So this is just throw around some ideas. We all know that a pond whose watershed is nutrient poor needs nutrients and that simply adding essential nutrients and preparing its chemistry (primarily liming) can double,triple, or even quadruple the production of fish in a pond of otherwise poor fertility. Under such a regimen, one basically changes the trophic status of the pond.
Ponds tend to accumulate nutrients naturally and so even a poor pond could (conceivably) reach the trophic status of a fertile pond given sufficient time. In as much as this is true, fertilization is a way to speed up a pond's trophic development to produce the desired production of fish at a much earlier pond age. So an area of interest for me is to understand how trophic status can be determined accurately. Another area of interest I have is the phosphorus reservoir needed to maintain a given weight of fish. IOWs if the goal is 300 #s of fish per acre and this dry weight of fish contains 2% or 1.2 lbs of phosphorous ... what minimum weight of phosphorus is needed to support the food chain that the 300#'s of fish need. Is it 5 times, 10 times, or 50 times, or even possibly more? To be sure, I have not been able to find references which can help with determining this minimum need. Clearly, there is probably is a minimum where below this native concentration ... even under optimum conditions ... the production of primary food is too low to support the goal weight.
By and large, going all the way back to Swingle, the procedure was to maintain bloom but since then we are finding that bloom clarity isn't always a good indicator of the weight of fish a pond can support or even its present primary production. When the clarity is low, the standing weight of the phytoplankton is high which means a lot of phytoplankton was produced but the condition isn't necessarily an indicator that daily primary production remains high. For example, it may represent a condition where there is insufficient phytoplankton grazing and where light and carbon are limiting factors. Furthermore, the quality of phytoplankton as food can be adversely affected by these limitations as well. This may in part explain why doubling the phytoplankton standing weight does not generally lead to a doubling of fish carrying capacity. Many authors have noted this, but many oligotrophic waters produce more fish than their clarity would otherwise suggest they could. In these waters there must be much more production of primary foods than any measure of secchi could adequately indicate. This may suggest that leaner systems (lower in nutrients) may be significantly more efficient at producing food with lower standing weights of primary producers. So a number of factors may contribute to this. Here are a few possibilities.
1. Less competition for sunlight, carbon, and oxygen (for respiration and cell division). Individually phytoplankton cells are able to produce more food internally and thus be able to replicate themselves at greater rates (individually producing more offspring than they can individually when standing weights are high). I have mentioned this before ... PRODUCTION IS MORTALITY ... and so conditions where mortality is high lead to higher rates of reproduction and more frequent and rapid cycling of nutrients. Keep in mind, nutrients can be used over and over again it is only sunlight that must be used in the present or be lost.
2. Improved water quality. This may help grazer's achieve higher standing weights. In theory, shouldn't grazers increase proportionately with the available food (standing weight of phytoplankton)? If they did, wouldn't the phytoplankton be adequately grazed? If you saw a pasture rank with tall grass, what would be your first thought be? Perhaps,"Ah it doesn't have sufficient grazing"?. Rank water ... in my way of thinking ... is water that isn't sufficiently grazed. A fair proportion of the production is going to sink to the bottom as food for bacteria instead of for free swimming zooplankton. There may be a point at which standing weights of phytoplankton become a deleterious influence for zooplankton (just as it can be for fish). One reference I read stated that the metabolic requirements for oxygen is much higher for zooplankton than it is for fish (on a biomass basis). The author suggested the need originates from higher rates of growth and reproduction. If this is so, then zooplankton may be be first to succumb when standing weights of vegetation become excessive.
I very much like feed as a way to introduce nutrients into nutrient poor water. The consumption produces immediate gain while the defecated feed is recycled into the ponds nutrient store producing many times it weight in primary production. Most of this primary production can be credited to sunlight and the minerals in feed that are defecated by the fish. In poor water, this production greatly enhances the food production of the whole pond. On the other hand, in very rich water it has little effect at all (at least for fish like CC, BG, and LMB). TP can gain a substantial amount on phytoplankton but they are exception to the rule. At high density and under intensive feeding, most fish only grow as fast as the feed will grow them and most all the primary production is ungrazed and is not utilized. IOWs there are limits to how productive a natural system can be for game fish when it is pushed beyond it's coping point. Beyond this point, the pond no longer hits on all cylinders, cascading effects diminish the food chain and the community is broken. Our fish become dependent on feed and everything we must do to keep feeding them.
When we try to maintain fish standing weights that are greatly above a natural systems ability to support them ... we create conditions that can actually limit natural production of foods for our fish. So if we can find ways to maximize natural productivity (particularly at the secondary trophic levels) then we have a very optimized, healthy, system with acceptable water quality. To me, this seems a very good goal to have for recreational fisheries. That said, I recognize that there are other ways to do this. For example, we could feed all the gain our fish achieve while only controlling nutrients when they threaten the lives of our fish.
A system that depends entirely on the sun but has sufficient nutrients to support a healthy population of fish and other creatures is not able to attain the standing weights of fish that are achievable with feed. The alternate good side to this equation (if fewer fish seems a really bad thing) is that the standing weights of vegetation will not be as high either. They will instead be balanced. The production of vegetation both phytoplankton and macrophytes will be just what it takes to support the community and nothing more.