Sedimentation in Chesapeake Bay
Sedimentation in Chesapeake Bay involves complex processes operating over different timescales with input from multiple sources consisting mainly of riverine, shoreline, in situ biogenic, and oceanic sources. Most researchers define sedimentation rate as the linear accumulation of sediment in centimeters per year (cm yr-1) and convert this rate into volumetric estimates of sediment flux, or mass accumulation rate (MAR), usually given in grams per square centimeter per year (g cm-2yr-1).
A variety of approaches have been used to study patterns and processes of bay sedimentation. These include the following methods used to study the geological record of sediment: geophysical surveys (determining Holocene rates of the past 7,500 years of estuarine sedimentation), bathymetric surveys (comparing 19th and 20th century bathymetry), short-lived radioisotopes (137Cs, 210Pb) and other chemical markers (useful for the past century), and pollen stratigraphy (mainly Ambrosia, ragweed pollen) correlated with land-use changes (documenting post-colonization – land clearance rates). Monitoring methods not included in the present summary include instrumental measurement of total suspended solids in water samples and satellite imagery and remote sensing. Radiocarbon (14C) dating has been used by some authors to estimate sedimentation rates in Chesapeake Bay. However, measured radiocarbon ages are a function of complex processes including cosmogenic radiocarbon production in the atmosphere, carbon cycling in the hydrosphere and biosphere, and the potential uptake of reservoir (“old” carbon) carbon into different types of carbonaceous material (i.e., wood, peat, total organic carbon, shells). Radiocarbon ages that have not been calibrated to global carbon cycle can provide inaccurate ages and, in the case of many ages published for Chesapeake Bay, lead to erroneous estimates of sedimentation rates. Consequently, “historical” rates of sediment flux (i.e., sediment that has accumulated over the past few centuries) estimated using other dating methods are the focus of part 1 of this section. Pre-historical rates are evaluated in part 2 where calibrated radiocarbon shell dates allow comparison between “natural” pre-land clearance sedimentation and historical rates.
In addition to primary sources of sediment derived from riverine input, coastal erosion, and ocean input, biogenic sediment is an important component of the total sediment budget in many parts of the bay and its tributaries. Biogenic sediment is usually discussed in the literature dealing with nutrient flux and cycling and, in some cases, researchers have provided estimates of particulate matter derived from biogenic processes in the context of measurements of total suspended solids (TSS). Despite the importance of biogenic sediment to water quality and sedimentation, these data are not usually taken into account in comparisons of sediment rates and fluxes based on sediment cores and surveys.
Although Chesapeake Bay is one of the most well-studied estuaries in the world, there are still many unknown aspects to sedimentation in estuaries in general, and Chesapeake Bay in particular. Nonetheless, three broad, well-supported conclusions can be drawn from the many studies carried out over the past 50 years. First, sedimentation rates are relatively high – on the order of 0.1 to 1 cm yr-1 -- compared to other aquatic environments such as most lakes, deep sea habitats, and bays and estuaries. Since the bay was flooded by sea level rise about 8,000-7,500 years ago, as much as 25-30 meters of sediment have accumulated in the main channel, and thick accumulations of sediment also characterize the bay’s marshes. High sedimentation rates in the channel reflect in part the sediment trapping capability of partially mixed estuaries.
Second, it is also clear from the literature that sedimentation rates vary widely depending on the region. For example, sedimentation rates can easily vary five- to ten-fold (0.1 to > 1.0 cm yr-1) over both small and large spatial scales. Spatial variability is especially evident throughout the main stem of the mid-bay and locally in small sub-estuaries such as the Rhode River and tributaries entering the Potomac.
Third, independent researchers using different methods have produced generally similar quantitative estimates of sediment rate and flux, regardless of the scope of the study. This fact indicates that there is a high degree of confidence that the estimates of sedimentation rates are relatively accurate.
Hypotheses to explain sediment accumulation
Two major hypotheses about the patterns of sedimentation in the Chesapeake Bay estuary have emerged over the past few decades and, because these are so pervasive in the bay community, it is useful to summarize their basic tenets and mention some uncertainties surrounding them.
The northern main stem bay and larger tidal tributaries have an Estuarine Turbidity Maximum (ETM) zone – a region where fine-grained particulate material is “trapped”, deposited, and, sometimes resuspended and redeposited. The nature of the ETM is a function of complex physical (freshwater inflow, tidal and wave-driven currents, gravitational circulation), particle flocculation, and biogeochemical processes that influence when and where sediment is deposited. It is generally believed that the majority of fine-grained river-borne sediment is trapped in ETMs and only escapes the upper reaches of the northern bay (or upper parts of tributaries) during extreme hydrological events. Most studies of the ETM have focused on relatively short timescales (sub-annual - interannual).
A related hypothesis about sedimentation can be referred to as the tributary “import-export” hypothesis. This hypothesis holds that there is a net import of sediment from the main stem bay into larger tidal tributaries except during extreme high flow events when some sediment is exported from tributaries to the main stem.
Whereas the ETM and import-export hypotheses explain many aspects of observed suspended material in the bay and its tributaries, it should be noted that other studies have found conflicting results. For example, the ETM hypothesis does not account for certain geochemical tracer data indicating that sediment has been transported over longer timescales from the northern bay at least to the mid-bay (Darby 1990, Helz et al., 2000). These data suggest that some sediment “escapes” the ETM, although the conditions causing this to occur are not yet known.
Similarly, the tributary import-export hypothesis is an idea that has not been tested in detailed field studies to the extent that one can derive quantitative estimates of import-export for each tributary. Moreover, the idea appears to be contradicted by satellite data showing export of suspended material from tributaries into the bay during the relatively wet periods. Another issue pertains to sedimentation in the mid-bay, a region where input of suspended material from the north (mostly Susquehanna River inflow) and ocean-source sediment from the south are believed to contribute relatively small proportions of the total sediment flux. If northern and southern sources are minor in the mid-bay, then it is difficult to account for the thick accumulation of sediment in parts of the main stem of mid-bay, even with large contributions from shoreline erosion.
These uncertainties indicate that further research is needed aimed at quantifying the relative contributions of various sources of sediment at the lower reaches of tributaries and the mid-bay region.
Land Use and Legacy Sediments
A second major hypothesis about sedimentation in Chesapeake Bay pertains to the impact of large-scale deforestation from agriculture and timber production on sediment flux to the bay. Several studies have concluded that sedimentation rates and fluxes have increased from between 3- to 10-fold as a result of extensive 18th and 19th century land clearance. It has been suggested that this human-induced historical increase in sediment flux is manifested in the high rates of sedimentation measured in the upper parts of tributaries where most of the eroded sediment is deposited. This hypothesis is often expressed in the idea of “legacy” sediment – a concept derived largely from studies of fluvial systems. Applied to Chesapeake Bay, it holds that sediment originating from cleared lands has been trapped in transit in rivers and the upper parts of tributaries, and has not yet reached the lower reaches of tidal tributaries or the Chesapeake Bay itself.
Although land clearance in the Chesapeake Bay watershed has no doubt led to large-scale erosion, it is still not clear what proportion of sediment eroded during colonial land clearance has been trapped and how much has been transported to the lower tidal reaches of tributaries or to the main stem. There is good paleoecological evidence from the mid-bay for historical degradation in phytoplankton communities (diatoms in Cooper and Brush 1993, dinoflagellates in Willard et al., 2003). However some of phytoplankton communities are strongly influenced by eutrophication and the effects of increased turbidity are unclear. Except for a few studies of the paleoecology of the main stem bay, field studies of sedimentation during the past few centuries have focused on relatively small sub-estuaries and include little or no data on sediment accumulation downstream in more distal regions. Moreover, published pre- colonial rates comparisons are based on poorly dated Holocene sediments and should not be used for evaluating temporal variability in rates. Consequently, there is a great deal of uncertainty about the impact of land clearance on diminished water clarity and bay faunas and floras.
Comparison of pre- and post-colonial sedimentation rates in middle Chesapeake Bay
Analyses comparing pre-colonial “natural” and historical (since 1880) sedimentation flux calculated from sediment cores obtained on cruises have been carried out between 1996 and 2002, as part of a larger ongoing study of sedimentation in Chesapeake Bay by the U. S. Geological Survey. The results show that historical sedimentation rates vary by about an order of magnitude throughout the bay area (from < 0.1 to 0.8 g cm-2 yr-1). Some of the highest rates are found in Pocomoke Sound, the northern main channel, and off the mouth of the Little Choptank River; some of the lowest were in the Potomac River tributary.
At some sites there was a ~4-fold greater sediment flux during the last century than during the prior 1000 years. These results confirm the general conclusions of other studies of sediment cores for the mid-bay main stem. Somewhat surprisingly, however, at many sites the historical rates are roughly equal to or exceeded pre-historical (1000-1880 AD) rates. Ongoing research is designed to evaluate the factors influencing prehistorical and historical sedimentation rates in the bay.
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