Stephen V. Smith

Biophysical and socio-economic assessments of the coastal zone: the LOICZ approach

Abstract The Land–Ocean Interactions in the Coastal Zone Project of the International Geosphere–Biosphere Programme focused on quantifying the role of the global coastal zone in the cycling of carbon and nutrients. From 1993 to date, it has developed protocols and tools that allow for site-specific and global assessments of coastal processes and their drivers. Indicators used in coastal assessments include the contribution of population and economic activities to waste load generation, and the resulting coastal system states relative to net production and nitrogen cycling.

SOIL EROSION AND SIGNIFICANCE FOR CARBON FLUXES IN A MOUNTAINOUS MEDITERRANEAN-CLIMATE WATERSHED

In topographically complex terrains, downslope movement of soil organic carbon (OC) can influence local carbon balance. The primary purpose of the present analysis is to compare the magnitude of OC displacement by erosion with ecosystem metabolism in such a complex terrain. Does erosion matter in this ecosystem carbon balance? We have used the Revised Universal Soil Loss Equation (RUSLE) erosion model to estimate lateral fluxes of OC in a watershed in northwestern Mexico. The watershed (4900 km2) has an average slope of 10 degrees +/- 9 degrees (mean +/- SD); 45% is >10 degrees, and 3% is >30 degrees. Land cover is primarily shrublands (69%) and agricultural lands (22%). Estimated bulk soil erosion averages 1350 Mg x km(-2) x yr(-1). We estimate that there is insignificant erosion on slopes < 2 degrees and that 20% of the area can be considered depositional. Estimated OC erosion rates are 10 Mg x km(-2) x yr(-1) for areas steeper than 2 degrees. Over the entire area, erosion is approximately 50% higher on shrublands than on agricultural lands, but within slope classes, erosion rates are more rapid on agricultural areas. For the whole system, estimated OC erosion is approximately 2% of net primary production (NPP), increasing in high-slope areas to approximately 3% of NPP. Deposition of eroded OC in low-slope areas is approximately 10% of low-slope NPP. Soil OC movement from erosional slopes to alluvial fans alters the mosaic of OC metabolism and storage across the landscape.

A Genetic Programming Approach to Estimate Vegetation Cover in the Context of Soil Erosion Assessment

This work describes a genetic programming (GP) approach that creates vegetation indices (VI’s) to automatically detect the sum of healthy, dry, and dead vegetation. Nowadays, it is acknowledged that VI’s are the most popular method for extracting vegetation information from satellite imagery. In particular, erosion models like the “Revised Universal Soil Loss Equation” (RUSLE) can use VI’s as input to measure the effects of the RUSLE soil cover factor (C). However, the results are generally incomplete, because most indices recognize only healthy vegetation. The aim of this study is to devise a novel approach for designing new VI’s that are better - correlated with C, using field and satellite information. Our approach consists on stating the problem in terms of optimization through GP learning, building novel indices by iteratively recombining a set of numerical operators and spectral channels until the best composite operator is found. Experimental results illustrate the efficiency and reliability of our approach in contrast with traditional indices like those of the NDVI and SAVI family. This study provides evidence that similar problems related to soil erosion assessment could be analyzed with our proposed methodology.

Carbon–Nitrogen–Phosphorus Fluxes in the Coastal Zone: The LOICZ Approach to Global Assessment

One of the major questions within the Land–Ocean Interaction in the Coastal Zone (LOICZ) Core Project is to evaluate the role of the coastal ocean in global carbon–nitrogen–phosphorus cycles. Carbon is generally considered to be the “major currency” within the International Geosphere-Biosphere Programme (IGBP), and the nitrogen and phosphorus cycles are intimately linked to carbon. Various authors (e.g., Smith and Mackenzie 1987; Smith and Hollibaugh1993; Sarmiento and Sundquist 1992) have argued that the ocean is net heterotrophic, and Smith and Hollibaugh (1993) have made the case that most of this net heterotrophy probably occurs in the coastal ocean. Evidence suggests that human activity can shift (indeed, is shifting) the trophic status of the coastal ocean toward increasing net heterotrophy (e.g., Rabouille et al. 2001). At the same time, other authors (Kemp et al. 1997) have argued that some parts of the coastal zone are presently clearly net autotrophic, arguing that this could represent a shift historically as a result of growing coastal eutrophication. Obviously, addressing this question becomes a major piece in the puzzle of evaluating the role of oceanic biological reactions in the global carbon cycle.

Carbon Flux of an Urban System in México

We estimated vertical and lateral fluxes of carbon for the isolated coastal city of Ensenada (Baja California, Mexico). In 2005, the city had a resident population of about 261,000, with tourism adding about 1.5%; it occupied an area of roughly 68 square kilometers (km2). Carbon (C) export was estimated at 400 gigagrams of carbon per year (Gg C/yr); notable sources to the atmosphere were combustion engines (42%), cement production (38%), water heating and cooking (7%), and human respiration (6%). Solid waste (6%) was exported for burial, but efflux to the bay was minor (about 0.1 Gg C/yr). Local deposition was limited to sewage sludge (about 2 Gg C/yr), asphalt, and extremely low primary production. Remote fluxes driven by local demand could be estimated only for electricity (61 Gg C/yr), but local flux from cement and other industrial production might be attributed largely to external demand. The urban system output to the atmosphere was about 6.4 kilograms of carbon per square meter per year (kg C/m2/yr), or roughly 23.6 kg/m2/yr in CO equivalence. By comparison, net ecosystem productivity in the surrounding watershed has been estimated at 0.04 kg C/m2/yr, so the citys atmospheric output of C might be balanced by productivity over about 11,000 km2 of the surrounding ecosystems. Between 2000 and 2005, C output increased faster than population growth, particularly from engine fuels.

Contrasting precipitation seasonality influences evapotranspiration dynamics in water-limited shrublands: SHRUBLAND EVAPOTRANSPIRATION DYNAMICS

Water-limited ecosystems occupy nearly 30% of the Earth, but arguably, the controls on their ecosystem processes remain largely uncertain. We analyzed six site years of eddy covariance measurements of evapotranspiration (ET) from 2008 to 2010 at two water-limited shrublands: one dominated by winter precipitation (WP site) and another dominated by summer precipitation (SP site), but with similar solar radiation patterns in the Northern Hemisphere. We determined how physical forcing factors (i.e., net radiation (R-n), soil water content (SWC), air temperature (T-a), and vapor pressure deficit (VPD)) influence annual and seasonal variability of ET. Mean annual ET at SP site was 45591mmyr(-1), was mainly influenced by SWC during the dry season, by R-n during the wet season, and was highly sensitive to changes in annual precipitation (P). Mean annual ET at WP site was 36352mmyr(-1), had less interannual variability, but multiple variables (i.e., SWC, T-a, VPD, and R-n) were needed to explain ET among years and seasons. Wavelet coherence analysis showed that ET at SP site has a consistent temporal coherency with T-a and P, but this was not the case for ET at WP site. Our results support the paradigm that SWC is the main control of ET in water-limited ecosystems when radiation and temperature are not the limiting factors. In contrast, when P and SWC are decoupled from available energy (i.e., radiation and temperature), then ET is controlled by an interaction of multiple variables. Our results bring attention to the need for better understanding how climate and soil dynamics influence ET across these globally distributed ecosystems.