Knysna Basin Project Utilizes C6 Multi-Sensor Platform with Chlorophyll and Turbidity Sensors

Parameters: In Vivo Chlorophyll, Turbidity

Introduction

Rhodes University Field Station is an adjunct of the Department of Zoology and Entomology. The University Department has made considerable advances in estuarine and oceanic studies over many years that have appeared in international peer reviewed journals. It is sited in the coastal town of Knysna, 400 km west of the university in Grahamstown. Estuarine research as such was initiated during my tenure as Professor and Head of the Department from 1963. Following my retirement in 1988 I set up the field laboratory to encourage graduate students and scientists to continue work on the water column, macroinvertebrates and saltmarshes of the Knysna estuary (34 04 46 S :: 23 03 29 E). The website gives some indication of the issues we consider important and the persons working on them.

Marine Embayment, Knysna estuary

Issue/Concern Being Addressed Early investigations of chlorophyll pigments in the water column of tidal estuaries in South Africa are reported by Lucas (1986) for the Great Fish River estuary, Allanson and Reed (1995) for the Kariega River estuary, and Grange and Allanson (1995) in the Great Fish River estuary. These studies provided clues as to what were the significant drivers in the pattern both spatially and seasonally in chlorophyll variation (see also Litaker et al. 1987; Wetz et al. 2006 and Cloern et al. 2010). Their development was directed to understanding the role of freshwater flow into the estuary and the consequences of long periods of zero flow, (Perissinotto 2010, Vorwerk et al. 2008, Snow and Adams 2005, Muir and Perissinotto 2011, Kotsedi et al. 2012). All of these studies assessed chlorophyll a distribution using grab water samples collected spatially over a period of time, usually a diurnal tidal cycle, and repeated seasonally. However, with the introduction of submersible digitally recording instruments such as the Turner Designs C3 or C6 Multi–Sensor Platform attached to Cyclops-7 sensors – one for chlorophyll and the second for turbidity (NTU), it is possible to visualise the links that occur between chlorophyll a and proximate factors such as temperature, turbidity and water depth continuously or over short time intervals during a tidal cycle or other changes, for example, river flow, Smith (2008). This paper reports the early findings arising from the deployment of a Turner Designs C6 platform equipped with chlorophyll and turbidity sensors at two stations in the marine embayment of the Knysna River estuary [34°:04’:46’’S 23°:03’:29’’E], (Figure 1) during the summer of 2012.

Installation and Sampling Protocol Calibration of the fluorometer followed the methods described by Turner Designs (2012) C-Soft Windows™ User Interface. Calibration begins with a blanking step in which the fluorometer is placed in glass-fibre filtered seawater and when the ‘Current RFU’ readings have stabilized, the instrument is set to blank. All further measures will be subtracted using this value. The fluorometer is then placed in solutions of known chlorophyll concentration (mg/l) and turbidity measured in nephelometric units (NTU). In this investigation a 2 L sample of seawater was collected at the site of the deployment and 1 litre filtered (GFC) and the filter content extracted in cold 90% acetone in the dark overnight, transferred to a centrifuge tube, stoppered, and left in the dark until centrifuged, (Aminot and Rey (2000). After centrifugation chlorophyll a was determined using a GBC UV-Visible Cintra 101 digital spectrophotometer. The concentration of chlorophyll is entered into the ‘Standard Value’ box in the ‘Calibration Screen’ and the remaining steps completed. The C6 platform is similarly calibrated for turbidity as NTU in which the turbidity of the water sample after blanking is measured using a standard nephelometric meter (Eutech Instruments TN-100/T-100) and the NTU value introduced via the ‘Calibration Screen’ during the calibration procedure. Time to start of the measurements in situ is chosen and the instrument deployed. View presentation for more information.

Turner Designs submersible fluorometer: left hand downloading data mode; right hand submersible mode with battery pack in place

Summary It is too early to talk about successes, other than to record that a paper was presented at the 49 Meeting of the Southern African Society of Aquatic Scientists in July of this year. Considerable interest was shown by a number of workers at the meeting.

Once the software has been worked through, the instrument is easy to use,but we have to be careful that dates and times are given in accordance with the C6 requirements. I mention this as we tend to use the British format of day-month-year.

Acknowledgements Our thanks are due to: Barloworld Rhodes University Marine remote sensing unit SANhydro tides Tokai, Cape Town MRSU 2012 Joclyn Fearon Neil Hart, Oceanography, University of Cape Town

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Authors: Brian Allanson, Inga Chinnery

Institution: Rhodes University Field Station, Knysna

Location: South Africa