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. Impacts , I . and Direct Effects of Sand Dredging , ,

for B~ach Renourishment on the "

Benthic Organisms and Geology of the

West! FloridaShelf,

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FINAL REPORT. Executive Summary I '

OCS Report

MMS 95·0004

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U.S. Department of the Interior

Minerals Management Service

Office of International Activities and Marine Minerals,.

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FINAL REPORT

Des Report MMS 95-0004

REPORT TO:

Office of Intemational Activities and Marine Minerals

Minerals Management Service

U.S. Department of the Interior

on

Impacts and Direct Effects of Sand Dredging

for Beach Renourishment on the

Benthic Organisms and Geology of the West Florida Shelf

Contract Number 14-35-0001-30644

EXECUTIVE SUMMARY

Principal Investigators:

. Notman J. Blake, University of South Florida

Lar(y J. Doyle, University of South Florida

JartJes J. Culter, Mote Marine Laboratory

Program ~anager: Sa~dra L. Vargo, Florida Institute of Oceanography

Submitted by:

Florida Institute of Oceanography

830 First Street South

51. Petersburg, Florida 33701

(813)893-9100

CITATION

REPORT AVAILABILITY

DISCLAIMER

Extra copies of the report may be o'btained from the Public Information Unit (~ail Stop 4030) at the following address: '

Telephone Number: 1-703-787-1300

U.S. Department of the Interior

Minerals Management Service

Office of International Activities and Marine Minerals

Public Information Unit (MS4030)

381 Elden Street

Herndon, Virginia 22070

This report was prepared under contract between the Minerals Managemerit Service

(MMS) artd the University of South Florida, Florida Institute of Oceanography (FlO). This

report hcls been technically reviewed by the MMS and approved for publication.

Approvalidoes not signify that the contents necessarily reflect the views and policies of

the Service, nor does mention of trade names or commercial products eonstitute

endorsement or recommendation for use. It is, however, exempt from review and

compliance with MMS editorial standards. '

Suggested citation:

I , Blake, N.J., L.J. Doyle, and J.J. Culter. 1995. Impacts and Direct effects :of sand

dr~ging for beach renourishment on the benthic organisms and geology of the

We~t Florida Shelf, Final Report - Executive Summary. OCS Report MMS 95-0004.

U.S: Department of the Interior, Minerals Management Service, Qffice of

International Activities and Marine Minerals, Herndon, Va. 23 pp.

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ES4.0 SUMMARY AND CONCLUSIONS 19, I

ES5.0 LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 21.

ES1.0

ES1. ~I' INTRODUCTION t. . . .. .1

Jt! I ES1.2, OBJECTIVES OF THIS STUDY : . . . . . :3

ES1.3I SCOPE OF STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

ES1·1 METHODS AND APPROACH . . . . . . . . . . . . . . . . . . . . . . .. 4

ES1.4.1 Geophysical Surveys . . . . . . . . . . . . . . . . . . . . . . . . . ..

ES1.4.2 Benthic Community Survey 7

FINAL REPORT

oes Report MMS 95-0004

i J._ ,- h _ •

TABLE OF CONTENTS

BASELINE INFORMATION 9

ENVIRONMENTAL CONDITIONS AT THE STUDY SITES 9

DREDGING TECHNIQUES EMPLOYED AT THE EGMONT i

AND LONGBOAT KEY SITES 10

SEDIMENT AND GEOPHYSICAL RESULTS . . . . . . . . . . . .. 11

Sediment Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

Geophysical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12, EPIBENTHIC RESULTS 12

Trawls 12

Underwater Video . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

INFAUNAL COMMUNITY RESULTS 13

Pre-Dredging Faunal Community Conditions. . . . . . . . . . .. 13

Egmont Key. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14

Faunal Abundance, Richness, and Diversity 14

Faunal Composition 15:

Faunal Similarity 17

Sarasota n

Faunal Abundance, Richness, and Diversity 17:

Faunal Composition 18'

Faunal Similarity 18;

Longboat Key. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18:

Manasota Key 19:,

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ES1.1 INTRODUCTION '

Complementary to the 1953 OCS Lands Act, the National Environmental policy Act

of 1969!requires that MMS use a systematic interdisciplinary approach, to plan

develoPlTlent which may have an impact on the environment. The Environmentfl,llmpact

Statements (EIS), Environmental Assessment Teams, data acquisition and ,analysis,

public hJarings, and special studies are mechanisms used by MMS to fulfill this . i t requlremen . ' , As an expression of continuing concern about the effect on the environment of

man's d~velopment of the OCS, the OCS Lands Act Amendments of 1978 were

promulgated. These Amendments authorize the Secretary to conduct studies to obtain I

the information , required to predict, assess, and manage the coastal environments of the

OCS and ithose coastal areas that may be affected by mineral mining activitie$.

To meet its responsibilities the MMS has four priority goals of oes non-energy

minerals I~asing: 1. to safeguard the ocean and coastal environments by assuring that

oes minJral activities are environmentally sound and acceptable, 2. to assure that oes

mineral activities are fUlly coordinated and compatible with other uses of the sea, 3. to

provide a~ effective consultation process for coastal States and the Federal government

on offsho~e minerals, and 4. to evaluate and achieve the potential of the U.S. GleS as a

domestic bupply source for strategic and other minerals.

To fulfill its environmental protection responsibilities the MMS has used 'a variety

of appro~ches, primarily related to the oes oil and gas leasing program.: These

approach~s have included EISs, marine environmental data acquisition and 'analysis , '

studies, literature surveys, socioeconomic analysis studies, public conferenCes, and

special stJdies (toxicity trials, modeling of spill trajectories, etc.). With the recent, location

and evalua.tion of various oes hard mineral deposits of potential economic value, there

exists the!, possibility of offshore hard mineral development for various: mineral

commodities. Therefore, environmental studies are being developed in order to evaluate

potential adverse impacts of mineral leasing or development and to identify mitigation I

measures Ito minimize these impacts. ,

Environmental disturbance is an important fact of life for shallow water organisms.

Storm wa~es, shifting substrata, longshore currents, temperature variations, and other

physical factors, as well as various biological influences' (e.g., competition and

predation)Iplay an important role in determining the structure and composition o~ benthic

communities (Bowen and Marsh, 1988). Species living in this environment must either

be capabl~ of withstanding these influences or of quickly recolonizing locally perturbed

areas from: which they have previously been extirpated. For most benthic invert~brates,

the latter response is more common.

Benthic organisms are commonly monitored because of their intimate contact with

the dredgeid sediments. Organisms living in the immediate vicinity of the dredging and

disposal a~eas either are transported from one site to the other or are buried by the

spoils. AI~O, organisms that feed on suspended and deposited materials in the~e areas

ingest dredged sediments resuspended and redeposited during dredging and spoil

disposal. '

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A second reason for studying benthic organisms stems from their utility as indicator

organisml

. Being less mobile than fish, they are not likely to escape or selectively avoid

the affect!=ct areas. Their relatively long life-spans make them useful as indicators of past

perturbations. Mollusk shell fragments provide a long-term record of changes in birth

and death rates (Gordon et a/., 1972). Benthic macrofauna are relatively easy ito collect

and identify in the field. The benthic community makes up a spectrum of species that , vary grea~ly in habitat, modes of feeding, and tolerance of environmental stress (Gordon

et a/., 197,2; Taylor et a/., 1970). ,

A third reason for monitoring benthos is that they provide a crucial link in the

detritus-b~sed food chain. They eat organic matter and thus 'recycle nutrients that

otherwise; would collect and remain trapped in the sediments. Benthic orgariisms are

food fot demersal fish and other predatory aquatic organisms. :

Fin~lIy, clams, mussels, oysters, scallops and lobsters are important as commercial , '

fishery resources.

Smkll-scale, short-term projects have less impact on benthic communities than do , '

large-scale, long-term projects. The hydrology of the disposal and dredging ~ites also

seemsiniportant. A high degree of flushing rapidly dilutes and carries away the

suspendeid solids, minimizing the physical and chemical effects on the: benthic

community. If, on the other hand, stagnant areas are created by dredging and :spoiling,

greater cHanges in the benthic community can be expected. The effect of dredging and

dumping 'on the hydrology of a system is a function of the ratio of the areas of the

dredging land , disposal sites to the size of the bay in which dredging and dumping

occurs. t'-!atural fluctuations in population size caused by seasonal migration or rapid

repopulation by opportunistic species can mask the immediate effects of dredging and

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dumping ~>n the distribution of benthic organisms. '

Sediment grain size, chemical composition of sediments, and organic content have

been fount! to influence recruitment of benthic organisms(McNulty et a/., 1962; Thorson,

1966;.Gra'y, 1974). McCall (1977), in a study of a disturbed bottom in Lon'g Island

Sound, found considerable local differences in community species composition caused

by sedim~nt grain size heterogeneity. Conversely, Zajac and Whitlatch (1982), in a

disturbande study at Alewife Cove, in southeastern Connecticut, found that sediment

characteri~tics did not strongly influence the end product of recolonization. ,

Rat~s and patterns of colonization are also affectect by a number of biotiC factors

such as aJ,ailability of larvae, competition (Rhoads et a/. , 1977), life histOries (Grassle and

Sanders, 1973), presence of adults of the same species, sediment microbes (Gray,

1974), and "trophic group amensalism" (Rhoads and Young, 1970). Amensalism refers

to the ass6ciation to two different species whereby one species is not affected ,and the

other is h~rmed or inhibited. An example is the association of an antibiotic-prpducing

mold and bacteria. In benthic communities it refers to inhibitory or competitive effects

between species, excluding predation. ,

Rate's of recolonization have been demonstrated in studies of disturbed soft bottom

habitats (Simon et al., 1976; Oliver et aI., 1977; Conner and Simon, 1979; Van Dolah et

al., 1984) and in studies using defaunated sediment (McCall, 1977; Rhoads et a/., 1978;

Zajac and Whitlatch, 1982; Bowen and Marsh, 1988). These studies showed that

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FINAL REPORT

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recolonization is extremely rapid, and that community characteristics such faunal

abundanCe, species diversity (Shannon Index H'), equitability (J'), and' species

composition, were equivalent to those in the surrounding community within as little as

3 month~ post-disturbance (Saloman et al., 1982). ;

Qualitative and quantitative differences between disturbed and undisturbed (or

recently disturbed) areas may reflect the difficulty in selecting a "control" site for

assessing the effects of disturbance in the offshore sandy bottom community. Even

slight differences in depth, current patterns, or sediment characteristics may create

condition~ that are more or less conducive to the settlement of highly selective planktonic

larvae. I~ areas of high species diversity, such as the coastal waters of southeastem

Florida, the resulting differences in community composition could be especially I ,

pronounced.

ES1.2 OBJECTIVES OF THIS STUDY

Thei main purpose of this study was to evaluate and gain insight into the /c;mg-term

effects of iseabed mining operations and the degree to which benthic organisms may

repopulate the area after a mining activity takes place in a coastal or shallow oP'iln ocean

area sucH as near the mouth of Tampa Bay on the west coast of Florida. This

informatioh is necessary to at least provide a preliminary assessment of the impacts of , ,

mining activities. Specifically the objectives were as follows: '

1. To evaluate and assess the possible impacts of sand dredging operations

on the benthic communities (epifaunal and infaunal) before, during and after

dredging operations. ;

2. To evaluate and assess the possible' changes to the geology and

topography as a result of the dredging operations.

The information produced by this study can be used to assess the: level of

dredging or mining impacts to the benthic communities and the suitability of a proposed

area of deVelopment in relation to the ability of the biota to survive these oper~tions or

to repopulkte after completion of dredging or mining activities.

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ES1.3 SCOPE OF STUDY , ' The immediate effects of dredging for sand and gravel or other offshore: mineral

deposits ate disruption or destruction of the benthic organisms and habitat in the path

of the dredge. Sessile or slow-moving organisms may be especially impacted: Some

potential impacts are: '

1. Changes in trophic dynamics

2. Disruption of spawning

3. Permanent changes in benthic community structure and form due ,to

substrate modification :

4. Modification of sediment grain size distribution resulting in changes in

benthic community composition

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Only a small number of studies available on the impacts of offshore dredging have

focused on the seafloor impacts on the dredged site itself. They place primary emphasis

on the extent and character of sediment resuspension created by the dredging operation,

impacts clf sediment plumes from surface discharge, and influences of these factors on

local fish land benthic communities. Little information exists regarding the degree to

which changes in substrate (I.e. grain size profile, etc.) induced by mining activity affect

benthic communities and their ability to repopulate an area atter the direct effeCts of the

dredging or mining operations cease. This project was designed to address th'is lack of

informatit· ;

ES1.4 METHODS AND APPROACH

Survey oltline ;

The: sampling program was conducted at each of four sites (Figures and 2)

selected latter consultation with MMS, the Army Corps of Engineers,principal

investigators, and the program manager. The general station locations in lati~ude and

longitude, site designations, and number of samplings are as follows:

Egmont Key Site I 2r37' N, 82°49'W 1-pre, 3-post

Sa~sota Site II 27°15' N, 8~35'W 3-pre, never dredged

Marlasota Site III 26°29' N, 82°2TW 1-pre, never dredged

Longboat Site IV 27°14' N, 82°36'W 1-post ,

ES1 A 1 deoPhYSiCal Surveys :

Priot to each benthic sampling event a geophysical survey was conducted at each

site. A m!bdmum of 12 nautical miles of high resolution seismic reflection profiling, side

scan son~r data, and continuous video tape of the bottom was collected simultaneously.

Line spac!ng was as close as possible and depended on the site sizes. Li'1es were

adjusted in surveys following the pre-dredging survey in order to pass over actual

dredged ~reas. Navigation was by LORAN C with fixes marked at 5 minute intervals on

the high r~solution seismic reflection profiling and side scan sonar surveys and every 10

minutes oh the video system. Surveys were conducted at two knots or less (the ROV

towing speed was the limiting factor). ' :

Seistnic reflection data was collected using an ORE Geopulse profiling' system

consistingIof an ORE power supply, acoustic source, 20 element hydrophone array,

receiver/filter/amplifier, and an EPC 1650 graphic recorder. The seismic profiles were

recorded ~sing a 62 ms sweep with band pass filters typically set for 700-3000 Hz.

Experience has shown that these settings are optimal for the west Florida shelf, but of

course, were adjusted during the survey in order to optimize the records. 'rhe raw

unfiltered l!tnalog signal was recorded on magnetic tape. '

Sidel scan sonar records were obtained using an EG&G model 260 Sea Floor I ,

Mapping System with automatic slant range correction. Sonographs were typically

recorded With a horizontal range of 75 m on a side (150 m total swath). Processed

digital data was recorded on magnetic tape as well as on paper record.

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hatched area on large map).

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Site III - Manasota, and Site IV - Longboat.

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recorder with a hull mounted transducer. '

'ES1.4.2! Benthic Community Survey :

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'A video camera mounted on a PHANTOM 500 ROV was towed a few feet above

the bottoT behind a torpedo-like weight that helps maintain depth control. Th'e system

I was remotely flown from the vessel laboratory. Video camera output was rec~rded on

IVCR tapel

. Observations of bottom type and biology were logged in real time aboard

j ship everY 10 minutes or as specific features or changes in bottom type or epibenthos

; were encountered. A LORAN fix was taken at the time of each log entry. On-board

video obJervations of epibenthos included the presence of dominant epifaunal species

: such as bbttom fish, crabs, sponges, corals, and shellfish as well as major attached and I " ,rolling algae. " ' !

, After each video camera/side scan/seismic site survey, three trawls were ~ken for

!epibenthic species verification using a 30 foot otter trawl. The location of each 2.0 minute

trawl waJ determined by the chief-scientist and depended upon the on-board

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observatidns with the video camera. The individuals collected in each trawl sam'ple were

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counted ahd representative individuals were preserved and returned to the laboratory for

analysis to the lowest practical taxon. :

I Thelbenthic infaunal and sediment samples weretaken with a box corer, A total

lof three stations were sampled at each site (two in the dredge area and one outside).

lThe contrbl station was outside the dredge area and one mile removed from the limits

•of the disp,osal area, but in a sediment environment similar to the dredge area. Although

'an attempt was made for the first sampling period to be prior to any dredging operation

~and the re'maining three periods after dredging, this did not prove practical for all four

'locations. ISite I was the only site sampled pre- and post-dredging. :

I Each box core removed a sediment plug measuring 21.3 cm x 30.5 cm in surface

dimension~ with a maximum depth of penetration of 32 cm. The box corer: gives a

'relatively Jndisturbed sample, both surficially and stratigraphically. Nine replicate cores

'were coliePted at each station. An additional replicate was collected, archil!ed and

'maintained as a backup. Previous sampling in the area has shown that nine cores are

'required t6 obtain a statistically valid description of the benthic macroinfauna. '

i Afte~ each box core was brought on board, it was catalogued and labelled 'with the

'appropriate sample number. Salient visual characteristics, including color, ,texture,

stratigraphy, and conspicuous epibiota, were recorded. '

i EacH box core was sub-sampled with a 5 cm diameter coring device to the depth

of core pe~etration. The resultant samples were placed in clearly labelled containers for

later sedirhent analyses.

i After! a 5 cm subsample was extracted for sediment analysis, the top 15 cm of

sediment remaining in each box core was removed for faunal processing. The,sample

Was placed in a 50 gallon drum mounted on a swivel and frame and subjected to a

gentle stream of running seawater. As the can filled with water, the sample was gently

stirred to tiring the soft-bodied organisms out of the Sediment. The water Was then - 1 '

poured thr0ugh a spout in the can chute onto a 500p mesh screen.

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I FINAL REPORT I OCS Report MMS 95-0004

I ,

I The material from the sieve was rinsed into a pre-labelled plastic quart jar with

filtered s~awater. A buffered forrnaiin and rose bengal solution was then added to the

I

jar to bri1g the final ~ormalin concentration to ten percent. The sediment left in the can

(along With the remaining fauna) was transferred to prelabelled cotton bags arid placed

I in a drum' of 15% magnesium chloride for fifteen minutes to relax the organisms. Finally,

, the bags! were transferred to a drum of 10% formalin. After each cruise, all infaunal

j samples re~e inventoried (each sample recorded on custody sheets) and transported

to Mote Manne Laboratory. '

, As 'part of the framework portion of the study to determine recolonization after

, dredging) the standard sedimentary parameters of grain size, carbonate content, and

! organic cbntent were determined for each of the ten samples at each station: for each

sampling [period. Grain size (texture) was measured by the standard technique of sieving

I an aliquot through sieves nested at 1 ¢ intervals; that is -1 ¢ (2mm), 0 ¢ (1mm), 1 ¢

\ (500 micr?ns), 2 ¢ (250 microns), 3 ¢ (125 microns), 4 ¢ (62 microns), and pan « 62

I microns) in the method of Folk, 1980. This whole range is considered to be ,the sand

i sized fraction. For Site I, the only one in which we have sampling before, during and

j after dredging, measurements of all ten replicate samples were combined at each of the

I

three statf,ons and histograms were constructed for comparison of sampling p'eriods.

Carbonate content was determined by acidifying a weighed aliquot with 10% HCI,

'

dissolving the carbonate minerals, drying the sample and re-weighing. Organic content

was deter'mined by combustion of a carbonate free subsample. ;

I Aft~r 48 hours, the infauna samples were transferred to 70% isopropyl alcbhol and

,stored until processing. Samples were first rough sorted to major categories (annelids,

!crustaceahs, molluscs, echinoderms, hemichordates and miscellaneous) and then

i identified to the lowest practical taxonomic level, which for most fauna was either the

Ispecies o~ genus level. '

I Wet weight biomass (g) was measured for each major taxonomic group. Wet

lweights.w,ere obtained by pouring the sample into a small sieve (250 micron mesh)

which separated the specimens from the preservative. The samples were rinsed with tap

;water to remove excess isopropyl alcohol. Specimens were blotted dry, placed in a

'small alurtJinum pan and weighed to the nearest mg. ':

: S~uence boundaries at each site were mapped from the high resolution' seismic

reflection Clata. Isopach maps of the sequences present, including the sand bo~y to be

'mined, w~re constructed for each sampling period. Side scan sonographs were

.'analyzed fbr topographic changes, bedform character, and sediment textural variation.

'Seismic aMd side scan records were correlated with bathymetry as determined: by PDR

'records and with the observations taken from the video data. :

i Th~ macroinfauna and epifauna species were coded for computer entry' utilizing

National Oceanographic Data Center (NODC) numeric coding system. All computer

'entries weJe 100% verified for accuracy and completeness prior to analysis. Mote Marine

'Laborato~ analyzed the collected macroinfaunal data utilizing accepted statistical

·proCedure~. ' Emphasis was on statistical parameters that numerically defined species

compositidn and relative abundance and that showed whether or not correlations existed

between combinations of substratum characteristics (such as grain size) and faunal

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included the following when appropriate:i

,

j The sites studied in this survey all lie in the innermost portion of the west Florida

Continental, Shelf. Water depths ranged from 5-6 m at all sites with the exception. of the

ship channel off Egmont Key (Site I) where depths were 10-15 m, Doyle and Sparks

(1980) hav6 described the geology and sediments of this continental margin. The west

Florida ma1rgin, like the peninsula itself is made up of about 5 kilometers of carbonate

rocks interCalated with evaporites which have been deposited more or less continuously

since Juraksic Time. Most of the west Florida continental shelf is veneered with a

surficial sahd sheet of relatively coarse carbonate sands composed predominantly of

mollusk frabments. However, the Florida Peninsula is covered with a number of,Tertiary

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(5) Association of assemblages with substratum variables by

discriminate multivariate analysis.

f (6) Analysis of distribution of fauna by species and numbers amohg major

I faunal groups. The loss of a particular type of organism (example

t

amphipods) representing a particular functional morphology,' is often

indicative of an alteration of the habitat structure,

I Data analysis was conducted with Mote Ma~ine Laboratory developed s~perCalc

\

'5.0 sprea~sheet software and BioStat 1/, community analysis software (Pimei.ntel and

Smith, 1993) ,

Representative epibenthic samples from the trawl were identified to the lowest

:practical taxon. Combined with the counts from the trawl this provided data on

tnUmericallabundance of epibenthic species in the sampling area. i

r

;ES2.0

ES2.1 BASELINE INFORMATION

I The bottom topography at all the sampling sites is essentially a flat, sand sheet

;with no cdnsolidated substrate or hard-bottom communities observed. Turbidity made

.observatio'ns difficult during dredging at Site I but dredged holes could be obserVed from

)he clams~ell dredge used at this location. Small sand waves reflect the curren~ activity

.in the area and this current activity also occasionally caused an increase in turbidity. ! '

ES2.2 ENVIRONMENTAL CONDITIONS AT THE STUDY SITES

I

Icharacteristics. Analysis of the data

, (1)' Species diversity

I j I (2)1 Species evenness

1 (3)\ Species richness

'1· (4), Definition of community assemblages based on classification and qrdination

r analysis procedures, such as cluster analysis. '

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FINAL REPORT

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quartz sand terrace deposits brought south by long shore currents from the Appalachian

Province ~uring higher stands of sea level. The inner 10 miles or so of the west Florida , , '

continental shelf is also veneered with this quartz sand mixed with varying amounts of

! carbonat~s. Quartz sand also composes the majority of the west Florida beaches.

1Rivers th?t drain into the west Florida margin are small and slow flowing ~ith little

I capacity ~o carry suspended sand sized material. In fact, they carry a very small Isuspendekt load of any size. What the rivers do bring down is trapped in estuaries or

l

lagoons Behind barrier islands. The beaches are therefore sediment poor and somewhat

,ephemer¥ It is ther~fore probable that the only new material being added to ,the inner

west Flonda system IS carbonate, - ,

Th~ quartz sand band is thin and sand sources for beach renourishment, the

-dredging lof the west Florida Margin that led to the inception of this project, ,are rare.

, Most sources are found in the ebb tidal deltas that lie offshore associated with either

1active or blosed inlets in the barrier island system. The largest of these is the giant

,feature atlthe mouth of Tampa Bay where the Egmont Key site (Site I) is located. The

only new sand coming into the system is from the erosion of headlands or from retreat

:of the du?eSalong the barrier islands. Many of the narrow barrier islands along the

lstudy area coast have been undergoing erosion and exhibit the bluff formation

characteriktic of a retreating coast. '

) ,

,ES2.3 DREDGING TECHNIQUES EMPLOYED AT THE EGMONT AND

LONGBOAT KEY SITES

\ It should be noted that the dredging techniques and equipment employed at the

,two studyl sites that were dredged were quite different. These differences :may be

Iresponsible for some of the observed differences in seafloor impacts _ '

I At the Egmont Key site (Site I), a clamshell dredge was used to excavate the sand

deposits Which were then placed in a hopper barge for transport into shallower waters.

:Cla.mshelll dre~g~s use buckets (sizes 1-8 cUbi~ met~rs) which are mechanically

activated to "bite Into" the seabed to remove matenal (typically for low-costilow-volume

'mining clbse to shore). Material may then be loaded into its own hopper Of onto a

'separate barge. Resulting small pits usually leave an irregular seafloor topography and • I

turbidity plumes throughout the water column are common (Groat et al.,1993)"

1 At tfue Longboat Key site (Site IV), Bean Dredging Corporation's Beachbuilder was

'employed.1 The Beachbuilder is a dustpan dredge. It has a wide, flat head designed to

skim thin layers of sand from the bottom and uses high-pressure jets of seaWater to

loosen sahd in a wide, shallow path. Because it's removing a much thinner layer' of sand

than a con'ventional dredge, it tends to leave some of the substrate intact (Tayldr, 1993). F

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11

,SEDIMENT AND GEOPHYSICAL RESULTS

•,

; Sediment Analvses "

j Gr~in size (texture) was measured by the standard technique of sieving Em aliquot

through Sieves nested at 1 I/> intervals; that is -1 I/> (2mm) , 0 I/> (1 mm), 1 I/> (500 p), 2

i I/> (250 p)l, 3 I/> (125 p), 41/> (62 p), and pan « 62 p) in the method of Folk, 1980. This

I whole range is considered to be the sand sized fraction. For Site I, the only one in which lwe have before, during and after dredging sampling, measurements of all ten' replicate

samples Were combined at each of the three stations and histograms were corstructed

1

for comp~rison of sampling periods. ';

Th~re was no significant difference in texture through time. At Stations Ii1 and 1-3

the majority of the particles are concentrated in the 2-3 I/> (250-125 p) size class with a

1

strong sdcondary concentration in the 3-4 I/> (125-62J.J) size range. These sizes are

, commonl~ described as fine sand and very fine sand, respectively. Station 1-2 shows an

! anomalous concentration in the coarse sand range 0-1 I/> (1 mm-500 p) range in the

101/93 sfl,rhpling as well as more sediment in the coarser fractions as well. Station 1-2

I generally!shows more variation between sampling periods as well. ,

! Normally it would be expected that a newly created depression in relatively I • ' I shallow water where waves and current affect the bottom, would begin to' be filled

immediat~IY with sediment moved by these forces around the inner portion of the shelf.

I,

,

Normally depressions , are thought to be sinks for fine grained sediments (finer than 'sand

I sized) first. Apparently that is not the case here. A probable reason for this is that there iis only a tiny finer than sand sized fraction present in the inner west Florida shelf surficial

sand she~t. There is simply virtually no fine fraction with which to fill the depression.

: Th~ sediment on the inner few miles of the west Florida shelf is dominantly fine

: to very fihe quartz sand, thus the heavy concentration in the 2-41/> size fractions. '

1

However, 'there is a second important component, calcium carbonate, composed mostly

'of mOllusdan shell fragments. These make up the coarser size fractions. In sOl1)e places

they can be quite concentrated and their distribution is patchy and can vary even within

lla station Wide area. Station 1-2 has a higher carbonate content that accounts for its

variatiility·l· •

, Examination of the within station variability for Station 3-1-09-3 shows the ~xpected

1

', concentrations in the 2-3 I/> and 3-41/> size fractions. It also shows that when the coarser

carbonat~ fractions increase there is a concomitant decrease in the finer quartz sand

ifractions.jTable 3.1-1 shows the carbonate content for two stations at Site II fort,he same

1

,sampling periods. Station 11-1-054, composed predominantly ofquartz sand shows heavy

,concentrations within the 2-4 I/> size ranges while Station 11-2-072 has a: coarser

distributioh, reflecting higher carbonate content. , :

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OCS Report MMS 95-0004

'ES3.2 EPIBENTHIC RESULTS

, Although epibenthic surveys were done extensively on the outer continental shelf

\of tlieeastern gulf of Mexico in the early 1970's as part of baseline surveys conducted

by the Bureau of Land Management, little to no information exists for the epibenthos of

'the nearsHore, subtidal, sand environment. The epibenthos of this area are highly patchy , , and mobile. In addition they are often nocturnal and may dig into the sediment during

.the daylight hours. Because of these factors, the epibenthic surveys of this study were

:qualitative!and relied upon the use of otter trawls and a towed, underwater video.

i Geophysical Results

The geophysical surveys provided a framework for the benthic sampling. Several

techniqueh were used - side scan sonar, high resolution seismic profiling, and precision

I depth recprding. Precision depth recording (PDR) of bathymetry provided the most

useful data. Even as late as 1994, after several storm and frontal system seasons, the

•excavatio? is clearly visible. It is clear that the excavation was below local wave base

I and that even in this shallow system sediment supply is insufficient to quickly hide the

,dredge star. Should such a study be undertaken again, it would be wise to dispense

;with the high resolution seismic profiling and side scan sonar work and concentrate on

repeating :with precise navigation a complete bathymetric survey of the excavation.

,.Trawls

A total of 41 different taxa were observed during the study, alow species richness

'reflecting t~e constancy of the habitat in this area. The crab, Portunus gibbesii, and sand

'dollar, Mel/ita tenuis were by far the most dominant species present during the study.

lAlthough both species were absent from Site II during the May, 1994 sampling, this site

had not ~een dredged as planned by the Army Corps of Engineers. The shrimp,

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TABLE 3.1-1

Weight Percent CaC03

Site/Location/Date Site/Location/Date

11-2-054 11-1-072

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i

j Penaeus ,duorarum a predominantly nocturnal species, was a distant third and showed

I

great variability between samples. Although an attempt was made to make each trawl

location 60nstant, current and wind conditions resulted in some variation. However,

I Portunus 1gibbesii and Mellita tenuis remained the dominant epibenthic species Ithroughout the course of the study at both dredged and undredged locations. The

t

smallest humber of epibenthic species and the smallest number of individuals were

collected :on the last cruise in May, 1994. This probably reflects seasonal changes and

I not a resylt of dredging since on this date only Sites I and II were sampled and only Site

II had been dredged.

I 1 , Underwater Video

j UnClerwater video was recorded over each transect at each location for each

Isamplingicruise. This represents a total of approximately 120 hours of video recorded

:during the course of the study. There was little variability observed between locations

ior betwe~n sampling dates.

I Ob~ervations of flora and fauna were rare. The flora observed was unattached,

jdrifting m~croalgae and not seagrass. Occasional worm tubes belonging to the infaunal

,species, Diopatra cuprea, were observed protruding above the sediment and an

\occasional portunid crab was observed swimming away from the light. These

observatidns reflect the constancy of this very dynamic sand habitat and the low

:epibenthic diversity of the area.

ES3.3 INFAUNAL COMMUNITY RESULTS

I

'Pre-Dredging Faunal Community Conditions

Thrle of the four study sites were sampled in July, 1992, prior to the

.commenc+ment of any dredging activity. These three sites were: Egmont Key (Site I),

Sarasota (Site II), and Manasota Key (Site III). Manasota Key had the highest mean

humber Of\:taxa per station (142 :t 29 taxa), followed by Sarasota (101 :t 9) and Egmont

Key (77 :t 26). The difference in number of taxa between Manasota Key and Egmont

Key was statistically significant (p < 0.05). Manasota Key also had the highest mean

~umber of!individuals per square meter (3635 :t 1175). Sarasota and Egmont Key had

;;imilar fau~al abundance, with mean numbers of individuals per square meter of 1832

:t 812 and 2296 :t 1030, respectively. None of the differences in mean number of

individuals\per square meter were statistically different (p = 0.05). 1 Man,asota Key had the highest species diversity (Shannon Index) among the three

stations (3~67), followed by Sarasota (3.19) and Egmont Key (2.47). There was a

statisticallylsignificant difference (p < 0.05) in Shannon-Weiner Index between Manasota

Key and Egmont Key. Equitability (a measure of the evenness of species distribution)

Was highest at Manasota Key (0.743 :t 0.12), followed by Sarasota (0.697 :t 0.09) and

Egmont K~y (0.573 :t 0.12). There were no statistical differences in equitability among

stations., ljJsually, a positive relationship exists between diversity and equitability Le., a

high equitability would indicate a high diversity and probably a "healthy condition" of the ,

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j

fauna, This was evident amo(lg all stations during July 1992, where there was a

1significan,t positive relationship (p < 0,001; r = 0,953) between species diversity and

, equitability.

j An1nelids comprised approximately 50% of the total fauna at all sites. At two

I stations (bne Sarasota and one Manasota Key), oligochaetes contributed over 10% of the

ttotal fau~a. The contribution by molluscs varied greatly within and between sites.

I Molluscs yaried from 14 individuals (2.1 %) at one Egmont Key station to 1125 individuals

I (73.6%) at another Egmont Key station. This fluctuation in molluscan fauna was due to

, differenc$s in the distribution of the bivalve, Parvilucina multilineata. Gastropods were

I slightly mbre abundant at Manasota Key with respect to the other two sites. Arthropods

\

' were con~istent in abundance at all sites, contributing between five and ten percent of

the total!fauna. Amphipods and decapods were the most abundant arthropods.

I

,Echinoderms were rare at Egmont Key, while they represented one to three percent of

the fauna 'at Sarasota and Manasota Key. Finally, miscellaneous taxa, dominated by the

, lancelet Branchiostomma, showed high variability within each site, ranging from 0.2 to

117.8% of the fauna at any given station.

, Th~ composition of fauna during pre-dredging sampling shows slight differences

!among stktions in dominant taxa. Tubificid oligochaetes were the dominant taxon at

IManasota\ Key, followed by the lancelet Branchiostomma sp. and the polychaete

I

prionosPio cristata. The bivalve, Parvilucina multilineata, was dominant at Egmont Key

and SaraSota during this period. Other differences in species composition among the

\three statibns include: higher relative abundance of the polychaete Aglaophamus verrilli

at Sarasota and Manasota Key; higher relative abundance of Mooreonuphis nebulosa,

!/solda pulChella, Goniadides carolinae, and Aricidea cf. suecica (polychaetes), Anadara

'transversa!(bivalve), at Manasota Key; and a high percentage of the gastropod Ceacum

,bipartitum iat Egmont Key. Overall, faunal richness and abundance at each of the three

! borrow sites indicate healthy, diverse infaunal communities along the west Florida coastal

!zone. \

Egmont Key

I Egmont Key (Site I) was sampled four times during this study. This included a

pre-dredgihg sampling (July, 1992), one sampling during dredging activities (January, , 1993) and: two post-dredging sampling events (September, 1993 and May, 1994,

respectively). This section discusses the temporal trends in faunal community response

to dredging at Egmont Key. It will also look at differences over time among the two

aredged and one control stations as determined by two-wayal'lalysis of-variance.

I Fau~al Abundance. Richness and Diversitv. '

I Tem'poral responses in number of taxa and faunal abundance (individuals/square

meter) were distinct for each station at this site. The control station (I-G) experienced a

slight decr~ase in both parameters shortly after dredging operations were completed

(between JLly, 1993 and January, 1994). One of the two dredged stations (1-02) showed

a much mdre drastic decline in number of taxa and individuals during this same period.

From Janu~ry to September, 1993, station I-G experienced a substantial increase in both

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I parameters, while the aforementioned dredged station (1-02) underwent an increase in

, number bf taxa, but a decrease ih faunal density. By the end of the study, station 1-02 Ihad not r~overed to faunal levels experienced by the control station. The other dredged

, station (I~D1), however, showed no apparent declines in faunal parameters associated

• with dred~ing activities. This station showed a continual increase in both faunal diversity

, and abundance. At the end of the study, station 1-01 had far more taxa present and

t more indiriduals than either of the other stations at the Egmont Key site.

: Sp~ies diversity and equitability at Egmont Key were fairly constant at the control

I station. These parameters showed wide fluctuations at station 1-02, especially during

: postdredging sampling events. The other dredged station (1-01), which had lower

diversity and equitability values prior to dredging, showed, a continua.! ihcrease in these

I parametets until the end of the study.

j A *,"o-way analysis of variance (ANOVA) was performed on all siation and date

j combinations to look at the interaction between these two variables. The test was

Idesigned Ito detect differences in mean number of taxa and faunal density. Although

, .there are inherent inaccuracies associated with applying analysis of variance to replicate

'benthic s~mples through space and time (see Hurlbert, 1971 for discussion), the test

lallows usl to determine if there ,is an appreciable interaction between stations and

{samPling dates. If there is, then it makes it inappropriate to discuss differences between

jcontrol an'd dredged stations at anyone time as statistically significant. In any case,

,there la significant (p < 0.001) interaction effect between sampling time and station

lat the Eg~ont Key site. Temporal trends in number of taxa and faunal density behaved

differently at each of the three stations over time. This makes it impossible to attribute

~any of thelobserved changes to the effects of dredging alone. This finding, along with

.the obserYjed differences in response of the two dredged stations, does not allow for an

'accurate analysis of dredging effects at Egmont Key. However, a look at changes in

faunal composition at these stations should help pinpoint which species were mainly

'responsiblb for the observed community oscillations.

! Faulal Composition. Annelids contributed the lowest percent of total fauna

throughout the study at the second dredge station (1-02). Typically, annelids constituted

20% or le~s of the total fauna (compared to roughly 60% at the other two stations) at

station 1-02 throughout the study. This was countered by a substantially higher percent

of mollusd at station 1-02 compared to the other two stations (70% to 25%, respectively). , This differe'nce in the composition of the fauna at station 1-02 compared to the other two

~tations at iEgmont Key could play an important role in the ability of fauna to recolonize

the second dredge site. This is due to differences in the reproductive and dispersal

potential bktween polychaetes and molluscs. '

f Oiff~rences in total and percent of taxa in major taxonomic groups among the •

three Egmbnt Key stations were less evident. While the total number of annelid and

molluscan taxa at the second dredge station were lowest, the percent of total taxa for

annelids a~d molluscs among all stations were similar. This indicates that, although

overall fauhal conditions were poorest at the second dredge station, the number of taxa

were in pro1portion to the number of individuals present. In other words, the fauna were

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i not limite<:! to a few taxa, but rather were spread among a number of taxa comparable

j to the other two stations.

Se,'veral species contributed to the high faunal density at the control station during

i July, 1992. Tubificid oligochaetes, Prionospio cristata (polychaete) and Caecum

, bipartitun!, (gastropod) were found almost exclusively at the control station, while the

, bivalve, Parvilucina multilineata was much more plentiful at the second dredge station.

During J~nuary, 1993, faunal abundance dropped off noticeably at the second dredge

, station b~cause of the absence of this bivalve. The higher total abundance at the control

, site was partly due to tubificid oligochaetes and nemerteans. Other significant species

, at the cohtrol station include Cylichnel/a bidentata, Caecum bipartitum, Mediomastus

, spp., PriohospiO pygmaea, and Aricidea cf. suecica. Caecum"jOhnsoni and Acteocina

'sp. were found exclusively at the first dredge station, while Myriowenia sp. was present

.only at ,tHe second dredge station. A number of species contributed to the large , '

Iincrease in faunal abundance at the control station during September, 1993. These

species Were: Parvilucina multilineata, Prionospio cristata, Prionospio pygmaea,

lNemertea? sp. B, Cyclaspis cf. varians, and Paraprionospio pinnata. These species are

lall surface-deposit feeding organisms (three are polychaetes in the family Spionidae).

iThiS suggbsts a stable surface with a sufficient organic component to support such a

,diverse a~d abundant fauna. The dredged stations may have had a disturbed surface

,which pre~ented the early colonization of these stations. By May, 1994, the distribution

of specie~ between the control and the dredge stations revealed some interesting

~distinction~. The second dredge station, which had depressed faunal abundance

throughout the study, had a large abundance of the bivalve Tellina sp. This species

:represent~ 70% of the fauna at this station (no other species contributed more than 4%

to the total fauna). This disparity in species distribution led to the low species diversity . , and equitability at this station during May, 1994. The other two stations (control and first

dredge st~tion) had several dominant species, although this dominance was distinct for

each station. Parvilucina multilineata, Prionospio cristata, Cylichnel/a bidentata - , (gastropoq), and Spiophanes bombyx (polychaete) were abundant at the control station.

Branchiost'omma sp., Prionospioperkinsi, Mooreonuphis nebulosa, Carazziel/a hobsonae,

and Erichthonius brasiliensis were specifically dominant at the first dredge station.

Overall div~rsity and equitability at these two stations were high, reflecting the abundant

and divers~ species composition.

I An ihteresting shift in faunal composition was observed at the second dredge

station (1-0'2). ' Bivalves contributed a much greater percent of the fauna at this station

~han the ot~er two. Before dredging, the bivalve Parvilucina multilineata represented 64%

of the total fauna. The next two sampling periods (immediately post dredging) revealed

very low faGnal , abundance with no clear dominance by any organism. By May 1994, the

bivalve Tel/ina sp. comprised 69% of the fauna. This clear shift in species composition

was not ob'served at the other two stations. '

! This!slow recolonization by bivalves has been demonstrated in other studies.

Bowen and Marsh (1988) found that periodic molluscan settlement in a Delray Beach,

Florida bo~row site was in most cases followed by nearly complete mortality. The

~uthors did' not speculate on the cause of this mortality but only noted that no adults

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. were fourd. Oliver et a/. (197.1) reported similar molluscan mortality in a study of

. dredged lmaterial disposed at various depths in Monterey Bay, California. Simon and

! Dauer (1977) also noted the slow appearance of molluscs in a disturbed area in Tampa

I

Bay. Th~ authors surmised that the phenomenon may be related to their more limited

dispersalIabilities and reproductive seasonality. The cause of the sharp decline in

Parvilucin,a multilineata at the second dredge station combined with the slow, but steady

, increase in Tellina sp. may reflect these inherent patterns of recolonization exhibited by

1molluscs.

Faunal Similarity. Distinctions in faunal composition among Egmont Key stations

; are also ~eflected in faunal similarity values. Not surprisingly, only two combinations

j showed "/oderate similarity, and they were temporal similarities at the same station. The

\first dredge station (1-01) showed moderate similarity between January, 1993 and

ISeptember, 1993, while the control station showed moderate similarity between

I

•September, 1993 and May, 1994. When a presence/absence data transform was

Iperformed, several more Egmont Key station combinations showed moderate similarity

1through space and time. Moderate similarity occurred between: dredge station two and

!control stMion, July, 1992; control station, July, 1992 and January, 1993; and control

jstation, Jb.nuary, 1993 and dredge station two, July, 1992. All other station-date

icombinatibns showed low to very low faunal similarity.

I In $ummary, each station at the Egmont Key site underwent different temporal

'patterns in benthic community composition. The two dredged stations responded quite

:differently Ifrom each other, making data interpretation difficult. Most of the difficulty

,involves the distinct contrast in faunal community response between the two dredged

'stations. lh general, the control station exhibited less variable temporal trends, while the

"two dredgbd stations showed widely fluctuating and contrary faunal responses. I' . Sarasota

1 The Sarasota site was sampled on three occasions: July, 1992, January, 1993

and May, 1994. This site was not dredged during the course of the project, so this site

can serve ks an illustration of natural temporal trends in community parameters. These

trends ca~ then be compared to trends from Egmont Key to delineate impacts

aSSOCiated! with dredging at Egmont Key.

I Faunal Abundance, Richness and Diversity. Both number of taxa and number of

individualslper square meter increased through time at the Sarasota site. This trend was

evident at lill three stations. Species richness was similar at all three stations in July, , .

1992, ranging from 92 to 110 taxa (mean = 101). By January , 1993, mean species

richness inbreased to 137 taxa. The station with the least number of taxa in July (11-01)

had the m6st taxa in January, an increase from 92 to 166 taxa. This station showed a

fess drama'tic increase in the number of individuals. Species diversity at Sarasota was

highest in ~anuary, 1993. This is due to a substantial increase in species richness • I

coupled with only a slight increase in faunal density from July, 1992 to January, 1993.

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IBy May, 1994, there was a further increase in species richness and a large increase in

i faunal d~nsity. This resulted in a: decrease in species diversity from January, 1993 to , I .

, May, 199f'

I Faunal Composition. The relatively low faunal abundance at station 11-01

: compared to the other two stations during July, 1992 was due to the absence of several

dominant!species at this station, most notably the bivalve Parvilucina multilineata and the

I polychaete Aglaophamus verrilli, which were numerous at the other two stations. By

January, 1993, these two species were no longer dominant. They were replaced by

tubificid dligochaetes and a suite of lesser dominant taxa, all of which were equally

,abundantlat all three stations. Consequently, faunal density was similar among all sites

by January, 1993. Between January, 1993 and May, 1994, there was more than a

,twofold in9rease in faunal density, from a mean of 2,383 individuals per square meter in

,January, 1993 to 5,698 individuals per square meter by May, 1994. An increase in the

;abundanck of a variety of organisms contributed to this overall rise in faunal density.

1 FaJnal Similaritv. Stations II-C and 11-0 showed moderate similarity during July,

'1992 and 'May, 1994 for both normal and transformed data, These stations were the

'most simil~r in faunal composition of any station-date combinations for the entire study.

'All other Sarasota station comparisons showed low similarity.

: ThiS pattern of change in faunal communities at Sarasota is consistent with models

of temporal variability in natural systems (Maurer et al., 1993; Rakocinski et al., 1991;

Boesch et ~/., 1976), Faunal density and diversity from estuarine systems showed peaks

during spri'ng and early summer (Leverone et al., 1991; Mahoney and Livingston, 1982;

Boesch et!aI., 1976). Differences in population-based parameters (e.g., density and

9iversity) tlj1rough space and time are often quite variable (Osenberg et al., 1994). This

variable pattern in faunal density and species diversity was also observed at the Sarasota

tite duringlthe present study.

LongboatKey' '

I I '

I The Longboat Key site was sampled once (a six month post-dredging event in

September! 1993) during the present study. However, an independent Mote Marine

Laboratory !study was conducted within the proposed borrow site during June, 1991,

Despite differences in sample size and sampling frequency between the two studies,

results frorh the MML study can be used to make qualitative observations and

compariso~s of faunal composition at this site. Also keep in mind that there was over

Ii two yeat difference between the two sampling events, rendering any temporal

relationship~ between the two studies inappropriate, I No ahalysis on faunal abundance, species richness, diversity and equitability are

available fd this study. A total of fifty species were found in this limited study. The

dominant species during the 1991 MML study were the lancelet, Branchiostomma sp.

(CompriSing\65% of,the total fauna), followed by, in decreasing abundance: Tellina sp.

(bivalve), Spio pettiboneae (polychaete), Rhepoxynius epistomus (amphipod), Olivella

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, 90% of the total fauna, which iridicates a relatively high level of oligomixity. This may,

however,l be a reflection of the limited sampling effort during this study. A' more

j comprehensive study might have revealed a higher degree of diversity and equitability.

! 13r~nchiostomma sp. was also the dominant species at Longboat Key during the

I present study. This species was followed by Actiocina caniliculata (gastropod), Tel/ina

,sp. (bivalve) and Rhepoxynius epistomus (amphipod). Several other species were

I, relatively labundant during the present study. These species include: Eudevenopus

, honduran~s, Acanthohaustorius uncinus, and Paraonis fulgens (polychaete).

Th~se cursory comparisons between the two studies suggest a consistency in

. faunal COTposition over time at this site. The dominant fauna present are characteristic

'of well sprted sandy environments typical of the nearshore coastal environment.

; Because of the relative similarity in fauna between the two studies, it appears that any

effects of \dredging at the Longboat Key borrow site were minimal.

IManasota, Key

I Manasota Key was sampled during July, 1992. Faunal conditions at this site have

been preJiously discussed in the section "Pre-Dredging Faunal Community Conditions".

JThese res~lts are briefly reiterated here. Manasota Key had the highest mean number

lof taxa per station (142 ± 29 taxa). The difference in number of taxa between Manasota

IKey and Egmont Key was statistically significant (p < 0.05). Manasota Key also had the

lhighest m:ean number of individuals per square meter (3635 ± 1175). Manasota Key

"also had the highest species diversity (Shannon Index) during July (3.67). There was a

'statistically significant difference (p < 0.05) in Shannon Index between Manasota Key

,and Egmdnt Key. Equitability (a measure of the evenness of species distribution) was

highest at Manasota Key (0.743 ± 0.12). There were no statistical differences in

'equitabilitY among stations. These findings indicate that Manasota Key sustained a rich

'and diverSe fauna during the present study.

t Th~ results support the contention of Osenberg et al. (1994) in that relatively few

of the pop~lation parameters provide adequate power given the time constraints of the

study. This indicates that greater emphasis should be placed on individual-based

'parameterS in field assessments of environmental impacts. It will be critical to develop

and test pfedictive models that link these impacts with effects on populations. In order

to provide\accurate descriptions of temporal variability in the soft-bottom communities

'of estuaries and coastal embayments, low bias and highly precise estimates from each

sampling 6ccasion are required.

I

4.0 SUMMARY AND CONCLUSIONS

3 The10bjectives of this study were to assess the possible impacts of sand dredging

on the berithic communities, both epifaunal and infaunal, and the possible changes to

the geolog1y and topography asa result of the dredging operations. To achieve this

objective g'eophysical and benthic surveys were conducted. I The Igeophysical surveys included precision depth recording, high resolution

seismic reflection profiling, side scan sonar, and box coring for sediment samples. The

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I precision, depth recording was the most useful geophysical data collected due to the

I persistent nature of the excavation left by the dredging operation. On the last cruise in

I 1994, aln'lost 2 years after dredging and after several storm and frontal system periods

, the exca~ation at the Egmont Key Site (Site I) was still clearly visible. It is clear that the

I

excavation was below the local wave base and that even in this shallow system sediment

_supply is 'insufficient to quickly hide the dredge scar. There was no significant difference

! in sedimJnt grain size at Site I comparing pre- and post dredging samples. However,

, grain siz~ determinations did offer an explanation why the depressions created by the

j dredging 'operation were not filled with finer grain sediments as would be expected due

, to wave ahd current action. The reason for this is that the finer than sand fraction of the

, sedimen~ is very small for the inner west Florida shelf surficial sand sheet. Therefore,

•there is nb fine fraction to fill the depressions. The variation in the coarser grain sizes

: is explainbd by increased amounts of calcium carbonate composed primarily of shell

fragmentl

I Th~ epifaunal community was surveyed by otter trawl and video camera. The

epibentho's sampled by ottertrawl displayed some seasonal variability. Portunus gibbesii

l

and Mel/ilk tenuis were the dominant species at both the dredged and undredged sites.

There was no detectable difference between pre- and post-dredging at Site I. The

'underwater video also showed little variability between sites or sampling dates and

observati9ns of flora and fauna were rare except for an occasional portunid crab or

surface tubes of infaunal species. This reflects the constancy of this very dynamic sand

habitat and the low epibenthic diversity.

t The infauna was sampled by box corer. The infauna at all sites was much more , I

abundant than the epibenthic fauna. A between site comparison of species richness and

:diversity pte-dredging indicated healthy, diverse infaunal communities typical of the west

Florida cobstal zone. There was some variation in the dominant taxa between sites.

1 As Egmont Key (Site I) was the only site with pre- and post-dredging sampling,

the infaum:t analysis focused on this location. Each station at this site underwent different

temporal pattems in benthic community composition. Compounding the difficulty of

interpreting the data is the observation that the two stations in the dredged area

respondedl quite differently from each other. In general, the control station exhibited

much less\temporal variability. Therefore, it was not possible to attribute differences

between stations solely to dredging.

! Tem'poral natural trends in infauna community parameters were determined from

the three sampling dates at the Sarasota site (Site II), which was not dredged. Faunal

density and diversity showed peaks in spring and early summer, although there was • !

considerable variability in the pattem.

! The \Longboat Key site was only sampled post-dredging during this study.

Comparison with data from other areas indicates a consistency in faunal composition

over time elt this site, with infauna exhibiting characteristics common to a well sorted

sandy envitonment typical of the nearshore coastal zone. Because of this consistency, - !

it appears that any effects of dredging at the Longboat Key site were minimal. Overall

the analysi~ of the infauna samples indicate that relatively few of the population

parameters provided adequate power to detect dredging impacts given the time and

!

I 20

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spatial constraints of the study. Greater emphasis should be placed on individual-based

parametets in field assessments of environmental impacts. No dredge hole or pits were

observed lin the geophysical records.

1 In conclusion, the only demonstrable impact of the dredging from this study was

1the physidal alteration of the bottom topography which persisted two years after dredging

lactivities ~sing a clamshell dredge ceased at the Egmont Key site (Site I). There were

,no obse~able physical alterations at the Longboat Key site (Site IV) where a dustpan

ldredge was used.

\ I

5.0 LITERATURE CITED

I

Boesch, OF, ML Wass and RW Virnstein. 1976. The dynamics of estuarine benthic

corilmunities. Pp. 176-196 In M.L. Wiley (ed.) Estuarine Processes, Vol 1: Uses,

1 Str~sses, and Adaptations to the Estuary. Academic Press, New York. , 'Bowen, PR and GA Marsh. 1988. Benthic faunal colonization of an offshore borrow pit

; in ~outheastern Florida. Miscellaneous Paper 0-88-5, US Army Engineer

i Waterways Experiment Station, Vicksburg, MS. 43pp.

Conner, 'W.,G and JL Simon. 1979. The effects of oyster shell dredging on an estuarine

benthic community. Estuarine and Coastal Marine Science. 9:749-758.

Doyle,

, LJ lind , T Sparks. 1980. Sediments of the Mississippi, Alabama, and Florida

I (MAFLA) continental shelf. J. Sed. Pet. 50: 905-915. , Folk, RL. 1980. Petrology of Sedimentary Rocks. Hemphill Publishing Co., Austin,

I Tex~s. 185 pp.

Gordon, RB, DC Rhoads and KK Turekian. 1972. the environmental consequence of

{ dre<jge spoil disposal in central Long Island sound: I. The New Haven spoil

ground and New Haven Harbor. Department of Geology and Geophysics, Yale

I Uni~. 39pp. (mimeogr).

~rassle, J~ and HL Sanders. 1973. Life histories and the role of disturbance. Deep-Sea

I Research. 20:643-659.

Groat, G, GBaninb, C Gardner, GK Magnuson, P Rusanowski, and K Weaver. 1993. U.S.

'I' Outer Continental Shelf Sand and Gravel Resources, Programs, Issues and

Rec6mmendations. Final Report of the OCS Policy Committee on OCS Sand and I GraVel Resources. 43pp. plus appendices.

Gray, ,JS. 1974. , Animal sediment relationships. Oceanogr. Mar.Biol. 12:223-261

Hurlbert, SH. 1971. The nonconcept of species diversity: a critique and alternative

I parafneters. Ecology 52: 577·586. , Leverone, JR, JK Culter, JS Sprinkel, MR Milligan and S Mahadevan. Evaluation of long- I term! studies of the benthic community in the vicinity of Big Bend, Tampa Bay.

1 Pp. 219-235 In S.F. Treat and P.A. Clark (eds.) Proceedings, Tampa Bay Area

I SCie?tific InfOrmation Symposium 2. 1991 February 27- March 1; Tampa, FL.

I 528pp.

Mahoney, ~MS and RJ Livingston. 1982. Seasonal fluctuations of benthic macrofauna I in th~ Apalachicola estuary, Florida USA: The role of predation. Mar. BioI. 69: I 207·214.

•

I 21

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I FINAL REPORT

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i

I

1

i Maurer, 0, G Robertson and T Gerlinger. 1993. Long-term temporal and spatial

j fJubtuations of soft bottom infaunal invertebrates associated with an ocean outfall

I fro'm the San Pedro Shelf, California. Int. Revue ges. Hydrobiol. 78: 535-555.

; McCall, pL. 1977. Community pattems and adaptive strategies of the infaunal benthos

I of Long Island Sound. J. Mar. Sci. 35: 221-266.

McNUlty, UK, RC Work, and HB Moore, 1962. Some relationships between the infauna I

, of the Jevel bottom and the sediment in South Florida. Bull. Mar. Sci. Gulf Carib.,

12:322-332.

; Oliver, JS! PT Morgan and WW Smith. 1977. Patterns of succession in benthic infaunal

I communities following dredging and dredged material disposal in Monterey Bay.

Teth. Rpt. 0-77-27, U.S. Army Engineer Waterways Experiment Station, Vicksburg, , MS. ,

Osenbergl CW, RJ Schmitt, SJ Holbrook, KE Abu-Saba and AR Flegal. 1994. Detection

of environmental impacts: natural variability, effect size, and power analysis. I

Ecol. Appl 4: 16-30.

Pimentel, RA and JD Smith. 1993. Biostat II, A Univariate Statistical Toolbox. Bersion

: 2.0.\ Tutorial Manual. Sigma Soft. Placentia, CA. 239 pp., '

Rakocinski, C, RW Heard, T Simons and 0 Gledhill. 1991. Macroinvertebrate

: assbciations from beaches of selected barrier islands in the northern Gulf of

1 Me~ico: Important environmental relationships. Bull. Mar. Sci. 48: 689-701.

Rhoads, D:C, RC Allen and MB Goldhaber. 1977. The influence of colonizing benthos

I

, on physical properties and chemical diagenesis of the estuarine sea floor. Pp.

11~138 l.!L(BC Coull, ed.) Ecology of Marine Benthos. University of South

I CarQlina Press, Columbia, SC.

Rhoads, 9C, PL McCall and JY Yingsi. 1978. Disturbance and production on the

: estuarine sea floor. Amer. Sci. 66: 577-586.

Rhoads, dc and OK Young.1970. The influence of deposit-feeding organisms on

1 seditnent stability and community structure. J.Mar.Res. 28:150-178.

Saloman, <t:H, SP Naughton and JL Taylor. 1982. Benthic community response to - , , dredging borrow pits, Panama City Beach, Florida. Misc. Rep. 82-3, U.S. Army

I Engi'neer Coastal Engineering Research Center, Fort Belvoir, Va.

Shannon, <l::E and W Weaver. 1972. The Mathematical Theory of Communications.

\ Univl of III. Press, Urbana, III. 117p.

Simon, JL 'and OM Dauer. 1977. Rs-establishment of a benthic community following

- natul-al defaunation. pp. 139-154. In (B.C. Coull, ed.) Ecology of Marine Benthos. ! Univ~rsity of South Carolina Press. Columbia, S.C.

Simon, JL,!LJ Doyle, and WG Conner. 1976. Environmental impact of oyster shell

\

dredging in Tampa Bay, Florida. Report No.4: Final Report to Florida

, Department of Environmental Regulation. 104pp.

Taylor, AS.! 1993. Coastal restoration activities and capabilities of bean dredging.

I Presentation to Coastal Zone '93.

Taylor, JL, UR Hall and CH Saloman. 1970. Mollusks and benthic environments in 1 '

Hillsborough Bay, Florida. U.S. Fish. Wildl. Serv., Fish. BulL 67(2): 213-241.

I

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oes Report MMS 95-0004

t Thorson, G. 1966. Some factors influencing the recruitment and establishment ofmarine

I . benthic . , communities. Netherlands J. Sea. Res. 3:267-293.

Van Oolah, AF, DR Calder, and OM Knott. 1984. Effects of dredging and open water i di~posal on benthic macroinvertebrates in a South Carolina estuary. Estuaries · 7(1):28-37.

IZajac, AN and RS Whitlatch. 1982. Response of estuarine infauna to disturbance. I.

I Spetial and temporal variation of recolonization. Mar. Ecol. Prog. Ser. 10: 1-14. I

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As. bureau of the Depllrtrnllnt of the Interior, lhe MV'lll",a looUlnasemDrH Service'. (101:101:5) primary nll!'QmDl.1irin I"' To

man.eg. me minllnllrMo\lfCllS Jocal1ld on the Hllrion'. Ourer ConrinenQI Shelf (OCS). .:oJillc:r I'alenUlI from the Federal OCS aDd onshore FeGeraland Indian lands., .rxI. di:r;trouhI tbGze I'lMlnues.

AI: fbi Nati~n'1 pmc:ipal c:onsewation -seney. The DeF'lnn»nt of the Inluior he nspormbvi!y for mon of oW' nationa1~ ~ci ~Iic lanch. and.naN~ ".SOUI'QII. T.m. ind~ fort.~ sound use of our land and watar _urcm; prol1l~ our

r.m. widlife, and. biologall cliven:iry, ~MnB the erMr'OlUnmt!ll and eulru:a.l values of our rmrionlll p1rla and bittoriad plaee.;

aDd pn:widing for the Ilr:l;.:,.m,nt of life thnn.gh outdoor rtlCnlltion. Tbll Depanl1'lllDt &Delle:: our energy and mineral ftlSOUl'alS

aDd IIIOrD to M1Nre that their darelopmmt is in the belt mt8rats of an our peopilloly OQl:OunlsUv stM'archhip lad citiuon

participlI:ion in thllir C&ftI. The DlpartlDlltlt also has. m:tjor ftlsponD,i.liry for Amltrian IndUn nlRlVlltion cornmunitie. Ind for people who We in island tenitoAlJ uDder US. adrni.ai..stnrion. "

Mote<Mlr. in weorkilll to mIIet ia: ~ilitieLthe Offabo." Mbr.... MaNtez:m:nl Profn.m administe.., the OCS competitive

lenms: P'OSraR1 and _esthe we and ~t3l1lysound upiom-ion IDd production ,)f our Narion'a oli.ahou narur.ti

Sa, oil and other mmlll"ll nDOUI'CIII. The ),{MS R.."...,. M.mtenRft P!"Otrlmmuta itS respo::uibiliries by .nsuring the

efJ"'lCieur,. litneJy arid ac:cunlte coUlICtton aDd disbvrmnwnr of rev.nUll fTOm miQenU lUring and p-oduc:rioo d~ 10 rndUln trhea .ad aUottee.. Stares and thll U.s. Treaaury.

TbII ~MS ItMt to fulfill ita rwponaibi!ities through tt. senetlll guiding prif'lCi?llll of:(l) bei~' rIIJpOnsw ro tim plbJic".

CQDCemll and inrenrm by maintaillirlga dia~UIl with.an pCIlmTi.a.lJy aITeCl1ld pln\u and (2) 'Inyi~ our in prognms with lin

empbasU on woriciq; to enhance the q~.Ii1y of life for aU AmericaQli by lending MMS assina~e and e:.:putise 10 economi<= d-Joptt'8l1t and IInvironmenQI protection.

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