Every number here is a modelled archetype. There is no such coast. The five sites are composites of what the literature says each kind of activity does, expressed as a departure from the reference embayment at the far end, which is the only honest way to compare them: coastal systems differ so much between places that an absolute number for "healthy" would be meaningless, while a departure from a nearby unimpacted site is how real monitoring programmes actually work.
Why the signatures differ in kind, not just in size. A salmon farm ADDS: dissolved nitrogen from fish excretion, particulate organic waste from faeces and uneaten feed, and copper from net antifouling. A port adds metals continuously and holds a legacy in its sediment, and it removes habitat outright through armouring and dredging. Agricultural runoff adds nutrients and sediment, and the sediment is what does the damage, because it takes the light that kelp and seagrass need. A shellfish farm is the odd one out: it is net EXTRACTIVE, filtering particles out of the water and exporting nitrogen when the crop is harvested. Treating all four as "pollution" hides the only fact that matters for deciding what to do.
The catch in every intervention on this page. Shellfish and macroalgae improve the water by moving contaminants into tissue. That is a real improvement to the water column and it is also a transfer, not a destruction. Where the tissue goes, and whether it is clean enough to sell, is the question the trophic column exists to answer, and it is why an intervention layer without a tissue-chemistry layer is only half a plan.
| Site | What it does to the system | Net direction |
|---|---|---|
| reference | The yardstick. Everything else is read against it | baseline |
| shellfish | Filters particles from the water column, exports nitrogen at harvest, and concentrates organics in the sediment directly beneath through biodeposition | extractive, with a local benthic cost |
| river mouth | Delivers dissolved nutrients and suspended sediment from the catchment. The sediment limits light, and light is what sets whether kelp and seagrass can hold | additive, and the light effect dominates |
| salmon farm | Dissolved nitrogen and phosphorus, particulate organic waste, and copper from net antifouling | additive across three channels at once |
| port | Continuous copper and zinc from hull coatings, a legacy metal burden in sediment, low flushing, and habitat removed by armouring and dredging | additive in metals, subtractive in habitat |
Sources for the mechanisms sit in projects/biofouling.evidence.md, which carries a verdict on every load-bearing claim in this project. The magnitudes on this page are illustrative and are not drawn from it.