DEEP: Temperature Monitoring 2015

Water Density and Hypoxia
Water temperature plays a major role in the timing and severity of the summer hypoxia event. The temperature difference between the bottom waters and the surface waters is known as "delta T". This delta T, along with salinity differences, creates a density difference, or "density gradient" resulting in a separation or "stratification" of water layers that hinders the oxygenated surface waters from circulating downward and mixing with the oxygen starved bottom waters. The greater the delta T the greater is the potential for hypoxia to be more severe. The DEEP's monitoring program also records water temperatures and salinity during its hypoxia monitoring cruises to help estimate the extent of favorable conditions for the onset and ending of hypoxia. During the summer, two sampling surveys are conducted each month. The WQ surveys are conducted near the beginning of the month and the HY surveys are conducted in the latter of each month.

Water temperature differences in the western Sound during the summer months are particularly influential in contributing to the difference in dissolved oxygen content between surface and bottom waters. The density stratification of the water column creates a barrier between the surface and bottom waters, and it is this barrier, the pycnocline (where the change in density with depth is at its greatest), that prevents mixing between the layers. Long Island Sound is a thermally stratified estuary. The more rapid the change in temperature with depth, the stronger a barrier the thermocline presents to mixing of the water column. One of the sampling sites in the western Sound chosen as a representative site is station D3 located mid-Sound between Eatons Neck, Long Island and Norwalk, CT.

Select a sampling date below to view graphs showing the temperature gradient trends over the last 17 years in the west-central Sound during that time of the summer.

    July 6-8, 2015            Delta T vs Dissolved Oxygen graph Western Sound

    July 20-23,2015

    August 3-6, 2015

    August 17-19, 2015

    August 31- September 2, 2015




Updated January 2016