There is a great amount of variation in the productivity of trout streams and the reasons why are quite complex and interesting. Productivity is essentially a measure of how much life there is in a waterbody. Not all waterbodies / watersheds are created equal. There are any number of reasons for this but many of them come back to geologic and land use influences.
In terrestrial landscapes, we tend to talk mostly about primary productivity which is a measure of how much photosynthetic biomass was produced over the course of a year (annual net primary productivity or ANPP). Globally, primary productivity is highly variable and is most dependent upon temperature and precipitation. Warmer and wetter places - tropical rain forests - are the most productive and dry (desert and tundra) and/or cold (tundra, taiga) places are the least productive. Here in Wisconsin and the Upper Midwest, productivity is generally fairly high. It is maybe a little higher further south (warmer, on average) and from east to west (wetter, on average, to the east) but it does not vary nearly as much as it does globally. There is, of course, local variation not tied to temperature or precipitation that affect productivity. Here in La Crosse, I can see the "goat prairies" on the south-facing bluffs which have thin soils that are further dried by their southern sun exposure. But in general, we live in a fairly productive place. Our summers are pretty green...
In addition to temperature and precipitation, nutrients are a further limiting factor on productivity. Nitrogen (N) and phosphorous (P) are often co-limiting, meaning that additions of N and P increase productivity. This is why most fertilizers are heavy on N and P and sometimes potassium (K) - fertilizers produce more photosynthetic growth. At times, calcium (Ca) or magnesium (Mg) can also be limiting as can some micronutrients but typically it is N and P that limit growth. Many of these elements are present in bedrock which is weathered and provides nutrients to soils and buffering capacity to streams. Some places have great amounts of calcium, magnesium, and other buffering minerals (like the Driftless Area) and others lack these rocks and thus lack much buffering capacity. Many of these places have issues with acid precipitation whereas the Driftless and other places have no issues with acid precipitation.
Productivity of Streams
In streams, measuring and comparing primary productivity tends to make less sense or at least is much more difficult to measure. There are two main reasons for this. First, productivity in aquatic systems turns over much more quickly compared to terrestrial landscapes. Primary productivity in many streams is the "slime on the rocks" - a mix of bacteria, algae, and other microorganisms that have many generations in a year and thus it is hard to estimate their annual productivity. In streams with less shade, filamentous algae and aquatic macrophytes (rooted vascular plants) comprise much of the in-stream (autochthonous) primary productivity. Second, many streams rely heavily upon organic matter that was produced outside of the stream (allochthonous production; read the river continnum concept post for a deeper dive). Many productive streams rely mostly on leaf fall as an energy source and most streams have at least some energy coming from allochthonous sources.
In streams, often the more meaningful measure of productivity is often secondary production - the biomass of animals. In streams, this starts with the "bugs" or more scientifically, the aquatic macroinvertebrates. This is where calcium is particularly important because macroinvertebrates must shed their exoskeletons (ecdysis) and replace them which requires more calcium. This is (a large part of why) streams in the Driftless Area are full of scuds (Amphipoda) which provide an abundant, high energy food source for trout. Fish biomass is also considered secondary production and as you probably well know, fish biomass in many Driftless Area streams is quite high (more on that in another post).
Source: US EPA - Alkalinity and Water
There are quite a number of ways that we measure how well streams are buffered against acids. Probably the simplest is pH which is a measure of the relationship between hydrogen ions (protons) and hydroxide ions (OH-) which at a neutral pH of 7 means that they are equal in concentration. One proton plus one hydroxide ion makes one water (H2O) molecule. Maybe the most direct measure is alkalinity, a measure of water's ability to neutralize acidity or acid neutralizing capacity (ANC, more or less...). Much of alkalinity comes from calcium - but not all. Hardness is a similar measure but depending upon geology and sources of buffering capacity, hardness may be different from ANC and alkalinity.
While the above paragraph describes the measures most directly linked to calcium in water, there are other commonly used measures. Two other commonly used measures of the amount of acid neutralizing components in water are total dissolved solids (TDS) and conductivity or specific conductance. Compared to the other measures I have written about, TDS and conductivity measure more ions and other chemical constituents than do ANC, hardness, and alkalinity. Conductivity is measured by sending a pulse of electricity through the water and measuring how much has been absorbed (conductivity meter). When I was doing my Ph.D. research on acid mine drainage impacted streams in West Virginia, we often had exceedingly high - mind-blowlingly high - conductivities because conductivity also measures ions of aluminum (Al), iron (Fe), manganese (Mn), and other quite significantly toxic ions. In fact, the conductivity of some of our streams were so high we could not electrofish those streams because the conductivity was too high (FWIW, low conductivity is more often an issue). Instead we seined them and caught no fish because the toxicity of the heavy metals associated with mining prevented all fishes from living there.
What About Wisconsin?
I always try to keep this blog somewhat local. We have quite a variation in productivity in Wisconsin; see the alkalinity map above, for starters. Of course, the larger story - which is MUCH harder to tell - is that there are so many other factors that affect secondary production.
To keep this fairly short, there were a few events of glaciation that had a big impact on the state. In the glaciated part of the state, a good bit of alkalinity comes from how glaciers ground rock into easily dissolved particles. In much of the glaciated part of the state, bedrock is buried quite deeply by glacial materials ("drift"; pdf link the the Wisconsin Geological and Natural History Survey efforts to map this). The soils created from this till are typically quite productive and high in calcium. In other parts of the state - northern Wisconsin, Minnesota, and the Upper Peninsula of Michigan, in particular - glaciers scoured to bedrock. Many of these areas have granitic rocks that are slow to weather and provide little calcium and other buffering materials. You will often see the acid stain indicative of streams with low buffering capacity.
Source: US EPA Lake & Stream Acidity
I know in helping the WDNR fisheries crew out of La Crosse with some electrofishing this past summer and our own research where we measured conductivity of our streams, there is a great variation across our Trout Unlimited chapter's area. In the sandbed streams north of I-90, we measured conductivities as low as 25 microsiemens / cm3 whereas dolomite-dominated areas to the south of the interstate, readings were well over 350 or 400. In the sand streams, we caught far fewer trout but often caught some larger fish. More on that in another post for another week. I still don't quite have my mind around the relationship between productivity and average or maximum trout size but some places known for large trout - like New Zealand - are fairly infertile streams.