Ecological History of the Chesapeake Bay
PSLA Seminar Series:
Dr. Grace Brush and the Ecological History of the Chesapeake Bay
Dr. Grace Brush presented her nearly four decades of research concerning the ecological changes seen in the Chesapeake Bay estuary over the last several hundred years, with a focus on the effects of early colonizing Europeans beginning in the 18th century.
Dr. Brush began with an explanation of the eminence of the Chesapeake Bay and the watershed that feeds it. She explained how the vast and highly ecologically variable watershed has a significant impact on the relatively shallow bay.
Dr. Brush next presented a question that had spurred the study: what had happened to the submerged aquatic plants that once flourished? An answer lay in the core sampling of various locations in and around the Chesapeake. Two of these core samples were subjected to significant analysis and gave 1,500 and 2000-year representations of their respective sampled location.
Dr. Brush and her colleagues implemented two methods to accurately date incremental depths along the core: Carbon-14 and lead-210 dating, and identification of relevant plant pollen as a time marker. To establish pollen as an effective marker, Dr. Brush and colleagues developed an apparatus to determine the fall velocity of ragweed, oak, willow, hickory, and chestnut pollen grains. The group concluded that transport behavior of both pollen and silt is nearly equivalent. The group also studied pollen movement in a local waterway to accurately estimate sedimentation rates.
Dr. Brush next discussed major findings. Firstly, sedimentation rates were found to coincide with increasing agricultural intensity seen after colonization. Significant salinity changes were noted by observing the distribution of foraminifera, protists with acute sensitivity to environmental changes, within the cores. An overall increase of total organic nitrogen and carbon was observed consistent with a known rise in eutrophication throughout the bay. Total “dry” herbs were found to majorly replace “wet” herbs beginning in 1800; Brush suggested this change was likely linked to the rise in hard surfaces from agricultural land, compounded by a declining beaver population.
Returning to the initial research question, Dr. Brush found that submerged aquatic vegetation had in fact declined since colonization: numerous local extinctions of submerged macrophytes were observed. Similarly, Dr. Brush and her colleagues surmised that the bay had seen an overall shift from a benthic to planktonic system, consistent with a loss in bottom-feeders and specific diatom species.
Dr. Brush and her colleagues were able to conclude that deforestation and a spike in agricultural land throughout the Chesapeake Bay watershed had a direct and significant impact on the ecosystem in numerous respects.
Edited by Alex Mahlandt
