Water Quality Trends and Management Implications from a Five-Year Study of a Eutrophic Estuary

• Published in: Ecological applications 2000-08, Vol.10 (4), p.1024-1046
• Main Authors: Glasgow, Howard B., Burkholder, JoAnn M.
• Format: Article
• Language: English
• Subjects: Albemarle-Pamlico ecosystem, North Carolina; algal bloom; Brackish; chlorophyll; Estuaries; estuary; fish kills; Freshwater fishes; Hypoxia; Invited Feature: Managing the Land-Water Interface; Neuse River and Estuary; Nitrates; nitrogen; phosphorus; Phytoplankton; Point sources; Pollution load; river; Surface water; suspended solids; toxic Pfiesteria complex; USA; USA, North Carolina, Neuse Estuary; Water quality; Watersheds
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/1051-0761(2000)010[1024:WQTAMI]2.0.CO;2

The Neuse River and Estuary, a major tributary of the second largest estuary on the United States mainland, historically has sustained excessive blooms of algae and toxic dinoflagellates, hypoxia, and fish kills. Previous attempts have been made to use short-term databases of 2-3 years, or data sets from infrequent (monthly) sampling, to assess whether nutrient inputs to the Neuse are increasing and supporting higher algal production. These previous efforts also have relied on single-point-determined flow velocity data, at upstream sites remote from the estuary, to estimate the volume of flow in quantifying nutrient loading to the estuary. We completed a five-year study of the Neuse, including a comparative inventory of nutrients to the watershed from point sources and from concentrated animal operations (CAOs) as recent nonpoint sources, as well as an intensive assessment of water quality over time in the mesohaline estuary. Estimates of nutrient loads were based on volume of flow data from shore-to-shore transect cross sections, taken with a boat-mounted acoustic Doppler current profiler at the westernmost edge of the estuary. A total of 441 point dischargers contributed at least 3.34 x 108L effluent/d to the Neuse system, much of which came from municipal wastewater treatment plants (2.03 x 108L effluent/d, excluding periods of plant malfunctions; total annual loadings of at least 9 x 105kg P and 2.1 x 106kg N, with a 17% increase in human population over the past decade). The Neuse basin also included 554 CAOs, with 76% in swine production (1.7 x 106animals, from a 285% increase in the past decade) and 23% in poultry (5.5 x 105animals). An estimated 5.9 x 109kg manure produced by swine and poultry during 1998 contributed ∼4.1 x 107kg N and 1.4 x 107kg P to the Neuse watershed. About 20% of the area in the watershed now has enough manure from CAOs to exceed the P requirements of all nonlegume crops and forages. About two-thirds of the N- and P-rich feeds for these animals are imported (with 4.0 x 107kg N and 1.6 x 107kg P in 1998); thus, the watershed increasingly has become a nutrient sink. Over the five-year study in the Neuse Estuary study area, P loading significantly declined (by an estimated 14%), whereas TN (total nitrogen) loading significantly increased (by an average of 16%) and TNi(total inorganic nitrogen) increased by ∼38%. The increased inorganic N (Ni), partly related to severe storms with high precipitation in years 4-5, coincided with a