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Neuse River & Pamlico Sound
Over the past several decades, increasing inputs of anthropogenic nutrients into coastal systems worldwide have been suggested as the main contributors to trends of declining bottom water oxygen concentrations. Specifically, high nutrient (nitrogen) loading rates promote high rates of algal production. When production exceeds consumption by animal grazers, plant matter (biomass) accumulates in the estuary. This imbalance leads to massive blooms of phytoplankton. Increased primary production and subsequent increased flux of organic carbon to bottom waters raises the oxygen demand. High bottom water oxygen demand combined with a stratified water column will set up hypoxic conditions (dissolved oxygen concentrations of < 2.0 mg/L).
The Neuse estuary and Pamlico Sound typifies the nutrient-sensitive estuaries of the southeast coast of the U.S.. These systems have lengthy water residence times so nutrients entering the system remain there for relatively long periods. The combination of the hydrological and nutrient cycling characteristics in these systems make them highly responsive to nutrient inputs and under excessive nutrient loading susceptible to algal blooms, hypoxia, and anoxia.
In North Carolina, 80-90% of commercial landings represent estuarine dependent species. The Neuse River and Pamlico Sound contribute to over half of the state’s fisheries landings, representing up to $85 million dollars of revenue. Although these estuaries are critical nursery habitat for juvenile and adult fish, little is known about the distribution of this estuarine community. The Neuse River fish community is composed of many commercially and ecologically important species, including spot (Leiostomus xanthurus), croaker (Micropogonias undulatus), menhaden (Brevoortia tyrannus), bay anchovy (Anchoa mitchilli), alewife (Alosa pseudoharengus), blueback herring (Alosa aestivalis), striped mullet (Mugil cephalus), pinfish (Lagodon rhomboides), southern flounder (Paralichthys lethostigma). sheepshead minnow (Cyprinodon variegatus), mummichog (Fundulus heteroclitus), and Atlantic silversides (Menidia sp.) (Nelson et al. 1991).
The intermittent hypoxia these systems experience may be affecting the fish community and the habitat they depend upon. Although direct mortality resulting from hypoxia is episodic and infrequent, sublethal effects associated with hypoxia altering fish habitat quality can result in changes in behavior, growth, and survival. Hypoxia can result in decreases in habitat quality through direct metabolic costs from altered habitat use, kill the invertebrates thus decrease prey resources for benthic feeding fishes, and alter distributions impacting overlap with competitors and predators.
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