Final--The Holocene Record of Benthic Communities in a Vibracore from Rice Bay

This discussion topic submitted by Rick Zimmerman (zimm@nsystems.wcp.miamioh.edu) at 4:44 pm on 7/28/99. Additions were last made on Wednesday, May 7, 2014.

The following is an overview of the first half of a presentation conducted for the 8th Symposium on the Natural History of the Bahamas.

Purpose:
The purpose of our study was to examine similarities and distinctions among the molluscan and foraminiferan death assemblages within a vibracore from Rice Bay. Then, using both death assemblages, combined with data from a sedimentology analysis, we hope to reconstruct what the past communities of Rice Bay were like over 6,000 years.


Our research takes us to Northeastern corner of San Salvador Island into a lagoon named Rice Bay. A mega-transect was performed between the beech on San Sal and a beech on Man Head Cay. The transect was approximately a half a kilometer long and our vibracore was pulled at about the midpoint of this transect in a dense seagrass bed in water approximately 3.2 meters deep. The core itself is 3.4 meters long and represents a time span of roughly 6,000 years. The final 10 centimeters of the core was peat, so our core provided a pretty comprehensive historical timeline. The peat was dated at 6,290 years.
The core was sawed in half from top to bottom and sub-samples were pulled every 20 centimeters throughout the core. This yielded 17 samples with a mean weight of 15.48 grams. Each sample was then wet sieved into six size fractions. All molluscs and foraminiferans were then identified in three size fractions: greater than 2mm, 2mm to 1mm, and 1mm to 0.5mm.

The identification process involved a binocular microscope and various identification manuals and books.

Rick and Margot deploy the sediment array in Rice Bay

Mollusc and Foraminiferan Research Questions for the Entire Core:
The major questions we looked to answer were: 1) Do the Molluscan and Foraminiferan records tell the same ecological story, and 2) Where and why do similarities and distinctions occur. Now, what Iím going to do is talk about the work I did with molluscs and what I found within the core, and then Margot is going to present her findings and provide some conclusions and recommendations for further study.

Mollusc Research Objectives
I had five major objectives: 1) To recognize the dominant species throughout the core, 2) to observe temporal changes in the molluscan community, 3) evaluate the relationship between grain size and the dominant species, 4) identify species richness, and finally, 5) observe if any taphonomic processes were occuring.

General Findings
A few of the common species of gastropods we were observing include Alvania aubriana, Dendropoma sp., Tricolia affinis, and Acteocina candei. Some of the most prevalent bivalve species we found were Lucina pensylvanica, Anadar sp., and Tellina angulosa.

Of the top twenty species in the rank order of abundance, the top six species are all gastropods and collectively make up over 65 % of the entire top twenty. So what we have is a core dominated by gastropods. This is important because past research has shown that we find more gastropods in seagrass beds as opposed to sand flats.

Our next goal was to identify temporal change in the community. This was accomplished using the Spearman Rank Test. The top ten Rank Order of Abundance for the entire core was applied to each sample and within core comparisons were performed. Our null hypothesis stated that one sample would be independent from another sample. If the comparison yielded a p-value of greater than 0.1, then the null hypothesis was accepted, if less than 0.1, it was rejected. The null hypothesis was accepted for 80 percent of the comparisons. Thus, the community had changed over the 6,000 year history. There were however, two points in the core were the community was unchanging. The first point occurs at the top of the core in samples one, two, and three. The second point occurs at samples nine and ten. What this means is that at these two points in time the community was remaining relatively the same.

The next observation we made was for species richness. We conducted a species count and found that the species richness for the core was increasing over time, and it hit an apex at samples four, five, and six. At these samples the species richness was up much higher as compared to the rest of the core. We compared this data to the sedimentology analysis we conducted. We observed the mean grain size throughout the core and found that the points in the core where the mean grain size was lowest were at samples four, five, and six. The mean grain size here was incredible low compared to a steady grain size for the rest of the core. The reason this is important is because we know that seagrass beds perform a baffling affect and trap the smaller sediments. We also know that we find more gastropods in seagrass as opposed to sand flats and that our core was dominated by gastropods and our species richness was highest at these samples. So, what we hypothesize is that at this point in the history of the bay, the lagoon was most likely a dense seagrass bed.

We also wanted to observe if any taphonomic processes were occuring. We tested this using bivalves. For each organism there is a right and a left valve, and we expected to find a one to one relationship, or close, of right to left vlaves. However, what we discovered was that there were only two points in the core where this ratio was achieved or even close. The other fifteen samples were saturated with right valves. What this tells us is that some processes and phenomena are acting on these shells.

Another relationship we identified was that of the percentage of bivalve and gastropods within each sample. What we found was that the entire core was dominated by gastropods. The bottom of the core, the beginning of the lagoon, had an equivalent percentage of bivalves and gastropods. However, as time continues and the gastropods take over and begin to dominate the core. Once again, this supports our observation that the lagoon was most likely seagrasss for the majority of its life.

There is however, one expectation we did not find. From other research conducted we usually observe a large number of Cerithium spp.in areas of dense seagrass. In the core however, we found very few Cerithium spp. The reason for this could be due to sampling, taphonomic processes, or other phenomena such as climate change.


References:

Abbott, R. Tucker. American Seashells. Van Nostrand Reinhold Company.
New York, NY. 1974.

Abbott, R. Tucker, Morris, Percy A. A Field Guide to Shells. Houghton Mifflin
Company. New York, NY. 1995.

Kaplan, Eugene H. A Field Guide to Southeastern and Caribbean Seashores.
Houghton Mifflin Company. New York, NY. 1988.



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