Chapter 5. Temporal Trends in Benthic Microfossils in Pocomoke Sound: Implications for Water Quality

Introduction

The impacts of post-colonial land-use change on the natural state of the Chesapeake Bay ecosystem has been documented through extensive paleoecological and geochemical analyses of fine-grained sediments from the mainstem bay. These "retrodictive" studies of ecosystem history include the application of a variety of proxies of ecosystem health such as diatoms (Cooper and Brush, 1991), foraminifera (Karlsen and others, 2000), dinoflagellates (Willard and others, 2003), ostracodes (Cronin and Vann, 2003), stable isotopes of sediments (Bratton and others, 2004), biogenic silica (Colman and Bratton, 2003), trace metals (Zheng and others, 2003), and organic biomarkers (Zimmerman and Canuel, 2000).

In addition to documenting the impacts of human activities, the sedimentary record of the bay has been instrumental in providing compelling evidence for the influence of climatological processes on water quality over various timescales. These studies demonstrate how regional precipitation influences freshwater river discharge, bay salinity, levels of dissolved oxygen, and turbidity over decadal and centennial timescales (Cronin and others, 2000; Karlsen and others, 2000; Cronin and Vann, 2003; Saenger and others, submitted). Moreover, these findings show that interannual and longer term variability in precipitation and river flow can exert dominant control over water quality, salinity, phytoplankton and submerged aquatic vegetation even in the absence of large-scale land clearance, an observation consistent with growing evidence from instrumental records (e.g., Tyler, 1986; Malone, 1991; Harding and Perry, 1997; Gibson and Najjar, 2000; Orth and others, 2002).

Due to their fine-grained lithology and rapid accumulation rates, sediments in the Pocomoke Sound channel also hold promise for reconstructing the faunal and water quality history of this region prior to 20th century monitoring programs. This chapter describes temporal patterns in two calcareous microfossil groups, benthic foraminifers and ostracodes, and their paleoecological significance based on cores from Pocomoke Sound from sites PC-2B, PC-3B, and PC-6B. The three sites comprise a north-south transect running from the proximal part of the channel just west of Long Point, to its distal end south of Watts Island (Figure 5.1). The sites sample the main channel at water depths of 7.9, 11.4, and 15.3 m, respectively, and thus provide a detailed history of the mesohaline region of Pocomoke Sound for the past several centuries.

Materials and Methods

Table 5.1 lists site information for cores and the sampling information for foraminifera and ostracodes in each core. Both short (~200 cm) and long (~450-500 cm) cores taken in May and September 2001 were used in the study. Microfaunal samples were taken at either 2-cm or 10-cm spacing depending on the core. Between 30 and 50 grams of wet sediment were washed through 63-µm sieves, oven dried and placed in vials. Approximately 100 specimens of each group were picked and from the > 150-µm size fraction. The picking and analytical procedures for foraminifers and ostracodes are described in detail in Karlsen and others (2000) and Cronin and Vann (2003), respectively. Species were identified using the published literature and scanning electron photomicrographs provided in Cronin and others (1999, see www: http://pubs.usgs.gov/pdf/of/of99-45/). Results are presented in terms of species’ relative frequencies (the species’ percent of the total assemblage from a sample), which are simply referred to as abundance.

The number of individual specimens used in quantitative microfaunal analyses depends on the objectives of the study, the abundance of the microfossils in the sediments and the time constraints. Time constraints limit the number of total samples one can reasonably analyze in a study, and there is a tradeoff between the number of specimens per sample and the number of samples and cores that can be studied. Our prior studies of Chesapeake Bay calcareous microfossils show that 100 specimens are usually sufficient to delineate primary trends in the major indicator species. For example, temporal variability in a species’ abundance, which reflects major environmental parameters such as salinity and dissolved oxygen, can be extremely large due to the dynamic nature of the estuarine system. It is not unusual for the most common species in Chesapeake Bay to fluctuate between 10 and > 80 % of the total assemblage within a core due to decadal to centennial changes in salinity, water quality, or both. Such variability is sufficiently large to obtain statistically significant confidence limits from which we can infer ecosystem change according to the statistical standards discussed by Buzas (1990). In addition, Karlsen and others (2000) compared faunal trends using 100 and 300 individual foraminifers in the same Chesapeake Bay sediment core and found no appreciable difference in terms of major faunal patterns. Thus, we chose to conduct analysis of multiple cores, using 100 specimens from closely spaced samples (usually 2 to 10-cm). For example, we examined the ostracode faunal patterns at PC-2B-1 every 2-cm in the upper 200 cm of sediment and crosschecked these results with data sampled every 10-cm from the upper 200 cm at PC-2B-3. A similar comparison of foraminifera from site PC-6B was carried out. Using multiple cores from a single site permitted us to replicate the trends and establish whether compression or extension of the sediments occurred during the coring process. Comparing faunal records from multiple cores at different sites within Pocomoke Sound also provided a level of reproducibility and a confidence that observed faunal patterns are representative of the entire sound, rather than unique to a single site.

Previous Studies of Chesapeake Bay Foraminifers and Ostracodes

A large literature on the ecology of benthic foraminifers from Chesapeake Bay and its major tributaries provides the basic information for reconstructing ecosystem history from foraminiferal assemblages. Important prior studies include the papers by Robert Ellison of the University of Virginia and Maynard Nichols of Virginia Institute of Marine Science (Ellison and others, 1965; Ellison and Nichols, 1970; Ellison, 1972; Ellison and Nichols, 1976; Nichols and Norton, 1969; Ellison and others, 1986), and Martin A. Buzas of the Smithsonian Institution (Buzas, 1969, 1974). The ecology of Chesapeake Bay foraminiferal species is summarized in Table 5.2 (see Karlsen and others, 2000).

Although Chesapeake Bay ostracodes have received less attention than foraminifera, the classic study of ostracodes from the Patuxent River pier at Solomon Island, Maryland by Tressler and Smith (1948) was one of the earliest studies of ostracode seasonal ecology. Their study provides detailed seasonal ecology of some common species encountered throughout the bay region, and serves as a baseline faunal reference point for the lower Patuxent area prior to large-scale anthropogenic nutrient influx into the bay. Elliott and others (1966) also provided important ostracode data from the Rappahannock River estuary. More recently, Cronin and Vann (2003) examined ostracodes from a wide range of habitats throughout the bay region and applied species’ ecological data to the paleoecological record from sediment cores. Their summary of ostracode ecology is given in Table 5.3.

In addition to describing microfaunal trends from Pocomoke Sound cores, the current chapter compares the Pocomoke Sound foraminiferal and ostracode records to those from the mesohaline region of the mainstem bay described in Karlsen and others (2000), Cronin and others (2000), Cronin and Ishman (2000), and Cronin and Vann (2003). In doing so, it allows us to make generalized conclusions about bay-wide faunal and ecosystem trends during the past few centuries.

Results

Foraminifera

Figure 5.2 illustrates trends in benthic foraminiferal species in cores PC-2B-1, PC-3B-1, and PC-6B-1 for the upper ~200 cm of the Pocomoke sedimentary record. The major features of the foraminiferal record are as follows:

• Two-step increase in Ammonia, most evident at 120-130 cm and 40-50 cm in PC-2B-1, and in PC-3B.
• Increased Ammonia parkinsoniana coinciding with decreases in Elphidium.
• Increase in abundance of Ammobaculites, at about 20-30 cm core depth in all three cores; Ammobaculites is absent to sparse in the lower 150 cm of cores.
• Miliammina fusca, a species that inhabits lower marsh habitats, is absent except in low numbers ~ 120-150 cm core depth. This observation supports the conclusions of Chapter 3 that there has been minimal transport of foraminiferal, and other sand-sized material, from marshes bordering Pocomoke Sound to the central part of the sound over the past few centuries.

We also found several important foraminiferal events in the long core PC-6B-2. Most notable is the first stratigraphic appearance of Ammonia at 220 cm; Ammonia increases in abundance in the upper 150 cm at PC-6B-1. Increased dominance of Ammonia in foraminiferal assemblages from the late 20th century in other bay sites has been interpreted to signify environmental degradation related to oxygen depletion especially during the 20th century (Karlsen and others, 2000). Such also seems to be the case in Pocomoke Sound. The other marker horizon identified in PC-6B-2 is the last stratigraphic appearance of Buccella frigida at 240 cm; this species also occurred at 340, 350, 400, and 410 cm in this core. Based on the analysis of cores from the mainstem bay, Cronin and Ishman (2000) estimated that B. frigida became extinct in Chesapeake Bay about 500-600 years ago. With the exception of the single sample at 240 cm, the foraminiferal data and radiocarbon dating (Chapter 4) from PC-6B-2 suggest that B. frigida also became extinct in the Pocomoke region perhaps several hundred years after disappearing from the central and northern bay.

Ostracodes

Figure 5.3, 5.4 and 5.5 illustrate temporal trends in 13 ostracode species in the three cores using 10-cm spaced samples. Notable ostracode faunal events include the following:

• Disappearance of A. captionis near 100-120 cm
• Oscillations in the abundance of C. mexicana
• Increase, then decrease of C. curta near 110-120 cm in PC-2B-1, 150-80 cm in PC-3B1, and 60-70 cm in PC-6B-1
• Disappearance of C. newportensis at 150 cm in PC-2B-1 and 100 cm in PC-6B
• Increase in M. repexa in all three cores in the upper 50-60 cm
• Decrease in P. brachyforma from 20-30 % to sparse or absent in the upper ~100 cm.
• Overall decrease in ostracode species diversity during 20^th century

Based on the encouraging results of the 10-cm spaced ostracode samples from the three short cores taken May 2001 and the high sedimentation rate and excellent chronology for site PC-2B (Chapters 3, 6), we chose to investigate temporal trends in ostracode assemblages at this site in more detail. Figure 5.6 compares patterns for six species in the upper 200 cm of PC-2B-1 (2-cm spacing) and PC-2B-3 (10-cm) showing very similar patterns of faunal variability in the two cores demonstrating the lack of any major compression or extension of the short or long piston cores during the coring process.

Figure 5.7 illustrates the combined 485-cm record from cores PC-2B-1 and PC-2B-3 showing abundances of nine indicator species using the age model described in Chapter 6. The two dominant species, C. mexicana and P. brachyforma, exhibit an inverse relationship in terms of their relative frequencies over the past 250 years. C. mexicana dominates the Pocomoke Sound assemblages in the early 1800s, briefly around 1900, and again in the post-1930s period. This species is most common in polyhaline salinities in other Chesapeake Bay regions; P. brachyforma, which is dominant during three multi-decadal intervals centered around 1800, 1860 and 1920-30, is more common in mesohaline conditions of coastal bays and lagoons. Although salinity is most likely the primary factor causing the fluctuations of these two dominant species, P. brachyforma is also known to be a detrital feeder and decreased salinity caused by increased freshwater runoff would have been accompanied by increased input of fine particulate organic material from land-derived sources may have accompanied decreased salinity. The inflow of detrital material into Pocomoke Sound also probably increased during the 19^th century when extensive land clearance occurred in many parts of the Chesapeake watershed.

Secondary species inhabiting Pocomoke Sound prior to about 1950 include A. captionis, C. newportensis, Cytherura sp., L. nikraveshae and Loxoconcha sp. Many of these species disappear or decline in abundances in the early and mid-20^th century, representing a decline in overall benthic species diversity. Twentieth century decline in diversity was also found off the Patuxent River mouth (Cronin and Vann, 2003).

After about 1950, the species C. curta appears abundantly for the first time in Pocomoke Sound channel, and this event is followed by an increase in M. repexa abundance around 1970. These species, which are rare to absent in Chesapeake Bay sediment cores as far back as 2000 years ago, are both tolerant of reduced oxygen levels and high turbidity.

Discussion

Resolving the issue of how human activity influenced Pocomoke Sound water quality was a primary objective of this study. Orth and others (2002) attempted to establish the long-term relationships in Pocomoke Sound between water quality (chlorophyll a, dissolved nitrogen and phosphorous, Secchi depth and total suspended solids) and SAV using data from the Chesapeake Bay Institute (CBI) for the period 1947-1980s and the Chesapeake Bay Program (CBP) from 1984-2000. The lack of CBI data prior to 1970 made it difficult to evaluate SAV coverage prior to the SAV decline of the 1960s known from anecdotal evidence. However, they were able to make the general conclusion that water quality appears to be worse for the 1984-2000 CBP monitoring period.

How and when human activity affected the ecosystem of Pocomoke Sound in particular, and Chesapeake Bay in general, can be addressed by comparing the microfaunal and paleoecological record of Pocomoke Sound to records available from the mesohaline region of the mainstem bay. Figures 5.8 and 5.9 plot four ostracode and two foraminiferal species in such a comparison, drawing on the excellent sedimentary records from off the Patuxent River mouth and the north central bay channel off the Rhode River (Figure 5.1; RD/2209 and PTXT-2 core sites of Cronin and others, 2000; Karlsen and others, 2000; Cronin and Vann, 2003). For the PTXT-2 site, data from two gravity cores and one piston core are shown in Figure 5.8 for the important species C. curta and M. repexa. The Pocomoke Sound ostracode data are those from the PC-2B site; the foraminiferal data in Figure 5.9 are taken from PC-2B-1 and PC-6B-1 short cores.

Figure 5.8 shows that most major faunal changes that occurred in the mainstem bay during the mid-to late 20^th century also occurred in Pocomoke Sound, although they may have occurred slightly later at the PTXT-2 site. These changes include a progressive decrease and ultimate disappearance of C. newportensis, fluctuations and the ultimate disappearance of P. brachyforma, an increase in C. curta, and subsequent increase in M. repexa, a species found in the uppermost 5-10 cm of sediment from almost all sites cored in the bay. Other faunal changes, such as the disappearance of A. captionis around 1900, also occur in other regions of the bay.

Figure 5.9 shows the temporal trends in two indicator foraminiferal species (Ammonia parkinsoniana, Ammobaculites salsum) during the 20th century in two Pocomoke Sound cores (PC-2B-1, PC-6B-1) compared with trends observed in the central mainstem bay at core sites RD98/2209 and PTXT-2P5. Despite different core locations, water depths, salinity regimes, and other physical and chemical conditions, all sites show generally similar patterns in Ammonia abundance. There is a progressive (though not uniform) increase in its relative frequency starting in the in the 1940-50s, reaching its greatest abundance in the 1980s and later. Increasing abundance of Ammonia was accompanied by deformed shell morphologies in the mid-bay; both trends were attributed to greater hypoxia during the post-1960s interval (Karlsen and others, 2000). The difference in abundances from site to site can be accounted for by other environmental factors, such as salinity. For example, the lowest abundances of Ammonia occur at RD98/2209, which is located in the channel of the mainstem at 25 m water depth has a mean salinity of 20-22 ppt, in contrast to lower salinity at the other sites. Regional differences in the levels of oxygen depletion might also be a factor in the variable dominance of this species.

Ammobaculites is a dominant species that today inhabits organic-rich, fine-grained sediments along the margins of Chesapeake Bay in lower mesohaline salinities. It does not typically inhabit mesohaline and polyhaline environments of the open bay such as the regions where our sediment cores are located. In contrast to results for Ammonia, trends in Ammobaculites are characterized by rare, intermittent occurrences in Pocomoke Sound and the mainstem bay (Figure 5.9). It is possible that this species only inhabits the open waters of the bay during brief periods when organic-rich pulses of sediment are discharged into the bay due to local climatological and hydrological processes or land use change processes.

The timing of faunal changes observed in the mainstem and Pocomoke Sound paleoecological records (Figures 5.8 and 5.9) raises the important issue as to whether 20^th century ecosystem degradation was synchronous throughout the bay, or whether local land-use histories exerted a greater influence on adjacent bay habitats. Several processes that introduce uncertainty in the chronology of the sedimentary record, which leads to uncertainty in correlation among core sites and might explain small age discrepancies in the major faunal events. These processes include temporally variable sedimentation rates at a site, bioturbation and mixing of sediments by benthos, analytical error in radio-isotopic dating, different sampling intervals for each core, and low microfaunal abundance in some core intervals. These factors combine to produce a potential uncertainty of about 10-20 years for the upper 100-200 cm of sediment at most core sites where sedimentation rates are ~1 cm yr^-1. Pending further paleoecological and radioisotopic dating of cores from regions with high sedimentation rates, it can be stated nonetheless that trends in foraminiferal and ostracode species with well-known ecological requirements provide strong evidence that 20^th century land-use changes altered the functioning of the Chesapeake Bay ecosystem from its natural state across a wide range of salinities (~ < 10 to > 22 ppt). Many changes in benthic communities were nearly synchronous throughout the bay within the limits of the available dating, although different regions of Chesapeake Bay may have been affected more than others.

Conclusions

The data presented above allows the following conclusions regarding the ecosystem history of Pocomoke Sound.

1. The middle to late 20^th century saw unprecedented changes in the benthic assemblages of both ostracodes and foraminifera in Pocomoke Sound that can be attributed to degradation in water quality. The increased abundance of Ammonia parkinsoniana and Cytheromorpha curta in the 1940-50s, followed by further increases in Ammonia and increases in Ammobaculites and Megacythere repexa in the 1970-80s, signify a two-phase shift in the ecosystem. These changes represent perturbations to the natural variability in faunal assemblages, which are normally driven by climatically influenced changes in salinity regimes. Changes in late 20^th century benthic communities included the rise to dominance of facultative anaerobic and detrital feeding species tolerant of hypoxia, increased influx of organic matter, and increased turbidity.
2. As environmentally tolerant species became numerically dominant in the Pocomoke Sound and other mesohaline and polyhaline regions of the bay, several species that had inhabited the bay for millennia became extinct between 1900 to 1950 in many regions.
3. Comparison of the Pocomoke paleoecological record with those from elsewhere in Chesapeake Bay provide strong evidence that environmental degradation during the 20^th century was nearly synchronous bay-wide within the limits of sediment core chronology (10-20 years). The evidence available so far indicates that bay-wide changes occurred from the Rhode River region of the mainstem in the north, to Pocomoke Sound in the south. These results suggest that the impacts of 20^th century land-use changes were felt throughout the bay, regardless of local land-use history.
4. Although the current study did not focus specifically on epiphytal species that inhabit shallow water grasses such as Zostera, our results support the hypothesis of Orth and others (2002, see also Orth and Moore, 1983) that unprecedented changes to the bay ecosystem affected submerged aquatic vegetation in the Tangiers-Pocomoke region prior to large-scale monitoring began in the 1970s and 80s. It is highly likely environmental changes that affected the channel of Pocomoke Sound would have also had severe impacts on the adjacent shallow water regions of Pocomoke Sound. It is not clear whether water quality changes were due directly to land use change in the local Pocomoke watershed or whether they were connected to broader changes in the bay and its watershed and estuarine circulation.

Acknowledgements

We are grateful to Captain Rick Younger for his stewardship of the R/V Kerhin during coring, Jeffrey Halka and Carl Hobbs, III for guidance on Pocomoke Sound stratigraphy and sedimentation, Christopher Nytch, Cheryl Eberth, and Meredith Robertson for assistance with sample processing.

References

U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/2004/1350/chapters/chapter5.htm
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