This post first appeared on Cold Weather Aquaponics.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.
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.
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).
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
http://www.ipm.iastate.edu/ipm/icm/2001/10-22-2001/why50.html[/caption] [caption id="" align="alignnone"
Various Nitrifying Bacteria Growth vs. Temperature in Aquaculture http://ejournal.sinica.edu.tw/bbas/content/2012/1/Bot531-12/Bot531-...
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).
This article suggests that nitrifying bacteria can operate at even lower temperatures than the studies listed above.
What this all means for you is that the rules of thumb for biofilter and/or grow bed sizing proposed by most of the sites and books about aquaponics may not serve you well when your water temperatures dip below about 50°F. Stay tuned for the next technical post on how to size your biofilter.