Hans H. Dürr

Global phosphorus retention by river damming

Significance Phosphorus is an essential nutrient for life. Humans have massively altered the global phosphorus cycle by increasing loading to river systems through fertilizer use, soil erosion, and wastewater discharges. River damming interacts with anthropogenic phosphorus enrichment by trapping a fraction of the phosphorus in reservoir sediments. We estimate that in 2000, 12% of the global river phosphorus load was retained in dam reservoirs. This fraction could increase to 17% by 2030, because of the construction of over 3,700 new dams. Although reservoirs represent a huge phosphorus sink, rising anthropogenic phosphorus emissions continue to outpace the addition of new retention capacity by river damming. The resulting growth in riverine phosphorus export likely contributes to the expanding eutrophication of surface waters worldwide. More than 70,000 large dams have been built worldwide. With growing water stress and demand for energy, this number will continue to increase in the foreseeable future. Damming greatly modifies the ecological functioning of river systems. In particular, dam reservoirs sequester nutrient elements and, hence, reduce downstream transfer of nutrients to floodplains, lakes, wetlands, and coastal marine environments. Here, we quantify the global impact of dams on the riverine fluxes and speciation of the limiting nutrient phosphorus (P), using a mechanistic modeling approach that accounts for the in-reservoir biogeochemical transformations of P. According to the model calculations, the mass of total P (TP) trapped in reservoirs nearly doubled between 1970 and 2000, reaching 42 Gmol y−1, or 12% of the global river TP load in 2000. Because of the current surge in dam building, we project that by 2030, about 17% of the global river TP load will be sequestered in reservoir sediments. The largest projected increases in TP and reactive P (RP) retention by damming will take place in Asia and South America, especially in the Yangtze, Mekong, and Amazon drainage basins. Despite the large P retention capacity of reservoirs, the export of RP from watersheds will continue to grow unless additional measures are taken to curb anthropogenic P emissions.

A scenario for impacts of water availability loss due to climate change on riverine fish extinction rates

1. Current models estimating impact of habitat loss on biodiversity in the face of global climate change usually project only percentages of species committed to extinction on an uncertain time-scale. Here, we show that this limitation can be overcome using an empirically derived background extinction rate-area curve to estimate natural rates and project future rates of freshwater fish extinction following variations in river drainage area resulting from global climate change. 2. Based on future climatic projections, we quantify future active drainage basin area losses and combine them with the extinction rate-area curve to estimate the future change in extinction rate for each river basin. We then project the number of extinct species in each river basin using a global data base of freshwater fish species richness. 3. The median projected extinction rate owing to climate change conditions is c. 7% higher than the median background extinction rate. A closer look at the pattern reveals great geographical variations highlighting an amplification of aridity by 2090 and subsequent increase in extinction rates in presently semi-arid and Mediterranean regions. Among the 10% most-impacted drainage basins, water availability loss will increase background extinction rates by 18.2 times (median value). 4. Projected numbers of extinct species by 2090 show that only 20 river basins among the 1010 analysed would experience fish species extinctions attributable to water availability loss from climate change. Predicted numbers of extinct species for these rivers range from 1 to 5. 5. Synthesis and applications. Our results strongly contrast with previous alarming predictions of huge surface-dependent climate change-driven extinctions for riverine fishes and other taxonomic groups. Furthermore, based on well-documented fish extinctions from Central and North American drainages over the last century, we also show that recent extinction rates are, on average, 130 times greater than our projected extinction rates from climate change. This last result implies that current anthropogenic threats generate extinction rates in rivers far greater than the ones expected from future water availability loss. We thus argue that conservation actions should be preferentially focused on reducing the impacts of present-day anthropogenic drivers of riverine fish extinctions.

Worldwide retention of nutrient silicon by river damming: From sparse data set to global estimate

Damming of rivers represents a major anthropogenic perturbation of the hydrological cycle, with the potential to profoundly modify the availability of nutrient silicon (Si) in streams, lakes, and coastal areas. A global assessment of the impact of dams on river Si fluxes, however, is limited by the sparse data set on Si budgets for reservoirs. To alleviate this limitation, we use existing data on dissolved Si (DSi) retention by dams to calibrate a mechanistic model for the biogeochemical cycling of DSi and reactive particulate Si (PSi) in reservoir systems. The model calibration yields a relationship between the annual in-reservoir siliceous primary productivity and the external DSi supply. With this relationship and an estimate of catchment Si loading, the model calculates the total reactive Si (RSiu2009=u2009DSiu2009+u2009PSi) retention for any given reservoir. A Monte Carlo analysis accounts for the effects of variations in reservoir characteristics and generates a global relationship that predicts the average reactive Si retention in reservoirs as a function of the water residence time. This relationship is applied to the Global Reservoirs and Dams database to estimate Si retention by damming worldwide. According to the results, dams retain 163 Gmol yr−1 (9.8 Tg SiO2 yr−1) of DSi and 372 Gmol yr−1 (22.3 Tg SiO2 yr−1) of RSi, or 5.3% of the global RSi loading to rivers.

Submarine groundwater discharge from tropical islands: a review

Submarine groundwater discharge (SGD) is a rarely recognized pathway for nutrients and other solutes from land to sea. The sensitive coastal ecosystems around tropical islands could be particularly affected by nutrient discharge associated with SGD in relation to surficial nutrient transport by rivers, but have received comparatively little attention to date. This study reviews the findings of local assessments of submarine groundwater discharge from tropical islands. In addition, the ratio of coast length and land area of individual land bodies is suggested as an appropriate first-order estimate of the relevance of SGD versus river discharge, demonstrating the potential relative importance of SGD from tropical islands over rivers. The review highlights the need for targeted research of submarine groundwater discharge from tropical islands and highlights its relevance for biogeochemical fluxes in these geographic settings.ZusammenfassungSubmariner Grundwasserabfluss (SGD) vom Land in die Ozeane ist ein wenig beachteter Transportweg für gelöste Stoffe. Existierende Studien betonen allerdings seine Wichtigkeit für die Nährstoffversorgung der Küstengewässer. Insbesondere in Küstenökosystemen im Bereich tropischer Inseln kann SGD im Verhältnis zum Flusseintrag die Nährstoffbudgets dominieren, dort hat SGD aber bisher nur wenig Interesse erfahren. Diese Studie beleuchtet die Erkenntnisse lokaler Untersuchungen des SGD tropischer Inseln. Darüber hinaus wird das Verhältnis von Küstenlänge und Landfläche einer Insel als grobe Abschätzung des Verhältnisses von SGD und Flusseinträgen vorgeschlagen und damit die potenzielle Wichtigkeit von SGD von tropischen Inseln gezeigt. Das Review zeigt den Bedarf an systematischen Feldstudien über SGD von tropischen Inseln und hebt dessen Bedeutung für biogeochemische Stoffkreisläufe dieser Regionen hervor.

Retention of dissolved silica within the fluvial system of the conterminous USA

Dissolved silica (DSi) is an important nutrient in aquatic ecosystems. Increased DSi retention within the fluvial system due to damming and eutrophication has led to a decrease in DSi exports to coastal waters, which can have severe consequences for coastal areas where ecosystem functioning depends on fluvial DSi inputs. The analysis of fluvial DSi fluxes and DSi retention at regional to global scales is thus an important research topic. This study explores the possibility to empirically assess regional DSi retention based on a spatially explicit estimation of DSi mobilization and fluvial DSi fluxes calculated from hydrochemical monitoring data. The uncertainty of DSi retention rates (rDSi) estimated for particular rivers is high. Nevertheless, for the St. Lawrence River (rDSixa0=xa091xa0%) and the Mississippi River (rDSixa0=xa013xa0%) the estimated DSi retention rates are reasonable and are supported by literature values. The variety of sources of the uncertainty in the DSi retention assessment is discussed.

Changing Land-, Sea-, and Airscapes: Sources of Nutrient Pollution Affecting Habitat Suitability for Harmful Algae

Globally, nutrient loading to surface waters is large and increasing, with sources from land-based pollution to aquaculture and atmospheric deposition. Spatial differences in amounts and forms of nutrients released to receiving waters are large, with Asia, Western Europe, and North America exporting the highest loads of nutrients, especially of inorganic nitrogen (N). Export of N is increasing more rapidly than that of phosphorus (P) on a global basis, leading to stoichiometrically imbalanced nutrient conditions. Under such conditions, some types of harmful algal blooms (HABs) can thrive. Differences in coastal typology affect the retentive nature of different coastal types, while dam and reservoir constructions have further altered riverine flows and differentially retain different nutrients. A coastal eutrophication index comparing information on the changes in N and P relative to silicon (Si) and modeling projections of future outcomes using several modeling approaches show that the likelihood for increased nutrient pollution and, correspondingly, for continued regional and global expansion of HABs is great.