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

Abstract

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 × 108 L effluent/d to the Neuse system, much of which came from municipal wastewater treatment plants (2.03 × 108 L effluent/d, excluding periods of plant malfunctions; total annual loadings of at least 9 × 105 kg P and 2.1 × 106 kg 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 × 106 animals, from a 285% increase in the past decade) and 23% in poultry (5.5 × 105 animals). An estimated 5.9 × 109 kg manure produced by swine and poultry during 1998 contributed ∼4.1 × 107 kg N and 1.4 × 107 kg 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 × 107 kg N and 1.6 × 107 kg 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 decrease in phytoplankton biomass (as chlorophyll a) that likely reflected displacement/washout of algal populations and cysts. Thus, while both N and P supplies have increased in the watershed, there is evidence for a significant increase in Ni loading but, as yet, no apparent signal for increased P in the lower estuary. Weather patterns ultimately control when/whether the elevated Ni supply will support increased algal production, so that estuarine algal blooms, hypoxia, and fish kills will remain difficult, at best, to predict in modeling efforts. We recommend that decadal data sets, with sufficient sampling frequency to capture nutrient loadings from major storm events, be used to assess fluctuations in algal production of lower rivers and estuaries, and relationships with changing nutrient inputs. Given increased N and P supplies in the Neuse watershed from ongoing growth of both human and swine populations, a current management goal of 30% N reduction should be altered to include increased focus on Ni and strengthened comanagement of P. As for estuaries in other regions, nutrient reduction goals should be interpreted as “moving targets” that likely will have to be substantially adjusted upward, over time, to accomplish noticeable reductions in algal blooms, hypoxia, and fish kills in the lower Neuse River and Estuary.