2.4 Water Temperature and Transparency
The vertical water temperature profile was continuously measured with two chains of temperature probes (HOBO TidBit UTBI-001, Onset, USA) that were designed with a higher resolution near the surface and a coarser resolution deeper in the water (Figs 3c and 4). These are referred to as thermistor chains (TCs). The first measurement site, TC1, consisted of a 15-m chain deployed in a 30-m deep section of the reservoir, in the vicinity of the raft. The second measurement site, TC2, consisted of a 70-m chain located in a 100-m deep section of the reservoir, 1 km south of the raft. At both sites, skin temperatures were obtained using floating sensors sheltered from incoming solar radiation by a white piece of foam. The chains were designed to withstand variations in water levels by using additional rope. A pressure sensor (HOBO water level logger u20-001-03, Onset, USA) was added to each chain to correct for the effects of rope tilt on the exact vertical position of the probes.
Water transparency was also periodically measured using a Secchi disc. The mean Secchi depth (SD ) was 4 ± 0.04 m. Assuming a mean product SD × Kd of 2.28 for water of moderate transparency following Koenings and Edmundson (1991), the vertical attenuation coefficient Kd becomes 0.57 m–1. Consequently, 50% of the incident energy flux density is absorbed in the first 1.2 m of water and 99% over the first 8.1 m of water.