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1-1-2015 A Comparison of Whole Body Mercury Concentrations of Mummichogs (Fundulus heteroclitus) and Atlantic Silversides (Menidia menidia) Daniel P. Ferons Coastal Carolina University
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A comparison of whole body mercury concentrations of mummichogs ( Fundulus heteroclitus ) and Atlantic silversides ( Menidia menidia )
By
Daniel P. Ferons ______Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Coastal Marine and Wetland Studies in the School of Coastal and Marine Systems Science Coastal Carolina University
2015 ______
Dr. Jane Guentzel, Major Professor Dr. Andrew Heyes ______
Dr. Juliana M. Harding Dr. Rachel Whitaker ______
Dr. Eric Koepfler Dr. Michael Roberts, Dean ______
Dr. Paul Gayes, SCMSS Director ACKNOWLEDGEMENTS
First and foremost, I would like to thank my advisor, Dr. Guentzel. She taught me how to collect mercury in the field and patiently corrected my writing. Without her guidance, this project would not have been possible. I would also like to thank Dr. Heyes for sample analysis. His help allowed me to expand the scope of this project.
Additionally, I would like to thank Dr. Harding for teaching me the laboratory dissection techniques that I used to evaluate the gut contents of mummichogs and Atlantic silversides. I also want to thank Dr. Whitaker and Dr. Koepfler for being on my committee.
Coastal Carolina University’s Professional Enhancement Grant awarded to Dr.
Guentzel and a Coastal Carolina University’s Undergraduate Research Council Grant funded this project.
I would like to thank Merrill Boyce for the use of his dock to launch a canoe for fish collection. I want to thank Richard Goldberg, Sam Gary, and Bryan Keller for driving the boat on mercury collection days. Another thank you to David Catalano,
Bradley Craig, Hunter Fiuzat, Andrew Frink, Roni Lance, Cory Rodgers, James
Rupinski, Dani Silva, Zach Smith, Lindsey Sherwood, Rob Schroeder Caleb Storovasik,
Kristen Trevey, and Cara Wilck for their help collecting fish from the field and/or dissecting the fish in the lab. Thank you to Dr. Jenna Hill for letting me use the sediment lab to measure grain size. Also, I want to thank Dr. Keshav for his help with my statistical analysis. Finally, I want to thank my parents, family, and friends for supporting me during this process.
ii ABSTRACT
Few studies have focused on mercury cycling within salt marsh estuaries.
Mummichogs ( Fundulus heteroclitus ) and Atlantic silversides ( Menidia menidia ) are two species of forage fish that live year round in South Carolina salt marshes. During high tide, mummichogs feed from the water column, sediment, and off of smooth cordgrass while Atlantic silversides prey upon zooplankton. In this study, total and methyl mercury within mummichogs and Atlantic silversides from Dunn Sound, SC were quantified and compared over four seasons throughout 2014. Gut contents were also quantified and compared to determine if there was a dietary impact on the concentrations of total and methyl mercury in these fish. Atlantic silversides (20-44.8 ng/g THg; 17.1-40.6 ng/g
MeHg) had significantly higher whole body total and methyl mercury concentrations than mummichogs (6.2-26.7 ng/g THg; 2.8-19.5 ng/g MeHg; p < 0.01, Two- way ANOVA,
Tukey’s Test). There was no significant difference between the percentages of methylmercury in mummichogs and Atlantic silverside (Two-way ANOVA). This suggests that mummichogs and Atlantic silversides assimilate mercury at the same rate.
The percent gut contents by weight (%W) and by number (%N) of Atlantic silversides were significantly different than mummichogs (p < 0.01, PERMANOVA). Mummichog gut contents were more diverse consisting of more than ten percent (%W and %N)
Arthropods, detritus, and miscellaneous throughout 2014. Atlantic silverside gut contents were dominated by planktonic Arthropods. The differences in mercury concentrations in these two fish may impact the bioaccumulation of mercury in higher trophic level organisms.
iii Table of Contents Acknowledgements…………………………………………………………………ii
Abstract……………………………………………………………………………..iii
List of Tables……………………………………………………………………….vi
List of Figures……………………………………………………………………...viii
Introduction A) Mercury Cycling………………………………………………………...1 B) Mummichogs ( Fundulus heteroclitus )…………....…………………….4 C) Atlantic Silversides ( Menidia menidia )………………………………....6 D) Objectives……………………………………………………………….8
Methods A) Site Selection……………………………………………………………10 B) Timetable………………………………………………………………..10 C) Fish……………………………………………………………………....10 D) Water………………………………….………………………………....12 E) Sediment………………………………………………………………....13 F) Smooth cordgrass ( Spartina alterniflora ) ………………………………13 G) Data Analysis a. Fish………………………………...... 14 b. Water ……………………………………………………………17 c. Sediment.………………………………………………………...17 d. Smooth cordgrass……………………………………..………....17
Results A) Fish a. Mercury………………………………….……………………....19 b. Gut contents………………………………...... 21 B) Water a. Mercury i. Unfiltered………………………………………………..23 ii. Filtered…..………………………………...... 23
Discussion…….………………………………...... ………………………………...25
References………………………………....………………………………...... 29
Appendix A) Water a. Unfiltered………………………………....……………………..38 b. Filtered………………………………....………………………..39
iv B) Sediment a. Mercury………………………………....………………………39 C) Smooth Cordgrass a. Mercury………………………………....………………………40
v
LIST OF TABLES
Table 1……………………………………………………………………………...41
Summary of the statistical tests used for fish total and methyl mercury by season.
Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Table 2……………………………………………………………………………...42
Summary of the statistical tests used for mummichog length, mummichog weight,
Atlantic silverside length, Atlantic silverside length, gut percent composition by weight
(%W), gut percent composition by number (%N), and percent ingestion by fish (%F) by season. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Table 3……………………………………………………………………………...43
Summary of the statistical tests used for unfiltered and filtered water total and methyl mercury by season. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Table 4……………………………………………………………………………...44
Summary of the statistical tests used for unfiltered and filtered water total and methyl mercury by season and tide. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
vi Table 5……………………………………………………………………………...45
Summary of the statistical tests used for sediment total and methyl mercury by season and tide. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Table 6……………………………………………………………………………...46
Summary of the statistical tests used for smooth cordgrass total and methyl mercury by season and living versus dead. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
vii LIST OF FIGURES
Figure 1………………………………………………………………………………47
SDHEC South Carolina Mercury Consumption Advisories Map
(http://www.scdhec.gov/FoodSafety/FishConsumptionAdvisories/AdvisoryMap/ ;
SCDHEC, 2014)
Figure 2………………………………………………………………………………48
Overview of mercury methylation in salt marsh sediments. The layers of brackish water, oxic sediments, and anoxic sediments do not represent actual depths. The sediment has been partitioned into an oxic region and a hypoxic region. Hg (II) = dissolved inorganic mercury; Hg i = undissolved inorganic mercury; MeHg = aqueous methylmercury.
Downward arrows represent flocculation and sedimentation processes. Arrows pointing from Hg (II) and MeHg toward Hg i represent oxidative and reductive demethylation.
Upward arrows represent passive aqueous diffusion and volatilization of mercury. The arrow pointing from MeHg to the diatom represents passive biological uptake by phytoplankton. Graphic is based on the information presented in Merritt and
Amirbahman (2009). The graphic was produced by Kathryn Ferons
Figure 3……………………………………………………………………………..49
Estuarine food web depicting the trophic levels occupied by mummichogs and Atlantic silversides in relation to other organisms. Drawing is courtesy of Kathryn Ferons.
Figure 4……………………………………………………………………………...50
The study site is located on the landward side of Waties Island in Dunn Sound, near
Cherry Grove, SC (33°51'13.17"N, 78°34'59.50"W). It consists of smooth cordgrass salt
viii marsh surrounded by tidal creeks, mud flats, and oyster reefs. © 2015 Google; Map Data:
SIO, NOAA, U.S. Navy, NGA, GEBCO; Image: © 2015 TerraMetrics
Figure 5………………………………………………………………………………51
Total and methylmercury concentrations in mummichogs and Atlantic silversides over the four seasons in 2014. Atlantic silversides had significantly higher concentrations of total (p < 0.01) and methyl (p < 0.01) mercury relative to mummichogs (Table 1). There was no significant difference between the percentage of methylmercury in Atlantic silversides and mummichogs (Table 1). However, the percent methylmercury, represented in the boxes above the bars, in mummichogs and Atlantic silversides was significantly higher in the fall relative to the spring (p < 0.05; Table 1). THg = Total mercury; MeHg = Methylmercury.
Figure 6……………………………………………………………………………...52
Mummichog and Atlantic silverside somatic weight (g) by total length (mm). The somatic weight to total length ratio was larger in mummichogs than Atlantic silversides.
Figure 7……………………………………………………………………………...53
The percent by weight of gut contents (%W) in mummichogs and Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides had significantly different gut contents by %W (p < 0.01; Table 2). Atlantic silverside gut contents were dominated by planktonic Arthropods throughout the whole year, whereas mummichogs gut contents varied by season.
Figure 8……………………………………………………………………………...54
The percent by number of gut contents (%N) in mummichogs and Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides had significantly
ix different gut contents by %N (p < 0.01; Table 2). The %N of Arthropods ingested by
Atlantic silversides was primarily zooplankton.
Figure 9……………………………………………………………………………...55
The percent of fish ingesting prey item of gut contents (%F) in mummichogs and
Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides did not have significantly different %F (Table 2). Over 90% of Atlantic silversides were captured with Arthropods in the gut contents throughout the year, while the percent of mummichogs captured with Arthropods in their gut contents was different by season.
Figure 10…………………………………………………………………………….56
Total and methylmercury concentrations in unfiltered water over the four seasons in
2014. Total mercury was not significantly different although the summer had a higher concentration than any other season (Table 3). Methylmercury concentrations were significantly different between the seasons (p < 0.01, Tukey’s test; Table 3). The percent methylmercury, represented in the boxes above the bars, was significantly different between seasons (p < 0.01, Dunn’s Test; Table 3). THg = Total mercury;
MeHg = Methylmercury.
Figure 11…………………………………………………………………………….57
Total and methylmercury concentrations in filtered water for four seasons in 2014. Total mercury was not significantly different. Methylmercury concentrations were significantly different between the seasons (p < 0.01, Dunn’s Test; Table 3). The percent methylmercury, represented in the boxes above the bars, was significantly different between seasons (p < 0.01, Dunn’s Test; Table 3). THg = Total mercury;
MeHg = Methylmercury.
x Figure 12…………………………………………………………………………….58
Total and methylmercury concentrations in unfiltered water by tide over four seasons in
2014. Total mercury remained constant by tide and season throughout the year. The one exception is the high total mercury concentration during summer low tide.
Methylmercury and the percent methylmercury, represented in the boxes above the bars, of the total mercury, were the same for winter and spring before decreasing through the fall. THg = Total mercury; MeHg = Methylmercury.
Figure 13…………………………………………………………………………….59
Total and methylmercury concentrations in filtered water for tides by the four seasons in
2014. Total mercury remained constant throughout the year. Methylmercury concentrations increased from the winter to the spring but decreased after spring through to the fall. There were no differences in the methylmercury concentrations by tide.
During the winter and spring, filtered total mercury concentrations were higher at high tide than low tide. In the summer and fall, high and low tide total mercury concentrations were the same. THg = Total mercury; MeHg = Methylmercury.
Figure 14…………………………………………………………………………….60
Total and methylmercury concentrations in sediment by tide over the four seasons in
2014. Total mercury concentrations were higher during low tide than during high tide from winter to summer. In the fall, high tide total mercury concentrations were significantly higher than low tide (Table 5). There was no change in the percent methylmercury. THg = Total mercury; MeHg = Methylmercury.
xi Figure 15……………………………………………………………………………..61
Total and methylmercury concentrations in living and dead smooth cordgrass other the four seasons in 2014. Sampling of the smooth cordgrass began in spring of 2014 because it was not considered a potential mercury contributor until after the first gut dissections.
A) Total mercury concentrations were higher in dead smooth cordgrass than living smooth cordgrass. Living smooth cordgrass total mercury concentrations did not change but dead smooth cordgrass decreased from the spring to the fall. B) Methylmercury concentrations were significantly higher in dead smooth cordgrass than in living
(Table 6). THg = Total mercury; MeHg = Methylmercury
xii INTRODUCTION
Humans are commonly exposed to mercury through the consumption of fish, shellfish, and marine mammals (Clarkson et. al., 2003). Concerns regarding health risks associated with the ingestion of mercury, specifically methylmercury (MeHg), have led to fish consumption advisories in 101 South Carolina rivers, lakes, estuarine, and marine ecosystems (SCDHEC, 2014). Since 2012, there have been three advisories within South
Carolina estuarine/marine waters and one in the Intracoastal Waterway (SCDHEC, 2014).
When estuarine waters are grouped with the coastal plain rivers, they constitute 94% of all current consumption advisories within South Carolina (Figure 1; Guentzel, 2009,
SCDHEC, 2014).
Mercury Cycling
Inorganic mercury (Hg i) is released into the environment through natural and anthropogenic sources. Natural sources include volcanic activity, rock weathering, and the degassing of mercury from water bodies (Schuster et. al., 2002, Pirrone et. al., 2010).
Anthropogenic sources of inorganic mercury come from the burning of fossil fuels, such as coal, and water runoff from mines (Schuster et. al., 2002, Pirrone et. al., 2010).
Inorganic mercury released into the atmosphere is later deposited to ecosystems through wet and dry deposition (Seigneur et. al., 2004, Miller et. al., 2005). Dry deposition usually consists of the settling of particulates and the adsorption of aerosols while wet deposition consists of rain and snow. Inorganic mercury can enter aquatic ecosystems directly via wet and dry deposition or it can be deposited on terrestrial ecosystems and enters aquatic systems via runoff. Once mercury is in aquatic ecosystems it can bioaccumulate at all trophic levels (Miller et. al., 2005).
1 Prior to bioaccumulation in organisms, atmospherically deposited inorganic mercury is converted into methylmercury (Figure 2). Inorganic mercury methylation is primarily mediated by iron and sulfate reducing bacteria present in soils and sediments with low oxygen (hypoxic) conditions that contain high concentrations of organic matter
(Compeau and Bartha, 1985, Gilmour et. al., 1992, King et. al., 1999, King et. al., 2000,
Kerin, 2006, Sunderland et. al., 2006). Production of methylmercury usually occurs in the top 1-2 millimeters of sediments within salt marshes (Compeau and Bartha, 1985,
Sunderland et. al., 2006, Guentzel, 2009, Yu et. al., 2012). If the sediments become completely anoxic (no oxygen), high concentrations of hydrogen sulfide can accumulate.
The hydrogen sulfide can bind to methylmercury creating mercury (II) sulfide chemical complexes, which are not bioavailable (Benoit a et. al., 1999, Benoit b et. al., 1999).
Mercury can be associated with dissolved organic matter (DOM) in the water column. Higher concentrations of DOM, total mercury, and methylmercury occur in
South Carolina Middle Atlantic Coastal Plain Blackwater Systems relative to South
Carolina Piedmont River Systems (Guentzel, 2009, Glover et. al., 2010, Guentzel, 2012).
South Carolina Middle Atlantic Coastal Plain River Systems have water column total mercury concentrations ranging from 2.0 ng/L to 13.6 ng/L, while the South Carolina
Blue Ridge and Piedmont River Systems have water column total mercury concentrations ranging from 2.0 ng/L to 4.0 ng/L (Guentzel, 2009). The mercury that is associated with
DOM may be more or less biologically available depending upon the type of chemical complexation it forms (Hurley et. al., 1998, Golden et. al., 2012). Reactions between mercury and DOM occur rapidly in water when there is a low mercury-to-DOM ratio. As mercury concentrations increase relative to DOM the percent of mercury that forms
2 chemical complexes with DOM decreases. These reactions are also influenced by pH
(Haitzer et. al., 2002, Haitzer et. al., 2003). At pH less than 4 there is an increase in the competition for binding sites between free protons and mercury. The free protons in acidic conditions bind to functional groups on DOM and the mercury becomes available for ionic interaction (Haitzer et. al., 2003).
Phytoplankton, such as diatoms, accumulate inorganic and methyl mercury by passive diffusion across the cell membrane (Mason et. al., 1996). The rate of mercury accumulation by phytoplankton is influenced by the surrounding pH, salinity, and available nutrients (Lawson and Mason, 1998, Haitzer et. al., 2002, Haitzer et. al., 2003).
When phytoplankton were ingested by copepods, a secondary consumer planktonic crustacean, and amphipods, a secondary consumer benthic crustacean, the methylmercury in the phytoplankton was assimilated more efficiently than the inorganic mercury.
Additionally, copepods assimilated less methylmercury than the amphipods (Lawson and
Mason, 1998). As total mercury concentrations decreased in the phytoplankton food source, the amphipods assimilated a high percentage of methylmercury relative to the copepods (Lawson and Mason, 1998).
Similar to copepods and amphipods, different fish species can assimilate different concentrations of mercury from the same food source (Stefansson et. al., 2013).
Stefansson et. al. (2013) compared the dietary methylmercury concentration in the sheepshead minnow ( Cyprinodon variegatuse ) and the inland silverside ( Menidia beryllina ). Inland silversides had significantly higher weight specific whole body total mercury concentrations than sheepshead minnows. Sheepshead minnows had significantly higher total mercury when comparing total body burdens of the mercury.
3 These findings suggest that differences in mercury concentrations can occur between species independent of the species diet (Stefansson et. al., 2013).
Mummichogs ( Fundulus heteroclitus )
Mummichogs are abundant in salt marshes from Nova Scotia to Florida (Hardy,
1978, Rosen, 1973). Schools can range from two fish to several hundred, and lengths
(mm) of individuals within the schools vary depending on the season (Hildebrand and
Schroeder, 1928, Kneib and Stiven, 1978). Spawning occurs from 12˚ to 26˚ C coinciding with the spring tides (Solberg, 1938, Taylor and DiMichele, 1980). Eggs are laid inside empty bivalve shells or inside the outer leaves of smooth cordgrass ( Spartina alterniflora ). After a minimum of eight days, when the salt marsh is inundated by high tide, the mummichog eggs hatch (Taylor et. al. 1977, Kneib and Stiven, 1978, Taylor and
DiMichele, 1983). Larval mummichogs hatch between 4.0-7.0 mm total length and are considered to be juvenile fish at 20.0 mm total length (Hardy, 1978). Typically, mummichogs do not reach maturity until they have over-wintered (Hardy, 1978, Kneib and Stiven, 1978).
Mummichog abundance in salt marshes varies by season. From the end of May through the end of July, the majority of mummichogs are adults with total lengths between 30 to 70 mm (Kelso, 1979). The highest densities of adult mummichogs over 70 mm total length occur during August. Even though individuals over 70 mm have reached maximum density, the population is dominated numerically by the young-of-the-year
(Kneib and Stiven, 1978, Meredith and Lotrich, 1979). After August, mummichogs over
70 mm migrate to deeper water or become prey (Meredith and Lotrich, 1979). When the water temperature decreases to less than 15˚C in the fall, some mummichogs will burrow
4 under the sediment while others will swim to deeper water at the mouth of a tidal creek to avoid the decreasing water temperatures (Hardy, 1978).
Mummichogs migrate from their resident tidal creek into the salt marsh during the flood tide and return with the ebb tide (Bigelow and Schroeder, 1953, Lotrich, 1975, Teo and Able, 2003). Although mummichogs move between tidal creeks and salt marshes, mummichogs do not swim more than 1 km (Teo and Able, 2003). There is some opportunistic feeding by mummichogs during low tide, however feeding usually takes place within the smooth cordgrass during the daylight high tide (Vince et. al., 1976,
Kneib and Stiven, 1978, Weisberg et. al., 1981). Mummichogs that move into the salt marsh with the flood tide have empty guts (Kneib and Steven, 1978). These fish feed during the slack high tide, and then return to the tidal creek with a full gut during the ebbing tide (Rozas et. al., 1988).
Mummichogs are opportunistic feeders that swallow prey whole. Therefore, the size of prey ingested by mummichogs is dependent on the size of their mouth (Vince et. al., 1976). Although mummichogs have an upturned mouth, which suggests surface feeding, they consume prey from the water column, subtidal benthos, intertidal mud flats, and off of smooth cordgrass (Baker-Dittus, 1978, Kneib and Stiven, 1978, Kneib et. al.,
1980, Weisberg and Lotrich, 1982, Allen et. al., 1994, Able et. al., 2007). The most frequent prey items found in the guts of mummichogs are amphipods, Annelids, benthic algae, copepods, fish eggs, grass shrimp, insects, juvenile fish, nematodes, planktonic algae, platyhelmenthes, ostracods, snails, tanaids, and detritus (Fritz, 1974, Vince et. al.,
1976, Kneib and Stiven, 1978, Kneib et. al., 1980, Heck and Thoman, 1981, Allen et. al.,
1994, Smith et. al., 2000, James-Pirri et. al., 2001, McMahon et. al., 2005, Able et. al.,
5 2007). Of the prey items listed, detritus is the most commonly ingested (Baker-Dittus,
1978, Kneib et. al., 1980, Allen et. al., 1994). Detritus that is ingested by mummichogs is not an energy source because it is not digested or assimilated into the tissues (Prinslow et. al., 1974, Baker-Dittus,1978, Kneib et. al., 1980.) The large quantity of detritus found in the gut contents of mummichogs can be explained as incidental ingestion during benthic feeding (James-Pirri et. al., 2001).
Blue crabs ( Callinectes sapidus ) are the chief predators of mummichogs
(Meredith and Lotrich, 1979, Kneib and Stiven 1982). Size-selective predation by blue crabs removes the largest mummichogs from an ecosystem (Kneib and Stiven 1982).
Wading birds, such as Great Egrets ( Ardea alba ) will prey on mummichogs during low tide when the mummichogs are confined to tidal pools and shallow creeks (Kneib, 1982,
Bent, 1963). Commercially important fish such as summer flounder ( Paralichthys dentatus ), red drum ( Sciaenops ocellata ), and American eels ( Anguilla rostrata ) feed on mummichogs while living in Atlantic wet coast salt marshes (Lotrich 1975, Meredith and
Lotrich 1979, Peterson and Peterson 1979).
Atlantic silversides ( Menidia menidia )
Atlantic silversides inhabit estuaries from Southern Canada to Florida (Robbins,
1969). This species is found within temperate and subtropical salt marshes, tidal creeks, and the shores of bays and inlets. When water is above 16 ˚C, silversides can be the most abundant species living within these habitats (Mulkana, 1966, Richards and Castagna,
1970, Anderson et. al., 1977, Hillman et. al., 1977). Silversides inhabiting coastal waters from Canada to northern North Carolina will migrate to deeper, offshore waters when the water temperature falls below 16˚C (Conover, 1982, Conover and Ross, 1982).
6 Silversides from southern North Carolina to Florida can be found in coastal habitats year round (Cain and Dean, 1976).
Atlantic silversides, in South Carolina, begin spawning during the spring when water temperatures are between 16-20 ˚C (Middaugh, 1981). Larval silversides hatch during late spring and throughout the summer when water temperatures are between 16-
30˚C (Austin et. al., 1975, Middaugh, 1981). Individuals typically hatch between 5.5-15.0 mm total length (Wang, 1974). Transformation into juvenile fish usually takes place at
20.0 mm TL when the anus moves from under the pectoral fin to approximately the midpoint of the fish. Atlantic silversides typically grow between 10-20 mm/month (Astin et. al., 1973, Conover and Ross, 1982). Silversides remain immature until males reach
91.0 mm TL and females reach 98.0 mm TL (Leim and Scott, 1966, Wang, 1974,
Conover and Ross, 1982). Growth rate decreases during October as the water temperature begins to decrease to 16˚C. Atlantic silverside growth stops in the beginning of November when water temperature is below 16 ˚C. Growth begins again at the end of
March, when water temperatures are greater than 16 ˚C, in preparation for spawning
(Middaugh, 1981, Conover, 1982, Conover and Ross, 1982). Spawning occurs during the day and follows high tide every 14 to 15 days (Middaugh, 1981, Conover, 1982). During spawning, silversides group into schools of several thousand and can deplete the dissolved oxygen in the water column around the school (Middaugh, 1981).
There is little information regarding silverside feeding periodicity. Silversides are often captured feeding in large schools over gravel, oyster reefs, and mud flats during the flood and ebbing tides (Conover and Ross, 1982). Silversides are opportunistic planktivores feeding on prey filtered from the water column. Common prey items include
7 algae, amphipods, cladocerans, copepods, detritus, diatoms, insects, phytoplankton, juvenile squids, and worms (Bigelow and Schroeder, 1953, Thomson et. al., 1971,
Bengtson, 1984, Allen et. al., 1995, Dutton and Fisher, 2010). Phytoplankton are ingested during the consumption of larger prey and are metabolized (Dutton and Fisher, 2010).
Atlantic silversides are planktivorous and target prey in the water column during high tide (Conover and Ross, 1982, Allen et. al., 1995, Dutton and Fisher, 2010). Blue crabs and wading birds, such as Great Egrets, prey on juvenile silversides (Figure 3;
Middaugh, 1981, Conover, 1982). Adult silversides remain in the tidal creeks during high tide and are more important prey species to swimming birds such as Double-crested
Comorants (Phalacrocorax auritus ) and terns, such as the Least Tern (Sternula antillarum antillarum ; Middaugh 1981, Conover, 1982). Adult silversides are also prey to commercially important fishes, such as striped bass ( Morone saxatilis ; Harding and
Mann, 2003), Atlantic mackerel ( Scomber scombrus ; Schaefer, 1970), and bluefish
(Pomatomus saltatrix ; Harding and Mann, 2001) that inhabit deeper areas of estuaries and tidal creeks.
Objectives
The primary objectives of this study are to quantify and compare the concentrations of total and methyl mercury within mummichogs and Atlantic silversides from a South Carolina salt marsh. Mummichog and silverside gut contents were quantified and compared to determine if there is a dietary impact on the total and methyl mercury concentrations in these fish. Total and methyl mercury concentrations in unfiltered water, filtered water, sediment, and smooth cordgrass were quantified and compared to determine if any of these ancillary parameters impacted fish mercury
8 concentrations. These data represent the findings from four quarterly sampling events as they relate to fish mercury concentrations and gut contents. The data in the appendix reports the ancillary parameters that were measured but did not have statistical significance, with respect to fish mercury and gut contents.
9 METHODS
Site Selection
The study site (33°51'13.17"N, 78°34'59.50"W) was in Dunn Sound, South
Carolina on the landward side of Waties Island, South Carolina. Waties Island is within the Anne Tilghman Boyce (ATB) Coastal Reserve. The ATB is owned by the Coastal
Education Foundation and, unlike Myrtle Beach, Surfside Beach, and Cherry Grove, is an undeveloped, natural barrier island (Chasten and Seabergh, 1993, Viso et. al., 2010). At mean low tide the water is one meter deep and the tidal range is from two to six meters.
There are no tidal current data available for the Dunn Sound channel. The study site is adjacent to the Dunn Sound main channel in the low marsh (Figure 4). There is a tidal creek running through the site that flows around a raised mudflat and an oyster reef during ebb tide. At high tide, the mudflat and oyster reef are completely covered by water and the smooth cordgrass marsh is flooded with water. At low tide, the water drains out of the main tidal creek leaving behind several pools of water.
Timetable
All samples (fish, water, sediment, and smooth cordgrass) were collected on a seasonal basis during 2014. The first sample was collected during the winter (February;
Water temperature: 8-11˚C) with three additional sampling events during spring (April;
Water temperature: 19-26˚C), summer (July; Water temperature: 27-29˚C), and fall
(October; Water temperature: 17-24˚C).
Fish
Mummichogs were captured in conical minnow traps (419 mm length, 228 mm width, 11 mm x 6.4 mm mesh size, 2 openings) placed along the edge of inundated tidal
10 creeks approximately half a meter into the low marsh smooth cordgrass and three- quarters of a meter below the water surface. Conical minnow traps were set one hour after high tide and collected two hours before low tide (Teo and Able, 2003). Raw chicken thighs were placed in plastic bags with holes. The holes allowed for the scent of the chicken to permeate the water column but prevented consumption of the chicken by mummichogs (McMahon et. al., 2005). During the ebb tide, a 6.1 m by 1.2 m seine net with 6.4 mm mesh was used to capture silversides. The seine was pulled across mudflats and oyster reefs with a minimum of one meter of overlaying water (Bengston, 1984).
Upon capture, mummichogs and silversides over 50 mm total length (TL) were sacrificed according to approved IACUC protocols (CCU 2013.02). Individuals that were
<50mm TL were released. Both species were euthanized using scissors to cut the spinal cord (Leary et. al., 2013). After cutting the spinal cord, a scalpel was used to pith the mummichogs because mummichogs have a high tolerance for low DO and are able to breathe air (Halpin and Martin, 1999, Leary et. al., 2013).
After being sacrificed, 18 individuals, nine from each species, were placed on dry ice and shipped overnight to the University of Maryland’s Center for Environmental
Science (UMCES) for mercury analysis. Once the fish arrived at UMCES, the individual mummichogs or silversides were composited and homogenized to test whole body mercury concentrations. Digestion was performed following the method of Taylor et al
(2008). Total mercury was quantified with a Tekran 2600 using EPA method 1631E
(USEPA, 2001). Methylmercury was quantified with a Tekran 2700 using EPA method
1630 (USEPA, 1998).
11 An additional 20-40 mummichogs and 20-40 silversides were sacrificed for gut content analysis. After an individual was sacrificed in the field, the abdominal cavity was filled with 95% ethanol. The ethanol preserved the gut contents for analyses. The fish were then placed on ice, returned to the laboratory, and frozen.
Total length (mm) and whole fish weight (g) was measured. Then, the intact gut was removed from the fish, weighed (g), and the gut length was measured (mm). After removing the gut, fish were sexed and the gonads and whole fish without the gut were weighed (g). Beginning with the hindgut organisms and parts of organisms within the gut were removed, taxonomically identified as specifically as possible, counted, and placed into plastic weigh boats (Bergman and Greenberg, 1994). Dissection began with the hindgut because water within the hindgut evaporated faster than the midgut and foregut.
Each taxonomic weigh boat and the empty gut were then weighed (g). These weights and counts were used to calculate the composition of each taxa within the guts of mummichogs and Atlantic silversides.
Water
Water samples for total and methyl mercury analysis were collected at high and low tide following clean collection protocols from United States Environmental
Protection Agency (USEPA) Method 1669, (USEPA, 1996). Four unfiltered water samples were collected at high tide and low tide to test for total and methyl mercury.
After collection, the water was placed on wet ice and shipped overnight to UMCES for mercury analyses. Upon arrival in the laboratory, two water samples were filtered. Total mercury was quantified with a Tekran 2600 using USEPA method 1631E (USEPA,
12 2001). Methylmercury was quantified with a Tekran 2700 using USEPA method 1630
(USEPA, 1998).
Sediment
Sediment samples for total and methyl mercury were collected at high and low tide. All samples were collected using a ponar dredge bottom sampler and following clean collection protocols as applied to USEPA Method 823-B-01-002 (USEPA, 2001).
The sediment was placed on dry ice and shipped overnight to the UMCES for mercury analysis. Total mercury was quantified with a Tekran 2600 using USEPA method 1631E
(USEPA, 2001). Methylmercury was quantified with a Tekran 2700 using USEPA method 1630 (USEPA, 1998).
Smooth cordgrass ( Spartina alterniflora )
Smooth cordgrass was collected at low tide from the lower salt marsh. Data collection began in spring of 2014 because smooth cordgrass was added as a potential mercury contributor after appearing in mummichog gut contents from the winter. Two samples were cut from the shoots and leaves of living smooth cordgrass 1m above the marsh surface. Two other samples were collected from the shoots and leaves of decaying smooth cordgrass within a wrack pile supported by living smooth cordgrass . Samples were placed on dry ice and shipped overnight to the UMCES for mercury analysis. Total mercury was quantified with a Tekran 2600 using USEPA method 1631E (USEPA,
2001). Methylmercury was quantified with a Tekran 2700 using EPA method 1630
(USEPA, 1998).
13 Data Analyses
All statistics were run using the R console (R Development Core Team, 2014).
Statistical significance was determined as p = 0.05 a priori for all statistical tests.
Levene’s Test was used to test for homogeneity of variance (Levene, 1960, R package:
CAR; Fox and Weisberg, 2011). For Levene’s Test, if the p-values was less than p = 0.05 then the null hypothesis was rejected, Levene’s Test was considered failed, and non- parametric statistical tests were used. If the p-value was greater than p = 0.05 for
Levene’s Test then the null hypothesis was not rejected, Levene’s Test was considered passed, and parametric statistical tests could be used (Levene, 1960). The Shapiro-Wilk
Test was used to test for normality (Shapiro and Wilk, 1965). For the Shapiro-Wilk Test, if the p-values was less than p = 0.05 then the null hypothesis was rejected, the Shapiro-
Wilk Test was considered failed, and non-parametric statistical tests were used. If the p- value was greater than p = 0.05 for Shapiro-Wilk Test then the null hypothesis was not rejected,Shapiro-Wilk Test was considered passed, and parametric statistical tests could be used (Shapiro and Wilk, 1965). Parametric post hoc multiple comparisons were done using a Tukey’s test. Non-parametric post hoc multiple comparisons were done using
Dunn’s Test.
Fish
Total mercury, methylmercury, and percent methylmercury passed Levene’s Test and the Shapiro-Wilk test. Three separate two-way ANOVAs were used to compare across species and season total mercury, methylmercury, or the percent methylmercury.
Mummichog length, mummichogs weight, Atlantic silverside length, and Atlantic silverside weight were compared by season using four separate Kruskal-Wallis tests.
14 Comparisons of fish gut contents by species and month were done using PERMANOVA
(1000 permutations) utilizing distance matrices (R package: VEGAN; Anderson, 2001,
Brosse et. al., 2011, Kopp et. al., 2013, Bohorquez-Harrera et. al., 2015, Oksanen et. al.,
2015). A PERMANOVA was used because the gut content data were nonparametic
(Anderson, 2001). PERMANOVA is a non-parametric multivariate statistical tool that uses a multivariate analogue similar to Fisher’s F-ratio (Anderson, 2001, Oksanen et. al.,
2015). The R VEGAN code does not have a post-hoc test for PERMANOVA (Oksanen et. al., 2015)
Prior to using a PERMANOVA, several assumptions were made. In regards to the availability of prey, it was assumed that there was no difference in prey species abundance or distribution across the study site. Additionally, individual fish had equal opportunity to encounter prey and equal success rates of prey capture. Assumptions were also made about the digestion of captured prey. Mummichogs and Atlantic silversides were assumed to have the same digestion rates and any prey captured would experience the same rate of digestive breakdown. Utilizing these assumptions, three separate
PERMANOVA tests were run. All gut contents were broken down into five major taxonomic categories, Arthropods, worm-like, Molluscs, other, and detritus. The worm- like category was a grouping of any organism that had a worm shape such as nematodes, platyhelminthes, cestods, and oligochaetes.
Mummichog and Atlantic silverside gut contents were compared using three different parameters, percent of fish gut by weight (%W), percent of fish gut by number of prey (%N), and the percent fish gut by frequency of ingestion (%F). These comparisons were tested for significance using PERMANOVA.
15 The percent of fish gut by weight (%W) was calculated using the following equation:
% = ∗ 100 ∑ Where X’ represents the weight of each individual category (Arthropods, worm-like,
Molluscs, other, algae/detritus), ∑ X represents the sum of the weight of all five individual taxonomic categories, and Y represents the somatic weight of an individual fish. The somatic weight of the fish (Y) was determined by subtracting the fish gonad weight from the fish total weight. For each season, the %W represents the average of the individual fish.
The percent of fish gut by number of prey (%N), was calculated using the following equation:
% = ∗ 100 ∑ Where X’ represents the number of prey consumed for each individual category
(Arthropods, worm-like, Molluscs, other, algae/detritus) and ∑ X represents the sum of the number of prey consumed for all five individual categories. For each season, the %N represents the average of the individual fish.
The percent fish gut by frequency of ingestion (%F) was calculated using the following equation:
% = ∗ 100 ∑
16 where X’ represents the number of individual fish that had consumed prey from each individual category (Arthropods, worm-like, Molluscs, other, algae/detritus) and ∑ X represents the sum of fish that consumed prey items during a season.
Water
Unfiltered water total mercury failed Levene’s Test so a Kruskal-Wallis test was used to compare the total mercury to season. A one-way ANOVA was used to compare methylmercury across seasons because unfiltered water methylmercury passed both
Levene’s test and the Shapiro-Wilk test. Percent methylmercury failed the Shapiro-Wilk test so the Kruskal-Wallis test was used to compare the percent methylmercury to season.
Filtered water total mercury and methylmercury failed Levene’s Test and filtered water percent methylmercury failed the Shapiro-Wilk test. Three different Kruskal-Wallis tests were used to separately compare filtered water total mercury, methylmercury, and percent methylmercury to season.
Sediment
Total mercury passed Levene’s test and the Shapiro-Wilk test and a one-way
ANOVA was used to compare total mercury across seasons. Methylmercury and the percent methylmercury both failed Levene’s test. Two different Kruskal-Wallis tests were used to separately compare methylmercury and percent methylmercury across seasons.
Smooth cordgrass
Total mercury and methylmercury passed Levene’s test and the Shapiro-Wilk test.
Two different one-way ANOVAs were used to separately compare total mercury and
17 methylmercury across seasons. The percent methylmercury failed the Shapiro-test. A
Kruskal-Wallis test was used to compare the percent methylmercury across seasons.
18 RESULTS
Fish
Mercury
Total and methyl mercury concentrations in mummichogs ranged from 6.2-26.7 ng/g and 2.8-19.5 ng/g respectively (Figure 5). Total and methyl mercury concentrations in Atlantic silversides ranged from 20-44.8 ng/g and 17.1-40.6 ng/g, respectively (Figure
5). There was significantly more total and methyl mercury in the whole body tissues of
Atlantic silversides relative to mummichogs for all seasons (p < 0.01, Tukey’s test; Table
1). Although mercury concentrations in mummichogs and silversides were significantly different, there was no significant difference between season or species-season interaction (Table 1).
Although not statistically significant the concentrations of total mercury in
Atlantic silversides were highest during the spring (mean mercury concentration of 34.64
± standard error of 0.91 ng/g) and lowest during the summer (22.1 ± 0.91 ng/g; Figure 5).
The decrease in whole body total mercury concentrations coincided with silverside life history. Atlantic silversides were significantly larger in the winter (91.91 ± 1.63 mm, 4.64
± 0.25 g) and spring (91.90 ± 2.10 mm, 4.70 ± 0.20 g) than in the summer (51.83 ± 0.66 mm, 0.83 ± 0.0.10 g) and fall (67.32 ± 1.26 mm, 1.64 ± 0.07 g, p < 0.01, Dunn’s Test;
Table 2; Figure 6). Although Atlantic silversides captured in the summer and fall were significantly smaller than Atlantic silversides captured winter and spring, the Atlantic silversides captured during the fall were significantly larger than Atlantic silversides captured during the summer (p < 0.01, Dunn’s Test; Table 2; Figure 6). This suggested
19 that silversides captured during the summer of 2014 are the young of the year from the
2014 cohort and did not accumulate as much mercury (Figure 6).
Although not statistically significant the concentrations of total and methyl mercury concentrations in mummichogs were highest during the spring (19.45 ± 3.87 ng/g THg, 13.60 ± 4.55 ng/g MeHg) and the lowest during the winter (6.42 ± 0.22 ng/g
THg, 3.35 ± 0.52 ng/g MeHg; Figure 5). Mummichog total length was significantly shorter during the winter (54.97 ± 1.01 mm), than spring (59.75 ± 2.04 mm), summer
(70.50 ± 1.61 mm), and fall (59.97 ± 1.91 mm, p < 0.01, Dunn’s Test; Table 2; Figure 6).
However, mummichog somatic weight was not significantly different between the winter
(2.55 ± 0.14 g) and the fall (2.88 ± 0.31 g; Dunn’s Test; Table 2; Figure 6). Mummichog somatic weight was significantly greater during the spring (3.31 ± 0.28 g) and summer
(5.46 ± 0.42 g) than during the winter and fall (p < 0.01; Dunn’s Test; Table 2; Figure 6).
The percentage of methylmercury in mummichogs and Atlantic silversides ranged from 52-79% and 57-89%, respectively (Figure 5). There was no significant difference in percent methylmercury between species or the species-season interaction (Table 1). The percent methylmercury of mummichogs and silversides was significantly different between seasons (p < 0.05, Tukey’s test; Table 1). The percent methylmercury was significantly greater in the fall (77% mummichogs, 89% Atlantic silversides) relative to the spring (67% mummichogs, 57% Atlantic silversides; Table 1, Figure 6). Percent methylmercury was similar between winter (52% mummichogs, 79% Atlantic silversides) and as well as between summer (79% mummichogs, 86% Atlantic silversides) and fall (Table 1; Figure 6). Although not statistically significant, there was
20 an increase in the percent methylmercury between the spring and summer (Table 1,
Figure 6).
Gut Contents
The %W of gut contents for Atlantic silversides was significantly different than mummichogs (p < 0.01;Table 2). Mummichog gut contents were more diverse with three or more taxonomic categories consisting of more than 10% of the total %W, except during the spring when Arthropods and detritus were the main contributors (Figure 8).
Arthropods dominated the %W of Atlantic silverside gut contents, ranging from 70-90% for all seasons (Figure 8). When combined, mummichog and Atlantic silverside gut contents by %W were significantly different by season (p < 0.01; Table 2). Both species ingested more miscellaneous prey items during the winter (42.2% mummichogs, 28.3%
Atlantic silversides) than any other month (Figure 8). The %W of the miscellaneous prey from winter was predominantly juvenile fish and fish bones. The subsequent decrease of miscellaneous %W after winter was attributed to the decrease in the number of fish and fish bones found within the gut contents. Considering each species alone, the %W was significantly different by season (p < 0.01; Table 2). With respect to mummichogs, the percentage of detritus increased from the winter (27.9%) to fall (59.6%). The %W of
Arthropods consumed by mummichogs was similar for winter (22.4%), summer (23.5%), and fall (22.9%) but increased during the spring (60.9%; Figure 8). With respect to the
Atlantic silverside gut contents there was a decrease in miscellaneous %W from the winter (28.3%) to the spring (6.5%; Figure 8).
The %N of gut contents for mummichogs was significantly different than Atlantic silversides (p < 0.01; Table 2). Atlantic silversides gut contents %N for Arthropods
21 ranged from 84.5% to 96.2%, while mummichogs Arthropod %N ranged from 20.7% to
60.2% (Figure 9). When combined, mummichog and Atlantic silverside gut contents %N were also significantly different by season (p < 0.01; Table 2). Miscellaneous prey %N was the largest during the winter in mummichogs (65.7%) and Atlantic silversides
(15.2%; Figure 9). The %N of Arthropods was the highest in both species during the spring (60.2% mummichogs, 96.2% Atlantic silversides) and summer (37.2% mummichogs, 95.9% Atlantic silversides; Figure 9). %N of detritus was the highest in the fall (71.5% mummichogs, 2.5% Atlantic silversides; Figure 9). Considering each species alone, the %N was significantly different by season (p < 0.01; Table 2). With respect to mummichogs, the percentage of detritus increased from the winter (0.0%) to fall (71.5%).
The %N of Arthropods consumed by mummichogs was similar for winter (23.7%) and fall (20.7%) but increased during the spring (60.2%) before decreasing again during the summer (37.2%; Figure 9). The main difference in the %N of Atlantic silversides was the decrease in miscellaneous %N from the winter (15.2%) to the spring (3.7%; Figure 9).
Besides the %N of miscellaneous prey consumed by Atlantic silversides during winter
(15.2%), no other gut category constituted more than 5% (Figure 9).
There were no significant differences between mummichogs and Atlantic silverside gut contents by %F in terms of species, season, or the interaction of species with season (Table 2). One hundred percent of mummichogs and Atlantic silversides consumed Arthropods in the spring (Figure 10). Throughout the four seasons more than
90% of Atlantic silversides were captured with Arthropods in their gut contents (Figure
10). Miscellaneous prey %F was the highest in the winter (85.7% mummichogs, 54.5%
Atlantic silversides) and the spring (37.5% mummichogs, 30.0% Atlantic silversides;
22 Figure 10) The %F of detritus was the greater in mummichogs (25.0-88.6%) than
Atlantic silversides (0.0%-16.0%; Figure 10).
Water
Unfiltered
The concentrations of total mercury in unfiltered water ranged from 1-4.2 ng/L and were not significantly different with respect to season (Table 3; Figure 10). Although not significantly different, total mercury concentrations during the summer (mean mercury concentration of 2.54 ± standard error 0.88 ng/L) were higher than winter (1.15
± 0.03 ng/L), spring (1.24 ± 0.04 ng/L), and fall (1.32 ± 0.20 ng/L; Figure 10).
Methylmercury concentrations in unfiltered water ranged from >0.1-0.4 ng/L and were significantly different with respect to season (p < 0.01, Tukey’s test; Table 3; Figure 10).
Spring (0.28 ± 0.03 ng/L) methylmercury concentrations were higher than the summer
(0.07 ± 0.02 ng/L; Figure 10). Spring and winter (0.22 ± 0.06 ng/L) methylmercury concentrations were higher than the fall (0.02 ± 0.00 ng/L; Table 3, Figure 10). The percentage of methylmercury relative to the total in unfiltered water ranged from 1.3-
23.0% and were significantly different by season (p < 0.01, Dunn’s Test; Table 3; Figure
10). The percentages of methylmercury in the spring (23.0%) and winter (18.9%) were significantly higher than the summer (2.6%) and fall (1.3%; Table 3, Figure 10).
Filtered
The concentration of total mercury in filtered water ranged from 0.28-0.77 ng/L and were not significantly different with respect to season (Table 3; Figure 11).
Methylmercury concentrations in filtered water ranged from 0.01-0.34 ng/L and were significantly different with respect to the seasons (p < 0.01, Dunn’s Test; Table 3; Figure
23 11). Spring (0.25 ± 0.05 ng/L) methylmercury concentrations were higher than the summer (0.03 ± 0.01 ng/L). Winter (0.10 ± 0.01 ng/L) and spring methylmercury concentrations were higher than the fall (0.01 ± 0.00 ng/L; Table 3, Figure 11). Percent methylmercury in filtered water ranged from 2.1-45.6% and were significantly different by season (p < 0.01, Dunn’s Test; Table 3; Figure 11). Spring (45.6%) percent methylmercury were significantly higher than the summer (7.0%). Winter (21.1%) and spring percent methylmercury were significantly higher than the fall (2.1%; Table 3,
Figure 11).
24 DISCUSSION
Atlantic silversides (20-44.8 ng/g THg; 17.1-40.6 ng/g MeHg), from Dunn Sound
SC, had significantly higher whole body total and methyl mercury concentrations than mummichogs (6.2-26.7 ng/g THg; 2.8-19.5 ng/g MeHg) over four seasons in 2014. The total and methyl mercury concentrations in mummichogs and Atlantic silversides are comparable to other studies. In an unimpacted salt marsh in Rhode Island found that
Atlantic silversides (54 ± 2 ng/g) had higher total mercury concentrations than mummichogs (16± 1 ng/g; Szczebak and Taylor, 2011). Weis and Ashley (2007) reported total mercury concentrations in Atlantic silversides (480 ± 210 ng/g) and mummichogs (250 ± 160 ng/g) from an anthropogenically impacted salt marsh in New
Jersey. Although the total mercury concentrations in the fish from New Jersey are an order of magnitude higher than those measured from Dunn Sound, SC, Atlantic silversides still have a higher concentration than mummichogs.
Although Atlantic silversides have significantly higher concentrations of total and methyl mercury, there is no significant difference in the percent methylmercury between Atlantic silversides (57-89%) and mummichogs (52-79%) from Dunn Sound,
SC. Chen et. al. (2009) found similar percent methylmercury (45-82%) in mummichogs from a Maine salt marsh.. In laboratory studies, mummichogs and Atlantic silversides collected from South Carolina assimilated 92% and 82-89%, respectively, of prey methylmercury (Dutton and Fisher, 2010, Dutton and Fisher, 2011). Instead, of a difference in percent methylmercury by species, there was a difference in percent methylmercury by season. This suggests that mummichogs and Atlantic silversides assimilate mercury at the same rate. The percent methylmercury can constitute more than
25 50% of the total mercury in a fish due to the digestive solubility of methylmercury
(Rouleau et. al., 1999, Oliveira Ribeiro et. al., 1999, Leaner and Mason, 2004).
Gut contents by %W and %N were significantly different between mummichogs and
Atlantic silversides (p < 0.01; Table 2). Atlantic silversides gut contents were dominated by planktonic Arthropods throughout 2014, while mummichogs gut contents changed by season. Mummichog gut content by %F for detritus averaged 64.1% over the course of this study. These values are similar to mummichogs captured in Connecticut (48.4 %F) and Rhode Island (50.0-82.2 %F; Allen et. al., 1994, James-Pirri et. al., 2001).
Miscellaneous %F was the highest for mummichogs during winter (85.7%) and the spring (37.5%). The miscellaneous prey in the gut contents of mummichogs during these two seasons can be attributed to whole teleosts and teleost bones. In New Jersey found that 4.3-24.7% of mummichogs guts contents contained evidence of piscivory (Able et. al., 2007). Arthropods, mainly amphipods and copepods, were another commonly ingested prey item by mummichogs (22.4-60.8 %W, 20.7-60.2 %N, 28.6-100 %F).
Amphipods and copepods have been found in the gut contents of mummichogs along the entire east coast of North America and constituted 1.1-48.8 %F (Baker-Dittus, 1978,
Kneib et. al., 1980, James-Pirri et. al., 2001, McMahon et. al., 2005). Atlantic silverside gut contents, in this study, were dominated by Arthropods (71.3-89.5 %W, 84.5-96.2
%N, 92.9-100 %F) that were mainly primarily copepods and ostracods. Studies from
Connecticut (Thomson et. al., 1971) and Rhode Island (Bengston, 1984) salt marshes found that more than 90% of captured Atlantic silversides consumed zooplankton. A study conducted in North Inlet, SC found that Atlantic silversides consumed larger
26 species such as juvenile fish and shrimp year-round (Allen et. al., 1995) in contrast to this study, which reported juvenile fish consumption during the winter.
The concentrations of total mercury in unfiltered water ranged from 0.96-4.18 ng/L throughout this study. These values are similar to concentrations of total mercury in unfiltered water collected from Winyah Bay, SC (1.45-2.1 ng/L; Guentzel et. al., 2001) and Dunn Sound, SC (0.88 ± 0.40 ng/L; Guentzel unpublished data, 2001).
Concentrations of methylmercury in unfiltered water in this study ranged from 0.01-0.36 ng/L, and are similar values of unfiltered methylmercury from Winyah Bay, SC (0.015-
0.036 ng/L; Guentzel et. al., 2001). The percentages of methylmercury in unfiltered water during the winter and spring ranged from 18.9-23.0%, while the percentages in summer and fall ranged from 1.3-2.6% in this study. The summer and fall percentages are similar to percentages of methylmercury (0.96-1.70%) reported in unfiltered water collected in
Winyah Bay, SC during the summer (Guentzel et. al., 2001).
Concentrations and percentages of methylmercury in filtered water samples collected at the mouth of the Ashepoo River, SC ranged from 0.08-0.236 ng/L and 4.6-5.7%, respectively (Guentzel, 2012). Throughout this study, the concentrations and percentages of methylmercury in filtered water ranged from 0.01-0.34 ng/L and 2.1-45.6%, respectfully. Methylmercury concentrations in the water column are influenced by phytoplankton (Luengen and Flegal, 2009). Phytoplankton, such as diatoms, can accumulate concentrations of methylmercury in their tissues by a factor of 10 4 relative to the surrounding water (Hammerschmidt and Fitzgerald, 2006, Pickhardt and Fisher,
2007). Methylmercury bioaccumulation does not occur instantaneously. Higher concentrations may appear in the water during the winter but elevated concentrations in
27 phytoplankton and higher trophic organisms may not occur until the summer (Cardoso et. al., 2014). For this study, the filtered water percent methylmercury was the highest during the winter (18.9%) and spring (23.0%), however percent methylmercury was the highest in the fish during the summer (79% mummichogs, 86% Atlantic silversides) and fall
(77% mummichogs, 89% Atlantic silversides). Although this study did not measure mercury concentrations in phytoplankton or zooplankton, the results of Cardoso et. al.
(2014) may explain the percent methylmercury trend in our data.
Mummichogs and Atlantic silversides are prey to commercially important species. If
Atlantic silversides have higher concentrations of total and methyl mercury than mummichogs throughout the year, then predators of Atlantic silversides should have higher mercury concentrations relative to predators of mummichogs. Bluefish and striped bass consume Atlantic silversides (Harding and Mann, 2001, Harding and Mann, 2003,
Sxcxebak and Taylor, 2011) and have total mercury concentrations ranging from 260-520 ng/g and 370 ± 20 ng/g, respectively (Burger and Gochfeld, 2011, Staudinger, 2011,
Szczebak and Taylor, 2011). Summer flounder and blue crab are the primary predators of mummichogs (Meredith and Lotrish, 1979, Peterson and Perterson, 1979, Kneib and
Stiven, 1982) and have total mercury concentrations of 140 ± 10 ng/g and 60-110 ng/g, respectively (Jop et. al., 1997, Burger and Gochfeld, 2011, Staudinger, 2011). Although predators of these fish consume other prey items in addition to mummichogs and silversides, the data suggests that organisms associated with the planktonic food web will accumulate higher concentrations of total and methyl mercury than organisms associated with the detrital food web.
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APPENDIX
This appendix includes ancillary parameters that were measured as a part of the initial four quarterly sampling events. The data includes total and methyl mercury concentrations by season and tide for water and sediments, sediment %LOI, sediment grain size, and total and methyl mercury concentrations for living and dead smooth cordgrass.
Water
Unfiltered
The concentration of total mercury in unfiltered water at high and low tide ranged from 1.08-1.68 ng/L and 0.96-4.18 ng/L, respectively (Figure 12). Although there were no significant differences by season or tide, the total mercury was the highest during the summer low tide (4.06 ± 0.02 ng/L; Table 4; Figure 12). Methylmercury concentrations in unfiltered water at high and low tide ranged from 0.01-0.36 ng/L and 0.01-0.39 ng/L, respectively (Figure 14). Methylmercury concentrations in unfiltered water were significantly different between the seasons (p < 0.01, Dunn’s Test; Table 4). Spring (0.28
± 0.03 ng/L) methylmercury concentrations are higher than the summer (0.07 ± 0.02 ng/L; Figure 14). Spring and winter (0.22 ± 0.06 ng/L) methylmercury concentrations were higher than the fall (0.02 ± 0.00 ng/L; Table 4; Figure 12). The percentage of methylmercury in unfiltered water at high and low tide ranged from 0.9-24.3% and 2.0-
28.7% respectively (Figure 12). The percent methylmercury was significantly different by season (Table 4). The percentages of methylmercury in the spring (23.0%) and winter
38 (18.9%) were significantly higher than the summer (2.6%) and fall (1.3%; Table 4,
Figure 12).
Filtered
The concentration of total mercury in filtered water at high and low tide ranged from 0.40-0.77 ng/L and 0.28-0.56 ng/L, respectively (Figure 13). Total mercury concentrations at high tide were significantly greater than low tide (p < 0.05, Tukey’s test; Table 4). Methylmercury concentrations in filtered water at high and low tide ranged from 0.01-0.29 ng/L and 0.01-0.34 ng/L, respectively (Figure 13). Methylmercury concentrations in filtered water were significantly different between the seasons (p <
0.01, Dunn’s Test; Table 4). Spring (0.28 ± 0.03 ng/L) methylmercury concentrations were significantly higher than the summer (0.07 ± 0.02 ng/L; Figure 13). Spring and winter (0.22 ± 0.06 ng/L) methylmercury concentrations were significantly higher than the fall (0.02 ± 0.00 ng/L; Table 4; Figure 13). The percentage of methylmercury in filtered water at high and low tide ranged from 0.9-24.3% and 2.0-28.7% respectively
(Figure 12). The percent methylmercury was significantly different by season (p < 0.01,
Dunn’s Test; Table 4). The percent methylmercury in the spring (23.0%) was significantly higher than the summer (2.6%). Summer and winter (18.9%) percent methylmercury were significantly higher than the fall (1.3%; Table 4, Figure 13).
Sediment
Mercury
The concentration of total mercury in sediment at high and low tide ranged from
3.22-43.89 ng/g and 7.36-36.35 ng/g, respectively (Figure 14a). There were no significant differences between the sediment total mercury by season or tide (Table 5).
39 However, there was a significant difference by season within individual tide. (p < 0.05,
Dunn’s Test; Table 5). Total mercury concentrations during fall high tide (35.16 ng/g) were significantly higher than fall low tide (0.09 ng/g; Table 5; Figure 14a).
Methylmercury concentrations in the sediment during high and low tide ranged from
0.01-0.10 ng/g and 0.01-0.17 ng/g, respectively (Figure 14b). The percentage of methylmercury in sediment during high and low tide ranged from 0.1-1.2% and 0.1-
0.6%, respectively (Figure 14b). Methylmercury concentrations and the percent sediment methylmercury were not significantly different by season or tide (Table 5).
Smooth cordgrass
Mercury
Smooth cordgrass mercury concentrations were not collected during the winter because smooth cordgrass was not seen as a potential mercury contributor to the fish until after initial dissections. The total mercury concentrations in living and dead smooth cordgrass ranged from 0.96-6.26 ng/g and 6.52-21.02 ng/g, respectively (Figure 15a).
Total mercury concentrations in smooth cordgrass had no significant differences by the season, status (living vs dead), or the interaction between season-status (Table 6).
Although not significant, dead smooth cordgrass had higher total mercury concentrations than living. Methylmercury concentrations in living and dead smooth cordgrass ranged from 0.01-0.22 ng/g and 0.04-0.21 ng/g, respectively (Figure 15a). Dead smooth cordgrass did have significantly more methylmercury than living smooth cordgrass (p <
0.01, Tukey’s test; Table 6; Figure 15b). The percentage of methylmercury in living and dead smooth cordgrass ranged from 0.1-8.6% and 1.0-2.1%, respectively, and was not significant (Table 6; Figure 15a).
40 Table 1: Summary of the statistical tests used for fish total and methyl mercury by season. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Test Sample Factor Test Statistic df p-value post-hoc analysis Season 1.95 3 0.16 Species Two-way 28.62 1 p<0.01 Atlantic silversides>mummichogs Fish THg Season- ANOVA Species 0.67 3 0.58 Season 0.99 3 0.43 Species Two-way 18.12 1 p<0.01 Atlantic silversides>mummichogs Fish MeHg Season- ANOVA Species 0.73 3 0.55 Season 5.79 3 p<0.05 fall>spring, winter=spring, summer=fall Fish Species Two-way 4.12 1 0.07 %MeHg Season- ANOVA Species 2.23 3 0.14
41
Table 2: Summary of the statistical tests used for mummichog length, mummichog weight, Atlantic silverside length, Atlantic silverside length, gut percent composition by weight (%W), gut percent composition by number (%N), and percent ingestion by fish (%F) by season. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non- parametric post hoc multiple comparisons. Test Sample Factor Test Statistic df p-value post-hoc analysis Mann-Whitney Fish Length Species U 2873.00 - p<0.01 Atlantic silversides > mummichogs Mann-Whitney Fish Weight Species U 5791.50 - p<0.01 mummichogs > Atlantic silversides Fall > Winter; Spring > Winter; Summer > Winter; Mummichog Length Season Kruskal-Wallis 33.21 3 p<0.01 Summer > Fall; Summer > Spring Spring > Winter; Summer > Winter; Summer > Mummichog Weight Season Kruskal-Wallis 35.42 3 p<0.01 Fall; Summer > Spring; Spring > Fall Atlantic Silverside Winter > Summer; Winter > Fall; Spring > Length Season Kruskal-Wallis 85.55 3 p<0.01 Summer; Spring > Fall; Fall > Summer Atlantic Silverside Winter > Summer; Winter > Fall; Spring > Weight Season Kruskal-Wallis 85.84 3 p<0.01 Summer; Spring > Fall; Fall > Summer Season 7.98 3 p<0.01 Species 93.08 1 p<0.01 Gut %W PERMANOVA Season- Species 1.99 3 p<0.05 Season 20.87 3 p<0.01 Species 33.90 1 p<0.01 Gut %N PERMANOVA Season- Species 12.49 3 p<0.01 Season <0.01 3 0.39 Species <0.01 1 0.37 Gut %F PERMANOVA Season- Species <0.01 3 0.24
42
Table 3: Summary of the statistical tests used for unfiltered and filtered water total and methyl mercury by season. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Test Sample Factor Test df p-value post-hoc analysis Statistic Unfiltered Water THg Season Kruskal-Wallis 0.90 3 0.82 One-way spring>fall, winter>fall, Unfiltered Water MeHg Season 11.11 3 p<0.01 ANOVA spring>summer Unfiltered Water spring>fall, winter>fall, Season Kruskal-Wallis 12.29 3 p<0.01 %MeHg spring>summer, winter>summer Filtered Water THg Season Kruskal-Wallis 1.02 3 0.8 spring>fall, winter>fall, Filtered Water MeHg Season Kruskal-Wallis 13.31 3 p<0.01 spring>summer spring>fall, winter>fall, Filtered Water %MeHg Season Kruskal-Wallis 11.98 3 p<0.01 spring>summer
43 Table 4: Summary of the statistical tests used for unfiltered and filtered water total and methyl mercury by season and tide. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Test Sample Factor Test df p-value post-hoc analysis Statistic Season Kruskal-Wallis 0.90 3 0.82 Unfiltered Water THg Tide Mann-Whitney U 32.00 14 0.99 spring>fall, winter>fall, Season 11.11 3 p<0.01 spring>summer Unfiltered Water MeHg Tide Two-way ANOVA 0.10 1 0.76 Season- 1.60 3 0.26 Tide spring>fall, winter>fall, Unfiltered Water Season Kruskal-Wallis 12.29 3 p<0.01 spring>summer, %MeHg winter>summer Tide Mann-Whitney U 32.00 14 0.99 Season Kruskal-Wallis 1.02 3 0.8 Filtered Water THg Tide t-test 2.19 14 p<0.05 high tide > low tide spring>fall, winter>fall, Season Kruskal-Wallis 13.31 3 p<0.01 Filtered Water MeHg spring>summer Tide Mann-Whitney U 30.50 14 0.92 spring>fall, winter>fall, Season Kruskal-Wallis 11.98 3 p<0.01 Filtered Water %MeHg spring>summer Tide Mann-Whitney U 26.50 14 0.6
44 Table 5: Summary of the statistical tests used for sediment total and methyl mercury by season and tide. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Test Sample Factor Test df p-value post-hoc analysis Statistic Season 0.48 3 0.91
Sediment Tide 1.27 1 0.29 Two-way ANOVA THg Season- 7.52 3 p<0.05 October high tide > October low tide Tide Sediment Season Kruskal-Wallis 0.96 3 0.81
MeHg Tide Mann-Whitney U 37.00 14 0.64
Sediment Season Kruskal-Wallis 2.01 3 0.57
%MeHg Tide Mann-Whitney U 37.00 14 0.64
45 Table 6: Summary of the statistical tests used for smooth cordgrass total and methyl mercury by season and living versus dead. Tukey’s test was used for parametric post hoc multiple comparisons and Dunn’s test was used for non-parametric post hoc multiple comparisons.
Test post-hoc Sample Factor Test df p-value Statistic analysis Season 1.28 2 0.35
Status 2.74 1 0.15 Smooth cordgrass THg Two-way ANOVA Season- 2.46 2 0.17 Status Season 1.77 2 0.25
Status 13.94 1 p<0.01 dead > living Smooth cordgrass MeHg Two-way ANOVA Season- 2.67 2 0.15 Status Season Kruskal-Wallis 4.04 2 0.13 Smooth cordgrass %MeHg Status Mann-Whitney U 18.00 14 0.99
46
Figure 1: SDHEC South Carolina Mercury Consumption Advisories Map (http://www.scdhec.gov/FoodSafety/FishConsumptionAdvisories/AdvisoryMap/ ; SCDHEC, 2014)
47
Figure 2: Overview of mercury methylation in salt marsh sediments. The layers of brackish water, oxic sediments, and anoxic sediments do not represent actual depths. The sediment has been partitioned into an oxic region and a hypoxic region. Hg (II) = dissolved inorganic mercury; Hg i = undissolved inorganic mercury; MeHg = aqueous methylmercury. Downward arrows represent flocculation and sedimentation processes. Arrows pointing from Hg (II) and MeHg toward Hg i represent oxidative and reductive demethylation. Upward arrows represent passive aqueous diffusion and volatilization of mercury. The arrow pointing from MeHg to the diatom represents passive biological uptake by phytoplankton. Graphic is based on the information presented in Merritt and Amirbahman (2009). The graphic was produced by Kathryn Ferons
48
Wading Bird Common Tern
Phytoplankton Mummichogs
Atlantic silversides Zooplankton Larval Fish
Grass Shrimp
Amphipods Striped Bass
Blue Crab
Summer Flounder
Figure 3: Estuarine food web depicting the trophic levels occupied by mummichogs and Atlantic silversides in relation to other organisms. Drawing is courtesy of Kathryn Ferons.
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Figure 4: The study site is located on the landward side of Waties Island in Dunn Sound, near Cherry Grove, SC (33°51'13.17"N, 78°34'59.50"W). It consists of smooth cordgrass salt marsh surrounded by tidal creeks, mud flats, and oyster reefs. © 2015 Google; Map Data: SIO, NOAA, U.S. Navy, NGA, GEBCO; Image: © 2015 TerraMetrics
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Figure 5: Total and methylmercury concentrations in mummichogs and Atlantic silversides over the four seasons in 2014. Atlantic silversides had significantly higher concentrations of total (p < 0.01) and methyl (p < 0.01) mercury relative to mummichogs (Table 1). There was no significant difference between the percentage of methylmercury in Atlantic silversides and mummichogs (Table 1). However, the percent methylmercury, represented in the boxes above the bars, in mummichogs and Atlantic silversides was significantly higher in the fall relative to the spring (p < 0.05; Table 1). THg = Total mercury; MeHg = Methylmercury.
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10 y = 0.1512e 0.0492x R² = 0.8714 Mummichogs Winter 8 Mummichogs Spring Mummichogs Summer 6 Mummichogs Fall Silversides winter 4 y = 0.0956e 0.0415x silversides spring Fish Somatic FishSomatic Weight (g) R² = 0.897 silversides summer 2 silversides fall
0 0 20 40 60 80 100 120 Fish Total Length(mm)
Figure 6: Mummichog and Atlantic silverside somatic weight (g) by total length (mm). The somatic weight to total length ratio was larger in mummichogs than Atlantic silversides.
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Figure 7: The percent by weight of gut contents (%W) in mummichogs and Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides had significantly different gut contents by %W (p < 0.01; Table 2). Atlantic silverside gut contents were dominated by planktonic Arthropods throughout the whole year, whereas mummichogs gut contents varied by season.
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Figure 8: The percent by number of gut contents (%N) in mummichogs and Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides had significantly different gut contents by %N (p < 0.01; Table 2). The %N of Arthropods ingested by Atlantic silversides was primarily zooplankton.
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Figure 9: The percent of fish ingesting prey item of gut contents (%F) in mummichogs and Atlantic silversides over the four seasons in 2014. Mummichogs and Atlantic silversides did not have significantly different %F (Table 2). Over 90% of Atlantic silversides were captured with Arthropods in the gut contents throughout the year, while the percent of mummichogs captured with Arthropods in their gut contents was different by season.
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Figure 10: Total and methylmercury concentrations in unfiltered water over the four seasons in 2014. Total mercury was not significantly different although the summer had a higher concentration than any other season (Table 3). Methylmercury concentrations were significantly different between the seasons (p < 0.01, Tukey’s test; Table 3). The percent methylmercury, represented in the boxes above the bars, was significantly different between seasons (p < 0.01, Dunn’s Test; Table 3). THg = Total mercury; MeHg = Methylmercury.
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Figure 11: Total and methylmercury concentrations in filtered water for four seasons in 2014. Total mercury was not significantly different. Methylmercury concentrations were significantly different between the seasons (p < 0.01, Dunn’s Test; Table 3). The percent methylmercury, represented in the boxes above the bars, was significantly different between seasons (p < 0.01, Dunn’s Test; Table 3). THg = Total mercury; MeHg = Methylmercury.
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Figure 12: Total and methylmercury concentrations in unfiltered water by tide over four seasons in 2014. Total mercury remained constant by tide and season throughout the year. The one exception is the high total mercury concentration during summer low tide. Methylmercury and the percent methylmercury, represented in the boxes above the bars, of the total mercury, were the same for winter and spring before decreasing through the fall. THg = Total mercury; MeHg = Methylmercury.
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Figure 13: Total and methylmercury concentrations in filtered water for tides by the four seasons in 2014. Total mercury remained constant throughout the year. Methylmercury concentrations increased from the winter to the spring but decreased after spring through to the fall. There were no differences in the methylmercury concentrations by tide. During the winter and spring, filtered total mercury concentrations were higher at high tide than low tide. In the summer and fall, high and low tide total mercury concentrations were the same. THg = Total mercury; MeHg = Methylmercury.
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Figure 14: Total and methylmercury concentrations in sediment by tide over the four seasons in 2014. Total mercury concentrations were higher during low tide than during high tide from winter to summer. In the fall, high tide total mercury concentrations were significantly higher than low tide (Table 5). There was no change in the percent methylmercury. THg = Total mercury; MeHg = Methylmercury.
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Figure 15: Total and methylmercury concentrations in living and dead smooth cordgrass other the four seasons in 2014. Sampling of the smooth cordgrass began in spring of 2014 because it was not considered a potential mercury contributor until after the first gut dissections. A) Total mercury concentrations were higher in dead smooth cordgrass than living smooth cordgrass. Living smooth cordgrass total mercury concentrations did not change but dead smooth cordgrass decreased from the spring to the fall. B) Methylmercury concentrations were significantly higher in dead smooth cordgrass than in living (Table 6). THg = Total mercury; MeHg = Methylmercury
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