Dinoflagellates, often referred to as "dinos" by aquarium enthusiasts, are a group of single-celled organisms classified within the phylum Dinoflagellata. Scientifically, they are characterized by two flagella that enable a distinctive spinning motion. They can be both photosynthetic and heterotrophic, meaning they can make their food from light and/or consume organic matter as a means of obtaining nutrients.

In the context of saltwater reef aquariums, dinoflagellates manifest as unsightly brown, green, or reddish-brown films or slimes that coat surfaces such as rocks, substrate, and corals. These outbreaks can lead to significant issues, including the smothering of corals, depletion of oxygen levels, and the release of toxins harmful to marine life. Despite the challenges they present, it's important to note that with informed strategies and consistent effort, overcoming a dinoflagellate infestation is entirely achievable.

Dinoflagellates

How to Identify Dinoflagellates

When taking an exhausting deep dive into the topic of "dinoflagellates in a saltwater aquarium" on the internet, much of the advice will start with recommending that you look at a sample of your particular dinos under a microscope in order to properly ID them so can custom tailor a treatment plan based on the type of dinos occurring in your tank. While this process can be fun and interesting, most of us don't have a high-quality microscope or the necessary experience to properly ID microscopic single-celled organisms. The good news is that you don't need to species ID your dinos to overcome an outbreak and stop them from returning.  

You do, however, need to distinguish dinoflagellates from other common nuisances like cyanobacteria and diatoms.

Here are the key differentiators:

  • Appearance: Dinoflagellates typically form slimy, mucous-like layers that may exhibit a bubbly texture. In contrast, cyanobacteria often appear as a dense, mat-like slime, while diatoms present as a fine, dusty brown coating.
  • Behavior: Some dinoflagellate species display a diurnal pattern, diminishing or disappearing at night and reappearing during daylight. Cyanobacteria and diatoms generally maintain a consistent presence regardless of the time of day.
  • Response to Disturbance: When agitated, dinoflagellate filaments tend to disperse into the water column, only to regroup shortly after. Cyanobacteria mats are more cohesive and may detach in larger pieces, whereas diatoms, being more particulate, will resettle without re-coalescing.
Diatoms
Dinoflagellates

Observing a water sample under a microscope can be revealing for a definitive identification. Under magnification, Dinoflagellates exhibit a characteristic spinning or erratic swimming motion due to their flagella, a trait not seen in cyanobacteria or diatoms. Diatoms have rigid silica-based shells that appear as rigid plates or boxes, while Dinoflagellates are often ovals, circles, or more irregularly shaped, depending on the exact type.

Why Do Dinoflagellates Appear?

There is no way to keep dinoflagellates out of your aquarium; they exist in ALL saltwater aquariums, but typically in small amounts. The problem starts when the conditions in your tank promote an excessive proliferation of dinos. A dinoflagellate takeover that typically happens when the microbiome is out of balance.

Vulnerable Conditions:

Low Nutrients: Undetectable nitrate and phosphate levels can lead to dino outbreaks. Many competing microorganisms might struggle to survive in these low-nutrient conditions, whereas dinoflagellates can thrive.

Lack of Biodiversity: Common in new aquariums, a lack of copepods, bacteria, and other beneficial micro-organisms can allow for a dinoflagellate takeover. Again, the lack of competition can lead to a dinoflagellate explosion.

Drastic Changes: Large water changes, overuse of UV sterilizers, and aggressive nutrient control can create prime conditions for dinos.

How To Prevent Dinoflagellates in a Saltwater Aquarium

Well, here's the good news. It's fairly easy to avoid a dino outbreak altogether. Just avoid the conditions previously discussed, and here's how:

  1. Add copepods early in the cycle process: Live pods like those from Algaebarn can be added early in the cycling process and then replenished annually to ensure a robust population. 
  2. Diversify microbiome: Adding live ocean-sourced bacteria, sand, or rubble helps to quickly establish microbiome diversity and keep dinos in check.
  3. Keep nutrient levels detectable: Maintain nitrate levels 5-10 ppm and phosphates at 0.02 - 0.06 ppm. If you're dealing with undetectable nutrient levels, simply increase the frequency of feeding, turn off your protein skimmer, or remove filter socks until nutrient levels are detected again. Do not use carbon dosing products and don't overdo it; elevated nutrient levels can lead to equally frustrating algae growth and various other ailments. 

 5-Step Approach To Eliminating a Dinoflagellate Outbreak

Should you find yourself with a snotty blanket of blasted dinoflagellates, we have just found it best to take this comprehensive 5-step approach that will help eliminate the dinos, regardless of the exact type. If you stick to the process, you will have some level of success.  It may require persistence, but like they say, persistence pays off! 

1. Check and correct your nutrient levels.

Shoot for nitrate levels at 5-10 ppm and phosphates at 0.02 - 0.06 ppm. Increasing nutrient levels is fairly easy, just turn off your skimmer or increase the frequency of feeding until the desired nutrient levels are maintained.  Do not use phosphate removal media or carbon dosing products.

2. Add competing organisms

Live bacteria, copepods, and other beneficial microbes will compete with dinoflagellates for resources. The following products are readily available and can all be safely added to your aquarium. While it may take a couple of weeks for the microbiome to improve, this is one of the most critical steps in the fight against dinos. 

3. Manually remove dinos

You want to physically remove the dinoflagellate growth as frequently as possible. Use a siphon to suck the dinos off your rocks and sandbed. You might also consider using a recirculating reactor with a 1 micron sediment filter that has a sufficient pore size to trap free-floating dinos. Use a bulb syringe to stir up and suspend dinos in the water column, they can then be filtered out by the recirculating reactor.

4. Add a UV Sterilizer

Adding a UV sterilizer can kill off free-floating dinoflagellates and will be one of the most highly effective means of controlling the dinoflagellate growth. 

5. Blackout the aquarium

Turn off the lights and wrap the aquarium in a blanket to block out ambient light for 3-4 days. This will starve the photosynthetic dinoflagellates of light and knock back the population drastically.