August is generally the month when Norwalk Harbor water is under stress from rising temperature. Marine water known as the tidal wedge underlies the incoming fresh river water. We rely on the river to move the oldest water of the wedge seaward so that clean, well-oxygenated marine waters can enter the harbor mouth and work their way upstream to replenish older marine water which is low on dissolved oxygen and subsequently dangerous to schools of fish.
When the harbor landscape was originally comprised of many acres of marshland, millions of gallons of water were distributed across this entire area on flood tide. That water was available on ebb tide in combination with flow from the Norwalk River to flush the harbor. Those marshes have been replaced with structures and extensive bulkheading which now leaves us with the river and north winds to help flush the estuary. This process is inefficient and consequently the Norwalk River flow harbor does not remove older marine waters quickly. The area with the oldest, oxygen-depleted tidal water is found at the far northern end at Wall Street. Dissolved oxygen levels at a depth of 2 feet below the surface are too low to sustain schools of fish. With help of a greatly improved waste water treatment plant, pump-out boats from East Norwalk Blue, water quality expertise from the Harbor Watch staff at Earthplace and Norwalk’s Public Works Dept. the amount of impaired water in the harbor has been held to the last hundred yards of harbor waters at Wall Street for the last 10 years. This is a major increase in water quality from events in the 1990s when the oxygen depleted area advanced downstream as far as the Maritime Center resulting in large fish kills.
Nevertheless, elevated water temperatures in August and the presence of very large schools of Menhaden (Bunker) stretching from the harbor mouth all the way upstream to Wall Street can pose a threat to a large school of fish now residing in the surface waters at Wall Street. Bunker crowd into this area at the end of summer for reasons known only to them. This phenomena has been observed for the last three years by Harbor Watch staff. The danger exists in that the fish are mainly relying on the oxygen in the surface flow from the Norwalk River and there are little in the way of oxygen reserves below the top layers of the water column to sustain them. One strong factor in their favor is that water temperatures are two to three degrees lower than last year at this time based on Harbor Watch records. Water temperature on 8/11/16 was 75 degrees at Wall Street and on 8/10/17 it was observed at 72 degrees due to the cool evenings. This drop in temperature has helped maintain dissolved oxygen in the marine water at Wall Street although still not at the levels to sustain a large school of bunker. It presently looks promising that Norwalk Harbor will avoid a large fish kill at Wall Street. The last major fish kill in Norwalk Harbor was in 2005.
-From the Desk of Dick Harris, ENB Marine Specialist