Discussion

Home River Estuary Offshore Habitat Map References

Falmouth 2017 - Group 12 7TITLE

Estuary

Results

Top

Station 26

The thermocline, which is at 4 m (shown by the t/s depth profile) is related to the increase in chlorophyll at the same depth. This is because the deep chlorophyll maximum is associated with the thermocline (Fernand, et al., 2013). The thermocline separates the nutrient diminished water above and the nutrient rich water below. However, there is a higher concentration of phosphate in the surface water, so this may point to an input of phosphate at this station. This could be from run-off from farm lands that use fertilizers in order to grow their crops. The nitrate has a higher concentration below the thermocline than in the surface water, and that is expected.

Station 26 transect graphs. East to West, distance from shore 50m (start) and 30m (end).

Start Lat – 50012.215760’ N

Long – 502.148795’ W

End lat – 50012.269129’ N

Long – 502.392403’ W

The ship’s track shows that the estuary current was flowing in a North-northeast direction (~30deg direction). The average flow speed was ~0.33 m/s at the centre of the estuary.

The fact that the estuary should flow in a southerly direction towards the sea suggests that there was a flood tide in the estuary flowing from the larger section of the estuary to the small channel northeast of Station 26.

There was a low tide at 11:27 UTC on the 7/7/17 (the day of the data collection). This means that at the time of measurement there would have been a flood tide which supports the above data.

The width of the estuary is small compared with the other stations and in a smaller space water will travel faster due to increased pressure. This idea is supported in the fact that the water speed is higher at station 26 than the other stations.

Station 27

The thermocline at this station is a lot deeper (at 17 m). The chlorophyll maximum is 0.2µg/L and it reaches that value at around 7 m depth. This concentration extends to 30 m, so the deep chlorophyll maximum must be located between 7 and 30 m. The nitrate concentration is higher in the surface than at 30 m, this could point to an input of nitrate at this location. The phosphate however is less concentrated in the surface water than the deeper water. This is because the thermocline stops the two layers from mixing (Taylor, 1988), so, there will be two different concentrations.

Station 27 transect graphs. East to West, distance from shore 35m (start) and 100m (end).

Start lat – 50010.880183’ N

Long – 501.510019’ W

End lat – 50010.465428’ N

Long 502.548517’ W

The ship’s track shows that the water flow was travelling in a north-westerly direction (310deg) which again supports a flood tide idea.

The station was in a wider part of the estuary which can explain the slightly slower average flow speed of 0.23 m/s as explained at station 26.

The time of collection of this data 13:37 UTC was well within the flood tide hours for that day so the flow direction makes sense.

Station 28

According to the t/s depth profile, the thermocline is at 5 m, the deep chlorophyll maximum is also at this depth, this is shown by the chlorophyll and nutrient depth graph. Nitrate is depleted in the surface layers (0.13 µmol/L), but below the thermocline the concentration 0.65 µmol/L this shows that the nutrient rich bottom layer is not mixing with the nutrient poor surface layer. This is also why the deep chlorophyll maximum is located at 5 m. The phosphate concentration however, does not follow the same trend. The concentration in the surface layers is much higher than the water below the thermocline this shows that phosphate is being utilized by phytoplankton in the bottom layers.

Station 28 transect graphs. West to East, distance from shore 40m (start) and 50m (end).

Start lat – 50010.030506’ N

Long – 502.826873’ W

End lat – 5009.791729’ N

Long – 501.578554’ W

The average water speed for Station 28 was 0.27 m/s which is again slower than the narrower estuary station 26. The flow direction average is northwards (356deg).

The high tide was at 17:17 and station 28 was measured at 14:00 UTC so it was well within the flood tide supporting the flow direction.

Station 29

as shown by the t/s depth profile, the thermocline is at 10 m. this corresponds with an increase in chlorophyll which shows the deep chlorophyll maximum. At this point the nitrate and phosphate concentrations are low. This indicates that the nutrients in the surface waters are being utilized. The phosphate concentration increases dramatically between 9 m and 11 m. This is expected as the phytoplankton will not be able to use the bio-available phosphate under the thermocline due to the strong stratification of the water column (Pingree & Pennycuick, 1975). The nitrate also follows this trend.

Station 29 transect graphs. East to West, distance from shore 24m (start) and 132m (end).

Start lat – 5008.943064’ N

Long – 501.127837

End lat – 5008.916549’ N

Long – 502.589653’ W

The average flow direction of station 28 was north west (334deg). The flow speed was much lower than the other stations at 0.19m/s. This is due to this being the mouth of the estuary and therefore it is much wider than the other 3 stations and the pressure is less.

The direction once again suggests a flood tide and the data was taken at 15:18 UTC which supports this.

Discussion of Conway results

Richardson Number

Looking at all the stations, there isn’t much variation between layers in the water column. The Richardson number shows that the flow is predominately turbulent and therefore very well mixed, which we would expect in summer due to tide. This also suggests that the vertical shear has overcome the layers in the water column to create this turbulence. Furthermore when calculating the densities down the water there was on average of 1 kg m3 change, making large enough stable density gradients, which supports the theory that significant turbulence has been created to disrupt the vertical stability and trigger mixing.

Three out of the 4 stations have a section which is in the transition zone, all at different depths so could transition to either laminar or turbulent depending on what has gone on before. This could be due to wind or tide most likely. However because the other consecutive layers are turbulent (implying the layers are very stratified which we would expect in summer), we would expect these to transition into turbulent flow also. Therefore this suggests that the estuary of the Truro is an overall turbulent water column .

Biological discussion

The diversity of zooplankton in the estuary increases when going from the river down to the sea. The top of the estuary was dominated by both copepods and copepod nauplii. This is supported by the research carried out by Lawrence et al, which states that copepod nauplii are not affected by salinity (Lawrence, et al., 2004). Therefore copepod nauplii can extend from the top of the estuary to the sea. This is shown by the pie charts as copepod nauplii is found at most of the stations. In addition, the ratio between adult copepods and copepod nauplii does not follow a significant trend therefore it can be concluded that the changes in salinity along the estuary do not affect the reproduction rates of copepods. Further emphasising the hypothesis that copepods are not affected by changes in salinity.