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Results:
Organisms were grouped based upon common names in order to provide a clear, concise picture of changes in composition. It should be noted that "worms" here refers to segmented worms from Phylum Annelida. Below you can find a table of all the species identified with links to the "Guide" pages.
*Each pie piece lists the number of species identified in that group and the percentage composition of the group.
Historically crabs were the most prevalent, with 8 species collected and identified. Groups unique to historical survey: -Shrimp
In recent surveys crabs were still the most prevalent group, however only 4 species were identified. Groups unique to current surveys: -Barnacles -Worms -Mussels -Oyster
Recent sampling found species in 7 out of 8 groups while the historical studies only found species in 4 out of the 8 groups. However, the overall number of species identified remained the same. Only 3 common species were identified in both surveys. The bar graph below illustrates the different number of species found per group in the two different studies.
Discussion:
There are several variables that may explain variability in the species found historically and today.
1) Saltwater Pond: The historical surveys included two saltmarsh surveys; one of which only included the saltwater pond and marsh edge. The current study did NOT survey the saltwater pond or the marsh edge surrounding it. This is a man-made pond that originated in the spoil area to the east of Grice Marine Lab. The photos below are evidence that the size of the pond has decreased since its creation in 1970.
(Left) This is an historical aerial photo of Ft. Johnson that can be found at the DNR website. (Below left) This photo taken in 2012 shows the current saltwater pond, which has reduced in size. The dock in the background no longer sits on top of water.(Below right) A google earth image of the saltwater pond area.
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The historical survey of the saltwater pond has several implications for our comparisons. First, the 3 shrimp species collected historically were from this particular survey. Since the pond was not surveyed in 2012 that may be why no shrimp species were collected in the current saltmarsh surveys. Shrimp species P.pugio and P. vulgarius were found in the 2012 mudflat surveys; therefore they are not absent from the Ft. Johnson area.
2) Sandy vs. Muddy substrates: The exact substrate bottom that is surveyed may also have implications for what species are collected. The historical surveys indicate that both muddy and sandy bottoms were sampled. However the 2012 surveys did not collect organisms from sandy substrates.
This picture is an example of a muddy saltmarsh substrate. The mussel Geukensia demissa can commonly be found attached to or burrowed next to Spartina alterniflora, the marsh cordgrass pictured here.
Fiddler crab species Uca minax and Uca pugnax have been noted as being more abundant in muddy saltmarsh habitats.
This photo was taken in April 2012 in the upper saltmarsh area, closest to the treeline.
Here is an example of a sandy marsh substrate; which was sampled in the historical surveys.
Fiddler crab species Uca pugilator has been noted to be more abundant in sandy marsh habitats.
This photo was taken in April 2012 along the marsh edge, to the east of the saltwater pond.
As indicated in the photos, the Ft. Johnson saltmarsh habitat can be characterized by both sandy and muddy substrates. Which "micro-habitat" is sampled may have an effect on which species are collected. This may account for Uca pugilator was not collected in the current marsh surveys. However, while casually walking around the saltmarsh edge we picked up several different fiddler crabs and were able to identify one as Uca pugilator. Therefore, even though not all 3 fiddler crab species were formally collected and identified in the current surveys, we do not think any are absent from the saltmarsh area.
In addition, surveying muddy habitats may result in the collection of mussels and oysters since they are more commonly found in muddy saltmarsh bottoms. The 2012 surveys collected 2 mussel and 1 oyster species while none were found in the historical surveys. Only one historical survey mentions sampling in mud; however only crab species were collected and identified. This is discussed further below.
3) Survey intent: Another interesting note is that one historical saltmarsh survey only recorded the collection of crab species. While we can only conjecture as to why this is the case, the absence of organisms other than crabs during a 1 hour and 40 minute survey makes us curious as to whether other organisms were present but just not collected and/or recorded at the time.
4) Comparison to Grice Beach: Several species found only in the historical saltmarsh survey were found in 2011 at Grice Beach, which is adjacent to the marsh. These include:
- Thinstripe hermit crab: Clibanarius vittatus
- Atlantic sand fiddler crab: Uca pugilator
- Blue crabs: Callinectes ornatus and Callinectes sapidus
- Northern quahog: Mercinaria mercinaria
Additional limitations for comparisons:
As mentioned in the introduction, it is important to note that the historical and recent surveys occurred at different times of the year (summer vs. late winter/early spring) and potentially with a different number of participants. We also do not know what identification methods were used historically. These differences may account for variability in the species collection and identification. Neither of these studies attempted to determine population abundances of these species, which could be a potential future project.
Conclusion:
At a first glance there appear to be marked difference between species found historically and today. However after reviewing collection methods, site location, and comparison limitations we cannot quantify any marked differences in species composition. While the data from both historical and current surveys serves as a baseline for organisms found in Ft. Johnson saltmarsh habitats, more intensive and organized surveys should be conducted. One recommendation is to be explicit in the exact location of the survey site, perhaps even recording GPS points.
*Note on Polymesoda caroliniana: During identification the clam was opened up however no discernible parts of the animal were seen as most of the shell was filled with mud.
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