This appeared to be red or brown tide in Flanders...

This appeared to be red or brown tide in Flanders Bay, just inside Great Peconic Bay. The Riverhead Business District is just visible in the upper left of the frame on Aug. 14, 2013. Credit: Doug Kuntz

The letter "Brown tide not fault of our septics" [Oct. 25] suggested that a lack of rainfall since late September and, in turn, a corresponding lesser delivery of groundwater nitrogen from septic tanks, caused the brown tide in Great South Bay.

The data do not support this hypothesis. While groundwater flow is generally proportional to rainfall on Long Island, there is a delay between these processes. Accordingly, the brown tide blooming in Great South Bay began in mid-September following an intense rain on Sept. 4.

Further, the intensity of this brown tide matches the sewer patterns of the watershed: low in the sewered Town of Babylon, but high in the unsewered towns of Islip and Brookhaven.

Last, the continuous delivery of septic tank nitrogen through the year ensures that Great South Bay always has a large reservoir of nitrogen stored in its sediments that can fuel brown tides, even when groundwater does not.

Christopher J. Gobler, East Quogue

Editor's note: The writer is a professor in marine and atmospheric sciences at Stony Brook University.

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