Cathy Holden (FPS) and Becky Lemon (Johns Associates)
Tom Styles, of Johns Associates, writes:
As you will have no doubt noticed, the next phase of the dredging to deepen Fleet Pond and island building to create more varied areas for the wildlife in and on the Pond has started in earnest. To check that the effects of the machinery working in the water and the inevitable disturbances are controlled and minimised, Johns Associates have been undertaking regular water quality testing, in addition to the routine monitoring carried out by the Environment Agency.
Two “lucky” Fleet Pond Society members joined Becky Lemon from Johns Associates on one extremely sunny Sunday in September to complete initial water quality sampling for the latest Autumn 2012 works. Monitoring for the current works is on-going and results will be issued once completed and reviewed.
Over the course of the Spring 2012 works, we sampled the water quality at least once a fortnight, taking with us two main pieces of kit. The first of these measures turbidity, which is the amount of light that can pass through a water sample; an indication of suspended particles. The second measures a large number of water quality factors including dissolved oxygen.
Figure 1. Map of Water Quality Sample Points At Fleet Pond
The results showed that at much of the lake the turbidity remains low. The results from March 2012 show that the highest levels of turbidity in the lake were at site 13 on the north eastern margins of the lake (shown ringed in the map above), where works were taking place at this time.
Despite these readings being high compared to normal levels, they were still below levels that would cause serious harm to fish (see Figure 2) unless maintained over a protracted length of time, measureable in weeks or months. They quickly returned to close to pre-works levels after the contractors left the lake in March 2012.
Figure 2. Effects Of Turbidity on Fish Activity
(Schematic adapted from “Turbidtiy: A Water Quality Measure”, Water Action Volunteers, Monitoring Factsheet Series, UW-Extension, Environmental Resources Centre)
The other main indicator is the amount of dissolved oxygen in the lake water. Figure 3 shows how levels of dissolved oxygen can affect fish in a lake. It is essential that this remains sufficiently high otherwise fish will start to suffer, and could result in fish deaths.
The European Union’s Water Framework Directive (EU WFD) details maximum acceptable levels for this indicator: if the levels fall below this, further action, firstly in the form of additional or changes to the existing silt curtain, introduction of portable aeration and potentially a temporary stopping the works.
Figure 3. Effect of Dissolved Oxygen on Fish Health
(Based on information from APHA.1995. Standard methods for the examination of water and wastewater. Amer. Publ. Health Assoc.)
The results from March 2012 show that all levels were high enough to support abundant fish populations, with even the lowest readings remaining above 9 parts per million (ppm). This is also firmly in the ‘Good’ category for the EU WFD.
These indicators will continue to be monitored by the team throughout the programme of works, and any causes for concern will be reported to the site management, the Environment Agency and or other relevant organisations to ensure that the quality of water in Fleet Pond is maintained.