Page Contents

Project Description
Hypotheses
General Methodology
Gears Used

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Introduction

The BITMAX project is designed to explore the physical and biological dynamics of the area known as the estuarine turbidity maximum (ETM) in Chesapeake Bay, Maryland.  Common in all coastal plain estuaries, the ETM is typically located near the foot of the fresh-saltwater interface, but can move up or down stream of this point.   In general, the ETM is characterized by high levels of suspended particles that are retained in the area due to physical forces.  The location and magnitude is determined from the changes in tides, wind events, and fluctuating river discharges that change water densities, circulation's, and velocities.  Sediments and organic matter that enter our estuaries from rainwater runoff are transported down estuary with the flow of the river and gradually settle to the bottom.  Where the river broadens out and this fresh-saltwater interface occurs, the river flow outward (to the ocean) slows and the settlement rate of the suspended particles increases.  Studies have shown that the physical forces acting in the ETM region resuspended and retain these settled particles and increase primary and secondary production making this area a potential important link to the estuaries food web.
Limited information and data are available on the dynamics of the Chesapeake Bay's ETM.  Schubel (1968) was one of the first to describe the Bay's ETM by collecting data on the mineralogy and concentrations and size compositions of the sediments and organic matter present in the upper bay from Turkey Point south to Tolchester Beach.  Results from this research described the mechanism of sediment entrapment that inspired future research on the importance of this dynamic feature within the Bay.  From 1996 through 2000, the ETM was studied as part of a multidisciplinary study that focused on the physical and biological interactions within the entire Chesapeake Bay under the name "Trophic Interactions Estuarine Systems" or TIES.  TIES was one out of three studies in the National Science Foundation's Land Margin Ecosystem Research program that investigated secondary production in the nations estuaries.  Preliminary data from the TIES study did support the hypotheses that the ETM is an area (1) of retention and resuspension of sediments and particles, (2) with increased zooplankton biomass (copepods), (3) that acts as a retention zone for certain fish larvae and/or nursery habitat, and (4) of increased feeding success for white perch young of the year (Boynton 1997).  North and Houde (2000) concluded larval and post larval striped bass and white perch survival maybe related to freshwater output which influences the ETM's magnitude.  A positive correlation was observed between river flow and young-of-the-year (YOY) abundance for striped bass and to a lessor extent white perch.

Work completed on the TIES cruises do not provide enough data to thoroughly explain the dynamics of ETM because that project focused on the entire bay as a whole and spent only a limited amount of time in the ETM region.  Objectives of the current study are to further explain the ETM's importance to the Chesapeake Bay's aquatic ecosystem over space and time and to evaluate the importance of the shoal areas that have not been sampled in the past.

Hypotheses

The specific hypotheses tested are listed below.

1. The ETM is maintained by tidal resuspension of an erodable pool of rapidly settling sediments and detritus.  The location of the nearbed pool is determined by convergent transport processes at the toe of the salt front. There is often a lag in response of the particle pool behind rapidly changing hydrodynamic forcing, leading to temporary spatial separation between the ETM and the salt front.
2. The copepod Eurytemora behaves as a passive settling particle with properties similar to sediments and detritus in the ETM.  This adaptation provides both a source of food, which enhances its production, and a refuge from predation.  Unlike sediments and detritus, Eurytemora does not adhere to the bottom.  Thus, it responds more quickly to rapidly changing hydrodynamic forcing.
3. Retention of fish larvae in the ETM region is dependent upon tidally timed and diurnal, vertical migratory behavior. The proximity of their freshwater spawning habitat (<1 psu) with the ETM and high concentrations of Eurytemora promotes recruitment of anadromous fish.
4. Most of the available freshwater habitat for anadromous fish larvae is over the lateral shoals, outside of the channel.  Nevertheless the quality of habitat is lower on the shoal because of physical separation from zooplankton prey that is most concentrated in the channel.

Methodology

Our study area is confined to the upper bay from the Chesapeake Bay bridge to the lower Elk River near Turkey Point.  Each year for two years starting in the spring of 2001, three-8 day cruises along the channel axis will be conducted (May- July-Oct) aboard the RV Henlopen.  In year 1, biological samples from the adjacent shoals will be taken during the July cruise only aboard the RV Orion, however, in 2002, the shoals will be sampled for biological data on all three cruises.  Physical data from the shoals will be collected from the RV Coot on all cruises (2000-01).

In general, each cruise will start with an axial hydrographic survey of the sampling area in order to locate the ETM and map the water properties using a series of CTD casts.  To help monitor the ETM throughout the cruise a bottom mounted ADCP (see below), a mooring with surface, mid-water conductivity, temperature, turbidity, and salinity instruments, and a tripod with and ADV, C/T, and turbidity sensors will be deployed at the estimated ETM location.  After the mooring deployment, the primary sampling begins with either mid-water trawling (juvenile fish), tucker trawling (larval fish), a 25 hour time series survey (combination of biological and physical sampling), or hydrographic survey including ADCP, TAPS, LISST, and pump samples (see below) depending on the time of day and cruise.  A CTD cast is made prior to any other samples at all stations.

Gears 

 CTD  (Conductivity-Temperature-Depth)

A CTD is an instrument that measures certain water properties.  Our CTD measures conductivity, temperature, depth, turbidity, florescence, irradiance, and dissolved oxygen.  At each station during our research cruises a CTD cast is made that provides us with a profile of the water column.  These data are extremely important in order to locate the salt front and ETM.

ADCP's  Acoustic Doppler Current Profiler's

An ADCP is an instrument that uses ultrasonic pulses to measure the current velocity and direction throughout the water column.  The major advantage of an ADCP compared to the traditional current meter is that the measurements are instantaneous and highly accurate.  There is no need to lower a unit to the bottom and retrieve it every time a current profile is needed.  ADCP's can be mounted on a boat permanently or temporarily, to a cage on the bottom, or to a buoy system.  On all cruises, an ADCP is mounted on the Bay floor and one is used onboard the RV Henlopen and RV Coot for surface to bottom measurements at selected stations.

LISST  Laser Particle Sizing Instrument

Suspended particle properties are measured with the LISST.  A state of the art instrument, the LISST uses laser diffraction to distinguish particle sizes and distributions within the entire water column.  These data are used to examine sediment transportation and the degree of sediment resuspension.

TAPS  Tracor Acoustic Profiling System

This instrument uses hydroacoustics to measure the size and number of zooplankton in the water column.  These data will be compared to results from Niskin bottle samples and tucker trawls (see below).  TAPS is connected to the CTD array and is used at nearly all stations.

Niskin Bottles

On the CTD array, a set of Niskin bottles are attached that can be opened and closed at depth to capture water samples.  These samples are usually taken at the bottom, mid, and surface waters.  Together with the TAPS data zooplankton abundance's and distributions can be determined.  These results will describe the prey available at depth to the larval and juvenile fishes.

Tucker Trawl

A tucker trawl is a gear that uses multiple nets to fish at discrete depths to catch zooplankton, and larval and juvenile fish.  We use three nets (28O microns) on a 1-m2 trawl to fish the bottom, mid, and surface waters.  To open and close the nets at depth a "messenger" is sent down the tow line that triggers the closing of a net and the opening of another net.  Each net is open or sampled at one of three depths (bottom,midwater,surface) for 2 minutes for a total of 6 minutes.  Once onboard, the sample is preserved in ethyl alcohol for future species identification and enumeration.

Mid-water Trawl

This gear is towed obliquely from the surface to the bottom at 2-minute intervals for a total of 20-minutes to capture young-of-the-year and juvenile fish.  The trawl mouth is 18-m2 and the codend mesh size is 6 mm.  Fish collected are measured and weighed and either discarded, frozen, or persevered in ethanol.  Any fish saved is taken back to the lab for stomach and aging analyses.  Data will be used to help describe the predatory-prey relationships, fish distribution in relation to physical forces, and fish survival.

Page created June 2001
Revised August 2003