LIS Water Quality Monitoring FAQs

The Connecticut Department of Energy and Environmental Protection (DEEP), on behalf of the Long Island Sound Study estuary program, conducts a Long Island Sound Water Quality Monitoring Program.  From October to May, water quality is monitored by collecting samples once a month from 17 sites by staff aboard the Department’s Research Vessel.   Bi-weekly hypoxia surveys start in mid-June and end in September with up to 48 stations being sampled during each survey.

How is Long Island Sound Water Quality Monitoring Done?

The Connecticut Department of Energy and Environmental Protection performs an intensive year-round water quality monitoring program on Long Island Sound. Water samples are collected using a Rosette Sampler that holds nine water sampling bottles. The bottles are remotely triggered to take a water sample at any depth. Dissolved Oxygen, temperature and salinity data are collected using an electronic instrument called a Conductivity Temperature Depth recorder (CTD), that takes measurements from the surface to the bottom of the water column. Samples are preserved aboard the mini laboratory for later analyses at a certified research laboratory. These data are used specifically to quantify and identify annual trends and differences in various water quality parameters and general conditions of LIS waters. Parameters for which surface and bottom waters are tested include water temperature, salinity, dissolved silica, particulate silica, dissolved nitrogen, particulate nitrogen, dissolved oxygen, chlorophyll a, and total suspended solids.

Why Do We Monitor Long Island Sound During the Summer?

Long Island Sound suffers from low oxygen levels in the bottom waters each summer, a condition scientists call hypoxia. Fish need oxygen to survive just as humans do. When levels of oxygen fall low enough, fish and other aquatic life in Long Island Sound become stressed and may die. The DEEP conducts a Summer Hypoxia Survey which is part of the broader Long Island Sound Water Quality Monitoring Program and has provided a description of the extent and duration of low dissolved oxygen concentrations for the summer months since 1991. Bi-weekly cruises by staff aboard the Department’s research vessel, John Dempsey, start in mid June and end in the middle of September with 284 stations sampled over an average of 7 cruises.

What is Hypoxia?

The term "hypoxia" means low dissolved oxygen ("DO") concentrations in the water. Marine organisms need oxygen to live, and low concentrations, depending on the duration and the size of the area affected, can have serious consequences for a marine ecosystem. As defined by the Long Island Sound Study, hypoxia exists when DO drops below a concentration of 3 milligrams per liter (mg/L), although ongoing national research suggests that there may be adverse affects to organisms even above this level. Low oxygen levels can occur naturally in estuaries during the summer, when calm weather conditions prevent the mixing of the water column that replenishes bottom water oxygen during the rest of the year. However, studies of the limited historical data base for the Sound suggest that summer oxygen depletion in Western Long Island Sound has grown worse since the 1950s. 

What Causes Hypoxia?

The primary cause of hypoxia is nutrient enrichment that comes from a variety of sources throughout the Long Island Sound watershed, in particular sewage treatment plants, stormwater runoff and atmospheric deposition.  The nutrients, in particular nitrogen, stimulate the growth of phytoplankton, microscopic plants that grow in the Sound.  When the phytoplankton dies it settles to the bottom of the Sound where it decays. Bacteria breaks down the organic material from the algae for food and fuel while using up oxygen. Nutrient enrichment leads to too much algae and high levels of oxygen consumption, leaving the Sound desperately short on oxygen needed by the fish and aquatic life.

To more fully understand hypoxia and its effects on water quality and the biology of Long Island Sound, CT DEEP receives funding from the EPA Long Island Sound Study to monitor the Sounds’ waters year round.  In addition to taking measurements of dissolved oxygen from New York City to Block Island Sound, CT DEEP measures nutrients (nitrogen, phosphorus, silica), phytoplankton and other related physical and chemical conditions.

How Does Water Temperature Contribute to Hypoxia Severity?

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 water quality (WQ) surveys are conducted near the beginning of the month and the hypoxia (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. 

Return to the Long Island Sound Water Quality Monitoring Program Page