Benthic Boundary Layer Geochemistry and Physics

PIs: G. Pawlak, F. Sansone, M. McManus, E. De Carlo, A. Hebert and T. Stanton
funded by National Science Foundation

3-d diagram of Kilo Nalu Offshore Observatory.

Coastal ocean shelf systems with permeable seabeds have been a focus of considerable recent interest. Despite this interest, there is still a limited understanding of the interactions between physical oceanography, changes in the chemical exchange between the sediment and seawater, and ecosystem response of these dynamic systems.

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UH/SOEST researchers are carrying out field research at Kilo Nalu as part of the National Science Foundation’s Coastal Ocean Processes (CoOP) inititiative on Coastal Benthic Exchange Dynamics (CBED). The project aims to significantly advance our ability to measure the transport of solutes into and out of permeable sediments, examine the seafloor’s response to a wide range of physical oceanographic forcing, and determine the pelagic ecosystem response to these processes.

The work encompasses:

  1. extension of the existing Kilo Nalu array to include an array of physical sensors over depths of 10-40 m;
  2. collection of high resolution temporal and spatial physical data over a range of oceanographic conditions;
  3. measurement of sediment biogeochemistry across the array;
  4. measurement of sediment-seawater chemical exchange under a range of conditions, at both sandy and hard-bottom permeable seafloors;
  5. comparison of these porewater-seawater fluxes with those calculated using eddy correlation and water column profiling techniques;
  6. examination of the biological response in the water column to physically driven sediment fluxes of nutrients;
  7. investigation of the effects of sediment exchange processes on the dynamics of the CO2 system in the surface water.

Together, these tasks aim towards resolution of the physical and geochemical responses of the benthic boundary layer (BBL) systems in response to environmental changes.

Detailed observations of the physical oceanography in the study domain are being carried out to obtain an accurate characterization of the BBL response to changes in the physical environment, An offshore array of thermistor chains and acoustic current profilers will be augmented by an autonomous vertical profiler and a high-resolution near-bed turbulence profiler. Monitoring of the spatial variability in the physical and chemical environment is being carried out using a combination of shipboard and autonomous underwater vehicle (AUV) surveys, supplemented by satellite based remote sensing.

Sandy sediments are important habitats for bottom fish, invertebrates, and other commercially important species. The Kilo Nalu BBL research will provide knowledge that can be used to support the management of these habitats to ensure their healthy ecological state. Thus, there is a potential economic benefit from the research, in that proper management of the environment of these species requires knowledge of sedimentary remineralization and the resultant nutrient and CO2 fluxes from the sediments.

The project also includes a significant educational program, Waves, Wind and Hawaii’s Coasts, to be developed by Bishop Museum in collaboration with project researchers. The goals for the outreach program are to make the research accessible to the local community and to exploit the real-time access to ocean conditions for educational purposes.  The program will take advantage of the Kilo Nalu observations to explore topics in science learning that are directly relevant to the everyday lives of Hawaii’s residents, to use actual data to understand earth and ocean processes, and to bring scientists and general public together in an educational setting.

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Wave Boundary Layer Processes over Rough Beds

PI: G. Pawlak
funded by Office of Naval Research

A primary goal of the Kilo Nalu observatory is to gain understanding of the hydrodynamic processes occurring at the rough coral reef bed under wave and tidal forcing. Our research group has developed an automated wave boundary layer profiler (pictured below) that resolves a spatial view of the turbulent boundary processes over a wave orbital excursion (~3m). The profiler moves a suite of acoustic velocimetry instrumentation that obtains a high resolution two-dimensional slice of the near-bed flow.

Wave Boundary Layer Profiling Platform schmatic   WBLPP deployed at Kilo Nalu

Wave Boundary Layer Profiling Platform schematic (left); WBLPP deployed at the Kilo Nalu Nearshore Reef Observatory (right). (Click on image to open a larger version in a new window.)


Nearshore Water Quality

PIs: G. Pawlak, E. DeCarlo
funded by UH Sea Grant

The coastal marine environment is a vital economic and cultural resource for the State of Hawaii. Water quality in the nearshore environment can have significant repercussions on tourism, fishing and public recreation activities. In terms of the physical environment, Hawaii’s exposure to an open ocean wave climate affects recreational and commercial aquatic safety issues that persistently confront government, residents and visitors alike. The south shore of Oahu is a focal region for the state in terms of economic importance, but is also one of the most heavily burdened in terms of societal impact with point and non-point source (NPS) pollution from urban Honolulu regularly affecting water quality in the nearshore. We are developing a set of observational tools based on those deployed on autonomous instrumented platforms such as CRIMP, that will enable a baseline, real-time characterization of the physical and biogeochemical environment of Oahu’s south shore.

The broad goal of the project is to provide a mechanism for access to real-time coastal data to scientists, coastal resource managers, navigators and recreational users. The specific objectives include:

  1. Real-time measurement of the physical conditions and water quality, including: swell height and direction, current velocity, temperature, conductivity/salinity, nutrients, chlorophyll, dissolved oxygen, pH and turbidity.
  2. Development of a web-accessible real-time and archived data system that will provide users with these variables in readily accessible formats
  3. Extension of existing wave modeling capabilities to Oahu’s south shore.
  4. Examination of the connections between the physical and biogeochemical environment in Mamala Bay with a focus on linking physical forcing and water quality. In particular, the study will determine temporal scales that affect water quality variability in relation to temporal scales of physical forcing.

The long-term scientific goal of the proposed observations is the development of a synoptic model of the physical and biogeochemical environment in the Waikiki – Ala Moana region including wave, tidal and wind-forcing and their effects on nearshore water quality.  The proposed project will lay the foundations for a broader coastal observing system for the south shore of Oahu that can have wide-ranging benefits for an array of users.

Current and acoustic echo intensity data at study site for June 2003.

Current and acoustic echo intensity data at study site for June 2003 corresponding with a period of significant swell activity. The lower two plots show tidal amplitude and significant wave height for the period of observations. (Click on image to open a larger version in a new window.)

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Sediment Porewater Processes

PIs: F. Sansone, M. Merrifield, G. Pawlak, I. Webster
funded by National Science Foundation

Shallow-water surface waves traveling above permeable sandy sediments (i.e., those with grains coarser than ~80 um) can induce greatly enhanced mixing across the sediment-water interface and rapid transport of water and particles within the sediment. Thus, hydrodynamic transport in sandy sediments in nearshore and continental shelf waters can be expected to (1) substantially increase the rates of biogeochemical processes occurring in sediments by increasing the supply of oxygen and particulate matter to the sediment, and (2) significantly increase the rate of return of degradation products such as nutrients to the water column, thereby increasing water-column and benthic primary production. The rapid transport of porewater and particles in sandy sediments renders the calculation of benthic fluxes using conventional diffusion-based models invalid and thus confounds the interpretation of chemical profiles collected in coastal areas and on the continental shelf.

The goal of the research is to significantly advance our ability to describe and quantify the transport of solutes and particulate matter in sandy sediments. Using field investigations, we intend to confirm (or refute) the validity of available theoretical models of hydraulic transport through sediment pores, and will evaluate the importance and timescales of the hydrodynamic pumping mechanisms. We propose to

  1. measure wave-induced porewater movement/dispersion under a range of wave conditions;
  2. evaluate current theories of wave-enhanced porewater movement/dispersion and exchange using the results of these measurements;
  3. establish an array of physical sensors at the field site (illustrated below), and collect synoptic data over a range of wave conditions;
  4. monitor porewater mixing of fluorescent dye by use of fiber optic array;
  5. compute porewater-seawater fluxes as functions of wave conditions;
  6. measure wave-induced particle transport rates into and within sandy sediments, as a function of particle size and wave conditions; and
  7. investigate the effect of bedforms on porewater and particle transport.

This research will be conducted in sandy sediment off the south shore of Oahu, Hawaii, a site that is subject to a wide and predictable range of wave conditions.

Sketch of field array for dye dispersion experiments.

(Left) Sketch of field array for dye dispersion experiments. (Click on image to open a larger version in a new window.)

Postdoctoral researcher Andy Hebert installs a “sediment window.”Closeup of sediment window installation.

(Above) Postdoctoral researcher Andy Hebert installs a “sediment window” to observe dye mixing in sandy sediment porewater. (Click on image to open a larger version in a new window.)

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