Where does sink collide?

In biogeochemistry, a sink is a reservoir that takes up more of a substance than it releases, so that on balance it draws that substance out of circulation. Sabine and colleagues, quantifying The Oceanic Sink for Anthropogenic CO2, show the ocean has absorbed roughly a third of the carbon dioxide emitted by human activity since the industrial era, acting as a net sink against the atmosphere. The referent is a compartment of the Earth system, and its measure is a net flux: the mass of carbon per year moving in minus the mass moving out. What makes something a sink here is the sign of that balance, not any physical container or terminal point. A forest, a soil, or an ocean is a sink only while it accumulates faster than it emits, and the same reservoir flips to a source if the balance reverses.[1]

Unit

A net flux into a reservoir (mass of carbon per year, uptake minus release)

Recovery means

Not applicable

Measured by

The net exchange between a reservoir and the atmosphere, from inventories, flux towers, or ocean carbon surveys

In graph theory and combinatorial optimisation, the sink is a distinguished vertex of a flow network: the single node into which flow drains and out of which none departs. Ford and Fulkerson, in Maximal Flow Through a Network, pose the problem of pushing as much flow as possible from a source vertex to a sink vertex across edges of bounded capacity, and prove that the maximum flow equals the minimum cut separating the two. The referent is a labelled position in a directed graph, defined purely by its role in the problem, not by any substance it holds. It conserves nothing and stores nothing; it is simply where the accounting ends. The sink is a formal terminus, and nominating a different vertex as the sink redefines the whole optimisation.[2]

Unit

A distinguished vertex in a directed flow network

Recovery means

Not applicable

Measured by

The value of the maximum flow reaching it, equal to the minimum source-sink cut

In population ecology, a sink is a habitat in which a local population's deaths exceed its births, so it would dwindle to nothing without immigration from elsewhere. Pulliam, in Sources, Sinks, and Population Regulation, formalised source-sink dynamics: a source habitat produces a surplus that disperses into sink habitats, where the birth rate falls short of the death rate and the population persists only because that surplus keeps arriving. The referent is a subpopulation and the demographic balance sheet of the patch it occupies, and the unit is a per-capita growth rate below replacement. A sink can be crowded and look healthy while quietly consuming animals faster than it produces them, which is why counting individuals can mislead about which habitats actually sustain a species.[3]

Unit

A local population whose intrinsic growth rate is below replacement

Recovery means

Not applicable

Measured by

The finite rate of increase of the local population; below one marks a sink, sustained only by immigration