So why that oxygen “bump” at about 8-9 meters depth? That “bump” corresponds to a high density layer of water at a point where water temperature changes rapidly with depth. The water temperature starts dropping quickly at about 5 meters (16 feet) and temperature continues dropping until about 9 meters (29 feet) when it reaches 10°C (50°F). That zone of rapid temperature drop is referred to as the metalimnion. Water at the surface dropped from 78°F (26°C) to 75°F (24°C) at 5 meters, and then abruptly dropped to 50°F (10°C) at 9 meters, reaching as low as 44°F (6.9°C) at the bottom. Certain photosynthetic algae that are adapted to low temperatures and low light intensity are found in high abundance in the metalimnion (the thermocline) and their photosynthetic activity result in greater than 100% oxygen saturation, in this sample it reached 122% saturation (13 mg/L)! All that photosynthetic plankton is food for another type of plankton – zooplankton. If you were a trout or Salmon – all that food (zooplankton) and oxygen rich water – make that the place to be!
Photosynthesis during the day and respiration at night by phytoplankton can result in major changes in oxygen concentrations in the metalimnion. Next weekend we’ll hope to do both a mid-day sample and a night-time sample to get a better comparison of what actually happens at West Hill Pond in high summer.