Tamar Estuary

Aim to…

Method

100ml of seawater was added to a brown bottle containing 1ml of Lugols Iodine, which stains and preserves the sample. This was recorded on a sheet with the site, depth, temperature, salinity and time the sample was taken.

At the lab, the sample was poured into a settling tube and left overnight. To prepare the samples for analysis, a Pastor pipette with a curved end, connected to an electric pump sucked the overlying liquid out of the settling tube. This meant that the sample was concentrated by a factor of 10.

Using a light microscope, 1ml of the concentrated sample was pipetted onto a 1000 square Sedgewick–Rafter cell. 100 squares were counted so that the phytoplankton counts are per ml.

For any phytoplankton chains or groups, each cell was counted in the chain and the phytoplankton families were noted and typed into an Excel Spreadsheet.

Findings

Falcon spirit – lower estuary

Species composition changes dramatically down the estuary. Chaetoceros species were the most abundant phytoplankton across all station in the estuary. Chaetoceros abundance was highest at station i18 with 36 cells/ml, then 15 cells/ml at station h11. Station h11 had the highest abundance of Thalassiosira and Cletarala species with 20 cells/ml and 18 cells/ml respectively and a single Gymnodiniaceae cell. Station d13 had the highest abundance of Guinardia with 12 cells/ml. Skeletonema species were the second most abundant species across all stations and the most abundant at station i18. Ciliates were not possible to identify to species level and were found in small numbers of 1 - 2 cells/ml at stations d13, b14, c15 and j19.

WTP – upper estuary

The most abundant phytoplankton in the upper estuary was Coscinodiscus, with the majority of cells at Station a4 and were the only phytoplankton found here. Chaetoceros species were found at Station a9 as well as small numbers of Odentella, Ditylum, Thalassiosira, Skeletonema, Ciliates, Leptocylindrus, and Dinoflagellates (Ciliates and dinoflagellates were not possible to identify to species level). Station a9 had the highest abundance of phytoplankton in the upper estuary with 30 cells/ml and the highest diversity. Pseudo-nitzschia species were found at Station a14 with 4 cells/ml, the only phytoplankton found here.

PHYTOPLANKTON

Aim to identify the changes in the species diversity and distribution of zooplankton in Plymouth, from the river head (Calstock) to the estuary mouth (Plymouth Sound).

Method

On the vessels “Falcon Spirit” and “Winnie the Pooh”, the zooplankton samples were collected using a 0.50 m diameter net with a 200mm mesh which decanted into a one litre plastic bottle.  The net, attached to a rope was cast in the direction of the current and held just below the surface of the water for 120 seconds. After the net and bottle were retrieved from the water, the outside of the net was washed with the river water at that site so that any zooplankton caught on the net were washed into the bottle. Formalin was added to the zooplankton sample to preserve it.

During the Lab analysis, 10ml of the Zooplankton sample were pipetted into an Aliquot and 5ml of this was pipetted into a Bogorov chamber to analyse under a stereo microscope. The number of each individual zooplankton group/order were counted by eye and written down on a sheet.

Findings

There was a wide spread of zooplankton found. The most common zooplankton found at all sites were copepods and their larval nauplii, which were more common throughout the river. Adult, copepods were at high densities at the head of the river near Calstock at 84% of total zooplankton sampled decreasing in abundance in the middle of the river to 28% whereas their nauplii increased in abundance from 14% to 71% [Figure 1&2].

Cladocera densities increased by the pontoon, however, copepods dominated the sample. Falcon Spirit surveyed the estuary, in the lower estuary there was a greater diversity of groups with nauplii dominating the sample at 41% of the total abundance, there were higher abundances of cirriped larvae (21%) and hydromedusae (17%) compared to the river samples. The upper estuary had high abundances of cirriped larvae, copepods and nauplii.  Therefore, a clear species distribution gradient can be seen from the river to the mouth of the estuary.

ZOOPLANKTON

Method

On Winnie the Pooh, samples were collected by hand using a syringe, 50ml of water were collected from a sample of surface water and forced through a filter which collected all the chlorophyll cells. The filter paper was removed from the syringe, folded in half so that the side containing cells were folded in on itself and stored in a labelled bottle of acetone which was recorded on a sheet.

On the Falcon Spirit the samples were taken by a mechanical pump which forced water through a filter paper which was then preserved the same way as on Winnie the Pooh.

Chlorophyll cells were collected on filter paper and then preserved in labelled Acetone tubes. They were assessed using a Fluorometer which measured the absorbency of the acetone sample. The filter paper was discarded before being analysed in the fluorometer. Some samples were difficult to analyse as the filter paper either deteriorated in the acetone so pieces of paper were still contained

Findings

Chlorophyll concentrations near the surface are highest at station b14 at 1.0 ug/L, around mid-way up the estuary. The lowest surface concentration was 0.55 ug/L at station i18, near the mouth of the estuary. Most of the stations including c15, d13, e16, g17, j19, have the highest concentrations of chlorophyll near the surface of the water column, peaking between 3-5m. This will be where there are sufficient light levels to fuel photosynthetic reactions within phytoplankton.

There is an anomaly at station h11 at 27m, which shows a sharp spike in chlorophyll concentrations from 0.6 to 1.3 micrograms per litre. Even near the mouth of the estuary at h11, mixing, turbulence and flocculation means light is scattered and does not penetrate deep into the water column. This is reflected by the results from the Secchi disk, which can only be seen down to 4m. This means the idea that phytoplankton are thriving at 27m below site h11, is very unlikely/impossible, and can be assumed to be an anomaly.

It can be seen below 10m at most sites the concentrations of chlorophyll remain relatively constant at a concentration of around 0.7 micrograms per litre. This reflects a lack of light at depth, which limits phytoplankton growth and reduces the effect of differing nutrient levels between the sites.

CHLOROPHYLL

Biological Findings…

Above Figure 1. The abundance of Zooplankton (m-3) at each site along the Tamar Estuary from the top of the estuary to the end of the estuary. (05/07/18)

Above Figure 3. The abundance of Phytoplankton (ml-1) in the Upper Tamar Estuary. (05/07/18)

Above Figure 2. The abundance of Phytoplankton (ml-1) in the Upper Tamar Estuary. (05/07/18)

Above Figure 4. Chlorophyll concentrations (µg/L) from calibration of CTD fluorometer against chlorophyll samples taken up the estuary plotted against depth (m) for the lower Tamar estuary, as sampled by RV Falcon Spirit on 05/07/18.