I like your analogy to an engine Eric. Two things in the graph stood out to me. One was the slope of the regression. If it had been a slope of -1.0, we could say that halving the secchi depth (essentially doubling the plankton) would double the standing weight. But the slope is shallower and this tells us that when we double the primary production ... we get less than double the standing weight. Speaking to your point of a well tuned engine perhaps we can say the lesser production allows the fish to breathe easier
? I don't know maybe in terms of efficiency its like the porting the heads of a stock 283. I've probably offered a bad example because the efficiency improves the less we ask from our water. Probably the better way to say it is that putting more fuel through the ports may result in more carbon deposition where the engine producing more horsepower might not breath as efficiently as one producing less.
The other thing that spoke to me was just how variable the data was. Perhaps none of the water was phosphorus limited but more than half of the lakes had secchi >36". Some had secchi depth >16 ft. For any given sechi depth the range of standing weights range ~ 9 times the minimum. This contrasts with each of the functions (min - mean - max). For example, if a pond is hitting all cylinders at 16 ft secchi depth we should expect it to be limited to around 147 lbs/acre. If one were to push it 8 times harder through fertilization (2 ft sechi) he might likely get around 390 lbs/acre. An improvement of 2.65 times. So there is as much room to improve a mean condition to the max condition as there is from pushing the water 8 fold with fertilization. For example the mean SW at 16 ft secchi depth is 47 lbs. Managing to maximize production with the same fertility could add 100 lbs. But all other things being equal ... 8 tupling the algal biomass to 2 ft secchi can only be expected to increase standing weight by 76 lbs. So perhaps this in part speaks to what esshup earlier said about brush and cover. I can see other things affecting the variability as well. For example, when macrophytes produce most of the primary production ... probably little of it feeds fish. Removing them might significantly improve the production of food for fish. The fish mix also. When fish that are present have shorter food chains the assemblage may weigh more. Some of the low SW lakes may have had stunted LMB populations that prevented prey species from obtaining their SW potential.
I was very encouraged by the range of data in that the max function which bounds the data appears to be a legitimate goal weight to work with for ponds maintaining some time weighted secchi depth through the growing season. I was further encouraged that the standing weights achievable with 6 to 4 ft secchi depth was something I would be able to work with.
