Ft. Johnson Mudflat

Results:

Organisms were grouped by common names in order to provide a clear, concise picture of changes in composition. It should be noted here that "worm" refers to segmented worms of Phylum Annelida. You can find a table at the bottom of this page that lists all species identified in both the historical and recent surveys. Links will take you to the Guide page for that species.

*For each pie "piece" the number of species and percentage composition are listed.

In the current surveys crabs, shrimp and amphipod are the three most prevalent groups with 3 species of each identified.

Recent sampling found species in 9 out of 10 groups while the historical studies only found species in 7 out of the 10 groups. However, the overall number of species identified dropped from 17 historically to 15 currently. The bar graph below illustrates the different number of species found per group in the two different studies. The blue bar represents organisms found in the historical surveys and the red bar represents organisms found in recent surveys.

Discussion:

There are several variables that may explain differences in the species found historically and today.

1) Location: It is important to note that surveys were not conducted in the same exact area. The historical surveys were conducted in front of the spoil area, east of Grice Marine Lab (photo: below left). However the current survey was conducted at the mudflat in front of the SC DNR building which is west of GML (photo: below right). The bottom substrate at the historical site is currently covered with rocks and shells and is a mixture of sand and mud; while the bottom substrate of the current site is almost completely mud. (Has the historical site changed since the 1970's? When were those rocks put there? Erosion?)  There are 5 common species between these two locations. The absence or presence of several other species may be due to collection methods or micro-habitat preferences and are discussed below.

*Both pictures were taken on April 26, 2012 at low tide.

2) Worm Collection: Historical surveys identified 4 worm species and the current surveys only identified one (Dioptra cuprea). During the 2012 survey we did encounter multiple worm individuals; however identification was not possible for most collected specimens because the heads or distinguishing body parts were lost/separated during collection. This may have skewed our results for how many worm species currently inhabit the mudflat. It should also be noted that both surveys found nemertean worms; the 2012  study was unable to identify to species level.

3) Amphipod Collection: As a part of our collection methods for the 2012 surveys we picked up seaweed from the mudflat and shook it to collect any organisms attached to it. This method in particular resulted in the collection of our amphipod species. However, it does not explicitly mention if this method was used in the historical survey, which may be why no amphipod species were collected and identified historically.

4) Crabs: Sand fiddler crab Uca pugilator was collected in the historical study. This species is noted as being most abundant in sandy saltmarsh areas; which is the habitat that lines the historical mudflat site. There is no sandy marsh area at the current site which may explain the absence of this species in the 2012 collection.

5) Comparison to Grice Beach The species Tagelus plebeius (Stout Razor clam) and Clibinarius vittatus (Thinstripe Hermit Crab) were recorded at the historical mudflat site but not at the current site. However in 2011 both were recorded at Grice Beach which is adjacent to the historical mudflat site. Therefore, these species should not be considered absent from the area.

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.

Another important note is the exclusion of oyster species collection in the current mudflat survey. The oyster beds located at the current site are a part of the South Carolina Department of Natural Resources Oyster Restoration Project and were first built in 2001. Since these oyster beds have not naturally developed at this site, we deliberately decided to exclude the oyster species in the survey. Even though oysters are found on rocks in this area, we cannot discern if they settled there naturally or because of the restoration project. The pictures below indicate the man-made oyster bed structures. Link: SCDNR Oyster Restoration.

*Both photos were taken on April 26, 2012 at low tide.

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 mudflat 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.