Fate and transport of Irgarol 1051 in a modular estuarine mesocosm

Fate and transport of Irgarol 1051 were assessed using a modular estuarine mesocosm containing natural seawater, saltmarsh sediments, marsh grass, shrimp, clams, snails, and naturally derived planktonic and benthic microorganisms. The mesocosms were enclosed in a greenhouse under near ambient conditions, and included a saltwater sump and a simulated tidal flux. The exposure was comprised of four replicate treatments of 0, 100, 1,000 and 10,000 ng/L Irgarol. Solid-phase extraction (SPE) was used to extract Irgarol and its major metabolite M1 (aka GS26575) from water samples. Sediment samples were extracted using Accelerated Solvent Extraction (ASE) with 100% dichloromethane. Irgarol and M1 were separated and quantified utilizing Liquid Chromatography Tandem Mass Spectrometry (LC-MS-MS) with ElectroSpray Ionization in Multiple Reaction Monitoring mode. Aqueous concentrations of Irgarol declined rapidly (average 93% loss) over the course of the 35 day experiment while Irgarol was accumulated in the sediments (average mass balance of 75 ± 5%). Loss of aqueous Irgarol occurred in two distinct phases, a relatively rapid phase up to 96 hours post-dose, and a slower phase following 96 hours. The rate constants of the initial rapid degradation phase (k₁) for treatments 100 and 1,000 ng/L Irgarol were 2–3 times higher than the rate constants of the subsequent slow degradation phase (k₂) for these treatments. The average half-life of Irgarol in water was 7 ± 3 days. The aqueous concentration of Irgarol's metabolite, M1 increased over the course of the experiment. By 35 days post-dose, M1 concentrations in water averaged about 3% of the parent compound. M1 also accumulated in mesocosm sediments (average 16.7 ± 2.5% of total Irgarol dose). Mass balance calculations showed that by 35 days, Irgarol and M1 amounts in mesocosm water and sediments were close to 100% of the initial Irgarol dose. Average partition coefficient normalized for organic carbon (log K_oc) calculated for Irgarol was 3.2 ± 0.1 for 35 days post-dose. Accumulation of Irgarol and M1 in mesocosm sediments may warrant further study to assess toxicity for benthic communities.