This article originally appeared in the September 2018 issue of Interstate Waters, a magazine published by the New England Interstate Water Pollution Commission. Dr. Ammerman is the Science Coordinator for the Long Island Sound Study.
By James Ammerman, PhD
Should we worry about phosphorus in estuaries? For overworked estuary managers around the world, including the Northeast, the short answer is generally no. However, one answer does not fit all estuaries in all seasons.
Debates about nitrogen versus phosphorus as the most important (limiting) nutrient in freshwaters versus estuarine and coastal waters go back decades and continue today. While both nutrients are essential to growth, in estuaries nitrogen is more likely to be the critical, limiting factor.
Humans are conducting a great global experiment with our rapid acceleration of the nitrogen and phosphorus cycles. The current water-quality impacts of this acceleration are likely more apparent than the current effects of climate change. Runoff, and loading of excess nitrogen and phosphorus to rivers, lakes, and coastal waters, cause excessive algae growth and degrade water quality.
The resulting negative impacts include nearly five hundred coastal low-oxygen, or hypoxic, zones around the world, sometimes known as “dead zones,” where bottom-water oxygen is very low. Excess nutrients also lead to global distributions of both marine and freshwater harmful algal blooms (HABs). Blooms may include the toxic freshwater cyanobacterium Microcystis. Additional impacts can include disruption of food chains and the loss of seagrass, fish, and shellfish.
Nitrogen fixation is the process that converts atmospheric nitrogen gas, which is largely inert, into ammonia or other chemical forms usable by living organisms. The Haber-Bosch process, developed in 1909, allowed the industrial fixation of nitrogen, a process previously limited to bacteria or lightning strikes. Nitrogen fertilizer and explosives are two of the most important products from this process. The former enabled the “Green Revolution” and the expansion of global agriculture that feeds the 7.6 billion people on earth today.
Phosphorus, in contrast, is mined from ancient marine sediments. While phosphorus is also an essential component of fertilizer, its use has accelerated more slowly. There are serious concerns that global phosphorus deposits may eventually be depleted.
Nitrogen and phosphorus are released into fresh and marine waters from both point-source inputs, such as wastewater treatment plants or industrial facilities, and nonpoint-source inputs, such as agriculture, stormwater, and atmospheric deposition. While there have been numerous successes in decreasing point source loads of both nitrogen and phosphorus to various water bodies, nonpoint-source load reductions have proven more challenging. That issue is discussed in the September 2017 Interstate Waters (“Pollution from Everywhere: States Confront Nonpoint Source Pollution”).
The concept of a limiting nutrient has a long history in both the agricultural and aquatic sciences. The limiting nutrient is the first of the essential nutrients to disappear from the environment, usually due to plant use of that nutrient, thus limiting plant growth. Without the limiting nutrient, plants cannot use other nutrients even when those others are abundant. The plants of interest are either agricultural crops or the algae in aquatic environments. These algae can live either in the water or on the bottom of the water body.
For estuary and water body managers, the question of which nutrient is limiting to growth is critical. The answer will inform decisions about how to expend resources to limit nutrients in the environment.
Early studies, where entire Canadian lakes were doused with phosphorus in the 1960s and 1970s, suggested phosphorus was the limiting nutrient. Reductions in phosphorus loading in Lakes Erie and Washington (near Seattle) during the same period also greatly improved water quality. Studies that added nitrogen to samples of estuarine water on the South Shore of Long Island and elsewhere suggested that nitrogen was the limiting nutrient in marine waters. Therefore, phosphorus is generally seen as limiting in lakes and streams, and nitrogen in estuaries and coastal waters, though there are important exceptions.
A common metric used in evaluating nutrient limitation is the Redfield ratio, which is the ratio of the molar concentration of nitrogen to phosphorus in the water. (A mole of nitrogen and phosphorus each has the same number of molecules, regardless of the differences in molecular weight.) When the ratio exceeds 16 to 1, which is the typical balanced ratio of nitrogen to phosphorus in many plants, the system may be phosphorus-limited, depending on the overall nutrient concentrations and other factors. The opposite situation, potential nitrogen limitation, may occur when the ratio is less than 16 to 1.
While there are still academic debates about nitrogen versus phosphorus limitation in estuaries and coastal waters, nitrogen is by far the most commonly limiting nutrient there. This includes estuaries in the Northeast. Coastal waters are naturally enriched in phosphorus relative to nitrogen, with a Redfield ratio of less than 16. Furthermore, in most experimental estuarine and coastal studies where water samples containing algae were incubated with added nutrients, the samples showed greater biological responses to nitrogen additions than phosphorus additions. As we succeed in reducing the nitrogen loading to many estuaries, the Redfield ratio in these estuaries declines further and nitrogen limitation becomes even stronger.
Some unfamiliar with the mechanism of a limiting nutrient express concerns that phosphorus will become more important as nitrogen loading is reduced, since phosphorus concentrations may remain high. In fact, the opposite is true: as nitrogen becomes more limiting, the likelihood of phosphorus limitation declines even more.
However, there are important examples of river-induced seasonal phosphorus limitation in some major estuaries and coastal regions. These occur largely where excessive nitrogen loading from rivers overwhelms any phosphorus enrichment. Two such examples occur in the two largest estuaries in the United States, the Chesapeake Bay and the Albemarle-Pamlico Sound (North Carolina). Examples also include the two largest dead zones in the world, in the Baltic Sea in Europe and the Louisiana coastal region of the Gulf of Mexico.
Neither the Baltic nor the Louisiana coast is a conventional estuary. The Baltic is a large inland sea with limited ocean exchange and is therefore brackish (with low salinity), and the Louisiana coast is a river-dominated coastal margin.
All four of these seasonally phosphorus-limited large aquatic ecosystems suffer significantly from excessive inputs of nitrogen and phosphorus, resulting in harmful algal blooms, hypoxia, and loss of fish and shellfish. In 2017, the Gulf of Mexico dead zone was the largest measured there since the beginning of monitoring in 1985. It was almost nine thousand square miles, about the area of New Jersey. The Baltic dead zone averages twice that size. Mathematical models suggest that significant reductions in both nitrogen and phosphorus could decrease the area of hypoxia in the Gulf of Mexico by more than reductions in nitrogen or phosphorus alone.
In most of these systems, phosphorus limitation occurs in the late spring to early summer, when river nitrogen loading is highest, and in an intermediate region of salinity between freshwater and seawater, where the growth of algae is greatest.
Both the EPA and the National Oceanic and Atmospheric Administration conduct periodic coastal and estuarine assessments that have a significant focus on the degree of eutrophication, that is, excess nutrients linked to blooms and oxygen depletion. The EPA’s National Coastal Condition Assessment, which is conducted every five years, is the broader of the two. The most recent report is the 2010 assessment, which was released in 2016. NOAA’s is the more narrowly focused National Estuarine Eutrophication Update, which is sporadic. It was last published in 2007.
An important component of the EPA’s assessment is its Water Quality Index, which incorporates nitrogen and phosphorus concentrations along with other parameters. The assessment rates each parameter as Good, Fair, or Poor. For both the Northeast and the national water quality indices, the assessment rates phosphorus concentrations as Fair or Poor much more often than nitrogen concentrations. This assessment implies, contrary to most research, that phosphorus is a greater threat to coastal water quality than nitrogen. The EPA’s phosphorus threshold needs to be re-evaluated. The agency has considered this issue, but so far has deferred change in order to maintain consistency with prior assessments.
In contrast, the NOAA update, which focuses on specific estuaries, dismisses nitrogen and phosphorus concentrations as unreliable eutrophication indicators. It includes only nitrogen loads, not concentrations, in its analyses. The rationale is that nitrogen is the primary limiting nutrient in estuaries.
So, what is the bottom line for estuary managers in the Northeast whose estuary has excess nitrogen and phosphorus? How should they spend limited funds available for nutrient control?
Study the Estuary: Nitrogen is probably the limiting nutrient in virtually all Northeast estuaries, unless there is extreme nitrogen loading from rivers. In that case, phosphorus limitation may occur. In many Northeast estuaries, nitrogen is declining or stable. As nitrogen declines, the possibility of phosphorus limitation becomes less likely.
Nonetheless, the nitrogen or phosphorus limitation in an estuary of interest should be demonstrated by research studies in that specific estuary. Investigation will establish the best cleanup needs and methods. This is particularly important where wastewater is a major source of nitrogen and phosphorus to an estuary. Wastewater treatment plants can more readily remove phosphorus, which is mostly in solids and sediments. Nitrogen is generally dissolved and requires extra treatment and expense to remove.
Phosphorus Reductions: Though every system is different and one size does not fit all, both the EPA and the European Union recommend both nitrogen and phosphorus reductions, or dual nutrient control, in many aquatic environments. This is particularly important when considering the entire freshwater–marine continuum of an aquatic ecosystem, since the limiting nutrient often shifts from phosphorus to nitrogen along the salinity gradient.
A good example is the Neuse River Estuary, which is a tributary of the Albemarle-Pamlico Sound. In the late 1980s, a ban on phosphorus detergents, combined with better wastewater treatment, improved the upstream water quality of the Neuse. However, the absence of phosphorus in the river also meant that nitrogen no longer fed aquatic plants there but instead flowed downstream to the higher-salinity region. There the nitrogen caused algal blooms and other problems. More recent nitrogen controls in this region may resolve the problems.
Similarly, phosphorus controls on the Seine and Scheldt Rivers in Europe, which drain to the English Channel and North Sea, respectively, have greatly reduced phosphorus concentrations in the rivers and improved their water quality. However, the phosphorus controls have done nothing to limit the algal blooms in the coastal waters, which are still nitrogen-limited.
In extreme cases of a heavily modified freshwater–marine continuum, such as the Florida Everglades, freshwater cyanobacterial blooms fed largely by excess phosphorus can flow downstream and directly invade coastal estuaries. In 2016 (and again this year), phosphorus- and nitrogen-laden Lake Okeechobee waters in Florida spawned massive cyanobacterial blooms that, due to heavy rains, were diverted to prevent flooding into rivers flowing both east and west to the coasts. Significant additional nitrogen and phosphorus added by septic systems in some urbanized downstream watersheds further intensified these blooms.
The photo below shows the original 2016 cyanobacterial bloom in Lake Okeechobee. This bloom was exported towards Florida’s east coast by the St. Lucie Canal, intensified by additional downstream nutrients, and ultimately flooded the St. Lucie River estuary with green slime. Unlike other systems described above, the major modification of the hydraulic flow regime in the Everglades exported the freshwater blooms themselves, and not just the excess nutrients, directly to the coast. This clearly demonstrates the need for dual nutrient control of both nitrogen and phosphorus at the freshwater source and downstream.
Keep Monitoring: Finally, estuary managers need to continue diligent monitoring and oversight efforts even in environments where nitrogen and phosphorus are declining. Phosphorus reductions and other improvements cleaned up Lake Erie in the 1960s and 1970s. However, inattention in later years has resulted in both increased bottom water hypoxia and major blooms of toxic cyanobacteria (Microcystis) from 2000 to the present.
Lake Erie is the drinking water source for eleven million people. Toledo, Ohio, a city at the western end of Lake Erie, shut down its drinking water intake for three days in 2014 in response to a cyanobacteria bloom. It was not the first Lake Erie community to do so. Though both total and point-source phosphorus loading to Lake Erie have remained within the target range, a highly bioavailable form of phosphorus has increased in the nonpoint source loads, probably due to changing agricultural practices in the Maumee River basin to the west of Lake Erie. Climate-change-induced warming of the lake may have also intensified these toxic blooms.
Ongoing Science: Nitrogen remains the major limiting nutrient and, therefore, the primary nutrient of concern in most estuaries. However, as our global experiment with the nitrogen and phosphorus cycles continues, with loading increases in some locations and declines in others, the best nutrient-control policies will continue to depend on current and robust research and monitoring efforts focused on each local estuary of concern.
James Ammerman, a NEIWPCC environmental analyst, is the Long Island Sound Study Science Coordinator. He also is on the adjunct faculty of Stony Brook University.