How Does a Refugium Work?

A refugium is an auxiliary tank that is connected to the
“main” tank. While the main tank is used mostly for display, the refugium
serves mainly utilitarian purposes. In short, refugia naturally turn unwanted
materials such as dissolved and particulate wastes into valuable live foods,
namely “pods” (especially copepods) and macroalgae.

Some aquarists are satisfied with the positive results and
leave it at that; some are more curious and wonder, how exactly do refugia make
this magical transformation?

To understand how refugia work, one must understand how
primary production and primary consumption work in Nature. Primary production
is carried out by autotrophic plants and bacteria, which generate harness
energy and synthesize biomass from inorganic sources of carbon and nitrogen
(most often through photosynthesis). Primary consumers simply are the organisms
that eat primary producers (e.g. herbivores). Primary consumers therefore
transfer food energy and biomass from plants/certain bacteria to carnivores. As
such, primary producers/consumers form the very base of every food pyramid!

Macroalgae take up excesses of dissolved nutrients (e.g.
nitrate and phosphate) that commonly build up in closed systems such as
aquaria. In so doing, they compete with less desirable benthic microalgae (e.g.
film-forming or filamentous types). Whatever nuisance algae do grow can be
controlled by copepods, which are naturally abundant in most all marine
ecosystems. In addition to “bad” algae, copepods (and many other pods) consume
detritus.

Now, it’s great if your aquarium animals eat your pods and
macro--these are nutritious, natural live foods, after all! But in the tiny
confines of an aquarium, these food sources can get exhausted quickly; hence
the refugium! Refugia provide a comfy refuge where the pods and macro can get
pampered, safe from the hungry jaws of your livestock. The pods simply drift
into the main tank little by little, whereas the macros can be offered as a
treat whenever you harvest (to keep the macroalgal bed in a state of constant
growth!).