ROBUST: The ROle of BUffering capacities in STabilising coastal lagoon ecosystems

“Buffer capacities” has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that “buffering capacities” can be depleted progressively, and, therefore, we make a distinction between current and potential “buffering capacities”. We have applied this concept to understand the limited “local stability” in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant–microbe interactions in the seagrass meadows.     

Air Quality,Climate, and Policy: A Case Study of Ozone Pollution in Tucson,Arizona

This article addresses the need to better understand the complex interactions between climate, human activities, vegetation responses, and surface ozone so that more informed air‐quality policy recommendations can be made. The impacts of intraseasonal climate variations on ozone levels in Tucson, Arizona from April through September of 1995 to 1998 are determined by relating variations in ozone levels to variations in atmospheric conditions and emissions of ozone's precursor chemicals, volatile organic compounds (VOCs) and nitrogen oxides (NO_x), and by determining month‐specific atmospheric conditions that are conducive to elevated ozone levels. Results show that the transport of ozone and its precursor chemicals within the Tucson area causes the highest ozone levels to be measured at a downwind monitor. The highest ozone levels occur in August, due in part to the presence of the North American monsoon. Atmospheric conditions conducive to elevated ozone concentrations differ substantially between the arid foresummer (May and June) and the core monsoon months ( July and August). Transport of pollution from Phoenix may have a substantial impact on elevated ozone concentrations during April, May, and June, while El Paso/Ciudad Juarez –derived pollution may contribute significantly to elevated ozone concentrations in August and September. Two broad policy implications derive from this work. Regional pollutant transport, both within the U.S. and between the U.S. and Mexico, is a potential issue that needs to be examined more intensively in future studies. In addition, spatiotemporal variations in sensitivities of ozone production require the adoption of both NO_x and VOC control measures to reduce ozone levels in the Tucson area.     

An approach to estimation problems containing uncertain parameters

We consider how to treat a finite-dimensional linear inverse problem when the form of the forward problem is known exactly, but is dependent upon some parameters whose exact value is uncertain and which enter the forward problem multiplicatively. We show one way to proceed when the uncertainty is treatable in a statistical manner. Predicting the secular variation ∂_tB(t) produced by a particular fluid flow V at the core-mantle boundary (when magnetic diffusion is ignored) is one such example, because the results depend on the main magnetic field B(t) originating in the core which is improperly known because of contamination by the crustal magnetic field. This infinite-dimensional inverse problem is often solved by projection on to a finite-dimensional basis, and the resulting parameters found by a maximum likelihood technique. If the main field is contaminated with errors from a Gaussian distribution, this paper describes how the maximum likelihood solution can take this into account, and we show the probability density function that must be maximised in this case. We give an example of the effects for a simple model system, and suggest possible areas of application.     

An assessment of boundary-layer air mass characteristics associated with topographically-induced local wind systems

Measured and derived meteorological parameters are used to examine the changes in air mass associated with local winds of differing origin. Research conducted explores the usefulness of wet bulb potential temperature, equivalent potential temperature, virtual potential temperature and relative humidity as indicators of air mass change during sea breeze, foehn wind and nocturnal drainage flow events. The complex interactions between these different topographically-induced wind systems in South Canterbury, New Zealand, provide an environment in which marked changes in air mass characteristics are common. Results demonstrate that under a considerable range of boundary-layer conditions, measured wet bulb potential temperature when used in conjunction with windspeed and direction enables quick and accurate determination of air mass origin. Relative humidity was also found to respond closely to changes in local air mass type, but its dependence on air temperature makes it a less reliable indicator.     

Climate stability as deduced from an idealized coupled atmosphere-ocean model

The stability of an idealized climate system is investigated using a simple coupled atmosphere-ocean box model. Motivated by the results from general circulation models, the main physical constraint imposed on the system is that the net radiation at the top of the atmosphere is fixed. The specification of an invariant equatorial atmospheric temperature, consistent with paleoclimatic data, allows the hydrological cycle to be internally determined from the poleward heat transport budget, resulting in a model that has a plausible representation of the hydrological cycle-thermohaline circulation interaction. The model suggests that the stability and variability of the climate system depends fundamentally on the mean climatic state (total heat content of the system). When the total heat content of the climate system is low, a stable present-day equilibrum exists with high-latitude sinking. Conversely, when the total heat content is high, a stable equatorial sinking equilibrium exists. For a range of intermediate values of the total heat content, internal climatic oscillations can occur through a hydrological cycle-thermohaline circulation feedback process. Experiments conducted with the model reveal that under a 100-year 2 × CO₂ warming, the thermohaline circulation first collapses but then recovers. Under a 100-year 4 × CO₂ warming, the thermohaline circulation collapses and remains collapsed. Recent paleoclimatic data suggest that the climate system may behave very differently for a warmer climate. Our results suggest that this may be attributed to the enhanced poleward freshwater transport, which causes increased instability of the presentday thermohaline circulation.