The importance of fine-scale flow processes and food availability in the maintenance of soft-sediment communities

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1993Although the association between soft-sediment invertebrates and a specific sediment type has been documented for many habitats, most studies have been correlative and have failed to convincingly demonstrate any single mechanism to explain this association. Sediment type has generally been characterized by grain size, however, many other potential causal factors correlate with grain size, including organic content, microbial content, stability, food supply, and larval supply. One hypothesis for animal-sediment associations is that settling larvae are transported as passive particles and are sorted into different sedimentary habitats much like sediment grains. To test the hypothesis that near-bed hydrodynamics may modify larval settlement, field and flume experiments were conducted where larval settlement was compared between microdepositional environments (small depressions) and non-trapping environments (flush treatments). Depressions have been observed to trap passive particles, and these experiments were therefore designed to test whether settling larvae would be trapped in depressions like passive particles. Flume flow simulations were carried out with the polychaete Capitella sp. I and the bivalve Mulinia latera/is. Experiments with flush and depression sediment treatments were conducted in the absence of the potentially confounding effects of suspended sediment and organic matter and therefore offered a highly controlled, explicit test of passive hydrodynamic deposition of larvae in depressions. Although larvae of both species were generally able to actively select a high-organic sediment over a low-organic alternative with a comparable grain size, elevated densities of both species were observed in depressions for a given sediment treatment. Thus, both species appeared to be vulnerable to hydrodynamic trapping. M. latera/is larvae, however, often made a "poor choice" by settling in high numbers in depressions containing the low-organic sediment while Capitella sp. I larvae were generally able to "escape" from depressions if the sediment was unsuitable. In field experiments carried out at Station R in Buzzards Bay, Massachusetts, significantly higher densities of Mediomastus ambiseta juveniles, spionid polychaete juveniles, bivalves, gastropod larvae, and nemerteans were observed in depressions compared with flush treatments over 5 relatively short experimental periods (3 or 4 days each) during the summer of 1990. Of the abundant taxa, only Capitella spp. was not significantly more abundant in depressions compared with flush treatments, although numbers tended to be higher in depressions. Experiments were conducted over a short time period to minimize potential biological interactions between taxa and reduce the likelihood that organic material would accumulate in depressions and provide a cue for settling larvae. Thus, higher numbers in depressions suggest that larvae were passively entrained. These flume and field experiments suggest that near-bed hydrodynamics may modify settlement at some scales, and that both active and passive processes may operate in determining larval distributions in shallow-water, muddy habitats. In deep-sea ecosystems, the role of near-bed hydrodynamics is also of interest because of the potential role that larval settlement in organic patches may play in maintaining the immense species diversity characteristic of many deep-sea ecosystems. To try to understand the role of organic patches in deep-sea communities, several investigators have used colonization trays containing sediments that have been treated in different ways. These experiments have been criticized in the past because the sediment surface in the trays was elevated above the bottom and may therefore have interfered with natural boundary layer flow. Flume simulations of flow over these colonization trays revealed serious flow artifacts generated by the trays, and that flow across the sediment surface of the trays was characterized by turbulent eddies, accelerated velocities and boundary layer thickening. These sorts of flow characteristics would not be expected over natural sediments, and an alternative colonization tray was designed to eliminate these artifacts. To test the hypothesis that different types of food patches would result in different types of larval response, and determine how near-bed hydrodynamics may influence larval settlement, flush colonization trays filled with prefrozen sediment were deployed in tandem with artificial depressions south of St. Croix, U.S.V.I at 900 m depth. Colonization trays and artificial depressions were either unenriched or enriched with Thalassiosira sp. and Sargassum sp. two types of algae chosen to mimic natural food patches on the sea floor. Unexpectedly high densities of organisms colonized trays after only 23 days. The Thalassiosira trays were colonized by high densities of a relatively low diversity, opportunistic fauna, Sargassum trays were colonized by lower densities of a higher diversity fauna, and unenriched trays were colonized by very low numbers of a very diverse fauna. All tray faunas were markedly different in composition from the natural, ambient fauna. These fmdings suggest that different patch types did, indeed, result in a specialized faunal response to each of the "patch" types. Depressions on the sea floor provide a natural mechanism for food patch formation because passive particles such as detritus and algae tend to be entrained in the depressions. To determine whether dominant colonizers would be entrained in depressions like passive particles or could differentiate between depression "patch" types in a flow environment that might be expected to make active selection more difficult, artificial depressions were unenriched or enriched with Sargassum sp. or Thalassiosira sp. Total densities of organisms and densities of the most abundant species were substantially lower in artificial depressions than in trays. Densities in Thalassiosira depressions were lower than in Sargassum depressions and densities in unenriched depressions were extremely low, suggesting that dominant colonizers were not passively entrained in depressions and that colonization was specialized and highly active for these taxa. A different fauna was also observed in natural depressions compared with flush sediments, suggesting that natural depressions do contribute to species coexistence. Long-term tray deployments designed to test whether different faunas would be present in "patches" of different ages indicated that time may also play an important part in a deep-sea patch mosaic.This was funded by NSF and ONR, NOAA, NSERC (Canada), WHOI Ocean Ventures Fund and the WHOI Ditty Bag Fund

Spatial linkages between decapod planktonic and benthic adult stages in a Newfoundland fjordic system

The relative importance of predatory decapod crustaceans in sedimentary communities depends on the spatial variability in their abundance and composition. At the scale of a fjord, such spatial patterns could be related to sill-mediated larval supply. This study examines larval and adult distributions of abundant predatory decapods at six representative sites in a sub-arctic Newfoundland fjord during three consecutive summers. Adult snow crab (Chionoecetes opilio) and toad crab (Hyas coarctatus, H. araneus) characterized outer areas of the fjord, whereas pandalid shrimp (Pandalus montagui) dominated inner areas, and rock crab (Cancer irroratus) showed only minor spatial differences. Multivariate analysis and nonparametric comparisons of larval abundance and composition suggest that the sill separating inner and outer areas of the fjord results in differences in larval supply that correspond to adult abundances for at least two of the species analyzed here: snow crab and pandalid shrimp. Although larval abundance was not related to adult distribution when all zoeal stages were considered, correspondence between larval and adult patterns emerged when only late stages (zoeae ≥ II) were included in the multivariate analyses. Nonparametric comparisons supported these results, indicating significant differences in larval abundance inside and outside the sill for corresponding species and stages. Our results suggest that larval supply may play a critical role in establishing adult spatial patterns at the scale of the entire fjord for some species, but a less relevant role at the finer scale represented by the sites and habitats located at each side of the sill

Macrofaunal response to artificial enrichments and depressions in a deep-sea habitat

To test whether colonizing macrofauna specialize on different types of small-scale patches of food and disturbance in the deep sea, sediment tray and artificial depression colonization experiments were conducted on the deep-sea floor at 900-m depth, south of St. Croix, U.S. Virgin Islands. Trays and depressions were unenriched (Unenriched Controls) or enriched with either Thalassiosira sp. or Sargassum sp. Concurrent deployment of different types of enrichment and disturbance made it possible to evaluate whether macrofauna specialize on different patches, and thus avoid species interactions that might lead to competitive exclusion. Depressions create a hydrodynamic regime that traps passive particles, allowing tests of the relative importance of active selection of different patch types versus passive deposition for abundant colonizers. After 23 d, total densities and densities of the four abundant colonizers (Capitella spp., Nereimyra punctata, Cumella sp. and Nebalia sp.) were extremely high in enriched trays, despite relatively low ambient densities. Densities in Unenriched Control Trays were very low, and did not attain ambient densities. After 24 d, total densities in all depression treatments were considerably lower than in enriched tray treatments, and only Sargassum Depression densities exceeded those in the ambient environment. Lower densities of organisms in depression treatments compared with trays and differences in densities among depression treatments suggest that the dominant colonizers were highly active and selective, and were not passively entrained in depressions. Faunal analysis indicated that trays and depressions were very different, and Sargassum Depression fauna was very different from other depression types. A strong difference was not observed between fauna in ambient sediments and Thalassiosira sp. or Unenriched Control Depressions, perhaps because Thalassiosira sp. was dropped in depressions on the sediment surface and may have been more readily available to consumers and more rapidly consumed than in trays. Thalassiosira Trays were colonized by a lower diversity fauna than Sargassum Trays, and Unenriched Control Trays were colonized by very low densities of a fauna that was comparable in diversity to the ambient community. Diversity in Sargassum Depressions was higher than in enriched trays but lower than in other artificial depressions and the ambient fauna. Thalassiosira Depressions and Unenriched Control Depressions were comparable in diversity to ambient fauna and natural depressions, which were highly diverse. These experiments suggest that fauna may respond quickly and selectively to artificial food patches and disturbance, and this fauna is different from that observed in the ambient sediment. Thus, a patch mosaic may be part of the reason for the high species diversity that is observed in deep-sea ecosystems. The different, highly diverse, fauna observed in natural depressions compared with flat ambient sediment suggests that natural analogs of these experiments have unique faunas that may contribute to the species richness of deep-sea habitats

Adult macrofauna effects on Capitella sp. I larval settlement: A laboratory flume study

The opportunistic, deposit-feeding polychaete Capitella sp. I is the overwhelming numerical dominant in disturbed and enriched sediments and rarely co-occurs in appreciable numbers with other abundant mud-dwelling macrofauna. Rapid colonization and population increase in organicrich sediments is typically followed by subsequent sharp decline. The mechanistic basis for these characteristics was explored in flume-flow experiments that tested whether settling Capitella sp. I larvae avoid sediments inhabited by macrofaunal adults or sediments reworked by them. The first set of experiments consisted of four treatments: conspecific adults or no adults in reworked or non-reworked sediment. Capitella sp. I settlement was significantly altered (depressed) only by pelletized sediment of conspecific adults. The second set of experiments involved similar treatments, but with adults of the deposit-feeding bivalve Tellina agilis. Neither adult presence nor sediment reworking significantly affected settlement of Capitella sp. I larvae. A third set of experiments that compared settlement in sediments with and without the suspension-feeding bivalve Mulinia lateralis demonstrated no significant treatment effect. These results suggest that larval settlement behavior could contribute to population growth in a boom and bust species when a critical limiting resource is overexploited. That is, sediments completely pelletized by Capitella sp. I adults may signal settling larvae that organic matter is depleted. Larvae may therefore settle in smaller numbers and are more likely to be dispersed away from abundant populations of adults. Active avoidance of conspecific adults or adults of other taxa is unimportant for the taxa at the densities tested here

Potential flow artifacts associated with benthic experimental gear: Deep-sea mudbox examples

In response to the growing recognition of the potential effects of near-bed hydrodynamics on various benthic processes, flume studies were conducted to document fine-scale flow patterns over several types of mudboxes that have been used to study colonization by deep-sea organisms. Mudboxes are typically filled with natural sediments or sediment treatments and placed in the field to observe how timing, larval supply and sediment composition may affect larval settlement. This study addresses potential hydrodynamic biases of mudbox structures as obstructions to the near-bed flow. Detailed velocity profiles were made over two types of “free vehicle” mudboxes that could be deployed and recovered from a surface vessel. One of these (“Old Free Vehicle”) was not designed with regard for potential hydrodynamic biases whereas the other (“New Free Vehicle”) was designed specifically to minimize flow disturbances and maintain a realistic boundary-layer flow over the mudbox sediments. Flume velocity profiles also were made over two smaller mudboxes designed to be deployed by a submersible, one (“Flush Sediment Tray”) which was designed to be placed flush with the ocean bottom, thus minimizing flow disturbance, and another (“Raised Mudbox”) which was not. Flume simulations indicated that the Old Free Vehicle and the Raised Mudbox cause considerable disturbance to the near-bed flow regime; flows over the mudbox sediment surface differed markedly from those predicted for the natural seabed and those observed over the flume bed in the absence of the mudboxes. Flow accelerations, growing secondary boundary layers and eddy formation were observed over these mudbox sediments, and vertical velocity profiles varied considerably in the along-channel direction. The alternative mudbox designs (New Free Vehicle and Flush Sediment Tray) were largely successful in reducing or eliminating these flow artifacts. Boundary-layer flows over both the New Free Vehicle and the Flush Sediment Tray were much more uniform, and velocity profiles over the sediment surfaces were very similar to those in the empty flume channel and those predicted for a natural deep-sea habitat. In addition, there was no evidence of eddy formation and other major flow disturbances. These flume studies underscore the benefit of considering potential hydrodynamic effects in designing benthic experimental sampling gear to reduce potential flow disturbances that may bias data collections and confound data interpretation

Performing under pressure: Quiet eye training improves surgical knot-tying performance

Background: We examined the effectiveness of traditional technical training (TT) and quiet eye training (QET) on the performance of one-handed square knot tying in first-year surgery residents under normal and high anxiety conditions. Methods: Twenty surgery residents were assigned randomly to the two groups and completed pretest, training, and simple and complex retention tests under conditions of high and low anxiety. The TT group received traditional instruction on improving hand movements; the QET group received feedback on their gaze behaviors. Participants wore an eye tracker that recorded simultaneously their gaze and hand movements. Dependent variables were: knot tying performance (%), quiet eye duration (%), number of fixations, and total movement time (s). Results: Both groups improved their knot tying performance (p 0.05). The QET group also demonstrated more efficient gaze and hand movements post training. Conclusions: These data demonstrate the effectiveness of training gaze behaviors, not only to improve the effectiveness and efficiency of performance, but also to mediate any negative effects of anxiety on performance. These findings may have important implications for medical educators and practitioners, as well as surgeons who may be (re)training or learning new procedures

Quiet eye training improves surgical knot tying more than traditional technical training: A randomized controlled study

Background We examined the effectiveness of technical training (TT) and quiet eye training (QE) on the performance of one-handed square knot tying in surgical residents. Methods Twenty surgical residents were randomly assigned to the 2 groups and completed pretest, training, retention, and transfer tests. Participants wore a mobile eye tracker that simultaneously recorded their gaze and hand movements. Dependent variables were knot tying performance (%), QE duration (%), number of fixations, total movement time (s), and hand movement phase time (s). Results The QE training group had significantly higher performance scores, a longer QE duration, fewer fixations, faster total knot tying times, and faster movement phase times compared with the TT group. The QE group maintained performance in the transfer test, whereas the TT group significantly decreased performance from retention to transfer. Conclusions QE training significantly improved learning, retention, and transfer of surgical knot tying compared with a traditional technical approach. Both performance effectiveness (performance outcome) and movement efficiency (hand movement times) were improved using QE modeling, instruction, and feedback. © 2014 Elsevier Inc. All rights reserved

Environmentally mediated trends in otolith composition of juvenile Atlantic cod (Gadus morhua)

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in ICES Journal of Marine Science 72 (2015): 2350-2363, doi: 10.1093/icesjms/fsv070.We evaluated the influence of environmental exposure of juvenile Atlantic cod (Gadus morua) to inform interpretations of natal origins and movement patterns using otolith geochemistry. Laboratory rearing experiments were conducted with a variety of temperature (~ 5, 8.5 and 12 °C) and salinity (~ 25, 28.5 and 32 PSU) combinations. We measured magnesium (Mg), manganese (Mn), strontium (Sr) and barium (Ba), expressed as a ratio to calcium (Ca), using laser ablation inductively coupled plasma mass spectrometry (ICP-MS), and stable carbon (δ13C) and oxygen (δ18O) isotopes using isotope ratio monitoring mass spectrometry. Temperature and salinity significantly affected all elements and isotopes measured, with the exception of salinity on Mg:Ca. We detected significant interactions among temperature and salinity for Mn:Ca and Ba:Ca partition coefficients (ratio of otolith chemistry to water chemistry), with significant temperature effects only detected in the 32 and 28.5 PSU salinity treatments. Similarly, we detected a significant interaction between temperature and salinity in incorporation of δ13C, with a significant temperature effect except at intermediate salinity. These results support the contention that environmental mediation of otolith composition varies among species, thus limiting the ability of generalized models to infer life history patterns from chemistry. Our results provide essential baseline information detailing environmental influence on juvenile Atlantic cod otolith composition, punctuating the importance of laboratory validations to translate species-specific otolith composition when inferring in situ life histories and movements.Research funding and support was provided by a Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Grant on Connectivity in Marine Fishes. R. Stanley was supported by an NSERC Postgraduate Scholarship and a Research and Development Corporation of Newfoundland (RDC) student fellowship.2016-04-2

The role of colonization in establishing patterns of community composition and diversity in shallow-water sedimentary communities

To determine whether pattern and diversity in benthic sedimentary communities are set primarily at colonization or by post-settlement biological interactions, we collected faunal cores and conducted reciprocal sediment transplant experiments at a sandy and a muddy site at 12 m depth, ~3 km apart off New Jersey. Multivariate analyses of cores collected at these sites in September 1994 indicated differences in the taxa determining local pattern, with the bivalve Spisula solidissima and the polychaete Polygordius sp. being dominant at the sandy site, and oligochaetes, several polychaete species and the bivalve Nucula annulata dominant at the muddy site. Individual cores from the sandy site were significantly less diverse than those at the muddy site. Short-term experiments (3-5 d) were deployed by divers at three different times (August-September, 1994). Replicate trays (100 cm2) filled with azoic sand or mud were placed flush with the ambient seafloor at both sites. Multivariate comparisons indicated that sediment treatment in trays played a greater role in determining colonization patterns in the first experiment, site played a greater role in the second, and both variables contributed in the third. This pattern suggests that larval settlement and habitat choice played an important role in the first and third experiments, and that local transport of recently settled juveniles from the surrounding sediments was important in the second and third experiments. Sandy-site trays had significantly lower diversity than muddy-site trays, but there was no effect of sediment type in trays on diversity of colonizers. These experiments focused on small spatial scales and three short time periods, but they demonstrate that species patterns in some environments may be set by habitat selection by larvae and by juvenile colonization from the surrounding community. Post-colonization processes such as predation and competition likely play a major role for some species, but patterns of initial colonization corresponded well with those in the local community

Greek research reactor performance characteristics after addition of beryllium reflector and LEU fuel

The GRR-1 is a 5-MW pool-type, light-water-moderated and-cooled reactor fueled with MTR-type fuel elements. Recently received Be reflector blocks will soon be added to the core to add additional reactivity until fresh LEU fuel arrives. REBUS-3 xy fuel cycle analyses, using burnup dependent cross sections, were performed to assist in fuel management decisions for the water- and Be-reflected HEU nonequilibrium cores. Cross sections generated by EPRI-CELL have been benchmarked to identical VIM Monte Carlo models. The size of the Be-reflected LEU core has been reduced to 30 elements compared to 35 for the HEU water-reflected core, and an equilibrium cycle calculation has been performed