EVALUATING MOSS AS BIOMONITORS OF HEAVY METAL DEPOSITION USING HIERARCHICAL SAMPLING AND RAIN LEACHING EXPERIMENTS

Heavy metal pollution is of increasing concern across the globe. Terrestrial epiphytic moss can be used as a biomonitor to estimate atmospheric heavy metal concentrations. However, much remains unknown about the relationship between metal content in moss tissues and atmospheric concentration. Using metal concentrations in moss to map the spatial distribution of metal deposition depends on a thorough understanding of the spatial and temporal variability within a moss sample and among samples. Physiology, precipitation events and rates, and other environmental conditions contribute to metal accumulation rates and spatial variability of metal concentrations in moss plants in the same vicinity. Quantifying and understanding spatial variability is vital to the use of moss as biomonitors. A hierarchical sampling method is used to model spatial variation of heavy metal content in a terrestrial epiphytic moss, Orthotrichum lyellii. Four samples are collected from each of four trees at several sites in Oregon representing an unpolluted site, urban sites, and a polluted parkland site downwind of Portland. The moss is acid digested and analyzed by ICP-MS for Pb, Cd, and Zn concentrations. These results enable us to quantify the variation in metal concentrations at spatial scales ranging from one meter to one hundred kilometers.

A rain leaching experiment is also used to assess how precipitation affects loss of Pb in the three cellular compartments of O. lyellii tissues. Moss tissue consists of the inter-, extra-, and intracellular compartments, each with different rates of uptake and loss of trace elements. Moss samples are subjected to an artificial rain solution of 0.01 M CaCl₂ for 15, 30, 60, and 120 minutes using a gravity-fed rain device. A sequential elution procedure is used to quantify the metal concentrations in each cellular compartment. The pre-rain and post-rain moss tissue concentrations are compared to determine how metal concentrations in moss change as a result of rain events of different duration.