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The hypothesis that planktonic larvae of benthic invertebrates sink through the water like passive particles in turbulent flows near the seabed was tested in the field using several groups of geometrically different sediment trap designs. A priori predictions regarding the rank order that the various traps would-coiiect larvae in the field were dictated from laboratory flume experiments to determine the relative particle collection efficiencies of traps. The flume flow was seeded with particles having fall velocities similar to those measured, in the laboratory, for nonswimming polychaete larvae. The flume flow speed (of 10 cm!sec) was within the range of near-bottom current velocities measured during trap collecting intervals at the study site. In seven field experiments, each lasting from one to eleven days, trap collections of Mediomastus ambiseta (a polychaete worm) postlarvae, total bivalve larvae and postlarvae, sabellariid polychaete larvae, and enteropneust postl a rvae generally fi t the patterns predi cted for pass i ve particle collections between or among the trap designs. While the results were statistically more significant during some intervals than during others, the rank order of larval collections within each group of trap desi gns tested nearly always corresponded preci sely to the rank order of passive particle collections by the traps in the flume experiments. Thus, the hypothesis that larvae sinking toward the seabed in the field and passive particles (with fall velocities similar to larvae) sinking in a fl ume are coll ected in the same rank order of abundance by near-bottom traps could not be falsified for collections of organisms from three invertebrate phyl a. Collections of the polychaete, Pectinaria gouldii, and of metamorphosing seastar larvae between or among trap designs significantly di ffered from the patterns predi cted for passi ve parti cl e coll ecti ons. A testable hypothesis to explain the Pectinaria collections involves unique hydrodynami c properti es of these postl arvae, rel ati ve to the other organisms collected, and is consistent with the passive sinking 3 ABSTRACT (cont.) hypothesis. Trap collections of the seastars may have resulted, at least in part, from larvae adhering to solid trap surfaces during metamorphosis. The passive sinking hypothesis could not be falsified in most of the field experiments conducted in this study. Thus, hydrodynamical processes must be i ncl uded in any future studies of processes that determine patterns of larval settlement. However, passive sinking by larvae is not the explicit result of this experimental study. Other processes that could have produced the observed patterns of larval collections among the trap designs now must be tested against the passive sinking alternative hypothesis. However, much more information on the biology and ecology of the larvae collected in this study is required before future processori ented experiments can be desi gned. If larvae sink like passive particles to heights of 50-cm above the seabed, as the results of this study suggest, then it is possible that larvae initially reach the seafloor at sites where particulates, with fall velocities similar to larvae, initially settle. This hypothesis requires experimental testing. Larvae may not remain at these initial settlement sites; however, after larvae initially reach the seafloor via passive physical processes, the larvae may redistribute by actively choosing a preferred microenvironment within that location, by actively swimming above the bottom or remai ni ng on the sediment surface to be resuspended and transported away, by resuspensi on only duri ng storm events, and/or by passively accumulating around microtopographic structures. As a precursor to the flume tests of traps, a theoretical analysis of the physical nature of trap biases was conducted. A dimensional analysis of the independent variables involved in the process of trapping particulates suggested that trap collection efficiencies should be a function primarily of trap Reynolds number, trap aspect ratio, the ratio of the fluid velocity to the particle fall velocity, and trap geometry. A review of data from previous studies that flume-tested various trap designs further suggested that particle collection efficiencies of cylindrical traps should decrease over some range of increasing trap Reynolds number, decrease over some range of decreasing particle fall velocity and increase over some range of increasing trap aspect ratio. Theoreti cal model s were then provi ded to account for these effects. Fl ume tests, in the present study, of cylinders varying by one order of magni tude in trap Reynol ds number supported one of the predi cti ons: particle collection efficiencies of the cylinders decreased by a factor of two over this range of increasing trap Reynolds number. Results of this theoretical and experimental study of trap collection characteristics suggest that more fl ume experiments to quantitatively determine the nature of trap biases are required before flux estimates, using traps in the field, can be adequately interpreted. Thesi s Supervi sor: Ti tl e: J. Frederick Grassl e Seni or Sci enti st Woods Hole Oceanographic Institution ~_4 ACKNOWLEDG EMENTS By spanning two disciplines, this research also spanned a wide range of peopl e and I here thank all my fri ends, coll eagues, and new fami ly who touched the perimeter or dove deep into the center of thi s research. Without a doubt, my dissertation research would not have been the same without the collaboration and friendship of five persons, whose contributions I will describe in the schmaltzy account that follows. Brad Butman assisted in many aspects of the flume experiments and he deployed a bottom tripod system at my field study site, providing physi cal measurements critical to thi s work. I thank Brad for hi s unending curiousity, his uncanny probing questions, his stamina for discussions and the warmth and sincerity of his smile. Char Fuller provided technical assistance for the very thankless aspects of the flume experiments and was my chief dive buddy and righthand-woman duri ng the fi el d experiments ~ In fact, Char cheerfully volunteered to do things most people would not have done if you paid them~ I thank Char for her thoroughness, her fri endship, and her maturity. Bill Grant motivated and executed many aspects of the theoretical analysis of sediment trap biases. In addition, Bill saved my life a mill ion times with his enthusiasm, his support, his great ideas and his gentleness during all aspects of this research. Fred Grassle, my major advisor, tried to teach me the classics and I regret havi ng often been such a dense and uncooperative student. Fred IS breadth of knowledge as an ecologist and a biological oceanographer was always an invaluable resource; the field study could not have be~n done wi thout his knowl edge of the invertebrates. I thank Fred for hi s generous nature, hi s encouragement, and for forgivi ng me for bei ng so impatient all the time. Keith Stolzenbach participated, in a big way, in every aspect of the flume experiments from the very beginning to the very end. He was also there for me during the seemingly endless writing stage. I thank him for calling at all the right times and knowing what to say. I thank the other members of my committee, Bob Whitlatch and Larry Madin and also Nancy Marcus, who chaired my defense, for reading and commenting on this thesis. Valuable discussions are often hard to identify, except after-thefact, but I know that I have yet to realize just how many live had with Brian Binder, Jim Carlton, Gary Tagon, Arthur Nowell and Pete Jumars. I especially appreciate the duet of encouragement I received from Pete and Arthur; they also supervised the first set of larval fall velocity measurements conducted at Fri day Harbor Laboratori es, WA, and I thank Arthur for the use of his settling chamber and for helping to design mine. 5 ACKNOWLEDG EMENTS I thank the suite of divers who put up with the miserable pea-soup conditions at my study site: Mel Briscoe, Cathy Cetta, Jerry Cheney, Paul Dragos, Char Fuller, Scott Gallagher, Ann Giblin, Scott Glenn, Bill Grant, George Hampson, Larry Madin, Betsey Pratt, Laurie Raymond, Maggie Rioux, Terry Rioux, Larry Sanford, Bob Spindel, Rod Taylor, and Nina Vassalotti. George Hampson loaned me his own Boston Whaler for the 1980 fi el d experiments, Bi 11 Grant provided use of the RV Sea Truck and Brad Butman provided use of the Whi tefoot for the 1982 experiments, all for which I am very grateful. I also thank the W.H.O.I. shop and, especially Bobby Weeks, for all their hel p wi th structures and equipment. Gretchen McManami n ski 11 fully typed several versi ons of thi s dissertation. Her cheerfulness and diligence are greatly appreciated. The U.S.G.S. and the W.H.O.I. graphics departments drafted some of the figures and W.H.O.I. graphics gave valuable advice and assistance for the ones that I drafted myself. I thank W.H.O.I. graphics for doing everythi ng on short noti ce. This research was funded by a National Science Foundation Dissertation Improvement Grant, A National Ocean Survey/Sea Grant Fellowship (NOAA, Dept. of Commerce Grant No. NA80-AA-D00077), a grant from the Coastal Research Laboratory at W.H.O.I., a grant from the Diving Equipnient Manufactures Association, an Association for Women in Science Predoctoral Award, a National Science Foundation Grant (No. OCE-78-19820) to Dr. J. Frederick Grassle, the W.H.O.I. Education Program, and the U.S. Geological Survey (Woods Hole, MA). 6 BIOGRAPHICAL NOTE The author was born on 26 November 1954 in Albany, California and was raised in Concord, California where she graduated from Clayton Valley High School in June 1972. She then attended San Jose State University (San Jose, CA), studying both botany and zoology, and eventually graduated with a B.A. in Zoology in June 1976. The scale was t

Initial settlement of marine invertebrate larvae : the role of passive sinking in a near-bottom turbulent flow environment