Estuary discussion

Biology

Phytoplankton

The greatest abundance of phytoplankton species were found middle to lower estuary stations (4-6), with a maximum of about 2000 cells per ml. In these stations, diatom species were proportionally dominant, especially Chaetoceros. Members of Thallasiosira, Guinardia, and Rhizosolenia made up the remainder, with some dinoflagellate species. This matches the findings of the offshore research, where diatoms were also dominant, possibly due to the turbid conditions.

Diatoms also dominated stations 2 and 3, located further up the estuary, however total abundance was less than 1000 cells per ml. The more dynamic environment of an estuary, with changing salinity and temperature, may inhibit the growth of planktonic blooms.

Station 1, furthest up the estuary (freshest water) saw a total abundance of less than 500 cells per ml. In this case, dinoflagellates made up the greatest proportion, especially Karenia mikimotoi and Prorocentrum micans. Dinoflagellates tend to dominate diatoms in conditions of lower turbidity, and higher nutrients, which may be expected further up an estuary.

Zooplankton

Station 3, located furthest down the estuary, showed the greatest abdundance of zooplankton, extrapolated to a total of about 900,000 per m3. Of these, hydromedusae were the most abundant, making up more than 50% of the total. Copepods, copepod nauplii, polychaete and gastropod larvae were the other major groups. This is somewhat different to the findings of the offshore research, where copepods were in all cases dominant.

Further up the estuary (stations 1 -2), where phytoplankton cells were fewer, total zooplankton number at around 500000 per m3. Of these the dominant group was copepod nauplii. This finding may indicate the use of the estuary as a spawning ground for copepods. The nauplii are not as freely motile as the adults of the species so their position may have been determined by tidal forcing. Other larval forms, those of polychaetes, echinoderms and gastropods make up the remainder of the count.

The sample was taken from just beneath the surface, and therefore the samples may have missed those members of the zooplankton that have vertically migrated down the water column during the sample period.

Figure 21.

Figure 22.

Figure 23.

Physical

As Falmouth is a macro-tidal estuary it is well-mixed. As it is a tidally dominated, this can be seen in the changing physical structure of the water column as it transitions from the head of the estuary to the mouth. The trends observed in the CTD profiles from Station 1 through to Station 6 demonstrate this (figures 13-19). Sampling was undertaken as the tidal flood occurred which meant that as we progressed downstream tidal mixing was increasing.

At Station 1 there is a temperature decrease with depth and a salinity increase (figure 13). A thermocline is present at 3m and a very shallow halocline at 0.2m. The combined effect of these changing properties with depth results in weak stratification. Between station 1 and 6 the water column is well mixed only showing very slight stratification towards station 1 where there is a larger freshwater influence. Stratification breaks down fairly quickly as you move downstream due to the aforementioned effects of the dominating tide in the Fal estuary. The freshwater initially causes a shallow halocline as its lower density causes it to be restricted to the upper layer before it is mixed in.

Station 6 located at Black Rock (figure 18) was the most offshore station, and therefore most strongly influenced by tidal forcing. The CTD profile provides evidence for this, as temperature and salinity remain relatively constant with depth. The lack of a thermocline and halocline suggest a very well mixed structure most likely due to mixing by tidal energy.

Figure 19 indicates that turbulent mixing was dominant at all 6 stations. All 6 profiles show that a majority of Richardson numbers of below 0.25. This turbulence leads to efficient mixing of the freshwater inputs and coastal seawater, leading to the well-mixed physical structure observed in the CTD profiles.

The fluorometry readings from figures 13-18 show that chlorophyll remain relatively constant with depth at each station. Between stations 1 and 4 there is a slight peak of chlorophyll near the surface. Figure 21 shows that at all stations the composition of phytoplankton is dominated by diatoms; this is to be expected as diatoms prefer well mixed conditions.

Transmisson remained fairly constant at 4v in the vertical profiles at every station except Stations 1 and 2. Station 1 was very shallow at the time of sampling and so here the increased transmission and thus turbidity could be as a result of the disturbance of sediments by the strengthening tidal flood currents.

The ADCP transect taken at station 6 (Figure 20) shows the tidal flood heading northward up the estuary, and that it is concentrated on the right hand side of the estuary due to the effects of coriolis.

Estuary Discussion, (Continued)