The objective was to observe temporal changes in different physical, chemical and biological parameters across the tidal cycle at a fixed point in the Falmouth Estuary.

Measurements were taken at the King Harry Ferry Pontoon (at 50°12’58.1’’ N, 005°01’40.8’’ W). The pontoon’s slightly offshore location is ideal for a time series analysis of particular physical and biological parameters. A series of recordings were taken at 30 minute intervals, over the total three hours (from 12:00 UTC until 15:17 UTC on the 23rd June 2015). The water depth was at a maximum of 3.3m at 13:20 UTC. The weather was warm, dry throughout the investigation and there were relatively strong winds during certain periods. Falmouth has a semi-diurnal tidal cycle and on June 23rd, during the spring cycle, high tides occurred at 06:58 UTC and 19:08 UTC, and low tides occurred at 01:04 UTC and 13:20 UTC.

Current, depth, salinity, temperature, pH, chlorophyll concentration and oxygen saturation were the main biological and physical parameters considered for the investigation. The systematic sampling allowed a time series that showed the influence of tides on the various parameters investigated.  

Methodology

Instruments were set up at mini-stations along the pontoon, and calibrated where necessary. Once assembled, they were tested prior to scheduled times for recording the raw data. Measurements were scheduled around ferry arrival and departure times.

Exo probe

Exo probe measurements of temperature, salinity, dissolved oxygen, pH and depth were taken at 0.5m intervals throughout the water column. The probe was partially submerged throughout the course of the investigation, to ensure that sensors were calibrated and worked correctly. Five minutes prior to the scheduled time for vertical depth measurements, the display interface was switched on to allow sufficient time for the Exo probe to be calibrated. However, a depth lag between the display interface and the Exo probe remained consistent throughout the investigation.

Flowmeter

The flow meter was deployed and measurements were taken throughout the water column at 0.5m intervals. Water movement (flow) was measured past the meter by recording the rate of impellor rotations. The direction and magnitude (in m/s) of the flow were measured.

Lightmeter

On the pontoon, two light sensors were deployed to obtain the light attenuation. The first reading was measured at the surface, accounting for the ambient irradiance (on-deck), whilst the second sensor accounted for the water column (in-situ) reading, at 0.5m depth intervals. The initial surface and subsurface irradiance values from both sensors were processed to calculate the correction ratio. For the surface ambient irradiance for each deployment, a vertical depth profile of irradiance was obtained, considering a temporal variation for each profile throughout the day.

Niskin Bottle

A horizontal Niskin bottle was deployed and fired at 0.5m intervals, in order to collect water samples. The Niskin bottle was deployed approximately every 30 minutes, between scheduled ferry times to avoid disturbances from its wake. Furthermore, 60ml from each water sample was passed through a filter. The filters were then placed in labelled test tubes containing 6ml of 90% acetone and placed in a freezer overnight. Chlorophyll concentrations (in µg/L) were calculated from measurements made with a Turner 10-AU Fluorometer.

Temperature

There was a general decrease of temperature with increasing depth. The highest temperature readings were observed in the surface layers (0 - 1.25 m), between the time of 13:30 and 15:17 (UTC). The maximum temperature reading, 17.55˚C, was recorded at 15:00 (UTC) at a depth of 0.02m and the minimum temperature reading, 15.12˚C, was recorded at 15:09 (UTC) at a depth of 2.01m. The surface layers were expected to be the highest in temperature, as the level of irradiance is at its highest here. Cooler waters have a higher density than warmer waters, thus would sit below the warmer waters, which is another reason the surface layers were expected to be of a higher temperature than the deeper layers1.

Salinity

The general pattern of salinity was opposite to temperature, here the salinity increased with increasing depth, with the exception of the earliest readings taken between 12:00 and 12:22 (UTC). Between this time, the surface salinity reached a maximum of 31.44 at 12:22 UTC. The surface salinity therefore appeared to decrease over the three hour sampling period. Low tide on this day was 13:21 (UTC), this coincides with the time period that the surface salinity was at its minimum reading. This suggests that the observed low surface salinity at this time was due to the dominance of the fresh water as the sea water retreated back down the estuary. More saline water, in a similar fashion to cold water, is of a higher density than fresher water. This means the water of highest salinity was expected to be in the deeper layers2.  

PH

The pH remained at a similar value vertically throughout the water column for the entire sampling period. While it remained similar vertically, it was shown to vary horizontally between the start (12:00 UTC) and end time (15:17 UTC), where the pH increased with time. The reason the pH was lower at the beginning of the sampling period could be due to the change in the level of respiration by phytoplankton. At the start of the sampling period, 12:00 (UTC), the level of irradiance would be expected to be at its highest and would have decreased from here with time, therefore the level of respiration by phytoplankton would have been at its highest, from here respiration would decrease and the pH would start to increase and become more basic3. The change in pH could also be caused by the changing tides, as fresh water generally has a lower salinity than salt water.

ODO% saturation

The Oxygen saturation was shown to increase over the sampling period of 12:00 to 15:17 (UTC). The minimum value was 119.1% and this was recorded at 14:41 (UTC), 3.1m depth. The maximum value was 130.0% and this was recorded at 15:04 (UTC), 0.52m depth. The time of the highest ODO% saturation value was around a similar time to the highest water temperature readings, so the high readings taken here could be due to increased photosynthetic activity of phytoplankton in the water column4.

ODO mg/l – Follows a very similar patter to ODO% Saturation.

Irradiance

The general trend of the irradiance level throughout the sampling period was a decrease with increasing time and increasing depth, with a couple of exceptions. One of the exceptions was the second sample taken between 12:30 to 12:33 (UTC), here the irradiance level exceeds the initial reading, at this time the irradiance peaked to 65.11, where for the initial sample the highest reading was only 47.94. Both of these peak readings were taken at the surface, 0.0m depth. The second exception to the general pattern was the reading taken between 13:02 to 13:07 (UTC), here the irradiance level begins to decrease a peak in irradiance of 35.17 which surpasses both previous readings. Irradiance decreases exponentially with depth, therefore the readings were expected to be lowest at the deeper layers. The exceptions to the general pattern could be explained by the varying cloud coverage throughout the day, thus the high peaks observed for 12:30 to 12:33 (UTC) and 13:02 to 13:07 (UTC) could be due to a burst of sunlight at this time.

Flowmeter

The beginning of the sampling period was close to the transition between high water (06:56 UTC) and low water (13:20 UTC). During high water the flow was expected to be lower - this explains the initial reading taken at 12:00, as the flow reading at this time was much lower than the preceding two readings at 12:30 and 13:01 (UTC), however, the surface reading was similar to these two preceding which could be an indicator of the changing tide and the flow beginning to increase for low water. At 12:30 (UTC) the flow had increased dramatically from 0.5 to 3m depth. At this sampling time, a wake from a passing boat reached the pontoon when the flow meter was between the depths of 1.5 to 2.5m. This would explain the peak seen between these depths on the plot - the peak reached 0.27m/s. At 13:01 (UTC) there was a dramatic decrease in flow from 0.30m/s to 0.0m/s. At this time slack tide was occurring – as at slack tide the water is unstressed and no flow occurs, these readings were expected. The flow speed at 13:01(UTC) remained low through increasing depth. At 13:29 (UTC), the flow was higher than the previous reading - the tide had turned at this point and was in low water, during low water the flow is expected to be higher so this explains the pattern observed. All samples from here then began to decrease in flow speed as the low water had occurred and was now approaching the next high water.

Chlorophyll

The general trend throughout out the sampling period was an increase in chlorophyll with increasing time. The concentration of chlorophyll remains fairly constant vertically throughout the water column and just varies horizontally with time. The highest chlorophyll reading was 12.7 ug/l taken at 14:59 (UTC), at 1.5m depth. The lowest chlorophyll reading was -0.15 ug/l taken at 12:04 (UTC), at 1m depth, however, with this value there was an issue with the calibration of the fluorometer, therefore it is possible that there is an inaccuracy. The next lowest chlorophyll reading was 0.13 ug/l, taken at 12:18 (UTC), at 2.5m depth. The chlorophyll increasing with time suggests there was more phytoplankton in the water at this time. This could be due to the tides, the period leading up to low tide is when the chlorophyll was at its minimum, this could mean the ebb tide was flushing the chlorophyll out, and the flood tide after the tide had turned began bringing chlorophyll back in, which is the period when the chlorophyll was at its highest.

[1] "The Ocean and Temperature - MarineBio.org". MarineBio Conservation Society. Web. Accessed 15:47 PM 6/30/2016. http://marinebio.org/oceans/temperature/

[2] Talley, LD. (2002). Salinity patterns in the ocean. In Encyclopedia of global change. Volume: the earth system: physical and chemical dimensions of global environmental change (eds MacCracken MC, Perry JS), pp.629–640. Chichester, UK: John Wiley & Sons.

[3] Jacobson, M. (2005). Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry. J. Geophys. Res., 110(D7).

[4] Coles, J. and Jones, R. (2000). Effect of temperature on photosynthesis-light response and growth of four phytoplankton species isolated from a tidal freshwater river. Journal of Phycology, 36(1), pp.7-16.

References

Introduction

Initial Results