), indicated areas of high productivity. This correlates with the high level of heterotrophs found. (Figure 9).

Offshore - biological