Specifications of the monitoring system
Turbidity is recorded at 5 depths with Wetlab’s light scattering sensors attached to a Keithley digital multimeter. Conversion equations to concentration of suspended solids and to coeffcicient of vertical light attenuation have been obtained in the laboratory. The equations were verified in the field by comparing turbidity time series with weekly manual measurements of suspended solids (Whatman GF/F glass fibber filters, 105˚C, 24 h) and light extinction (10 cm intervals, 4π Li-Cor light sensor). See details here
From 2003 we adopted a sampling frequency of 6 s. Rough data are averaged for 10 min intervals using a median filter.
Sensors are cleaned twice a week. Their performance is highly reliable even after 7 years of field use.
The reason for recording turbidity is that underwater light is influenced much more by wind-induced sediment resuspension than by incident solar radiation due to the shallowness of the lake.
Dissolved oxygen is recorded by a WTW Oxi 340i oxygen meter equipped with a CellOx 325 electrode at 80 cm above the sediment surface (about the middle of the water column). The sensor is calibrated weekly. This sensor is one of the cheapest but not the optimal solution for recording oxygen.
Meteorological variables are recorded by the Mettech system. We record data every 10 minutes. The wind sensor is located 3 m above the water surface. The following variables are detected:
- Current wind speed and direction
- Average wind speed and direction
- Wind gust (speed and direction)
- Global radiation
- Total rain and rain rate
- Temperature at 5 depths in the water column and at 2 depths in the sediments
Biomass and photosynthesis of living phytoplankton. On-line delayed fluorescence spectroscopy has been used in Lake Balaton to assess biomass and composition of phytoplankton between 2001 and 2004. (See details here, modeling results here). We have planned a new instrument with programable excitation light intensity to record photosynthesis-light curves with high frequency. Currently the new equipment is under construction.
Data aquisition and communication. Data from the various sensors is collected by an Apple Mac Mini computer. All sensors are attached to the computer via serial (RS-232) connections. The computer stores all data locally and sends it immediately to the server located at the Department in Budapest. When the internet connection is living, visitors can examine the latest results usually within the very same second of recording. The data collector program can also tolerate pauses in the internet service, all records are delivered to the server once the connection is re-established.
High frequency data measured by other organisations:
- Daily water level: last month; since 1867
- Wind record at 8 stations around the lake every 5 min
- Daily nutrient loads through the largest tributary since 1975 (yearly averages can be found on the web)
- Hydrological data in every few hours at 6 stations on the main tributary of Lake Balaton (Zala River)