2009 NORTHEASTERN NATURALIST 16(4):595–606
Microscopic Past of Poutwater Pond
Adrienne P. Smyth1 and Peter M. Bradley1,*
Abstract - Poutwater Pond Bog is a National Natural Landmark located in
Holden, MA. As present-day species inhabiting this bog have been described, this
study presents an insight into earlier inhabitants of the area during the mid- to late
Holocene. A 5-m coring of peat was collected 10 m from the pond edge. Radiocarbon
analysis of 10 sections of the core shows a nearly linear accumulation rate
of peatland from 8500 years ago to the present. The presence of mineral matter
at the base of the core suggests paulidification as the mechanism for formation
of the bog. Microfossils were isolated from sections of the peat core by densitybuoyant
centrifugation, and examined using a scanning electron microscope.
High-resolution images of pollen, sponge remains, and algae are presented. Circumneutral
and acidobiontic diatoms were found at different depths of the core,
indicating a changing water environment over time. Arboreal pollen grains were
documented spanning 8500 years of vegetative history, permitting insight into the
biodiversity that once existed.
Introduction
Peatlands preserve a record of environmental history. Pollen from the
vegetation and the surrounding landscape of a peatland are preserved in
the upper layer of peat sediment. Initial peat growth can be approximated at
1 cm per year, although 90% of this is compacted (Lagerås 2003). Examination
of a sediment core of 1 m potentially contains 100–1000s of years
of paleoecological data, in the form of radiocarbon dating and analysis of
microfossils, some of which is presented here.
Peatland development results from either terrestrialization, filling in of
a lake or water body, or paulidification, a result of an increase in the water
table that triggers peat growth on solid ground inhibiting water drainage.
In an examination of three peatlands in New England, it was determined
that topographical influences and paulidification were instrumental in their
formation (Anderson et al. 2003). A determination of the mechanism of
peatland initiation can be inferred by examination of the core for lake mud
or silt beneath the peat, indicating terrestrialization in contrast to paulidification,
which would have mineral deposits directly underneath the peat
(Anderson et al. 2003). Therefore, a single coring may provide insight into
peatland development.
Classification of bogs can be based on water source or supply, surrounding
landforms causative of bog development or landform produced
1Department of Biology, Worcester State College, 486 Chandler Street, Worcester,
MA 01602-2597. *Corresponding author - peter.bradley@worcester.edu.
596 Northeastern Naturalist Vol. 16, No. 4
by the bog (Damman and French 1987), or by the presence of vegetative
indicator species (Popp 1997). Poutwater Pond Bog has been classified as
a “quaking bog” and “an early stage ombrotrophic mire” (Schofield 1971).
Favour (1971) agrees with this classification due to its sphagnum coverage,
ericaceous shrubs, coniferous trees, and other indicator species. This site
has also been described as an “undisturbed sphagnum-heath bog” (USDOI
NPS 2001), and a level bog (BioMap and Living Waters 2004, Salett 2002).
The US Department of the Interior, National Parks Service (2001) describes
Poutwater Pond as an “excellent example of ecological succession from
open water in a glacial depression to upland forest.” Poutwater Pond’s acidic
water and vegetation are all indications of an ombrotrophic bog or mire;
however, the water supply from an ombrotrophic bog is exclusively from
precipitation (Damman and French 1987), and the surface water runoff from
the surrounding uphill landscape must contribute some nutrients and minerals
and as such should be considered a minerotrophic, topogenous peat bog
lake system of the moat bog type (Salett 2002).
Analysis of microfossils preserved in a bog can provide much paleoecological
data. Pollen analysis from sediment was first recorded by Swedish
geologist Lennart von Post in the early 1900s (West 1971). An analysis of
pollen in increasing depths of sediment can provide a history of vegetation
from a time when glaciers covered the land to the present. Shuman et al.
(2004) provide an analysis of regional forest development in New England,
by comparison of pollen abundance to moisture availability and temperature
for the past 15,000 years. Diatoms, unicellular algae, are also useful
ecological indicators as they are sensitive to temperature, pH, conductivity,
and salinity and exhibit species-selectivity for habitats depending on climate
(Smol and Cumming 2000). Diatom assemblies have been used to track the
development of a bog from an open-water environment to a raised bog over
7200 years (Rüland et al. 2000).
This research aimed to discover the age and depth of the peatland of
Poutwater Pond Bog, determine the timeline of peatland development,
isolate microfossils for qualitative scanning electron microscopy, and identify
microfossils to a species level for subsequent analysis as proxies of
environmental change and quantitative analyses. Our preliminary paleoecological
findings result from a 5-m core collected in the fall of 2005, from
the bog mat at the edge of Poutwater Pond. Radiocarbon dating provides a
timeline for approximately the last 8500 years. In another study using this
core, bacteria have been isolated and identified (Fynan et al. 2009). Images
of microfossils produced by scanning electron microscopy provide insight
into the biological diversity of the historic past and allows for identification
to species level. Diatoms can be used to determine changes in peatland
development, and arboreal pollen dispersed within cores can indicate past
climatic changes.
2009 A.P. Smyth and P.M. Bradley 597
Site Description
Poutwater Pond Bog is located in northern Holden, MA, with coordinates
of 42º25΄30˝N, 71°50´20″W on the Sterling quadrangle (Fig. 1). It is an
11.33-ha (28-acre) site with an elevation of 213 m (700 ft). The 2-ha (5-acre)
pond is surrounded by a floating sphagnum bog mat of 6–30 m (20–100 ft).
It has also been known as Wonketopick or Rutland Pond. Wonketopick is
most likely the Nipmuc “Wonchatopek” or “Wonketopic” meaning “Pond,
where to get roots” or “crooked roots place” (NIAC 1995). This etymology
Figure 1. Map showing the location of Poutwater Pond, Holden, MA. Map courtesy
of The National Natural Landmarks Program, National Park Service.
598 Northeastern Naturalist Vol. 16, No. 4
is supported by the observed presence of Sagittaria latifolia Willd. (Broadleaved
Arrowhead or Wapato), which was most likely harvested for its
underwater edible tubers by Native Americans (Darby 1996), in the shallow
waters surrounding the floating bog mat.
In 1971, the site was nominated by Ed Schofield and subsequently evaluated
and recommended by Paul Favour for designation as a National Natural
Landmark, which was awarded in 1972. In 1994, the Metropolitan District
Commission (MDC) purchased 200 ha (493 acres) of which 86 ha (213
acres) includes Poutwater Pond and its surrounding peatland, for watershedprotection
purposes. A subsequent evaluation (O’Conner et al. 1997) of
Poutwater Pond’s unique geographical makeup, the kettlehole depression,
soil variation, esker, and plant communities prompted its nomination as
Massachusetts’ first Nature Preserve and designation as such in 1998. The
90.7-ha (224-acre) nature preserve is jointly owned by the Department of
Fisheries and Wildlife and the MDC.
In 1999, an inventory of vascular plants at Poutwater Pond described 76
plant species in 56 genera and 34 families (Searcy and Hickler 1999). The
pond is surrounded by a floating bog mat on which dwarf and tall shrubs
infiltrate. On the westerly side, the peatland is wider and the slope to upland
more gradual. Typical bog plants are found in the peatlands of Poutwater
Pond. These include the families Cyperaceae (sedges), Orchidaceae
(orchids), and Ericaceae (the heath family—e.g., rhododendron, azalea,
blueberry, and cranberry bushes).
Trees that are able to root and grow in bog-like conditions are found in
Poutwater, including Picea mariana P. Mill (Black or Bog Spruce), Larix
laricina (DuRoi) K. Koch, (Tamarack or American Larch), and Acer rubrum
L. (Red Maple).
Methods
Sediment coring at a location 10 m from the open edge of Poutwater
Pond was done with a Russian Peat Corer. The corer was pushed into the
ground with the chamber open to the desired depth. It was then twisted
180º clockwise, trapping the sediment within the chamber. The corer was
then withdrawn, and the sediment sample retrieved by turning the chamber
counterclockwise. Sediment was collected every 50 cm, to a maximum depth
of 5 m with the use of extension rods. The samples were covered in plastic
wrap and aluminum foil and stored at 4 °C until subsequent sectioning into
5- to15-cm samples for pollen extraction, microscopic analysis, and radiocarbon
dating (Smyth 2006).
Microfossils were extracted by modification of a density-buoyant centrifugation
method developed by Morgenroth et al. (2000). One gram of
wet sediment sample was added to 5 ml of ZnCl2 (2 g/ml), vortexed, and
centrifuged for 10 minutes at 400 g. Microfossils were collected from the
lighter organic top layer onto a Nalgene 0.2-μm filter, and washed with 50 ml
2009 A.P. Smyth and P.M. Bradley 599
of water. The filter was removed and air dried. Microfossils were collected
from the filter onto double-stick carbon tape on 8-mm mounting stubs, gold
coated using an Anatech Hummer 6.2 sputter coater following the manufacturers
instructions, and examined with a JEOL JSM - 5600LV scanning
electron microscope (SEM) (Smyth 2006).
Radiocarbon dating was performed by Geochron Laboratories, Billerica,
MA on 10 samples. Nine samples were processed using 30 g of peat sediment
treated with 1M hot HCl for 1 h, followed by drying and combustion
in oxygen to generate CO2 for the analysis. The basal age was determined
by accelerated mass spectrometry. The age is referenced to the year A.D.
1950. The 14C data obtained was then input into 2 calibration programs for
conversion into calibrated years before present (ca BP) using the programs
of Fairbanks et al. (2005) and Ramsey’s OxCal 4 (Bronk 1995).
Identification of pollen and diatoms from 13 core sections was done by
comparison to the online keys of PalDat (Buchner and Weber 2000), USDAARS
(2001), and ADIAC (2006) in addition to consulting the literature
of Wodehouse (1965), Krammer and Lange-Bertalot (1986), Bassett et al.
(1978), Camburn and Charles (2000), and Faegri and Iversen (1989).
Results
Figure 2 plots 10 uncalibrated and 9 calibrated samples from sections
of the core that were radiocarbon dated. The most recent sample extends
Figure 2. Radiocarbon dating of the peat core from Poutwater Pond showing the
relationship between depth and age. Error bars are standard deviation in years, and
range of sediment sampled in cms.
600 Northeastern Naturalist Vol. 16, No. 4
outside the range of both calibration programs (Bronk 1995, Fairbanks
et al. 2005) and is omitted from the calibrated data in the figure. The two
calibration programs have similar plots; however, a notable difference of
approximately 1000 years is observed between the raw 14C date and either
of the calibrated dates for the oldest sample. The basal dating of peatland
initiation at the site is approximately 8500 ca BP.
Paulidification is the inferred mechanism of peatland initiation from
the observation of mineral deposits, which lack any lake silt or mud
and which have peat deposits immediately above, at the basal depth of
the 5-m core collected 10 m from the lake margin of Poutwater Pond.
However, further coring is needed to confirm this preliminary evidence
for paulidification at this site. There was no observed change in the peat
appearance of the core over time except the presence of the additional
mineral matter at the base. The peatland accumulation rate is estimated
to be between 0.5–0.58 mm/yr. As mineral matter impeded deeper sample
collection, this result is only suggestive and warrants further coring closer
upland.
Visualization of the isolated microfossils by scanning electron microscopy
has resulted in many collected and stored images, a few of which we
present in this paper (Figs. 3–5). Figure 3 is a low-magnification image
Figure 3. Scanning electron microscope (SEM) images of 4500 ca BP sample containing
pollen, diatoms, and sponge remains. (a) pine and (b) alder pollen, (c) sponge
spicule, (d) sponge gemmosclere, (e) centric diatom, and (f) pennate diatom.
2009 A.P. Smyth and P.M. Bradley 601
Figure 4. Freshwater microfossils as examined with the SEM. Ages of samples are in
brackets in ca BP. (a) Cyclotella sp., a centric diatom (8250), (b) Eunotia sp., a pennate
diatom (3800), (c) Aulacoseira sp., a centric diatom (5400), (d) unidentified
Dinoflagellate (3800), and (e) Eunotia glacialis F. Meister, a pennate diatom (5400).
captured by SEM of isolated pollen, sponge spicules, and diatoms from 4500
ca BP, demonstrating a good recovery and specimen isolation using the outlined
methodology. Higher magnification and resolution by SEM (Figs. 3–5)
aids species identification of microfossils using morphological distinctions.
Centric and pennate diatoms were found at all depths sampled. Although
many diatom species have been identified, only a few are shown in Figures
3 and 4 as examples. Other microfossils isolated include freshwater sponge
spicules and gemmoscleres, and dinoflagellates (Figs. 3c, 3d, and 4d).
Arboreal pollen was found throughout the core, and representative images
of Pinus (pine), Carya (hickory), Tsuga (hemlock), Betula (birch), Salix
(willow), Alnus (alder), Fagus (oak), and Acer (maple) are shown in Figure
5. Pinaceae family and Fagaceae pollen were found throughout the depth of
the core. The more temperate species of hickory, willow, and maple were
found at depths central to the sediment sampling, from 246 to 325 cm, and
dated around 3–6000 ca BP; these were not as abundant as pine and oak species.
Non-arboreal pollen of Alismaceae, plant members of which are found
602 Northeastern Naturalist Vol. 16, No. 4
in ponds and streams, was found in the most recent deposition of peat, dating
less than 125 BP, but at no other depth sampled. As BP refers to dates before 1950,
this is a time in the early 1800s.
Figure 5. Pollen isolated from different depths of peat: SEM images. Ages of samples
are in brackets in ca BP. (a) American Water-plantain (<125), (b) pine on left and
hickory on right (4000), (c) hemlock on left and birch on right (7250), (d.) maple at
top and Aulacoseira sp. on bottom (7250), (e) alder (4500), (f) oak (8500), and (g)
willow (4500).
2009 A.P. Smyth and P.M. Bradley 603
Discussion
Peatlands, in addition to preserving a record of biological diversity,
are becoming more important as climate-change studies are increasing in
significance and attention. Peatlands are a carbon sink, and as such, scientists
differ in their opinions as to whether the demise of peatlands by
elevating temperatures would result in them becoming a carbon source
or remaining as a carbon sink by the initiation of new forest cover. Some
literature suggests the accumulation of peat has begun to slow down in the
West Siberian Lowland and that these areas have been methane sources
since the early Holocene (Smith et al. 2004). Our data of a New England
temperate peatland provide evidence of its existence since 8500 ca BP,
which compares to 3 other peatlands in New England that have been dated
at 7250, 8420, and 9600 ca BP, respectively (Anderson et al. 2003). The
accumulation rate of peat over time at Poutwater Pond of 0.5–0.58 mm/yr
does appear to lag during more recent times, but this interpretation is not
conclusive and falls within the data presented from the three previously
mentioned New England peatlands.
Analysis of the diatom data suggests a changing lake environment. Cyclotella
and species of other diatom genera of low acid-neutralizing capacity
were found at the deeper sampling depths and are indicative of a deep-water
oligotrophic lake environment prior to or during the early stages of peatland
initiation. Acidophilic or acidobiontic species of diatoms (e.g., Eunotia and
Aulacoseira) are found as time progresses, the sphagnum peatland accumulates,
and the water chemistry is altered and becomes more acidic (Brugam
and Swain 2000).
General climate trends during the time of initiation at the site approximately
8500 ca BP are associated with a change from a warm, dry to a cooler,
wetter climate suitable for the accumulation of peatland species (Davis et
al. 1980). Vegetative pollen recovered is mainly from tree species (Figs. 5
b–g). Pine, hemlock, and oak pollen were found at 8500 ca BP, suggesting a
thriving forest landscape. Hickory, birch, and maple pollen grains were identified between 4–7400 ca BP. The most infrequently encountered pollen was
willow, at 4500 ca BP, followed by alder. The observation of pollen from the
more temperate tree species during the mid-Holocene agrees with a warming
after 8200 ca BP and with moisture availability decreasing until 3000 ca BP
(Shuman et al. 2004). The non-arboreal Alisma plantago L. (Water Plantain)
pollen (Fig. 5a) may be indicative of more recent human influences. Native
Americans may have cultivated this edible plant in addition to the presentday
Arrowhead found in the shallow waters of the moat adjacent to the
floating bog mat.
Freshwater sponges and dinoflagellates (Fig. 4) were also a part of this
habitat, as evidenced by sponge spicules and gemmoscleres (Fig. 3), and
their presence is in agreement with the ending of a drier climate trend of
604 Northeastern Naturalist Vol. 16, No. 4
the Holocene and the beginning of a modern cooler and wetter climate trend
around 3200 ca BP (Newby et al. 2000).
This study, derived from the preliminary analysis of microfossils and
radiocarbon dating, of a coring from the peatlands of the National Natural
Landmark and Massachusetts’ first Nature Preserve, Poutwater Pond Bog,
Holden, MA, presents a timeline for peatland initiation and provides insight
into the palynological diversity during the Holocene (Figs. 3–5); our
results offer evidence for changing climate trends spanning approximately
8500 years.
Acknowledgments
The authors are grateful to the Worcester State College faculty mini-grant program
for financial assistance to obtain the radiocarbon data. We thank Wyatt Oswald
of Harvard Forest for assistance with pollen identification and core sampling. We are
also grateful to the guest editor, Dr. G.A. Langlois, and the reviewers for many valuable
suggestions that improved the manuscript.
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