Anthropogenic hypoxia: A Summary of the eutrophication of the Northeastern United States’ principal

Updated: Aug 27, 2021

Why are Northeastern American waters deteriorating? Find out in this article.


“The principal consequences of eutrophication include the formation of anoxic “dead zones” in the bay, the death of aquatic plant life, and heightened levels of water toxicity

The Chesapeake Bay, being the largest estuary in the United States and the third largest in the world, continues to be of paramount importance to the health of the 10 million people who call the region home1. The health of the bay is responsible for the production of over 500 million pounds of seafood every year1, approximately 23% of the total yearly seafood production of the United States3. Its watershed, composed of over 100,000 rivers and streams, stretches through not only Maryland, but Delaware, Pennsylvania, New Jersey, and even the state of New York.2 Despite its paramount importance to the economic and ecological vitality of the Northeast, the health of the Chesapeake bay continues to be in decline. The populations of many ecologically essential fish species, such as the Menhaden, have declined by as much as 85% on an historical scale.4 Algal blooms, habitat destruction, and lower life expectancies in local wildlife have become the regional standard, and many of these issues are a symptom of a much larger problem: eutrophication.


Causes

Eutrophication is, in general terms, the significant and environmentally destructive increase in the consumption of ecologically available nutrients on an ecosystem-level scale. Eutrophication is typically used to describe this phenomenon in rivers and estuaries; however, eutrophication may occur in any body of water; including creeks, lakes, and even seas. While eutrophication is the result of the complex evolution of several environmental parameters, its primary cause is increased nutrient concentrations, most notably Nitrogen and Phosphorus. Excessive concentrations of these nutrients, which may originate from a wide variety of sources, promote the growth of single-celled and simple multi-celled microbial organisms such as algae and phytoplankton. As these organisms Begin to dominate the microbial biome of a given body of water, a variety of consequences ensue; including hypoxic or anoxic conditions due to increased microbial oxygen uptake, and even the death of aquatic plant life due to reduced water clarity, a case in which algal blooms are largely to blame. Thus, this largely anthropogenic phenomenon may disrupt or outright destroy the delicate ecosystems within and throughout many of our most ecologically critical water bodies, including the Chesapeake Bay.

The primary culprits of this destructive phenomenon are heightened levels of Nitrogen and Phosphorus. While a significant portion of these nutrients originate from natural sources (such as the atmosphere, in the case of Nitrogen), it has been estimated that most of these nutrients are somehow associated with anthropogenic origins. These origins largely include agricultural runoff and wastewater treatment plant effluent. Agricultural runoff, being the largest constituent of anthropogenic nutrient sources, is estimated to be responsible for up to 39% of the total nutrient influent in the Chesapeake Bay for Nitrogen, and 49% for Phosphorus.­5 Much of this agricultural runoff may be traced back to fertilizers and manures, both of which contain high concentrations of bioavailable Nitrates and Phosphates. While these nutrients largely promote the health of many economically important crops, a significant proportion of these nutrients, over time, find their way to creeks, streams, and groundwater. As these nutrients are transmitted via these seemingly insignificant water bodies, they eventually flow into larger bodies, such as rivers and wetlands, many of which happen to be tributaries of the Chesapeake Bay. Thus, every farm, ranch, or agricultural production center located within the watershed of the Chesapeake may contribute to this excess of nutrients, and the environmental damage which ensues.

While wastewater treatment plant effluent had, historically, been a principal source of organic pollutants in the Chesapeake, its culpability has been diminished in more recent times, largely due to advances in wastewater treatment technology in addition to large-scale federal and state initiatives. For reference, the proportion of Nitrogen loading due to wastewater treatment plant effluent decreased from 28% of all Nitrogen loading in 1985 to 16% in 2015.6 A more prominent trend can be seen in the loading of Phosphorus; the proportion of Phosphorus loading decreased from 39% in 1985 to 16% in 2015.6 While the progress seen thus far in the wastewater treatment industry has played a critical role in the restoration of the bay, the adoption of new technologies in regional wastewater treatment plants and collection systems, including bioremediation programs, in addition to the continuation of large-scale federal and state programs may be paramount to maintaining the progress attained thus far.


Impacts

The eutrophication of the Chesapeake Bay is responsible for a wide variety of consequences, both economical and ecological. In terms of ecological impact, the principal consequences of eutrophication include the formation of anoxic “dead zones” in the bay, the death of aquatic plant life, and heightened levels of water toxicity. Hypoxic or anoxic “dead zones” are volumes of water which exhibit critically low concentrations of dissolved oxygen (DO) and, despite their extreme destructive potential, are largely the result of something rather benign and inconspicuous: algae. When the concentrations of organic pollutants reach sufficient levels, and other environmental factors (temperature, pH, etc.) lend their favor, there is then a high likelihood of an algal bloom. As the population of algae (as well as other cyanobacteria and phytoplankton) increases at exponential rates, so does the consumption of suspended nutrients, until those nutrients have been largely consumed. It is at this stage that the algae population decreases at a rate not unlike the initial rate of population growth. With the algae population significantly declined, the nutrients stored in the form of dead algae cells are then decomposed – a process which, at scale, consumes large quantities of oxygen, enough to form hypoxic dead zones. The lack of oxygen in these volumes of water dissuade and even prohibit the growth of larger, oxygen-consuming organisms, including fish, crabs, and even non-aquatic organisms whose diet is largely composed of these complex aquatic lifeforms.

In addition to the formation of hypoxic dead zones, such algal blooms oftentimes induce a secondary ecological consequence: the decay of aquatic plant populations, as well as the populations of the organisms which critically depend upon these plants for sustenance and shelter. The Chesapeake bay region includes over 2,700 species of plants1 – many of which are aquatic, which serve as the foundation of the ecosystem’s food web, in addition to a variety of other ecological functions. When an aquatic biome undergoes an algal bloom, the thick layers of algae reduce the transmittance of sunlight to the floor of the lake, river, or in the case of the Chesapeake, the bay. Without the presence of this sunlight, aquatic plant populations may, depending upon the severity of the algal bloom, decline to near-critical levels. Consequently, the populations of herbivorous fish, mollusks, and various invertebrates, as well as the organisms which rely on them, soon follow. Thus, the absence of these critical plants may propagate echoes which reverberate throughout the ecosystem, cleaving the network of life built upon the collimation and exchange of energy, of which these aquatic plants act as a focal point. It is by this mechanism that algal blooms of sufficient intensity may disassemble and reorganize entire ecosystems, in ways which are usually inconsistent with long-term biodiversity and ecological sustainability.

While most of the consequences of large-scale algal blooms allow for a considerable degree of ecological plasticity, the most severe consequence of these events may also be the most deadly and irreversible: toxicity. The production of toxic substances known as Cyanotoxins is oftentimes a result of the natural decay of dead algae cells. As the population of algae in an algal bloom begin to diminish, the concentration of cyanotoxins may reach levels high enough to endanger both aquatic and land animals -- including humans8. Ecological effects aside, the human health effects associated with the ingestion of these toxins, including seizures, paralysis, and liver failure, are potentially severe enough to compel local authorities in affected areas to shut down fisheries and recreational areas, such as beaches9. While high levels of cyanotoxins are rarely associated with fish deaths, the ecological concentration of such toxins towards the top of the local food chain may pose a threat to local seabird populations, as suggested by a recent study pertaining to the poisoning of Alaskan seabird hatchlings due to large concentrations of ecologically available cyanotoxins10. In addition to seabirds, high concentrations of cyanotoxins due to algal blooms have been associated with the death of several land mammals, including livestock and pets – particularly dogs.


Treatment

Due to the increasing frequency and severity of algal blooms in waterbodies throughout the United States, a variety of options to prevent these events, as well as treating the underlying issue of eutrophication, have undergone extensive development and evolution throughout recent years. One such option involves the application of the biochemical principles which govern Sequencing Batch Reactors (SBR’s) used in many modern WWTP’s to sedimentary denitrification and Anaerobic Ammonium Oxidation (Anammox) processes, both of which oftentimes occur in estuarine and pre-estuarine water bodies11. In an SBR designed to treat wastewater for suspended nitrates, DO content is fluctuated between high and low concentrations, where ammonia is converted to nitrate during high DO intervals, and nitrate is converted to nitrous oxide and, consequentially, diatomic nitrogen gas during low DO intervals. Throughout this entire process, these functions are carried out by a set of naturally-occuring anaerobic microbes. Similar microbes can be found in the sediments found within estuarine waters, where obligate and facultative anaerobes engage in a similar denitrification process.

Thus, a means to eliminate suspended nitrogenous compounds present in small bodies of moving water presents itself in the form of a Sequencing Sedimentary Denitrification Reactor (SSDR). These reactors are composed of an initial aeration stage, in which the DO content of incoming water is increased such that aerobic metabolism of suspended carbon sources may be sustained for a short time. Following a section in which DO is allowed to gradually return to near-hypoxic conditions, is a section consisting of a porous substrate located on the floor of the water body. If sufficiently low DO levels are maintained within this section, obligate and facultative anaerobes present in the porous substrate, as well as local sediments, may engage in the biochemical processes which facilitate the operation of denitrifying SBR’s, thus eliminating a portion of the nitrogen present in the incoming water volume. By constructing repeating sections of SSDR’s, it may be possible to decrease nitrogen concentrations to levels which dissuade explosive algae growth. While the SSDR is currently an untested technology, the underlying principle which governs its operation, sedimentary denitrification, is believed to be responsible for much of the natural denitrification which occurs in many of our world’s waterways; for example, it has been estimated that approximately 31% of all natural denitrification performed in the Pearl River Estuary located in Southern China is largely caused by Sedimentary Denitrification11.

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