The largest fish kill on the Pamlico River in more than a decade could be linked to a potentially toxic algae that researchers have only recently begun to study in North Carolina waters.
State investigators collected water samples after the death of an estimated 3.9 million fish on Aug. 5-6 on the Pamlico River near Broad Creek. The preliminary lab results show significant numbers of a potentially toxic microscopic algae, Karlodinium. The algae is not known to affect human health, but has been linked to fish kills in the Chesapeake Bay and elsewhere.
Karlodinium, a one-celled algae that propels through water with a whip-like motion, is common in brackish waters along the Atlantic coast. In large numbers, the algae can produce brownish or “mahogany” tides and can release toxins that are lethal to fish. In North Carolina, blooms associated with fish kills have been rare.
“It's hard to say whether Karlodinium was the culprit,” said Jill Paxson, leader of the state Division of Water Quality's Pamlico River rapid response team, which investigated the fish kill. “It was a large number of Karlodinium. It's important to continue to look into it and not overlook it.”
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Harmful strains of Karlodinium release a toxin that damages the gills of fish and can cause them to suffocate. The fish kill consisted primarily of one species, juvenile menhaden, suggesting the event was more likely caused by a toxin rather than a drop in oxygen levels in the water, which would kill multiple species, Paxson said.
Hans Paerl, a professor at UNC-Chapel Hill Institute of Marine Sciences in Morehead City, said Karlodinium blooms have been recorded since the 1960s in the Chesapeake Bay where water monitoring has been in place longer. In a recent paper, Paerl and other scientists linked a dense bloom of Karlodinium in October 2006 to four subsequent fish kills on the Neuse River.
Paerl said the Karlodinium blooms require certain natural conditions to develop, such as stratified layers of fresh water and salt water. Under those conditions, excessive nitrogen and phosphorus that washes into the river in runoff and treated sewage fuel growth of algae.
Heather Jacobs, the Pamlico-Tar riverkeeper, said the fish kills are a symptom of a larger problem – an ecosystem out of balance.
State regulators declared the Tar-Pamlico River basin sensitive to nutrients in 1989 after algal blooms, low oxygen levels and increased numbers of fish kills pointed to excessive amounts of nitrogen and phosphorus in the water. Regulators required reductions in nitrogen discharged by sewage treatment plants and limits on runoff.
“We've been working on reducing nutrients in the Tar-Pamlico,” Jacobs said. “I think we're going to find we're going to have to do a lot more.”