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Final Report - Synthesis and Analysis of Historical Hypoxia Data in the Western Narrows of Long Island Sound

April 1, 2018
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This report describes the compilation of Interstate Environmental Commission’s (IEC) 25 years of hypoxia-related water quality data from the Western Narrows of Long Island Sound (LIS) into a relational database management system (RDBMS) with a common data model (CDM) using Microsoft Access. The result was the compilation of approximately 25 thousand water quality sampling and meteorological records and the creation of a geodatabase. To facilitate future database efforts, numerous tools and tables were created that will allow for efficient updates and expansions. In addition, analyses were run on the data to describe and identify trends with hypoxia in the Western Narrows of LIS.

Queries of the RDBMS were performed to extract data for the purposes of identifying temporal and spatial trends in dissolved oxygen (D.O.) concentrations and hypoxic conditions. In general, the observed trends were consistent with the published literature and previous IEC reports. There was no overall annual trend across the 25 year dataset but hypoxic conditions tended to increase from 1991-2004 and then decrease from 2005-2015 (Exhibit ES-1). Over the 25 year period, hypoxic conditions generally began in July, peaked in August, and retreated in September (Exhibit ES-1). D.O. was lower more frequently at bottom depths, which indicates higher frequency of hypoxia relative to surface waters. Hypoxic conditions primarily decreased along a west to east axis. Generally, as nutrient concentrations increased, D.O. concentrations decreased. However, nutrient data were limited to select years. Additional data and increased data resolution may further elucidate these relationships and enable forecasting of hypoxic conditions.

In addition to incorporating data collected post-2015, analogous data from eastern LIS and additional environmental data may be incorporated into the database to further investigate hypoxic conditions. Future efforts may include analyses at a finer scale and/or focus on specific areas within the Western Narrows. Also, utilizing this compilation of data and existing hydrodynamic and D.O. models, a 3-D model could be developed to provide operational water quality forecasts and inform ecological management decisions.