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The effects of oyster shell dredging on an estuarine benthic community
Estuarine and Coastal Marine Science (1979) 9, 749--758
T h e Effects o f Oyster Shell D r e d g i n g on an Estuarine B e n t h i c C o m m u n i t y a
William G. Conner b and Joseph L. S i m o n Department of Biology, University of South Florida, Tampa, Florida 33620, U.S.A.
Keywords: dredging; oysters; shells; benthic fauna; species composition; Florida coast This paper describes the extent and nature of the effects on the benthos of physical disruptions associated with dredging fossil oyster shell, qhvo dredged areas and one undisturbed control area in Tampa Bay, Florida, were quantitatively sampled before dredging and for one year after dredging. The immediate effects of dredging on the soft-bottom community were reductions in numbers of species (4o% loss), densities of macroinfauna (65% loss), and total biomass of invertebrates (90% loss). During months 6-12 after dredging, the analysis used (Mann-Whitney U "lest, a = 0"05) showed no difference between dredged and control areas in number of species, densities, or biomass (except El). Community overlap (Czeckanowski's coefficient) between dredged and control areas was reduced directly after dredging, but after 6 months the pre-dredging level of similarity was regained.
Introduction Mining of dead oyster shell is an economically and ecologically important industry on the Gulf Coast of the United States. Oyster shell is currently being removed from Gulf Coast estuaries at a rate in excess of2o milliort cubic yards each year (Esprcy, 1975). Dredged shell is used for the production of cement, masonry block, road beds, poultry feed, or as cultch in the establishment of new oyster beds. During shell dredging, the upper 1- 3 m of the substratum is disrupted by a cutter head and hydraulically transported to a hopper on the top of the dredge. The hopper separates larger shell fragments from the finer sediment particles and water which pass through the hopper. Fragmems larger than 6.35 mm (88inch) are retained by the hopper and deposited onto a barge which transports the shell to shore. The balance of the material passes through the hopper and is returned to the water through an effluent pipe. Oyster shell dredging involves physical disruption of benthic systems and produces a visible silt plume (Simon & Dyer, 1972). This paper documents the immediate and long-term effects of shell dredging on a soft-bottom community in Tampa Bay, Florida. Previous investigations dealing with the impact of shell dredging on the benthos have presented very limited m o u n t s of data (1V[ay,1973), or compared dredge cuts of different ages with adjacent control areas (Taylor, I972a , I972b, 1973; Army Corps of Engineers, 1974). These previous aDissertation research conducted in" partial fulfillment of requirements for the degree of Doctor of Philosophy at the University of South Florida (WGC). bCurrent address: The MITRE Corporation, x8zo Dolley Madison Blvd., i%IcLean,Virgiania 22IO2. 749 .
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studies have failed to demonstrate the adequacy of the sample size used, or to evaluate the appropriateness of the control areas before dredging. By contrast, the current study uses a sample size found large enough to collect the majority of benthic macroinfaunal species present. Two dredged areas and a reasonably good control area were sampled before dredging and monitored for one year after dredging. A nonparametrie statistic was applied to biomass and density data to demonstrate the effects of one shell dredging operation in Tampa Bay, Florida on the benthos.
27050 .
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Figure I. The dredging study area was in Tampa Bay which is located on the Gulf Coast of Florida. Et and E: represent dredged areas; Ct, Cz and Cs represent potential control areas. C3 was chosen as the best control. Materials and methods The study was conducted in Tampa Bay, Hillsborough County, Florida (Figure x). Samples were taken at two experimental sites which were dredged and at one control site which was not affected by dredging (F_a, E2 and C3 respectively). All three sites were located in about 7 m of water and were characterized by muddy bottoms and the presence of Chaetopterus tubes in densities of 3o-5o/mL Selection of the most suitable control site was accomplished in a two step process. First, fifteen areas were visually inspected by divers and qualitatively compared to the experimental site F_a. The two experimental sites were located about ISO m apart. Visual inspection by divers showed no apparent difference between the experimental sites, so in the procedure of control site selection, Ea was used to represent both experimental sites. After this qualitative comparison, the three sites which appeared to be the most similar (Ca, C2 and C a in Figure x)
The effects of oyster shell dredging on an estuarine benthic community
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F i g u r e 2. Species-area curves were u s e d to d e t e r m i n e t h e n u m b e r o f cores r e q u i r e d to adequately s a m p l e m a c r o b e n t h i e species. T w o r a n d o m o r d e r i n g s o f cores are
shown. were subjected to quantitative sampling and compared to the experimental site using Czeckanowski's coefficient which considers species overlap and relative abundances at the sites. Czeckanowski's coefficient was calculated using the following formula:
c,=
ZW
a+b '
where 'to' is the sum over species of the lesser numerical scores between samples, ' a ' is the total number of individuals in one sample, and 'b' is tile total number of individuals in the other sample. This index has been previously referred to as a modification of Jaccard's Coefficient of Community, Gleason's Index, and Kulczynski's Index (Bray & Curtis, 1957). Using Czeckanowski's coefficient, the community similarity indexes between the experimental and the potential control areas were calculated as: Cx C2 C3
60.8% 58"3% 67.o %
Although statistical methods are not available to objectively test the significance of the observed differences in similarity between the experimental and control sites, Cs was chosen from the three potential control areas for use in the balance of the study. Dredging operations were not identical at the two experimental sites ( ~ , Ez, Figure 1). The following table summarizes the differences between dredging treatments: Hopper used Time dredged Area dredged El Ez
No Yes
4 Hours 1o Days
5~ m X 5 ~ m ,oo m • 300 m
For the first 3 months after dredging (May-Au .gust 1975), samples were taken at z-week intervals, as it was thought that the fastest rates of change would be observed immediately after dredging. Subsequently, monthly samples were taken until April 1976, 12 months after dredging.
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IV. G. Conner C.~ft. L. Simon
Benthic samples were taken by divers with hand-held PVC cores. Each core sampled an area of o'oo45 m 2 to a depth of 2z cm. To determine the number of cores necessary to sample all species, a large number of cores (2o) was taken in an area similar to the experimental area. Figure 2 shows the cumulative number of species plotted against the number of cores in the manner of a standard species-area graph for two random orderings of the cores. The chosen sample size, x6 cores (o.725 m2), appears adequate to sample the majority of the species and all numerically important species in the pre-dredging system. Species-area curves were also used to check adequacy of sample size in two phases of the recovery system. It should be noted that this method of determining sample size is based on the objective of sampling most or all of the species present. No confidence limits on measurements of density or biomass arc implied. The sampled cores were extricated from the sediment, capped at both ends, and returned to the surface where the core contents were sieved using a o-5 mm screen. Reish (i959) showed that this screen aperture size should retain nearly xoo% of the macrofaunal species and over 95% of the biomass of the macrofaunal species. The material retained on the sieve
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Figure 3. Bottom water salinity and sediment temperature determinations made at experimental and control areas from February x975 to April x976 showed no appreciable difference between experimental and control areas.
The effects of oyster shell dredging on an estuarine benthic community
753
TABLE I. Immediate effects of dredging at El and E, on total fauna, polycbaetes, bivalves, amphipods, and ophiuroids as measured by; (a) Number of invertebrate species, (b) Densities of invertebrates, and (c) Invertebrate biomass. Densities of the cumacean Cyclaspis tended to disguise dredging effects through dramatic temporal and spatial variation and have, therefore, been disregarded (a) Number of invertebrate species El
Total fauna Polychaetes Bivalves Amphipods Ophiuroids
El
Before
After
% Loss
Before
After
% LOss
78 22 x4 x2 5
47 16 7 7 4
4o% 27% 50~ 42% 2o%
8x 3o Iz xx 5
48 2o 5 xo z
42 % 33% 58% 9% 6oO
(b) Densities of invertebrates (Ind m-I)
Et Before Total fauna x665I (without Cydaspis) Polychaetes 5647 Bivalves 3767 Amphipods 3245 Ophiuroids 467
Ea
After
% Loss
Before
After
~o Loss
3627
78%
20448
9443
54%
2329 6x7 274 83
59% 85% 92% 82%
zx239 24oo 2960 328
47oi 356 89r 4x
58% 85~/O 7o% 88%
(c) Invertebrate biomass (gm -~)
El
El
Total fauna Polychaetes Bivalves Amphipods OPhiuroids
Before
After
% Loss
Before
After
% Loss
26"3o 4"93 x'64 o'x5 7"98
3"z9 o'42 o'3x o.ox x.74
88% 92% 82% 93% 78%
28"7x 4"72 3"a6 o'25 7'68
4"4z x.to o'H o'x3 o'63
85% 77% 96% 48% 92%
was treated with a o'xS~/o solution of propylene phenoxytol (McKay & Hartzband, i97o ) to minimize muscular contraction of the organisms during subsequent fixation with a xO~/o solution of formalin. Rose bengal was mixed with the formalin to stain the organisms red ~ [ a s o n & Yevitch, x976). "The organisms were picked from the samples, sorted to major taxa, and stored in 7o% isopropyl alcohol for later identification to species. After species identification and counting of individuals by cores, biomass was determined for each species from samples pooled by station. Biomass was defined as the loss on combustion in a muffle furnace at 55 ~ ~ for z 4 h (Paine, x97x ). Sediment samples were taken using a x.75 cm diameter core to a depth of 15 em. Cores were returned to the laboratory on ice and frozen fintil processed. Three replicates were wet sieved and then dried at 60 ~ for 24 h to determine particle size frequency distribution. Sediment parameters (median phi, mean phi, sorting coefficient) were calculated (Inman,
754
W. G. Conner ~ J. L. Simon
z952 ). Four aliquots of a fourth core were used to measure the organic content of the sediment by the ash-free-weight method (Crisp, x97x ; Paine, I97x ). The temperature at each station was measured by a diver inserting the bulb of a thermometer approximately z cm into the substratum and reading the thermometer under water. Salinity of a bottom water sample was determined using a refractometer.
nJ.d. 5O
50 n.r,.d_
Number
of Species 4O
40 52.52 5.9 n=6
51.22 .2
8.8 n-6
30
30
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E2
EI
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n.$.d.
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1=6
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E2
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20
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E2
E1
E2
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"i" 25516"~ oI
5
n'~ c
I E1 RECOVERY PHASE I (Months 1-6)
0 E2
C
RECOVERY PHASE II (Months 6-12)
Figure 4. During the first 6 months after dredging, both experimental sites showed reductions in number of species, invertebrate densities and biomass. However, during months 6-x2 significan~ differences between control and experimental sites were not found except in biomass at Es. *Denotes a significant difference was shown by the i~,lann-X,Vhitney U Test ((z = 0"05) between the experimental and the appropriate control, 'n.s.d.' signifies no significant differences.
The effects of oyster shell dredging on an estuarlne benthic community
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1
Dredging
l
I
/ 1075 Sampling Dates
1078
Figure 5. Community similarity as measured by Czeckanowski's quantitative index
between the experimental sites (E~ and E2) and their time-appropriate controls (12) showed a reduction immediately after dredging and a recovery within 6 months.
Results
Physical parameters Salinity and temperature readings show inconsequential differences between control and experimental sites (Figure 3). Differences between sites were much less than the seasonal variation at each site, and temperature and salinity differences were assumed not to cause biological differences between the dredged and control areas. Dredging caused significant though temporary changes in sediment parameters. The immediate effects of dredging on the upper 15 cm (the biologically important sediment layer) were: increased particle size, reduced organic content, reduced silt-clay content, and a more poorly sorted sediment. However, within 6-x~ months the surface sediment had reverted to one not significantly different from predredging sediments.
Immediate biological effects of shell dredghlg Within 3~ min after dredging was performed at Ea, inspection of the bottom by divers revealed a poorly consolidated sandy substratum which was very different from the predredging muddy bottom. Tubes of the polychaete Chaetopterus variopedatus (Reiner) were dislodged and laying on the bottom and individuals of Pinnixa spp. (a decapod crab often commensal with Chaetopterus) were mobile on the substratum surface. Several large errant polychaetes were found in the water column and on the bottom (Nephtyidae, Nereidae). The bottom topography was altered from flat to troughs and gouges x-2 m in relief. Dredged areas were sampled within minutes (El) or days (E2) after dredging. Losses in numbers of species ( 4 o ~ loss) were less than losses in numbers of individuals (66% loss) and losses in biomass ( 8 7 ~ loss) were greater than either losses in species numbers or densities (Table I). These generalizations hold for total fauna and for most of the major taxa. Amphipods were least affected and bivalves most affected, while polychaetes and ophiuroids sustained moderate losses.
756
IV. G. Conner ~.~ft. L. Simon
Community recovery after dredging To estimate the duration of statistically significant dredging effects, the one year study period was divided into two 6-month recovery phases. For each phase the two experimental sites were statistically compared to the control site using the biological parameters of number of species present, densities of invertebrates, and biomass (Figure 4). In each case, there was a significant quantitative reduction during the first six months after dredging. However, during Recovery Phase II, the benthic community had returned to control levels in number of species, densities, and biomass (except El). This analysis indicates that although there were significant negative effects on the community, these effects were temporary, lasting less than i2 months. The analysis presented above considers only the quantitative aspects of the system without addressing problems of changes in species composition and community structure. Similarity analysis was used to look for measurable changes in community structure (Figure 5). Comparisons were made between dredged areas and their contemporary controls using Czcckanowski's coefficient. According to this analysis, the immediate effect of dredging on community similarity was a reduction of about I5% from the pre-dredging condition. This drop was followed by a trend of increasing similarity until November I975, when it appeared that predredging levels of similarity were reached. A rebound period of about six months was also observed for infaunal densities and species composition at both Fa and E z. Therefore, it appears that the post-dredglng community structure was not grossly different from that of the control area or the pre-dredging system. Fa was dredged for a shorter period of time than E a and the hopper, which was used at E2, was not used at 'F~. No consistent or meaningful differences were observed between dredging effects at E x and E 2 relative to the differences in dredging procedures.
Discussion and conclusions
May (i973) concluded that the effects of hydraulic dredging on the maerobenthos were limited and temporary in Alabama estuaries. However, the major thrust of his work involved water quality effects and the ultimate fates of rcsuspended sediments. Biological portions of the study were not well documented by E. E. Jones (University of South Alabama, unpublished) who sampled around a shell dredge and in control areas. Research concerned with shell dredging was performed for the Army Corps of Engineers in San Antonio Bay by Texas A&M Research Foundation (I974). The scope of this project was very broad and the treatment of shell dredging effects"on maerobenthic communities was not decisive. Different age cuts (I95o-x972) were compared to control areas, but sample size may not have been adequate and control areas were not shown to be appropriately similar to dredged areas before the dredging occurred. Analyses of biomass effects were not included. However, the study concluded that in new dredge cuts, defaunation was virtually complete. After four years, dredged areas sustained macrofaunal densities at least 80% of those found in undredged areas. The Army Corps of Engineers (x974) reached the same basic conclusions based on similar work done by the Taylor Biological Compahy in Tampa Bay. This study generally supports the conclusions of the previous studies, with the exception that, in this case, defaunation resulting from dredging was not total. Oyster shell dredging has been shown to have a significant impact "on the benthic community of a dredged area. However, the observed effects of dredging on the benthos were temporary and within twelve months a near total recovery was observed in this case.
The effects of oyster shell dredging on an estuarine benthic community
757
Shell dredging need not be detrimental to the recreational industrial, or ecological functions of an estuary. However, in a review, Clark (x977) concludes that shell dredging has the potential to cause problems if mining activities are excessively heavy or poorly managed. Dredging should not be allowed to disrupt bottom topography to the extent that the water circulation patterns of the estuary would be altered. This could seriously affect patterns of sediment deposition and water quality (Schubel 86 ~ e a d e , I975). Problems might also arise from the nature of rcsuspended sediments which may seriously alter water quality if significant amounts of chemical contaminants are released (Lee, x977). Ultimate fates of suspended sediments sl~ould be considered witI~ respect to areas such as grass beds, oyster beds and coral reefs that might be sensitive to increased sedimentation.
Acknowledgements We would like to acknowledge S. Bloom, J. Cultcr, D. Dabaets, D. Dauer, D. ~IeKirdy, R. Mumme, S. Santos, and L. Weiner for special assistance in the field and in the laboratory. Without the cooperation of the dredging industry through Benton and Company, St. Petersburg, Florida, this research would not have been possible. This study was supported by funding from the State of Florida Department of Environmental Regulation. We are also grateful to the M I T R E Corporation, ~Ietrek Division for providing facilities needed to prepare the final manuscript.
References Army Corps of Engineers I974 Draft Environmental Impact Statement--Oyster Shell Dredglng--Tampa and llillsborough l~ays, Florida. U.S. Army Engineer District, Jacksonville, Florida. xzx pp. Bray, J. R. & Curtis, J. T. x957 An ordination of the upland forest communities of southern "~Viseonsin. Ecological 3Ionographs 27, 325-349. Clark, J. R. x977 CoastalEcosystem l~lanagement. John Wiley and Sons, New York. 9z8 pp. Crisp, D. J. x97x Energy flow measurement. In l~lethodsfor the Study of 2~larine Benthos (IBP Handbook No. x6, I/olme, N. A. & Mclntyre, A. D., eds). Blackwell, Oxford. pp. x97-279. Esprey, W. IL Jr. x977 Environmental aspects of dredging in the Gulf Coast Zone with some attention paid to shell dredging, bz Estuarb, e Pollution Control and Assessment. EPA Office of Water Planning and Standards, '~Vashington, D.C. No. 44olx--77-oo7. 755 PPInman, K. L. z95z i'~easures for describing the size distribution for sediments. Journal of Sedimentary Petrology 2z, zz5-z45. Lee, G. F. x977 Significance of chemical contaminants in dredged sediment on estuarine water quality. In Estuarlne Pollution Control and Assessment. EPA Office of "~Vater Planning and Standards, Washington, D.C. No. 44o--x-77--oo7. 755 PP. Mason, W. T. & Yevitch, P. P. x967 The use of Phloxine B and Rose Bengal stains to facilitate sorting benthic samples. Transactions of the American 2~IicroscoplcalSociety 86, 2zI-223. May, E. B. x973 Environmental effects of hydraulic dredging in estuaries. Alabama ~[arine Resource 13ulletbz No. 9. PP- x-85. McKay, C. R. & IIartzband, D. J. x97o Propylene phenoxytol: narcotic agent for unsorted benthic invertebrates. Transactiolfs of the American ~llcroscopical Society 89, 53-54. Paine, R. T. x97r The measurement and application of the calorie to ecological problems. Annual Revler~'s of Ecology and Systematics 2, x45-I64. Reish, D. J. z959 A discussion of the importance of screen size in washing quantitative marine bottom samples. Ecology 40, 307-309. Schubel, J. R. &/~eade, R. H. x977 l~'~art'simpact on sedimentation. In Estuarlne Pollution Controland Assessment. EPA Office of Water Planning and Standards, Washington, D.C. No. 44o/x'-'77/oo7. 755 Pp. Simon, J. L. & Dyer, J. P. III. I97z An Evaluation ofSiliation Createdby Bay Dredging and Construction Company Durhzg Oyster Shell Dredgbtg Operations b~ Tampa Bay, Florida, January x, x97z to 3larch 3x, x97z. Final Research Report, Department of Biology, University of South Florida, Tampa. 60 pp.
758
IV. G. Conner & f t . L. Simon
Taylor, J. L. x972a Some Effects of Oyster Shell Dredging on Benthic Invertebrates in Tampa Bay, Florida. Taylor Biological Company, St. Petersburg Beach, Florida, z6 pp. Taylor, J. L. z972b Some Effects of Oyster Shell Dredging on Benthic Invertebrates in ~Iobile Bay, Alabama. Taylor ]3iologlcal Company, St. Petersburg Beach, Florida, x6 pp. Taylor, J. L. x973 Studies of the Effects of Oyster Shell Dredging in Tampa Bay, Florida. Taylor Biological Company, St. Petersburg ]3each, Florida, 7 PP. Texas A & ~,~ Research Foundation z973 Environmental Impact Assess,neat of Shell Dredging in San Antonio Bay, Texas. U.S. Army Engineer District, Galveston, Texas, ~3x pp.
T h e Effects o f Oyster Shell D r e d g i n g on an Estuarine B e n t h i c C o m m u n i t y a
William G. Conner b and Joseph L. S i m o n Department of Biology, University of South Florida, Tampa, Florida 33620, U.S.A.
Keywords: dredging; oysters; shells; benthic fauna; species composition; Florida coast This paper describes the extent and nature of the effects on the benthos of physical disruptions associated with dredging fossil oyster shell, qhvo dredged areas and one undisturbed control area in Tampa Bay, Florida, were quantitatively sampled before dredging and for one year after dredging. The immediate effects of dredging on the soft-bottom community were reductions in numbers of species (4o% loss), densities of macroinfauna (65% loss), and total biomass of invertebrates (90% loss). During months 6-12 after dredging, the analysis used (Mann-Whitney U "lest, a = 0"05) showed no difference between dredged and control areas in number of species, densities, or biomass (except El). Community overlap (Czeckanowski's coefficient) between dredged and control areas was reduced directly after dredging, but after 6 months the pre-dredging level of similarity was regained.
Introduction Mining of dead oyster shell is an economically and ecologically important industry on the Gulf Coast of the United States. Oyster shell is currently being removed from Gulf Coast estuaries at a rate in excess of2o milliort cubic yards each year (Esprcy, 1975). Dredged shell is used for the production of cement, masonry block, road beds, poultry feed, or as cultch in the establishment of new oyster beds. During shell dredging, the upper 1- 3 m of the substratum is disrupted by a cutter head and hydraulically transported to a hopper on the top of the dredge. The hopper separates larger shell fragments from the finer sediment particles and water which pass through the hopper. Fragmems larger than 6.35 mm (88inch) are retained by the hopper and deposited onto a barge which transports the shell to shore. The balance of the material passes through the hopper and is returned to the water through an effluent pipe. Oyster shell dredging involves physical disruption of benthic systems and produces a visible silt plume (Simon & Dyer, 1972). This paper documents the immediate and long-term effects of shell dredging on a soft-bottom community in Tampa Bay, Florida. Previous investigations dealing with the impact of shell dredging on the benthos have presented very limited m o u n t s of data (1V[ay,1973), or compared dredge cuts of different ages with adjacent control areas (Taylor, I972a , I972b, 1973; Army Corps of Engineers, 1974). These previous aDissertation research conducted in" partial fulfillment of requirements for the degree of Doctor of Philosophy at the University of South Florida (WGC). bCurrent address: The MITRE Corporation, x8zo Dolley Madison Blvd., i%IcLean,Virgiania 22IO2. 749 .
.
.
.
.
.
.
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. . . . . .
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.
.
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IV. G. Conner & J. L. ,Simon
studies have failed to demonstrate the adequacy of the sample size used, or to evaluate the appropriateness of the control areas before dredging. By contrast, the current study uses a sample size found large enough to collect the majority of benthic macroinfaunal species present. Two dredged areas and a reasonably good control area were sampled before dredging and monitored for one year after dredging. A nonparametrie statistic was applied to biomass and density data to demonstrate the effects of one shell dredging operation in Tampa Bay, Florida on the benthos.
27050 .
p 0
82030 . 4
8 12 Km
16
Figure I. The dredging study area was in Tampa Bay which is located on the Gulf Coast of Florida. Et and E: represent dredged areas; Ct, Cz and Cs represent potential control areas. C3 was chosen as the best control. Materials and methods The study was conducted in Tampa Bay, Hillsborough County, Florida (Figure x). Samples were taken at two experimental sites which were dredged and at one control site which was not affected by dredging (F_a, E2 and C3 respectively). All three sites were located in about 7 m of water and were characterized by muddy bottoms and the presence of Chaetopterus tubes in densities of 3o-5o/mL Selection of the most suitable control site was accomplished in a two step process. First, fifteen areas were visually inspected by divers and qualitatively compared to the experimental site F_a. The two experimental sites were located about ISO m apart. Visual inspection by divers showed no apparent difference between the experimental sites, so in the procedure of control site selection, Ea was used to represent both experimental sites. After this qualitative comparison, the three sites which appeared to be the most similar (Ca, C2 and C a in Figure x)
The effects of oyster shell dredging on an estuarine benthic community
75 x
90 qu
80 70
0
E
6O
5O U~ed
i/
40 30
.. j , , - /
=0
/
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I
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f
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f
I
I
2
3
4
5
6
7
8
9
Number
I
f
10 I I
I
~
!
V
f
I
!
12 13 14 15 16 17 18 19 20
of Cores
F i g u r e 2. Species-area curves were u s e d to d e t e r m i n e t h e n u m b e r o f cores r e q u i r e d to adequately s a m p l e m a c r o b e n t h i e species. T w o r a n d o m o r d e r i n g s o f cores are
shown. were subjected to quantitative sampling and compared to the experimental site using Czeckanowski's coefficient which considers species overlap and relative abundances at the sites. Czeckanowski's coefficient was calculated using the following formula:
c,=
ZW
a+b '
where 'to' is the sum over species of the lesser numerical scores between samples, ' a ' is the total number of individuals in one sample, and 'b' is tile total number of individuals in the other sample. This index has been previously referred to as a modification of Jaccard's Coefficient of Community, Gleason's Index, and Kulczynski's Index (Bray & Curtis, 1957). Using Czeckanowski's coefficient, the community similarity indexes between the experimental and the potential control areas were calculated as: Cx C2 C3
60.8% 58"3% 67.o %
Although statistical methods are not available to objectively test the significance of the observed differences in similarity between the experimental and control sites, Cs was chosen from the three potential control areas for use in the balance of the study. Dredging operations were not identical at the two experimental sites ( ~ , Ez, Figure 1). The following table summarizes the differences between dredging treatments: Hopper used Time dredged Area dredged El Ez
No Yes
4 Hours 1o Days
5~ m X 5 ~ m ,oo m • 300 m
For the first 3 months after dredging (May-Au .gust 1975), samples were taken at z-week intervals, as it was thought that the fastest rates of change would be observed immediately after dredging. Subsequently, monthly samples were taken until April 1976, 12 months after dredging.
75z
IV. G. Conner C.~ft. L. Simon
Benthic samples were taken by divers with hand-held PVC cores. Each core sampled an area of o'oo45 m 2 to a depth of 2z cm. To determine the number of cores necessary to sample all species, a large number of cores (2o) was taken in an area similar to the experimental area. Figure 2 shows the cumulative number of species plotted against the number of cores in the manner of a standard species-area graph for two random orderings of the cores. The chosen sample size, x6 cores (o.725 m2), appears adequate to sample the majority of the species and all numerically important species in the pre-dredging system. Species-area curves were also used to check adequacy of sample size in two phases of the recovery system. It should be noted that this method of determining sample size is based on the objective of sampling most or all of the species present. No confidence limits on measurements of density or biomass arc implied. The sampled cores were extricated from the sediment, capped at both ends, and returned to the surface where the core contents were sieved using a o-5 mm screen. Reish (i959) showed that this screen aperture size should retain nearly xoo% of the macrofaunal species and over 95% of the biomass of the macrofaunal species. The material retained on the sieve
-
...4It-- 4 r s ~
I sJ
\
as
, . Control . . . .
Experimental
25
10
F
I M
0 j
•
I j
IAJSIOINIDJ
1975
j I FIM
l
1976
Sampling Dates
Figure 3. Bottom water salinity and sediment temperature determinations made at experimental and control areas from February x975 to April x976 showed no appreciable difference between experimental and control areas.
The effects of oyster shell dredging on an estuarine benthic community
753
TABLE I. Immediate effects of dredging at El and E, on total fauna, polycbaetes, bivalves, amphipods, and ophiuroids as measured by; (a) Number of invertebrate species, (b) Densities of invertebrates, and (c) Invertebrate biomass. Densities of the cumacean Cyclaspis tended to disguise dredging effects through dramatic temporal and spatial variation and have, therefore, been disregarded (a) Number of invertebrate species El
Total fauna Polychaetes Bivalves Amphipods Ophiuroids
El
Before
After
% Loss
Before
After
% LOss
78 22 x4 x2 5
47 16 7 7 4
4o% 27% 50~ 42% 2o%
8x 3o Iz xx 5
48 2o 5 xo z
42 % 33% 58% 9% 6oO
(b) Densities of invertebrates (Ind m-I)
Et Before Total fauna x665I (without Cydaspis) Polychaetes 5647 Bivalves 3767 Amphipods 3245 Ophiuroids 467
Ea
After
% Loss
Before
After
~o Loss
3627
78%
20448
9443
54%
2329 6x7 274 83
59% 85% 92% 82%
zx239 24oo 2960 328
47oi 356 89r 4x
58% 85~/O 7o% 88%
(c) Invertebrate biomass (gm -~)
El
El
Total fauna Polychaetes Bivalves Amphipods OPhiuroids
Before
After
% Loss
Before
After
% Loss
26"3o 4"93 x'64 o'x5 7"98
3"z9 o'42 o'3x o.ox x.74
88% 92% 82% 93% 78%
28"7x 4"72 3"a6 o'25 7'68
4"4z x.to o'H o'x3 o'63
85% 77% 96% 48% 92%
was treated with a o'xS~/o solution of propylene phenoxytol (McKay & Hartzband, i97o ) to minimize muscular contraction of the organisms during subsequent fixation with a xO~/o solution of formalin. Rose bengal was mixed with the formalin to stain the organisms red ~ [ a s o n & Yevitch, x976). "The organisms were picked from the samples, sorted to major taxa, and stored in 7o% isopropyl alcohol for later identification to species. After species identification and counting of individuals by cores, biomass was determined for each species from samples pooled by station. Biomass was defined as the loss on combustion in a muffle furnace at 55 ~ ~ for z 4 h (Paine, x97x ). Sediment samples were taken using a x.75 cm diameter core to a depth of 15 em. Cores were returned to the laboratory on ice and frozen fintil processed. Three replicates were wet sieved and then dried at 60 ~ for 24 h to determine particle size frequency distribution. Sediment parameters (median phi, mean phi, sorting coefficient) were calculated (Inman,
754
W. G. Conner ~ J. L. Simon
z952 ). Four aliquots of a fourth core were used to measure the organic content of the sediment by the ash-free-weight method (Crisp, x97x ; Paine, I97x ). The temperature at each station was measured by a diver inserting the bulb of a thermometer approximately z cm into the substratum and reading the thermometer under water. Salinity of a bottom water sample was determined using a refractometer.
nJ.d. 5O
50 n.r,.d_
Number
of Species 4O
40 52.52 5.9 n=6
51.22 .2
8.8 n-6
30
30
El
C
E2
E2
EI
n.s.d. 20,000
n.$.d.
20,000
Dens~t~ Ind./m 2 10,000
10,000 1404: 4562 n=6
16885• 4313 rl=6
198362 6307 n-6 '
22182"* 10548
1=6
0 El
E2
20-
20
15-
15
EI
E2
E1
E2
Biomass
Q,'m2
,
10"
10
"i" 25516"~ oI
5
n'~ c
I E1 RECOVERY PHASE I (Months 1-6)
0 E2
C
RECOVERY PHASE II (Months 6-12)
Figure 4. During the first 6 months after dredging, both experimental sites showed reductions in number of species, invertebrate densities and biomass. However, during months 6-x2 significan~ differences between control and experimental sites were not found except in biomass at Es. *Denotes a significant difference was shown by the i~,lann-X,Vhitney U Test ((z = 0"05) between the experimental and the appropriate control, 'n.s.d.' signifies no significant differences.
The effects of oyster shell dredging on an estuarlne benthic community
l
5o
J..,
755
1
Dredging
l
I
/ 1075 Sampling Dates
1078
Figure 5. Community similarity as measured by Czeckanowski's quantitative index
between the experimental sites (E~ and E2) and their time-appropriate controls (12) showed a reduction immediately after dredging and a recovery within 6 months.
Results
Physical parameters Salinity and temperature readings show inconsequential differences between control and experimental sites (Figure 3). Differences between sites were much less than the seasonal variation at each site, and temperature and salinity differences were assumed not to cause biological differences between the dredged and control areas. Dredging caused significant though temporary changes in sediment parameters. The immediate effects of dredging on the upper 15 cm (the biologically important sediment layer) were: increased particle size, reduced organic content, reduced silt-clay content, and a more poorly sorted sediment. However, within 6-x~ months the surface sediment had reverted to one not significantly different from predredging sediments.
Immediate biological effects of shell dredghlg Within 3~ min after dredging was performed at Ea, inspection of the bottom by divers revealed a poorly consolidated sandy substratum which was very different from the predredging muddy bottom. Tubes of the polychaete Chaetopterus variopedatus (Reiner) were dislodged and laying on the bottom and individuals of Pinnixa spp. (a decapod crab often commensal with Chaetopterus) were mobile on the substratum surface. Several large errant polychaetes were found in the water column and on the bottom (Nephtyidae, Nereidae). The bottom topography was altered from flat to troughs and gouges x-2 m in relief. Dredged areas were sampled within minutes (El) or days (E2) after dredging. Losses in numbers of species ( 4 o ~ loss) were less than losses in numbers of individuals (66% loss) and losses in biomass ( 8 7 ~ loss) were greater than either losses in species numbers or densities (Table I). These generalizations hold for total fauna and for most of the major taxa. Amphipods were least affected and bivalves most affected, while polychaetes and ophiuroids sustained moderate losses.
756
IV. G. Conner ~.~ft. L. Simon
Community recovery after dredging To estimate the duration of statistically significant dredging effects, the one year study period was divided into two 6-month recovery phases. For each phase the two experimental sites were statistically compared to the control site using the biological parameters of number of species present, densities of invertebrates, and biomass (Figure 4). In each case, there was a significant quantitative reduction during the first six months after dredging. However, during Recovery Phase II, the benthic community had returned to control levels in number of species, densities, and biomass (except El). This analysis indicates that although there were significant negative effects on the community, these effects were temporary, lasting less than i2 months. The analysis presented above considers only the quantitative aspects of the system without addressing problems of changes in species composition and community structure. Similarity analysis was used to look for measurable changes in community structure (Figure 5). Comparisons were made between dredged areas and their contemporary controls using Czcckanowski's coefficient. According to this analysis, the immediate effect of dredging on community similarity was a reduction of about I5% from the pre-dredging condition. This drop was followed by a trend of increasing similarity until November I975, when it appeared that predredging levels of similarity were reached. A rebound period of about six months was also observed for infaunal densities and species composition at both Fa and E z. Therefore, it appears that the post-dredglng community structure was not grossly different from that of the control area or the pre-dredging system. Fa was dredged for a shorter period of time than E a and the hopper, which was used at E2, was not used at 'F~. No consistent or meaningful differences were observed between dredging effects at E x and E 2 relative to the differences in dredging procedures.
Discussion and conclusions
May (i973) concluded that the effects of hydraulic dredging on the maerobenthos were limited and temporary in Alabama estuaries. However, the major thrust of his work involved water quality effects and the ultimate fates of rcsuspended sediments. Biological portions of the study were not well documented by E. E. Jones (University of South Alabama, unpublished) who sampled around a shell dredge and in control areas. Research concerned with shell dredging was performed for the Army Corps of Engineers in San Antonio Bay by Texas A&M Research Foundation (I974). The scope of this project was very broad and the treatment of shell dredging effects"on maerobenthic communities was not decisive. Different age cuts (I95o-x972) were compared to control areas, but sample size may not have been adequate and control areas were not shown to be appropriately similar to dredged areas before the dredging occurred. Analyses of biomass effects were not included. However, the study concluded that in new dredge cuts, defaunation was virtually complete. After four years, dredged areas sustained macrofaunal densities at least 80% of those found in undredged areas. The Army Corps of Engineers (x974) reached the same basic conclusions based on similar work done by the Taylor Biological Compahy in Tampa Bay. This study generally supports the conclusions of the previous studies, with the exception that, in this case, defaunation resulting from dredging was not total. Oyster shell dredging has been shown to have a significant impact "on the benthic community of a dredged area. However, the observed effects of dredging on the benthos were temporary and within twelve months a near total recovery was observed in this case.
The effects of oyster shell dredging on an estuarine benthic community
757
Shell dredging need not be detrimental to the recreational industrial, or ecological functions of an estuary. However, in a review, Clark (x977) concludes that shell dredging has the potential to cause problems if mining activities are excessively heavy or poorly managed. Dredging should not be allowed to disrupt bottom topography to the extent that the water circulation patterns of the estuary would be altered. This could seriously affect patterns of sediment deposition and water quality (Schubel 86 ~ e a d e , I975). Problems might also arise from the nature of rcsuspended sediments which may seriously alter water quality if significant amounts of chemical contaminants are released (Lee, x977). Ultimate fates of suspended sediments sl~ould be considered witI~ respect to areas such as grass beds, oyster beds and coral reefs that might be sensitive to increased sedimentation.
Acknowledgements We would like to acknowledge S. Bloom, J. Cultcr, D. Dabaets, D. Dauer, D. ~IeKirdy, R. Mumme, S. Santos, and L. Weiner for special assistance in the field and in the laboratory. Without the cooperation of the dredging industry through Benton and Company, St. Petersburg, Florida, this research would not have been possible. This study was supported by funding from the State of Florida Department of Environmental Regulation. We are also grateful to the M I T R E Corporation, ~Ietrek Division for providing facilities needed to prepare the final manuscript.
References Army Corps of Engineers I974 Draft Environmental Impact Statement--Oyster Shell Dredglng--Tampa and llillsborough l~ays, Florida. U.S. Army Engineer District, Jacksonville, Florida. xzx pp. Bray, J. R. & Curtis, J. T. x957 An ordination of the upland forest communities of southern "~Viseonsin. Ecological 3Ionographs 27, 325-349. Clark, J. R. x977 CoastalEcosystem l~lanagement. John Wiley and Sons, New York. 9z8 pp. Crisp, D. J. x97x Energy flow measurement. In l~lethodsfor the Study of 2~larine Benthos (IBP Handbook No. x6, I/olme, N. A. & Mclntyre, A. D., eds). Blackwell, Oxford. pp. x97-279. Esprey, W. IL Jr. x977 Environmental aspects of dredging in the Gulf Coast Zone with some attention paid to shell dredging, bz Estuarb, e Pollution Control and Assessment. EPA Office of Water Planning and Standards, '~Vashington, D.C. No. 44olx--77-oo7. 755 PPInman, K. L. z95z i'~easures for describing the size distribution for sediments. Journal of Sedimentary Petrology 2z, zz5-z45. Lee, G. F. x977 Significance of chemical contaminants in dredged sediment on estuarine water quality. In Estuarlne Pollution Control and Assessment. EPA Office of "~Vater Planning and Standards, Washington, D.C. No. 44o--x-77--oo7. 755 PP. Mason, W. T. & Yevitch, P. P. x967 The use of Phloxine B and Rose Bengal stains to facilitate sorting benthic samples. Transactions of the American 2~IicroscoplcalSociety 86, 2zI-223. May, E. B. x973 Environmental effects of hydraulic dredging in estuaries. Alabama ~[arine Resource 13ulletbz No. 9. PP- x-85. McKay, C. R. & IIartzband, D. J. x97o Propylene phenoxytol: narcotic agent for unsorted benthic invertebrates. Transactiolfs of the American ~llcroscopical Society 89, 53-54. Paine, R. T. x97r The measurement and application of the calorie to ecological problems. Annual Revler~'s of Ecology and Systematics 2, x45-I64. Reish, D. J. z959 A discussion of the importance of screen size in washing quantitative marine bottom samples. Ecology 40, 307-309. Schubel, J. R. &/~eade, R. H. x977 l~'~art'simpact on sedimentation. In Estuarlne Pollution Controland Assessment. EPA Office of Water Planning and Standards, Washington, D.C. No. 44o/x'-'77/oo7. 755 Pp. Simon, J. L. & Dyer, J. P. III. I97z An Evaluation ofSiliation Createdby Bay Dredging and Construction Company Durhzg Oyster Shell Dredgbtg Operations b~ Tampa Bay, Florida, January x, x97z to 3larch 3x, x97z. Final Research Report, Department of Biology, University of South Florida, Tampa. 60 pp.
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Taylor, J. L. x972a Some Effects of Oyster Shell Dredging on Benthic Invertebrates in Tampa Bay, Florida. Taylor Biological Company, St. Petersburg Beach, Florida, z6 pp. Taylor, J. L. z972b Some Effects of Oyster Shell Dredging on Benthic Invertebrates in ~Iobile Bay, Alabama. Taylor ]3iologlcal Company, St. Petersburg Beach, Florida, x6 pp. Taylor, J. L. x973 Studies of the Effects of Oyster Shell Dredging in Tampa Bay, Florida. Taylor Biological Company, St. Petersburg ]3each, Florida, 7 PP. Texas A & ~,~ Research Foundation z973 Environmental Impact Assess,neat of Shell Dredging in San Antonio Bay, Texas. U.S. Army Engineer District, Galveston, Texas, ~3x pp.