Background

Secchi depth is a parameter used to determine the clarity of surface waters. The measurement is made with a “secchi” disk, a black and white disk that is lowered into the water and the depth is recorded at which it is no longer visible. A secchi depth recording of 5 ft indicates that the device was last visible at 5 ft below the surface. High secchi depth readings indicate clearer water that allows sunlight to penetrate to greater depths. Low readings indicate turbid water which can reduce the passage of sunlight to bottom depths. Limited light penetration can be a factor in diminished aquatic plant growth below the surface, thus reducing biological reaeration at lower depths. Secchi depth measurements, in feet (ft), are reported from 1986 to 2003 in this report.

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Results

In general, the Hudson River has the highest turbidity in the harbor. The Hudson carries high levels of sediment, organic materials, and other suspended solids until they reach the Upper Bay by lower Manhattan. As flows slow in the broader channel, sediments settle out. For the summer 2003, the Hudson River sampling locations Mt. St. Vincent (N1), 125th Street (N3B), 42nd Street (N4), and The Battery (N5) had a mean summer secchi depth of 3.2+0.3 ft. In comparison, the Upper Bay had a mean summer secchi depth of 6.1+0.4 ft. And in the Kills, secchi depth averaged 4.2+0.1 ft for 2003 and is comparable in turbidity to the Upper East River/WLIS region, also a tidal straight.

The Lower Bay, in contrast, has the best water clarity in the harbor, where 2003 summer secchi depth averaged 7.2±1.6 ft. Within the Lower Bay, Rockaway (N16) has the clearest waters in the harbor, averaging 10.6+1.3 ft secchi depth in summer 2003. In July, secchi depth averaged 14.9+2.7 ft at Rockaway, well above its ten-year July mean of 9.62+1.29 ft. Chlorophyll-a and total suspended solids levels were significantly below their ten-year monthly averages for much of the summer at the Rockaway sampling location.

The waters towards the western end of the Lower Bay are more turbid than the Rockaway region. Raritan River (K5A) carrying riverine sediments has the highest turbidity in the Lower Bay, averaging 4.0+0.3 ft in 2003.

Throughout the Harbor, Jamaica Bay has suffered the most significant decline in water clarity. High levels of algae growth in specific regions of Jamaica Bay are largely responsible for this decline. From 1986-1990, mean summer secchi depth in Jamaica Bay was 5.72±0.83 ft. By 2001-2003, this had declined to only 4.12±0.05 ft.

In 2003, water clarity improved somewhat over 2001 and 2002 in Jamaica Bay which had a mean summer secchi depth of 3.8±0.3 ft (Figure 7–1). August was a particularly poor month during which time monthly secchi depth averaged 2.89 ft, compared to ten-year monthly means of 4.11±0.89 ft. August also saw an increase in chlorophyll-a in Jamaica Bay which likely explains the decrease in water clarity. At Bergen Basin (J7), secchi depth fell to a low of 2.2 ft on average for the month of August, at one point falling to 1.5 ft, coinciding with major bloom events in that area.

Other regions in the Harbor have experienced declines in water clarity, though not to the extent of Jamaica Bay. Average secchi depth for New York Harbor from 1986-1990 was 5.15±0.5 ft and declined to 4.5±0.23 ft between 2001 and 2003. The Inner Harbor and Upper East River/WLIS have declined slightly while the Lower Bay has maintained its clarity, showing no deterioration.

Much of the decline in water clarity in New York Harbor and Jamaica Bay can be attributed to algae growth which is correlated to declines in water clarity (Figure 7–2).

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