desorption of physically adsorbed C,H6 are approximately 7.7 kcallmol and 1013 s-', respectively (7) , the activation energies, E,, and preexponential factors, kr(0), for disso- ciative chemisorption may be kvaluated eas- ilv (these values are also listed in Table 1). , \
From these results, the electronic and eeometric effects on C-H bond activation "
of these four surfaces of Pt and Ir can be discussed quantitatively. The E, values giv- en in Table 1 provide the magnitudes by which the Ir surfaces of a given geometry are more active than the corresponding Pt surfaces and by which the corrugated (1 10) surfaces of each metal are more reactive than their atomically flat (1 11) counter- parts. By comparing samples of the same crystallographic orientation, we find that the Er value on Ir is 5.0 to 6.3 kcal/mol lower than that on Pt. For each metal, the corrugated (1 10)- (1 x 2) surfaces have E, values that are 4.8 to 6.1 kcallmol lower than those of the close-packed (1 11) sur- faces. Consequently, the Pt(l l0)-(1 x 2) and Ir( l l1) surfaces have approximately equal activity toward C,H6 activation, demonstrating that the difference in geo- metric structure compensates for the intrin- sic electronic difference between the two metals. It is known from photoemission and x-ray absorption studies that changes in sur- face structure induce subtle, and yet signifi- cant, changes in the electronic structure of the catalyst surface (20). Hence, at the core of the structure sensitivity issue is the pro- found influence microscopic surface geome- try has on surface electronic structure, and the continued study of this important effect is of great interest in the heterogeneous activation of saturated hvdrocarbons.
REFERENCESANDNOTES 1 J. H. Sinfelt. Bimetallic Catalvsts. Discoveries.
Concepts, and Applications (wiley, New York, 1983), pp. 130-157.
2. A. E. Shilov Activation of Saturated Hydrocarbons b y Transition Metal Complexes (Re~del, Dor- drecht, Netherlands, 1984), R. H. Crabtree, Chem. Rev 85, 245 (1985), B. Meunier and B. Chaudret, Eds., Perspectives in the Selective Ac- tivation of C H and C-C Bonds in Saturated Hydrocarbons, vol CXXX of the NATO Advanced Study Institutes Seres (Redel, Dordrecht, Neth- erlands, 1987).
3. D. F McMillen and D. M. Golden, Annu. Rev Phys. Chem 33, 493 (1982)
4. J. C. Rasser Platinum-Iridium Reforming Cata- lysts (Delft Univ, Press, Delft, Netherlands, 1977) pp 156-193
5. M. A Van Hove, W H Weinberg, C.-M. Chan, Low-Energy Electron Diffract~on. Experiment, Theo- ry, and Suiface Structure Determination (Springer- Verlag, Heidelberg, Germany, 1986), pp. 265272.
6. T. S. Wlttrig, P. D. Szuromi W. H. Weinberg, J. Chem. Phys. 76, 3305 (1982); P. D Szuromi, J. R Engstrom, W H. Weinberg, ibid 80, 508 (1984).
7. P. D. Szuromi, J. R. Engstrom. W H. Weinberg, J. Phys Chem. 89, 2497 (1 985).
8. L. E Firment and G. A. Somorjai, J. Chem Phys. 66, 2901 (1 977).
9 J A. Rodrlquez and D W. Goodman, J. Phys Chem. 94, 5342 (1 990)
10. Y.-K Sun and W H Weinberg, J. Vac Sci. Technol. A 8, 2445 (1 990)
11. C. B. Mullins and W H. Weinberg, J Chem Phys. 92, 4508 (1 990)
12 J. J. Vajo, W. Tsai, W. H. Weinberg, Rev. Sci Instrum. 56, 1439 (1 985). The I r ( l l 1 ) crystal was cut, pollshed, and prepared with standard tech- nques. The crystal surface was cleaned between experiments by heating to 1000 K for 5 min at an 0, pressure of 5 x l o - @ torr flowing through the mcroreactor, followed by annealng to 1625 K for 1 mln to desorb the surface oxygen. Thermal desorption spectra of CO, which were n complete agreement with previously published ones from the clean I r ( l l 1 ) surface [C M. Comrie and W H. Weinberg, J. Chem Phys 64, 250 (1976)], were measured frequently to verlfy the cleanliness of the surface. Furthermore, because klnetic data are known to be extremely sensitive to surface conditions, the h g h degree of reproducbllity of the data presented here Indicates the absence of surface contaminatlor- The 13C-labeled ethane [I,2-di-'3C]C2H,, 99 atom % 13C) was obtaned from Icon S e ~ c e s and was introduced lnto the m~croreactor froni a gas-handling manifold that was pumped by a dlffuslon pump to a base pressure below lo- ' torr
13 Under these expermental condtlons the dlsso- ciative chemsorption of C,H, I S irreversble, and no gas-phase carbon-contaning products of a surface self-hydrogenolysis reaction are formed. The ttration condtions and procedure were se- lected to ensure the complete ox~dat~on of the surface carbon to 13C02.
14. The measurements of C2H, activation at defect sltes on our I r ( l l1) sample have been done separately and will be reported elsewhere (D. F. Johnson and W. H. Weinberg, In preparation)
15. C T. Rettner, H E. Pfnur, D J. Auerbach, Phys Rev Lett 54, 271 6 (1 985); A C Luntz and D. S
Bethune, J Chem Phys. 90, 1274 (1989); M B Lee, Q . Y Yang, S. T. Ceyer, ibid. 87, 2724 (1 987).
16 W H. Weinberg, in Dynamics of Gas-Surface Collisions, C. T Rettner and M N. R. Ashfold, Eds. (Royal Society of Chemistry, Cambridge, 1991), pp. 171-220; G. Ehrlich, In Chemist~yandPhysics of Solid Surfaces Vll, R Vanselow and R F. Howe, Eds (Springer-Verlag, Heldelberg, Germany, 1989), pp 1-64.
17. W H Weinberg, in Kinetics of Interface Reac- tions, H. J Kreuzer and M Grunze, Eds. (Spring- er-Verlag, Heldelberg, Germany, 1987), pp. 94- 121, W. H. Welnberg, Langmuir9, 655 (1993)
18. C T. Campbell, Y.-K. Sun, W. H. Welnberg, Chem Phys. Lett. 179, 53 (1991).
19 C. T Rettner, E. K. Schweizer, H Stein, D. J. Auerbach, Phys. Rev. Lett. 61, 986 (1988).
20 M. Cardona and L Ley, Eds., Photoemission in Solids I and 11, vols. XXVl and XXVll of Topics in Applied Physics (Springer-Verlag, Heldelberg, Germany, 1978); J. W. Gadzuk, in Electronic Structure and Reactivity of Metal Surfaces, vol. XVI of the NATO Advanced Study Institutes, Se- ries 0, E. G. Derouane and A. A. Lucas, Eds. (Plenum, New York, 1976), pp. 341-387; J. C. Fuggle and J E Inglesfield, Eds., Unoccupied Electronic States Fundamentals for XANES, EELS, IPS, and BIS, vol LXlX of Topics in Applied Physics (Springer-Verlag, Heidelberg, Germany, 1992); J H Sinfelt and G D Meitzner, Acc Chem. Res. 26, 1 (1 993)
21 Th~s work was supported by the Department of Energy (grant DE-FG03-89ER14048), the Donors of the Petroleum Research Fund adminstered by the Amercan Chemical Socety (grant ACS-PRF- 23801-AC5-C) and the Unlversltywide Energy Research Group of the University of California.
1 March 1993, accepted 27 Apr l 1993
Ecological Roulette: The Global Transport of Nonindigenous Marine Organisms James T. Carlton and Jonathan B. Geller
Ocean-going ships carry, as ballast, seawater that is taken on in port and released at subsequent ports of call. Plankton samples from Japanese ballast water released in Oregon contained 367 taxa. Most taxa with a planktonic phase in their life cycle were found in ballast water, as were all major marine habitat and trophic groups. Transport of entire coastal planktonic assemblages across oceanic barriers to similar habitats renders bays, estuaries, and inland waters among the most threatened ecosystems in the world. Pres- ence of taxonomically difficult or inconspicuous taxa in these samples suggests that ballast water invasions are already pervasive.
Biological invasions are a great threat to the integrity of natural communities of plants and animals and to the preservation of endangered species (1). Most invasion studies have focused on terrestrial and freshwater systems in which one or a few successful invaders have had a catastrophic impact on native species (2). Island ecosys- tems. such as New Zealand and the Hawai-
J. T Carlton, Maritime Studles Program, Willlams College, Mystc Seaport, Mystic, CT 06355, and De- partment of B~ology, Williams College, Williamstown, MA 01267. J. B. Geller, Department of Biologcal Sciences, Unl- versity of North Carolina at Wilmington, Wilmington, NC 28403.
ian Islands, have in particular been devas- tated by the invasion of nonindigenous species (1-3). Invasions in marine systems have been less studied (4), but are of such magnitude that marine invasions may be leading to profound ecological changes in the ocean.
Any mechanism for rapidly transport- ing large volumes of water containing plankton from shallow, coastal waters across natural oceanic barriers has the potential to facilitate massive invasions of entire assemblages of neritic marine orga- nisms. Such a mechanism exists in the transport of ballast water and plankton by ocean-going ships (5), a dispersal mecha-
SCIENCE VOL. 261 2 JULY 1993
We sampled ballast water from 159 cargo ships in Coos Bay, Oregon. The ships and their ballast water originated from 25 Japanese ports (9). Plankton from these vessels included 16 animal and 3 protist phyla, and 3 plant divisions (Table I) . All major and most minor phyla were represented (1 0) , including 47 ordinal or
higher taxa and a minimum of 367 dis- tinctly identifiable taxa (11). The su- praspecific diversity demonstrates the wide taxonomic spectrum represented and em- phasizes the broad implications of this phenomenon (12).
All major marine trophic groups were represented (Table 1) including carnivores,
herbivores, omnivores, deposit feeders, scavengers, suspension feeders, primary producers, and parasites, although the last were rare. Taxa characteristic of most tem- perate shallow-water marine communities were represented, including those from in- faunal, soft and hard bottom epifaunal, epibiotic, and planktonic habitats. The bal-
Table 2. Examples of recent invasions probably mediated by ballast water
Year introduction Introduced to first recognized
(reference) Higher taxon Taxon Species
Dinoflagellata Alexandrium catenella Alexandrium minutum Gymnodinium catenatum Phyllorhiza punctata (*,t) Cladonema uchidai ( t) Mnemiopsis leidyi Teneridrilus mastix (*) Desdemona ornata (*)
Japan Europe? Japan Indo-Pacific Japan, Chlna Western Atlantic China South Africa,
Australia U.S. Atlantic Europe Indian Ocean
Australia Australia Australia California California Black Sea California Italy
Cnidaria Scyphozoa Hydrozoa
Ctenophora Annelida Oligochaeta
Marenzelleria viridis Bythotrephes cederstroemi Rhopalophthalmus
tattersallae (*) Neomysis japonica Neomysis americana (*) Nippoleucon hinumensis
Germany Great Lakes Kuwait
Crustacea Cladocera Mysidacea
Japan U.S. Atlantic Japan
Australia Argentina, Uruguay California Oregon Columbia River California California California California Chile California Chile Texas Chile New Jersey
Copepoda Pseudodiaptomus inopinus Pseudodiaptomus marinus Pseudodiaptomus forbesi Sinocalanus doerrii Oithona davisae
Asia Japan China China Asia
Limnoithona sinensis Centropages abdominalis Centropages typicus Acarfia omorii Hemigrapsus sanguineus
China Japan U.S. Atlantic Japan Asia Decapoda,
Brachyura Charybdis helleri Indo-Pacific, Israel
~, (Caribbean) Decapoda: Salmoneus gracilipes (*) Japan, Micronesia California 1986 (45)
Caridea Hippolyte zostericola (*) Exopalaemon styliferus (*)
Gastropoda Tritonia plebeia Bivalvia Potamocorbula amurensis
Dreissena poiymorpha Dreissena sp. Rangia cuneata (*) Theora fragilis Musculista senhousia ( t )
Western Atlantic Indonesia, India Europe Asia Eurasia Eurasia Southern U.S. Asia Japan
Colombia (Atlantic) Iraq, Kuwait Massachusetts California Great Lakes Great Lakes New York California New Zealand Australia Germany New Hampshire,
Maine Great Lakes Great Lakes Great Lakes Nigerla, Cameroon Panama Canal Arabian Gulf Hawaii
Ensis americanus Membranipora
membranacea (*) Gymnocephalus cernuus Proterorhinus marmoratus Neogobious melanostomus Butis koilomatodon
U.S. Atlantic Europe Ectoprocta
Pisces Europe Black Sea Mediterranean Indo-west Pacific
Rhinogobius brunneus Mugiligobius sp.
Philippines Arabian Sea Philippines,
Sparidentex hasta Parablennius thysanius
*Suggested herein as a ballast-mediated invason. tAn alternative means of dispersal includes transport as external foulng on sh~ps' hulls. Here we suggest that transport as ephyrae (for Scyphozoa) and hydromedusae (for Hydrozoa) are as probable as transport as fouling polyps
80 SCIENCE VOL. 261 2 JULY 1993