Hans H. Dürr

Global carbon dioxide emissions from inland waters

Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8  petagrams of carbon (Pg C) per year from streams and rivers and 0.32  Pg C yr−1 from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr−1 is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.

Anthropogenic perturbations of the silicon cycle at the global scale: Key role of the land‐ocean transition

Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century.

Global monthly water stress: 2. Water demand and severity of water stress

[1] This paper assesses global water stress at a finer temporal scale compared to conventional assessments. To calculate time series of global water stress at a monthly time scale, global water availability, as obtained from simulations of monthly river discharge from the companion paper, is confronted with global monthly water demand. Water demand is defined here as the volume of water required by users to satisfy their needs. Water demand is calculated for the benchmark year of 2000 and contrasted against blue water availability, reflecting climatic variability over the period 1958–2001. Despite the use of the single benchmark year with monthly variations in water demand, simulated water stress agrees well with long‐term records of observed water shortage in temperate, (sub)tropical, and (semi)arid countries, indicating that on shorter (i.e., decadal) time scales, climatic variability is often the main determinant of water stress. With the monthly resolution the number of people experiencing water scarcity increases by more than 40% compared to conventional annual assessments that do not account for seasonality and interannual variability. The results show that blue water stress is often intense and frequent in densely populated regions (e.g., India, United States, Spain, and northeastern China). By this method, regions vulnerable to infrequent but detrimental water stress could be equally identified (e.g., southeastern United Kingdom and northwestern Russia). Citation: Wada, Y., L. P. H. van Beek, D. Viviroli, H. H. Durr, R. Weingartner, and M. F. P. Bierkens (2011), Global monthly water stress: 2. Water demand and severity of water stress, Water Resour. Res., 47, W07518, doi:10.1029/2010WR009792.

Evaluation of sinks and sources of CO2 in the global coastal ocean using a spatially-explicit typology of estuaries and continental shelves

Received 21 April 2010; revised 25 June 2010; accepted 7 July 2010; published 13 August 2010. [1] The exchange of CO2 between the atmosphere and the global coastal ocean was evaluated from a compilation of air‐water CO2 fluxes scaled using a spatially‐explicit global typology of inner estuaries (excluding outer estuaries such as large river deltas) and continental shelves. The computed emission of CO2 to the atmosphere from estuaries (+0.27 ± 0.23 PgC yr −1 )i s∼26% to ∼55% lower than previous estimates while the sink of atmospheric CO2 over continental shelf seas (−0.21 ± 0.36 PgC yr −1 )i s at the low end of the range of previous estimates (−0.22 to −1.00 PgC yr −1 ). The air‐sea CO2 flux per surface area over continental shelf seas (−0.7 ± 1.2 molC m −2 yr −1 )i s the double of the value in the open ocean based on the most recent CO2 climatology. The largest uncertainty of scaling approaches remains in the availability of CO2 data to describe the spatial variability, and to capture relevant temporal scales of variability. Citation: Laruelle, G. G., H. H. Durr, C. P. Slomp, and A. V. Borges (2010), Evaluation of sinks and sources of CO2 in the global coastal ocean using a spatially‐explicit typology of estuaries and continental shelves, Geophys. Res. Lett., 37, L15607, doi:10.1029/2010GL043691.

Partitioning global patterns of freshwater fish beta diversity reveals contrasting signatures of past climate changes

Here, we employ an additive partitioning framework to disentangle the contribution of spatial turnover and nestedness to beta diversity patterns in the global freshwater fish fauna. We find that spatial turnover and nestedness differ geographically in their contribution to freshwater fish beta diversity, a pattern that results from contrasting influences of Quaternary climate changes. Differences in fish faunas characterized by nestedness are greater in drainage basins that experienced larger amplitudes of Quaternary climate oscillations. Conversely, higher levels of spatial turnover are found in historically unglaciated drainage basins with high topographic relief, these having experienced greater Quaternary climate stability. Such an historical climate signature is not clearly detected when considering the overall level of beta diversity. Quantifying the relative roles of historical and ecological factors in explaining present-day patterns of beta diversity hence requires considering the different processes generating these patterns and not solely the overall level of beta diversity.

River hydrological seasonality influences life history strategies of tropical riverine fishes

Under a particular set of selective forces, specific combinations of traits (strategies) will be favored in a given population, within the particular constraints of the considered species. For fishes, three demographic strategies have been suggested to result from adaptive responses to environmental predictability (i.e., seasonality): periodic, opportunistic and equilibrium [Winemiller KO, Rose KA (1992) Patterns of life-history diversification in North American fishes: implications for population regulation. Can J Fish Aquat Sci 49:2196–2218]. These strategies optimize fitness within predictable, unpredictable and stable systems, respectively. We tested these predictions of life history trait distribution along a gradient of hydrologic seasonality in West African tropical rivers at the drainage basin scale. We used logistic regression of species presence–absence data to test whether dominant life history traits of species caused community compositional change in response to a gradient of seasonality in hydrologic regime across basins. After accounting for taxonomic relatedness, species body size and statistical redundancy inherent to related traits, we found a higher proportion of species producing a great number of small oocytes, reproducing within a short period of time and presenting a low degree of parental care (the periodic strategy) in highly seasonal drainage basins (e.g., rivers with a short and predictable favorable season). Conversely, in more stable drainage basins (e.g., rivers with a wet season of several months), we observed a greater proportion of species producing small numbers of large oocytes, reproducing within a long period of time and providing parental care to their offspring (the equilibrium strategy). Our results suggest that distributions of tropical freshwater fishes at the drainage basin scale can be partly explained by the match between life history strategies and seasonality gradients in hydrological conditions.

Non-native species disrupt the worldwide patterns of freshwater fish body size: implications for Bergmann’s rule

In this study, we test whether established non-native species induce functional changes in natural assemblages. We combined data on the body size of freshwater fish species and a worldwide data set of native and non-native fish species for 1058 river basins. We show that non-native fish species are significantly larger than their native counterparts and are a non-random subset of the worldwide set of fish species. We further show that the median body size of fish assemblages increases in the course of introductions. These changes are the opposite of those expected under several null models. Introductions shift body size patterns related to several abiotic factors (e.g. glacier coverage and temperature) in a way that modifies latitudinal patterns (i.e. Bergmanns rule), especially in the southern hemisphere. Together, these results show that over just the last two centuries human beings have induced changes in the global biogeography of freshwater fish body size, which could affect ecosystem properties.

Global and regional patterns in riverine fish species richness : a review

We integrate the respective role of global and regional factors driving riverine fish species richness patterns, to develop a synthetic model of potential mechanisms and processes generating these patterns. This framework allows species richness to be broken down into different components specific to each spatial extent and to establish links between these components and the processes involved. This framework should help to answer the questions that are currently being asked by society, including the effects of species invasions, habitat loss, or fragmentation and climate change on freshwater biodiversity.

Fish-SPRICH: a database of freshwater fish species richness throughout the World

There is growing interest in large-scale approaches to ecology, for both plants and animals. In particular, macroecological studies enable examination of the patterns and determinants of species richness of a variety of groups of organism throughout the world, which might have important implications for prediction and mitigation of the consequences of global change. Here, we provide richness data for freshwater fishes, which, with more than 13,000 described species, comprise a quarter of all vertebrate species. We conducted an extensive literature survey of native, non-native (exotic), and endemic freshwater fish species richness. The resulting database, called Fish-SPRICH, contains data from more than 400 bibliographic sources including published papers, books, and grey literature sources. Fish-SPRICH contains richness values at the river basin grain for 1,054 river basins covering more than 80% of the earth’s continental surface. This database is currently the most comprehensive global database of native, non-native and endemic freshwater fish richness available at the river basin grain.

The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust

The terrestrial surface, the “skin of the earth”, is an important interface for global (geochemical) material fluxes between major reservoirs of the Earth system: continental and oceanic crust, ocean and atmosphere. Because of a lack in knowledge of the geochemical composition of the terrestrial surface, it is not well understood how the geochemical evolution of the Earth’s crust is impacted by its properties. Therefore, here a first estimate of the geochemical composition of the terrestrial surface is provided, which can be used for further analysis. The geochemical average compositions of distinct lithological classes are calculated based on a literature review and applied to a global lithological map. Comparison with the bulk composition of the upper continental crust shows that the geochemical composition of the terrestrial surface (below the soil horizons) is significantly different from the assumed average of the upper continental crust. Specifically, the elements Ca, S, C, Cl and Mg are enriched at the terrestrial surface, while Na is depleted (and probably K). Analysis of these results provide further evidence that chemical weathering, chemical alteration of minerals in marine settings, biogeochemical processes (e.g. sulphate reduction in sediments and biomineralization) and evaporite deposition are important for the geochemical composition of the terrestrial surface on geological time scales. The movement of significant amounts of carbonate to the terrestrial surface is identified as the major process for observed Ca-differences. Because abrupt and significant changes of the carbonate abundance on the terrestrial surface are likely influencing CO2-consumption rates by chemical weathering on geological time scales and thus the carbon cycle, refined, spatially resolved analysis is suggested. This should include the recognition of the geochemical composition of the shelf areas, now being below sea level.