Expert picks of journal articles, non-peer reviewed articles, or other scientific publications that have bearing on nutrient criteria development with associated summaries. Click on the article title to access its abstract and the review provided by an expert.

• Experimental nutrient additions accelerate terrestrial carbon loss from stream ecosystems. Rosemond, A.D., et al. 2015. Science 347(6226), 1142–1145.

  Abstract: Nutrient pollution of freshwater ecosystems results in predictable increases in carbon (C) sequestration by algae. Tests of nutrient enrichment on the fates of terrestrial organic C, which supports riverine food webs and is a source of CO₂, are lacking. Using whole-stream nitrogen (N) and phosphorus (P) additions spanning the equivalent of 27 years, we found that average terrestrial organic C residence time was reduced by ~50% as compared to reference conditions as a result of nutrient pollution. Annual inputs of terrestrial organic C were rapidly depleted via release of detrital food webs from N and P co-limitation. This magnitude of terrestrial C loss can potentially exceed predicted algal C gains with nutrient enrichment across large parts of river networks, diminishing associated ecosystem services.

  Full Report: Experimental nutrient additions accelerate terrestrial carbon loss from stream ecosystems
  Picked by Stephen Maurano (maurano.stephen@epa.gov), U.S. EPA, Region 4

  Review: Aquatic life in streams are fueled by carbon from algal photosynthesis, and from terrestrial sources, such as leaves and wood. Nutrient criteria are commonly derived to prevent an excess in algal carbon production, and now researchers are demonstrating nutrient impacts on terrestrially derived carbon. As reported in this recent Science article, terrestrial organic carbon residence time in forest streams was cut approximately in half (from 167 to 75 days) as a result of experimental nitrogen and phosphorus additions. The accelerated loss of terrestrial carbon outweighed the increase in algal carbon sequestration – resulting in a net carbon loss with the potential to disrupt riverine food webs.

  Although excess algal biomass in streams via the “green” pathway of primary producers may be more visually obvious, this research demonstrates the importance of nutrient effects via the “brown” detrital pathway. The publication also highlights the importance of dual nutrient control, as nitrogen and phosphorus not only co-limit algae in many waterbodies, but as these results indicate, they appear to co-limit loss rates of terrestrial carbon from streams as well. Ecosystem functions, like litter decomposition rates, provide promising endpoints to be integrated into conceptual models for nutrient criteria.

• Evolving paradigms and challenges in estuarine and coastal eutrophication dynamics in a culturally and climactically stressed world. The H.T. Odum Synthesis Essay. Paerl, H.W. et al. 2014. Estuaries and Coasts 37:243-258.

  Abstract: Coastal watersheds support more than one half of the world’s human population and are experiencing unprecedented urban, agricultural, and industrial expansion. The freshwater–marine continua draining these watersheds are impacted increasingly by nutrient inputs and resultant eutrophication, including symptomatic harmful algal blooms, hypoxia, finfish and shellfish kills, and loss of higher plant and animal habitat. In addressing nutrient input reductions to stem and reverse etrophication, phosphorus (P) has received priority traditionally in upstream freshwater regions, while controlling nitrogen (N) inputs has been the focus of management strategies in estuarine and coastal waters. However, freshwater, brackish, and full-salinity components of this continuum are connected structurally and functionally. Intensification of human activities has caused imbalances in N and P loading, altering nutrient limitation characteristics and complicating successful eutrophication control along the continuum. Several recent examples indicate the need for dual N and P input constraints as the only nutrient management option effective for long-term eutrophication control. Climatic changes increase variability in freshwater discharge with more severe storms and intense droughts and interact closely with nutrient inputs to modulate the magnitude and relative proportions of N and P loading. The effects of these interactions on phytoplankton production and composition were examined in two neighboring North Carolina lagoonal estuaries, the New River and Neuse River Estuaries, which are experiencing concurrent eutrophication and climatically driven hydrologic variability. Efforts aimed at stemming estuarine and coastal eutrophication in these and other similarly impacted estuarine systems should focus on establishing N and P input thresholds that take into account effects of hydrologic variability, so that eutrophication and harmful algal blooms can be controlled over a range of current and predicted climate change scenarios.

  Full Report: Evolving Paradigms and Challenges in Estuarine and Coastal Eutrophication Dynamics in a Culturally and Climatically Stressed World
  Picked by Jacques L. Oliver (jacques.oliver@epa.gov), U.S. EPA, Office of Water

  Review: Synthesis is the process of identifying pieces of information, sometimes seemingly disparate and unrelated, and weaving them into an integrated, coherent thesis. Hans Paerl and colleagues have published one such synthesis on the topic of nutrient pollution, what those of us in the nutrient criteria world commonly (albeit inaccurately) refer to as “eutrophication” (see Nixon 1995 and references therein for a closer look at the term). The theme Paerl et al. 2014 offers is a familiar one: nutrients pollute the aquatic continuum stretching from headwaters all the way to coastal estuaries. Indeed, similar papers (Lewis et al. 2000, Elser et al. 2007, Conley et al. 2009) and special volumes (Limnology and Oceanography Special Issue 51(1), 2006) have been dedicated to subject. High level government science panels have also weighed in (NRC 2000, USEPA 2007, USEPA 2011). Dr. Paerl himself has published extensively on nutrient pollution and eutrophication evidenced by the numerous citations of his papers, particularly as they relate to nutrient limitation by both nitrogen and phosphorus. So why publish another paper telling us what has already been said?

  The paper is peppered with citations that illustrate the larger point that the nutrient limitation paradigm, i.e., freshwater primary productivity is limited by phosphorus and estuarine/marine primary productivity are limited by nitrogen, is not supported upon closer examination of the scientific record. As one would expect, the studies cited by Paerl et al. 2014 were conducted in a specific context (e.g., Mallin et al. 2005, Finlay et al. 2010). Yet when the results of these papers are viewed in the aggregate a different paradigm is revealed – one that simply does not align with the phosphorus-freshwater and nitrogen-estuarine/marine echo chamber of the past 50 years. Controlling both nitrogen and phosphorus is not the only message implied in these papers. Controlling both pollutants at the scales at which the effects of nitrogen and phosphorus occur is also important. Paradigms that are scaled broadly in time (e.g., decades) and space (e.g., continents), such as the phosphorus-freshwater and nitrogen-estuarine/marine paradigm, lack the resolution to accommodate the exceptions that occur at smaller temporal and spatial scales. As it turns out, Paerl et al. 2014 provides compelling evidence of the growing number of “exceptions” to the paradigm such that ignoring them will likely make water quality management of nutrient pollution at the state and local levels increasingly difficult (also see Glibert et al. 2014 and Mike Paul’s associated review).

  Perhaps what distinguishes Paerl et al. 2014 from other synthesis papers is what distinguishes our current period in the climate record: rapid climate change. In reminding us of what we already observe – atmospheric temperatures are rising, droughts are becoming increasingly more frequent and intense, and altered watershed hydrology, or rather the changes in the variability in watershed hydrology, is changing before our eyes – the authors weave the myriad climatic stressors together with the persistent nutrient pollution stress that has been occurring across the freshwater-marine continuum (and will likely increase with human population growth). The resulting predictions the authors make are intuitive and consistent with documented effects observed on smaller scales. For example, warming of aquatic systems, and the growing duration of those warmer conditions, will likely create more opportunities for algae that have higher temperature optima for growth, particularly the harmful algae (see Figure 2). Coupling algal growth optima at higher temperatures to hydrologic variability and high nutrient loads could yield algal community responses manifested as larger and more frequent blooms (see Figure 8). These would not only adversely affect resident aquatic life, but other important human uses such as drinking water sources and recreation (see Yuan et al. 2014, Yuan and Pollard 2014). While the predictions Paerl et al. 2014 make are global, the management actions they recommend ultimately have to be translated at the state and local level, specifically setting water quality expectations for both nitrogen and phosphorus and the subsequent reductions in their inputs across the freshwater-marine continuum. With climate change upon us, the authors argue, the time to act is now.

  Conley, D.J., H.W. Paerl, R.W. Howarth, D.F. Boesch, S.P. Seitzinger, K.E. Havens, C. Lancelot, and G.E. Likens. 2009. Controlling eutrophication: nitrogen and phosphorus. Science 323:1014-1015.

  Elser, J.J., M.E.S. Bracken, E.E. Cleland, D.S. Gruner, W.S. Harpole, H. Hillebrand, J.T. Bgai, E.W. Seabloom, J.B. Shurin, and J.E. Smith. 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters 10:1124-1134.

  Finlay, K., A. Patoine, D.B. Donald, M. Bogard, and P.R. Leavitt. 2010. Experimental evidence that pollution with urea can degrade water quality in phosphorus-rich lakes of the northern Great Plains. Limnology and Oceanography 55: 1213-1230.

  Glibert, P.M., R. Maranger, D.J. Sobota, and L. Bouwman. 2014. The Haber-Bosch-harmful algal bloom (HB-HAB) link. Environmental Research Letters 9:105001.

  Limnology and Oceanography. Special Issue 51(1). 2006. Eutrophication of freshwater and marine ecosystems.

  Mallin, M.A., M.R. McIver, H.A.Wells, D.C. Parsons, and V.L. Johnson. 2005. Reversal of eutrophication following sewage treatment upgrades in the New River Estuary, North Carolina. Estuaries 28:750-760.

  National Research Council. 2000. Clean coastal waters: understanding and reducing the effects of nutrient pollution. Ocean Studies Board and Water Science and Technology Board, Commission on Geosciences, Environment, and Resources. Washington, DC: National Academy Press. 405 p.

  Nixon, S.W. 1995. Coastal marine eutrophication: a definition, social causes, and future concerns. Ophelia 41:199-219.

  USEPA. 2007. Hypoxia in the Northern Gulf of Mexico: An Update by the EPA Science Advisory Board. Scientific Advisory Board Publication No. EPA-SAB-08-003. U.S. Environmental Protection Agency. Washington, DC.

  USEPA. 2011. Reactive nitrogen in the United States: an analysis of inputs, flows, consequences, and management options. Scientific Advisory Board Publication No. EPA-SAB-11-013. U.S. Environmental Protection Agency. Washington, DC.

  Yuan, L.L and A.I. Pollard. 2014. Classifying lakes to quantify relationships between epilimnetic chlorophyll a and hypoxia. Environmental Management December 2014.
  Yuan, L.L, A.I. Pollard, S. Pather, J.L. Oliver, and L. D’Anglada. 2014. Managing microcystin: identifying national-scale thresholds for total nitrogen and chlorophyll a. Freshwater Biology 59:1970-1981.

• The Haber-Bosch-harmful algal bloom (HB-HAB) link. Glibert, P.M. et al. 2014. Environmental Research Letters 9:105001.

  Abstract: Large-scale commercialization of the Haber–Bosch (HB) process is resulting in intensification of nitrogen (N) fertilizer use worldwide. Globally N fertilizer use is far outpacing that of phosphorus (P) fertilizer. Much of the increase in N fertilizers is also now in the form of urea, a reduced form of N. Incorporation of these fertilizers into agricultural products is inefficient leading to significant environmental pollution and aquatic eutrophication. Of particular concern is the increased occurrence of harmful algal blooms (HABs) in waters receiving nutrient enriched runoff. Many phytoplankton causing HABs have physiological adaptive strategies that make them favored under conditions of elevated N : P conditions and supply of chemically reduced N (ammonium, urea). We propose that the HB-HAB link is a function of (1) the inefficiency of incorporation of N fertilizers in the food supply chain, the leakiness of the N cycle from crop to table, and the fate of lost N relative to P to the environment; and (2) adaptive physiology of many HABs to thrive in environments in which there is excess N relative to classic nutrient stoichiometric proportions and where chemically reduced forms of N dominate. The rate of HAB expansion is particularly pronounced in China where N fertilizer use has escalated very rapidly, where soil retention is declining, and where blooms have had large economic and ecological impacts. There, in addition to increased use of urea and high N:P based fertilizers overall, escalating aquaculture production adds to the availability of reduced forms of N, as does atmospheric deposition of ammonia. HABs in both freshwaters and marginal seas in China are highly related to these overall changing N loads and ratios. Without more aggressive N control the future outlook in terms of HABs is likely to include more events, more often, and they may also be more toxic.

  Full Report: The Haber Bosch–harmful algal bloom (HB–HAB) link (PDF) (14 pp, 1 MB, About PDF)
  Picked by Michael J. Paul (Michael.Paul@tetratech.com), Senior Scientist, Center for Ecological Sciences, Tetra Tech, Inc.

  Review: This paper proposes hypotheses explaining the apparent linkage between reduced nitrogen forms and harmful algal blooms/cyanotoxin production. The authors review the role of the Haber-Bosch process, the chemical process by which synthetic fertilizer is made, and the proliferation of reduced nitrogen (especially urea) in the biosphere, which is now many times what it was historically. They also dispute many traditionally held opinions regarding nutrient limitation by P and N, arguing that N limitation is more common than believed across a range of aquatic systems. The propose that the HB-HAB link is based on 2 major factors: (1) the inefficiency of incorporation of N fertilizers in the food supply chain, the leakiness of the N cycle from crop to table, and the fate of lost N relative to P to the environment; and (2) the adaptive physiology of many HABs to thrive in environments in which there is excess N relative to classic nutrient stoichiometric proportions and where chemically reduced forms of N dominate.

  Some highlights include the following:

  • Not all cyanobacteria are nitrogen fixers, including many toxin forming taxa (e.g., Microcystis);
  • Nitrogen fixation does not offset N limitation and denitrification does not remove all nitrogen;
  • Some toxins are more common under high N:P ratios and are N rich compounds;
  • Many cyanobacteria have low P requirements;
  • Diatoms prefer oxidized forms of nitrogen; cyanobacteria prefer reduced forms of N;
  • Reduced nitrogen species, especially urea, stimulates cyanobacteria and other HAB taxa;
  • Many HABs have alternate strategies to acquire P, therefore are less dependent on dissolved P;
  • Eutrophication management requires both N and P management.
• Land use patterns, ecoregion, and microcystin relationships in U.S. lakes and reservoirs: a preliminary evaluation. Beaver, J.R. et al. 2014. Harmful Algae 36:57-62.

  Abstract: A statistically significant association was found between the concentration of total microcystin, a common class of cyanotoxins, in surface waters of lakes and reservoirs in the continental U.S. with watershed land use using data from 1156 water bodies sampled between May and October 2007 as part of the USEPA National Lakes Assessment. Nearly two thirds (65.8%) of the samples with microcystin concentrations ≥1.0 μg/L (n = 126) were limited to three nutrient and water quality-based ecoregions (Corn Belt and Northern Great Plains, Mostly Glaciated Dairy Region, South Central Cultivated Great Plains) in watersheds with strong agricultural influence. canonical correlation analysis (CCA) indicated that both microcystin concentrations and cyanobacteria abundance were positively correlated with total nitrogen, dissolved organic carbon, and temperature; correlations with total phosphorus and water clarity were not as strong. This study supports a number of regional lake studies that suggest that land use practices are related to cyanobacteria abundance, and extends the potential impacts of agricultural land use in watersheds to include the production of cyanotoxins in lakes.

  Full Report: Land use patterns, ecoregion, and microcystin relationships in U.S. lakes and reservoirs: A preliminary evaluation
  Picked by Mario Sengco (mario.sengco@epa.gov), U.S. EPA, Office of Water

  Review: This paper is a preliminary assessment of relationships between microcystin concentrations and common environmental variables available from the EPA National Lakes Assessment (location, land use, total nitrogen, total phosphorus, temperature, etc.). Primarily using the canonical correlation analysis, Beaver et al. found a significant relationship between microsystin concentration and the dependent variables temperature, total nitrogen, and dissolved organic carbon. Further, control of these dependent variables appears to be driven by land use type and ecoregion. Land use type only partially reflected microcystin concentrations, with the strongest land use–microcystin relationships existing in the Midwest, where total nitrogen concentrations (driven by agriculture) were highest. The results of this study do not allow for prediction of microcystin concentrations based on the causal variables, but in time, it may provide a stepping stone for predicting the potential impacts of microcystin when developing numeric nutrient criteria.

• Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Elser, J.J. et al. 2007. Ecology Letters 10:1135-1142.

  Abstract: The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.

  Full Report: Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems (PDF) (8 pp, 210 K, About PDF)
  Picked by Michael J. Paul (Michael.Paul@tetratech.com), Senior Scientist, Center for Ecological Sciences, Tetra Tech, Inc.

  Review: This paper is a meta-analysis of nitrogen (N) and phosphorus (P) enrichment experiments from terrestrial, freshwater, and marine ecosystems across the world. It was the largest such meta-analysis of its kind and incorporated more than 1000 individual studies. The goal of the synthesis was to determine if there were broad ecosystem differences in N and P limitation (or co-limitation) or whether they were generally similar, as theory might predict given that all phototrophs use similar biochemical processes. This study used the response ratio (RR = ln(E/C), where E is measures value of the response variable in enrichment treatment and C is the value for the control; a value above 0 indicates an enrichment response) as the metric to compare effects across disparate studies. It is a commonly used meta-analysis metric when comparing across disparate studies, measures, habitats, etc. They used Analysis of Variance (ANOVA) to compare RR across ecosystem types and sub-habitats.

  The authors found the following:

  • N and P limitation is broad and widespread across major habitats of the biosphere;
  • The average strength of P limitation is similar across the three ecosystem types;
  • The average strength of N limitation is similar to P limitation in freshwaters, but greater than P limitation in marine waters (i.e., experiments in freshwater show equal enrichment response to N alone as they do to P alone; marine systems show P enrichment effects, but greater N enrichment effects);
  • Simultaneous additions of N and P produce synergistically higher response ratios than single nutrient additions and this effects is greater in freshwaters (i.e., adding N and P together causes a greater enrichment effect than either alone or even additively, and it is very large in freshwaters).

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