Relative contribution of food and water to the Cd burden in Balanus amphitrite in an urban tidal creek discharging into the Great Barrier Reef lagoon

The barnacle biomonitor Balanus amphitrite, its food sources (two class sizes of suspended particulate material, SPM, and microzooplankton) and water (dissolved phase) were sampled weekly for Cd concentrations along a tidal creek discharging into the Great Barrier Reef lagoon (Ross Creek, Townsville, Queensland). The Cd concentration in the dissolved phase increased upstream, ranging from 1.7 to 283 ng L 1. The Cd concentration in the barnacle’s food sources exhibited the same patterndranging, for the small size class of SPM (0.45e50 mm), from !0.01 to 1.15 mg kg 1, for large SPM (50e200 mm) from 0.07 to 1.62 mg kg 1, and for microzooplankton (50e200 mm) from 0.03 to 0.80 mg kg 1. More than 95% of the total Cd in the Ross Creek water (!200 mm) was in the dissolved phase (!0.45 mm). A simple diffusion coefficient model estimated an export during the dry season from Ross Creek into the GBR lagoon of around 6 g Cd d 1. The Cd concentration in two populations of B. amphitrite increased upstream between two sites 2.2 km apart and ranged from 2.15 to 4.69 mg kg 1 and from 5.47 to 12.81 mg kg 1. In addition, their Cd concentrations varied over the three sampling months, exhibiting specific patterns for this variation, which suggests that changing the food sources’ Cd concentration and relative abundance may result in a specific Cd concentration in B. amphitrite. Therefore, for monitoring programs, the interpretation of variations in metal concentration in a biomonitor would be improved by looking at its metal sources and their temporal variability.