The Comprehensive Conservation and Management Plan for Long Island Sound, produced in 1994 under the EPA’s Long Island Sound Study, identifies persistent summertime hypoxia (low dissolved oxygen) as the principal environmental problem affecting the Sound. The hypoxia problem is primarily affects the western and central portions of the Sound. The eastern Sound is more vigorously flushed with ocean water and experiences only isolated instances of hypoxic water.
Fig. 1 Bathymetry of Long Island Sound
Hypoxia occurs when the water becomes depleted of dissolved oxygen. Aquatic animals require oxygen for respiration, just as terrestrial animals do. Sources of the oxygen dissolved in seawater include diffusion from the air, physical aeration and photosynthesis by marine plants. In bottom waters, the oxygen must come from either vertical mixing of the water column or the horizontal advection of oxygenated waters. If the removal of dissolved oxygen from bottom waters through animal respiration occurs more rapidly than replenishment by mixing and/or advection, dissolved oxygen levels will decline, stressing marine animals that are unable to escape the deteriorating conditions. If oxygen levels fall low enough, the results can be fatal.
Hypoxia occurs naturally, given the right combination of physical and biological processes such as vertical and horizontal mixing, air-sea exchanges, nutrient loading, photosynthesis, respiration and oxygen demand. In the traditional view, hypoxia is caused by vertical stratification of the water column, which prevents oxygen-rich surface waters from mixing with deeper water, and the delivery to deep waters and subsequent decomposition of organic material resulting from biological productivity in surface waters. The degree of vertical stratification depends on the temperature and/or salinity differences between surface and bottom layers as well as the strength of physical forces available to induce vertical mixing. When there is abundant organic matter falling to deeper waters and there undergoing decay and the water column is highly stratified, hypoxic or even anoxic (no dissolved oxygen) conditions in bottom waters can ensue.
Hypoxia is a nationwide phenomenon, occurring seasonally in the Gulf of Mexico, the Neuse River in North Carolina and Chesapeake Bay, among other places. Long Island Sound has suffered from summertime hypoxic events for decades. Public attention to this problem was galvanized in the late 1970’s by large fish kills in the western Sound caused by hypoxia. In 1986, Congress created the Long Island Sound Study under the aegis of the U.S. Environmental Protection Agency (EPA) to assess environmental conditions in the Sound and to develop a management plan that identifies threats to the Sound’s health and remedial actions to counter those threats. Field surveys conducted under the auspices of the LISS identified hypoxia linked to nutrient enrichment as the greatest environmental threat to the Sound.
Figure 2: A bi-plot of principle components and empirical orthogonal function vectors of bottom dissolved oxygen (DO) in summer, spring chlorophyll a (CHLA), spring total nitrogen (TN), summer bottom temperature (BT), summer wind speed (WIND), summer density stratification (DS), and spring river discharge (RIVER) at major stations from 1995 to 2004. Stations in the Narrows are represented by N, C is for the Central Basin, W is for the Western Basin, and E is for the Eastern Basin. There is a good separation between the Narrows stations, the eastern stations, and the rest of the sound.
In 1998, the states of Connecticut and New York and EPA adopted a plan for hypoxia management, with the aim of managing nitrogen targets through development of a total maximum daily load (TMDL). The Long Island Sound TMDL (LIS TMDL) management team recognized that hypoxia is not driven by daily or short term nitrogen loadings, but may be a function of annual loading rates. As a result, the LIS TMDL is defined as an allowable annual load of nitrogen into the Sound. In many places, the degree of hypoxia is strongly correlated to nutrient loading and the resultant decay of phytoplankton blooms. It is estimated that human sources have approximately doubled the yearly amount of nitrogen introduced into the Sound from natural sources. The plan for the Sound is to reduce the total nutrients and, therefore, to reduce the severity and extent of hypoxia in the bottom waters of the Sound
However, despite efforts to reduce nitrogen inputs and levels in the Sound, severe hypoxic events continue to occur. The likely reason is that hypoxia in the Sound appears to be the result of factors other than stratification and nutrient loading. Dr. Kamazima Lwiza and a team of researchers from SoMAS are attempting to elucidate the specific factors that contribute most to hypoxia in each of the Sound’s basins.
Understanding hypoxia in Long Island Sound requires knowledge of the Sound’s unique geological attributes. The Sound is approximately 90 miles long and 12 miles wide, and averages about 60 feet deep. The west end connects to the Hudson River estuary via the East River. The east end connects to the Atlantic Ocean through the Race. The Sound’s bottom topography divides it into 4 distinct basins separated by sills and shoals. The East Basin has a narrow, deep channel. The Central Basin is characterized by a V-shaped channel, deeper along one edge and gradually sloping up to the other side. The Western Basin has a deep channel, while the Narrows is the westernmost and shallowest basin (Figure 1). The Sound’s complex and variable bathymetry and topography plays an important role in the susceptibility of each basin to hypoxia.
Figure3: Multiple linear regression showing the relationship between the maximum hypoxic volume as a percentage (%) and the combined effect of five variables from Fig. 10: (1) mean spring total chlorophyll a for the whole sound (CHLA), (2) mean summer wind speed (WIND), (3) maximum spring discharge into Long Island Sound (RIVER), fall precipitation of the preceding year (PRCP), and (5) mean spring total nitrogen (TN). The solid line represents the least-squares best fit from linear regression with the coefficient of determination (r2) shown in the upper-right.
Dr. Lwiza’s research indicates that the primary factor(s) controlling the dissolved oxygen content of bottom waters of the Sound varies from one location to another. For example, the westernmost basin and locations with shallow water depths are strongly influenced by density stratification. The variability of bottom oxygen levels in other areas is mainly driven by biological processes, with the exception of the Eastern Basin, where the amount of dissolved oxygen is determined by solubility (see Figure 2). Lwiza’s findings show that variability in the volume of the area subjected to seasonal hypoxia is mainly a function of primary production during spring, which acts as a source of organic carbon. Previous investigations have maintained that hypoxia is initiated by strong stratification and weak wind speed. When the wind increases in early fall, the ensuing mixing allows bottom waters to recover from hypoxia. But, Dr. Lwiza’s team found that bottom oxygen replenishment is not necessarily controlled by increased mixing. They have identified three mechanisms besides vertical mixing that might influence oxygen recovery: (i) horizontal exchange; (ii) background vertical diffusion and (iii) diminished microbial activity. While the first mechanism can act alone to bring in oxygenated water to a hypoxic location, the latter two must operate in tandem. Neglecting the influence of spring bloom production and basing a hypoxia control policy on nitrogen limits load alone makes it difficult to evaluate the response of the Sound to those limits. Lwiza emphasizes the need to include biogeochemical processes missing in the current models, e.g., the role of the spring bloom, microbial activity in the water column and the sediments and diagenetic mobilization. Understanding hypoxia dynamics in the Sound requires an understanding of how these processes interact with physical factors and the competition between vertical mixing and stratification.
Based on these results, Dr. Lwiza and PhD student Younjoo Lee wrote a paper explaining why, despite efforts to limit nitrogen inputs, Long Island Sound continues to experience severe hypoxic conditions. The LIS TMDL management initiative calls for a reassessment phase to evaluate the response to nitrogen reduction. However, the large interannual variability in summertime bottom dissolved oxygen levels detailed by Lwiza and his colleagues complicates this assessment and necessitates an examination of factors that control the observed variability. The paper by Lwiza and Lee notes that variability in late winter and spring total nitrogen inputs, spring chlorophyll a, spring river discharge and summer wind condition together can explain more than 90% of the variability in summertime hypoxia in the Sound (Figure 3). Their analysis exposes a major flaw in current TMDL policy formulation and evaluation- – a singular focus on nitrogen loadings to the exclusion of other factors now known to be important in the onset and progression of seasonal hypoxia. To be able to properly assess the response of the Sound to nitrogen reduction, we must have a systematic way of accounting for the effect on hypoxia of a variety of other factors, especially meteorological forcing (e.g., wind, cloud cover, fall and spring precipitation).
The paper on Long Island Sound hypoxia by Lee & Lwiza (2008) was listed in the Top 25 Hottest Articles in the Journal of Estuarine, Coastal and Shelf Science (http://top25.sciencedirect.com/subject/earth-and-planetary-sciences/9/journal/estuarine-coastal-and-shelf-science/02727714/archive/14/ )
