Metalimnetic oxygen minimum and the presence of Planktothrix rubescens in a low-nutrient drinking water reservoir

Abstract

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A followup monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overseen by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

Gas Pressure Dynamics in Small and Mid-Size Lakes

“…Both in Arendsee and Rappbode Reservoir, a reduction of oxygen concentrations could be measured: both lakes formed a metalimnetic oxygen minimum. This could possibly be attributed to the decomposition of organic material below the epilimnion, while the metalimnetic and hypolimnetic water remained disconnected from the supply with new oxygen from the atmosphere (see also [54,55] for Rappbode Reservoir; [56,57] for Arendsee).…”

High frequency data provide new insights into evaluating and modeling nitrogen retention in reservoirs

“…The outlets are discharging either into the downstream river Bode towards Wendefurth Reservoir (gauge Hirtenstieg) or into a water transfer gallery towards Rappbode Reservoir. The water gallery delivers approximately one third of the annual inflow into Rappbode Reservoir, which is the largest drinking water reservoir in Germany in terms of volume, providing drinking water to more than one million people (Wentzky et al 2018a).…”

S-type Dissolved Oxygen Distribution along Water Depth in a Canyon-shaped and Algae Blooming Water Source Reservoir: Reasons and Control

“…Effler et al have introduced the characteristics and origin of MOM in an eutrophic reservoir in New York, and found that MOM is a recurring phenomenon due to the respiration of relatively high concentrations of phytoplankton biomass within the metalimnion [10]. Wentzyk et al also reported the MOM phenomenon in Rappbode Reservoir located in the Harz Mountains in Northern Germany, and found that the respiration of phycoerythrin-rich Planktothrix rubescens was responsible for the oxygen minimum in metalimnion in a low-nutrient drinking water reservoir [15]. Kreling et al further revealed the function of physical transport and oxygen consumption for the development of MOM in a lake, which concluded that more DO was consumed in the metalimnion than that transported to this layer at the same time [16].…”

“…Kreling et al further revealed the function of physical transport and oxygen consumption for the development of MOM in a lake, which concluded that more DO was consumed in the metalimnion than that transported to this layer at the same time [16]. The previous studies have preliminarily summarized the formation of MOM mainly due to the following reasons: (1) Higher DO consumption in the metalimnion was mainly attributed to the enhanced decomposition of dead algae or respiration of algae beyond their photosynthetic activity in the metalimnion [15]; (2) the interflow of DO depleted or depleting water also has been claimed to be an important influencing factor for MOM formation [17]; (3) MOM has been related to oxygen depletion at the side wall in some situation, which was advected into the water body of the reservoir on isopycnal water layer [18]; (4) finally, vertical gradient in the water column due to the retardation of diapycnal transfer and vertical exchange led to the reoxygenation difficulty [19]. Although those studies have reported MOM phenomenon in different areas and stated the reasons, the characteristics including the MOM formation mechanisms, the developed thickness and ecological risk of this phenomenon in reservoirs are of great difference with regard to hydrological and climatic conditions [10,15,20,21,22,23].…”