Lake metabolism

Lake metabolism represents a lake's balance between carbon fixation (gross primary production) and biological carbon oxidation (ecosystem respiration). Whole-lake metabolism includes the carbon fixation and oxidation from all organisms within the lake, from bacteria to fishes, and is typically estimated by measuring changes in dissolved oxygen or carbon dioxide throughout the day. Ecosystem respiration in excess of gross primary production indicates the lake receives organic material from the surrounding catchment, such as through stream or groundwater inflows or litterfall. Lake metabolism often controls the carbon dioxide emissions from or influx to lakes, but it does not account for all carbon dioxide dynamics since inputs of inorganic carbon from the surrounding catchment also influence carbon dioxide within lakes.

Concept Estimates of lake metabolism typically rely on the measurement of dissolved oxygen or carbon dioxide, or measurements of a carbon or oxygen tracer to estimate production and consumption of organic carbon. Oxygen is produced and carbon dioxide consumed through photosynthesis and oxygen is consumed and carbon dioxide produced through respiration. Here, organic matter is symbolized by glucose, though the chemical species produced and respired through these reactions vary widely.

Photosynthesis: 6 C O 2 + 6 H 2 O →

l i g h t C 6 H 12 O 6 + 6 O 2 {\displaystyle 6CO_{2}+6H_{2}O{\xrightarrow[{}]{light}}C_{6}H_{12}O_{6}+6O_{2}}

Respiration: C 6 H 12 O 6 + 6 O 2 →

6 C O 2 + 6 H 2 O {\displaystyle C_{6}H_{12}O_{6}+6O_{2}{\xrightarrow[{}]{}}6CO_{2}+6H_{2}O}

Photosynthesis and oxygen production only occurs in the presence of light, while the consumption of oxygen via respiration occurs in both the presence and absence of light. Lake metabolism terms include:

GPP - gross primary production (e.g. total photosynthesis) R - total respiration ( R h + R a ) {\displaystyle (R_{h}+R_{a})}

R h {\displaystyle R_{h}} - heterotrophic respiration

R a {\displaystyle R_{a}} - autotrophic respiration NEP - net ecosystem production = GPP - R NPP - net primary production = GPP - R a {\displaystyle R_{a}}

Measurement techniques Estimating lake metabolism requires approximating processes that influence the production and consumption of organic carbon by organisms within the lake. Cyclical changes on a daily scale occur in most lakes on Earth because sunlight is available for photosynthesis and production of new carbon only for a portion of the day. Researchers can take advantage of this diel pattern to measure rates of change in carbon itself or changes in dissolved gases such as carbon dioxide or oxygen that occur on a daily scale. Although daily estimates of metabolism are most common, whole-lake metabolism can be integrated over longer time periods such as seasonal or annual rates by estimating a whole-lake carbon budget. The following sections highlight the most common ways to estimate lake metabolism across a variety of temporal and spatial scales and go over some of the assumptions of each of these methods.

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