Long Term Nutrient and Chlorophyll a Dynamics across Long Island Sound and Impacts on Dissolved Oxygen Conditions within the Western Sound (1991-2019)
Nitrogen overload, eutrophication, and hypoxia have been challenging and persistent water quality problems in Long Island Sound (LIS) over the past decades with major impacts on commercial industries, ecology, and recreational activities in the region. Recognizing these problems, the EPA enforced three phases of the Clean Water Act (CWA) to reduce nitrogen loads in an effort to improve this important estuary. This study examines how nitrogen (NH3, NOx & TDN), chlorophyll a (CHLA), and dissolved oxygen (DO) concentrations changed in LIS over the past 30 years, in response to water quality regulations as well as changes in environmental conditions (i.e., temperature and precipitation). To address this overarching goal, we used water quality monitoring data collected by the Connecticut Department of Energy and Environmental Protection (CTDEEP), precipitation data from National Oceanic and Atmospheric Administration (NOAA) and discharge data from United States Geological Survey (USGS) from 1991 to 2019. Three main questions are addressed: 1) How do nitrogen loads and CHLA concentrations vary across the Sound and how have they changed over the past 30 years in response to regulations? 2) How representative is the CTDEEP "spring chlorophyll bloom" of CHLA conditions across the Sound and how does it relate to the development of hypoxic conditions in summer months? 3) What factors might explain observed high anomalies in CHLA concentrations in certain years, despite ongoing efforts to control excessive nutrient pollution? These questions focus on changes in nitrogen and CHLA considering regulation timestamps, spring chlorophyll bloom effects on summer DO, and environmental factors that might explain occurrences of anomalous spring chlorophyll blooms despite nutrient regulations. Spatially from western to eastern LIS there was a decreasing longitudinal gradient for both CHLA and nutrients, and an increasing gradient for bottom DO concentrations, inferring that where nutrients and CHLA are high, water quality conditions are affected and DO is reduced. Likewise, the converse is also expected. Temporally, there was a definitive decline in nutrients from 2001 to 2019 with spring NOx declining by 53% from 2001-2016 when PIII of the CWA was in effect (relative to 1995-2000 levels), then by 74% in 2017-2019 (relative to 2001-2016 levels). Spring NH3 declined by 40% from 2001-2016 (relative to 1995-2000 levels), then an additional 40% within 2017-2019 (relative 2001-2016 levels). Spring TDN declined by 17% then 32% in the same manner as the NOx and NH3. The largest reductions are evidenced in the 2017-2019 period, after the Total Maximum Daily Load (TMDL) goal was achieved, but it is only after PIII was effected in 2000 that nutrient declines appeared substantial. Despite these reductions, CHLA and DO fluctuated with strong (>10µg/L) spring phytoplankton blooms occurring from 2001-2005 and 2007-2011, whereby 2008, 2010 & 2011 achieved “poor” or >/=20 µg/L status according to National Coastal Condition Report standards and particularly large hypoxia zones developing 2003, 2012 & 2016. Poor correlations for both simple and multiple linear regressions between spring CHLA and nutrients were found. Summer bottom DO was found to be weakly correlated to spring CHLA, spring NOX, spring NH3, spring TDN, spring precipitation, spring discharge and summer delta density (r 2 < 0.2). A multiple linear regression, however, combining the effect of eight variables – including nutrients, precipitation, temperature, delta density, spring riverine discharge of Connecticut, Housatonic and Norwalk Rivers, and spring CHLA - allowed for good prediction of summer bottom DO in the Narrows (r 2 = 0.6), highlighting the additive or even synergistic effect of combined influences on DO conditions within the Narrows. We found that CHLA anomalies and intensity of spring phytoplankton bloom events were related primarily to (i) increases in nutrients preceding blooms, (ii) abrupt short-term temperature changes, (iii) precipitation and (iv) low delta densities. An anomalous jump in CHLA concentrations in western LIS after 2000 appeared to be the result of a shift in phytoplankton assemblages produced by increasing temperatures and changing nutrient stoichiometries facilitated by nutrient regulations.
Long Island, nitrogen, chlorophyll a, oxygen, precipitation, temperature, Biogeochemistry, Environmental Health, Environmental Health and Protection, Environmental Monitoring, Other Earth Sciences, Water Resource Management
City University of New York - City College
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