Regional seas and their interception of riverine fluxes to oceans

Regional seas (n = 19), archipelago coasts (n = 3) and extended coastal platforms (n = 2) are here considered as mega filters of land-ocean linkage through river fluxes. They are differentiated on the basis of morphological characteristics and they correspond to 7.2% of the global ocean area and to 2.4% of its volume. Their catchment characteristics (48 indicators for catchment relief, lithology, climate; river inputs of total suspended solids TSS, total N and dissolved SiO2; population) are determined on the basis of a recombination of 69 individual coastal segments and catchments (COSCATs, [Meybeck, M., Durr, H.H. and Vorosmarty, C.J., 2006. Global coastal segmentation and its river catchment contributors: a new look at land-ocean linkage. Global Biogeochemical Cycles, 20, GB1S90, doi:10.1029/2005GB002540.], Global Biogeoch. Cycles, 20, 1, GB IS90). Regional Seas (RS) alone intercept 44 Mkm2 of land-drainage, i.e. 39% of the exorheic area, corresponding to 52% of the population connected to oceans, to 35% of the river runoff, 38% of the global coastline (at 0.5 degrees resolution) and to 46%, 33% and 46% of the river fluxes to oceans of TSS, silica and total N, respectively. In this intercepted area, essentially developed in the N. Hemisphere, the wet tropical catchments are under-represented compared to other climate types, while its relief and lithology are similar to those of the direct drainage to the open ocean. The RS catchment area/RS area ratio ranges from 0.35 (close to the global ocean figure) to 10.3, the average RS depths from 42 to 2 500 in and theoretical river water renewal times from 50 to 52000 years. Together with the catchment population pressure per km2 of RS (from 1 to 670 p km-2) they are the first-order indicators that best differentiate the RS. The resulting river inputs per unit RS catchment area and per unit RS area and RS volume range over 2 to 4 orders of magnitude. The population pressure is also very variable. Each RS should therefore be characterized, protected and managed individually. If specific filter coefficients of river material were applied to the 3 types of mega filters, these filters could intercept about 5300, 15 and 100 10e12 g y-1 of TSS, total N and dissolved SiO2, respectively, thus lowering the present river inputs to the open ocean by 33%, 36% and 26%, respectively. Regional Seas and their catchments are very different from the rest of the oceans. Their average specific river loads per unit sea area or volume are higher by one order of magnitude than those of the open ocean and much higher for some individual seas. They are very sensitive to human pressures, and should now be treated separately and individually both in Earth System Science and Ocean protection

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 (rDSi = 91 %) and the Mississippi River (rDSi = 13 %) 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

Global patterns of dissolved silica export to the coastal zone: Results from a spatially explicit global model

We present a multiple linear regression model developed for describing global river export of dissolved SiO2 (DSi) to coastal zones. The model, with river basin spatial scale and an annual temporal scale, is based on four variables with a significant influence on DSi yields (soil bulk density, precipitation, slope, and area with volcanic lithology) for the predam situation. Cross validation showed that the model is robust with respect to the selected model variables and coefficients. The calculated global river export of DSi is 380 Tg a¿1 (340¿427 Tg a¿1). Most of the DSi is exported by global rivers to the coastal zone of the Atlantic Ocean (41%), Pacific Ocean (36%), and Indian Ocean (14%). South America and Asia are the largest contributors (25% and 23%, respectively). DSi retention in reservoirs in global river basins may amount to 18¿19

Global spatial distribution of natural river silica inputs to the coastal zone

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

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)

Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

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Global Ecology and Oceanography of Harmful Algal Blooms

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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.<br>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.<br>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).<br>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.<br>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