Fossil remains of dinoflagellates consist primarily of the nonmotile or cyst stage. Dinoflagellate cysts are common in most sediment samples from the Chesapeake Bay. They are useful in documenting temporal trends in Chesapeake phytoplankton during climatic changes in the Holocene, as well as during the postcolonial period of 19th century agricultural land clearance and 20th century fertilizer application and urbanization.
Dried sediment samples, weighing between 4.0 and 7.0 grams, were prepared for palynological analysis by decalcifying with dilute (i.e. 10%) hydrochloric acid and then neutralizing (to a pH of 7) with deionized water to remove carbonates. After neutralization, hydrofluoric acid (52% by volume) was added to each sample and the resulting mixture was allowed to rest at room temperature for ~48 hours. After resting, the samples were rinsed with deionized water into 50 milliliter (ml) centrifuge tubes. A 10 ml solution of acetic anhydride and sulfuric acid, in a ratio of 9:1, was added to each sample. Samples were heated in a water bath at 100°C for 10 minutes after which, 25 ml glacial acetic acid was added to each sample to arrest the reaction. The samples were then treated with 25 ml solution of 10% potassium hydroxide and heated to 70°C in a water bath.
The samples were then centrifuged with a Damer/IEC Model K centrifuge at 1500 rpm and washed until neutralized to a pH of 7. If the samples contained sand, heavy minerals, or organic material, they were centrifuged at 500 rpm for 20 minutes and then at 1500 rpm for 10 minutes with 3 ml zinc chloride, a heavy liquid with a specific gravity of 2.1. After neutralization, the samples were sieved through 1249 micron (µm) and 10µm nylon mesh sieves to remove plant material and clay-sized particles. The residue was mixed with warm (i.e. 5°C) glycerin jelly and placed on microscope slides for final preparation.
Two drops of the residue and glycerin jelly mixture was dropped onto a clean slide and placed on a warming table at ~40°C. The coverslips were carefully placed on the slides and allowed to cure for 48 hours on the warming table. The slides were cured upside-down (by supporting the edges of the slides with thin wood dowels) to bring the fossil cysts closer to the coverslip, making it easier to focus. The coverslip was then outlined with clear acrylic to seal the slide and each slide was labeled with location, sample date, and depth.
The slides were mounted on a binocular microscope to identify and count a minimum of 300 dinoflagellate cysts for meaningful statistical analysis (Imbrie and others, 1973). Scanning Electron Micrographs were taken of individual dinoflagellates to insure taxonomic identity (Verardo, 1999). The relative abundance (i.e. percent of total number of individuals) of each species was calculated by dividing the number of individuals by the total number of individuals in each sample.
The age model for MD99-22099 was developed from radiocarbon dating (see Colman and others, Chapter 6, this volume) and pollen biostratigraphy (Willard and Korejwo, Chapter 7, this volume). One datum of particular importance when interpreting dinoflagellate trends is the agricultural horizon, which occurs at ~250-260 cm in core MD99-2209. This horizon refers to the sharp rise in Ambrosia (ragweed) pollen to greater than 10% of the total pollen assemblage that occurred during the 1800's (Brush and others, 1982). Willard and others (in prep.) discuss recent data on the timing and impact of 19th century land clearance on Chesapeake Bay and its watershed.
The mean abundance of Spiniferites cysts in MD99-2209 varies from <30 to ~60% between the bottom of the core (~1700 cm) and the unconformity near 800 cm. Above the unconformity the abundance of Spiniferites progressively increases, reaching a peak above 80% near 200 cm, above which it varies between 50 and 80%. These variations in relative abundance presumably signify changes in temperature and salinity, although we cannot yet quantify these factors for this site. It is noteworthy that the relatively high proportions of Spiniferites in the uppermost 200 cm, which corresponds to the past 150 years, is reminiscent of the dominance of this genus between 1600 and 1720 cm.
One species in particular, Spiniferites mirabilis, seems to be an excellent indicator of the negative impact of 19th century agriculture and 20th century fertilizer application had on Bay dinoflagellates. Prior to the 19th century, this species varied from <4-7% of the total dinocyst assemblage, with the exception of an interval between 600-700 cm (about 1300-1600 yr BP). Pre-historical low abundance of S. mirabilis contrasts sharply with its abundance since the middle 1800's, when it increased to 10-15%, occasionally reaching 20% during the 1900's. Harland (1983) and Turon (1984) present evidence that S. mirabilis can tolerate increased nutrient concentrations of phosphate and nitrate and warm surface waters. Recent studies of the modern distribution of S. mirabilis in coastal bays and estuaries along the Atlantic and Gulf coasts of North America support the idea that this species is extremely tolerant of high nutrient levels, reduced and variable salinity, high turbidity, and perhaps reduced levels of dissolved oxygen (Verardo, unpublished data; Willard and others, in prep.). The results from MD99-2209 demonstrate that during the last 30 years, S. mirabilis has reached unprecedented levels of abundance in Chesapeake Bay, probably related to changing environmental conditions.
The second most abundant dinoflagellate genus in MD99-2209 is Operculodinium centrocarpum. O. centrocarpum has a modern distribution associated with estuarine and coastal waters in mild to cool temperate regions (Wall and others, 1977; Harland, 1983; de Vernal and others, 1992; Brenner, 1998). This species comprised 5 to 30% of early Holocene assemblages between 1600 and 1400 cm. The abundance then decreased to 10% before increasing to 25% by 900 cm. O. centrocarpum maintained a steady relative abundance of 5-15% for much of the past 2000 years (above 800 cm) although a small net decline can be observed and increased variability above the 200 cm level. Other species, such as Polysphaeridium zoharyi and Lingulodinium macherophorum are presently both found along the southeastern seaboard of the United States and the Caribbean and are considered tropical-subtropical marine species (Wall and others, 1977; Harland, 1983; Edwards and Anderle, 1992). Both of these species exhibit oscillating abundances between 5-10% during the early Holocene (between 1700 cm and 900 cm).
One of the noteworthy results to emerge from this preliminary data is the progressive decrease in the relative abundances of Polysphaeridium zoharyi, Lingulodinium macherophorum and Nematophaeropsis spp. in the last ~2300 years of Late Holocene deposition (the interval 800 cm to 0 cm). Nematophaeropsis spp., a genus found in the North Atlantic associated with the cold North Atlantic Gyre (Wall and others, 1977; Harland, 1983; Edwards and Anderle, 1992), decreased to ~1% at the surface. The only taxonomic groups that did not decrease in abundance during the Late Holocene were Spiniferites spp. and S. mirabilis, which presently is the dominant taxon in the Bay. These trends are also observed in dinoflagellate assemblages from other Chesapeake Bay cores (Verardo and others, 1998; Willard and others, in prep.) and suggest a diminished phytoplankton biodiversity during the post-Colonial period.
There are several reasons to support the argument that the cysts recovered from MD99-2209 and other Holocene sediments from Chesapeake Bay cores represent autochthonous assemblages that inhabited Chesapeake Bay. First, the excellent preservation of cysts (Verardo, 1999), including coloration similar to cysts from modern sediments, suggests they have not been transported. Second, although species such as P. zoharyi occur in sediments near the Bay (Edwards, 1986), P. zoharyi is also a component of the modern assemblage. Extinct Miocene species have rarely been encountered in the several hundred Holocene dinocyst samples from the Chesapeake Bay. Third, other microfaunal and floral paleoecological indicators (Cooper and Brush, 1991; Cronin and others, 2000) show concordant trends with those of the dinoflagellates, suggesting the entire ecosystem, including phytoplankton, respond to environmental change. Fourth, P. zoharyi and other Chesapeake Bay dinoflagellate taxa typically live in marine environments, phytoplanton populations could easily enter the Bay at its mouth where deep saline water flows into the main channel from the continental shelf. In sum, there is no evidence for significant reworking of Cenozoic dinoflagellates in the Marion-Dufresne and other Chesapeake Bay sediment cores.
Brush, G., Martin, E., DeFries, R., and Rice, C., 1982, Comparisons of 210Pb and pollen methods for determining rates of estuarine sediment accumulation: Quaternary Research, v. 18, p. 196.
Cronin, T., Verardo, S., Weimer, L., Ishman, S., and Dwyer, G., 1999, Holocene climatic variability in the eastern U.S. recorded in Chesapeake Bay sediments [abs.]: American Geophysical Union, San Francisco.
Cronin, T., Willard, D., Karlsen, A., Ishman, S., Verardo, S., McGeehin, J., Kerhin, R., Holmes, C., Colman, S., Zimmerman, A., 2000, Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments: Geology, v. 28, p. 3.
de Vernal, A., Londeix, L., Mudie, P., Harland, P., Morzadec-Kerfourn, M., Turon, J-L., and Wrenn, J., 1992, Quaternary organic-walled dinoflagellate cysts of the North Atlantic ocean and adjacent seas - ecostratigraphy and biostratigraphy, in Head, M., and J. Wrenn, eds., Neogene and Quaternary dinoflagellate cysts and acritarchs: American Association of Stratigraphic Palynologists, p. 289.
Edwards, L., 1986, Late Cenozoic dinoflagellate cysts from South Carolina, U.S.A., Wrenn, J.S., Duffied, Stein, J., eds., American Association of Palynologists, Contribution Series Number 17.
Edwards, L., and Anderle, V., 1992, Distribution of selected dinoflagellate cysts in modern marine sediments, in Head, M., and Wrenn, J., eds., Neogene and Quaternary dinoflagellate cysts and acritarchs: American Association of Stratigraphic Palynologists, p. 259.
Fensome, R., Riding, J., Taylor, F., 1996, Dinoflagellates, in Jansonius, J., and McGregor, D., eds., Palynology - principles and applications: American Association of Stratigraphic Palynologists, p. 107.
Harland, R., 1983. Distribution maps of recent dinoflagellate cysts in bottom sediments from the North Atlantic Ocean and adjacent seas: Paleontology, v. 26, p. 321.
Imbrie, J., van Donk, J., and Kipp, N., 1973, Paleoclimatic investigation of a late Pleistocene Caribbean deep-sea core: Quaternary Research, v. 3, p. 10.
Turon J-L., 1984, Le palynoplancton dans l'environnement de l'Atlantique nord-oriental. Evolution climatique et hydrologique depuis le dernier maximum glaciaire: Memoires de l'Institut de Geologie du Bassin d'Aquitaine, v. 17, p. 1.
Verardo, S., 1999, Dinoflagellates, in Cronin, T., Wagner, R., and Slattery, M., eds., Microfossils from Chesapeake Bay sediments - illustrations and species database: United States Geological Survey Open-File Report 99-145, 159 sp.
Verardo, S., Cronin, T., Willard, D. and Edwards, L., 1998, Catastrophic impact of 19th century environmental changes on Chesapeake Bay Dinoflagellate Cysts [abs.]: Geological Society of America.
Wall, D., Dale, B., Lohmann, G., and Smith, W., 1977, The environmental and climatic distribution of dinoflagellate cysts in modern marine sediments from regions in the North and South Atlantic Oceans and adjacent seas: Marine Micropaleontology, v. 2, p. 121.
Willard, D.A., Verardo, S., Cronin, T.M., in prep., Late Holocene climate variability and anthropogenic influence in Chesapeake Bay.
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