Stratification and respiration dynamics in shallow ponds: insights from continuous temperature measurements

In this study, we performed continuous high spatial resolution temperature measurements (six thermistors per 1–1.4 m water column) at six sites in three shallow temperate ponds during the warm season. These measurements were accompanied by experiments on plankton and sediment respiration rates, including the relationship between temperature and sediment respiration. Continuous temperature measurements allowed the construction of detailed stratification profiles, as well as the construction of seasonal trends in both plankton respiration and sediment oxygen demand, all with high temporal resolution (at 1-h intervals). A peak in plankton respiration was detected in mid-summer, while sediment oxygen demand showed two peaks, one before and one after the plankton peak. Sediment respiration made a significant contribution to total respiration in the water column and even exceeded plankton respiration during certain periods. The criteria for determining stratification and its thresholds, which differ for shallow and deep waters, are discussed in detail. We discuss the uncertainties in estimating respiration and explore contributing factors, such as limitation of sediment oxygen demand by low oxygen concentrations near the bottom and sampling time. In addition, we used statistics to analyze the effect of environmental factors on sediment surface oxygen. On the basis of statistical analysis, water column turbidity was found to be the most significant factor, followed by duration of stratification breakdown, dissolved oxygen (DO) in the surface layer, water column depth, chlorophyll a concentration in the water column, time of day, plankton respiration near the pond bottom, duration of stratification, and temperature near the pond bottom.