Bryan, good questions.
So nutrient dynamics is very complicated, and I am NOT an expert in it, but I know enough to be dangerous.
There are many nutrient/energy pathways in the lake, and they are complicated and have feedback loops. But to simplify... nutrients in the water (nitrogen, phosphorous, silica, etc) are used by phytoplankton. Phytoplankton are plants, and they need to be in the photic zone (sunlight to grow). However, just because they're in the upper part of the water column doesn't mean they are only using resources in that top slice of the column, or that those resources came from/stay in that area. Sort of like a tree isn't only using nutrients above ground.
There are multiple ways nutrients get cycled around the lake. There are external loadings (inputs from rivers, or via precipitation over the lake, for example). And internal loading (cycling of nutrients among the lake). And the nutrients also have to be in a form that can be used by specific animals, called "bioavailability"
Within the lake, there's energy pathways that go vertically (bottom to surface and vice versa) and horizontally (nearshore/offshore). And these pathways are both physical (currents/wind/etc) and biological. Biologically, an example of vertical movement might be an alewife foraging on
Diporeia, a bottom dwelling shrimp like critter, and then the alewife getting eaten by a salmon. Or, a zooplankton that stays deep in the water column for most of the day making a vertical migration at night to eat phytoplankton near the surface. Or, a salmon dying and sinking to the bottom to get decomposed. Just as a few examples.
Physically, nutrients are transported both vertically and horizontally in the lake, from normal circulation currents, wind/waves, upwelling events, downwelling events, mixing during seasonal lake turnover (going from stratified with a thermocline to evenly mixed).
Historically, nutrient inputs to the nearshore were carried into the offshore region via biological and physical processes, and were an important part of the offshore pelagic food web (think salmon/alewife). But mussels have broken that process.
A single quagga mussel can filter about a quart of water a day. There are
quadrillions of quagga mussels in Lake Michigan (a quadrillion is 1 million billion.... 1,000,000,000,000,000 or 10^15th, a thousand times larger than 1 trillion ), which is an almost inconceivable number. They're more than 80% of the animal biomass in the lake, and some estimates peg it at 90% of animal biomass in the entire great lakes (Superior excepted, because its water is too calcium poor for the mussels to make their shells... by and large).
Estimates say that mussels can filter the entire volume of Lake Michigan in about 2 weeks, although obviously if there is thermal stratification they are not filtering the entire water column at one time. But they are crazy efficient and the number of them is pretty much hard to even conceptualize. And they do have access to the entire water column via mixing, at the times of year when there is no thermal stratification.
What the mussels do is filter the nutrients out of the water column (by eating plankton) and then concentrating those nutrients in their shells, bodies, and feces. The mussels basically are capturing phosphorus in the nearshore zone, and keeping it hostage on the bottom. This prevents the historical transport of phosphorous to the offshore zone, crippling important food web processes. They have essentially completely disconnected internal and external nutrient loading, which is why they have effected the ecosystem way more than reductions to external loading by the clean water act.
Below is probably the best paper detailing how the mussels are changing the nutrient cycle. I pulled out a few quotes that really illustrate what is going on
This is from Li et al. 2021,
www.pnas.org/doi/10.1073/pnas.2008223118
Note that all bolding is mine, for emphasis
Having outcompeted zebra mussels, quagga mussels now are abundant in most of the bottom areas in all of the Great Lakes except Lake Superior, often at densities exceeding 10,000 individuals per square meter (8–11). The expansion of quagga mussels coincided with unexplained changes in the abundances and distributions of other benthos (12) and modifications to the structure and phenology of the phytoplankton community (13) and food web structure (14, 15). Less attention was given to observations that pelagic concentrations of phosphorus (P), the productivity-limiting nutrient in the Great Lakes, decreased even while external P inputs remained steady (16, 17).
The tissues and shells of quagga mussels now contain nearly as much phosphorus as the entire water columns of the impacted Great Lakes (Figs. 1 and 2 and SI Appendix, Table S5).... In Lake Michigan, for example, dreissenid filter feeding outpaced passive sedimentation as a sink (BEN comment: sink means they are trapped, essentially, not available for circulation in the ecosystem) for P around the year 2000 and now exceeds it >10-fold (Fig. 1D). A growing population can rapidly deplete phosphorus from the water column. In Lake Michigan, sequestration into mussel tissues and shells accounted for 20 to 40% of the total benthic P sink since around 2010 The cycling of large amounts of P through quagga mussel populations transfers the control of P dynamics away from external inputs. Mussel growth (or mortality) can take up (or release) large quantities of P, offsetting or overshadowing the effects of external loading. For example, in Lake Michigan, the entire annual input of P from watershed (4 Gg P) may be absorbed by an increase in mussel biomass of less than 50% (Fig. 4). Unimpeded growth of the population, if realized, would continue depleting TP from the water column even if the external P inputs returned to their 1970s levels (SI Appendix, Fig. S1A). Rapid increases in mussel biomass are not unrealistic, as they have been observed in the Great Lakes as well as in other systems (59). It is not yet clear at what levels the dreissenid populations will stabilize in Lakes Michigan and Huron, nor whether they are likely to experience strong fluctuations, such as the recent ones in Lakes Ontario and Erie (10, 11).
Here is another paper expanding and discussing Li's paper linked above, and has a nice diagram in it
I'm getting kicked out for having too many links, so I'll just cite this one. Vanni, M.J. Invasive mussels regulate nutrient cycling in the largest freshwater ecosystem on Earth,
Proc. Natl. Acad. Sci. U.S.A. 118 ( 8 )
Again, all bolding is mine
Using ecosystem-level simulation models, calibrated with an abundance of lake-specific data, Li et al. (1) demonstrate the remarkable effect dreissenid mussels (hereafter, mussels) have on P cycling in the Great Lakes. These effects are striking and result from the high population densities achieved by these invaders. Mussels affect P cycling in two main ways: Their populations sequester massive amounts of P in their biomass, and they mediate a large flux of P between pelagic and benthic habitats. The biomass of zebra and quagga mussels now represents >90% of benthic animal biomass in four of the Great Lakes (all but Lake Superior), and while several native animals have declined since the invasion, dreissenid biomass greatly exceeds that of preinvasion native benthic animals. Perhaps even more impressive, within each lake these mussel populations contain in their bodies about as much P as the entire water column above them (1). This is truly remarkable when one considers that the mussels live in a thin zone about 10 to 20 cm thick on the lake bottom, whereas in both Lakes Huron and Michigan the mean lake water depth is >80 m.Dreissenids also greatly alter biogeochemical cycling rates, in particular fluxes of nutrients between the water column and the benthos. In Lake Michigan, feeding by mussels represents a flux of P from the water to the lake bottom that exceeds natural sedimentation of particles by ∼10-fold . A little more than one-third of the P filtered by mussels is shunted to the sediments as feces and pseudofeces, and an even larger fraction is returned to the water column via excretion. Much of the P in feces is mineralized by microbes and subsequently released into the water column in dissolved form. The release of P by mussels into the water exceeds the preinvasion sediment-to-water P flux by about 10-fold and also exceeds P input from the entire lake’s watershed by about eightfold. Thus, the fluxes of P into and out of mussel populations now dominate the P cycle of these lakes. Some of the P in mussel bodies is also returned to the water column after mussels die and decompose Overall, dreissenid populations represent a net benthic sink for P, that is, they mediate a net P flux from the water to the benthos, especially when their populations are expanding, that is, after invasion. As a consequence, the steady-state mass of P in the water column is much lower in Lakes Michigan, Huron, and Ontario, where mussel populations are still expanding, than it would be in the absence of these invaders. Furthermore, because mussels now dominate P cycling in these lakes, the water column P mass is relatively insensitive to P inputs from the lakes’ watersheds
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