Our results indicate increased N loading to Chesapeake Bay started as early as the 19th century and substantially increased to the present. Time-series comparisons of the sampled shells display distinct trends, with both %N and δ15N values following a roughly exponential curve with higher concentrations of N and more 15N enriched shells occurring in the 19th century and modern samples. The %N remains more constant than the δ15N values, but increases ~0.07% from the 18th century to modern shells (Fig. 3). The δ15N values remain relatively constant from the Early Woodland until the first part of the late 18th to 19th century (i.e., 1750–1800) but then increase ~3‰ from the late 18th century to mid late 19th century (i.e., 1850–1900) (Fig. 2). There was another substantial increase in δ15N values (~1‰) between the mid to late 19th century shells and the modern collected shells, which is correlated to a 10-fold increase in reactive N species globally from 1860 to 199035.

δ15N values in mollusk shells have been shown to faithfully track the values in soft tissues and ambient POM26,27,36. In contrast, variation in mollusk shell %N has not been validated as a meaningful proxy for ambient N concentrations and additional analysis of modern samples is required before interpretation of such data is possible. Although the relatively constant %N suggests little to no N loss from the shells over time, our discussion will focus solely on δ15N values.

We interpret this substantial increase in δ15N values to be increased anthropogenic inputs into Chesapeake Bay during the 1800s. During the Woodland Period, it is unlikely that Native Americans37 had any significant impact on the N loading of the bay. While there was urban development during the 18th century, the impacts were likely more local in scale, as population levels were comparatively low in much of the region.

Our results are similar to several sediment nutrient loading studies in Chesapeake Bay over the last ~3000 years, with the exception of when δ15N values began to rapidly increase within the Bay38. Bratton et al.35 observed significant increases in δ15N values from 1750–1800 AD, whereas we determined a later time period of 15N enrichment. This is likely due to sampling location, however, since Bratton et al.’s study locations were closer to the Susquehanna River, which is a larger source of N for Chesapeake Bay35 than our study location. Zimmerman and Canuel38 also show increases in δ15N values beginning between the 18th and 19th centuries depending on study location within Chesapeake Bay, with their more northerly collected sediment core showing 15N enrichment ~100 years before their southern core. A study location farther north in the bay would also not have as long of a period to mix with estuarine waters before deposition, so it could potentially record higher δ15N values at an earlier time period. Another possibility for the timing differences between this study and Bratton et al.35 is due to the dating methods used. While our study solely used 14C from corrected and calibrated archaeological oysters21, Bratton et al.35 used pollen stratigraphy, 14C of shell material within the cores, and the short-lived radioisotopes 137Cs and 210Pb within recently deposited sediment4. Since pollen was primarily used to relatively date the sediment, it is possible that the absolute dates obtained by their 14C analysis could permit larger error in the dating due to reworking and bioturbation of the sediments.

During the 19th century, population sizes and industrialization in the northeast US expanded exponentially39, which likely had a profound impact on the ecological health of the Bay. Between 1830 and 1880, over 80% of the forest surrounding Chesapeake Bay had been cleared, which, in combination with plowing for agriculture, greatly increased the sediment accumulation in the bay4,5,20,35,39,40,41,42. During this time, the population size nearby Chesapeake Bay increased threefold39, and the increased amounts of sewage discharge and erosion caused by plowing likely increased the N inputs and 15N enrichment in the Bay24. The significant decrease in oyster populations at this time also contributed to the failing health of the Bay due to decreased filtration rates of the bay water39 and increased sedimentation rates which correlates to previous findings of decreased diatom diversity and increase in the centric:pennate diatiom ratio since the beginning of the 19th century4. Therefore, it is likely that the POM and sediment of the bay were more enriched in 15N. Consequently, we would expect a substantial shift in δ15N values during the 19th century due to increased amounts of sewage and eroded soil entering the bay, which was evident in the results of our shell study.

A similar pattern was present in size changes in archaeological oysters. Archaeological size data from throughout the Chesapeake Bay indicate a significant increase in oyster size during the Historic period (18th to 19th centures) when compared to earlier Woodland times34,37. Research at sites on the Potomac and Patuxent rivers documents this trend in detail, with oysters first increasing in size during the 18th and early 19th centuries and then decreasing dramatically after AD 1860, perhaps from intensive eutrophication34. It has been suggested that the degree of organic matter loading into Chesapeake Bay has significantly increased within the last two centuries38,43,44, however, Cornwell et al. did not determine substantial increases in N deposition within the sediment, which could be due to increased algal blooms in the water column38,44 and metabolic activity of estuarine organisms43. Our data support this hypothesis, providing the first direct evidence of N loading in this location of the bay during the 19th century, particularly in the latter half of the 19th century.

The potential effects of diagenetic alteration of %N and δ15N values must be considered. Loss of shell organic matter, and consequently N, likely occurs after burial and may be evident in the %N trends noted in our study. We argue, however, that such loss would not explain the observed δ15N trends. First, the observed trend in δ15N values would require preferential loss of 15N, which is contradictory to the kinetics of stable isotopes. Secondly, it is difficult to imagine why diagenetic influence of N isotopes would stop in pre-seventeenth century shells, and even reverse to some extent in the earliest samples. Third, Black28 was not able to influence shell δ15N values during a series of prehistoric cooking methods studies unless the shell was heated to the point at which it began to physically disintegrate. Finally, the close correspondence between our data and the sediment core-based proxies of anthropogenic N noted above indicates that the shell data trends accurately follow levels of environmental N. We argue that the trends in δ15N values noted in this study are primarily the result of changes in the sources contributing to the oysters’ habitat. The source for the somewhat higher δ15N values in the Early Woodland samples is not readily apparent.

This study demonstrates the potential utility of shell δ15N proxies. The abundance of this and similar species in archaeological sites worldwide suggests that detailed records of anthropogenic N impacts can be created from shell midden deposits. Such baseline data are valuable not only in addressing modern pollution concerns, but also in assessing the extent of ancient human habitation, land-use, agricultural practice, and related activities. This approach may be less expensive than sediment core analysis and therefore permit wide geographic coverage to facilitate regional N reconstructions. Additionally, archaeological contexts may allow reliable age control of samples, which is sometimes difficult in cores with disturbed sediments. Sclerochronological shell sampling also permits seasonal analysis of δ15N values in large, fast growing species27, which provides even more detailed information. Further research is needed to better assess the validity of shell %N data, the impacts of diagenesis and prehistoric cooking methods, and the possible transport of shells before burial.

For the δ15N values in C. viriginica shells from Chesapeake Bay, there are significant changes in δ15N values of the shells over time. The δ15N values remain relatively constant from ~3,200 years ago until 1750–1800, but significantly increase after 1850 to the live-collected shells. While there is a possibility that diagenetic processes may have influenced these data, the sharp increase in δ15N values between ~1750–1800 and 1850-1900 appears to be primarily due to the industrial revolution, animal wastes, and soil erosion. The subsequent increases from 1850 to 2013 are due to a significant increase in human population, synthetic fertilizer use, sewage discharge, erosion, and other anthropogenic pollution sources in Chesapeake Bay over the last two centuries. Different N pollutants have varying δ15N values (i.e., animal wastes range from ~10–20‰ and synthetic fertilizer ~0‰), which explains the large increase in δ15N values between ~1750–1800 and 1850–1900 and the smaller increase from ~1850–1900 to modern samples. Between ~1750–1800 and 1850–1900, the main N pollutants in Chesapeake Bay were isotopically enriched, leading to a significant increase in δ15N values from earlier periods. The widespread use of synthetic fertilizers in the 20th century likely caused the reduced increase in δ15N values between ~1850–1900 to modern samples since synthetic fertilizer has more depleted δ15N values than earlier sources while increasing the %N in the Bay.