ALBERT

Description: 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 1993

Description: Although 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.

Description: This was funded by NSF and ONR, NOAA, NSERC (Canada), WHOI Ocean Ventures Fund and the WHOI Ditty Bag Fund.

Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 57 (2012): 945-958, doi:10.4319/lo.2012.57.4.0945.

Description: Oxygen availability restricts groundfish to the oxygenated, shallow margins of Saanich Inlet, an intermittently anoxic fjord in British Columbia, Canada. New and previously reported 210Pb measurements in sediment cores compared with flux data from sediment traps indicate major focusing of sediments from the oxygenated margins to the anoxic basin seafloor. We present environmental and experimental evidence that groundfish activity in the margins is the major contributor to this focusing. Fine particles resuspended by groundfish are advected offshore by weak bottom currents, eventually settling in the anoxic basin. Transmittance and sediment trap data from the water column show that this transport process maintains an intermediate nepheloid layer (INL) in the center of the Inlet. This INL is located above the redox interface and is unrelated to water density shifts in the water column. We propose that this INL is shaped by the distribution of groundfish (as resuspension sources) along the slope and hence by oxygen availability to these fish. We support this conclusion with a conceptual model of the resuspension and offshore transport of sediment. This fish-induced transport mechanism for sediments is likely to enhance organic matter decomposition in oxygenated sediments and its sequestration in anoxic seafloors.

Description: The VENUS Project and University of Victoria supported the ship and submersible time for field experiments, and the U.S. Geological Survey and Coastal and Marine Geological Program generously supported J.C. The project was supported by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada to V.T. and P.S. and a Yohay Ben-Nun fellowship and Moshe Shilo Center for Marine Biogeochemistry Fund award to T.K.

Description: Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 23 (2009): GB4032, doi:10.1029/2008GB003416.

Description: In this study, we link groundfish activity to the marine silica cycle and suggest that the drastic mid-1980s crash of the Baltic Sea cod (Gadus morhua) population triggered a cascade of events leading to decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension associated with reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, British Columbia, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that dwindling groundfish populations caused by anthropogenic perturbations, e.g., overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities.

Description: We acknowledge the VENUS Project, University of Victoria, for supporting the ship and submersible time for field experiments and USGS, CMGP, for support to J.C. Additional funding from NSERC Canada and from the Canada Research Chairs Foundation to V.T.; a Rothschild fellowship to G.Y.; and a Yohay Ben-Nun fellowship and Moshe Shilo Center for Marine Biogeochemistry fund to T.K. are also acknowledged.