Do Your Bacteria Get Cold?

Keeping up our trek through the list of aquaponics winterization techniques (in no particular order), this week we turn to water chemistry. This post will provide an explanation for why nitrification in cold water offers us a challenge.  It assumes you’ve digested the basics of nitrification.  If you haven’t, read up here.  The next technical post will give you a guide for sizing your biofilter.

As you read this, keep in mind that I’m not a chemist, a biologist, or a bio-chemist, and this is not my area of expertise.  I’d recommend taking this post with about 500 ppm of salt. I’m only going into this issue because it’s essential for cold weather operation.

  • Passive Solar Greenhouse Design
  • Insulated and Air-Sealed Fish Tanks and Grow Beds
  • Insulated Piping
  • Multiple Layers of Thermal Protection for Plants
  • Fish Selection for Cold Hardiness
  • Plant Selection for Cold Hardiness and Freeze/Thaw Tolerance
  • Efficient Water (Not Air) Heating
  • Programmable Temperature-Dependent Pumping Controls
  • Strategies for Maximizing Nitrification in Cold Water
  • Aquaponics-Integrated Hot Tubs (Seriously)

Cold Water Nitrification

In some ways this post puts the cart before the horse because we haven’t gotten into seasonal water temperatures and the reasons for them. For the meantime, I hope you’ll just trust me that keeping warm water in a backyard aquaponics greenhouse in a climate like mine is foolish. Lots of people do it. But then again, lots of people buy huge off-road vehicles and never go off road. Go figure.

Hello. My name is Nitrosomonas.

Anyhow, assuming that you’ll be lowering your water temperatures down to 50°F or lower in winter, you’ll need to start thinking about bacteria.

If you’re not familiar with the bacteria that live in your aquaponics system, welcome to the club! Some may try and pretend that we understand the bacteria in our systems, but in reality it’s 99.9% mystery. We know that some things work, and that’s what we do. But in truth, humans know more about the farthest reaches of space than we do about our own soil bacteria.  For more on this, read Teaming with Microbes: A Gardener’s Guid to the Soil Food Web.

But I digress.  For the purposes of nitrification (converting ammonia to nitrite then to nitrate), there are two types that do the work.  Or, rather, we group bacteria into these two types depending on the work that they do. Nitrosomonas bacteria convert ammonia (toxic to fish) to nitrites (even more toxic to fish).  Once that step is done, Nitrobacter then convert nitrites to nitrates (not very toxic to fish).

And I’m Nitrobacter. I eat Nitrosomonas’ poop.

By the way, “convert” as it’s used here is a euphemism for “eat the one thing and poop out the other,” in the same way that we “convert” Cracklin’ Oat Bran into little rocks. There are two things that get otherwise squeamish people comfortable talking about poop. One is gardening. The other is raising children.

This beautiful, magical process works wonderfully and (usually) reliably in our aquaponics systems. Mine has gotten so reliable that I only check my ammonia and nitrate levels once a month (or whenever the neighbor kids want something to do).

However, like most things, this slows down when it gets cold. I can relate to this. When it’s 10°F outside, it takes a lot to get me off the couch and out the door for a run. I have to put on a jacket, a hat, gloves, maybe a mask, and the whole darned thing is uncomfortable. Bacteria is the same way. Converting ammonia is such a bother. I need a nap.

Fortunately, this cold-weather slowdown is predictable, as the following charts show us. I’ve copied a bunch of them because the results don’t all agree with one-another and I don’t have a clear way of determining which is most accurate.

Nitrification Rates by Temperature in Soil

Various Nitrifying Bacteria Growth vs. Temperature in Aquaculture

Nitrification Rates by Temperature in Aquariums

According to the US EPA, most strains of nitrifiers grow optimally at temperatures between 77 and 86°F, but nitrification has occurred over a wide range of temperatures (46-79°F).

Stay tuned for the next technical post on how to size your biofilter.

What do you think – have you had issues with cold weather nitrification?