Neuse River Estuary Water Quality and Fish Kills Advisory for Summer/Fall 1998
The rivers and estuaries of coastal North Carolina have experienced unprecedented increases in nutrient loading over the last 50 years. Changes in watershed land use, increased human population and exponential growth of intensive animal operations have all contributed to nutrient over-enrichment. A proliferation of water quality problems have resulted in our rivers and estuaries, including nuisance algal blooms, anoxic water and fish kills.
While there are currently only limited data on which to make conclusions, it appears that severe water quality problems in the Neuse River Estuary originate with excessive winter and spring rains and are exacerbated by above average rains during the summer and early fall (Paerl et al., 1998). The rains deliver large pulses of nutrient-rich discharge to the estuary and algae "bloom" rapidly in response to these nutrients.
One mechanism for fish kills occurs when the algal blooms die and sink to the bottom where degradation and decomposition consume oxygen at a high rate. Fresh water runoff coupled with hot and calm weather encourages stagnation and stratification (fresh water floating downstream over the top of saltier water). The stratification slows the rate of oxygen replenishment from the atmosphere. If these conditions persist for long periods of time (days to weeks), the water column, particularly the bottom waters, become oxygen depleted. Waters with low or no dissolved oxygen are know as "dead waters" because they are usually devoid of fin fish and shellfish. Large dead water zones can overlap directly with critical fin fish and shellfish feeding and nursery areas, resulting in a reduction of suitable habitats. When fin fish and shellfish are exposed to these stressful conditions, they become more susceptible to parasites, disease and infections and they also die directly from a lack of oxygen (Winn and Knott, 1992; Lenihan and Peterson, 1998). Although fish will avoid dead water, strong winds can quickly push this water into creeks and shallow areas near shore. Fish that are fleeing the dead water may become trapped along the shoreline or in the creeks with no way to escape.
Sudden kills of certain fish species (primarily menhaden) have also been correlated with the presence of the dinoflagellate Pfiesteria piscicida and similar Pfiesteria-like organisms. While a wide variety of fish have been shown to be susceptible to Pfiesteria piscicida when they are cultured together in the laboratory, little is definitively known about the interplay between fish species and Pfiesteria in the wild (The Raleigh Report 1998). It is known, however, that these organisms graze on other algae and therefore that their growth can be stimulated by nutrient induced algal blooms. It is also quite possible that stress caused by low oxygen conditions makes fish more susceptible to attacks from organisms such as Pfiesteria, (Burkholder et al., 1995).
We believe that the summer and fall of 1998 may be shaping up as one of significant water quality problems since the record El Nino rainfalls of this winter and spring have provided a strong nutrient push to the Neuse River Estuary.
We strongly recommend continuation of an intensive long term monitoring and modeling program for the Neuse. Only with a well coordinated and thorough data collection effort spanning a number of different runoff/loading seasons coupled with the quantitative analysis that rigorous modeling can provide, can we hope to sort out the mechanisms and chain of events that lead to fish kills, be able to predict with some degree of confidence how, why and where these events will occur, and be able to separate the effects of natural weather variations from nutrient management actions in stimulating or controlling these events.
References:
Burkholder, J., H.B. Glasgow and C.W. Hobb. 1995. Fish kills linked to a toxic ambush-predator dinoflagellate: distribution and environmental conditions. Marine Ecology Progress Series 124:43-61.
Lenihan, H.S. and C.H. Peterson. 1998. How habitat degradation through fishery disturbance enhances impacts of hypoxia on oyster reefs. Ecological Applications, 8(1):128-140.
Paerl, H., J. Pinckney, J. Fear and B. Peierls. 1998. Ecosystem responses to internal and watershed organic matter loading: consequences for hypoxia in the eutrophying Neuse River Estuary, North Carolina, USA. Marine Ecology Progress Series, in press.
The Raleigh Report 1998: Pfiesteria research needs and management actions, Recommendations
of an expert panel convened December 4-5, 1997, Research Triangle Park, NC, Special
Report Series No. 19, The Water Resources Research Institute, Raleigh, NC, 17p.
Winn, R. and D. Knott. 1992. An evaluation of the survival of experimental populations exposed to hypoxia in the Savannah River Estuary, Marine Ecology Progress Series, 88:161-179.
Return to Press Release Page.