Sherri R. Cooper
Duke University Wetland Center
Nicholas School of the Environment, Durham, NC

In this two year study of sediment cores collected from the Pamlico and Neuse River estuaries, the researcher has begun to recreate the history of water quality in these estuaries by dating sediment core samples and analyzing indicators of water quality, nutrient and trace metal flux, diatom assemblages and pollen through time.

Sedimentation rates have generally increased three to 10-fold in the past 50 years over previous sedimentation rates, and metal and sulfur flux to the sediments has increased over the past 50 years. Surface sediments often contain heavy metal concentrations that exceed "Threshold Effects Levels" (TEL) as reported by the U. S. Environmental Protection Agency. Cadmium shows highest levels in the Pamlico estuary at the core collection site nearest the phosphate mining operations. Most other metals show higher concentrations in the Neuse River estuary.

Diatom and pollen assemblages have changed through time. The most dramatic assemblage changes in the diatoms appear to have occurred in the past 30-50 years in the Pamlico and Neuse estuaries, possibly associated with industrial activity, increasing population, and land-use changes. Recent assemblages are composed of higher abundances of small planktonic taxa that are often found in large blooms in higher nutrient waters. These samples exhibit relatively low species richness and diversity compared to older (pre-1950) samples. Older diatom assemblages are composed of more benthic and epiphytic taxa. Changes likely reflect eutrophication, increased turbidity and sedimentation, and increased freshwater flow to the estuaries, as well as an increase in industrial activities. They may also reflect declines in submerged aquatic vegetation in these estuaries. 

Pollen assemblage changes include an increase in ragweed pollen over the past several hundred years, signifying increased land disturbance by humans. Pollen count results also show an increase in nut tree pollen (walnut and pecan) over the last several hundred years and an increase in sweetgum tree pollen in the past 50 years.
The biogenic silica (BSi) results and the determination of diatom valve flux to the sediments both show that more diatom frustules are being deposited to the sediments in recent years. These results indicate higher diatom production, most likely due to increased nutrient inputs to the estuaries. As production increases, dissolved silica in the waters may become limiting, especially if diatoms frustules are preserved in the sediments and not recycled. As silica becomes limiting in the water column, diatoms may be outcompeted by other algal species, including dinoflagellates. Diatoms are generally better food sources in the estuarine food web than other algal species, so this change could potentially cause problems in higher trophic levels.

The researcher points out that the time frame of water quality changes seen in the Pamlico and Neuse estuaries occurs more recently than similar changes in the Chesapeake Bay. She says that these differences may be due to several factors, including slower population growth, different land use near the estuaries, and the geomorphology of the estuaries. She also says that these conditions may indicate that proper management could reverse trends in water quality changes more quickly for these estuaries than for the Chesapeake Bay.