Grace S. Brush

Down by the riverside: urban riparian ecology

Riparian areas are hotspots of interactions between plants, soil, water, microbes, and people. While urban land use change has been shown to have dramatic effects on watershed hydrology, there has been surprisingly little analysis of its effects on riparian areas. Here we examine the ecology of urban riparian zones, focusing on work done in the Baltimore Ecosystem Study, a component of the US National Science Foundation’s Long Term Ecological Research network. Research in the Baltimore study has addressed how changes in hydrology associated with urbanization create riparian “hydrologic drought” by lowering water tables, which in turn alters soil, vegetation, and microbial processes. We analyze the nature of past and current human interactions with riparian ecosystems, and review other urban ecosystem studies to show how our observations mirror those in other cities.

Long-Term History of Chesapeake Bay Anoxia

Stratigraphic records from four sediment cores collected along a transect across the Chesapeake Bay near the mouth of the Choptank River were used to reconstruct a 2000-year history of anoxia and eutrophication in the Chesapeake Bay. Variations in pollen, diatoms, concentration of organic carbon, nitrogen, sulfur, acid-soluble iron, and an estimate of the degree of pyritization of iron indicate that sedimentation rates, anoxic conditions and eutrophication have increased in the Chesapeake Bay since the time of European settlement.

A 2,500-Year History of Anoxia and Eutrophication in Chesapeake Bay

Ongoing monitoring programs and historical data are not sufficient to establish anthropogenic effects on the ecology of Chesapeake Bay. However, stratigraphic records preserved in the sediments can be used to reconstruct both prehistoric and historic sedimentation and water conditions of the bay, including anoxia and eutrophication. Pollen, diatoms, total organic carbon (TOC), nitrogen, total sulfur, and an estimate of the degree of pyritization of iron (DOP) are being used as paleoecological indicators in dated sediment cores for the purpose of reconstructing a long-term environmental history of the bay. Analysis of the data indicates that sedimentation rates, anoxic conditions, and eutrophication have increased in the Chesapeake Bay since the time of European settlement. For example, since initial land clearance around 1760, sedimentation rates have increased from as low as 0.02 cm yr−1 to an average 0.22 cm yr−1, and TOC from 0.14 mg cm−2 yr−1 to a high 4.96 mg cm−2 yr−1. Diatom community structure shows a steady decrease in overall diversity since 1760 and the centric:pennate ratio has increased significantly since 1940.

The natural forests of Maryland: an explanation of the vegetation map of Maryland.

The forests of Maryland have been mapped at a scale of 1:250,000 on the basis of 15 regional associations: (1) tamarack, (2) bald cypress, (3) hemlock-yellow birch-black birch, (4) river birch-sycamore, (5) sycamore-green ash-box elder-silver maple, (6) shingle oak, (7) chestnut oak-post oak-blackjack oak, (8) chestnut oak-bear oak, (9) chestnut oak, (10) loblolly pine, (11) basket oak-lobolly pine, (12) willow oak-loblolly pine, (13) basket oak, (14) sugar maple-basswood, and (15) tulip poplar. The associations differ from each other in species composition and in abundances of species common to many associations. Each was identified in the field by the presence of relatively few common discontinuous tree species referred to as characteristic species. Correlations between forest associations and geologic, topographic, and soils units mapped at a similar scale suggest that patterns of available water are important in controlling distribution of woody species throughout Maryland.

Comparisons of 210Pb and pollen methods for determining rates of estuarine sediment accumulation

Abstract Comparisons of sedimentation rates obtained by 210 Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods. Most of the discrepancy between the methods can be explained by the analytical precision of the 210 Pb method and by the exactness with which time horizons can be identified and dated for the pollen method. X-radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and/or mixing of sediments. Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., 210 Pb trends and/or “possible” horizon assignments). Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth. The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments. Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon.

Sedimentation Cycles in a River-Mouth Tidal Freshwater Marsh

Tidal freshwater marshes are critical buffers that exist at the interface between watersheds and estuaries. Little is known about the physical dynamics of tidal freshwater marsh evolution. Over a 21-mo period, July 1995 to March 1997, measurements were made of biweekly sediment deposition at 23 locations in a 3.8-ha tidal freshwater marsh in the Bush River subestuary of the upper Chesapeake Bay. Biweekly accumulation showed high spatial and temporal variability, ranging from −0.28 g cm−2 to 1.15 g cm−2. Spatial variability is accounted for by habitat differences including plant associations, elevation, and hydrology. Temporal variability is accounted for by interannual climate variability, the growth cycles of marsh plants, stream-marsh interactions, forest-marsh interactions, and animal activity.

Tidal wetland record of holocene sea-level movements and climate history

Abstract Paleontological, geochemical, and lithological indicators of former marine and nonmarine influence are correlated between 13 cores from a tidal wetland (Wolfe Glade) on the southeast coast of Delaware Bay. Twenty new radiocarbon dates are used to establish the chronostratigraphy of marsh facies. Sediment iron content, weight loss on ignition, and remains of plants, diatoms and foraminifera (known to tolerate specific tidal inundation cycles) are used to describe cored facies originating from supratidal, intertidal and subtidal salt marsh subenvironments. Pollen assemblages are used to infer local Holocene climate fluctuations and to reconstruct paleoenvironments during marsh evolution. The record of paleoclimatology suggests the Atlantic Chron was warm with variable humidity in the region, whereas the Subboreal was dominated by cool and humid conditions. The Subatlantic was warmer, and initially dryer, with a cool humid phase prior to about 1 ka, returning to warmer, humid conditions in the last several centuries. Local relative sea-level movements are characterized by five rapid, short-term episodes when the rate of sea-level rise accelerated relative to the rate of marsh aggradation. These episodes are recorded by transgressive facies contacts at 5.3 ± 0.2 ka, 4.4 ± 0.2 ka, 3.25 ± 0.2 ka, and 1.8 ± 0.2 ka, sidereal years. An earlier transgressive facies contact dated 6.9 ± 0.2 ka is probably not the product of a true sea-level movement. Sea-level movements are recorded throughout the marsh as palustrine or high marsh peats or peaty overlain by lower intertidal or subtidal marine deposits. Several features suggest that these episodes are local relative transgressions produced by short-term accelerations in the rate of sea-level rise relative to marsh aggradation: the contemporaneity and apparent suddenness of marine inundations: the sequence of facies indicating marine drowning; the presence of similar events in marshes elsewhere in Delaware Bay; and the marsh-wide extent of indicative facies transitions. We propose that the rapid and frequent sea-level movements observed in Wolfe Glade are the result of surges and relaxations in the Gulf Stream (and associated spin-up and partial collapse of the North Atlantic gyre) in response to winds generated by changes in the North Atlantic atmospheric thermal gradient associated with Holocene climate fluctuations.

Patterns of recent sediment accumulation in Chesapeake Bay (Virginia—Maryland, U.S.A.) tributaries☆

Abstract Sediment accumulation rates obtained from 39 cores in 10 tributaries of the western shore of Chesapeake Bay average 0.30 cm yr.−1 since European settlement, compared with pre-settlement rates in three tributaries of 0.14 cm yr.−1. Drainage areas of six of the tributaries extend from the Coastal Plain into the Piedmont and four are restricted to the Coastal Plain. Rates are highly variable, but higher in the upper parts of the tributaries and lower at the mouths. During the period of early agriculture, when Pre-settlement rates are based on unpublished reports of 14C dates, ranging from 670 to 5330 yr. B.P., of peat layers from three western shore tributaries where sediment deposition is believed to have been continuous. Post-settlement rates are average rates calculated between a maximum of five pollen horizons. The pollen horizons represent historically documented changes in the regional vegetation and include initial land clearance (1634, 1650, 1720 and 1730, depending on the tributary), shift to intensive agriculture (1780 and 1840), beginning of the chestnut blight (1910), demise of chestnut (1930) and the beginning of large-scale urbanization (1960).

Nutrient and Metal Accumulation in a Freshwater Tidal Marsh

Stratigraphic records from sediment cores collected in a freshwater tidal marsh and in the estuary upstream and downstream from the marsh were used to determine the accumulation of nutrients and trace metals over long time periods. Analysis of pollen and seeds show that the high marsh has formed only within the past 100 yr, following increased sedimentation rates in the area. Variations in nutrient and trace metal accumulations over several decades show that pollutants from agricultural runoff and wastewater discharge are stored to a greater extent in high-marsh than in low-marsh sediments. Greater accumulation rates in the high marsh are probably related to its greater sedimentary organic carbon concentration.

Stratigraphic evidence of human disturbance in an estuary

Abstract Prior to European settlement, oligohaline and mesohaline sections of Chesapeake Bay draining Piedmont saprolite supported diverse and abundant diatom and macrophyte populations. Compositional changes in diatoms and macrophytes in oligohaline sections correspond with 17th- and 19th-century deforestation and increased siltation, while effects on downstream populations were less notable. After deforestation, previously sparse diatom populations in a mesohaline estuary draining sandy Coastal Plain soils became more abundant. Fertilization of cultivated land was accompanied by increased production of both attached and free-floating diatoms. After the discharge of sewage, diatom populations increased enormously in the affected areas, followed by a dramatic decrease. The decrease suggests silica limitation after intense phosphorus enrichment. The loss of macrophytes and increase in planktonic diatoms in oligohaline areas in recent years resemble the historical sequences observed in lakes undergoing eutrophication. However, in the estuary, similar declines have also occurred in macrophyte populations in mesohaline areas where eutrophication is much less severe, but where chlorine and herbicide toxicity during the past 20 yr is similar to upstream areas.