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Biological

Zooplankton

Phytoplankton

The data for Stations A & B were both collected at Site 39, the furthest point offshore that was sampled. Station A was collected from 30-20m and Station B was collected from 15-0m. The species diversity for both depths was relatively low, with only 3 species present at both depths. At greater depth, Copepoda, Copepoda naupili and Cirripedia larvae were the only zooplankton present and all found at the density of 38 individuals m-3. Copepods were found in greater abundance in Station B along with Echinoderm Larvae at around double the abundance and Siphonophores at very high abundances (>500 individuals m-3). Station C was conducted Site 41, much closer to the shore that Site 39. The Station C data was collected from 15-0m and showed a higher number of species present (5 recorded) with Polychaete larvae being the dominant form of zooplankton at this Station. The Station D data was collected at Site 43, in the mouth of the Fal Estuary and again much closer to shore. The zooplankton were collected from 15-0m and here the highest diversity was observed with 10 types of zooplankton recorded. The following zooplankton were found in roughly the same abundances: Cladocera, Decapod larvae, Gastropod larvae, Siphonophores, Ctenephora and Echinoderm larvae were all found at an abundance between 22 and 47 individuals m-3. Cirripedia larvae, Polychaete larvae and Hydromedusae were also found at similar abundance of 189-212 individuals m-3. By far the most abundant zooplankton was Copepoda at 354 individuals m-3.

The general pattern seen in this data is that diversity of zooplankton decreases from 0-15m depth with an increase in distance from shore as Stations C & D were conducted close to the shore and had higher diversities than the offshore Site 39. This is likely due to the waters closer to the shore are more nutrient rich due to the riverine input from the Fal and Helford Estuaries being able to support more phytoplankton and offer more food to the zooplankton. For example the highest abundance of zooplankton was seen at Site 43, the mouth of the Fal estuary, where there was also the highest diversity of phytoplankton, which mean there was more variety of prey items to support a wider range of zooplankton.


The overall number of phytoplankton decreases as we move from the offshore stations to the estuarine stations however diversity increases (Fig. 2). For Station 40, in the deep water at 35m there were very high numbers of one species, however 29.5m seemed to have the greatest diversity. For Station 41 there was high abundance of Chaetoceros socialis at 1.3m, but more diversity in the middle depth at about 8.3m and finally fairly high abundance of Gyrosigma at the deepest around 15.2m. At Station 42 there was a high number of Chaetoceros socialis again at around 1.5m, then higher diversity of species in the middle depths of around 8.3m. For Station 43 there was greater diversity at the 1.7m than at the other stations, 8.41m had the largest diversity of phytoplankton types and finally at 19m, there was lower diversity. Overall phytoplankton diversity with depth seemed low in surface waters, high just below surface and low again at depth with often a high abundance of one species being found in very shallow and deep water.

This pattern could be because there is often nutrient remineralisation occurring at the depths below surface water and therefore more phytoplankton will be found here in order to gain both nutrients and light (Uitz et al, 2006). Fewer species will be found in shallow water because surface waters are often low in nutrients, and they will be low in deep due to not enough light penetrating. However the numbers here are likely to be high of one individual species as they may be specialised and adapted to deal with this environment. Some of the species found such as dinoflagellates may be able to migrate between depths (Olli, 1999).


Reference

K.Olli (1999) Diel vertical migration of phytoplankton and heterotrophic flagellates in the Gulf of Riga. Journal of Marine Systems, 23:145-163.


J.Uitz, H.Claustre, A.Morel and S.Hooker (2006) Vertical distribution of phytoplankton communities in open ocean: An assessment based on surface chlorophyll. Journal of geographical research, 111:


Fig. 1 Zooplankton abundance for four samples across three sampling sites in the Fal and Helford Estuary. Stations A & B were both taken at Site 39 but at different depths (30-20m for A and 15-0m for B). The data for Station C was collected at Site 41. The Station D data was recorded at Site 43.

Fig. 2 Phytoplankton abundance at stations 40, 41, 42 and 43 within the Fal and Helford Bay