Photos of the Long Island Sound

Research & Monitoring

Status and Trends: LISS Environmental Indicators

Western Sound Spring Bloom Chlorophyll a Concentration

LISS Water Quality Monitoring Program, CTDEEP

Chlorophyll a Concentration (ug/L)
1991 20.3
1992 23.7
1993 6.6
1994 16.6
1995 10.6
1996 8.2
1997 9.0
1998 4.0
1999 2.6
2000 1.7
2001 17.5
2002 12.1
2003 16.3
2004 15.1
2005 14.8
2006   6.7
2007 17.8
2008 21.6
2009 19.0
2010 20.0
2011 23
2012 48.5
2013 23.0


Chlorophyll is the green pigment in plants that is necessary to convert carbon dioxide into organic compounds during photosynthesis. There are several kinds of chlorophyll. Chlorophyll a is the predominant type found in land plants, as well as in microscopic plants called phytoplankton that live in aquatic environments.


The concentration of chlorophyll a reflects the amount of phytoplankton in surface water. High levels of chlorophyll a in the western Sound during spring are indicative of phytoplankton blooms, which historically have been linked to summertime declines in oxygen. This indicator seeks to characterize the spring bloom conditions in the western Sound each year.


Nutrients such as nitrogen promote phytoplankton growth, and an excess of nitrogen can lead to the overgrowth of phytoplankton and the formation of a phytoplankton bloom. From 1992-2000, chlorophyll a concentrations steadily declined at a similar rate as decreases in point source nitrogen loads from sewage treatment plants. However, chlorophyll a levels have risen since, and are now as high or higher than those observed in the early ’90s even though point source nitrogen loads have not increased. 2012 was a historically high year, with 2013 a fairly average year, which corresponds well to observed patterns in hypoxia.  The reason for the variability of chlorophyll a concentration in the last decade is not well understood, but may have to do with changes in temperature, sunlight, and/or predator (zooplankton) abundance.


Chlorophyll a is reported in units of micrograms per liter of seawater.  Chlorophyll a concentrations are measured at three stations in the western Sound (Long Island Sound Study water quality monitoring stations B3, D3, F3) for the three months during which the spring bloom is most likely to occur (February, March, April).  The reported indicator is the average of the maximum value at each of these three stations recorded during this time period. This value is intended to estimate the average chlorophull value across the western Sound during the peak of the spring bloom.

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Marine diatoms, a common type of phytoplankton found in Long Island Sound.

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