Weighting microplastics by polymer type reorders which Brisbane creek ranks worst

PERI, the index that carries the ranking, is toxicity-weighted, which means hazard scores for each polymer enter the calculation before any two sites can be compared ^[5]. The finding that composition matters more than abundance is therefore partly a statement about the instrument. Weight PVC heavily and a site whose particles are mostly PVC will outrank a site holding more particles of something inert, whatever is happening to the animals in either creek.

The part that is not arithmetic is the disagreement between the two orderings. Kedron Brook holds the heaviest particle load; Bulimba Creek holds the highest hazard and the highest toxicity-weighted risk ^[7]. Two defensible indices, two different worst creeks, and the shortlist a manager works from is settled by the choice of index rather than by the sampling ^[15].

The denominator is modest. Six sites on each of three creeks, resampled four times across a year ^[8], is 18 locations and 72 sediment collections ^[9]. Sites were placed to span the industrial, commercial and residential land uses each creek flows through, and Heshani Mudalige reports that land use shapes where the risk lands ^[14]. That is a described pattern across 18 points in one city, not a traced path from a particular land use to a particular polymer.

The writeup carries no PLI, PHI or PERI values for any site ^[17], so the size of the gap between Bulimba and Kedron is not readable from it. A rank order can hide a hair's difference or a wide one.

No organisms were sampled either ^[18]. The mechanisms quoted are general toxicology attached to the hazard weights: PVC-borne chemicals reaching fish growth, reproduction and survival as particles move up the food chain ^[10], PMMA causing physical stress and reduced feeding efficiency in whatever eats it ^[11]. The framework infers ecological risk from what the plastic is made of, without checking that inference against the invertebrates actually living in the sediment.

The human-health passage travels furthest from the mud. Mudalige says microplastics enter the body mainly through seafood and are linked to potential harm to the circulatory and immune systems and to neurodegenerative disorders ^[13]. A sediment core says nothing about dose in a person, or in a fish caught at Bulimba Creek, and the zoning result does not lean on that passage.

One inconsistency sits inside the quoted mechanisms. PVC appears twice, the second time as the polymer easily ingested in fibre form and able to ferry other pollutants on its surface ^[12], which is the property one would expect of polyester, the third of the three high-hazard polymers named ^[4]. The paper itself, in Marine Pollution Bulletin, is where that ambiguity resolves ^[16].

For a regulator, the authority of a zoning map like this rests on one untested premise: that the polymer hazard scores order real harm the way they order sediment. Bulimba Creek is top of the list because of how its plastic scores, and the ranking is worth exactly what the scores are worth.