Factors Controlling Growth Rates of Ribbed Mussels (Geukensia demissa) in New England Salt Marshes.
Patrick Doughty1 1Department of Biology, Lawrence University, Appleton, Wisconsin 54911 USA
Abstract. Ribbed mussels (Geukensia demissa) are the dominant shellfish of New England salt marshes. This paper seeks to identify how factors such as location relative to an ocean inlet and nutrient addition have affected recent yearly growth rates of demissa. I also looked at how carbon, nitrogen, and sulfur isotope composition could hint at habitat quality and relate to patterns found in growth rates. I found that growth rates were high in areas close to the inlet but decreased farther into the marsh. Sites with nitrogen addition were found to have high growth rates in comparison to untreated sites. Ultimately, sites with nitrogen addition far from an ocean inlet were found to have greater growth rates than untreated areas close to an ocean inlet.
Key words: Geukensia demissa; ribbed mussel; New England; growth rate; growth bands; salt marsh; gradient; sulfur isotopes; carbon isotopes; nitrogen isotopes; nutrient enrichment; prey quality; trophic cascade; bottom up
Introduction: The ribbed mussel (Geukensia demissa) is one of the few shellfish found in the salt marshes of New England and has come to dominate its habitat. Though other shellfish, such as oysters, are present close to the ocean inlet, they dwindle in size and number deeper into the marsh ultimately giving way to the mussel. Go farther into the marsh still and even the mussels begin to fade. This suggests a diminishing of habitat quality for all shellfish as distance into the marsh increases and that ribbed mussels although best adapted for the salt marsh are not immune to the effects. The question comes as to what may be causing this lowering of habitat quality. Peterson and Fry (1987) noted in a study looking at isotope patterns that ribbed mussels close to the marsh’s ocean inlet had relatively more depleted carbon and enriched sulfur than mussels further into the marsh (Figure 1). The isotope composition values of the mussels in close proximity to the ocean were very similar to their food source of plankton, which in turn has similar values to those found in seawater. The mussels deeper into the marsh had carbon isotope values closer to their food source, Spartina alterniflora detritus, and more depleted sulfur isotope values. These depleted sulfur isotopes are due to reduction of sulfate in the marsh sediments (Carlson and Forrest, 1982) into hydrogen sulfide, which is assimilated into the Spartina and then into the mussels through predation. Sulfate reducing bacteria tend to reduce sulfate only when there is a lack of better electron acceptors, like oxygen and nitrate
(Nelson, personal communication). This lack of good electron acceptors should factor into the efficiency of energy entering into the food web, resulting in a lower quality food source for the mussel. This lower quality food source is possibly a significant factor in determining growth.
Although this gradient of a high quality to low quality food source with distance into the marsh is important in finding a general case, nutrient addition is a possible confounding factor. Nutrients such as nitrogen when added into nitrogen-limited seawater have the potential to cause blooms of phytoplankton. This increase in productivity increases the amount of energy taken up in higher trophic levels in a bottom-up trophic cascade. In the case of nitrogen addition and the resulting phytoplankton bloom, the habitat quality for mussels would increase.
The purpose of this experiment was to attempt to see how changes in habitat quality affect growth rate of the ribbed mussel. The first problem to look at was differences in growth rate along a gradient starting at the marsh inlet and ending in the deep marsh, for which I studied Little Sippewissett marsh. The second was to look at how nitrogen additions affected growth rate, so I looked at two marsh creeks in Plum Island, one of them having a ten-year legacy of added nitrogen. For both these scenarios, I looked at the growth rate in terms of yearly growth band thickness. Growth band thickness gives a close representation to growth rates, as more energy is required to put thicker layers in place. The highest quality habitats will provide the mussel with the greatest levels of energy and this will recorded in their growth rates.
Materials and Methods:
Field:
Mussels were collected from Little Sippewissett Marsh and Plum Island marshes between mid-October and early-November. In Little Sippewissett, ten mussels were taken at six different sites along a gradient starting at the ocean inlet and ending in the deep marsh (Figure
2). I only selected mussels that were greater than 72mm in length because mussels at this length have reached sexual maturity and ended their period of rapid growth (Brousseau, 1984). Of the remaining mature mussels, I selected those with the lowest levels degradation just beyond the umbo. At Plum Island, mussels were collected from West Creek (Control) and
Sweeny Creek (10 years of nitrogen addition) (Figure 3). Sets of five mussels were taken at 50,
100 and 150 meters away from the main cut from the main stream hereafter referred to as the
“zero point”. Nitrogen additions in the form of nitrate occur during summer months at the
Sweeny Creek zero point where as West Creek remains relatively undisturbed. Mussels collected at Plum Island followed the same selection process as those at Little Sippewissett.
Lab:
The mussels I brought back to the lab were carefully opened with a hooked scalpel to avoid damaging the shell and the byssus threads removed. For five mussels of each Little
Sippewissett site, I collected only posterior abductor muscular tissue from each and combined them into composites for each site. For the five remaining mussels from Sippewissett sites and all mussels from Plum Island, I took composite samples for each site consisting of all tissue inside the shell. All samples were dried in a drying oven for two days, crushed into a fine powder, and then sent off for carbon, nitrogen and sulfur isotope analysis using a GC-MS.
Of the remaining shells, I took three that were the least degraded from each site for growth band analysis. A cross-section was taken from each individual by sectioning the shell longitudinally along a plane passing through the umbo and approximately bisecting the posterior margin as described by Rhoads and Lutz (1980) but using a tile saw, yielding thicker sections (Figure 4a). These sections were adhered to microscope slides using epoxy and once dry were sanded down and polished using progressively finer sand paper until the growth bands were easily visible under a dissecting microscope. Attempts to stain the cross-sections with Feigl’s solution, described by Warne (1962), failed to selectively target sequential growth bands and instead stained the entire surface. Light etching with dilute hydrochloric acid proved a better option by slightly dissolving the winter growth bands, causing them to shine white where as summer growth bands remained darker. Using a high-powered microscope with attachable camera, I took photos of the area just beyond the umbo for each cross-section and analyzed the photos with Imagej software. One summer and one winter band are grown every year with the bands on the inside of the shell being the most recent. I calculated yearly growth as being the combined thickness of the summer growth band (dark) and winter growth band
(light) for a given year.
Shells collected in 2012 from the Plum Island sites were donated to the dataset by Suzy
Ayvazian along with their carbon and nitrogen isotope values. I examined the growth rates of three shells from Sweeny Creek and three shells from West Creek following the previously detailed method. Unfortunately, this data set was not included due to damage making the shells unreadable.
Results:
Little Sippewissett.
Carbon isotope compositions for both whole tissue and exclusively muscular tissue
13 o become more enriched with distance into the marsh with Sipp 1( C /oo Whole= -19.4) being
13 o most depleted and Sipp 5 ( C /oo Whole= -17.6) being most enriched (Figure 5). The
13 o exception, in this case, being Sipp 6 ( C /oo Whole= -18.1) which has a more depleted value than expected by the trend. Large differences between whole and muscular tissue values occurred in most sites but was least pronounced in Sipp 6. Sulfur isotopes depleted with
34 o distance into the marsh with the most enriched value at Sipp 1 ( S /oo Whole= +14.2) and the
34 o most depleted value at Sipp 6 ( S /oo Whole= +4.2) (Figure 6). Nitrogen similarly followed a general trend of depletion with greater distance into the marsh (Figure 7).
In general, growth rates, indicated by yearly growth band thickness, for the past five years at Little Sippewissett were higher in sites closer to the ocean inlet than in the deep marsh
(Figure 8). From observations taken directly on site, Sites 1, 2, and 3 had the largest mussels on average. Sites 2, 3, and 4 had the greatest densities of mussels which generally formed clusters on bunches of Spartina facing the creek. Site 5 had an intermediate density of mussels and site
6 had a very low density of mussels with most being below mature length. Degradation of the umbo and surrounding area was highest at sites 2, 3, and 4 with sites 1, 4, and 5 having less severe degradation and site 6 having almost none at all.
Plum Island.
Carbon isotope compositions showed little difference or trend between treatment sites
0 or over distance into the marsh but all were closer to the value of sea water plankton (-22 /00)
o (Peterson and Fry, 1987) than the highest value from Little Sippewissett (-17.6 /oo) (Figure 9).
Sulfur isotope compositions fell between the extremes of Little Sippewissett but had a similar trend of depletion with distance into the marsh (Figure 10). The treated site had more depleted sulfur values than the control and a steeper decrease with distance. Nitrogen isotope compositions for both Plum Island sites fell below the average of Sippewissett sites close to the ocean and farther into the marsh (Figure 11). The treatment site had slightly more depleted values than the control site and appeared to have a downward trend with distance into the marsh. Growth rates between treated and untreated sites show a very promising trend (Figure
12). Treated sites had higher average yearly growth band thicknesses than the control site for the past five years. There was, however, little noticeable trend with distance into the marsh.
Overall, the Little Sippewissett sites near the ocean inlet were above average with sites in the deep marsh being roughly average (Figure 13). Interestingly, although the Plum Island control site was consistently below average, the Plum Island treated site was consistently above average and in most years surpassed the Little Sippewissett sites close to the ocean inlet. From observations taken directly on site at Plum Island, mussels at both sites tended not to cluster but burrowed into the mud of the creek wall, which was sloughing off in the treated site but remained intact in the control site. Degradation of the umbo and surrounding area was almost nonexistent on the Plum Island shells much like Little Sippewissett site 6.
Discussion:
Little Sippewissett.
Just as expected, mussels farther into the marsh tend to have lower growth rates on average and the reason for this is found in the isotope data. Carbon isotopes become enriched across a gradient starting at the ocean inlet. This means that the mussels are progressively shifting to a diet that is ultimately based on Spartina detritus. The exception in the trend lies at site 6, which has a more depleted value than the trend suggests, but this is likely due to inputs from terrestrial C-3 plants, which have more depleted carbon values. This diet is reflected in the sulfur, which shows depletion of sulfur isotopes with increasing distance into the marsh. This means that either the Spartina is taking up more highly depleted sulfur deeper into the marsh, that the mussels are consuming a greater diet based in Spartina, that mussels select for ocean- based productivity but it is consumed on the way into the marsh, or all of the above. The first suggests that sulfate-reducing bacteria have less highly-efficient electron acceptors like oxygen deeper into the marsh and will therefore be reducing sulfur more often. The second assumes that sulfate-reducing bacteria have an equal amount of good electron acceptors at each site giving the mud and Spartina equal sulfur values throughout the marsh. The third suggests that mussels select food originating from ocean sulfate if available and will actively remove it from the water column preventing mussels in the deep marsh from obtaining it. It is likely that not one, but all three contribute to the overall trend. The nitrogen isotopes in particular shed light on the last of the three by showing roughly what trophic level the mussels are feeding on.
Mussels close to the ocean have more enriched nitrogen isotope compositions than those in the deep marsh because they are feeding higher on the food web. This suggests that mussels close to the ocean are likely feeding more on zooplankton coming in from the ocean than on phytoplankton or Spartina detritus. Zooplankton are ultimately a better food source due to higher transfer efficiencies which lead to more energy and growth. Presence of zooplankton, in providing a rich energy source, also provides a better habitat. One possible aspect of the data that warrants more study is calculating of fat content based on differences in carbon isotope composition of muscular and whole animal tissue. Mussels with more fat should have a greater difference in values as carbon in fat is much more depleted and more fat in a sample would greater reduce the total value in comparison to the muscular tissue value. As fat is a sign of health it would also be another way to determine habitat quality.
Plum Island: As expected, sites treated with nitrogen had higher growth rates than those left undisturbed. The carbon isotope compositions of the mussels in Plum island were very close to
0 values found in plankton from sea water (-22 /00) (Peterson and Fry, 1987) and are much closer than values found in Little Sippewissett, even at sites close to the ocean. This indicates that almost the entire make up of the mussels’ diets in Plum Island are based in seawater. This would be surprising based on how far into the marsh the sites tested are, but makes sense considering Plum Island has a nine foot tide in comparison to a two foot tide for Little
Sippewissett. This larger tide brings vast quantities of water into the marsh and carries with it production made at sea. The sulfur isotope data for these sites slightly disagree with the carbon data, because the sulfur values lie between the extremes found at Little Sippewissett and therefore Spartina, which takes in the light sulfur, should be part of the diet. There are two possibilities to explain this inconsistency. First, Spartina (more enriched than plankton) is part of the diet but so too are terrestrial C-3 (less enriched than plankton) and the combination of these two happen to provide a value close to plankton. Second, the depleted hydrogen sulfide from reduced sediments leeches into the stream and any phytoplankton growing in the nearby area take it up and have more depleted values than those raised in the ocean. I believe the second to be much more likely especially because the sulfur values in the enriched plot begin lower than the control plot suggesting a greater percentage of the diet being formed nearby with local sulfur. Another possibility for the lower values could be due to anoxic conditions brought about by decomposition of phytoplankton blooms. Nitrogen isotope composition contributes a very important piece to the puzzle. Nitrogen values for both control and treatment are lower than both the Little Sippewissett mussels close to the ocean and those in the deep marsh. This means that mussels in Plum Island are feeding lower on the trophic scale and eating lower quality food. Since the carbon values show a food web based on seawater, the low quality food they are consuming is most likely phytoplankton. Zooplankton are likely rare in both treatment sites as they travel in from the ocean and must cover quite a bit of distance with other shellfish having the opportunity to weed out high value prey. It is likely that a few zooplankton will make it to both sites but the sites treated with nitrogen likely have higher populations of phytoplankton supported by the excess nitrogen. Although this would have the potential to cause a bottom-up trophic cascade causing zooplankton population to rise, the population is likely kept below sustainable levels due to high predation pressure. The resulting depleted nitrogen values and higher growth rates for treated sites, therefore, can be attributed to higher concentrations of low quality food compared to the control where there are very low concentrations of slightly higher quality food.
Comparing Little Sippewissett and Plum Island.
The Plum Island control site had the lowest growth rate of all the sites compared. This was likely due to very low abundances of prey resulting from location in the marsh. As these growth rates were lower than those found in the Little Sippewissett deep marsh, the deep marsh is the better habitat. This is likely due to the deep marsh Spartina detritus providing a greater base for productivity than can be matched by very low abundance of a higher quality food source. In the treatment plots, however, nitrogen gave mussels in Plum Island high growth rates similar to if not higher than those found a short distance from the ocean in Little Sippewissett.
Conclusions:
In Little Sippewissett, I found that growth rates on average decrease with distance into a marsh.
This is likely due to deceasing food quality from a diet rich in ocean zooplankton to diets based on Spartina detritus. In Plum Island, I found that sites treated with nitrogen had higher growth rates than those left untreated. This was likely due to stimulated growth in the water column of low-quality phytoplankton. I also found that nitrogen addition to a site deep into the marsh resulted in growth rates similar to those found in mussels located closer to an ocean inlet.
Acknowledgements: I would like to thank Jimmy Nelson of the Marine Biological Laboratory for advising me on this project. Suzy Ayvazian collaborated and provided samples. Nick Uline assisted in the field. Rich McHorney, Fiona Jevon, Sarah Nalven, and Alice Carter assisted in the lab.
Literature Cited:
Brousseau, D. J. 1984. Age and Growth Rate Determinations for the Atlantic Ribbed Mussel,
Geukensia demissa Dillwyn (Bivalvia: Mytilidae). Estuaries 7(3):233-241.
Carlson, P. R. 1982. Uptake of dissolved sulfide by Spartina Alterniflora: Evidence from natural sulfur abundance ratios. Science 216: 633-35.
Peterson, B. J., & Fry, B. 1987. Stable Isotopes in Ecosystem Studies. Annual Review of Ecology,
Evolution, and Systematics 18:293-320.
Rhoads, D., and R. A. Lutz. 1980. Skeletal growth of aquatic organisms. Plenum Press, New York,
New York, USA.
Warne, S. J. 1962. A quick field or laboratory staining scheme for the differentiation of the major carbonate minerals. Journal of Sedimentary Petrology , 32(1):29-38.
Figures and Tables:
Figure 1. Carbon-Sulfur cross plot for the ribbed mussel and corresponding map of origin in Great Sippewissett Marsh. Numbers 1-9 indicate isotope values in cross plot and location in map. Data retrieved from Peterson, B., and Fry, B. 1987. Stable Isotopes in Ecosystem Studies. Annual Review of Ecology, Evolution, and Systematics 18:293-320
Figure 2. Map indicating sampling locations in Little Sippewissett Salt Marsh. Map courtesy of Google Earth.
Figure 3. Map indicating sampling locations in Plum Island. Map courtesy of Google Earth.
A
B Figure 4a. Uncut and cut mussel shells showing area collected for cross-section Figure 4b. Photograph of finished mussel cross-section. Both summer (dark) and winter(light) growth bands shown. Red line combines both bands from a given year to give total growth in said year. Sipp 1 Sipp 2 Sipp 3 Sipp 4 Sipp 5 Sipp 6
-11.0
-13.0
-15.0
oo /
o Whole
C C 13
-17.0 Tissue
-19.0
-21.0
-23.0 <--- To Ocean To Marsh---> Site
Figure 5. Carbon isotope composition of all internal tissue (whole) and muscular tissue (tissue) of Ribbed Mussels at Little Sippewissett sites.
16.0
14.0
12.0
10.0
oo
/ o
S S 8.0 34
Whole
6.0 Tissue
4.0
2.0
0.0 Sipp 1 Sipp 2 Sipp 3 Sipp 4 Sipp 5 Sipp 6 <--- To Ocean To Marsh---> Site
Figure 6. Sulfur isotope composition of all internal tissue (whole) and muscular tissue (tissue) of Ribbed Mussels at Little Sippewissett sites.
14.0
13.0
12.0
11.0
10.0
oo
/
o N N
15 Whole
9.0 Tissue 8.0
7.0
6.0
5.0 Sipp 1 Sipp 2 Sipp 3 Sipp 4 Sipp 5 Sipp 6 <--- To Ocean To Marsh---> Site
Figure 7. Nitrogen isotope composition of all internal tissue (whole) and muscular tissue (tissue) of Ribbed Mussels at Little Sippewissett sites.
1000
900
800
m)
m 700
600 Sipp 1 Sipp 2 500 Sipp 3 Sipp 4 400 Sipp 5
300 Sipp 6 Yearly Growth Band ThicknessGrowth ( Yearly 200
100
0 2008 2009 2010 2011 2012 2013 2014 Year
Figure 8. Average thickness of growth bands for the five most recent years (2009-2013) of growth in Ribbed Mussels at Little Sippewissett sites. Blue markers indicate sites closer to the ocean; green markers indicate sites farther into the marsh.
Control Control Control Treatment Treatment Treatment 50 100 150 50 100 150 -15.0
-16.0
-17.0
-18.0
oo /
o -19.0
C C
13
-20.0
-21.0
-22.0
-23.0 Site
Figure 9. Carbon isotope composition of all internal tissue of Ribbed Mussels at Plum island sites along a gradient of 50, 100 and 150 meters from the zero point. Sites shown are control 13 (Blue) and treatment (Red). Dark blue line represents Sippewissett’s most depleted C value 13 (Sipp 1 whole= -19.4). Green line represents Sippewissett’s most enriched C value (Sipp 5 whole= -17.6).
16.0
14.0
12.0
10.0
oo
/ o
S S 8.0
34
6.0
4.0
2.0
0.0 Control Control Control Treatment Treatment Treatment 50 100 150 50 100 150 Site
Figure 10. Sulfur isotope composition of all internal tissue of Ribbed Mussels at Plum island sites along a gradient of 50, 100 and 150 meters from the zero point. Sites shown are control 34 (blue) and treatment (red). Dark Blue line represents Sippewissett’s most enriched S value 34 (Sipp 1 whole= +14.2). Green line represents Sippewissett’s most depleted S value (Sipp 1 whole= +4.2).
Control Control Control TreatmentTreatmentTreatment 50 100 150 50 100 150 10.0
9.5
9.0
8.5
8.0
oo /
o 7.5
N N
15 7.0
6.5
6.0
5.5
5.0 Site
Figure 11. Nitrogen isotope composition of all internal tissue of Ribbed Mussels at Plum island sites along a gradient of 50, 100 and 150 meters from the zero point. Sites shown are control (blue) and treatment (red). Dark blue line represents Sippewissett’s sites close to the ocean 15 inlet average N value (Sipp 1,2,3 whole avg = 8.4). Green line represents Sippewissett’s 15 deep marsh sites average N value (Sipp 4,5,6 whole avg = 7.4). 600
500
m) m 400
Treatment 50 Treatment 100 300 Treatment 150 Control 50 Control 100
200 Control 150 Yearly Growth Band ThicknessGrowth ( Yearly
100
0 2008 2009 2010 2011 2012 2013 2014 Year
Figure 12. Average thickness of growth bands for the five most recent years (2009-2013) of growth in Ribbed Mussels at Plum Island sites along a gradient of 50, 100 and 150 meters from the zero point. Sites shown are control (blue) and treatment (red). 500
450
400
m)
m 350 Sipp Near Ocean (1,2,3) 300 Sipp Deep Marsh (4,5,6) 250 Plum Treatment
200 Plum Control
150 Yearly Growth Band ThicknessGrowth ( Yearly
100
50
0 2008 2009 2010 2011 2012 2013 2014 Year
Figure 13. Average thickness of growth bands for the five most recent years (2009-2013) of growth in Ribbed Mussels at Little Sippewissett and Plum Island sites. Little Sippewissett sites close to the ocean (dark blue) and deep into the marsh (green) are shown along with the Plum Island control (light blue) and treatment (red) sites. The black line indicates the average growth band thickness of all shells for each given year.