Hi, everyone. Long time no see.
I've had phosphorous on my mind for a while, now, and I've come up with a notion or two that might be of interest in these parts. As before, apologies if what I'm pondering has already been pondered, but I do like to geek on this stuff...
I strongly believe the LED gurus are on to something with their recognition of the need to provide a broader spectrum to facilitate bacterial photosynthesis. More on this later, but first, here's my take on the P problem...
I suspect the root of the problem is that we're trying to pull P out of our systems using organisms that are "P-lite" -- that is, the N:P ratio in the algal cells is higher than the Redfield ratio of 16:1 (...which, it should be recalled, is the average N:P ratio of all the stuff scooped up by plankton nets and is most likely not the N:P ratio that would be found by sampling any particular species, or any particular cell, of bacteria or algae). This makes sense in that cyano dominates at low N:P ratios (like 10:1 and below) while green algae outcompetes cyano for P at high N:P ratios (above 20:1). In other words, the problem is the scrubber itself in that it creates an environment so favorable to green algae that it can outcompete cyano even under conditions (low N:P ratio) that should strongly favor cyano. But even so, it's still just green algae, so it simply can't absorb P fast enough to keep up with what's going into the system.
Put simply, if the food going into a system is around 16:1 N:P, then algae with an internal N:P ratio of 18:1 or 20:1 or even higher (see Redfield Ratio is not always 16:1 in phytoplankton) can't possibly keep up. It N-limits itself.
The obvious solution here is to dose N, like the FW guys who practice estimative index dosing, but I would argue that dosing macronutrients in a reef tank is inherently risky. To err is human, after all. Besides, from what I've read around here, that doesn't seem to work... My guess is that the problem isn't a micronutrient deficiency; rather, the problem is that not just the algae but the entire microbial loop is starved for nitrogen, and because bacteria are much, much better than algae at capturing nutrients during times of surplus and storing them for future use, that's where most of the N ends up.
So like I said, seems to me the LED guys are on to something with the idea of including bacteria-friendly wavelengths -- if you can't beat 'em, join 'em... I'm thinking, though, that maybe we, as algae-friendly folk, should be looking beyond the display tank to consider the lighting needs of cyanobacteria.
I'm thinking that perhaps what's called for here is a cyano scrubber. Cyano totally hearts low N:P ratios to begin with, so why fight it? The tinkerers and LED guys out there (Floyd, I'm looking in your direction...) may want to consider two-stage scrubbers, with a "normal" scrubber emptying into a low-flow area (maybe even just a sump) that's lit to foster the growth of cyano after N zeros out, or perhaps building scrubbers that can be transitioned from high flow and lighting for green algae to low flow and lighting that's optimized for cyano.
But until somebody figures out whether or not we're all going to have to become cyano farmers, too, as a stopgap solution for P accumulation, I've been considering plumbing a "remote deep mud bed" in a five-gallon plastic bucket into my system. I've been running an algae scrubber since Day 1 and because my DT is small, I don't feed heavily lest my corals outgrow their happy home, so my P levels are still quite low, but "low" isn't undetectable... I can see the writing on the wall at this point.
The reason I've got mud on my mind is that in an anaerobic environment, chemical conditions change such that bacteria can reduce P compounds to orthophosphate (PO4) which persists in the absence of oxygen and, being readily soluble, accumulates to concentrations far higher than what's detectable in the overlying water. Orthophosphate likes to adhere directly to mineral surfaces, so the optimal nutrient sink for P should combine anaerobic reducing conditions with a lot of available surface area per unit volume -- ie, a small grain size, like fine sand or mud... So it's no surprise that "Miracle Mud" and similar products are out there -- not only are they better simulations of the seafloor than pool filter sand, but they're excellent nutrient sinks, as well.
But I'm looking for an inexpensive alternative -- has anyone ever tested river clay from the local art store for P loading? -- because my idea is to let the new mud bed cycle and come into equilibrium with the rest of the system, and then dump it, along with any benthic organisms that managed to colonize it (...just more nutrients to export!), and start over with fresh material. Obviously, I'll never zero out P this way, but P should migrate -- very slowly, over the course of months -- from the established substrate into the RDMB; that is, phosphorous will gradually diffuse out of the area of high concentration and into the area of low concentration. I'm not really worried about P accumulation in my substrate yet, but if you are, this might be worth trying, as it's simple, comparatively cheap, and very low maintenance.
The basic idea is that much like a scrubber establishes a "sweet spot" in the system where the environment is optimal for the growth of green algae, a remote deep mud bed would be a sweet spot where phosphorous will want to accumulate. And like an algae scrubber, it has to be cleaned regularly to be effective... Happily, "regularly" in this context probably means something on the order of once or twice a year. Like I said: low maintenance.