Managed Nitrogen Load Decrease Reduces Chlorophyll and Hypoxia in Warming Temperate Urban Estuary

Abstract

Many urban estuaries worldwide suffer from excess phytoplankton and hypoxia (low oxygen) due to high nutrient loads. A common water quality management strategy is to require wastewater treatment facility upgrades. This case study examines Narragansett Bay, a warming temperate mid-latitude urban estuary with seasonal periodic hypoxia, during June through September from 2005 to 2019. Within this period, numerous facilities were upgraded to nitrogen removal over several years. The response of the bay is more consistent with “textbook” expectations for reduced chlorophyll and hypoxia than what was seen in many other systems—despite its complex coastline geometry, numerous river inputs, and widely-distributed treatment facilities. River flow drives inter-annual variability with increased load, density stratification, chlorophyll, and hypoxia in wet years. Mean 2013-2019 bay-wide total nitrogen load was 34% less than the 2005-2012 mean, a reduction of about 106 kg yr-1, comparable to the range of flow-driven inter-annual variations. Chlorophyll Index and Hypoxia Index event-based metrics applied to high-frequency time series observations at eight sites quantify exceedances of severe and moderate thresholds. Relatively steady 33% and 16% Chlorophyll Index declines, for severe and moderate thresholds, occurred from about 2007 to 2019. The Hypoxia Index declined markedly by 2009 and 2014 for severe and moderate thresholds, respectively, and remained at or near zero from 2014 to 2019. The load reduction explains chlorophyll and hypoxia declines better than physical processes including river flow, stratification, tidal variations, winds, sea level differences, and temperatures. River flow about 55% higher than the 2005-2019 mean would increase non-treatment facility loads by an amount comparable to the managed load decrease, so future wet summers could partially reverse the improvements. Long-term trends include warming of about 0.5°C decade-1, which reduces oxygen saturation by 0.1 mg l-1 decade-1. This rate is likely a lower bound for temperature-driven oxygen decreases, because warming can also accelerate phytoplankton growth and bacterial consumption. Without warming, the managed load decrease would have curtailed hypoxia more effectively. Climate trends should be at least as important to future eutrophication as the managed load decline because, in addition to warming influences, long-term increases in river flow would increase load and stratification.

Long-term declines in chlorophyll a and variable phenology revealed by a 60-year estuarine plankton time series

“…Phytoplankton-derived chl a concentrations in NBay exhibited long-term annual declines from 1968 through 2019, regardless of season, and provide context for reports of summertime declines in NBay chl a concentrations ( 14 , 32 ). Furthermore, the annual maximum chl a concentration decreased over time suggesting a waning intensity of phytoplankton bloom events.…”

“…1 A ) whose production has important implications for fisheries and human use along the coastal US ( 14 , 29 ). NBay is a highly seasonal system experiencing long-term (1950 to 2015) warming waters and more recently, shifts in anthropogenic nutrient loading through upgrades in wastewater treatment ( 30 – 33 ). Prior analyses of the partial NBPTS chl a dataset indicated declines of annual and seasonal means using linear regression ( 14 , 34 , 35 ), but no formal time-series analysis has been conducted on the entire 60-y NBPTS to test hypotheses regarding either the influence of environmental parameters on changing chl a concentrations or long-term shifts in phenology.…”

Effect of nutrient reductions on dissolved oxygen and pH: a case study of Narragansett bay

“…At a basic level, hypoxia is driven by organic matter respiration which, in strongly seasonal and temperature-stratified systems, tends to be decoupled in space and time from production (Cloern and Jassby, 2008;Rabalais et al, 2010). Nutrient-fueled eutrophication has generally been the major driver of coastal hypoxia, and efforts are ongoing in several coastal systems to mitigate eutrophication through reduction of external nutrient loading, particularly from wastewater treatment facilities (WWTF) (Diaz and Rosenberg, 2008;Boesch and Goldman, 2009;Taylor et al, 2011Taylor et al, , 2020Greening et al, 2014;Staehr et al, 2017;Lefcheck et al, 2018;Boesch, 2019;Rabalais and Turner, 2019;Codiga et al, 2022). Nutrient load reductions reduce hypoxia by decreasing primary production and the subsequent delivery of organic material to bottom waters.…”

“…Thanks to the aggressive management actions that have been implemented throughout the NB watershed, including nitrogen reduction upgrades at eleven WWTFs within Rhode Island, the total nutrient reduction from prior 2005 to 2012 was greater than 50% (Oviatt et al, 2017) and the mean bay-wide nitrogen loading to NB in 2013-2019 was 34% less than the mean loading from 2005-2012 (Codiga et al, 2022). The primary productivity and occurrence of hypoxia has markedly declined after the application of these managed nutrient reductions (Codiga et al, 2022). Nutrient reduction should theoretically alleviate OA in bottom water because of reduced organic carbon respiration.…”

“…The aggressive nutrient reduction provides a rare opportunity to understand the feedback of OA to intentional decadal nutrient reduction in the context of global warming and climate change (Codiga et al, 2022). Codiga (2020) has reported that in NB, nearbottom water (0.1°C yr -1 ) was warming twice as fast as the surface (0.05°C yr -1 ) and suggested the unexpected faster warming trend near the bottom has less to do with local air sea interaction and the influence of warming air temperatures and is more likely the result of the intrusion of deep shelf waters (Pfeiffer-Herbert et al, 2015) that have warmed as a result of the warming in northwest Atlantic shelf water (Wu et al, 2012;Goncalves Neto et al, 2021).…”