Probably the most labor-intensive method of estimating a metric of lake metabolism is by measuring all the inputs and outputs of either organic or inorganic carbon to a lake over a season or year, also known as a whole-lake carbon budget. Measuring all the inputs and outputs of carbon to and from a lake can be used to estimate net ecosystem production (NEP). Since NEP is the difference between gross primary production and respiration (NEP = GPP - R), it can be viewed as the net biological conversion of inorganic carbon to organic carbon (and vice versa), and can thus be determined through whole-lake mass balance of either inorganic or organic carbon. NEP assessed through inorganic (IC) or organic carbon (OC) can be estimated as:
where E is export of OC through fluvial transport and IC through fluvial transport and carbon gas (e.g. CO, CH) exchange between the lake surface to the atmosphere; S is storage in the lake sediments and water column for OC and water column for IC; and I is the input of OC and IC from fluvial, surrounding wetland, and airborne pathways (e.g. atmospheric deposition, litterfall). A lake that receives more OC from the watershed than it exports downstream or accumulates in the water column and sediments (I > E + S) indicates that there was net conversion of OC to IC within the lake and is thus net heterotrophic (negative NEP). Likewise, a lake that accumulates and exports more IC than was received from the watershed (S + E > I) also indicates net conversion of OC to IC within the lake and is thus net heterotrophic.