PATTERNS OF CHANGE IN THE WETLANDS AT MILITARY CANAL

The wetland cores were collected to examine changes in the wetland ecosystems bordering the bay, and to identify changes in surficial water availability in the terrestrial near-shore portion of the ecosystem over time.

Military Canal North Core

Core SEI97-BW1, collected in a mangrove swamp on the north side of Military Canal, reveals sedimentological changes throughout the history of the site.  A basal black peat (38-48 cm) is overlain by a peaty marl (24-38 cm), which grades into a tan marl (12-24 cm).  A thin transitional peaty marl (10-12 cm) is overlain by an upper brown peat (0-10 cm).  There is some correspondence of the three pollen assemblage zones with the lithologic changes.  Zone I (24-48 cm) includes the basal peat and peaty marl and is dominated strongly by Pinus pollen, which comprises >75% of assemblages (Figure 11; Appendix H).  Myrica and Cyperaceae pollen and fern spores also are common in this interval.  Zone II (12-24 cm) consists of the tan marl and also is dominated by Pinus pollen (>80%), but Casuarina pollen is present, albeit in low percentages (<2%).  Zone III includes the transitional peaty marl and uppermost peat and is characterized by Casuarina pollen abundance >5% and is divided into two subzones.  Zone IIIa (8-12 cm), corresponding to the transitional peaty marl and basal peat, consists of a transitional pollen assemblage, with Pinus pollen and fern spores comprising 50-70% and up to 23% of assemblages, respectively.  In Zone IIIb (0-8 cm), Pinus pollen is much less abundant (3-30%), and fern spores dominate the assemblages strongly (41-77%).  Other taxa that reach their greatest abundances in Zone III are are Quercus, Casuarina, and the Chenopodiaceae/Amaranthaceae (Figure 11).  Total pollen concentration fluctuates throughout the core.

Military Canal South Core

Figure 12. Percent abundance of pollen of major plant types from Military Canal South core (SEI-97-BW2). Lithology is shown on left; pollen concentration in grains/g/sample on the right. (Data in Appendix H.) [larger version]

Core SEI97-BW2, collected in a mangrove swamp on the south side of Military Canal, includes a basal marl layer (40-44 cm), overlain by black peat (36-40 cm).  This grades into a peaty marl (30-36 cm) then a tan marl (20-36 cm), which grades back into a brown peat in the upper 10 cm.  There is less correspondence between pollen assemblages and lithology than in the BW1 core, and three pollen assemblage zones are present.  The basal interval, Zone I (14-44 cm) is dominated strongly (76-98%) by Pinus pollen and members of the Asteraceae are common (Figure 12; Appendix H).  The boundary between Zones I and II corresponds to a change from predominantly marl to peaty marl.  Zones II (6-14 cm) and III (0-6cm) are distinguished primarily by differences in abundance of Pinus pollen, which comprises 33-52% of Zone II assemblages and <25% of Zone III assemblages.  Otherwise, both zones are characterized by the presence of Casuarina pollen and greater abundances of pollen of the Euphorbiaceae, Solanaceae, Typha, Rhizophora, Ambrosia, Chenopodiaceae/Amaranthaceae, and Apiaceae (Figure 12).  Total pollen concentration fluctuates throughout the core.

Discussion of Military Canal Cores

Figure 13. (A, top) Aerial photographs of Military Canal, 1940, from Smith (2002); and (B, bottom) digital orthophoto quadrangle, 1999, from EarthExplorer (http://edcsns17.cr.usgs.gov/EarthExplorer/). Red circles show core locations. [larger version]

Although both Military Canal cores were collected in modern mangrove forests, their pollen records indicate that this was the site of a complex history of vegetation and hydrology.  Different understory species associated with different hydrologic regimes existed throughout the time of deposition at the sites.  Recent assemblages from the site north of the canal (SEI97-BW1) are characterized by abundance of an unidentified fern species and poor representation of weedy taxa such as the Asteraceae or Chenopodicaeae/Amaranthaceae or marsh taxa (Cyperaceae, Poaceae).  The site south of the canal, on the other hand, has abundant weedy and marsh taxa and ferns typical of drier conditions, similar to conditions found on tree-island heads (Acrostichum, Thelypteris).  The greater abundance of weedy taxa, in particular the Asteraceae, is a long-term feature of the southern site, persisting throughout the entire core (Figure 12).  We interpret these differences as indicative that the southern site has always been relatively drier than the northern site.  Examination of aerial photographs from 1940 (Figure 13) shows a flow pattern discharging near the northern site; the distance between the two core sites apparently is great enough to result in hydrologic differences even before construction of Military Canal early in the 20th century.  After its construction, the hydrology was further altered, increasing the abundance of ferns and other taxa characteristic of shorter hydroperiods (Willard, Weimer, and Riegel, 2001).