Introduction

On 4th July 2017 between 12:00 UTC – 16:00 UTC we took measurements half hourly from the pontoon near King Harry Ferry. Taking vertical measurements from a set location allowed us to investigate how changing physical conditions, specifically temperature and flow speed at different depths throughout the tidal cycle, influenced factors such as chlorophyll concentration, which can be used as an approximation for phytoplankton abundance.

Our aim was to observe part of a coastal ocean  system, in this case the influence of tidal range on stratification, light and nutrient concentration on a stationary point in the river Fal over a set period of time.

Map 1. Map displaying the location of the pontoon in the Fal estuary used for data collection on 04/07/2017.

Discussion

With King Harry Ferry’s pontoon located relatively close to the mouth of the estuary, tide has a significant influence on most water properties. High tide at the day of the measurements was at 14:00 UTC, and just before and during that time the nutrient concentrations change significantly (Fig. 1).  The decrease in temperature and increase of salinity, dissolved oxygen and chlorophyll suggest a single, tidal-front surface wave, affecting the freshwater surface layer (≤ 2 m) for a short period of time.

The salinity measurements suggest a slower high-energy tidal flow, temporarily stratifying the estuary (Fig. 1).  Change in flow direction supports this suggestion, as the tidal flow in at 12:40 UTC has enough energy to push the natural river flow back and change the overall flow direction to about 330° (Fig. 2), with the flow speed at 0-3m being significantly lower than 3-5m, indicating a freshwater top layer pushing back against the tide. After that, around high water at 14:00 UTC, the flow energy decreases, creating slack water in the estuary with the freshwater flow dominating overall flow direction again.

The change in direction likely causes mixing areas throughout the river, especially after high tide at 14:35 UTC, just as the vertical increase of turbidity shows (Fig. 1). The increase in light attenuation at 14:32 UTC (Fig. 3) would support this theory, since the cloud cover was mostly consistent at the day measurements were taken. Instead, the cause for reduced surface light levels is probably a temporary increase in free particles in the water column.

The isolated pH increase at 4m/13:00, which seems to be correlated with turbidity increase (Fig. 1), likely has a temporary source of anthropogenic pollution, e.g. a mining leakage or a passing boat or vessel disposing of their sewage.

Figure 1 also shows higher phytoplankton abundance during high tide via chlorophyll measuring, suggesting the tidal wave swept in a number of organisms. However, comparing the chlorophyll concentration taken with the probe to the chlorophyll measured in the water samples shows different results for the same depth and time (Fig. 4). Plotted against each other it becomes apparent that there is no pattern in the value difference of both chlorophyll measurements (Fig. 5), meaning that the Exo-Sounder chlorophyll data is not portraying true trends in chlorophyll concentration and thus phytoplankton abundance. Likely, the probe was calibrated incorrectly, plus the Niskin bottle was lowered to sampling depth by marks on the rope it was attached to, thus the depth might be inaccurate as well.

Method

Light meter (left)

Sensor is lowered into the water column once per hour,  with irradiance each time measured first on the pontoon and then each meter in the water. Light attenuation can be determined by ratio of surface to depth irradiance.

Exo-Sounder (back)

The probe is lowered every half hour into the water column, where it takes a variety of measurements such as temperature, salinity and dissolved oxygen every meter. It is connected to a digital display (right) on the pontoon.

Niskin bottle (front)

Deployed meter by meter  and closed at  1 m and  4 m depth each to take water samples. From both samples, 3 x 50 ml are syringed through filters, which then are placed in acetone and cooled over night. Fluorescence analysis later will determine chlorophyll concentration.

Flow meter (back)

The device is lowered into the water column once per hour, measuring flow by rate of impellor’s rotation caused by water movement in meters per second, and flow direction in degrees every meter.

Results

There were changes in temperature, salinity, dissolved oxygen and chlorophyll concentration from 13:00/13:30 UTC until 14:30 UTC, and another concentration change around 14:30 UTC, particularly at surface levels ≤ 2 m depth. Salinity concentration increases with depth to a time-constant level of 34.0 to 43.4 ppt. There was also a prominent change in pH and turbidity at 4 m/13:00 UTC (Fig. 1).  

Flow direction changed from 292°-359° at 12:40 UTC to 222°-110° from 13:35 UTC onwards, with changes in flow speed more prominent in surface levels ≤ 2 m depth (Fig. 2). Changes in light attenuation, especially in surface levels to 2 m depth, ranged from over 30% light received to less than 10% at immediate surface, while ≥ 3 m depths remain mostly constant at 10-5% (Fig. 3).

Disclaimer: The views and opinions expressed are solely those of the contributors, they do not reflect the views and opinions of the University of Southampton.

Limitations

Every device was lowered into the water column by hand, the only indicator for depth being marks every meter on the rope the devices were connected to (except for the exo-sounder), resulting in inaccurate depth measurements.

Table 1. Table displaying the environmental conditions and times (UTC) of data collection within the estuary.

Fig. 1  Depth profiles of nutrients once every half hour over a four-hours period, measured  stationary with an Exo-Sounder at King Harry Ferry’s pontoon in the Fal estuary (50⁰12.958 N, 005⁰01.671 W). Note that the gaps between measurements were  filled  with estimates and might be inaccurate.

Fig. 2   Depth profiles of water flow speed and direction once every hour over a four-hours period from 12:40 to 15:35 UTC, measured  stationary with a flow meter at King Harry Ferry’s pontoon  in the Fal estuary (50⁰12.958 N, 005⁰01.671 W).

Fig. 2   Depth profiles of water flow speed and direction once every hour over a four-hours period from 12:40 to 15:35 UTC, measured  stationary with a flow meter at King Harry Ferry’s pontoon  in the Fal estuary (50⁰12.958 N, 005⁰01.671 W).

Fig. 4    Comparison of measured chlorophyll  concentrations over time at 1 m and 4 m depth each, with the half-hour measurements of the Exo-Sounder plotted in a line, and the hourly filter analysis results plotted as scatter.

Fig. 5    Chlorophyll  concentrations  plotted against each other for calibration. No trend suggests the chlorophyll measurements from the Exo-Sounder differ too much from the actual chlorophyll concentration in the water  (measured with filter analysis) to be reliable.

PONTOON

Falmouth 2017 - Group 7