Ya know... over the last two years, in my quest to better understand the purpose of refugia for our systems, I've seen no better definition than Calfo's as written in the Reef Invertebrates book (and quoted above). It is clear, concise, leaving room for interpretation or specialized implementation by the end user. By thinking of his definition on a broad level, the marine aquarist can easily understand the principles behind refugiums.
For all intensive purposes, our displays are "refugia" for those species of fish, coral, and inverts that we wish to display. If you had a nice display with just corals and small gobies, it is a safe haven for both. Add in the grouper, eel, or xymonocanthus longirostris (orange spot filefish) and it is no longer refugia for the corals or fish. The filefish is a consumer of corals, while the grouper and/or eel will make a quick snack of your gobies. So, in principle, your display is the refugium in which you created and controlled to support the long life and hopeful reproduction of the housed species.
Now, with that said, there are situations where the inhabitants of your display may be dependant upon a food source that is quickly depleted in your tank (i.e. pods, mysid). In this case, aquarists have created additional real estate for the production of such critters, that can be either in-line with your system or external, that is void of species that would prey upon them. This too is refugia. If you put a school of Banggai Cardinals in that space, they would quickly deplete or reduce the pod/mysid population there and therefore upset the balance and be cause for removal of the label "refugium".
Others may set up a similar system, but rather than external to the main display, it is in-line and contains just macro algae. This system is typically used for nutrient export and the increase of total water volume in the system, therefore assisting in stability of water quality. This too is a refugium. Add in that one little tang, and the macro algae is "threatened" and is no longer considered "safe".
Basically, the need for refugiums, and the type of refugium utilized is driven by the overall needs of the system as a whole, and the inhabitants of that system. If you have little or no nutrient load, you may have no NEED for a macro refugium. If you do not keep species that are dependant on ispods, copepods, or mysid shrimp, you may not NEED a crustacean producing refugium. One thing that I can attest to and would like to pass along to my peers, is whether or not you NEED such an addition, they can't really hurt.
My 75 Gallon display tank is young in the grand scheme of things. I dealt with the newbie issues of cyano, diatom blooms, etc. My tank then became stable and actually quite enjoyable to own/work with. After a year of bliss, I started to have some "issues". After implementing some corrective actions (dosing/lighting etc.) I noticed SOME improvement. However, after cutting in a new sump, with a large refugium chamber, my reef has taken off. My refugium is a mix and match of all the components I listed above. I have a deep sand base, live rock and rubble for aid in the production of pods and mysid and also keep a healthy population of macro alga for nutrient export. Finally, the increase in total water volume of my system has helped me keep my chemistry in check, without major swings.
I'm sure you've seen my other posts, but I am just amazed with my fuge. While the kids, and sometimes company stare at the pretty colored coral, fish and clams in my display, I can be found on my knees staring in to a box of muck, mire, macro and magic. The abundance and diversity of life in a proper refugium is simply amazing.
Dave
