Acute waterborne and chronic sediment toxicity of prometryn to three marine benthic species using microcosms.

Prometryn, a triazine herbicide detected in near-shore waters, sorbs strongly to and persists in sediments, yet its toxicity to marine benthic organisms remains poorly characterised. We assessed acute (96-h, waterborne) and chronic (28-d, spiked-sediment microcosm) toxicity to the Manila clam (Ruditapes philippinarum), the nereid (Perinereis aibuhitensis) and the ghost shrimp (Callianassa japonica) using procedures adapted from OECD and U.S. EPA guidance. Experimental toxicity endpoints are based on nominal concentrations because exposure concentrations were not verified throughout the tests. Effects may therefore occur at actual concentrations below the nominal effect concentrations reported here. The 96-h LC50 values were 25.3, 11.8 and 2.40 mg/L and the 28-d LC50 values were 8.67, 3.51 and 1.42 mg/kg dry weight, respectively. Growth responses occurred together with substantial mortality at the lowest sediment concentrations tested for the nereid and ghost shrimp, so a consistent ordering of growth and survival sensitivity could not be established across species. An exploratory acute-data species sensitivity distribution for 21 marine species yielded an HC5 of 2.06 µg/L, with primary producers more sensitive than the animals tested. Screening quotients reached 0.31 for marine or coastal water and 0.064 for marine sediment, whereas fresh or estuarine sediment reached 1.2 and a freshwater agricultural drainage canal 2.1. On the marine data alone, therefore, no quotient approaches 1; but the only marine sediment survey available covers nine stations in a single bay, and the sediment benchmark itself rests on an extrapolated ghost-shrimp growth EC10. In both settings, the margin between the most sensitive laboratory threshold and the highest reported concentration was several-fold narrower in sediment than in the water column: about 1000-fold versus about 3800-fold for marine observations, and about 50-fold versus about 550-fold once river-estuary and freshwater observations are included. Sediment therefore requires explicit, compartment-specific treatment in the risk assessment of prometryn.