Nortek Vectors reveal turbulent water flow effects in marine ecosystems

Researchers at the Georgia Institute of Technology employed six Nortek Vector velocimeters to better understand how turbulent water flow and currents in intertidal salt marsh and oyster reef systems impact foraging by predators and the wider community-level effects.

Lab groups led by Marc Weissburg and Donald Webster at the Georgia Institute of Technology set out in 2010 to understand how physical processes influence odor-mediated interactions in natural environments. Specifically, research focused on how turbulent water flow, measured using Nortek Vector velocimeters, in intertidal salt marsh and oyster reef systems impacts foraging by predators and the subsequent community-level effects on population abundances and distributions.

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For example, blue crabs show reduced foraging efficiency in high-velocity and high-turbulence flows, which has been shown to enhance survival of bivalve prey in the field. Unfortunately, relatively little is known about the velocity and turbulence characteristics in the highly variable flow environments where these organisms forage, which prevents adequate formulation of hypotheses on community-level effects.

To enhance understanding of odor-mediated processes in these environments, the Weissburg and Webster lab groups developed three main goals:

1. To document the spatial and temporal variation in turbulent flow parameters that odor-mediated predators may be exposed to while foraging. To this end, they looked at: a) How similar turbulent flow parameters were within individual sites versus between sites, and whether they could use this information to suggest sampling regimes that encompass variation that organisms may encounter in the field. b) How turbulent flow parameters varied between sites and under different large-scale tidal forcing (i.e. neap versus spring tides).

2. To determine the influence of wave-induced error on measurements of turbulent flow parameters in intertidal salt marsh systems.

3. To investigate long-term (over three months and between years) changes and variation in turbulent flow parameters within salt marsh systems.

To explore these issues, graduate student and 2009 Nortek Student Equipment Grant winner Miranda Wilson deployed six Nortek Vector velocimeters at multiple intertidal sites within Wassaw Sound, Georgia during June to August 2010.

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Throughout the sampling period, two instruments were located at a reference site concurrent to the other four instruments being rotated between three other sites. In order to assess differences in turbulent flow within each site, they placed all four instruments within each site simultaneously, arranged 1 m, 5 m and 10 m from a reference instrument. They collected time series of three velocity components for four complete tidal cycles at each site for three tidal types (neap tide, normal tide and spring tide). For each deployment, they sampled three-dimensional velocity components at 16 Hz during 5-minute bursts, separated by 10 minutes. All instruments, regardless of site or deployment, were mounted such that the sampling volume was located approximately 10 cm above the substrate in order to examine the near bed region inhabited by blue crabs and other foragers.

As shown below, there were significant differences (analyzed using a two-way analysis of variance) in values of mean u-velocity as a function of site and tidal type, as well as a significant interaction between site and tidal type. Mean values of u-velocity are lower during neap tide and increase with the strength of tidal forcing (agreeing well with previous studies). However, this pattern was site specific, with the SN site showing similar values for u-velocity for neap and normal tides. Mean values of u-velocity were also smaller at the DMH site. Site- and tidal-type-specific patterns of turbulent flow properties may result in highly context-specific impacts on predator foraging efficiency and their subsequent effects on prey populations. For example, blue crabs may exhibit higher predation rates at DMH and during neap tide because of decreased foraging efficiency in the higher-velocity flow regimes found at other sites and during other tidal types.

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Thank you to Miranda Wilson for sharing the group’s work with Nortek USA.


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