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

Phytoplankton

Zooplankton

Estuary - Biological

Results - Figure 1 shows the distribution and abundance of phytoplankton at Stations 1, 4, 5 and 7. Nine genera of phytoplankton were identified in the samples from Station 1, 7 of which were diatoms. The dominant diatom species were Chaetoceros, Guinardinia and Thalosiossira all of which had an abundance of >2 x 105  cells L-1. A bloom of the protist Euglena was found at this site with an abundance of 3.1 x 105 cells L-1. Unidentified ciliates and diatom individuals reached a collective mass of 3,4 x 10 3 cells L-1 (Figure 1a)

At Station 4 six genera were found, 5 of which were diatoms and 1 of which was Tintinnina. Abundance was dominated by Chaetoceros with 1.1 x 105 cells L-1. Evidence of a Euglena bloom which was observed at the previous station was significantly reduced. Unidentified ciliates and diatom individuals reached a collective mass of 1.6 x 103 cells (Figure 1b).

Nine genera were found at Station 5, of which 7 were diatoms, 1 was a dinoflagellate and 1 ciliate genus. The water column at this site was dominated by Thaliossiosira with an abundance of 1.5 x 105 cell L-1. At this point some succession of the less abundant plankton genus is seen (Figure 1c).

A notable drop in diversity of genera is seen at Station 7. 4 genera are found overall, 3 of which are diatoms and 1 of which is a dinoflagellate. Abundance was also notably smaller, the highest biomass from Leptocylindrius being 3.6 x 104 cells L-1 (Figure 1d).

Figure 2 shows some images of the phytoplankton found.

Discussion -At Station 1 the Euglena bloom may be explained by increased rainfall causing increased organic runoff from nearby farms. The high diatom presence will be due to the increased geochemically weathered silicon and the lesser proportion of phytoplankton species that can tolerate low salinity (31.19 at sample location) resulting in reduced competition. However the equally large numbers of Tholassiosira and Chaetocerus indicate and amount of competitive succession as no one genus has dominated the water body. It is expected that phytoplankton abundance, and diversity due to competition, would be greater at the riverine end of the system due to the increased amount of nutrient inputted to the system (add measurement), this would also lead to the inference of silicon removal and thus a non-conservative estuarine system. The increased DO saturation levels (103.9 % at 31.926 salinity) at the source of the system correlates with an increased level of primary production at this point.

As the salinity increases the biodiversity stays equal to that seen at station one however at station 5 the biomass has reduced to ~50% of that seen at Station 1. Station 7’s significant drop in both abundance and diversity is likely due to the salinity changes (∆s=1.669 from station 5-7) found at the mouth of the estuary and the rising tide which will have brought the off shore turbidity into the estuary causing migration of species not able to tolerate the increased turbidity.

Results - Figure 3 shows the distribution and abundance of zooplankton at Stations 1, 4 and 7. Station 1, the highest position upstream sampled, generally had the highest number of each genus of zooplankton. However, there was a lower abundance of Cladocera (1.1m3 compare to 12.7m3 at Station 7) and less Chaetognatha (1.1m3 compared to 3.3m3 at Station 4). Station 1 was dominated by copepod (8.9m3), Copepod nauplii (4.5m3) and polychaeta larvae (4.5m3). The highest abundance of larvae was found in this area of the estuary, with fish larvae only being present in samples from Station 1.

Station 4 showed intermediate numbers of each zooplankton genus with the exception of the copepod abundance which was lowest at Station 4. No Hydromedusae were present in samples taken from Station 4. The most dominate genus at Station 4 was Chaetognatha (3.3/m3).

At Station 7 cladocera abundance was high(approximately 12 times greater than at Station 1 and 4). Copepoda were also present in high numbers (6.4/m3). However, all other recorded zooplankton genera were found in low numbers, no greater than 1/m3. Gastropod larvae were only found at this station but Chaetognatha, echinoderm larvae and fish larvae were not present.

Discussion - Zooplankton numbers peak in the summer months at the end of the phytoplankton spring bloom, which are their main food source and when light is at its highest. The data should therefore represent the highest number of zooplankton production in the course of a year for the Falmouth area.

Our results show that larvae tend to reside higher up stream within the estuary. This is typical as larvae require high nutrient concentrations and low turbidity (conditions found within the upper Fal estuary).

Copepoda are found throughout the estuary however their numbers are lowest at Station 4. This may be due to the fact that Station 4 was sampled during high tide. This could have resulted in zooplankton migrating from the mid estuary in order to graze on phytoplankton being flushed upstream. Chaetognatha numbers increase more than twofold from Stations 1 to 4 but they were not present at Station 7. This could be due to an increase in predation as Chaetognatha are an important prey for fish

Larink & Westheide, 2011). At Station 7 the estuary widens and more rivers feed into the Fal estuary therefore there would be a greater diversity of predators. Chaetognatha also prefer temporal and permanent stagnant waters (Forro et al., 2008). This could also explain the lack of Chaetognatha at Station 7 as turbidity, and thus mixing, increased from the head to the mouth of the estuary (from station 1-7).

Cladocera numbers are approximately 12 times greater at Station 7 than at the other stations. Cladocera are found in almost all ponds, lakes and slow flowing rivers but only a few species secondarily invade the marine pelagia (Larink & Westheide, 2011). Therefore, it is unusual that an increased number was found at the mouth of the estuary. This could be due to a number of variables, the first being the recent rainfall. After a period of dry weather, several days of rainfall may have resulted in an increase of riverine flow. This could have caused flushing of the estuary and thus a shift downstream of zooplankton. This would affect this species, Cladocera, in particular due to its small size, 0.2-6.0mm. Another explanation may be that the validity of our data was affected by few repeats and multiple investigators identifying species.

Figure 4 - zooplankton images

Figure 2 - phytoplankton images

Click to enlarge

Click to enlarge