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NOAA Technical Memorandum GLERL-116

SETTLING PARTICLE FLUXES, AND CURRENT AND TEMPERATURE PROFILES

IN GRAND TRAVERSE BAY, LAKE MICHIGAN

Brian J. Eadie1

Gerald S. Miller1

Margaret B. Lansing1

Andrew G. Winkelman2

1NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105

2NOAA GLERL/University of Michigan, Cooperative Institute for Limnology and Ecosystems Research

Ann Arbor, MI 48105

Great Lakes Environmental Research Laboratory

Ann Arbor, Michigan

February 2000

UNITED STATES

DEPARTMENT OF COMMERCE

William Daley

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

D. James Baker

Under Secretary for Oceans

and Atmosphere/Administrator

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S.DEPARTMENT OF COMMERCE
2

NOTICE

Mention of a commercial company or product does not constitute an endorsement

by NOAA. Use of information from this publication concerning proprietary

products or the tests of such products for publicity or advertising purposes is not

authorized. This is GLERL Contribution No. 1156.

This publication is available as a PDF file and can be downloaded from GLERL’s

web site: www.glerl.noaa.gov. Printed can be requested from the GLERL Publications Unit, 2205 Commonwealth Blvd., Ann Arbor, MI 48105.

pubs.glerl@noaa.gov
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Table of Contents

Abstract .............................................................................................................................................. 5

Introduction ........................................................................................................................................ 5

Methods.............................................................................................................................................. 7

Results and Discussion .................................................................................................................... 12

References........................................................................................................................................ 22

Appendix 1.--Values of all trap data collected in Grand Traverse Bay, Lake Michigan

during 1997-1999 ....................................................................................................... 24

Appendix 2.--Acoustic Doppler Current Profiler (ADCP) current meter and temperature

data collected in Grand Traverse Bay, Lake Michigan during 1997.......................... 25

Tables

Table 1.--Sample Splitting Data ....................................................................................................... 10

Table 2.--Accuracy and precision of sample splitting (n=5; all mixtures)....................................... 10

Table 3.--Mooring information and Acoustic Doppler Current Profiler setup parameters .............. 11

Figures

Figure 1.--Bathymetry of Grand Traverse Bay (Holcombe, et al., 1996) .......................................... 6

Figure 2.--Comparisons of replicate trap flux measurements ............................................................ 8

Figure 3.--Locations of sediment traps (Station numbers 4-9) and ADCP (stations 5 and 7) ......... 11

Figure 4.--Contours of backscatter strength, U and V current components, and water temperature

from mooring C5. Positive U is towards the east; positive V towards the north. The

white areas in the U and V panels in June indicates that the acoustic backscatter was

insufficient to compute a credible current velocity......................................................... 13

Figure 5.--Sequencing trap deployment schedules. Symbols represent times of deployment

and retrieval .................................................................................................................... 17

Figure 6. --A) Mass flux profiles for stations 4, 5, and 6 for the spring-summer of 1997 .............. 18

Figure 6. --B) Mass flux profiles for stations 7, 8, and 9 for the same period. Split panels

represent two deployments, the data for stations 4,5, and 7 are a combination of

non-sequencing traps and sequencing traps (see Appendix I) ........................................ 19
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Figure 7.--Mass fluxes measured at the sequencing traps from stations 4, 5, and 7. All trap

intervals were 7 days during these deployments ............................................................ 20

Figure 8.--Mass fluxes at all sequencing trap sites for the entire period of deployment,

May 1997 through August 1999 ..................................................................................... 21
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Settling Particle Fluxes, and Current and Temperature Profiles in Grand Traverse Bay

Brian J. Eadie

Gerald S. Miller

Margaret B. Lansing

Andrew G. Winkelman

Abstract. Settling particle fluxes and mass flux profiles are reported for trap samples

collected at five stations in Grand Traverse Bay, Lake Michigan during 1997-1999. Trap

collection precision is estimated, and 10 cm and 20 cm diameter sequencing traps are

inter-calibrated using data from traps deployed in replicate on specially constructed

brackets. Temperature data is reported for two stations (during June-September 1997).

Acoustic Doppler Current Profilers (ADCP) current meter data and contours of backscatter strength, U and V current components, and water temperature are included for the 95

m station. All data is available online in ASCII and MS Excel formats at ftp://

ftp.glerl.noaa.gov/publications/tech_reports/glerl-116.

INTRODUCTION

In 1997 the EPA-Ecological Assessment Program funded a 3-year coordinated proposal from GLERL, the University of Maryland, and Michigan State University whose overall objective was to quantify the absolute and relative

flows of bioaccumulative organic contaminants through the pelagic, epi-benthic, and benthic food webs of Grand

Traverse Bay in northern Lake Michigan. We hypothesize that efficient scavenging of atmospheric-derived

contaminants from surface waters delivers large chemical fluxes seasonally to the epi-benthic food web, and that

this process pumps recent atmospheric loadings into the Great Lakes fisheries. Specific objectives, with abbreviated approaches in italics, were:

1. To quantify the seasonal fluxes of organic carbon and associated contaminants from the surface waters to near

the sediment-water interface. Sequencing sediment traps will be deployed below the thermocline and near the

lake floor to collect settling particles for chemical characterization.

2. To quantify trophic linkages in the epi-benthic food web, with emphasis on the relative importance of seasonally-dependent settling fluxes from surface waters and ingestion of infaunal organisms. Sculpins and other

epi-benthic fishes, macrozooplankton, and infaunal organisms collected seasonally will be analyzed for gut

contents, stable isotope composition, and organic contaminant levels.

3. To quantify, through statistical analysis of contaminant fingerprints and bioenergetics modeling, the relative

magnitudes exposure of sedimentary- and atmospherically-derived contaminants to the Great Lakes fisheries.

Principal components analysis will be used to compare the PCB congener distributions in water, settling

particles, sediments, and biota. A bioenergetics model of the benthic food web will be used to estimate transfer of carbon from infaunal organisms and settling particles.

The overall strategy of this field study was to carefully characterize the settling flux of organic matter and chemical contaminants from surface waters during three seasons in the northern Great Lakes and to assess the impact of

this flux on carbon and contaminant flows through the benthic food web. Stable isotopes of carbon and nitrogen

combined with classical gut content analysis of epi-benthic fishes are being used to characterize the benthic food

web. Polychlorinated biphenyl congeners are being studied as representatives of the larger class of

bioaccumulative organic contaminants and as important pollutants in their own right. This study has been conducted in Grand Traverse Bay (Figure 1), a deep, oligotrophic embayment of Lake Michigan that provides both

typical northern Great Lakes conditions and logistical convenience.
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For many constituents in the Great Lakes, the resuspension of surface sediments, (which contain large inventories

of certain nutrients and contaminants deposited over the past few decades) presently results in much greater

fluxes than from external inputs (Eadie et al., 1984, 1989; Eadie and Robbins, 1987; Robbins and Eadie, 1991;

Brooks and Edgington, 1994). For example, Lake Michigan is the sixth-largest lake in the world and has a

hydraulic residence time of about 62 years (Quinn, 1992). For particle-reactive constituents, internal removal

through sedimentation is much more rapid than this. Radiotracer studies with 239Pu (t1/2 = 25,000 years) and 137Cs

(t1/2 = 30.2 years) show that >95% of these tracers were removed from the water and transferred to sediments

within a few years (Wahlgren et al., 1980; Eadie and Robbins, 1987). Although initial removal of particle-reactive

tracers from the water is rapid (a few years), a small residual concentration remains in the water (either on

particles, in biota, or in solution) and now diminishes exponentially on a time scale of decades. Studies of

Thomann and DiToro (1983), Eadie et al. (1984), and Robbins and Eadie (1991), have shown that the small

amount remaining in the system is primarily the result of an annual cycle of sediment resuspension and redeposition that releases constituents from sediments back into the water. The long-term decline of 239Pu and (decaycorrected) 137Cs in the lake has about a 20-year time constant (Wahlgren et al., 1980), which probably characterizes the net rate of incorporation of these tracers into permanent sediments (Robbins, 1982).

The processes of particle flux and resuspension has been examined in Lake Michigan through the use of sediment

traps since the mid 1970s (Wahlgren et al., 1980; Eadie et al., 1984). These cylindrical devices are moored at

Figure 1. Bathymetry of

Grand Traverse Bay

(Holcombe, et al., 1996)
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selected depths to intercept materials settling to the bottom. Traps provide an efficient tool for the collection of

integrated samples of settling materials for detailed analysis. Measuring the mass collected allows us to calculate

the gross downward flux of particulate matter and associated constituents and to calculate both mass and constituent settling velocities.

GLERLs primary role in the study was to provide expert sampling advice, to collect settling materials using our

autosequencing sediment traps and provide subsamples to co-investigators for constituent analysis, and to participate in the ultimate analysis of all of the data collected in the project. During the summer of 1997, GLERL needed

to test two new Acoustic Doppler Current Profilers (ADCP) and these, along with temperature recording equipment, were deployed in the western arm of the bay providing several months of data where no other current data

were available.

METHODS

Two types of sediment traps were used in this study. Our simple trap, based on reviews of various designs

(Bloesch and Burns, 1980; Gardner, 1980a,b), is a cylinder 10 cm in diameter with an aspect ratio of 5:1 above

the funnel opening to a 500 ml polyethylene bottle. In 1990 we developed a sequencing trap modified from the

designs of Baker et al. (1988), and Jannasch et al. (1980). These are also cylindrical, but with an inner diameter of

20 cm and an aspect ratio of 8:1 above the funnel. A computer-controlled carousel contains 23 polyethylene

bottles (60 ml), which rotate under the funnel at preprogrammed intervals. An electric motor rotates the carousel

and uses a single-pole detent switch to provide position feedback. A microprocessor-based controller, developed

in house, runs the motor based on a schedule and records confirmation of each rotation using nonvolatile memory.

A battery pack allows up to 2 years of operation. Cylindrical traps have a high collection efficiency in low current

lake environments and have proved satisfactory in many lake studies (Bloesch and Burns, 1980; Eadie et al.,

1984). The accuracy of calculated fluxes is poorly understood, but depends on the trap design, the types of

particles in the fluid, and the currents at the site (Gardner, 1980b; Hawley, 1988; Gardner, 1996).

Trap Sampling Precision: To estimate trap collection precision and intercalibrate between the 10 cm diameter

traps and the 20 cm diameter sequencing traps, a series of deployments were made between 1984 and 1989 in

regions with a wide range of fluxes on specially constructed brackets to assure identical depth and exposure. The

20 cm traps used in these tests did not have sequencing capability, but were identical in other aspects. The traps

were deployed as anchored arrays using subsurface buoyed 1/4” steel cable. The 500 ml bottles in the simple traps

were poisoned with 25 ml of chloroform and filled with distilled water prior to deployment. The 60 ml polyethylene collection bottles in the sequencing trap were poisoned with 6 ml of chloroform and filled with distilled water

immediately prior to deployment. This concentration of chloroform is an effective preservative (Lee et al., 1989)

and results in a supersaturated solution, with beads of chloroform remaining after retrieval. The sequencing traps

are deployed with the collection funnel feeding to an empty opening (no collection bottle). After a preprogrammed period of time, the carousel will move the first collection bottle under the funnel. The remaining 22

bottles will follow in a preprogrammed sequence. After retrieval, the sample bottles are removed from the traps

and transported to the laboratory in cold storage (4o

C). The traps have on-board intelligence that records the time

of each sequence and various system checks.

When deployed in replicate, both the 10 cm and 20 cm traps showed good repeatability with paired t-test showing

equal means (P<0.05) in all four comparisons (Figure 2). The 10 cm traps replicate with an average difference

between pairs of ±11%, and the 20 cm traps (with the 8:1 aspect ratio) replicate with an average difference

between pairs of ±14%. An intercomparison of capture efficiency between the 10 and 20 cm traps resulted in a

design change from an aspect ratio of 5:1 to 8:1 for the larger diameter traps. A larger trap diameter results in a

higher trap Reynolds number, with presumably lower collection efficiency. There was little bias (slope = 1.05)

between the two types of traps, and the scatter was much reduced with the extended aspect ratio, which became

our standard for 20 cm diameter traps. All sequencing traps in this study have an aspect ratio of 8:1.
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Figure 2. Comparisons of replicate trap flux measurements. (a) Replicate 10 cm (4”) diameter traps placed on

brackets for simultaneous deployments. Correlation coefficient is high and the traps replicate with an average

difference between pairs of ± 11%. (b) A similar treatment of 20 cm (8”) diameter traps with similar results. (c)

A comparison of 10 cm and 20 cm traps with 5:1 aspect ratios. The scatter was worse than for individual

pairs of the same size. (d) Comparison of 10 cm (5:1 aspect) with 20 cm (8:1), extended aspect ratio. There

was little bias (slope = 1.05) between these pairs and the scatter was much reduced with the higher aspect

ratio.
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Sample Handling: After arrival at the lab, the trap samples are allowed to settle in a refrigerator for 1 day, then

overlying water is carefully siphoned off and the residual is split into two equal portions. One half of the sample is

transferred into a precleaned glass jar and frozen for subsequent PCB analysis. The other half of each sample is

freeze dried, then weighed and transferred into precleaned scintillation vials for storage in a freezer. This fraction

is used for analysis of nutrients, stable isotopes, etc. All trap samples were weighed on an analytical balance

calibrated to within ±1 mg with known standard weights during each weighing session. Flux is equal to the mass

collected divided by the length of collection and the trap cross section. To calculate fluxes from the trapped

material, a reliable measurement of the total weight is required. In previous studies we always split sediment trap

samples after they were freeze dried and weighed.

Because this project involves quantification of PCBs and other trace organic contaminants that require wet

extraction procedures, it is necessary to accurately divide the samples into two fractions while still wet. A total

weight of the sample can then be estimated from the freeze-dried split of the total material.

After further literature and catalog searches we purchased a stainless steel dry sediment sample micro-splitter

(Model SP-241x; Gilson Co. Inc., PO Box 677, Worthington, OH, 43085-0677). This model has a reservoir of

approximately 80 ml into which the sample is poured. A bottom vent is then opened, and the sample can be

poured into 30 evenly spaced (1 mm) slots. The even numbered slots empty into a stainless steel tray on the left,

and the odd numbered slots empty on the right. We then tested this device for our wet sample splitting requirements and came up with the following satisfactory results.

Sample Matrices: Our objective was to determine the precision of splitting and the ratio of the two samples. To

do this, four samples were examined:

1. Distilled water (DDW).

2. Distilled water (55 ml) + chloroform (6 ml); our standard trap poison solution.

3. Ground Lake Michigan sediment in #2.

4. A sediment trap sample from Lake Michigan near LMMB station 6; 5 m above bottom from a 100 m deep

station.

Five replicates of each matrix were made. The samples were poured into the splitter, and the left and right trays

were weighed for matrices 1 and 2. For matrices 3 and 4, the left and right trays were emptied into preweighed

beakers that were dried at 90o

C then weighed. The data are presented in Table 1.

Excellent replication was obtained in the tests (Table 2). Matrices 3 and 4, with sediment or trap materials, were

split into two equal portions without bias. In other studies we have determined that replicate traps placed side by

side have a coefficient of variation (100*sd/mean) of a little less than 10%. The splitting errors appear substantially smaller and will not degrade our interpretation of the data.

Our standard splitting procedure is:

1. Allow the 60 or 500 ml trap bottles to settle for approximately 24 hours in refrigeration.

2. Extract approximately 25 ml of the overlying water with a syringe, remove excess overlying water from 500

ml trap bottles.

3. Pour the remaining trap sample through a 500 um (355 um for year 2) screen into the splitter reservoir.

4. Split by opening the bottom valve.

5. Rinse with the water from step #2.

6. Further rinse (if needed) with (CHCl3) pre-extracted DDW.

7. Pour right tray back into trap sample bottle for PCB, etc.

8. Pour left side into pre-cleaned beakers for freeze drying, weighing, and analyses.

9. Transfer >500 um (>355 um for year 2) materials to precleaned, preweighed filters.

10. Rinse screen and splitter under faucet, then rinse with pre-extracted DDW.
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Total Dry Wt (left) Wt (Right) Fract left Fract Rt

 Wt (g) (g) (g)

DDW 33.4473 31.4184 0.516 0.484

DDW 32.5575 30.962 0.513 0.487

DDW 32.9653 30.9628 0.516 0.484

DDW 32.2945 29.296 0.524 0.476

DDW 31.7108 29.3542 0.519 0.481

DDW(55):CHCl3(6) 31.6683 33.0099 0.490 0.510

DDW(55):CHCl3(6) 30.2318 31.3103 0.491 0.509

DDW(55):CHCl3(6) 31.2056 31.5524 0.497 0.503

DDW(55):CHCl3(6) 30.8368 31.6704 0.493 0.507

DDW(55):CHCl3(6) 31.0031 33.3368 0.482 0.518

Grnd Sed in DDW(55):CHCl3(6); DRY 0.5639 0.2779 0.286 0.493 0.507

Grnd Sed in DDW(55):CHCl3(6); DRY 1.387 0.6952 0.6918 0.501 0.499

Grnd Sed in DDW(55):CHCl3(6); DRY 2.9349 1.5035 1.4314 0.512 0.488

Grnd Sed in DDW(55):CHCl3(6); DRY 3.9479 1.9049 2.043 0.483 0.517

Grnd Sed in DDW(55):CHCl3(6); DRY 5.1343 2.5843 2.55 0.503 0.497

Trap from 5m AB @ 100m sta.; DRY 0.4434 0.2224 0.221 0.502 0.498

Trap from 5m AB @ 100m sta.; DRY 0.7476 0.367 0.3806 0.491 0.509

Trap from 5m AB @ 100m sta.; DRY 1.2745 0.6423 0.6322 0.504 0.496

Trap from 5m AB @ 100m sta.; DRY 1.3124 0.648 0.6644 0.494 0.506

Trap from 5m AB @ 100m sta.; DRY 2.2998 1.1689 1.1309 0.508 0.492

Table 1. Sample Splitting Data

Mixture Left Side Right Side P (paired t)

Fraction Fraction

DDW 0.518 ± 0.004 0.483 ± 0.004

DDW + CHCl3 0.491 ± 0.005 0.509 ± 0.005

Ground Sediment 0.498 ± 0.010 0.502 ± 0.010 0.77

Trap 0.500 ± 0.006 0.500 ± 0.006 0.93

Table 2. Accuracy and precision of sample splitting (n=5; all mixtures).

Current and Temperature Measurements

Two current meter/water temperature moorings were deployed 18 June 1997 (Figure 3) in the western arm of

Grand Traverse Bay. A 300 kHz RDI Workhorse Acoustic Doppler Current Profiler (WH-ADCP) was moored 9 m

below the surface in a downward-looking mode at each site. A 810 mm syntactic foam subsurface buoy, designed

for the WH-ADCP, provided about 100 kg reserve buoyancy. Mooring information and ADCP setup parameters

are given in Table 3. On the southern mooring, C5, an Aanderaa 40-m thermistor chain measured hourly water

temperatures at 4 m intervals from depths of 13 m to 53 m. Five Brancker T-1000 temperature loggers were

attached to the mooring line at C7 with a 4 m spacing beginning at -13 m. The ADCPs also measured water

temperatures at the transducer head at the 9 m depth. A hardware-limiting problem in both WH-ADCP’s resulted

in low echo intensities for cell #1 that caused the internal data quality checking routines to reject the velocity data

from that cell. At mooring C7, the current and echo electronics in the ADCP failed after 6 days, but the water

temperature measurements continued for the entire deployment and are included with the temperature logger data.
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Moorings C5 C7

Location 44o49.39’ 85o37.41’ 45o01.68’ 85o33.05’

Water Depth 94.5 m 118.0 m

Deployed 06/18/97 1030 EST 06/18/97 1300 EST

Retrieved 09/16/97 1700 EST 09/17/97 1100 EST

Current meters:

ADCP 300 kHz 300 kHz

No. cells 45 55

Cell size 2 m 2 m

# pings 400 400

Std deviation 0.5 cms-1 0.5 cms-1

Sampling interval 30 min 30 min

ADCP Depth 8.8 m 8.8 m

Depth first good cell 15 m 15 m

Depth last good cell 89 m

Temperature:

Instrument Thermistor chain Temperature loggers

Sampling interval 1 hour 15 min

Depths 13-53 m, 4 m intervals 13-29 m, 4 m intervals

Available Data

Currents 06/18/97-09/16/97 06/18/97-06/24/97

Temperature 06/18/97-09/16/97 06/18/97-09/16/97

Table 3.--Mooring Information and Acoustic Doppler Current Profiler Setup Procedures.

Figure 3. Locations of sediment traps (Station numbers 4-9) and ADCP (stations 5 and 7).
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An ADCP transmits acoustic pulses from a Janus configured four-transducer assembly and receives return echoes

reflected from scatterers in the water, such as plankton, sediment, or bubbles. Current velocities are determined

using the Doppler principle. Two acoustic beams are required to compute one horizontal current component and

one vertical velocity. A second pair of transducers computes the other horizontal component and a second vertical

velocity. The difference between the two independent measurements of vertical velocity is termed the error

velocity and is a useful data quality indicator. Nonzero error velocity values indicate either that the equipment is

malfunctioning or that there is horizontal nonuniformity in the water. Range-gating the echo signal into successive

segments are processed independently thereby producing a profile. Parameters routinely recorded by ADCPs are

vertical profiles of horizontal velocity, vertical velocity, error velocity, and echo intensity. Vertical velocity

accuracy is not as readily verifiable because of the small magnitudes and difficulty of making validating measurements and have not been included in this report. Comparisons of ADCPs and conventional mechanical current

meters, for example Savonius rotor meters, show very good correlation of horizontal velocities (Miller and Saylor,

1993; Appell et al., 1991; Pettigrew et al., 1986).

Echo intensity, a measure of the returning signal strength reflected by suspended particles, is generally reported in

terms of the volume scattering strength (Medwin and Clay, 1998).

Sv

 = 10log10( IR / II )

where Sv

 is the volume backscattering strength in dB, IR is the returned intensity, and II the incident or transmitted

energy. Received backscatter power is a nonlinear function of the strength of the transmitted power, properties of

the receivers, the loss of energy due to sound absorption, beam spreading, and the effective area of the reflecting

particles. Obtaining absolute backscatter values, that is, values that are instrument independent, require that each

unit go through extensive calibration procedures, procedures that have not been routinely available.

Using a working version of the sonar equation (Deines, 1999) and ‘typical’ values for strength of the transmitted

power and properties of the receivers characteristic of the WH-ADCP, the echo intensity output was adjusted for

power attenuation losses due to beam spreading and water absorption.

The ADCP current and temperature data are contained in ASCII and MS Excel files and can be found at: ftp://

ftp.glerl.noaa.gov/publications/tech_reports/glerl-116. A data description is given in Appendix II.

Contour plots of data collected at mooring C5 are shown in Figure 4.

RESULTS AND DISCUSSION

Prior to this effort, there was no trapping done in Grand Traverse Bay, thus we relied on our 15-year record of

sampling in reasonably similar environments for selecting locations for trap placement. Long-term average mass

fluxes measured from 1978 to 1992 at a 100 m deep station, 25 km offshore in southeastern Lake Michigan

exhibit profiles of mass flux with an exponential increase toward the bottom. From late December through early

June, Lake Michigan is virtually isothermal and well mixed. Average fluxes during this period are high throughout the water column, but there is clear evidence of a benthic nepheloid layer (BNL). During the stratified period

(June-December), the upper half of the water column becomes isolated from the large inventory of materials in

the sediments, although episodic mixing does occur during upwellings. A BNL is still clearly evident from the

mass flux profile.
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Figure 4. Contours of backscatter strength, U and V current components, and water temperature from mooring

C5. Positive U is towards the east; positive V towards the north. The white areas in the U and V panels in June

indicates that the acoustic backscatter was insufficient to compute a credible current velocity.
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Figure 4 (cont). Contours of backscatter strength, U and V current components, and water temperature from

mooring C5. Positive U is towards the east; positive V towards the north. The white areas in the U and V panels in

June indicates that the acoustic backscatter was insufficient to compute a credible current velocity.
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Figure 4 (cont). Contours of backscatter strength, U and V current components, and water temperature from

mooring C5. Positive U is towards the east; positive V towards the north. The white areas in the U and V panels in

June indicates that the acoustic backscatter was insufficient to compute a credible current velocity.
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All trap data are presented in Appendix 1.

Overall, five (week-long) cruises of the R/V SHENEHON were conducted in Grand Traverse Bay.

Co-Principal Investigators and graduate students participated in all cruises, collecting water,

particulate, and biological samples as part of their overall sampling strategy. Cruises left Muskegon on 5/5/97, 6/16/97, 9/15/97, 8/31/98, and 8/2/99, stopping overnight in Frankfort and arriving

in the bay on the afternoon of the second day.

Results: Spring-Summer 1997

In early May 1997 profiles of non-sequencing traps (4” diameter) were deployed at stations 6, 8, and 9, along with

profiles of sequencing and non-sequencing traps at stations 4, 5, and 7 (see Figures 3 and 5). The primary objective of this extensive sampling was to get some preliminary flux data from a system where none had been previously collected.

The non-sequencing trap arrays at stations 6, 8, and 9 were retrieved and redeployed after the onset of thermal

stratification, approximately 40 days after deployment, and mass fluxes calculated. All traps were retrieved in

mid-September, and sequencing traps were redeployed at stations 5 and 7. Subsamples were distributed for

constituent analysis.

Results from the first deployments are displayed in Figure 6 (a and b). Measured fluxes for the unstratified MayJune period at stations 6, 8, and 9 were low compared to open Lake Michigan values (average 5 g/m2

/d near the

surface), and appeared more like open lake flux profiles during the stratified period when the lake is decoupled

from sediment resuspension. All of the profiles exhibit the exponential increase in mass flux near the bottom

observed in all Great Lake profiles and attributed to a benthic nepheloid layer. The samples from trap 6 exhibit a

peculiar profile -- some sample was lost from the bottom trap during retrieval in June, but the low flux values at

75 m are unexplained at this time.

The sequencing trap mass flux values from relatively shallow (45 m) station 4 were unexpectedly low for the

entire spring-summer period (Figure 7). The three samples immediately after the onset of stratification were the

highest recorded for the entire period of deployment and will be examined for biogenic silica. This may be the

spring bloom in the southeastern portion of the bay.

There was a small peak in mass fluxes at the near-surface of station 5 and 7 in late spring, but except for the initial

interval immediately after deployment, the qualitative pattern of mass fluxes were not synchronous within the

eastern arm of the bay. Near-bottom fluxes were several times higher at station 7 than at station 5 and may point

to a region of sediment focusing near station 7.

Results: Sequencing traps 1997-1999

Two sequencing traps were deployed in mid-September 1997 at station 5 (30 and 91 m) and three at station 7 (15,

30, and 115 m). Trap sampling intervals were set at 15 days. These stations were selected by collaborators as the

primary water column stations and were sampled approximately monthly during the 1997-98 season. These traps

were retrieved and others redeployed at 30 m below the surface at stations 5, 7, 8, and 9 in early September 1998.

These final deployments had collection intervals of 12-15 days and were retrieved in early August 1999. All of the

successful samples are illustrated in Figure 8.

These deployments have provided 2 complete years of mass fluxes and samples for analyses from 30 m below the

surface at stations 5 and 7 in the eastern and nearly 1 full year from the same depth in the western arm (station 9).

There is clear evidence in these records of strong resuspension during the unstratified periods in the 2 years.

These fluxes are substantially higher at the more southern station (5) than at station 7. The qualitative flux patterns do not appear to be well synchronized between these two stations implying a complex transport of particu-
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Figure 5.--Sequencing trap deployment schedules. Symbols represent times of

deployment and retrieval.
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FIGURE 6.-- a) Mass flux profiles for stations 4, 5, and 6 for the spring-summer of 1997.
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FIGURE 6.--b) Mass flux profiles for stations 7, 8, and 9 for the same period. Split panels represent two

deployments, the data for stations 4,5, and 7 are a combination of non-sequencing traps and sequencing

traps (see appendix I).
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Figure 7.-- Mass fluxes measured at the sequencing traps from stations 4, 5, and 7. All trap intervals were 7 days

during these deployments.
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Figure 8.--Mass fluxes at all sequencing trap sites for the entire period of deployment,

May 1997 through August 1999.
22

late matter within the western arm of the bay. Peak mass fluxes at station 9, in the eastern arm, are about the same

magnitude during the unstratified period, but not well correlated temporally.

Our results clearly support the hypothesis that there is significant sediment-water exchange of chemical constituents during the unstratified periods when massive sediment resuspension events occur.

REFERENCES

Appell, G.F., P.D. Bass, and M.A. Metcalf. Acoustic Doppler current profiler performance in near surface and

bottom boundaries. IEEE J. of Oceanic Engineering 16:390-396 (1991).

Baker, E.T., H.B. Milburn, and D.A. Tennant. Field assessment of sediment trap efficiency under varying flow

conditions. Journal of Marine Research 46:573-592 (1988).

Bloesch, J., and N.M. Burns. A critical-review of sedimentation trap technique. Schweiz. Z. Hydrol. 42(1):15-55

(1980).

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Appendix 1

Values of all trap data collected in Grand Traverse Bay, Lake Michigan

during 1997–1999.

Data are contained in both ASCII and MS Excel files at:

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-116.
25

Appendix 2

Acoustic Doppler Current Profiler (ADCP) current meter and temperature

data collected in Grand Traverse Bay, Lake Michigan

during summer 1997.

Data are contained in both ASCII and MS Excel files at:

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-116.

File format is shown below.

Water temperature data mooring C7 (30-minute data; the ADCP temperatures at the 9m depth are included):

gtb_c7allt.txt

Information header

Day-of-yr, time(est), year, rec #, water temp(1-6) (deg C)

 Water temperature data mooring C5 (hourly data; the ADCP temperatures at the 9 m depth are included):

gtb_c5allt.txt

Information header

Day-of-yr, time(est), year, rec #, id #, watertemp(1-12) (deg C)

(Missing data = -0.45)

ADCP current velocity and backscatter data mooring C5 (hourly data):

gtb_c5_97.txt and gtb_c5_97a.txt

Header: rec #, year, mon, day, hour(est), min, water temp(C)

Depth1

(m), U1(cm/s), V1(cm/s), Backscatter1(dB)

Depth2(m), U2(cm/s), V2(cm/s), Backscatter2(dB)

 ... ... ... ...

 ... ... ... ...

Depthn

(m), Un(cm/s), Vn(cm/s), Backscattern(dB)

Header: rec #, year, mon, day, hour, min, water temp(C)

Depth1(m), U1(cm/s), V1(cm/s), Backscatter1(dB)

Depth2(m), U2(cm/s), V2(cm/s), Backscatter2(dB)

 ... ... ... ...

 ... ... ... ...

Depthn(m), Un(cm/s), Vn(cm/s), Backscattern(dB)

Etc.

(Missing data –999.0)