The Refugium: Technical aspects and set-up.

A refugium is a separate section of the aquarium, which functions both as a component of the filtration system , and as a sort of “nature reserve” of beneficial benthic and planktonic organisms .

This is usually a dedicated section of the sump or a small “satellite” aquarium, set up to provide an area protected from predators, with the best conditions for proliferation, an abundance of substrates suitable for the various benthic organisms that will colonize it.

This area, in addition to providing a safe habitat for the more vulnerable and beneficial organisms that might be disturbed in the main aquarium environment, serves several purposes, among which the following are essential:

• Promoting biodiversity

• The settlement of the algal component

• Reduction of dissolved nutrients and suspended organic matter

• Biological filtrationThe production of live food

Setup

Refugiums can be set up in different ways depending on the needs.
The type of substrates and their thickness, the lighting, the water circulation, the algal population can be varied, but we will try to illustrate a relatively standard system, with a good balance of all the components and which allows for the hosting of a large variety of macroalgae and benthic organisms.

The technical equipment of a refugium is decidedly minimal and is generally limited to a glass or plastic container, lighting and heating (in “satellite” systems).
Movement is generally very limited and to protect the organisms that populate it, it is not advisable to insert movement pumps (except in special cases).
The water flow is guaranteed by a small dedicated pump or by the fall, depending on the positioning.

As for lighting, if we were to approach the topic with a meticulous and scientific approach, the article would become very complicated, without any real practical utility.
To oversimplify an extremely vast topic concerning a sector (LED lighting) in continuous and rapid transformation, I will limit myself to giving you a few practical examples.

For a hypothetical 30 x 30 x 30 (h) refugium, we could evaluate the use of a simple 30-40W LED ceiling light, natural white 4500-6500 °K.
For those who prefer to work with “phytostimulants”, reducing the wattage but increasing the available PAR, I recommend selecting reliable instruments from selected suppliers.
In the case of high-quality diodes, working with specific spectra, the wattage can easily be halved.

Unfortunately, the immense differences between the various lighting technologies currently available do not allow me to give you more detailed information without writing a treatise, probably obsolete in a few years.

Soft substrates

The choice of soft substrates (sand and mud), their technical characteristics and their thickness depends on the specific objectives we wish to achieve.
These parameters can be varied to better match the needs of the system, within certain limits.

Generally we recommend setting up with the SSB technique, therefore with a layer of fine oolitic aragonite, with a mixed “sugar” grain size between 500 um and 2 mm, and an average thickness of 4 cm .
This type of setup is the simplest and most “universal”.

In advanced techniques, different substrates can be used within the same refugium and the thicknesses can be varied (even with the help of RDSB): depending on what we use, we will favor one type of organism rather than another.
Calcite in granules, pearls or flakes, micronized or sugarsize, clay or calcareous muds, maerl and shells, (…) the substrates that can be used successfully are numerous.

In that case, our advice is to design the system very carefully and to always vary the grain size of the substrates both along the thickness of the SSB and in the different areas of the refugium.
The more variability of zones and substrates we offer, the greater the capacity of the system to host a high biodiversity.

Solid substrates

Even for solid substrates, the choice of type, quantity and arrangement will depend on our objectives.
If we want to enhance the nitrification and denitrification capacities of the system we will opt for substrates that are more suitable for bacterial colonization and microfauna .
If, on the other hand, we want to increase the system's capacity to produce benthic organisms, improve the disposal of organic waste and organic particulate matter, we will opt for substrates that are more suitable for colonization by small detritivorous worms, gammarids, amphipods such as rocks or specific shelters.

We generally advise against adding too many non-specific hard substrates for a number of reasons, but primarily to avoid limiting the space for macroalgae, which will act both as a substrate for associated organisms and as an essential component in nutrient reduction.

Specific substrates

In recent years, shelters for benthos and specific substrates for colonization by benthic mesofauna have appeared on the market.
These substrates are generally made of plastic or ceramic material.
These are honeycomb structures, which exponentially increase the available colonizable surfaces and allow the effectiveness of the system to be greatly improved.

Population

When setting up a refugium in a marine reef aquarium, the choice of organisms that will inhabit it is crucial to its success.
Preferred species are divided into several groups, each with specific ecological roles and benefits for the system.

Welcome species

Detritivorous Crustaceans:
These small organisms such as amphipods, mysids and isopods, play a key role in the process of decomposition of organic debris. They are essential for keeping substrates clean and reducing the accumulation of decaying organic material.

Polychaetes and Annelids:
Small polychaete worms, or annelids, are extremely efficient in the process of bioturbation, helping to keep substrates aerated and prevent the formation of anoxic zones. In addition, many polychaetes are useful in controlling algae, suspended particles, food residues and organic debris.
Copepods: These small planktonic crustaceans are important both as a food source for fish and corals, and for their role in controlling microalgae. They also contribute to nutrient cycling by breaking down organic debris.

Molluscs:
Small benthic gastropods of the genus Stomatella, Euplica, Peringia or Collonista, are extremely effective in controlling algae and cleaning surfaces.
Their grazing activity helps keep substrates clean, limiting the growth of unwanted algae. Bivalve molluscs are also extremely useful thanks to their frequent and abundant production of gametes and their ability to filter water, helping to reduce the presence of suspended particulate matter.

Small echinoderms:
Some sea urchins, brittle stars and other small echinoderms contribute to the cleaning of surfaces and the bioturbation of the substrate, helping to prevent the formation of anoxic zones.
Their activity helps to reduce organic debris and maintain ecological balance in the system.

Harmful species

While the presence of some species within the refugium is welcome and indicative of the stability of the system, the presence of some organisms should be avoided.

Rapidly growing and opportunistic organisms can quickly become invasive, crowding out beneficial species and disrupting the ecological balance of the refugium.
We can include in this category some radiolarians, some types of planarians and other flatworms, some predatory eunicids or nereids.

Predators:
It is not advisable to include predators and other planktonic or benthos-feeding organisms for prolonged periods.
Predatory species such as most shrimp, crabs or hermit crabs can significantly reduce the population of beneficial organisms in the refugium and lead to an imbalance in the system.
The same goes for the vast majority of fish and corals.

Infesting microorganisms:
Some microorganisms (such as some cyanobacteria or dinoflagellates) can produce harmful substances or significantly alter the chemical parameters of the water, such as pH or nutrient levels, compromising the health of the entire system.

Colonization Times

Full colonization may take several weeks or months, depending on aquarium conditions and the biodiversity introduced.
Generally in an adequate system, complete colonization of the substrates is observed in a period of between 30 and 60 days.
This lag time can be significantly reduced by adding abundant inocula of mature substrates, benthic fauna and zooplankton.

Diet

In the initial stages of colonization of the system, it may be useful to add specific feeds (freeze-dried, phytoplankton concentrates or live microalgae), with the aim of improving the reproductive performance of the microfauna and facilitating their increase in number.
Generally, once on a diet, no additional food is given in a refugium.
The organisms that colonize it feed on microalgae, organic debris, biofilm and food residues, therefore requiring few food supplements, except for specific needs.

Cleaning and maintenance

Cleaning must be minimal so as not to disturb the biological balance.
We recommend limiting yourself to pruning with periodic removal of excess algae and control of invasive species.
Removal of excess debris is generally performed on an annual basis and only when its presence may be problematic.


See you for the second part of the article:
“Macroalgae: choice, selection and care”

Stay tuned!!!