Pacific Coastal and Marine Science Center
Unlike the major anthropogenic changes that terrestrial and coastal habitats underwent during the last centuries such as deforestation, river engineering, agricultural practices or urbanism, those occurring underwater are veiled from our eyes and have continued nearly unnoticed. Only recent advances in remote sensing and deep marine sampling technologies have revealed the extent and magnitude of the anthropogenic impacts to the seafloor. In particular, bottom trawling, a widespread industrial fishing practice that involves dragging heavy nets, large metal doors, and chains over the seafloor to catch fish. Most attention to the direct impacts of bottom trawling has previously been given to the alterations of fish populations and benthic communities. Few studies have, in contrast, addressed the modifications of the seabed sediment itself and associated changes in sediment fluxes. Here we show that chronic bottom trawling leads to a six-fold increase in off-shelf directed sediment transport when compared to the mass resuspended by natural processes. We also provide a universal quantitative approach to calculating global bottom trawling-induced sediment resuspension and export that can be used to predict future man-made sediment dispersal. Combining global soft-sediment distribution data on all continental shelves with worldwide bottom trawling intensity estimates, we show that bottom trawling-induced resuspended sediment mass amounts to approximately the same as the sediment mass supplied to the continental shelves through the world's rivers.
Globally, reef building corals face elevated extinction risk from climate change and reef-building corals have experienced global declines resulting from bleaching events caused by week to month-long warm-water exposure. We present results of the thermodynamics and hydrodynamics of an atoll system and their effect on coral cover based on field measurements from 2012 to 2014 on Palmyra Atoll in the central Pacific. We found that spatial variations in coral cover were correlated with temperature variations on time scales of days to weeks. Shallow terrace and backreef sites with high coral cover (> 50%) had a highly variable temperature distributions, but their average weekly temperature distributions were lower and similar to offshore waters. The mechanism for maintaining this low weekly temperature was mean advection, which varied on a weekly timescale in response to wave forcing. Tides were also important in driving flow on the atoll, but their contribution to the net transport of heat was not significant. Wind and regional forcing were generally not important in driving flow inside the atoll. Buoyancy-driven flows were important within the lagoons, and in driving cross-shore exchange on forereef environments. The physical factors favoring high coral cover percentage varied according to the different prevailing hydrodynamic regimes: low temperatures in backreef habitats, short travel times in lagoon habitats (days since entering the reef system), and lower wave stress on forereef habitats. In light of future warming from climate change, local areas of reefs which maintain lower temperatures through wave-driven mean flows will have the best likelihood of promoting coral survival.
Abstract: Marshes attenuate incoming wave action by providing friction through vegetative drag. This reduction in wave energy allows sediments to drop out of suspension and settle onto the marsh platform, where they contribute to vertical accretion and marsh sustainability. The ability to reduce wave energy also makes marshes an appealing option for nature-based coastal protection. For these applications and others, quantifying the amount of attenuation and understanding its drivers is critical. For this talk, I will present results from two field campaigns at the China Camp State Park salt marsh in San Pablo Bay. Here the tide flows over mudflats and through three consecutive vegetation zones: cordgrass (Spartina foliosa), cordgrass with pickleweed (Salicornia pacifica), and pickleweed. Measurements were collected during perigean spring tides in winter 2014 and summer 2016. We examined the within marsh variations, as well as the seasonal variation driven by the differing weather regimes, wave climates, and vegetation characteristics. The results highlight the effectiveness of vegetative drag in attenuating wave action under a variety of conditions. This work was performed as part of the NSF/USGS Graduate Research Internship Program advised by Jessie Lacy.
Abstract. Submarine canyons can have significant influence on the hydrodynamic distribution of sediments and organic matter (OM) eroded and deposited from the continents. In Baltimore Canyon, on the U.S. Atlantic margin, loci of net erosion, winnowing, and deposition emplace a complex mixture of marine and terrestrial OM along the canyon axis. The resulting sedimentary deposits of OM represent both a potential carbon sink through long-term burial and a food source for unique benthic communities. However, understanding the provenance and depositional history of OM is a continual challenge: commonly-measured bulk geochemical properties often provide insufficient information to distinguish end-member sources. Utilizing resources at PCMSC and UCSC, this study took a novel approach to separate functional classes of OM and investigate sources and degradative pathways of OM in Baltimore Canyon. I will review how compound-class separation, biomarker analysis, bulk and compound-specific stable isotope analysis, and radiocarbon analysis each contributed to a new interpretation of organic matter dynamics in this system, and I will speculate on the relative utility of various organic analyses across broader studies.
Abstract. Models quantifying the onset of sediment motion do not typically account for the effect of biotic processes because they are difficult to isolate and quantify in relation to physical processes. Here we investigate an example of the interaction of kelp (Order Laminariales) and coarse sediment transport in the coastal zone, where it is possible to directly quantify and test its effect. We develop a physical model to explore the reduction in critical shear stress of large cobbles colonized by Nereocystis luetkeana, or bull kelp. Observations of coarse sediment motion at a site in the Strait of Juan de Fuca (northwest United States–Canada boundary channel) confirm the model prediction and show that kelp reduces the critical stress required for transport of a given grain size by as much as 92%, enabling annual coarse sediment transport rates comparable to those of fluvial systems.
Abstract. Coastal dunes provide important ecosystem services such as habitat for endangered species, sites of high tourism value, groundwater recharge, and protection against coastal erosion, storm surges, and sea-level rise. Despite these values, fundamental understanding of their erosion and rebuilding cycles following natural storms or enhanced climatic variability forcing is limited. Similarly, efforts to restore dune ecosystems in North America tend to ignore required littoral-terrestrial sediment exchanges and resulting morphodynamics that govern broader ecological function. This talk reviews emerging technologies and techniques being used to study coastal dune systems such as unmanned aerial systems (UAS), terrestrial laser scanning (TLS), digital photogrammetry (e.g., Structure-from-Motion, SfM), and spatial analysis methods. A case study of dynamic dune restoration project from British Columbia will be presented to demonstrate applicability for this research.
Three months of field observations along the inner-continental shelf at Fire Island, NY were used to investigate cross-shore transport of sediment as a significant source of beach sand. Fire Island is a 50 km long barrier island along the southern shore of Long Island, NY which serves to protect the heavily developed mainland during storms. Sediment budget analyses over the past 50 years repeatedly come up short of nearly 200,000 m3/yr of sand which is required to account for accretion along the central segment of the island. Cross-shore sediment fluxes from beyond the ‘depth of closure’ on the inner-continental shelf were hypothesized to be the likely source.
To investigate this hypothesis and identify mechanisms for cross-shore sediment flux, instrument frames were deployed in 12-15 m water depth and equipped with acoustic sensors to concurrently measure surface waves, hydrodynamic flows, near-bed wave velocity and turbulence, suspended sediment, and bedform morphology and migration. Measurements were made at these depths to quantify cross-shore transport in response to wave forcing at depths where both wave skewness and wave-current interactions should significantly contribute to cross-shore sediment transport.
Bedform migration and geometry were computed from rotary sonar images and consisted of orbital-scale ripples with a slight onshore-directed asymmetry. Ripple migration occurred in response to wave events and was used to estimate bedload transport, which was directed onshore at a rate of 1.1 m3/yr per meter alongshore during the deployment. These rates were compared with migration at other locations (MVCO and LEO15) and found to be quite different: at Fire Island, ripple migration was consistently onshore despite the common occurrence of significant negative wave-skewness events.
Near-bed profiles of velocity structure from a convergent pulse-coherent acoustic Doppler profiler revealed that wave-boundary layer streaming and mean-flows from ripple vortex entrainment played a significant role in bedload transport and causing us to rethink our conceptual model of wave-induced bedload transport. The sources of negative wave-skewness at Fire Island were also explored.
Estuaries are key-environments influencing exchange at the continent-ocean interface. Whether an estuary is a source or sink of a scalar depends on a great number of variables. Some variables derive from natural causes (e.g., tides), while other are associated with human activities (e.g. dredging). Many formerly natural variables are being affected by human activities, either locally (e.g. freshwater inflow regulation) or globally (e.g. climate change). In this seminar I present an overview of the physical settings governing estuarine circulation and fine sediment transport along the Brazilian shores.
Many estuaries around the world work as sediment filters or traps, reducing sediment flux to coastal areas. Previous studies of estuaries in Palau have presented similar trends of reduced sediment flux; thus leading to long term coral degradation within the estuary and sheltering of the offshore reef. This study shows that the sediment storage of Ngermeduu Bay, Palau, Micronesia has reached a maximum and the bay is no longer collecting sediment. Net flux is out of the system, with most of the sediment going offshore towards the reef and some making its way back towards the mangroves and rivers. The authors describe the physical mechanisms responsible for sediment transport in the system and discuss the implications of net outward sediment flux on the offshore environmental systems.
River inflows are often the dominate source of sediment, contaminants, and nutrients into a system and the dynamics of these flows dictate the fate of introduced constituents and their impact on an ecosystem. The form of the river plume depends on the density of the river flow relative to the ambient surface waters. For instance, a dense river plume will plunge beneath the surface and, in a deep stratified system, can intrude into the water column at a depth of neutral buoyancy. While near-field mixing between the plume and ambient water masses dictate the depth of an inflow intrusion, once the intrusion forms, the plume is influenced by buoyancy and rotational forces. The rotationally influenced plume dynamics in the mid- and far-field regions dictate the trajectory and lateral variability sediment deposition in the system although these regions have received limited attention in the literature. We have employed both field observations and numerical modeling to investigate the fate and transport of sediments emanating from river plumes into a stratified, rotational lacustrine system (Pallanza Bay of Lake Maggiore, Italy). The present talk will focus on the role of ambient stratification on the plume dynamics in the mid and far-field regions though a three-dimensional numerical modeling study of the system, which was extensively validated with observations. We find that the ambient stratification controls the intrusion dynamics and therefore significantly impacts the extent and lateral variability of sediment deposition in the system. An analytical expression was developed to predict the extent of sediment (in the along-plume direction) using a priori known flow rate and ambient stratification. Furthermore, the lateral variability of sediment deposition is affected by the ambient stratification and we found that a counter-clockwise recirculation region could generate across-bay transport of sediment. These mechanisms can be used to understand the varying rates of contaminant recovery in sediments across Pallanza Bay and to predict timescales of recovery for regions of interest.