Because “particles per km² of surface” is a sneaky metric, and SF Bay is basically a stormwater funnel with tides, while the Great Lakes are an inland ocean with a huge dilution tank and a lot of places for particles to disappear into.
1) SF Bay gets hit with massive, fast urban inputs, especially stormwater
The SF Bay regional work estimated ~7 trillion microplastics/year entering via stormwater, about ~300× the wastewater pathway.
That matters because stormwater is a “now” input: rain washes whatever’s on roads and hard surfaces straight into drains, creeks, and then the Bay, often as pulses that can spike surface counts.Also, Sutton et al. (2016) explicitly point to stormwater runoff as an additional pathway, consistent with the high fragment loads in surface water. (San Francisco Estuary Institute)
2) Scale and dilution: the Great Lakes are gigantic, so averages look smaller
Eriksen et al. (2013) measured Great Lakes surface microplastics (333 µm manta trawl) and found an average ~43,000 particles/km², with a hotspot >466,000 particles/km² downstream of major cities. (PubMed)
Sutton et al. (2016) report SF Bay surface water averaging ~700,000 particles/km². (San Francisco Estuary Institute)Those two “averages” are not sampling the same kind of system:
- The Great Lakes include vast open-water areas that can be relatively lower than nearshore plumes, so a basin-wide mean gets pulled down.
- SF Bay is a smaller, heavily urbanized estuary where a much larger fraction of the water surface is “near a source” most of the time.
Also, the Great Lakes system contains the majority of North America’s available freshwater (it’s just… a lot of water).
A big bowl needs a lot more confetti before the surface looks “packed.”3) A lot of Great Lakes microplastics don’t stay floating at the surface
Even buoyant plastics don’t stay buoyant forever: biofouling, aggregation, and mixing push material downward. The Great Lakes monitoring/risk report notes that in aquatic systems, the majority of particles are transported to water bodies and then settle into benthic sediments.
So the Lakes can be “enclosed and accumulating” while still showing lower surface-sheet density than an urban estuary that’s constantly being re-seeded at the surface.4) Hydrodynamics: SF Bay tends to concentrate and resuspend
Estuaries like SF Bay are shallow, tidally energetic, and full of depositional zones. That combination can:
- Keep particles in play (resuspension),
- Move them back and forth (tidal pumping),
- Concentrate them in embayments and along shorelines.
The Great Lakes have convergence zones too (Eriksen’s hotspot is basically that story), but they also have enormous open-water mixing and long shorelines that spread sources out. (PubMed)
5) “Heterogeneous and enclosed” is exactly why Great Lakes have hotspots, not necessarily a higher average
Your instinct is right: they’re heterogeneous, and enclosure favors retention. Eriksen’s data show intense local maxima near urban influence. (PubMed)
But if you average a few nasty plumes into a whole inland sea, the number can look smaller than an estuary where “plume” is basically the default condition.6) Bonus annoyance: cross-study comparisons are fragile
Same unit (particles/km²) helps, but details still matter: tow conditions, season, wind-driven mixing, how fibers were counted/verified, and what got excluded as “not plastic” (Eriksen famously found a chunk of visually-identified particles were actually coal ash). (PubMed)
So treat “SF > Great Lakes” as plausible and supported in these datasets, not as a universal law of nature.Net: SF Bay can look higher than the Great Lakes in surface counts because it’s smaller, more source-proximate, stormwater-dominated, and tidally resuspensive, while the Great Lakes are huge, dilute, and bleed a lot of particles out of the surface layer into sediments.
