research watch: where does all the mercury go?

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Autumn leaves release VOCs Autumn frosts in deciduous forests trigger significant releases of volatile organic compounds (VOCs) from leaves and may contribute to ground- level ozone, according to new findings from the Austrian Alps. The concentra- tion of natural VOCs after a hard frost was 2-1/2 to 10 times more than the concentration of VOCs from burning fossil fuels over one week in mid- November 1999. After the first hard frost in Novem- ber 1999, scientists recorded daily peaks of plant VOCs and found that they correlated with warm daytime and freezing nighttime temperatures. The same pattern of VOC release was observed in the lab when scien- tists froze and thawed leaves of clover, grass, fir, and larch from the forests and fields surrounding Sonnblick. The researchers propose that frost destroys plant cells, which release wound com- pounds, such as hexanals, pentenol, and methylbutanals, in large amounts when they thaw (Geophys. Res. Lett. 2001, 28 (3), 507–510). Modeling biodiversity Given fears that biodiversity may be declining precipitously worldwide, ecologists have been scrambling to de- termine how numbers of various species are changing. One of ecology’s more fundamental measures of biodi- versity is the mathematical relation- ship connecting “species richness” and the test area in which a particular species has been laboriously counted. However, M. Crawley and J. Harral of Imperial College in the United Kingdom find that the relationship is not as simple as has been believed. In this study, the British researchers studied the flora in Berkshire County in southeastern England. All vascular plants, including native and alien, were counted, except those clearly planted. Replicated and randomized plant data were collected in 11 differ- ent spatial scales, ranging from 0.01 to 10 8 m 2 . These zone data were fitted to the model, in which species richness S is given by S = cA z , where c is the con- centration of species in an area A. The exponent z is typically believed to be ~0.25. However, the researchers found a greater variation in the expo- nent. At small scales (<100 m 2 ), z was 0.1–0.2 and strongly habitat-depen- dent; at intermediate scales (1 hectare to 10 km 2 ), z jumped to 0.4–0.5; and at the largest scales, z re- turned to the 0.1–0.2 range. These variations suggest that different pro- cesses determine plant diversity at various spatial scales (Science 2001, 291, 864–868). Where does all the mercury go? Total mercury concentrations in tribu- taries of the Chesapeake Bay tend to be higher under high-flow conditions, whereas methylmercury, the form that accumulates in fish, is less dependent on flow rate, according to a new study from the University of Maryland. The finding suggests that resuspension of bed sediments or erosion of river- banks during high-flow conditions is contributing inorganic mercury but little methylmercury. Analyzing flow conditions, sus- pended particulate concentrations, and watershed characteristics of six representative Chesapeake Bay tribu- taries varying both in size and the ex- tent of urbanized areas surrounding them, Nicole Lawson and colleagues find that the two most urbanized rivers—the Potomac, which flows through the Washington, DC, area and Herring Run, which flows through Baltimore, MD—showed the strongest correlation between flow rates and total mercury concentrations. Concentrations of other metals with substantial anthropogenic sources—such as lead, nickel, and zinc—also seemed to depend on river flow, whereas cadmium, copper, and chromium concentrations increased only slightly with flow rate. For most of the rivers, mercury was the metal most strongly retained within the wa- tershed. Overall, the study results con- firm the importance of including high-flow events in any sampling pro- tocol to determine the flux and water- shed yields of metals in rivers (Water Res. 2001, 35 (2), 501–515). Undigested particles in the ocean Nearly all organic matter formed at the surface of the ocean is biodegrad- ed as it sinks toward the ocean floor. A small fraction of sinking particles, however, resists degradation and gets buried in the sediment, locking carbon away from the atmosphere. New evi- dence reported by John Hedges of the University of Washington and col- leagues could explain why some parti- cles remain untouched by hungry microbes, while others are ultimately recycled into CO 2 . Using solid-state 13 C nuclear mag- netic resonance spectroscopy, the re- searchers characterized particulate organic matter at ocean depths lower than possible using conventional ana- lytical techniques. Bulk samples of plankton (0–100 m depth) and organic particles in the upper (500–1000 m) and lower (3000–3500 m) water col- umns of the Pacific Ocean and the Arabian Sea were found to be sur- prisingly similar in composition. The researchers suggest that some particu- late organic matter is shielded by sink- ing inorganic particles, like opal, calcium carbonate, and aluminosili- cates, which made up 80% of the mass of all of the particles sampled (Nature 2001, 409, 801–804). Research Watch 146 A ENVIRONMENTAL SCIENCE &TECHNOLOGY / APRIL 1, 2001

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Autumn leaves release VOCsAutumn frosts in deciduous foreststrigger significant releases of volatileorganic compounds (VOCs) fromleaves and may contribute to ground-

level ozone, according to new findingsfrom the Austrian Alps. The concentra-tion of natural VOCs after a hard frostwas 2-1/2 to 10 times more than theconcentration of VOCs from burningfossil fuels over one week in mid-November 1999.

After the first hard frost in Novem-ber 1999, scientists recorded dailypeaks of plant VOCs and found thatthey correlated with warm daytimeand freezing nighttime temperatures.

The same pattern of VOC releasewas observed in the lab when scien-tists froze and thawed leaves of clover,grass, fir, and larch from the forestsand fields surrounding Sonnblick. Theresearchers propose that frost destroysplant cells, which release wound com-pounds, such as hexanals, pentenol,and methylbutanals, in large amountswhen they thaw (Geophys. Res. Lett.2001, 28 (3), 507–510).

Modeling biodiversityGiven fears that biodiversity may bedeclining precipitously worldwide,ecologists have been scrambling to de-termine how numbers of variousspecies are changing. One of ecology’smore fundamental measures of biodi-versity is the mathematical relation-ship connecting “species richness” andthe test area in which a particularspecies has been laboriously counted.However, M. Crawley and J. Harral ofImperial College in the United

Kingdom find that the relationship isnot as simple as has been believed.

In this study, the British researchersstudied the flora in Berkshire Countyin southeastern England. All vascularplants, including native and alien,were counted, except those clearlyplanted. Replicated and randomizedplant data were collected in 11 differ-ent spatial scales, ranging from 0.01 to108 m2.

These zone data were fitted to themodel, in which species richness S isgiven by S = cAz, where c is the con-centration of species in an area A.The exponent z is typically believedto be ~0.25. However, the researchersfound a greater variation in the expo-nent. At small scales (<100 m2), z was0.1–0.2 and strongly habitat-depen-dent; at intermediate scales (1hectare to 10 km2), z jumped to0.4–0.5; and at the largest scales, z re-turned to the 0.1–0.2 range. Thesevariations suggest that different pro-cesses determine plant diversity atvarious spatial scales (Science 2001,291, 864–868).

Where does all the mercury go?Total mercury concentrations in tribu-taries of the Chesapeake Bay tend tobe higher under high-flow conditions,whereas methylmercury, the form thataccumulates in fish, is less dependenton flow rate, according to a new studyfrom the University of Maryland. Thefinding suggests that resuspension ofbed sediments or erosion of river-banks during high-flow conditions iscontributing inorganic mercury butlittle methylmercury.

Analyzing flow conditions, sus-pended particulate concentrations,and watershed characteristics of sixrepresentative Chesapeake Bay tribu-taries varying both in size and the ex-tent of urbanized areas surroundingthem, Nicole Lawson and colleaguesfind that the two most urbanizedrivers—the Potomac, which flowsthrough the Washington, DC, area andHerring Run, which flows through

Baltimore, MD—showed the strongestcorrelation between flow rates andtotal mercury concentrations.

Concentrations of other metalswith substantial anthropogenicsources—such as lead, nickel, andzinc—also seemed to depend on riverflow, whereas cadmium, copper, andchromium concentrations increasedonly slightly with flow rate. For most ofthe rivers, mercury was the metalmost strongly retained within the wa-tershed. Overall, the study results con-firm the importance of includinghigh-flow events in any sampling pro-tocol to determine the flux and water-shed yields of metals in rivers (WaterRes. 2001, 35 (2), 501–515).

Undigested particles inthe oceanNearly all organic matter formed atthe surface of the ocean is biodegrad-ed as it sinks toward the ocean floor. Asmall fraction of sinking particles,however, resists degradation and getsburied in the sediment, locking carbonaway from the atmosphere. New evi-dence reported by John Hedges of theUniversity of Washington and col-leagues could explain why some parti-cles remain untouched by hungrymicrobes, while others are ultimatelyrecycled into CO2.

Using solid-state 13C nuclear mag-netic resonance spectroscopy, the re-searchers characterized particulateorganic matter at ocean depths lowerthan possible using conventional ana-lytical techniques. Bulk samples ofplankton (0–100 m depth) and organicparticles in the upper (∼500–1000 m)and lower (3000–3500 m) water col-umns of the Pacific Ocean and theArabian Sea were found to be sur-prisingly similar in composition. Theresearchers suggest that some particu-late organic matter is shielded by sink-ing inorganic particles, like opal,calcium carbonate, and aluminosili-cates, which made up 80% of the massof all of the particles sampled (Nature2001, 409, 801–804).

Research�Watch

146 A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / APRIL 1, 2001

APRIL 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 147 A