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

Vertically thin layers of zooplankton were found to be common and recurring features in Mamala Bay on the south shore of the island of Oahu, Hawaii. The formation, maintenance, and vertical displacement of these thin layers are, in part, a function of regional physical oceanographic processes. The purpose of this study was to quantify general thin zooplankton layer characteristics in Mamala Bay and the underlying physical environment in which they occurred. We utilized a 2 mo time series of acoustic backscatter measurements from a calibrated acoustic Doppler current profiler (ADCP) to identify thin scattering features; took biological samples; and collected vertical profiles of physical, optical, and biological characteristics of the water column. In general, thin zooplankton layers were associated with increased water column stability. Stratification at the study site was low relative to other coastal regions around the continental USA where thin layers have been observed. Instances of significant stratification were short-lived, and possibly as a result, thin layers were shorter in duration. Diurnal surface heating accounted for much of the observed stratification, and the breakdown of stratification during civil twilight corresponded with a decrease in thin zooplankton layer formation. Biological and optical measurements taken during a focused shipboard profiling experiment over the course of the study suggested a mechanism for zooplankton layer formation in which zooplankton converged to graze on a thin phytoplankton layer. 409: 95-106, 2010 changes in the underlying physical environment. Thin layers are often found in conjunction with vertical gradients of some physical property, such as water density or current velocity. Franks (1992) and Stacey et al. (2007) proposed a buoyancy-driven mechanism for thin layer formation in which non-motile, or passive, organisms settle at densities equal to their own density; therefore one would expect a higher concentration of organisms near large density gradients. This theory has been supported by observational work by Dekshenieks et al. (2001), who found that a large percentage (71%) of thin layers were associated with the pycnocline. Another purely physical mechanism that has been proposed to contribute to thin layer formation is strain by a sheared velocity profile (Franks 1995, Stacey et al. 2007. To this end, Ryan et al. (2008) found strong statistical relationships between shear in the water column and the presence of thin layers. Thus, an understanding of the regional physical processes that contribute to local stratification and shear may help to predict the occurrence of conditions that are favorable to thin layer development and maintenance. KEY WORDS: Thin layer · Physical processes · Stratification · Zooplankton Resale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESSMar Ecol Prog SerIn addition, motile species of phytoplankton and zooplankton can actively converge into thin layers seeking nutrients, ...

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“…2) (Sansone et al 2008). Additional temperature data were provided by a second thermistor chain deployed on the 40 m isobath.…”

Section: Methodsmentioning

confidence: 99%

Effects of physical structure and processes on thin zooplankton layers in Mamala Bay, Hawaii

Sevadjian

McManus²,

Pawlak³

2010

Mar. Ecol. Prog. Ser.

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“…The wave conditions were chosen based on 2 years of data from the Kilo Nalu observatory (Samsone et al, 2008) presented in Fig. 2.…”

Section: Model Experimentsmentioning

confidence: 99%

“…Although the Kilo Nalu cabled reef observatory once provided real-time observations of several physical and biogeochemical parameters (Samsone et al, 2008), the lack of continuous measurements of the nearshore currents in Honolulu during the modeled period (and in general) makes it impossible to properly validate the results and quantify the performance of each modeling strategy. Unfortunately, there are no results from the atmospheric model and the ocean model system used to provide surface forcing and boundary conditions to the nearshore domain during the period of the Kilo Nalu experiment.…”

Section: Introductionmentioning

confidence: 99%

Different approaches to model the nearshore circulation in the south shore of O'ahu, Hawaii

Souza

Powell

2017

Ocean Sci.

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Abstract. The dynamical interaction between currents, bathymetry, waves, and estuarine outflow has significant impacts on the surf zone. We investigate the impacts of two strategies to include the effect of surface gravity waves on an ocean circulation model of the south shore of O'ahu, Hawaii. This area provides an ideal laboratory for the development of nearshore circulation modeling systems for reef-protected coastlines. We use two numerical models for circulation and waves: Regional Ocean Modeling System (ROMS) and Simulating Waves Nearshore (SWAN) model, respectively. The circulation model is nested within larger-scale models that capture the tidal, regional, and wind-forced circulation of the Hawaiian archipelago. Two strategies are explored for circulation modeling: forcing by the output of the wave model and online, two-way coupling of the circulation and wave models. In addition, the circulation model alone provides the reference for the circulation without the effect of the waves. These strategies are applied to two experiments: (1) typical trade-wind conditions that are frequent during summer months, and (2) the arrival of a large winter swell that wraps around the island. The results show the importance of considering the effect of the waves on the circulation and, particularly, the circulation-wave coupled processes. Both approaches show a similar nearshore circulation pattern, with the presence of an offshore current in the middle beaches of Waikiki. Although the pattern of the offshore circulation remains the same, the coupled waves and circulation produce larger significant wave heights ( ≈ 10 %) and the formation of strong alongshore and cross-shore currents (≈ 1 m s −1 ).

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“…The mTc and supporting instruments were deployed at KNO (Sansone et al 2008a;Pawlak et al 2009) in late January 2011 in 13-m deep water, 400 m from shore ( hole, and then spreading the excavated sand over the instrument until the original sediment height was reestablished (Fig. 3).…”

Section: Field Deploymentmentioning

confidence: 99%

“…0.5-1 sand-ripple wavelength. There are pronounced opposing gradients in dissolved oxygen and other important biogeochemical constituents at these depths (e.g., Falter and Sansone 2000;Sørensen et al 2007;Pawlak et al 2009), and vertical oscillations of these gradients can vary rapidly at a given site in response to changes in the physical environment (Sansone et al 2008a;Fogaren 2010, Fogaren et al 2013). Precht and Huettel used dye to characterize this advective mechanism in the lab (2003) and in the field (2004), and Hebert et al (2007) measured transport from this mechanism by injecting fluorescent dye in the field at one depth over a range of surface wave conditions.…”

mentioning

confidence: 99%

Untitled

2014

Limnology & Ocean Methods

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A miniature thermistor chain (mTc) was developed to measure the subdiurnal variability of temperature in the upper layers of subtidal coastal permeable (sandy) sediments and across the sediment-water interface (SWI). The mTc has 15 precision thermistors (0.002°C accuracy) attached by narrow tines to a stainless steel backbone that connects to an electronics module, all of which is buried in the top 20 cm of the sediment. Instrument performance was tested by deploying the mTc in nearshore permeable sediment at the Kilo Nalu Observatory, Oahu, Hawaii over an 80-d period. The mTc reached thermal equilibrium with the adjoining sediment within a few days after deployment and then recorded the advective propagation of the sub-daily water-column temperature variation into the sediment. The data produced are consistent with predicted effects of surface waves on advective porewater transport: transport rate increased with wave height and decreased with depth below the SWI, and temperature time lag increased with depth below the SWI. Data from an independent, more deeply buried thermistor are in good agreement with the mTc time-series data, showing attenuated temperature variability and similar (but longer, as expected) thermal time lags. Because thermal variations in surficial sediments is dominated by advection in wavy environments, mTc subdiurnal temperature propagation data can be used to calculate advective transport across the SWI and as deep as 20 cm into the sediment (i.e., over depths where advection dominates over thermal diffusion).

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

Cited by 14 publications

References 11 publications

Effects of physical structure and processes on thin zooplankton layers in Mamala Bay, Hawaii

Effects of physical structure and processes on thin zooplankton layers in Mamala Bay, Hawaii

Different approaches to model the nearshore circulation in the south shore of O'ahu, Hawaii

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