Kilo Nalu: Physical/Biogeochemical Dynamics Above and Within Permeable Sediments

Special Issue on Coastal Ocean Processes, Observing Technologies and Models, Oceanography, Volume 21, No. 4. December 2008.

tropical reef seabed at this site consists of live coral, a fossil limestone reef, and carbonate sands. The slope of the seafloor is 1:30 from the shore to 40-m water depth, 1:2 from 40-100-m depth, and 1:1 from 100-250-m depth. The latter depth is located ~ 2 km offshore, reflecting the extremely narrow coastal shelf at this site.
The goals of the Kilo Nalu Coastal Benthic Exchange Dynamics (CBED) project are to significantly advance our ability to measure the transport of solutes into and out of permeable sediments, to examine the seafloor's response to a wide range of physical forcing (such as surface waves, barotropic and baroclinic tides, currents, and near-bottom turbulence), and to determine the pelagic ecosystem's response to these processes. Here, we provide a brief description of new measurement capabilities already realized at the observatory and highlight a few recent results that demonstrate their importance.
This article has been published in Oceanography, volume 21, Number 4, a quarterly journal of The Oceanography Society. Copyright 2008 by The Oceanography Society. All rights reserved. Permission is granted to copy this article for use in teaching and research. republication, systemmatic reproduction, or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: info@tos.org or Th e Oceanography Society, PO box 1931, rockville, MD 20849-1931 Figure 1. (a) Seabed boundary layer instruments during installation. On the center beam, from left: the digital camera; a power/Ethernet infrastructure package; the stainless steel scanned laser housing and, behind it, the bCDvSP; and a commercial velocimeter used in turbulent flux calculations. The near beam of the frame supports the remote-controlled dye pump and dye reservoir. The dye injection rake can be seen on the seabed prior to burial 10 cm below the sand surface. (b) Seabed morphology measured remotely by the scanned laser system at 2-mm resolution within a 0.5 x 0.5-m square at the kilo Nalu 10-m sand site. Short-crested, cross-shore ripples are superimposed on a larger-scale feature in this example. The structure of these ripples has a dominating effect on the wave boundary layer structure and the transport of tracers out of the top few centimeters of the bed in response to wave forcing.

ADvANCINg ObSErvAtOry CAPAbILItIES
Wave-and current-generated turbulence  (Falter et al., 2004;Lowe et al., 2005); however, the results are comparable, given that the earlier observations were in shallower water and that the profiler observations represent a single location.
Although there are numerous mathematical treatments of wave-driven solute exchange between permeable sediment porewater and the overlying seawater, there have been only a few attempts to measure this exchange in the field (e.g., Webb and Theodor, 1968;Falter and Sansone, 2000;Precht and Huettel, 2004;Reimers et al., 2004). Hydrodynamic processes, including oscillating flows from passing surface waves and advection due to the interaction of sand ripples and bottom currents, significantly enhance porewater transport in Kilo Nalu sands.
We have found that porewater mixing at 10-m water depth and up to 15-cm sediment depth is enhanced over molecular diffusion by three to four orders of magnitude, with enhanced diffusivities positively correlated with wave height (Hebert et al., 2007).
We are testing the use of commercial optodes to measure DO concentrations within the benthic boundary layer at Kilo Nalu. Our goal is to use these rugged, stable devices to measure near-bed chemical concentrations, which, in combination with near-bed turbulence measurements, allow estimation of the sediment-seawater DO fluxes. DO was measured within the 50-cm layer above a 10-m-deep reef bed with an optode mounted on the vertical profiler    November 20, 2007 (all data, black), October 20-November 4 (before swell, blue), November 4-November 8 (during swell, red), November 8-12 (after swell, green), November 12-20 (long after swell, gray). Dashed lines indicate one standard deviation in gradient observations over the entire period. Data collected from the 10-m-depth reef site. (c) remotely acquired voltammetric microelectrode profile from the kilo Nalu 10-m-depth sand site. Sediment-water interface is located at depth = 0. Error bars represent +/-1 standard deviation of the mean.