Persistent eutrophication and hypoxia in the coastal ocean

Abstract

Nutrient loading (notably nitrogen and phosphorus) to coastal oceans from food production, fossil fuel burning, aquaculture operations, and wastewater from humans, livestock, and industry has accelerated during the past decades, causing over-enrichment of nutrients, or eutrophication. Eutrophication degrades coastal water quality with two most common symptoms, hypoxia and harmful algal blooms, creating profound ecological and societal consequences such as biodiversity decline, seagrass loss, coral bleaching, fish kills and marine mammal mortalities, and human health threats. Such marine pollution symptoms have persisted although billions of dollars have been invested in both research and management as well as efforts of restorations in many developed countries. Consequently, we are still witnessing trends in the expansion of coastal eutrophication and hypoxia from developed regions into developing regions. Though the limited efficacy of mitigation witnessed so far suggests the complexity of the issue, we contend that closing the knowledge gaps in the causality between eutrophication and hypoxia is essential and crucial towards making science-and evidence-based policies. We recognize that the non-linear cause-effect relationship in coastal marine ecosystem degradation caused by multi-stressors is complex.The strength and synergistic effect of multiple driving forces of coastal eutrophication is dependent on regional geographic feature, economic development, and societal management, while the long-term trends of eutrophication and hypoxia are subject to the control of the trends in nutrient loadings and physical dynamics under a changing climate. This review also examines lessons from past eutrophication management practices, and advocates for a better, more efficient indexing system of coastal eutrophication and an advanced regional earth system modeling framework to facilitate the development and evaluation of effective policy and restoration actions.

Increasing eutrophication driven by the increase of phosphate discharge in a subtropical bay in the past 30 years

“…Coastal regions, situated between the land and the ocean, are rich in biological resources and diverse ecosystems, and are also the center of global economic development and human activities (Winther et al, 2020;Dai et al, 2022;Dai et al, 2023). However, the rapid development of socio-economy and climate change have continuously intensified the environmental loads on this vital area since the Industrial Revolution, which in turn has had a negative impact on human society and has damaged the sustainability of the region and global oceans (Doney et al, 2012;Winther et al, 2020;Dai et al, 2023).…”

“…Coastal regions, situated between the land and the ocean, are rich in biological resources and diverse ecosystems, and are also the center of global economic development and human activities (Winther et al, 2020;Dai et al, 2022;Dai et al, 2023). However, the rapid development of socio-economy and climate change have continuously intensified the environmental loads on this vital area since the Industrial Revolution, which in turn has had a negative impact on human society and has damaged the sustainability of the region and global oceans (Doney et al, 2012;Winther et al, 2020;Dai et al, 2023). Among many factors, pollution of nutrients, primarily due to massive discharges from agriculture and industry, has caused worldwide expansion of eutrophication in the coastal regions, which can directly endanger the biodiversity, habitat and even mass mortality of impacted marine organisms, and thus eventually impair marine ecosystem and human society (Carpenter, 2008;Deegan et al, 2012;Breitburg et al, 2018;Malone and Newton, 2020;Mauŕe et al, 2021).…”

“…Among many factors, pollution of nutrients, primarily due to massive discharges from agriculture and industry, has caused worldwide expansion of eutrophication in the coastal regions, which can directly endanger the biodiversity, habitat and even mass mortality of impacted marine organisms, and thus eventually impair marine ecosystem and human society (Carpenter, 2008;Deegan et al, 2012;Breitburg et al, 2018;Malone and Newton, 2020;Mauŕe et al, 2021). Although great efforts have been made to reduce nutrient (mainly nitrogen and phosphorus) loads and lighten related environmental damages, eutrophication and its eco-environmental effects remain a persistent environmental crisis in the coastal oceans (Kelly et al, 2021;Lao et al, 2021;Wang et al, 2021b;Dai et al, 2023).…”

Particle-associated denitrification is the primary source of N2O in oxic coastal waters

“…Coastal eutrophication was reported as early as a century ago, and the record has been expanding from western Europe to North America, then to East Asia. Eutrophication is predicted to occur in the rest of the world (e.g., South America and Africa) in the near future, becoming one of the most persistent and challenging global environmental issues 2 , 8 . While the global sediment loads have concomitantly increased 7 and a dramatic spatial change of sediment flux has been observed over the past decades 5 , the influences of the rapid human- and climate-induced changes of sediment delivery on marine biogeochemistry and N 2 O production have not been recognized.…”

“…superimposed on global change (e.g., ocean warming, acidification, deoxygenation etc.) 2 , 3 . The massive discharge of human-induced nutrients (nitrogen (N) in particular) and erosional sediments to the coastal ocean are among the most fundamental anthropogenic perturbations to the global coastal ecosystems.…”