A study of nutrient dynamics in the Atlantic Ocean

During the GEOSECS cruise of the R/V KNORR, July 1972-April 1973, a very complete and high quality nutrient data set was acquired for the Atlantic Ocean. One hundred and twenty-one hydrographic stations were occupied throughout the Atlantic providing an internally consistent picture of the nutrient dynamics for this ocean. The dynamic and biological controls on the nutrient distribution were viewed by means of horizontal distribution patterns, vertical profiling, and statistical modeling of relationships between oxygen, potential temperature, salinity, and nutrients. The general conclusions are summarized as follows: 1. The nutrient concentrations in the Atlantic exhibit the interplay at all depths of nutrient rich waters of South Atlantic origin with nutrient poor waters of the North Atlantic. This interrelationship of the two water sources manifests itself in numerous extrema (maxima and minima) in the water column. 2. For intermediate and deep waters, the strong predominance of lateral transport over processes of vertical dissipation are apparent in the Atlantic. Identifiable water types with only small variations of potential temperature (θ), salinity (S), and preformed nutrients can be characterized thousands of miles from their region of origin. 3. Silicate distribution in the Atlantic exhibits very marked gradients between waters of South and North Atlantic origin. Variations of up to 100 μm/kg occur where salinity differences are less than 0.3‰. Great potential exists for the use of silicate as a water mass tracer for Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). 4. The deep and bottom water nutrient distribution can be explained purely from hydrodynamic considerations. Nutrients, dissolved oxygen (O₂), and apparent oxygen utilization (AOU) behave like conservative parameters. The rates of oxidation in deep water are slow relative to the physical processes of mixing and advection, 5. The total organic carbon (TOG) is relatively invariant below a few hundred meters. Significant variation at the cores of NADW, AAIW, and at the ocean bottom is indistinguishable at the present analytical capability. This supports the observation of very low rates of oxidation in the abyssal waters of the Atlantic. 6. The use of statistical models of O₂ as a function of θ or S and a nutrient are consistent with θ-S diagrams in distinguishing the influence of various water types. In addition, a subsurface water type is seen in temperate and equatorial regions which is due to biochemical activity. This water type corresponds to the portion of the water column where rapid oxidation of organic carbon ceases. It is characterized by a low preformed nutrient concentration but a relatively high oxidative nutrient portion. 7. Statistical modeling for a series of stations in the Drake Passage shows the extent of biological depletion across the Passage and points out the influence of an oxygen rich bottom water in the southern reaches of the Drake Passage. This is bottom water from the South Scotia Sea observed by other authors. 8. An apparent breakdown of Redfield's ratio for the Δ O₂: Δ PO₄ and the Δ O₂:Δ NO₃ in the bottom waters of the Atlantic is seen. My analysis indicates that the variation is due not to an inconsistency in the Redfield ratio but to the very low rates of oxidation at great depths. Nearly all the variation in the oxygen content of the deep water at an equatorial station and a station in the Drake Passage can be explained by the use of a conservative variable such as θ or S. Significant oxidation larger than the analytical errors of the GEOSECS methods cannot be seen for the stations considered at present

Challenges Facing the Sacramento–San Joaquin Delta: Complex, Chaotic, or Simply Cantankerous?

Freshwater is a scarce and precious resource in California; its overall value is being made clear by the current severe drought. The Sacramento–San Joaquin Delta is a critical node in a complex water supply system that extends throughout much of the western U.S. wherein demand is exceeding supply. The Delta also underpins a major component of the U.S. economy, helps feed a substantial part of the country, is a unique and valuable ecological resource, and is a place with a rich cultural heritage. Sustaining the Delta is a problem that manifests itself in many dimensions including the physical structure of the Delta, the conflicting demands for water, changing water quality, rapidly evolving ecological character, and high institutional complexity. The problems of the California Delta are increasingly complex, sometimes chaotic, and always contentious. There is general agreement that current management will sustain neither the Delta ecosystem nor high-quality water exports, as required under the Delta Reform Act, so there is a renewed urgency to address all dimensions of the problem aggressively. Sustainable management of the Delta ecosystem and California’s highly variable water supply, in the face of global climate change, will require bold political decisions that include adjustments to the infrastructure but give equal emphasis to chronic overuse and misuse of water, promote enhanced efficiency of water use, and facilitate new initiatives for ecosystem recovery. This new approach will need to be underpinned by collaborative science that supports ongoing evaluation and re-adjustment of actions. Problems like the Delta are formally “wicked" problems that cannot be “solved” in the traditional sense, but they can be managed with appropriate knowledge and flexible institutions. Where possible, it is advisable to approach major actions incrementally, with an eye toward avoiding catastrophic unexpected outcomes. Collaborative analyses of risks and benefits that consider all dimensions of the problem are essential. Difficult as the problems are, California has the tools and the intellectual resources to manage the Delta problem and achieve the twin goals of a reliable water supply and an ecologically diverse Delta ecosystem

An independently corroborated, diatom-inferred record of long-term drought cycles occurring over the last two millennia in New Mexico, USA

We investigated late Holocene (2000 YBP to present) drought in northern New Mexico, USA, using diatom valves sampled from a lake sediment core. Diatoms were analyzed with a combination of multivariate ordination and time series analysis to identify significant changes in community dynamics and corresponding significant bifurcations between periods of increased and decreased precipitation. This diatom-inferred precipitation regime was statistically corroborated against an independently derived tree ring record of precipitation in northern New Mexico. Also, both the tree ring and diatom records were tested for concordance with indirect radiocarbon solar intensity data and were both significantly cross-correlated with solar intensity. Periods of drought aligned with periods of decreased solar intensity during ~1400–1000 and ~600–200 YBP; periods of increased solar activity aligned with periods of increased precipitation during ~1000–600 YBP and ~200 YBP to present day. These results suggest that longer-term drought regimes in northern New Mexico may have been modulated by solar activity

Water in a Changing World

Life on earth depends on the continuous flow of materials through the air, water, soil, and food webs of the biosphere. The movement of water through the hydrological cycle comprises the largest of these flows, delivering an estimated I 10,000 cubic kilometers (km^\u3e of water to the land each year as snow and rainfall. Solar energy drives the hydrological cycle, vaporizing water from the surface of oceans, lakes, and rivers as well as from soils and plants (evapotranspiration). Water vapor rises into the atmosphere where it cools, condenses, and eventually rains down anew. This renewable freshwater supply sustains life on the land, in estuaries, and in the freshwater ecosystems of the earth

Factors affecting ammonium uptake in streams - an inter-biome perspective

The Lotic Intersite Nitrogen experiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of \u275~-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 452 (2008): 202-205, doi:10.1038/nature06686.Worldwide, anthropogenic addition of bioavailable nitrogen (N) to the biosphere is increasing and terrestrial ecosystems are becoming increasingly N saturated, causing more bioavailable N to enter groundwater and surface waters. Large-scale N budgets show that an average of about 20-25% of the N added to the biosphere is exported from rivers to the ocean or inland basins, indicating substantial sinks for N must exist in the landscape. Streams and rivers may be important sinks for bioavailable N owing to their hydrologic connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favor microbial denitrification. Here, using data from 15N tracer experiments replicated across 72 streams and 8 regions representing several biomes, we show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of instream nitrate that is removed from transport. Total uptake of nitrate was related to ecosystem photosynthesis and denitrification was related to ecosystem respiration. Additionally, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.Funding for this research was provided by the National Science Foundation

Ecosystem Processes and Human Influences Regulate Streamflow Response to Climate Change at Long-Term Ecological Research Sites

Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Nino Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes

Challenges Facing the Sacramento–San Joaquin Delta: Complex, Chaotic, or Simply Cantankerous?

Freshwater is a scarce and precious resource in California; its overall value is being made clear by the current severe drought. The Sacramento–San Joaquin Delta is a critical node in a complex water supply system that extends throughout much of the western U.S. wherein demand is exceeding supply. The Delta also underpins a major component of the U.S. economy, helps feed a substantial part of the country, is a unique and valuable ecological resource, and is a place with a rich cultural heritage. Sustaining the Delta is a problem that manifests itself in many dimensions including the physical structure of the Delta, the conflicting demands for water, changing water quality, rapidly evolving ecological character, and high institutional complexity. The problems of the California Delta are increasingly complex, sometimes chaotic, and always contentious. There is general agreement that current management will sustain neither the Delta ecosystem nor high-quality water exports, as required under the Delta Reform Act, so there is a renewed urgency to address all dimensions of the problem aggressively. Sustainable management of the Delta ecosystem and California’s highly variable water supply, in the face of global climate change, will require bold political decisions that include adjustments to the infrastructure but give equal emphasis to chronic overuse and misuse of water, promote enhanced efficiency of water use, and facilitate new initiatives for ecosystem recovery. This new approach will need to be underpinned by collaborative science that supports ongoing evaluation and re-adjustment of actions. Problems like the Delta are formally “wicked" problems that cannot be “solved” in the traditional sense, but they can be managed with appropriate knowledge and flexible institutions. Where possible, it is advisable to approach major actions incrementally, with an eye toward avoiding catastrophic unexpected outcomes. Collaborative analyses of risks and benefits that consider all dimensions of the problem are essential. Difficult as the problems are, California has the tools and the intellectual resources to manage the Delta problem and achieve the twin goals of a reliable water supply and an ecologically diverse Delta ecosystem