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Dredging impact on an urbanized Florida bayou: effects on benthos and algal-periphyton
Environmental Pollution 115 (2001) 161–171 www.elsevier.com/locate/envpol
Dredging impact on an urbanized Florida bayou: effects on benthos and algal-periphyton M.A. Lewis *, D.E. Weber, R.S. Stanley, J.C. Moore US Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, 1 Sabine Island Drive, Gulf Breeze, FL 32561-5299, USA Received 25 August 2000; accepted 8 February 2001
‘‘Capsule’’: Impacts of dredging can be localized and of short duration. Abstract Environmental effects of dredging events have been uncommonly reported for shallow, residential estuaries characteristic of the Gulf of Mexico region. The objective of this study was to determine the impact of hydraulic dredging on an urbanized estuary. Physicochemical quality, benthic community composition, whole sediment toxicity, periphytic algal community composition and trace metal tissue quality were determined prior to and after dredging. The effects on surface water pH, dissolved oxygen, and temperature were negligible but photosynthetically active radiation was decreased at several stations. Dredging significantly reduced benthic diversity and density (P <0.05). However, the sediments were not acutely toxic to the epibenthic, Americamysis bahia (formerly Mysidopsis bahia); survival averaged 93% (post-dredging) and to 98% (pre-dredging). There were several post-dredging taxonomic structural changes in the diatom-dominated, periphyton community but differences in mean density and three diversity indices were not significant. Trace metal concentrations in periphyton after dredging were reduced from an average of 4–65% and significantly for mercury, zinc and chromium in several areas. It was concluded that the environmental impact of small-scale dredging events in urbanized near-coastal areas, based on the selected parameters, are likely to be localized and of short-term environmental consequence. The choice of the target biota, response parameters and chemical analysis are important considerations in the environmental impact assessment of these periodic episodic events. Published by Elsevier Science Ltd. Keywords: Dredging; Benthos; Periphyton; Florida; Bayou-Estuary
1. Introduction Approximately, 129 million m3 of sediment are dredged annually in the US from more than 400 ports and 25,000 miles of navigation channels (US Army Corps of Engineers, [USACOE], no date). Dredging is a relatively frequent event in many coastal areas of the Gulf of Mexico where about 100106 wet metric tons of sediment were removed during 1990 (Ferrario, 1990). Reports have been published in the scientific literature describing the in situ effects of dredging and dredged material disposal (Beyer and Stafford, 1993; Gibson and Looney, 1994; Onuf, 1994; Long et al., 1996). Effects of dredging have been reported, by among others, on birds (Howarth et al., 1982), oysters (Winger and Lasier, * Corresponding author. Tel.: +1-850-934-9382; fax: +1-850-9349201. E-mail address: [email protected] (M.A. Lewis). 0269-7491/01/$ - see front matter Published by Elsevier Science Ltd. PII: S0269-7491(01)00118-X
1995; Wirth et al., 1996), lobsters (Greig and Pereira, 1993), fish (Rice and White, 1987) and aquatic plants (Lee et al., 1982; Combs et al., 1983; Brookes, 1987). Environmental impacts observed in these studies included reduction in numbers of benthic species, increased turbidity, reduction of primary productivity and mobilization, and increased bioavailability of sediment trace metals. This latter effect on metal bioavailability is of particular concern in the Gulf of Mexico region, due to numerous fish consumption advisories for mercury in coastal areas (Paulic et al., 1996; USEPA, 1999). Bayou Texar is a shallow and relatively small estuary located in northwest Florida. It provides spawning habitat for fish and shellfish but also serves as a major urban recreational resource. It is a ‘‘residential’’ bayou with lawns maintained to the water edge for most of its shoreline. The environmental quality of the bayou is affected by extensive urbanization in its watershed (Lewis et al., 2001a,b); stormwater runoff enters the
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bayou from 68 culverts and storm drains and from Carpenter’s Creek. As a result, seasonal fish kills and hypoxic conditions due to eutrophication are common during summer months (Stone et al., 1990). In addition, sediment has been accumulating in Bayou Texar at a rate of approximately 19,000 yds3 annually and without dredging, the bayou has a predicted ‘‘life expectancy’’ of approximately 200 years (Stone et al., 1991). This high rate of deposition reduces water depth and circulation, and restricts recreational use. Deposition has reduced the depth 1 to 2 m depending upon the location. Efforts to eliminate this problem have included periodic small scale dredging near the bayous’s junction with Pensacola Bay and an area near Carpenter’s Creek. The objective of this study was to determine the impact of such an event that occurred between October (1994) and February (1995) when approximately 22,800 m3 of fluvially transported sediment were removed. This information is important to understand since despite the relative frequency of dredging events in this bayou, as well as in other small urbanized estuaries in the Gulf of Mexico region, their environmental impacts are relatively unknown for biota other than the benthic macroinvertebrate community. The autotrophic and heterotrophic components of periphyton serve as food resources for estuarine biota and they are considered relevant bioindicators of environmental condition (Phillips, 1977; Weitzel, 1979; Sladeckova, 1990). Furthermore, algal-periphyton are considered effective accumulators of trace metals (tenCate, 1991; Rankin et al., 1994; McCormick and Stevenson, 1998). Contaminant bioconcentration by periphyton however, has not been commonly used to monitor in situ dredging impacts despite a bioconcentration requirement for dredged spoil deposition (USACOE, 1992; Bridges et al., 1996). The primary objective of this baseline study was to determine the pre- and post-dredging effects of hydraulic dredging on the macroinvertebrate benthos and on periphytic-algae, including trace metal bioconcentration.
2. Materials and methods 2.1. Study area Bayou Texar is adjacent to Pensacola Bay, Escambia County, Florida (Fig. 1). It has a surface area of approximately 1.5 km2 and volume of 2.8106 m3 with a daily exchange of 11–34% (Stone et al., 1991). The length of the bayou is approximately 2 km and the width varies from 35 to 600 m. It has a mean depth of 2 m referenced to mean low water. The dredging area was located in the primary depositional zone between the Carpenter’s Creek delta and an area approximately 300 m south. Seven of the 10 sampling stations were located
in the dredging zone (DZ) and three were located outside the dredging zone (ODZ) at increasing distances seaward. Not all sampling stations were used for all analyses. 2.2. Physicochemical water quality Several physicochemical water quality parameters were determined in situ pre- and post-dredging at five sampling stations. These were pH, dissolved oxygen (mg/l), temperature ( C), salinity (psu) and photosynthetically active radiation (PAR). Portable analytical instrumentation was used for these measurements (Hydrolab Corp., Austin, TX) determined at 0.5 m incremental depths. Salinity was measured using a hand-held refractometer (Leica Co., Buffalo, NY). PAR measurements, as were the others, were made usually between 10:00 am and 2:00 pm using a Li-Cor Model LI-189 Quantum Radiometer/Photometer (Leica Co., Buffalo, NY). 2.3. Sediment collection Sediment samples were collected from each of the 10 stations prior to dredging and periodically thereafter up to a maximum of 12 months. Sediments were collected with a stainless steel ponar grab (volume — 2.1 l; surface area — 0.13 m2) to a depth of approximately 13 cm and were homogenized and then stored at 4 C until use for chemical analysis and toxicity testing. 2.4. Chemical quality of sediment A detailed description of the sediment chemical quality and analytical methodology has been reported for this bayou for 1993–1994 (Lewis et al., 2001a,b). In addition, samples of sediment were collected during this study and analyzed for cadmium, chromium, copper, nickel, lead, and zinc by inductively coupled plasma atomic emission spectroscopy (ICP). Samples were collected from one to four stations within the dredging zone before, during and after dredging. The digestion, clean-up and analysis procedures were based on EPA procedures (1994, 1995). An intracoupled plasma spectrophotometer (Fisher Scientific Co., Franklin, MA) was used for the analyses. Detection limits were between 0.7 and 4.0 mg/g dry weight. Mercury concentrations were determined post-dredging using an automated mercury analyzer (Leeman Labs, Hudson, NH) and mercury cold vapor atomic absorption with tin as the reductant. The detection limit was 0.2 ng/g. The mercury concentrations in sediments collected prior to dredging were analyzed by ICP. The detection limit was 3.0 mg/g. An instrument calibration verification solution, quality control solution, and laboratory fortified blank were analyzed to verify system performance. In addition,
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Fig. 1. Location of the dredging zone in Bayou Texar, Escambia County, Florida. Bayou Texar is located at 30 27.140 N and 87 12.260 W.
duplicate samples, matrix spiked samples, and a standard reference material (SRM 2704-Buffalo River sediment) were used for quality control. Surrogate and spiked compound recoveries were within acceptable limits (40–130%). The trace metal concentrations were compared with proposed sediment quality assessment guidelines for Florida coastal areas (MacDonald et al., 1996). Exceedance of these numerical, effects-based guidelines, such as the threshold effects level, suggests that the sediments are contaminated to such an extent that adverse biological effects may occur. 2.5. Whole sediment bioassays Twelve acute toxicity bioassays were conducted with the epibenthic invertebrate, Americamysis (Mysidopsis) bahia, and four whole sediment samples. A. bahia is considered a sensitive species and it has been recommended for use in toxicity tests (Nimmo and Hamaker,
1982; ASTM, 1995). Sediments were collected from four stations located within the dredging zone one month before, during, and 1 year after dredging. Particle size of the sediments was determined prior to dredging using a hydrometric technique (ASTM, 1992). The toxicity test methodology followed standardized procedures (ASTM, 1993; USEPA, 1996). A total of 30 organisms was exposed in six replicate test chambers to each undiluted whole-sediment sample in the seven-day static bioassays. A reference sediment was included which was collected from nearby Perdido Bay, Florida. Numerous bioassays have been conducted at the USEPA Gulf Ecology Division Laboratory with this sediment confirming its non-toxic nature to this test species as well as to others (unpublished data). 2.6. Benthic community composition Benthic community composition was determined 1 month prior to and 1 month after dredging for sediments
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collected from the same four sampling stations for which acute toxicity was determined. The benthos were removed on site from each of two grab samples collected from each station using a 30-mm sieve. Specimens were preserved in buffered formalin containing Rose Bengal stain (0.1 g/l) until identification. Taxonomic analysis was conducted according to standard procedures (APHA et al., 1995). A Leica Wild M 3 Z stereo zoom microscope was used to identify whole specimens. A Nikon Labophot phase contrast compound microscope was used to examine parapodia from nereid polychaetes. Organisms were identified to the lowest possible taxon using standard reference taxonomic keys, such as those of Cooley (1978), Heard (1982), and Uebelaker and Johnson (1984). Several taxonomic structural measurements of the benthic community were used as indicators of sediment contamination. These included taxa richness (number of species), species density (m2), and the Shannon–Wiener diversity index (Shannon and Weaver, 1949). This measure of diversity is the most widely used index of its type (Washington, 1984). The scarcity of benthic organisms precluded a more in-depth analysis using other numerical indices. 2.7. Periphyton colonization Periphyton were colonized in situ at each of the 10 sampling stations pre- and post-dredging. Periphyton were colonized on 12 acrylic substrates (one side=0.01 m2) contained in periphytometers (6020 cm). Periphytometers were submerged approximately 5 cm below the water surface for the 21 day colonization periods conducted at the same station. To minimize contamination from settled suspended solids, the periphytometers were cleaned once during each colonization. In addition, after each colonization period, the substrates were removed from the periphytometers and vigorously rinsed with deionized water. Substrates were placed individually in plastic bags and stored on ice during transport to the laboratory and stored at 15 C until analysis for trace metal concentrations and community composition. 2.8. Trace metal residue analysis Periphyton contained on substrates adjacent to those analyzed for community composition were analyzed for the same trace metals determined in sediment and by the same technique (ICP). Sample digestion, preparation and analysis were also similar and followed USEPA (1994, 1995) guidelines. Periphyton were dried at 105 C overnight and then digested with 10 ml of 1:1 nitric acid and deionized water using a microwave oven. Method detection limits (MDL) ranged between 0.7 to 2.9 ug/g dry weight for all metals but mercury (0.2 ng/g dry wt.).
The same quality assurance procedures used for sediments, were followed for the periphyton, including the standard reference material. All instrument performance criteria were met before sample analysis began. Surrogate and spiked compound recoveries were within acceptable limits (40–130%). 2.9. Algal-periphyton community composition The algal-periphyton were removed from the substrates and preserved in 10 ml of 3% buffered formalin containing 0.1 ml Lugol’s iodine solution. Samples were stored in the dark until analysis to the lowest possible taxon using several regional and national taxonomic keys which included, among others, those of Patrick and Reimer (1966), Campbell (1973), Marshall (1986), and Dillard (1989–1991). Species number and density (cells/mm2) were determined using a hemacytometer counting chamber. Several structural measures of community composition were calculated (Shannon and Weaver, 1949; Bray and Curtis, 1957; Margalef, 1958; Pielou, 1975) using commercially available software (Primer, Version 1, Plymouth Marine Laboratory, UK). A detailed description of these indices can be found in Clarke and Warwick (1994). Community composition (pre- and post-dredging) was also compared by using non-metric multidimensional scaling (MDS; Kruskal and Wish, 1978) and the calculation of an index of multi-variate dispersion (IMD; Clarke and Warwick, 1994). This index is a species dependent comparative index of multivariate dispersion for community stress. 2.10. Statistical analysis Differences in algal-periphyton and benthic community structure and the periphyton trace metal residues were compared pre- and post-dredging between and within the dredging and non-dredging zones using a two-way analysis of variance (ANOVA). Data were transformed to meet the normality assumption when necessary. One-way analysis of variance followed by post-hoc analysis were used to determine differences in photosynthetic active radiation. Statistical analyses were performed by using SAS (SAS Institute Inc., 1991). The threshold level of statistical significance for this study was a=0.05.
3. Results 3.1. Physicochemical water quality The physicochemical parameters at the surface were different during the 4 months of periodic dredging. Dissolved oxygen concentrations for all stations typically
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ranged from 6.3 to 9.6 mg/l during dredging. The pH ranged from 6.7 to 8.2. Surface salinity ranged from 0.5 to 15.9 ppt, and the temperature was between 15.1 to 23.4 C. Most of the differences in the parameters were likely the result of seasonal factors, tidal action and the variation in discharge from Carpenter’s Creek. Subsurface photosynthetically active radiation decreased significantly in the dredging zone at some stations compared with the non-dredged area (Fig. 2). The average reductions, relative to percent at surface, ranged from 49 to 61% at the 0.5 m depth for the five stations in the dredging zone. The reductions were significant at two stations at this depth when compared with the outside dredging zone area. PAR at the 1 m depth averaged 32% of surface values in the non-dredging area and ranged from 14 to 25% at the five dredging stations. The reductions at this depth were significant for one of the five stations in the dredging zone when compared with the average for the non-dredged area.
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for which the concentrations were similar. The average concentrations of all metals but nickel in sediment collected post-dredging in August were either similar or less than those during dredging. However, it is important to note that all but cadmium and copper were still elevated in August relative to pre-dredging levels. Mercury concentrations before dredging were below the detection limit of 3.0 mg/g dry weight but using a more sensitive analytical technique, averaged 668.2 and 498.0 ng/g dry weight during and after dredging, respectively. 3.3. Whole sediment toxicity Acute toxicity was uncommon despite the presence of potentially toxic concentrations of several trace metals as indicated by the exceedances of the sediment quality guidelines. Survival of A. bahia after exposure to the pre- and post-dredging sediments was similar statistically. Survival after exposure to the four sediments
3.2. Chemical quality of sediment The sediments in the dredging zone were considerably contaminated and remained contaminated despite the dredging based on the sediment quality guidelines proposed for Florida coastal areas (Fig. 3). The average concentrations of cadmium, copper, lead, zinc and mercury in most sediments, regardless of the sampling period, exceeded the numerical threshold effects guideline values of 0.68, 18.7, 30.2, 124.0 and 0.13 mg/g, respectively. There were temporal differences in the trace metal concentrations (Fig. 3). The average concentrations of most trace metals were greater during the latter stages of dredging (January). The exception was for cadmium
Fig. 2. Photosynthetically active radiation (PAR) measured at 0.5 and 1.0 m depths relative to surface. Values are for five sampling stations during dredging and represent mean ( 1 S.D.) of multiple measurements. DZ, dredging zone; ODZ, outside dredging zone. Different letters represent significant difference (a=0.05).
Fig. 3. Comparison of trace metal concentrations for sediments collected from the dredging zone before, during and after dredging. Values represent mean ( 1 S.D.) for sediments collected from one (pre-dredging) and four (post-dredging) stations. Values in mg/g dry weight except for mercury (ng/g dry wt.). BD, below detection limit of 3 mg/g dry weight. Exceedances of sediment quality guidelines proposed for Florida near-coastal areas (MacDonald et al., 1996) are also shown ().
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collected from the dredging zone 1 month before dredging averaged 98% ( 1 S.D.=4%; range=93–100%) relative to an average of 93% ( 14%; range=73– 100%) 1 year after dredging (Table 1). Survival of this epibenthic invertebrate after exposure to two sediments collected from these stations during dredging was 33 and 73%. Survival of mysids exposed to the reference sediment was 100%. The particle size distribution of the sediments collected during October and November from the four locations for which toxicity was determined appears in Table 1. Two sediments (Stations one and three) were sand-dominated (62 and 98% sand) and one (Station four) was comprised mainly of clay (86%). Station two sediment was comprised of 49 and 51% clay.
3.5. Algal-periphyton community composition The number of species and individuals of the algalperiphyton were similar, when compared pre- and postdredging within and between the dredging and non-dredging stations. For example, the number of periphytic algal species colonized on the substrates at the seven stations located in the dredging zone prior to dredging averaged 19.7 ( 1 S.D.=4.5; range=15–25) relative to 21.4 ( 2.4; range=16–25) 8 months afterwards at the same locations (Table 2). Likewise, the average number of species colonized at the three stations outside the dredging zone were similar predredging (23.3 5.7; range=17–28) and post-dredging (20.0 6.2; range=13–25). Algal cell density increased after dredging from an average of 166.1 ( 118.0; range=37–340) cells/ml to 471.6 ( 402.7; range=87– 1150) cells/ml in the dredging zone. Outside the dredging area, the increase was from a mean of 134.7 ( 101.4; range=42–423) cells/ml to 502.7 ( 391.8; range=52–762) cells/ml. Although increases were evident when based on average density values, they were not statistically significant due to the considerable variation in results for the specific colonization stations. Diversity indices for the algal-periphyton were less after dredging in the dredging and non-dredging zones but the decreases were not significant (Table 2). The decrease in values averaged 16% ( 7%; range=11– 30%) for the three indices. For example, the Shannon– Wiener index values for periphyton colonized in the dredging zone decreased approximately 13% from an average of 0.98 ( 1 S.D.=0.11; range=0.85–1.20) to 0.85 ( 0.10; range=0.07–1.00), after dredging. These average diversity index values were very similar to those for periphyton colonized at the three non-dredging stations, 1.08 ( 0.12; range=0.96–1.19; pre-dredging) and 0.87 ( 0.14; range=0.75–1.03; post-dredging). There was a difference in the pre- and post-dredging community composition based on multidimensional
3.4. Benthic community composition The benthic macroinvertebrate community in the dredging zone was not abundant nor diverse prior to dredging and was less so afterwards (Table 1). The number of species and individuals and the diversity index values decreased statistically as a result of the dredging. For example, species number before dredging ranged from 4 to 6 relative to 1 to 4 afterwards and density averaged 114 ( 1 S.D.=39) individuals/m2 before and 27 ( 18) individuals/m2 1 month after dredging. The Shannon–Wiener diversity index values pre-dredging ranged from 0.40 to 0.62 and were either not calculable due to low number of individuals or were relatively similar to those at the same stations, post-dredging. The more abundant species prior to dredging were the polychaetes, Laeonereis culveri, and Streblospio benedecti and a dipteran species (Palpomyia). Their combined relative abundance was 68% 1 month pre-dredging and 23% 1 month post-dredging. Two primary differences in abundance were the post-dredging increase of a harpacticoid copepod, from 6 to 69%, and the decrease in Palpomyia from 17 to 2%.
Table 1 Several structural characteristics of the benthic macroinvertebrate community and sediment toxicity in Bayou Texar pre-dredging (October 1994) and post-dredging (January 1995)a Sampling stations
1 2 3 4
Pre-dredging
Post-dredging
Particle size
No. species
No. individualsb
Diversity indexc
Invertebrate survival
No. species
No. individuals
Diversity index
Invertebrate survival
Sand (%)
Clay (%)
6 4 6 6
144 95 68 148
0.51 0.40 0.58 0.62
100 100 93 100
1 4 1 4
4 34 23 46
0d 0.50 0 0.44
73 100 100 100
62 49 98 14
38 51 2 86
a Values represent mean for two replicate grab samples collected from each station located in the dredging zone. Survival (%) of Americamysis bahia also shown after 7-day exposure to whole sediments. Particle size distribution is for pre-dredging condition. b per m2. c Shannon-Wiener diversity index (Shannon and Weaver, 1949). d Could not be calculated due to low numbers of individuals.
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Table 2 Comparison of taxonomic structural characteristics of the periphytic algae colonized in areas within the DZ and ODZ pre-dredging (August 1994) and post-dredging (August 1995)a Colonization location
Number species
Number individuals
Shannon-Wienerb
Margalef’s indexc
Pielou’s indexd
ODZ Pre Post
23.3 (5.7) 20.0 (6.2)
134.7 (101.4) 502.7 (391.8)
1.08 (0.12) 0.87 (0.14)
10.9 (0.10) 7.6 (0.80)
0.81 (0.13) 0.70 (0.20)
DZ Pre Post
19.7 (4.5) 21.4 (2.4)
166.1 (118.0) 471.6 (402.7)
0.98 (0.11) 0.85 (0.15)
9.2 (0.8) 8.2 (1.5)
0.76 (0.12) 0.64 (0.10)
a b c d
Values represent mean (1 S.D.) for the seven dredging zone (DZ), and three outside dredging zone (ODZ) colonization sampling sites. Shannon and Weaver (1949). Margalef (1958). Pielou (1975).
scaling ordination and transformed abundances and Bray–Curtis similarities (Fig. 4). The index of multivariate dispersion was 0.318 which indicates greater stress in the post-dredging periphyton community. The dissimilarity between the periphyton colonized pre- and post-dredging was 49.3%. The algal-periphyton were dominated by diatoms in the dredging zone where they comprised approximately 75% (pre-dredging) and 84% (post-dredging) of the taxa. The diatom, Neidium affine, was the dominant species comprising 30 and 33%, respectively, of the total taxa collected during the pre- and post-dredging colonization periods. In addition, Achnanthes minutissima was also relatively common before (15% of taxa) and after (8% of taxa) the dredging event as were species of the genus, Fragilaria which had a cumulative abundance of 21% (before) and 16% (after). The most obvious difference in species composition was for Nitzschia recta which was not colonized prior to dredging but comprised 21% of the taxa colonized 8 months afterwards. 3.6. Periphyton trace metal residues The trace metal residues for periphyton colonized in the dredging zone (DZ) averaged for seven stations were less 8 months after dredging (Table 3). The percentage reduction averaged 34.4 ( 1 S.D.=19.7) for the seven trace metals. The average decreases in contaminants in decreasing order were: mercury (65%) chromium (44%), zinc (44%), copper (38%), lead (25%), cadmium (21%) and nickel (4%). Depending upon the spatial scale of comparison, significant decreases occurred for mercury, chromium and zinc. The average mercury concentration in periphyton colonized at the seven stations in the dredging zone was 1427.7 ( 1 S.D.=389.1; range=984–2000) ng/g dry weight before dredging and 502.1 ( 146.8; range=242–639) ng/g dry weight for periphyton colonized at the same sites 8 months after dredging. The post-dredging reductions occurred at all colonization stations (Fig. 5).
Fig. 4. Difference in algal-periphyton community composition as indicated by two dimensional MDS ordination of the 10 stations preand post-dredging based on transformed algal-periphyton abundances and Bray–Curtis (1957) similarities. The stress value was 0.19. See Clarke and Warwick (1994) for a more detailed description of technique.
The trace metal residues for periphyton colonized at the three stations located outside the dredging zone (ODZ), on average, were usually greater post-dredging (Table 3). The most noticeable exception was mercury. Concentrations of mercury in the periphyton colonized outside the dredging zone averaged 1091.0 ( 380.5; range=653–1340) ng/g dry weight pre-dredging and 710.0 ( 139.6; range=553–820) ng/g post-dredging. The average 35% decrease was not significant.
4. Discussion The environmental effects of the dredging occurred within the dredging zone and did not appear to extend
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Table 3 Comparison of trace metal residues in periphyton colonized within the DZ and ODZ pre-dredging (August 1994) and post-dredging (August 1995)a Colonization location
Trace metals Cd
Cr
Cu
Ni
Pb
Zn
Hg
ODZ Pre Post
1.3 (0.4) 1.0 (0.14)
34.8 (16.5) 40.2 (5.3)
68.4 (48.8) 74.9 (3.3)
12.7 (4.4) 16.5 (1.9)
107.2 (40.6) 139.7 (9.5)
389.3 (239.1) 554.3 (82.2)
1091.0 (380.5) 710.0 (139.6)
DZ Pre Post
2.2 (0.7) 1.5 (1.1)
44.8 (25.4) 24.9 (4.0)
51.3 (53.9) 31.8 (10.1)
19.7 (7.6) 14.6 (4.0)
137.1 (60.5) 101.8 (17.2)
461.6 (455.3) 265.0 (45.2)
1427.7 (389.1) 502.1 (146.8)
a
Values in mg/g dry weight except for mercury (ng/g dry wt.). Values represent mean (1 S.D.) for three outside dredging zone (ODZ) and seven dredging zone (DZ) colonization sites.
Fig. 5. Comparison of total mercury concentrations in periphyton (ng/g dry wt.) colonized for 21 days before dredging and 8 months afterwards. Stations 4–10 are located in the dredging area.
seaward. Effects on physicochemical water quality were minor during dredging other than a reduction in the photic zone at a few sampling stations. The most obvious biological effect, as would be expected, was the reduction in the benthic macroinvertebrate community. However, this reduction needs to be considered in the context that the benthos in this sediment depositional zone was depauperate prior to dredging. Therefore, the ecological significance of its further depletion may have been minimal. There were indications that the dredging impacted the algal-periphyton causing a post-dredging compositional change. However, the magnitude of these effects were station-specific and when the data were combined and compared for the pre- and post-dredging time periods, many differences in the response parameters were not statistically significant. This includes the trace metal residues. Many residue concentrations decreased after dredging in the dredging zone, but the pre- and post-dredging differences were significant, only for mercury, chromium and zinc. The reduction in mercury
may have been the most positive ecological benefit of the dredging since it reduced possible trophic transfer to fish which, as stated earlier, is an important issue for many near-coastal areas of Florida (Paulic et al., 1996; USEPA, 1999). It was assumed that the residue results were for organic matter on the substrates. However, the effect of settled inorganic substances on these type of results is an important consideration (Newman and McIntosh, 1989). For perspective, the ash free dry weight (AFDW) to dry weight (DW) percentage for periphyton colonized in the dredging zone prior to the dredging event (1993–1994) averaged approximately 36% ( 1 S.D.= 6%; Lewis et al., 2001a,b). Dredging was assumed to be the cause of the differences in residue concentrations since no other episodic or unusual seasonal event occurred during this study. However, this could not be confirmed based on the sediment chemical analyses conducted during this baseline study. No insight for mercury can be provided due to differences in the analytical methodology. The
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sediment concentrations for chromium and zinc, the other two metals for which the residues significantly decreased, typically increased during and after dredging. The lack of congruity in these data sets may be due, in part, to the differences in the time of sediment collections and the periphyton colonizations which were not usually identical. This is particularly true for the pre-dredging samples which were collected approximately 1 year prior to dredging. Furthermore, a few samples of sediment may not be characteristic of the dynamic sediment conditions associated with dredging. Water quality in Bayou Texar has been reported (Lewis et al., 2001a,b). Concentrations of trace metals were below detection (range=7–40 mg/l) at most sampling areas including in the dredging zone. For example, based on six measurements of pre-dredging water samples, only zinc was detectable (mean=16.8 1 S.D. deviation=3.5) mg/l. Information for post-dredging samples is not available and consequently, no comparative insight can be provided. Based on all considerations, it is evident that the number of sampling stations, the type and frequency of chemical analysis, and the media analyzed, are important considerations even for small scale dredging events if a cause-effect relationship is to be firmly established for biota other than that associated with sediment. There is no available information in the scientific literature describing the environmental effects of dredging on the biota in this bayou or for periphyton in any other similar coastal area in Florida. However, several studies have been conducted elsewhere in which dredging effects were determined on aquatic flora such as seagrasses (Onuf, 1994; Long et al., 1996), seaweeds (Lyngby and Mortensen, 1996) and phytoplankton (Windom, 1975; Iannuzzi et al., 1996). The reported effects included increases in turbidity, reduction of primary productivity and changes in the structural characteristics of the plant communities. The above results were event-specific and dependent on the type of flora, response parameter, dredging methodology, volume dredged and type of habitat. In summary, the environmental impact of relatively small-scale dredging events in shallow water bayous, such as Bayou Texar, may not be a major factor in their long-term environmental condition. Urbanized bayous are impacted continuously by the development in their watersheds and by residential areas along their periphery. Therefore, infrequent and spatially-limited dredging may not be a cause of ecologically significant deterioration as it may be for larger scale events in more pristine near-coastal areas. The usual reduction in the benthos, likely already sparse in the areas to be dredged, may be ‘‘counteracted’’ by beneficial effects to other biota due to the removal of sediments and the increase in depth and circulation.
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Acknowledgements The trace metal concentrations in sediment and periphyton were analyzed by personnel from AVANTI Corp., (Gulf Breeze, FL). Particle size distribution was determined by Law Engineering and Environmental Sciences Inc. (Pensacola, FL). Val Coseo prepared the manuscript, graphics support was provided by Stephen Embry (OAO Corp., Pensacola Beach. FL) and statistical support by Dr. Michael Bundrick (University of West Florida, Pensacola, FL). Taxonomic identification of the benthos and periphyton was performed by the Institute of Coastal Sciences, University of West Florida, Pensacola, Florida. Thomas Roush and Larry Goodman (USEPA) provided field support. The US Environmental Protection Agency through its Office of Research and Development funded the research described here. It has been subjected to Agency review and approval for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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Patrick, R., Reimer, C.W. 1966. Diatoms of the United States, Vol. 1. The Academy of Natural Sciences of Philadelphia. No. 13, Philadelphia, Pennsylvania. Paulic, M., Hand, J., Lord, L., 1996. Water-Quality Assessment for the State of Florida (Section 305 (B) Main Report). Florida Department of Environmental Protection, Tallahassee. Phillips, D.J.H., 1977. The use of biological indicator organisms to monitor trace metal pollution in marine and estuarine environments — a review. Environmental Pollution 13, 281–317. Pielou, E.C., 1975. Ecological Diversity. Wiley, New York. Rankin, I., Kumar, P., Dushenkov, 1994. Bioconcentration of heavy metals by plants. Current Opinion in Biotechnology 5, 285– 290. Rice, C.P., White, D.S., 1987. PCB availability assessment of river dredging using caged clams and fish. Environmental Toxicology and Chemistry 6, 259–274. SAS Institute Inc, 1991. SAS/STAT Users Guide, Version 6, 4th Edition. SAS Institute Inc, Cary, NC. Shannon, C.E., Weaver, W., 1949. The Mathematical Theory of Communication. University of Illinois Press, Urbana, IL. Sladeckova, A., 1990. Periphyton as indicator of the reservoir water quality III. Biomonitoring techniques. Ergenbn. Limnol. 33, 775– 782. Stone, G.W., Morgan, J.P., Moshiri, G.A., Elewad, S., 1990. Physical, biological and environmental studies of Bayou Texar, Vol. V (Rep. 02:12-31-90 Research Paper Series). Institute of Coastal and Estuarine Research, University of West Florida, Pensacola, FL. Stone, G.W., White, D., Morgan, J.P., Moshiri, G.A., 1991. Sedimentations rates and macroinvertebrate distribution in Bayou Texar, Escambia County, Florida (Rep.-02). Institute for Coastal and Estuarine Research, University of West Florida, Pensacola, Florida. tenCate, J.H., Simons, J., Schreurs II., J., 1991. Periphytic micro- and macroalgae in Dutch freshwater ditches and their significance as ecological indicators of water quality. Archives of Hydrobiologie 122, 275–296. Uebelaker, J.M., Johnson, P.G., 1984. Taxonomic Guide to the Polychaetes of the Northern Gulf of Mexico. Minerals Management Service, U.S. Dept. of Interior, Washington, DC. US Army Corps of Engineers (USACOE), 1992. Critical Body Residue (CBR) Approach for Interpreting the Consequences of Bioaccumulation of Neutral Organic Contaminants. EEDP-04-17. US Army Corp of Engineers, Vicksburg, MS. US Army Corps of Engineers (No Date). Dredging: Building and Maintaining our Underwater Highways, USACOE, Washington, DC. US Environmental Protection Agency, 1994. Test Methods for Evaluating Solid Waste, Vol. 1A. Revision 0, Washington, DC. US Environmental Protection Agency, 1995. Test Methods for Evaluating Solid Waste, Vol. 1A. Revision 2, Washington, DC. US Environmental Protection Agency, 1996. Ecological Effects Test Guidelines OPPTS 850.1035. Mysid Acute Toxicity Test. EPA 712C-96-136, Washington, DC. US Environmental Protection Agency, 1999. Update: Listing of Fish and Wildlife Advisories. EPA 823-F-99-005. Office of Water, Washington, DC. Washington, H.G., 1984. Diversity, biotic and similarity indices — a review with special relevance to aquatic ecosystems. Water Research 6, 653–694. Weitzel, R.L., 1979. Periphyton measurements and applications. In: Weitzel, R.L. (Ed.), Methods and Measurements of Periphyton Communities: A Review. ASTM STP 690. American Society for Testing and Materials, Philadelphia, PA, USA, pp. 3–33. Windom, H.L., 1975. Water-quality aspects of dredging and dredgespoil disposal estuarine environments, in Estuarine Research, Vol. 2. In: Cronin, L.E. (Ed.), Geology and Engineering. Academic Press, New York.
M.A. Lewis et al. / Environmental Pollution 115 (2001) 161–171 Winger, P.V., Lasier, P.J., 1995. Sediment toxicity in Savannah Harbor. Archives of Environmental Contamination and Toxicology 28, 357–365. Wirth, E.F., Scott, G.I., Fulton, M.H., Van Dolah, R.F., Maier, P.P.,
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Dredging impact on an urbanized Florida bayou: effects on benthos and algal-periphyton M.A. Lewis *, D.E. Weber, R.S. Stanley, J.C. Moore US Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, 1 Sabine Island Drive, Gulf Breeze, FL 32561-5299, USA Received 25 August 2000; accepted 8 February 2001
‘‘Capsule’’: Impacts of dredging can be localized and of short duration. Abstract Environmental effects of dredging events have been uncommonly reported for shallow, residential estuaries characteristic of the Gulf of Mexico region. The objective of this study was to determine the impact of hydraulic dredging on an urbanized estuary. Physicochemical quality, benthic community composition, whole sediment toxicity, periphytic algal community composition and trace metal tissue quality were determined prior to and after dredging. The effects on surface water pH, dissolved oxygen, and temperature were negligible but photosynthetically active radiation was decreased at several stations. Dredging significantly reduced benthic diversity and density (P <0.05). However, the sediments were not acutely toxic to the epibenthic, Americamysis bahia (formerly Mysidopsis bahia); survival averaged 93% (post-dredging) and to 98% (pre-dredging). There were several post-dredging taxonomic structural changes in the diatom-dominated, periphyton community but differences in mean density and three diversity indices were not significant. Trace metal concentrations in periphyton after dredging were reduced from an average of 4–65% and significantly for mercury, zinc and chromium in several areas. It was concluded that the environmental impact of small-scale dredging events in urbanized near-coastal areas, based on the selected parameters, are likely to be localized and of short-term environmental consequence. The choice of the target biota, response parameters and chemical analysis are important considerations in the environmental impact assessment of these periodic episodic events. Published by Elsevier Science Ltd. Keywords: Dredging; Benthos; Periphyton; Florida; Bayou-Estuary
1. Introduction Approximately, 129 million m3 of sediment are dredged annually in the US from more than 400 ports and 25,000 miles of navigation channels (US Army Corps of Engineers, [USACOE], no date). Dredging is a relatively frequent event in many coastal areas of the Gulf of Mexico where about 100106 wet metric tons of sediment were removed during 1990 (Ferrario, 1990). Reports have been published in the scientific literature describing the in situ effects of dredging and dredged material disposal (Beyer and Stafford, 1993; Gibson and Looney, 1994; Onuf, 1994; Long et al., 1996). Effects of dredging have been reported, by among others, on birds (Howarth et al., 1982), oysters (Winger and Lasier, * Corresponding author. Tel.: +1-850-934-9382; fax: +1-850-9349201. E-mail address: [email protected] (M.A. Lewis). 0269-7491/01/$ - see front matter Published by Elsevier Science Ltd. PII: S0269-7491(01)00118-X
1995; Wirth et al., 1996), lobsters (Greig and Pereira, 1993), fish (Rice and White, 1987) and aquatic plants (Lee et al., 1982; Combs et al., 1983; Brookes, 1987). Environmental impacts observed in these studies included reduction in numbers of benthic species, increased turbidity, reduction of primary productivity and mobilization, and increased bioavailability of sediment trace metals. This latter effect on metal bioavailability is of particular concern in the Gulf of Mexico region, due to numerous fish consumption advisories for mercury in coastal areas (Paulic et al., 1996; USEPA, 1999). Bayou Texar is a shallow and relatively small estuary located in northwest Florida. It provides spawning habitat for fish and shellfish but also serves as a major urban recreational resource. It is a ‘‘residential’’ bayou with lawns maintained to the water edge for most of its shoreline. The environmental quality of the bayou is affected by extensive urbanization in its watershed (Lewis et al., 2001a,b); stormwater runoff enters the
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bayou from 68 culverts and storm drains and from Carpenter’s Creek. As a result, seasonal fish kills and hypoxic conditions due to eutrophication are common during summer months (Stone et al., 1990). In addition, sediment has been accumulating in Bayou Texar at a rate of approximately 19,000 yds3 annually and without dredging, the bayou has a predicted ‘‘life expectancy’’ of approximately 200 years (Stone et al., 1991). This high rate of deposition reduces water depth and circulation, and restricts recreational use. Deposition has reduced the depth 1 to 2 m depending upon the location. Efforts to eliminate this problem have included periodic small scale dredging near the bayous’s junction with Pensacola Bay and an area near Carpenter’s Creek. The objective of this study was to determine the impact of such an event that occurred between October (1994) and February (1995) when approximately 22,800 m3 of fluvially transported sediment were removed. This information is important to understand since despite the relative frequency of dredging events in this bayou, as well as in other small urbanized estuaries in the Gulf of Mexico region, their environmental impacts are relatively unknown for biota other than the benthic macroinvertebrate community. The autotrophic and heterotrophic components of periphyton serve as food resources for estuarine biota and they are considered relevant bioindicators of environmental condition (Phillips, 1977; Weitzel, 1979; Sladeckova, 1990). Furthermore, algal-periphyton are considered effective accumulators of trace metals (tenCate, 1991; Rankin et al., 1994; McCormick and Stevenson, 1998). Contaminant bioconcentration by periphyton however, has not been commonly used to monitor in situ dredging impacts despite a bioconcentration requirement for dredged spoil deposition (USACOE, 1992; Bridges et al., 1996). The primary objective of this baseline study was to determine the pre- and post-dredging effects of hydraulic dredging on the macroinvertebrate benthos and on periphytic-algae, including trace metal bioconcentration.
2. Materials and methods 2.1. Study area Bayou Texar is adjacent to Pensacola Bay, Escambia County, Florida (Fig. 1). It has a surface area of approximately 1.5 km2 and volume of 2.8106 m3 with a daily exchange of 11–34% (Stone et al., 1991). The length of the bayou is approximately 2 km and the width varies from 35 to 600 m. It has a mean depth of 2 m referenced to mean low water. The dredging area was located in the primary depositional zone between the Carpenter’s Creek delta and an area approximately 300 m south. Seven of the 10 sampling stations were located
in the dredging zone (DZ) and three were located outside the dredging zone (ODZ) at increasing distances seaward. Not all sampling stations were used for all analyses. 2.2. Physicochemical water quality Several physicochemical water quality parameters were determined in situ pre- and post-dredging at five sampling stations. These were pH, dissolved oxygen (mg/l), temperature ( C), salinity (psu) and photosynthetically active radiation (PAR). Portable analytical instrumentation was used for these measurements (Hydrolab Corp., Austin, TX) determined at 0.5 m incremental depths. Salinity was measured using a hand-held refractometer (Leica Co., Buffalo, NY). PAR measurements, as were the others, were made usually between 10:00 am and 2:00 pm using a Li-Cor Model LI-189 Quantum Radiometer/Photometer (Leica Co., Buffalo, NY). 2.3. Sediment collection Sediment samples were collected from each of the 10 stations prior to dredging and periodically thereafter up to a maximum of 12 months. Sediments were collected with a stainless steel ponar grab (volume — 2.1 l; surface area — 0.13 m2) to a depth of approximately 13 cm and were homogenized and then stored at 4 C until use for chemical analysis and toxicity testing. 2.4. Chemical quality of sediment A detailed description of the sediment chemical quality and analytical methodology has been reported for this bayou for 1993–1994 (Lewis et al., 2001a,b). In addition, samples of sediment were collected during this study and analyzed for cadmium, chromium, copper, nickel, lead, and zinc by inductively coupled plasma atomic emission spectroscopy (ICP). Samples were collected from one to four stations within the dredging zone before, during and after dredging. The digestion, clean-up and analysis procedures were based on EPA procedures (1994, 1995). An intracoupled plasma spectrophotometer (Fisher Scientific Co., Franklin, MA) was used for the analyses. Detection limits were between 0.7 and 4.0 mg/g dry weight. Mercury concentrations were determined post-dredging using an automated mercury analyzer (Leeman Labs, Hudson, NH) and mercury cold vapor atomic absorption with tin as the reductant. The detection limit was 0.2 ng/g. The mercury concentrations in sediments collected prior to dredging were analyzed by ICP. The detection limit was 3.0 mg/g. An instrument calibration verification solution, quality control solution, and laboratory fortified blank were analyzed to verify system performance. In addition,
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Fig. 1. Location of the dredging zone in Bayou Texar, Escambia County, Florida. Bayou Texar is located at 30 27.140 N and 87 12.260 W.
duplicate samples, matrix spiked samples, and a standard reference material (SRM 2704-Buffalo River sediment) were used for quality control. Surrogate and spiked compound recoveries were within acceptable limits (40–130%). The trace metal concentrations were compared with proposed sediment quality assessment guidelines for Florida coastal areas (MacDonald et al., 1996). Exceedance of these numerical, effects-based guidelines, such as the threshold effects level, suggests that the sediments are contaminated to such an extent that adverse biological effects may occur. 2.5. Whole sediment bioassays Twelve acute toxicity bioassays were conducted with the epibenthic invertebrate, Americamysis (Mysidopsis) bahia, and four whole sediment samples. A. bahia is considered a sensitive species and it has been recommended for use in toxicity tests (Nimmo and Hamaker,
1982; ASTM, 1995). Sediments were collected from four stations located within the dredging zone one month before, during, and 1 year after dredging. Particle size of the sediments was determined prior to dredging using a hydrometric technique (ASTM, 1992). The toxicity test methodology followed standardized procedures (ASTM, 1993; USEPA, 1996). A total of 30 organisms was exposed in six replicate test chambers to each undiluted whole-sediment sample in the seven-day static bioassays. A reference sediment was included which was collected from nearby Perdido Bay, Florida. Numerous bioassays have been conducted at the USEPA Gulf Ecology Division Laboratory with this sediment confirming its non-toxic nature to this test species as well as to others (unpublished data). 2.6. Benthic community composition Benthic community composition was determined 1 month prior to and 1 month after dredging for sediments
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collected from the same four sampling stations for which acute toxicity was determined. The benthos were removed on site from each of two grab samples collected from each station using a 30-mm sieve. Specimens were preserved in buffered formalin containing Rose Bengal stain (0.1 g/l) until identification. Taxonomic analysis was conducted according to standard procedures (APHA et al., 1995). A Leica Wild M 3 Z stereo zoom microscope was used to identify whole specimens. A Nikon Labophot phase contrast compound microscope was used to examine parapodia from nereid polychaetes. Organisms were identified to the lowest possible taxon using standard reference taxonomic keys, such as those of Cooley (1978), Heard (1982), and Uebelaker and Johnson (1984). Several taxonomic structural measurements of the benthic community were used as indicators of sediment contamination. These included taxa richness (number of species), species density (m2), and the Shannon–Wiener diversity index (Shannon and Weaver, 1949). This measure of diversity is the most widely used index of its type (Washington, 1984). The scarcity of benthic organisms precluded a more in-depth analysis using other numerical indices. 2.7. Periphyton colonization Periphyton were colonized in situ at each of the 10 sampling stations pre- and post-dredging. Periphyton were colonized on 12 acrylic substrates (one side=0.01 m2) contained in periphytometers (6020 cm). Periphytometers were submerged approximately 5 cm below the water surface for the 21 day colonization periods conducted at the same station. To minimize contamination from settled suspended solids, the periphytometers were cleaned once during each colonization. In addition, after each colonization period, the substrates were removed from the periphytometers and vigorously rinsed with deionized water. Substrates were placed individually in plastic bags and stored on ice during transport to the laboratory and stored at 15 C until analysis for trace metal concentrations and community composition. 2.8. Trace metal residue analysis Periphyton contained on substrates adjacent to those analyzed for community composition were analyzed for the same trace metals determined in sediment and by the same technique (ICP). Sample digestion, preparation and analysis were also similar and followed USEPA (1994, 1995) guidelines. Periphyton were dried at 105 C overnight and then digested with 10 ml of 1:1 nitric acid and deionized water using a microwave oven. Method detection limits (MDL) ranged between 0.7 to 2.9 ug/g dry weight for all metals but mercury (0.2 ng/g dry wt.).
The same quality assurance procedures used for sediments, were followed for the periphyton, including the standard reference material. All instrument performance criteria were met before sample analysis began. Surrogate and spiked compound recoveries were within acceptable limits (40–130%). 2.9. Algal-periphyton community composition The algal-periphyton were removed from the substrates and preserved in 10 ml of 3% buffered formalin containing 0.1 ml Lugol’s iodine solution. Samples were stored in the dark until analysis to the lowest possible taxon using several regional and national taxonomic keys which included, among others, those of Patrick and Reimer (1966), Campbell (1973), Marshall (1986), and Dillard (1989–1991). Species number and density (cells/mm2) were determined using a hemacytometer counting chamber. Several structural measures of community composition were calculated (Shannon and Weaver, 1949; Bray and Curtis, 1957; Margalef, 1958; Pielou, 1975) using commercially available software (Primer, Version 1, Plymouth Marine Laboratory, UK). A detailed description of these indices can be found in Clarke and Warwick (1994). Community composition (pre- and post-dredging) was also compared by using non-metric multidimensional scaling (MDS; Kruskal and Wish, 1978) and the calculation of an index of multi-variate dispersion (IMD; Clarke and Warwick, 1994). This index is a species dependent comparative index of multivariate dispersion for community stress. 2.10. Statistical analysis Differences in algal-periphyton and benthic community structure and the periphyton trace metal residues were compared pre- and post-dredging between and within the dredging and non-dredging zones using a two-way analysis of variance (ANOVA). Data were transformed to meet the normality assumption when necessary. One-way analysis of variance followed by post-hoc analysis were used to determine differences in photosynthetic active radiation. Statistical analyses were performed by using SAS (SAS Institute Inc., 1991). The threshold level of statistical significance for this study was a=0.05.
3. Results 3.1. Physicochemical water quality The physicochemical parameters at the surface were different during the 4 months of periodic dredging. Dissolved oxygen concentrations for all stations typically
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ranged from 6.3 to 9.6 mg/l during dredging. The pH ranged from 6.7 to 8.2. Surface salinity ranged from 0.5 to 15.9 ppt, and the temperature was between 15.1 to 23.4 C. Most of the differences in the parameters were likely the result of seasonal factors, tidal action and the variation in discharge from Carpenter’s Creek. Subsurface photosynthetically active radiation decreased significantly in the dredging zone at some stations compared with the non-dredged area (Fig. 2). The average reductions, relative to percent at surface, ranged from 49 to 61% at the 0.5 m depth for the five stations in the dredging zone. The reductions were significant at two stations at this depth when compared with the outside dredging zone area. PAR at the 1 m depth averaged 32% of surface values in the non-dredging area and ranged from 14 to 25% at the five dredging stations. The reductions at this depth were significant for one of the five stations in the dredging zone when compared with the average for the non-dredged area.
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for which the concentrations were similar. The average concentrations of all metals but nickel in sediment collected post-dredging in August were either similar or less than those during dredging. However, it is important to note that all but cadmium and copper were still elevated in August relative to pre-dredging levels. Mercury concentrations before dredging were below the detection limit of 3.0 mg/g dry weight but using a more sensitive analytical technique, averaged 668.2 and 498.0 ng/g dry weight during and after dredging, respectively. 3.3. Whole sediment toxicity Acute toxicity was uncommon despite the presence of potentially toxic concentrations of several trace metals as indicated by the exceedances of the sediment quality guidelines. Survival of A. bahia after exposure to the pre- and post-dredging sediments was similar statistically. Survival after exposure to the four sediments
3.2. Chemical quality of sediment The sediments in the dredging zone were considerably contaminated and remained contaminated despite the dredging based on the sediment quality guidelines proposed for Florida coastal areas (Fig. 3). The average concentrations of cadmium, copper, lead, zinc and mercury in most sediments, regardless of the sampling period, exceeded the numerical threshold effects guideline values of 0.68, 18.7, 30.2, 124.0 and 0.13 mg/g, respectively. There were temporal differences in the trace metal concentrations (Fig. 3). The average concentrations of most trace metals were greater during the latter stages of dredging (January). The exception was for cadmium
Fig. 2. Photosynthetically active radiation (PAR) measured at 0.5 and 1.0 m depths relative to surface. Values are for five sampling stations during dredging and represent mean ( 1 S.D.) of multiple measurements. DZ, dredging zone; ODZ, outside dredging zone. Different letters represent significant difference (a=0.05).
Fig. 3. Comparison of trace metal concentrations for sediments collected from the dredging zone before, during and after dredging. Values represent mean ( 1 S.D.) for sediments collected from one (pre-dredging) and four (post-dredging) stations. Values in mg/g dry weight except for mercury (ng/g dry wt.). BD, below detection limit of 3 mg/g dry weight. Exceedances of sediment quality guidelines proposed for Florida near-coastal areas (MacDonald et al., 1996) are also shown ().
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collected from the dredging zone 1 month before dredging averaged 98% ( 1 S.D.=4%; range=93–100%) relative to an average of 93% ( 14%; range=73– 100%) 1 year after dredging (Table 1). Survival of this epibenthic invertebrate after exposure to two sediments collected from these stations during dredging was 33 and 73%. Survival of mysids exposed to the reference sediment was 100%. The particle size distribution of the sediments collected during October and November from the four locations for which toxicity was determined appears in Table 1. Two sediments (Stations one and three) were sand-dominated (62 and 98% sand) and one (Station four) was comprised mainly of clay (86%). Station two sediment was comprised of 49 and 51% clay.
3.5. Algal-periphyton community composition The number of species and individuals of the algalperiphyton were similar, when compared pre- and postdredging within and between the dredging and non-dredging stations. For example, the number of periphytic algal species colonized on the substrates at the seven stations located in the dredging zone prior to dredging averaged 19.7 ( 1 S.D.=4.5; range=15–25) relative to 21.4 ( 2.4; range=16–25) 8 months afterwards at the same locations (Table 2). Likewise, the average number of species colonized at the three stations outside the dredging zone were similar predredging (23.3 5.7; range=17–28) and post-dredging (20.0 6.2; range=13–25). Algal cell density increased after dredging from an average of 166.1 ( 118.0; range=37–340) cells/ml to 471.6 ( 402.7; range=87– 1150) cells/ml in the dredging zone. Outside the dredging area, the increase was from a mean of 134.7 ( 101.4; range=42–423) cells/ml to 502.7 ( 391.8; range=52–762) cells/ml. Although increases were evident when based on average density values, they were not statistically significant due to the considerable variation in results for the specific colonization stations. Diversity indices for the algal-periphyton were less after dredging in the dredging and non-dredging zones but the decreases were not significant (Table 2). The decrease in values averaged 16% ( 7%; range=11– 30%) for the three indices. For example, the Shannon– Wiener index values for periphyton colonized in the dredging zone decreased approximately 13% from an average of 0.98 ( 1 S.D.=0.11; range=0.85–1.20) to 0.85 ( 0.10; range=0.07–1.00), after dredging. These average diversity index values were very similar to those for periphyton colonized at the three non-dredging stations, 1.08 ( 0.12; range=0.96–1.19; pre-dredging) and 0.87 ( 0.14; range=0.75–1.03; post-dredging). There was a difference in the pre- and post-dredging community composition based on multidimensional
3.4. Benthic community composition The benthic macroinvertebrate community in the dredging zone was not abundant nor diverse prior to dredging and was less so afterwards (Table 1). The number of species and individuals and the diversity index values decreased statistically as a result of the dredging. For example, species number before dredging ranged from 4 to 6 relative to 1 to 4 afterwards and density averaged 114 ( 1 S.D.=39) individuals/m2 before and 27 ( 18) individuals/m2 1 month after dredging. The Shannon–Wiener diversity index values pre-dredging ranged from 0.40 to 0.62 and were either not calculable due to low number of individuals or were relatively similar to those at the same stations, post-dredging. The more abundant species prior to dredging were the polychaetes, Laeonereis culveri, and Streblospio benedecti and a dipteran species (Palpomyia). Their combined relative abundance was 68% 1 month pre-dredging and 23% 1 month post-dredging. Two primary differences in abundance were the post-dredging increase of a harpacticoid copepod, from 6 to 69%, and the decrease in Palpomyia from 17 to 2%.
Table 1 Several structural characteristics of the benthic macroinvertebrate community and sediment toxicity in Bayou Texar pre-dredging (October 1994) and post-dredging (January 1995)a Sampling stations
1 2 3 4
Pre-dredging
Post-dredging
Particle size
No. species
No. individualsb
Diversity indexc
Invertebrate survival
No. species
No. individuals
Diversity index
Invertebrate survival
Sand (%)
Clay (%)
6 4 6 6
144 95 68 148
0.51 0.40 0.58 0.62
100 100 93 100
1 4 1 4
4 34 23 46
0d 0.50 0 0.44
73 100 100 100
62 49 98 14
38 51 2 86
a Values represent mean for two replicate grab samples collected from each station located in the dredging zone. Survival (%) of Americamysis bahia also shown after 7-day exposure to whole sediments. Particle size distribution is for pre-dredging condition. b per m2. c Shannon-Wiener diversity index (Shannon and Weaver, 1949). d Could not be calculated due to low numbers of individuals.
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Table 2 Comparison of taxonomic structural characteristics of the periphytic algae colonized in areas within the DZ and ODZ pre-dredging (August 1994) and post-dredging (August 1995)a Colonization location
Number species
Number individuals
Shannon-Wienerb
Margalef’s indexc
Pielou’s indexd
ODZ Pre Post
23.3 (5.7) 20.0 (6.2)
134.7 (101.4) 502.7 (391.8)
1.08 (0.12) 0.87 (0.14)
10.9 (0.10) 7.6 (0.80)
0.81 (0.13) 0.70 (0.20)
DZ Pre Post
19.7 (4.5) 21.4 (2.4)
166.1 (118.0) 471.6 (402.7)
0.98 (0.11) 0.85 (0.15)
9.2 (0.8) 8.2 (1.5)
0.76 (0.12) 0.64 (0.10)
a b c d
Values represent mean (1 S.D.) for the seven dredging zone (DZ), and three outside dredging zone (ODZ) colonization sampling sites. Shannon and Weaver (1949). Margalef (1958). Pielou (1975).
scaling ordination and transformed abundances and Bray–Curtis similarities (Fig. 4). The index of multivariate dispersion was 0.318 which indicates greater stress in the post-dredging periphyton community. The dissimilarity between the periphyton colonized pre- and post-dredging was 49.3%. The algal-periphyton were dominated by diatoms in the dredging zone where they comprised approximately 75% (pre-dredging) and 84% (post-dredging) of the taxa. The diatom, Neidium affine, was the dominant species comprising 30 and 33%, respectively, of the total taxa collected during the pre- and post-dredging colonization periods. In addition, Achnanthes minutissima was also relatively common before (15% of taxa) and after (8% of taxa) the dredging event as were species of the genus, Fragilaria which had a cumulative abundance of 21% (before) and 16% (after). The most obvious difference in species composition was for Nitzschia recta which was not colonized prior to dredging but comprised 21% of the taxa colonized 8 months afterwards. 3.6. Periphyton trace metal residues The trace metal residues for periphyton colonized in the dredging zone (DZ) averaged for seven stations were less 8 months after dredging (Table 3). The percentage reduction averaged 34.4 ( 1 S.D.=19.7) for the seven trace metals. The average decreases in contaminants in decreasing order were: mercury (65%) chromium (44%), zinc (44%), copper (38%), lead (25%), cadmium (21%) and nickel (4%). Depending upon the spatial scale of comparison, significant decreases occurred for mercury, chromium and zinc. The average mercury concentration in periphyton colonized at the seven stations in the dredging zone was 1427.7 ( 1 S.D.=389.1; range=984–2000) ng/g dry weight before dredging and 502.1 ( 146.8; range=242–639) ng/g dry weight for periphyton colonized at the same sites 8 months after dredging. The post-dredging reductions occurred at all colonization stations (Fig. 5).
Fig. 4. Difference in algal-periphyton community composition as indicated by two dimensional MDS ordination of the 10 stations preand post-dredging based on transformed algal-periphyton abundances and Bray–Curtis (1957) similarities. The stress value was 0.19. See Clarke and Warwick (1994) for a more detailed description of technique.
The trace metal residues for periphyton colonized at the three stations located outside the dredging zone (ODZ), on average, were usually greater post-dredging (Table 3). The most noticeable exception was mercury. Concentrations of mercury in the periphyton colonized outside the dredging zone averaged 1091.0 ( 380.5; range=653–1340) ng/g dry weight pre-dredging and 710.0 ( 139.6; range=553–820) ng/g post-dredging. The average 35% decrease was not significant.
4. Discussion The environmental effects of the dredging occurred within the dredging zone and did not appear to extend
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Table 3 Comparison of trace metal residues in periphyton colonized within the DZ and ODZ pre-dredging (August 1994) and post-dredging (August 1995)a Colonization location
Trace metals Cd
Cr
Cu
Ni
Pb
Zn
Hg
ODZ Pre Post
1.3 (0.4) 1.0 (0.14)
34.8 (16.5) 40.2 (5.3)
68.4 (48.8) 74.9 (3.3)
12.7 (4.4) 16.5 (1.9)
107.2 (40.6) 139.7 (9.5)
389.3 (239.1) 554.3 (82.2)
1091.0 (380.5) 710.0 (139.6)
DZ Pre Post
2.2 (0.7) 1.5 (1.1)
44.8 (25.4) 24.9 (4.0)
51.3 (53.9) 31.8 (10.1)
19.7 (7.6) 14.6 (4.0)
137.1 (60.5) 101.8 (17.2)
461.6 (455.3) 265.0 (45.2)
1427.7 (389.1) 502.1 (146.8)
a
Values in mg/g dry weight except for mercury (ng/g dry wt.). Values represent mean (1 S.D.) for three outside dredging zone (ODZ) and seven dredging zone (DZ) colonization sites.
Fig. 5. Comparison of total mercury concentrations in periphyton (ng/g dry wt.) colonized for 21 days before dredging and 8 months afterwards. Stations 4–10 are located in the dredging area.
seaward. Effects on physicochemical water quality were minor during dredging other than a reduction in the photic zone at a few sampling stations. The most obvious biological effect, as would be expected, was the reduction in the benthic macroinvertebrate community. However, this reduction needs to be considered in the context that the benthos in this sediment depositional zone was depauperate prior to dredging. Therefore, the ecological significance of its further depletion may have been minimal. There were indications that the dredging impacted the algal-periphyton causing a post-dredging compositional change. However, the magnitude of these effects were station-specific and when the data were combined and compared for the pre- and post-dredging time periods, many differences in the response parameters were not statistically significant. This includes the trace metal residues. Many residue concentrations decreased after dredging in the dredging zone, but the pre- and post-dredging differences were significant, only for mercury, chromium and zinc. The reduction in mercury
may have been the most positive ecological benefit of the dredging since it reduced possible trophic transfer to fish which, as stated earlier, is an important issue for many near-coastal areas of Florida (Paulic et al., 1996; USEPA, 1999). It was assumed that the residue results were for organic matter on the substrates. However, the effect of settled inorganic substances on these type of results is an important consideration (Newman and McIntosh, 1989). For perspective, the ash free dry weight (AFDW) to dry weight (DW) percentage for periphyton colonized in the dredging zone prior to the dredging event (1993–1994) averaged approximately 36% ( 1 S.D.= 6%; Lewis et al., 2001a,b). Dredging was assumed to be the cause of the differences in residue concentrations since no other episodic or unusual seasonal event occurred during this study. However, this could not be confirmed based on the sediment chemical analyses conducted during this baseline study. No insight for mercury can be provided due to differences in the analytical methodology. The
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sediment concentrations for chromium and zinc, the other two metals for which the residues significantly decreased, typically increased during and after dredging. The lack of congruity in these data sets may be due, in part, to the differences in the time of sediment collections and the periphyton colonizations which were not usually identical. This is particularly true for the pre-dredging samples which were collected approximately 1 year prior to dredging. Furthermore, a few samples of sediment may not be characteristic of the dynamic sediment conditions associated with dredging. Water quality in Bayou Texar has been reported (Lewis et al., 2001a,b). Concentrations of trace metals were below detection (range=7–40 mg/l) at most sampling areas including in the dredging zone. For example, based on six measurements of pre-dredging water samples, only zinc was detectable (mean=16.8 1 S.D. deviation=3.5) mg/l. Information for post-dredging samples is not available and consequently, no comparative insight can be provided. Based on all considerations, it is evident that the number of sampling stations, the type and frequency of chemical analysis, and the media analyzed, are important considerations even for small scale dredging events if a cause-effect relationship is to be firmly established for biota other than that associated with sediment. There is no available information in the scientific literature describing the environmental effects of dredging on the biota in this bayou or for periphyton in any other similar coastal area in Florida. However, several studies have been conducted elsewhere in which dredging effects were determined on aquatic flora such as seagrasses (Onuf, 1994; Long et al., 1996), seaweeds (Lyngby and Mortensen, 1996) and phytoplankton (Windom, 1975; Iannuzzi et al., 1996). The reported effects included increases in turbidity, reduction of primary productivity and changes in the structural characteristics of the plant communities. The above results were event-specific and dependent on the type of flora, response parameter, dredging methodology, volume dredged and type of habitat. In summary, the environmental impact of relatively small-scale dredging events in shallow water bayous, such as Bayou Texar, may not be a major factor in their long-term environmental condition. Urbanized bayous are impacted continuously by the development in their watersheds and by residential areas along their periphery. Therefore, infrequent and spatially-limited dredging may not be a cause of ecologically significant deterioration as it may be for larger scale events in more pristine near-coastal areas. The usual reduction in the benthos, likely already sparse in the areas to be dredged, may be ‘‘counteracted’’ by beneficial effects to other biota due to the removal of sediments and the increase in depth and circulation.
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Acknowledgements The trace metal concentrations in sediment and periphyton were analyzed by personnel from AVANTI Corp., (Gulf Breeze, FL). Particle size distribution was determined by Law Engineering and Environmental Sciences Inc. (Pensacola, FL). Val Coseo prepared the manuscript, graphics support was provided by Stephen Embry (OAO Corp., Pensacola Beach. FL) and statistical support by Dr. Michael Bundrick (University of West Florida, Pensacola, FL). Taxonomic identification of the benthos and periphyton was performed by the Institute of Coastal Sciences, University of West Florida, Pensacola, Florida. Thomas Roush and Larry Goodman (USEPA) provided field support. The US Environmental Protection Agency through its Office of Research and Development funded the research described here. It has been subjected to Agency review and approval for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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