When it comes to setting up your refugium, there are plenty of considerations to make. You must select macroalgae species, clean-up crew species, a type of substrate (or none?), lighting and so on. Most of this planning is pretty fun. But, with so much to think about, and so many options, it might be easy to overlook one of the most important factors of all: water flow.
Water flow characteristics can significantly impact macroalgal growth rates and, therefore, the refugium’s overall capacity for nutrient export. And controlling dissolved nutrient levels is one of the main reasons for installing a refugium in the first place, right?
Types of water flow
The first distinction aquarists typically make when discussing water currents is whether they are chaotic or laminar. Chaotic flows are most like those encountered over shallow reefs that are subjected to strong wave action. As the term suggests, chaotic flows are highly inconsistent with regards to direction and velocity. Laminar flows are more like those in rivers and estuaries, as flow direction and velocity are more or less constant (they may change direction with the tides, but are nevertheless comparatively stable and predictable).
So what does this look like in a refugium? A chaotic flow pattern might be created with the use of specialized water pumps. Usually controllable to some degree, these pumps can be adjusted for variable output, for movement (of the nozzle or the entire unit), or for both. In creating chaotic water movement, it might help to place the inlet from the main tank in the center of the refugium so that it may progress in all directions.
Laminar flows are a little more straightforward to create. In this case, you’ll want to direct the inlet to the far end of the refugium opposite the outlet. The water will simply flow from one side of the refugium to the other. Generally, for generating laminar flow, supplementary pumps are not necessary.
Which to use?
Type of flow is usually dictated by the kind of macroalgae to be kept. Velocity can be higher for rigid forms (e.g. Chaetomorpha), but must be subdued for softer, fleshier types (e.g. Ulva). In more special circumstances, this will also be additionally influenced by the type of animals that might be housed there. For example, when using a refugium as a place to keep seahorses, a relatively gentle, laminar flow is ideal.
Most seaweeds will grow either attached or unattached. But, many prefer to grow one way or the other. Laminar movement that flows through the crop are best for attached macroalgae. If the aquarist aims to cultivate types that form canopies (e.g. Ulva) or sheets (e.g. Chaetomorpha), laminar flows are likewise suggested. This is most easily accomplished with a longer refugium.
Chaotic flow can be used to tumble the macros and is highly effective with unattached macroalgae. This is most easily accomplished with a taller refugium. The reason for tumbling is that it is incredibly efficient, since all parts of every plant receive light. These kinds of currents are most appropriate for compact, globular macros that don’t readily form canopies or big, solid masses (e.g. Gracilaria and Halymenia). Lighter, fleshier algae such as ogo require only a moderate flow rate, whereas heavier, denser algae such as leafy sphere appreciate something more vigorous.
How much flow is enough flow? Really, if our primary purpose is to remove nutrients, the higher the turnover of water through the refugium, the better. There are, however, some physiological limitations here. First of all, there will be some maximum efficiency at which the macros are capable of taking up the nutrients. And then there are also physical limitations to how much flow can be applied. This is because the more delicate types like Ulva and ogo can be damaged from shearing or compaction in the face of strong currents.
If one chooses a softer macro (maybe because the intent is to feed harvested material to herbivorous livestock), an “oversized” refugium is a good idea. Why? Because you can pass more water through it even at a reduced velocity!
There is an interesting caveat to the more-is-better rule for velocity. In terms of nutrient sequestration, yes, you want an overall faster flow. Still, it is quite desirable to have small pockets of lesser flow here and there. The reason for this is that it allows suspended particulate matter to settle out onto the bottom of the refugium. This accomplishes two things. For one, it reduces detritus build-up in the display tank (where you really don’t want to see it). Secondly, this deposits it right where your pods (i.e. detritus eaters) will be most abundant.
The macros themselves help with this. Friction slows water currents as they hit the plant surfaces. But one can fiddle around with baffles, the rockwork, pump orientation and so on to maximize settling.
Going with the flow
No two refugia are exactly the same. A single refugium might even change considerably between growth/harvest cycles, as it biologically develops, etc. Therefore, unfortunately, there is no handy formula for the perfect flow pattern. But with some awareness of how water movement affects macroalgae growth (nutrient sequestration rates!), any aquarist can make adjustments as needed to get the most out of their planted refugium.