James J. McCarthy

A framework for vulnerability analysis in sustainability science

Global environmental change and sustainability science increasingly recognize the need to address the consequences of changes taking place in the structure and function of the biosphere. These changes raise questions such as: Who and what are vulnerable to the multiple environmental changes underway, and where? Research demonstrates that vulnerability is registered not by exposure to hazards (perturbations and stresses) alone but also resides in the sensitivity and resilience of the system experiencing such hazards. This recognition requires revisions and enlargements in the basic design of vulnerability assessments, including the capacity to treat coupled human–environment systems and those linkages within and without the systems that affect their vulnerability. A vulnerability framework for the assessment of coupled human–environment systems is presented.

Temperature effects on export production in the open ocean

A pelagic food web model was formulated with the goal of developing a quantitative understanding of the relationship between total production, export production, and environmental variables in marine ecosystems. The model assumes that primary production is partitioned through both large and small phytoplankton and that the food web adjusts to changes in the rate of allochthonous nutrient inputs in a way that maximizes stability, i.e., the ability of the system to return to steady state following a perturbation. The results of the modeling exercise indicate that ef ratios, defined as new production/total production = export production/total production, are relatively insensitive to total production rates at temperatures greater than ∼25°C and lie in the range 0.1-0.2. At moderate to high total production rates, ef ratios are insensitive to total production and negatively correlated with temperature. The maximum ef ratios are ∼0.67 at high rates of production and temperatures of 0°−10°C. At temperatures less than ∼20°C, there is a transition from low ef ratios to relatively high ef ratios as total production increases from low to moderate values. This transition accounts for the hyperbolic relationship often presumed to exist between ef ratios and total production. At low rates of production the model predicts a negative correlation between production and ef ratios, a result consistent with data collected at station ALOHA (22°45′N, 158°W) in the North Pacific subtropical gyre. The predictions of the model are in excellent agreement with results reported from the Joint Global Ocean Flux Study (JGOFS) and from other field work. In these studies, there is virtually no correlation between total production and ef ratios, but temperature alone accounts for 86% of the variance in the ef ratios. Model predictions of the absolute and relative abundance of autotrophic and heterotrophic microorganisms are in excellent agreement with data reported from field studies. Combining the ef ratio model with estimates of ocean temperature and photosynthetic rates derived from satellite data indicates that export production on a global scale is ∼20% of net photosynthesis. The results of the model have important implications for the impact of climate change on export production, particularly with respect to temperature effects.

Illustrating the coupled human- environment system for vulnerability analysis: Three case studies

The vulnerability framework of the Research and Assessment Systems for Sustainability Program explicitly recognizes the coupled human–environment system and accounts for interactions in the coupling affecting the systems responses to hazards and its vulnerability. This paper illustrates the usefulness of the vulnerability framework through three case studies: the tropical southern Yucatán, the arid Yaqui Valley of northwest Mexico, and the pan-Arctic. Together, these examples illustrate the role of external forces in reshaping the systems in question and their vulnerability to environmental hazards, as well as the different capacities of stakeholders, based on their access to social and biophysical capital, to respond to the changes and hazards. The framework proves useful in directing attention to the interacting parts of the coupled system and helps identify gaps in information and understanding relevant to reducing vulnerability in the systems as a whole.

Nitrogenous Nutrition of Marine Phytoplankton in Nutrient-Depleted Waters

Variability in the small-scale temporal and spatial patterns in nitrogenous nutrient supply, coupled with an enhanced uptake capability for nitrogenous nutrients induced by nitrogen limitation, make it possible for phytoplankton to maintain nearly maximum rates of growth at media nutrient concentrations that cannot be quantified with existing analytical techniques.

Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap-234Th approach

The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th and organic carbon analyses on water column and drifting sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12°N to 12°S at 140°W. Samples were collected during the Survey I (February–March 1992) and Survey II (August–September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/234Th ratio in trap samples. Assuming that the organic carbon/234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/234Th ratio for each station. The variability in the station-averaged POC/234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-trap approach ranged from 1 to 6 mmol C m−2 day−1 during Survey I, and from 2 to 30 mmol C m−2 day−1 during Survey II. The average uncertainty for the POC fluxes was ±60%. Primary and new production integrated to the depth of the 0.1 % light level varied by factors of 2–3 for Survey I and Survey II, respectively. The export of particulate organic carbon from the euphotic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10°N–10°S; 90°W–180°E) of new (0.47 Gt C year−1) and particulate export (0.42 Gt C year−1) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region.

Utilization of ammonium and nitrate during austral summer in the Scotia Sea

Abstract The nitrogenous nutrition of the phytoplankton in the Scotia Sea was investigated with 15 N tracer techniques during the austral summer of 1979. On a regional scale, ambient NH 4 + concentrations in the upper hundred meters were variable (0.1 to 2.5 μg-at. 1 −1 ) and one or more orders of magnitude lower than ambient NO 3 − concentrations (17 to 31 μg-at. 1 −1 ). Despite the abundance of NO 3 − , late summer phytoplankton showed a consistent preference for NH 4 + utilization relative to NO 3 − . This was determined by use of a relative preference index and the patterns are similar to those of other marine, estuarine, and fresh waters to which it has been applied. The ratio of NO 3 − utilization to total nitrogen utilization indicated that when ambient NH 4 + concentrations were > 1.0 μg-at. 1 −1 , NO 3 − accounted for s40% of the total nitrogen utilized. Thus, even when NO 3 − is present in quantities that approach the upper limits for near-surface open-ocean waters, the processes that recycle NH 4 + locally can make a major contribution to the nutrition of the phytoplankton. Data for NO 3 − uptake from the 0.1% light level at several of the southernmost stations appeared anomalous. The observed values were higher than would have been expected solely from phytoplakton uptake.

New production along 140°W in the equatorial Pacific during and following the 1992 El Niño event

Abstract This study was conducted as part of two JGOFS transects along 140°W between 12°N and 12°S during February–March 1992 and August–September 1992. Although its purpose was to investigate seasonal variability in nitrogenous nutrient availability and biological utilization in support of primary production, the occurrence of the 1992 El Nino during the first transect permitted us to compare El Nino and post-El Nino conditions. We had hypothesized that an El Nino-related reduction in upwelling of cold nutrient-rich water would lead to a reduction in surface nutrient concentrations and rates of new and primary production in the vicinity of the equator. However, during the height of the El Nino, NO3− concentrations from 2°N to 7°S remained high enough (> 2 μmol kg−1) to preclude nitrogen-limited primary production. Total nitrogen uptake rates measured 6 months after the El Nino were 2.4 times greater than those observed during the El Nino. On both transects, mean values for NH4+ uptake rates were 8 times those for NO3− uptake. Mean rates of new production integrated to the 1% light depth over the full transects were 4.3 mmol C m−2 day−1 during the El Nino, and 9.9 mmol C m−2 day−1 6 months later. Within the 2°N-2°S region, rates of new production were 4.8 and 18.5 mmol C m−2 day−1 for the first and second transects, respectively. Ratios of carbon fixed in primary production and nitrogen uptake averaged 7.7 and 5.1 (mole ratio) for the transects during and after the El Nino, respectively. Even though both the rates of primary production and NO3− concentrations were higher after the El Nino, there was a strong suppressing effect of NH4+ concentration on NO3− uptake. On both transects local minima in f-ratios (0.06) were evident within 1° of the equator. The mean f-ratio for 2°N-2°S was slightly lower and less variable (0.06-0.13; x =0.11 ) during the El Nino than after (0.08-0.20; x=0.13). Over a broader meridional band (5–7°N to 5–8°S) f-ratios during the El Nino were similar to values determined in 1988, a non-El Nino year, during the same season. Diel periodicity was evident in NO3− uptake between 2°N and 3°S, reaching 10- and five-fold day vs night enhancement during and after the El Nino, respectively. Following the El Nino, the diel cycle in NO3− uptake was strongly skewed to the early portion of the light day in the most NO3−-rich waters. These and other comparisons between the two transects serve to indicate that phytoplankton species assemblages and/or nutritional sufficiency of micro-nutrients were different during and after the El Nino. On both transects plankton nutritional preferences resulted in nitrate-sparing conditions in the vicinity of the equator. In spite of high primary productivity, f-ratio calculations and turnover times for NH4+ suggest that local rates of remineralization were sufficient to meet 87–90% of the nitrogen demand in the 2°N-2°S region, resulting in residence times for NO3− of 305 days during the El Nino and 190 days 6 months later. A potential implication of this condition is a correspondingly low export of the particulate product of photosynthesis to the deep ocean. Water column density structure and nutrient distributions argue for reduced rates of nutrient upwelling during the El Nino event. Altered upper-ocean physics and concomitant changes in plankton community structure and function allowed for more extensive upper-ocean nutrient recycling, and presumably reduced export, of primary production during the El Nino. As a consequence the depletion time of recently upwelled NO3− remained long, and thus this nutrient was conserved during the period of diminished supply from upwelling. While these patterns imply direct regulation of new production by the availability of NH4+, the role of a micro-nutrient such as Fe that influences (1) the species composition of the phytoplankton assemblage, and associated potential for export from rather than recycling within the euphotic zone or (2) the sensitivity of NO3− uptake to NH4+ presence, cannot at this time be properly evaluated. Significantly higher rates of new production with only a small increase in f-ratio in the period following the El Nino may constitute a more prominent feature in the ENSO cycle of equatorial biological production and export than the El Nino event per se. Whether this is a general feature in the ENSO cycle, or unique to the period of our study, which was one of unusual global atmospheric conditions, has yet to be established.

Coupled physical and biological modeling of the spring bloom in the North Atlantic (I): model formulation and one dimensional bloom processes

This is the first of two papers that introduce a mesoscale eddy resolving coupled physical and biological model system. The physical model consists of a quasigeostrophic interior with a fully coupled surface boundary layer. The nitrogen based biological model includes nitrate, phytoplankton, heterotroph and ammonium fields. This interdisciplinary model system is used to examine aspects of the 1989 JGOFS North Atlantic Bloom Experiment data set. This paper deals mainly with one dimensional processes and a companion paper addresses three dimensional phenomena. The data set consists of two time series of observations taken from different water masses in the mesoscale environment. The general features of the two time series are well represented by a one dimensional model when the mesoscale spatial variability in the initial condition is treated explicitly within the one dimensional framework. However, a significant bias is evident in the first time series as the sampling pattern began in a warm feature and moved toward colder ones. Mistaking spatial for temporal variability in this case results in an apparent sink of heat and source of nitrate in the data. Removing this bias with the one dimensional model results in an f-ratio that is almost a factor of two higher (0.64) than computed by other authors based on nutrient inventories and primary productivity measurements (0.37). The second time series was conducted in the interior of a mesoscale feature and spatial biasing is minimal. The model forms a seasonal thermocline and nitracline that compare quite well with the data in both magnitude and vertical extent. A subsurface ammonium maximum is generated by the model from an initially homogeneous profile that also agrees well with the data. Simulated primary productivity profiles match 14C incubations except on the final day of the simulation when surface nutrients appear in to have been exhausted slightly prematurely. Computed f-ratios are consistent with independent estimates based on uptake measurements. A systematic parameter dependence and sensitivity analysis is carried out on these results. The most sensitive parameters are the phytoplankton and heterotroph maximum growth rates. Detailed analysis of the behavior of the system indicates tight coupling between phytoplankton production and heterotrophic consumption even in the early stages of the bloom.

Temporal and spatial variations in the natural abundance of 15N in PON from a warm-core ring

Abstract Vertical patterns of the natural abundance of 15 N (δ 15 N) for suspended particulate organic nitrogen (PON) for warm-core ring 82-B were found to found reflect vertical patterns in the utilization of NO 3 − by phytoplankton and the presumed diagenesis of sinking PON on long and short time scales. Temporal variations in the δ 15 N of PON were strongly associated with the onset of stratification and the removal of NO 3 − from the euphotic zone. In April, before stratification, euphotic zone values ranged from −2.6 to +3.3 per mil. During stratified conditions, in June, euphotic zone δ 15 N values were between +4.0 and +11.0 per mil. This shift in the δ 15 N of PON is attributed, in part, to a decrease in the contribution of isotopic fractionation by phytoplankton NO 3 − assimilation. It is concluded, however, that the input of Shelf Water or Slope Water PON of high δ 15 approximately + 10 per mil) into the ring also could have made a significant contribution to the shift in δ 15 N values between April and June. The contribution of Shelf Water- or Slope Water-derived PON to ring PON could be as high as 50% in the euphotic zone. A shift in the δ 15 N of PON at depth in Ring 82-B (400 to 800 m) probably occured in response to the change in euphotic zone δ 15 N values via the downward particle flux. It is hypothesized that PON at this depth interval had a residence time of about one month.

Natural isotopic composition of dissolved inorganic nitrogen in the Chesapeake Bay

The natural abundances of 15N in the dissolved inorganic pools of nitrogen in the Chesapeake Bay were measured in the spring and fall of 1984. Changes in the δ15N of NH4+ and the combined pool of (NO3− + NO2−) reflected both seasonal and short-term changes in the estuarine nitrogen cycle. In the spring, oxidation of NH4+ at the head of the bay in the region of the turbidity maximum and in localized regions throughout the bay, led to elevated values of δ15N in the NH4+ pool. The δ15N of the (NO3− + NO2−) pool tended to increase toward the south, enabling an estimate of the isotopic fractionation factor for the consumption of NO3− to be derived; the estimate (1·0070), is similar to literature values of the fractionation factor for NO3− uptake by phytoplankton, supporting previous research suggesting that phytoplankton uptake is the major sink for NO3− in the bay. Denitrification led to elevated values of δ15N in the (NO3− + NO2−) pool in deep water. Over the course of the summer, the δ15N of NH4+ increased throughout the bay. A significant correlation was found between the δ15N of the NH4+ pool and the concentration of NO2− both above and below the pycnocline during the fall cruise, suggesting that the increase in the δ15N of the NH4+ pool was due to the oxidation of NH4+. In the fall, changes were also observed in the δ15N of both the NH4+ and (NO3− + NO2−) pools which were consistent with the occurrence of NH4+ oxidation. From these changes, a fractionation factor for NH4+ oxidation between 1·0120 and 1·0167 was derived, which is similar to values reported in the literature.