The development of hypoxia in the Long Island Sound (LIS; see Background) can arise from anthropogenic (i.e. man-made) and/or natural processes. Determining the contribution from each is important when it comes to water quality management. For example, when severe hypoxia occurs in a given summer, water management authorities seek to understand the causes. If it is determined that the causes were mainly anthropogenic in nature (e.g., increased affluent discharge), it might be necessary to put forth significant effort and financial/community resources to try and limit the occurrence in future years. On the other hand, if there appeared to be no change in the anthropogenic interactions with the LIS but rather if it could be determined that the weather/climate was particularly conducive to the development of hypoxia, there would be less or no reason to make any changes in water management practices.

At present, there is still a great deal of uncertainty associated with our knowledge of the manner that climate and weather variations affect the LIS and its water quality, and in particular the development of hypoxia. Knowing the variations in the exchanges of heat, momentum and salt/freshwater between both the LIS and the atmosphere via air-sea exchange and the LIS and the ocean via horizontal currents (see Objective 2) is important since both of these play a significant role in establishing the summertime pycnocline. There is strong evidence that natural variations in these physical processes play a role in the development of hypoxia. For example, the uppermost time series plot in the figure above shows dissolved oxygen (DO) in the bottom waters of the LIS (near Hart Island) for the period 1946-98. Evident are the year-to-year variations in bottom DO. The middle time series plot shows the associated year-to-year variations in the temperature difference between surface and bottom water. This quantity represents a measure of vertical stratification (i.e. strength of pycnocline), with greater stratification facilitating favorable conditions for hypoxia to develop. Two things are worth noting in regards to these time series and their correlation (shown in the lowest panel). First, variations in stratification are known to be associated with variations in heat, momentum and salt/freshwater exchange - and thus are considered to be driven by natural processes. Second, there appears to be a relatively strong relationship between bottom DO and vertical stratification indicating that natural processes do in fact play a significant role in year-to-year variations in bottom DO and thus the development of hypoxia.

To facilitate the interpretation of the historical record of hypoxia events and their underlying causes, one aim of this study is to develop a multi-decadal estimate of the exchange of heat, momentum and rainfall between the atmosphere and the LIS. This estimate will necessarily be developed from surrounding land-based observation sites where long records of observations are available (e.g., La Guardia Airport). However, these longer, land-based records can only depict the exchanges occurring over the LIS so accurately. The ferry-based observations provide actual over-water values of the above quantities so that the long-record estimate can be better constrained. Having such long-period records of air-sea exchange properties will help to determine the extent that natural variations played a role in past variations of LIS bottom DO and in turn help to guide future water quality management decisions.