Hearing on Non-point Source Pollution: The Impacts of Agriculture on Water Quality

Congressional testimony by Robert W. Howarth, before the U.S. House of Representatives subcommittee on Water Resources & the Environment - The Impact of Agriculture on Water Quality. April 19, 2007

The role of sulfur in salt marsh metabolism

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1979The rate of sulfate reduction in stands of dwarf Spartina alterniflora in the Great Sippewissett Salt Marsh is approximately 75 moles S04 m2 year-1. This is the highest rate reported for any natural ecosystem. Sulfate reduction is the most important form of respiration in the marsh and results in the annual consumption of 1800 g C m-2, approximately equivalent to net primary production. Sulfate reduction rates in the peat are high for at least three reasons: 1) the below-ground production of Spartina alterniflora provides a large, annual input of organic substrates over a depth of some 20 cm, 2) sulfate is rapidly resupplied to the peat in infiltrating tidal waters, so low sulfate concentrations never limit the rate of sulfate reduction, and 3) sulfide concentrations remain below toxic levels. The stable mineral pyrite is a major end-product of sulfate reduction in salt marsh peat while iron mono-sulfides are not. This is unlike most anoxic marine sediments and apparently results because iron mono-sulfides are undersaturated. The iron mono-sulfides are undersaturated in part because of the relatively low concentration of total soluble sulfides and in part because of the fairly low pH of the peat. Both of these conditions probably result from the activity of the Spartina roots. If the incorporation of 35S into pyrite were not measured, the S3504 reduction measurements would greatly underestimate the true rate of sulfate reduction. Pyrite acts largely as a temporary store of reduced. sulfur. The pyrite concentration of the peat undergoes seasonal changes. On an annual basis, the reduced sulfur which results from sulfate reduction is either re-oxidized to sulfate within the peat or is exported, much of it as thiosulfate or a similar intermediately reduced compound. Most of the energy which is originally in organic matters is stored in reduced sulfur compounds when the organic matter is respired by sulfate reducing bacteria. Consequently, the export of reduced sulfur compounds from the peat represents an energy export. The export of energy as reduced inorganic sulfur compounds is probably larger than the net above-ground production by Spartina. This is an important vector for moving some of the energy trapped by the below-ground production of Spartina to zones where it is available for coastal food webs.Later support was from the National Science Foundation Grant DEB 78-03557

Controls of benthic nitrogen fixation and primary production from nutrient enrichment of oligotrophic, Arctic lakes

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosystems 16 (2013): 1550-1564, doi:10.1007/s10021-013-9701-0.We examined controls of benthic dinitrogen (N2) fixation and primary production in oligotrophic lakes in Arctic Alaska, Toolik Field Station (Arctic Long-Term Ecological Research Site). Primary production in many oligotrophic lakes is limited by nitrogen (N), and benthic processes are important for whole-lake function. Oligotrophic lakes are increasingly susceptible to low-level, non-point source nutrient inputs, yet the effects on benthic processes are not well understood. This study examines the results from a whole-lake fertilization experiment in which N and P were added at a relatively low level (4 times natural loading) in Redfield ratio to a shallow (3 m) and a deep (20 m) oligotrophic lake. The two lakes showed similar responses to fertilization: benthic primary production and respiration (each 50–150 mg C m−2 day−1) remained the same, and benthic N2 fixation declined by a factor of three- to fourfold by the second year of treatment (from ~0.35 to 0.1 mg N m−2 day−1). This showed that the response of benthic N2 fixation was de-coupled from the nutrient limitation status of benthic primary producers and raised questions about the mechanisms, which were examined in separate laboratory experiments. Bioassay experiments in intact cores also showed no response of benthic primary production to added N and P, but contrasted with the whole-lake experiment in that N2 fixation did not respond to added N, either alone or in conjunction with P. This inconsistency was likely a result of nitrogenase activity of existing N2 fixers during the relative short duration (9 days) of the bioassay experiment. N2 fixation showed a positive saturating response when light was increased in the laboratory, but was not statistically related to ambient light level in the field, leading us to conclude that light limitation of the benthos from increasing water-column production was not important. Thus, increased N availability in the sediments through direct uptake likely caused a reduction in N2 fixation. These results show the capacity of the benthos in oligotrophic systems to buffer the whole-system response to nutrient addition by the apparent ability for significant nutrient uptake and the rapid decline in N2 fixation in response to added nutrients. Reduced benthic N2 fixation may be an early indicator of a eutrophication response of lakes which precedes the transition from benthic to water-column-dominated systems.This project was supported by NSF-OPP 9732281, NSF-DEB 9810222, NSF-DEB 0423385, and by a Doctoral Dissertation Improvement Grant NSF-DEB 0206173. Additional funding was provided by the Small Grants Program through the NSF-IGERT Program in Biogeochemistry and Environmental Change at Cornell University

Variation in sediment and seagrass characteristics reflect multiple stressors along a nitrogen-enrichment gradient in a New England lagoon

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Haviland, K. A., Howarth, R. W., Marino, R., & Hayn, M. Variation in sediment and seagrass characteristics reflect multiple stressors along a nitrogen-enrichment gradient in a New England lagoon. Limnology and Oceanography, 67, (2022): 660-667, https://doi.org/10.1002/lno.12025.We examined concentrations of organic carbon, dissolved sulfides, total sediment sulfur, and stable sulfur isotope ratios in seagrass leaf tissues across a nitrogen-enrichment gradient in a coastal marine ecosystem (Cape Cod, Massachusetts) in 2007–2010 and 2017–2019. We also measured seagrass aboveground and belowground biomass, epibiota biomass, and leaf chlorophyll content. Seagrasses were present at all sites in the former period but were lost at our most nitrogen-impacted site (Snug Harbor) by 2011. In 2007–2010, sediment organic carbon and dissolved sulfides were highest in Snug Harbor and decreased along the gradient; leaf tissues depleted in 34S also indicated higher sulfide intrusion into seagrass tissues in more eutrophic areas. By 2017–2019, sediment organic carbon and pore-water soluble sulfides had decreased in Snug Harbor, but had increased at the intermediate site, to levels found at the most impacted site prior to the seagrass die-off. Again, leaf tissue 34S depletion reflected this pattern, indicating seagrasses were exposed to the highest sulfides at the intermediate site. The decreases in sediment organic carbon and soluble sulfides in Snug Harbor years after the loss of the seagrasses illustrate a feedback between high organic matter in seagrass beds and increasing stressors like elevated soluble sulfides in nutrient-enriched systems. We found significant relationships between sediment conditions and seagrass responses, including greater aboveground to belowground biomass ratios, epibiota biomass, and 34S-depleted leaves at sites with high pore-water sulfide and highly organic sediments. Our research suggests that the reduction of anthropogenic nitrogen entering the harbor is necessary for improving sediment quality and preventing seagrass mortality.The authors thankfully acknowledge as our funding sources: The National Science Foundation Biocomplexity, GRFP, and LTREB programs (grants 0420575, 1654845, and 2018241438), and the Woods Hole SeaGrant program

Roads as nitrogen deposition hot spots

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Biogeochemistry 114 (2013): 149-163, doi:10.1007/s10533-013-9847-z.Mobile sources are the single largest source of nitrogen emissions to the atmosphere in the US. It is likely that a portion of mobile-source emissions are deposited adjacent to roads and thus not measured by traditional monitoring networks, which were designed to measure longterm and regional trends in deposition well away from emission sources. To estimate the magnitude of near-source nitrogen deposition, we measured concentrations of both dissolved inorganic nitrogen (DIN) and total (inorganic + organic) dissolved nitrogen (TDN) in throughfall (i.e., the nitrogen that comes through the forest canopy) along transects perpendicular to two moderately trafficked roads on Cape Cod in Falmouth MA, coupled with measurements of both DIN and TDN in bulk precipitation made in adjacent open fields at the same transect distances. We used the TDN throughfall data to estimate total nitrogen deposition, including dry gaseous nitrogen deposition in addition to wet deposition and dry particle deposition. There was no difference in TDN in the bulk collectors along the transects at either site; however TDN in the throughfall collectors was always higher closest to the road and decreased with distance. These patterns were driven primarily by differences in the inorganic N and not the organic N. Annual throughfall deposition was 8.7 (+0.4) and 6.8 (+0.5) TDN - kg N ha-1 yr-1 at sites 10 m and 150 m away from the road respectively. We also characterized throughfall away from a non-road edge (power line right-of-way) to test whether the increased deposition observed near road edges was due to deposition near emission sources or due to a physical, edge effect causing higher deposition. The increased deposition we observed near roads was due to increases in inorganic N especially NH4 +. This increased deposition was not the result of an edge effect; rather it is due to near source deposition of mobile source emissions. We scaled these results to the entire watershed and estimate that by not taking into account the effects of increased gaseous N deposition from mobile sources we are underestimating the amount of N deposition to the watershed by 13% - 25%.This research was supported by Woods Hole SeaGrant (grant NA06OAR4170021), NSF IGERT (grant DGE 0221658), an Edna Bailey Sussman Environmental Internship Award from Cornell University, and a Mellon Foundation award though Cornell University.2014-04-1

Role of external inputs of nutrients to aquatic ecosystems in determining prevalence of nitrogen vs. phosphorus limitation of net primary productivity

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Howarth, R. W., Chan, F., Swaney, D. P., Marino, R. M., & Hayn, M. Role of external inputs of nutrients to aquatic ecosystems in determining prevalence of nitrogen vs. phosphorus limitation of net primary productivity. Biogeochemistry, (2021), https://doi.org/10.1007/s10533-021-00765-z.Whether net primary productivity in an aquatic ecosystem is limited by nitrogen (N), limited by phosphorus (P), or co-limited by N & P is determined by the relative supply of N and P to phytoplankton compared to their elemental requirements for primary production, often characterized by the “Redfield” ratio. The supply of these essential nutrients is affected by both external inputs and biogeochemical processes within the ecosystem. In this paper, we examine external sources of nutrients to aquatic systems and how the balance of N to P inputs influences nutrient limitation. For ocean subtropical gyres, a relatively balanced input of N and P relative to the Redfield ratio from deep ocean sources often leads to near co-limitation by N and P. For lakes, the external nutrient inputs come largely from watershed sources, and we demonstrate that on average the N:P ratio for these inputs across the United States is well above that needed by phytoplankton, which may contribute to P limitation in those lake that experience this average nutrient loading. Watershed inputs are also important for estuaries and coastal marine ecosystems, but ocean sources of nutrients are also significant contributors to overall nutrient loads. The ocean-nutrient sources of N and P are very often at or below the Redfield ratio of 16:1 molar, and can be substantially so, particularly in areas where the continental shelf is wide. This large input of coastal ocean nutrients with a low N:P ratio is one factor that may make N limitation more likely in many coastal marine ecosystems than in lakes.Preparation of this manuscript was supported by a National Science Foundation Grant # 1654845 from the Long Term Research in Environmental Biology program, a grant from the Atkinson Center for a Sustainable Future at Cornell University, and by an endowment given to Cornell by David R. Atkinson to support a professorship held by RWH

A Century of Legacy Phosphorus Dynamics in a Large Drainage Basin

There is growing evidence that the release of phosphorus (P) from legacy stores can frustrate efforts to reduce P loading to surface water from sources such as agriculture and human sewage. Less is known, however, about the magnitude and residence times of these legacy pools. Here we constructed a budget of net anthropogenic P inputs to the Baltic Sea drainage basin and developed a three-parameter, two-box model to describe the movement of anthropogenic P though temporary (mobile) and long-term (stable) storage pools. Phosphorus entered the sea as direct coastal effluent discharge and via rapid transport and slow, legacy pathways. The model reproduced past waterborne P loads and suggested an similar to 30-year residence time in the mobile pool. Between 1900 and 2013, 17 and 27 Mt P has accumulated in the mobile and stable pools, respectively. Phosphorus inputs to the sea have halved since the 1980s due to improvements in coastal sewage treatment and reductions associated with the rapid transport pathway. After decades of accumulation, the system appears to have shifted to a depletion phase; absent further reductions in net anthropogenic P input, future waterborne loads could decrease. Presently, losses from the mobile pool contribute nearly half of P loads, suggesting that it will be difficult to achieve substantial near-term reductions. However, there is still potential to make progress toward eutrophication management goals by addressing rapid transport pathways, such as overland flow, as well as mobile stores, such as cropland with large soil-P reserves.Peer reviewe

Opportunities to reduce nutrient inputs to the Baltic Sea by improving manure use efficiency in agriculture

While progress has been made in reducing external nutrient inputs to the Baltic Sea, further actions are needed to meet the goals of the Baltic Sea Action Plan (BSAP), especially for the Baltic Proper, Gulf of Finland, and Gulf of Riga sub-basins. We used the net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI, respectively) nutrient accounting approach to construct three scenarios of reduced NANI-NAPI. Reductions assumed that manure nutrients were redistributed from areas with intense animal production to areas that focus on crop production and would otherwise import synthetic and mineral fertilizers. We also used the Simple as Necessary Baltic Long Term Large Scale (SANBALTS) model to compare eutrophication conditions for the scenarios to current and BSAP-target conditions. The scenarios suggest that reducing NANI-NAPI by redistributing manure nutrients, together with improving agronomic practices, could meet 54-82% of the N reductions targets (28-43 kt N reduction) and 38-64% P reduction targets (4-6.6 kt P reduction), depending on scenario. SANBALTS output showed that even partial fulfillment of nutrient reduction targets could have ameliorating effects on eutrophication conditions. Meeting BSAP targets will require addressing additional sources, such as sewage. A common approach to apportioning sources to external nutrients loads could enable further assessment of the feasibility of eutrophication management targets.Peer reviewe

Nitrogen as a threat to European water quality

Anthropogenic increase of nitrogen in water poses direct threats to human health and aquatic ecosystems. High nitrogen concentrations in drinking water are dangerous for human health. In aquatic ecosystems the nitrogen enrichment contributes to eutrophication, which is responsible for toxic algal blooms, water anoxia, fish kills and habitat and biodiversity loss. Nitrogen concentrations in European rivers, lakes, aquifers and coastal waters are generally high in many regions. In addition, nitrogen is probably accumulating in groundwaters, reducing the future reliability of the resource. In Europe, nitrogen pressures on water are wide spread, resulting in elevated costs. About 40% of the European population would potentially be exposed to high nitrates values in drinking water if adequate treatment was not in place, and a large proportion of European aquatic ecosystems is eutrophicated or at risk of eutrophication. Even under favourable land use scenarios the nitrogen export to European waters and seas is likely to remain significant in the near future. The effects of climate change on nitrogen export to waters are still unsure. Policy tools are available within the European Union and under international conventions to mitigate against nitrogen pollution in water, but their full implementation has not yet been achieved throughout Europe. In many cases a delay in the water quality response to implementation of management measures have been observed, due to historical nitrogen accumulation in soils, sediments or aquifers or to inadequate design of the mitigation plans. The issue of pollution swapping between environmental compartments has appeared as an important element to be considered by both the scientific and policy prospective. To support the sustainable management of water resources, positive effects could be obtained implementing existing policy tools, improving the integration in sectoral policies and promoting the interactions between science and policy.JRC.DDG.H.5-Rural, water and ecosystem resource

Nutrient Enrichment Drives Gulf of Mexico Hypoxia

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95312/1/eost16763.pd