The goal of the U.S. Climate Resilience Toolkit’s (CRT) Climate Explorer (CE) is to provide information at appropriate spatial and temporal scales to help practitioners gain insights into the risks posed by climate change. Ultimately, these insights can lead to groups of local stakeholders taking action to build their resilience to a changing climate. Using CE, decision-makers can visualize decade-by-decade changes in climate conditions in their county and the magnitude of changes projected for the end of this century under two plausible emissions pathways. They can also check how projected changes relate to user-defined thresholds that represent points at which valued assets may become stressed, damaged, or destroyed. By providing easy access to authoritative information in an elegant interface, the Climate Explorer can help communities recognize—and prepare to avoid or respond to—emerging climate hazards. Another important step in the evolution of CE builds on the purposeful alignment of the CRT with the U.S. Global Change Research Program’s (USGCRP) National Climate Assessment (NCA). By closely linking these two authoritative resources, we envision that users can easily transition from static maps and graphs within NCA reports to dynamic, interactive versions of the same data within CE and other resources within the CRT, which they can explore at higher spatial scales or customize for their own purposes. The provision of consistent climate data and information—a result of collaboration among USGCRP’s federal agencies—will assist decision-making by other governmental entities, nongovernmental organizations, businesses, and individuals.

Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a N-15/N-14 that is substantially higher than the N-15/N-14 of suspended particulate organic nitrogen (PNsusp) found in the same surface waters (with delta N-15 of similar to 4 to 5 parts per thousand and similar to-1 to 1%, respectively). Moreover, the concentration and N-15/N-14 of DON are much less dynamic than those of PNsusp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and N-15/N-14 of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 mu M, but the average delta N-15 of DON is significantly different, 3.9 parts per thousand versus air in the North Atlantic and 4.7 parts per thousand in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the delta N-15 of nitrate is 2.6 parts per thousand, while it is 4.0 parts per thousand in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the N-15/N-14 of DON relative to surface ocean PNsusp as well as the interbasin difference in the N-15/N-14 of DON. In this model, DON is produced from PNsusp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PNsusp pool, as an integral part of the ammonium-centered cycle that lowers the N-15/N-14 of PNsusp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the N-15/N-14 elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N-2 fixation rates and DON concentration and N-15/N-14 on short time scales.

The concentration and N-15/N-14 ratio of total nitrogen (TN) were measured in precipitation samples collected at Bermuda between January and December 2000. By correcting for nitrate, analyzed previously, the concentration and delta N-15 of "reduced" N (RN, i.e., ammonium + organic N) were also determined. The TN precipitation flux (similar to 10-19 mmol N m(-2) yr(-1)) is twice the NO3- precipitation flux, and the mass-weighted annual average delta N-15 of TN, 2.3 parts per thousand, is higher than the delta N-15 of NO3- in the same samples (-4.5 parts per thousand), indicating that RN has an annual average delta N-15 of -0.6 parts per thousand. While neither the concentration nor the flux of RN (6.8 mu M and 5.2 mmol N m(-2) yr(-1), respectively) in precipitation shows statistically significant seasonal variation, the delta N-15 of RN varies significantly from 2.7 parts per thousand in the cool season to 1.5 parts per thousand in the warm season. This seasonality in the delta N-15 of RN is similar to that of NO3-, implying that RN and NO3- in precipitation may have related sources or, more speculatively, mechanisms of inter-conversion. Additionally, the seasonality of the RN delta N-15 at Bermuda is similar to that of typical ammonium concentrations in precipitation at Bermuda, both showing maxima in the spring and late summer, raising the possibility that the maxima in the RN delta N-15 derives from ammonium at those times. Finally, the low delta N-15 of the TN flux will cause it to have an effect on the delta N-15 of Sargasso Sea thermocline NO3- that is in the same sense as the effect of N-2 fixation, with slightly greater isotopic leverage. If the atmospheric TN flux is not marine-derived, it could explain a substantial fraction of the previously documented upward decrease in NO3- delta N-15 from deep water into the thermocline of the Sargasso Sea, for example, similar to 20 to 35 parts per thousand of it, assuming a N2 fixation rate of 45 mmol N m-2 yr-1 as estimated by Hansell et al. [Hansell, D.A., Bates, N.R., and Olson, D.B., 2004. Excess nitrate and nitrogen fixation in the North Atlantic Ocean. Mar. Chem., 84:243-265]. (C) 2008 Elsevier B.V. All rights reserved.

In this paper, we present multi-parameter data on phytoplankton community composition, and its response to storm events in the Sargasso Sea in late February and early March of 2 years (2004 and 2005). Observed physical conditions spanned a continuum from pulsed destratification/stratification to continuous mixing, with a corresponding range of phytoplankton growth responses. The pulsed destratification/stratification condition resulted in a rapid (1–2d) doubling of euphotic zone chlorophyll (Chl-a) along with a rapid succession, days timescale, from diatoms to haptophytes and then to cyanobacteria. Deep (>300m) continuous mixing led to a slow (8–9d) doubling of autotrophic biomass with no observed succession in the phytoplankton community. These different temporal responses appear to be due to differences between nutrient-limited and light-limited phytoplankton growth, although differences in grazing rates or selective grazing cannot be ruled out. Unexpectedly, we found that flow cytometrically enumerated picoeukaryotes were not accounted for in HPLC-pigment derived phytoplankton classifications and did not covary with any of the pigments quantified. Yet, the picoeukaryotes were positively related to increases in total Chl-a and increased carbon export, suggesting an important but as yet unknown role in the Sargasso Sea carbon cycle.

This chapter covers improvements in tracer methods as applied to pelagic marine over the past decade and attempts to point out promising new directions for the future. Applicable methods from the large literature on benthic nitrogen cycling studies are highlighted. The focus is on the methods rather than on ecological or physiological applications of the methods, as the applications are covered in other chapters of this volume. However, salient applications of the methods will be mentioned to provide context as to why one might want to isolate and analyze the isotopic composition of a particular analyte. The review begins with major changes over the past years, discusses the basic experimental design of tracer experiments, then covers the analytical separation of the different constituents of the nitrogen cycle, and concludes with the 15N analysis itself and interpretation of the resulting isotope data using various models. Two major developments in analytical chemistry have greatly enhanced the ability to conduct nitrogen isotopic analyses in the ocean; improvements in the actual instrumentation for 15N analysis, and advances in chemical detection that provide an ability to more accurately measure low concentrations of ammonium and nitrate. Coupling of mass spectrometers to CHN (carbon, hydrogen, nitrogen) analyzers, combined with advances in computers that permitted extensive automation, has opened up the field to a far wider range of practitioners as well as permitted a vast expansion in the speed of analysis that can be completed.

Colonies of Trichodesmium spp. are conspicuous, macroscopic components of the life in tropical and subtropical oceans. The large size and the morphology of the colony raise questions regarding the mechanism of carbon supply for photosynthesis. Constraints on these mechanisms may be indicated by the stable carbon isotopic composition (delta(13)C) that reflects the balance between carbon supply and speciation, as well as the growth rate and colony size. The delta(13)C of Trichodesmium off Bermuda measured here revealed a strong correlation between size of individual colonies and season. The smallest colonies, 2-7 mu g C colony(-1), showed the lightest delta(13)C composition (similar to-19 parts per thousand), increasing to asymptotic values of similar to-12 parts per thousand above 7 mu g C colony(-1). The average delta(13)C of the colonies was lightest immediately after the onset of stratification in the Sargasso Sea, gradually increasing by similar to 4 parts per thousand to heavier values during the summer. We propose that the mass effect is due to increased use of HCO(3)(-) by the larger colonies, whereas the seasonal influence may be related to changes in irradiance and pCO(2) affecting the internal carbon cycling.

We have hypothesized that the weekly/biweekly passage of winter storms in the subtropical open ocean destabilizes the water column leading to pulsed NO3− inputs, resulting in new production that is not accounted for in most annual estimates. This paper presents data on nitrogen and carbon cycling in the Sargasso Sea at approximately daily resolution, during the period prior to seasonal stratification in 2004 and 2005; these data permit us to assess the importance of winter storms for introducing NO3− and the contribution of these inputs to annual new and export production. The two sampling years were in stark contrast to each other with 2004 characterized by periods of relative calm between winter storms, and 2005 characterized by nearly continuous storm activity. As a result, temporal variability in mixed layer depth (MLD) and euphotic zone [NO3−] were very different between years. MLDs in 2004 increased to >150m in response to the passage of storms and then rapidly shoaled to <100m leading to the pulsed injection of NO3− (∼100nmoll−1) into the lower half of the euphotic zone, while in 2005 MLDs were consistently >300m and euphotic zone [NO3−]>100nmoll−1. Despite the very different [NO3−], rates of daily NO3− uptake were similar from year to year because of significant nocturnal uptake in 2004. Similar rates of new production did not translate into similar rates of particulate nitrogen and carbon export however, as observed export from the upper 200m was 2–5-fold greater in 2004 than in 2005. Furthermore, the decrease of particulate nitrogen and carbon flux with depth between 200 and 400m in 2004 was substantially lower than in 2005; this is consistent with the observed biological response in which diatoms and coccolithophores exhibited rapid growth following pulsed NO3− inputs in 2004. A combination of data from the Bermuda Testbed Mooring, which provides a longer temporal record than the cruise, and the observations presented in this study show that in the winter of 2004, there were 8–10 storm events that likely resulted in pulsed NO3− inputs. Summed over all the events, new production prior to seasonal stratification was estimated to be ∼0.12–0.18molNm−2 or ∼14–21% of current annual estimates.

N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low‐δ15N nitrate throughout the Sargasso Sea thermocline, which we attribute primarily to the input of recently fixed N. Although previous work suggests southward increases in N2 fixation and ventilation age, no meridional trend in nitrate δ15N is apparent. In the upper 200 m, the algal uptake‐driven increase in nitrate δ18O is greater than in δ15N, because of (1) a higher fraction of nitrate from N2 fixation at shallower depths and/or (2) cycling of N between nitrate assimilation and nitrification. A mean depth profile of newly fixed nitrate estimated from the nitrate isotope data is compared with results from an ocean circulation model forced with different Atlantic fields of N2 fixation. The nitrate from N2 fixation is communicated between the model's North and South Atlantic and suggests a whole Atlantic N2 fixation rate between 15 and 24 Tg N a−1. One important caveat is that fixed N in atmospheric deposition may contribute a significant proportion of the low‐δ15N N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N2 fixation would still be too high.

The effects of several environmental variables on net nitrate uptake by the scleractinian coral Diploria strigosa were investigated under controlled flow conditions. D. strigosa exhibited nitrate uptake rates ranging from 1 to 5 nmol cm−2 h−1 at ambient concentrations of 0.1–0.3 μM that are typical of oligotrophic reefs such as Bermuda. Net uptake ceased at approximately 0.045 μM. The uptake was positively correlated with concentration up to a saturation concentration of approximately 3 μM. The uptake was also positively correlated with water velocity at 1 μM, but not at 6 μM, suggesting diffusional limitation at low concentrations and kinetic limitation at higher concentrations. Nitrate uptake by D. strigosa was not affected by light intensity or time of day, but was almost completely inhibited by 48 h exposure to ammonium levels found on many reefs.

As intermediary in a number of key biological processes, the dynamics of oceanic NO(2)(-) concentrations have historically been used as an indicator of the balance between oxidative and reductive pathways in the marine nitrogen cycle. As appreciation of the role of NO(2)(-) in the marine nitrogen cycle grew through the 1960s and 1970s, and data sets from different ocean basins became available, a common feature was observed in stratified water columns: a peak in NO(2)(-) concentrations at the base of the euphotic zone, with near zero concentrations both shallower and deeper. These concentrations are significant; they commonly range between 10 and 400 nmol L(-1) but as high as 4,500 nmol L(-1). This peak in NO(2)(-) concentration is termed the primary nitrite maximum (PNM). Since the 1960s, the mechanisms sustaining the ubiquitous PNM have remained uncertain, with available data supporting either bacterial nitrification or NO(2)(-) release by phytoplankton. Simple box models have reproduced the PNM feature with nitrification as the source of NO(2)(-), whereas others have succeeded solely with phytoplankton. Conclusive identification of the mechanism(s) maintaining the PNM in the world's oceans has yet to be achieved, but the preponderance of data supports phytoplankton excretion, with nitrification likely playing only a supporting role. Furthermore, there are a number of potentially important inconsistencies in the role of nitrification between culture studies and field observations. Biological-physical interactions are likely also important in controlling PNM formation and maintenance.

To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time‐series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000 through May 2001. The annually averaged TN δ15N in the upper 100 m is 3.9‰, which is greater than thermocline nitrate (2–3‰ at 250 m) and similar to literature values for shallow sinking nitrogen at BATS (3.7‰). We discern no seasonal variation in TN δ15N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a δ15N for the sinking flux that is similar to previous measurements, suggesting that N2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and 15N/14N of total organic nitrogen (TON) between the surface and subsurface (∼250 m) suggest that subsurface remineralization of ∼0.25 μM of the surface TON acts to lower the 15N/14N of nitrate in the thermocline at BATS.

The ingestion of two size classes of natural particulate matter (PM) and the uptake of the associated nitrogen by four species of scleractinian corals was measured using the stable isotopic tracer 15N. PM collected in sediment traps was split into <63 and >105 µm size fractions and labeled with (15N-NH4)2SO4. Siderastrea radians, Montastrea franksi, Diploria strigosa, and Madracis mirabilis were incubated in flow chambers with the labeled PM in suspension (<63 µm), or deposited onto coral surfaces (>105 µm). Ingestion was detected for all four species (98–600 µg Dry wt. cm–2 h–1), but only for D. strigosa was any difference detected between suspended and deposited PM. Only the three mounding species, S. radians, M. franksi, and D. strigosa showed uptake of suspended and deposited particulate nitrogen (PN); whereas, the branched coral M. mirabilis had no measurable PN uptake. Only coral host tissues were enriched with 15N, with no tracer detected in the symbiotic zooxanthellae. Uptake rates ranged from as low as 0.80 µg PN cm–2 h–1 in S. radians to as high as 13 µg PN cm–2 h–1 in M. franksi. M. franksi had significantly higher uptake rates than S. radians (ANOVA, p<0.05), while D. strigosa had a statistically similar uptake rate compared to both species. These results are the first to compare scleractinian ingestion of nitrogen associated with suspended and deposited particulate matter, and demonstrate that the use of PM as a nitrogen source varies with species and colony morphology.

The nutritional history of corals is known to affect metabolic processes such as inorganic nutrient uptake and photosynthesis, but little is known about how it affects assimilation efficiency of ingested prey items or the partitioning of prey nitrogen between the host and symbiont. The temperate scleractinian coral Oculina arbuscula and its tropical congener Oculina diffusa were acclimated to three nutritional regimes (fed twice weekly, starved, starved with an inorganic nutrient supplement), then fed Artemia nauplii labeled with the stable isotope tracer 15N. Fed corals of both species had the lowest assimilation efficiencies (36–51% for O. arbuscula, 38–57% for O. diffusa), but were not statistically different from the other nutritional regimes. Fed and starved corals also had similar NH4+ excretion rates. This is inconsistent with decreased nitrogen excretion and reduced amino acid catabolism predicted by both the nitrogen recycling and conservation paradigms. In coral host tissue, ~90% of the ingested 15N was in the TCA-insoluble (protein and nucleic acids) and ethanol-soluble (amino acids/low molecular weight compounds) within 4 h of feeding. The TCA-insoluble pool was also the dominant repository of the label in zooxanthellae of both species (40–53% in O. arbuscula, 50–60% in O. diffusa). However, nutritional history had no effect on the distribution of prey 15N within the biochemical pools of the host or the zooxanthellae for either species. This result is consistent with the nitrogen conservation hypothesis, as preferential carbon metabolism would minimize the effects of starvation on nitrogen-containing biochemical pools.