Decrease In River Discharge Research Articles

The effect of riverine discharge on biogeochemical processes in estuarine sediments

A recently developed autonomous benthic lander equipped with an underwater potentiostat and a micromanipulator for in situ voltammetric depth profiling of main redox species in pore waters was deployed for 3 yr at multiple stations along the Satilla River estuary (Georgia, U.S.A.). These measurements revealed that biogeochemical processes in estuarine sediments vary seasonally and are influenced by riverine discharge. A prolonged drought decreased river discharge, altered the salinity gradient in the estuary, and profoundly affected anaerobic respiratory processes in the underlying sediments. Under normal hydrologic conditions, iron reduction was the dominant anaerobic respiratory process across the estuary, likely due to the significant supply of iron from the coastal plane and low salinity of the estuary. Under drought conditions, the salinity of the estuary increased to full seawater strength and carbon remineralization was enhanced significantly. Yet, sulfate reduction was only observed near the mouth of the estuary, whereas a substantial increase in iron reduction was distinctive upriver. Evidence indicates that the increase in iron‐reducing activity during the drought resulted mainly from the deposition of mineral particles in the upper estuary when the salinity gradient increased. Altogether, this study demonstrates that the biogeochemical response of estuarine sediments to natural perturbations is rapid and that respiration processes are controlled by a combination of temperature, supply of inorganic and organic substrates, and hydrological processes. These findings also suggest that an increase in the frequency of droughts as a result of climate change may enhance the resiliency of iron‐reducing bacteria in river‐fed estuarine sediments.

A palynological and palaeoclimatological record from the southern Philippines since the Last Glacial Maximum

A palynological analysis of a marine sediment core in the southern Philippines, provides a detailed regional vegetation and climate history for the West Pacific Warm Pool (WPWP) since the Last Glacial Maximum (LGM). Chronology was determined by a detailed oxygen isotope record. A higher representation of pollen from tropical upper montane rainforest during the LGM indicate that this forest type moved down along elevation, probabaly due to the lowered temperature. During the last deglaciation and the early Holocene, mangroves were more expanded and tropical mid and upper montane rainforests were restricted, suggesting a rising sea-level and temperature increase. Herbaceous pollen and pteridophyte spore records indicate a much drier condition during the LGM than the Holocene. Mangrove development is controlled by conditions at the river mouth influenced by river discharge. Pteridophyte spores are abundant in wet conditions and are mainly transported by rivers. During the mid-Holocene, the reduction in mangrove pollen and pteridophyte spore appears to be a result of climate change: mainly decrease in river discharge. This may have been affected by the decreasing intensity of the Southeast Asian Monsoon, and the increasing frequency and intensity of warm ENSO events, El Nio, in this region.

New threats of an old enemy: The distribution of the shipworm Teredo navalis L. (Bivalvia: Teredinidae) related to climate change in the Port of Rotterdam area, the Netherlands

The effects of four climate change scenarios for the Netherlands on the distribution of the shipworm upstream of the Rhine–Meuse estuary are described. Global warming will cause dry and warmer summers and decreased river discharges. This will extend the salinity gradient upstream in summer and fall and may lead to attacks on wooden structures by the shipworm. Scenarios including one or two degree temperature increases by 2050 compared to 1990 with a weak change in the air circulation over Europe will lead to an increased chance of shipworm damage upstream from once in 36years to once in 27 or 22years, respectively; however, under a strong change in air circulation, the chance of shipworm damage increases to once in 6 or 3years, respectively. The upstream expansion of the distribution of the shipworm will also be manifested in other northwest European estuaries and will be even stronger in southern Europe.

Environmental and maternal effects on embryonic and larval developmental time until dispersal of lake sturgeon (Acipenser fulvescens)

For migratory fish like lake sturgeon ( Acipenser fulvescens ), the period from egg deposition through embryonic and larval development until dispersal (ELDTUD) contributes substantially to variation in survival at the individual level and to population levels of recruitment. Using genetically determined parentage, we examined the relative importance of environmental variables in a stream environment (e.g., temperature and discharge) and maternal effects (including individual female body size, spawning time, and location) to ELDTUD on an individual basis. Adult lake sturgeon (n = 208) spawning in the Upper Black River (Michigan, USA), and larvae (n = 1444) dispersing downstream were captured during the 2007 spawning season. We used generalized mixed models and multimodel inference based on Kullback–Leibler information-theoretic criteria to demonstrate that environmental variables and the maternal effects of individual female and spawning time were both important predictors of ELDTUD. Decreasing ELDTUD during the season resulted from linearly increasing temperature and nonlinearly decreasing river discharge. Spawning time and individual female explained a large proportion of variation in ELDTUD. The individual-based approach used in this study provided precise estimates of ELDTUD and also facilitated the partitioning of variation in ELDTUD of larvae produced by the same female and among females spawning at different times and different environmental conditions.

The response of salt intrusion to changes in river discharge and tidal mixing during the dry season in the Modaomen Estuary, China

The increase of salt intrusion in recent years in the Modaomen Estuary, one of the estuaries of the Pearl River Delta in China, has threatened the freshwater supply in the surrounding regions, especially the cities of Zhongshan, Zhuhai in Guangdong Province and Macau. A numerical modeling system using nested grids was developed to investigate the salt transport mechanisms and the response of salt intrusion to changes in river discharge and tidal mixing. The steady shear transport induced by estuarine circulation reaches maximum and minimum, respectively, during neap and spring tides, while the tidal oscillatory transport shows an opposite pattern. The net transport is landward during neap tides and seaward during spring tides. The salt intrusion length responding to constant river discharges generally follows a power law of −0.49. The dependence of salt intrusion on tidal velocity is less than that predicted by theoretical models for exchange flow dominated estuaries. The response of salt intrusion to change in tidal velocity depends largely on river discharge. When river flow increases, the impact of tidal velocity increases and the phase lag of response time decreases. The asymmetries of salt intrusion responding to increasing and decreasing river discharge (tidal velocity) are observed in the estuary.

Quantifying uncertainty in the impacts of climate change on river discharge in sub-catchments of the Yangtze and Yellow River Basins, China

Abstract. Quantitative evaluations of the impacts of climate change on water resources are primarily constrained by uncertainty in climate projections from GCMs. In this study we assess uncertainty in the impacts of climate change on river discharge in two catchments of the Yangtze and Yellow River Basins that feature contrasting climate regimes (humid and semi-arid). Specifically we quantify uncertainty associated with GCM structure from a subset of CMIP3 AR4 GCMs (HadCM3, HadGEM1, CCSM3.0, IPSL, ECHAM5, CSIRO, CGCM3.1), SRES emissions scenarios (A1B, A2, B1, B2) and prescribed increases in global mean air temperature (1 °C to 6 °C). Climate projections, applied to semi-distributed hydrological models (SWAT 2005) in both catchments, indicate trends toward warmer and wetter conditions. For prescribed warming scenarios of 1 °C to 6 °C, linear increases in mean annual river discharge, relative to baseline (1961–1990), for the River Xiangxi and River Huangfuchuan are +9% and 11% per +1 °C respectively. Intra-annual changes include increases in flood (Q05) discharges for both rivers as well as a shift in the timing of flood discharges from summer to autumn and a rise (24 to 93%) in dry season (Q95) discharge for the River Xiangxi. Differences in projections of mean annual river discharge between SRES emission scenarios using HadCM3 are comparatively minor for the River Xiangxi (13 to 17% rise from baseline) but substantial (73 to 121%) for the River Huangfuchuan. With one minor exception of a slight (−2%) decrease in river discharge projected using HadGEM1 for the River Xiangxi, mean annual river discharge is projected to increase in both catchments under both the SRES A1B emission scenario and 2° rise in global mean air temperature using all AR4 GCMs on the CMIP3 subset. For the River Xiangxi, there is substantial uncertainty associated with GCM structure in the magnitude of the rise in flood (Q05) discharges (−1 to 41% under SRES A1B and −3 to 41% under 2° global warming) and dry season (Q95) discharges (2 to 55% under SRES A1B and 2 to 39% under 2° global warming). For the River Huangfuchuan, all GCMs project a rise in the Q05 flow but there is substantial uncertainty in the magnitude of this rise (7 to 70% under SRES A1B and 2 to 57% under 2° global warming). Differences in the projected hydrological changes are associated with GCM structure in both catchments exceed uncertainty in emission scenarios. Critically, estimated uncertainty in projections of mean annual flows is less than that calculated for extreme (Q05, Q95) flows. The common approach of reporting of climate change impacts on river in terms of mean annual flows masks the magnitude of uncertainty in flows that are of most importance to water management.

Impact of climate evolution and land use changes on water yield in the ebro basin

Abstract. In this study the climatic and hydrological trends across 88 sub-basins of the Ebro River basin were analyzed for the period 1950–2006. A new database of climate information and river flows for the entire basin facilitated a spatially distributed assessment of climate-runoff relationships. It constitutes the first assessment of water yield evolution across the whole Ebro basin, a very representative example of large Mediterranean rivers. The results revealed a marked decrease in river discharges in most of the sub-basins. Moreover, a number of changes in the seasonality of the river regime was found, resulting from dam regulation and a decrease in snowpack in the headwaters. Significant and positive trends in temperature were observed across most of the basin, whereas most of the precipitation series showed negative coefficients, although the decrease in magnitude was low. The time evolution of the residuals from empirical models that relate climate and runoff in each sub-basin provided evidence that climate alone does not explain the observed decrease in river discharge. Thus, changes in water yield are associated with an increase in evapotranspiration rates in natural vegetation, growth of which has expanded as a consequence of land abandonment in areas where agricultural activities and livestock pressure have decreased. In the lowlands of the basin the decrease in water yield has been exacerbated by increased water consumption for domestic, industrial and agricultural uses. Climate projections for the end of the 21st century suggest a reduced capacity for runoff generation because of increasing temperature and less precipitation. Thus, the maintenance of water supply under conditions of increasing demand presents a challenging issue requiring appropriate coordination amongst politicians and managers.

Effects of Land use Change and Erosion on Physical and Chemical Properties of Water(Karkhe watershed)

Karkhe watershed (KW) with more than 5 million hectares recently encountered many problems in the natural, social and man-made habitats. Unsuitable use of land, soil erosion and lack of rainfall are the most destructive factors in this watershed affecting the whole region fundamentally. As expected, these problems have caused a considerable fall in biodiversity in fauna and flora, a decrease in volume and quality of water resources in watershed and a rise in probability of destructive floods. In this study the changes in land use levels and erosion were assessed in 5 sub-regions of Karkhe watershed in two periods of time ’1988 and 2002’ using images of LANDSAT. In addition, the effects of these manipulations on the physico-chemical qualities of water such as, cation and anion concentration, acidity and salinity were quantified. The results show that erosion changes in Karkhe Watershed are inconsiderable from 1988 to 2002. The extent of urban area and irrigated agricultural lands in Karkhe sub-regions (Gamasiab, Gharresu Kashkan ) are around 1000 and 2000 km2 respectively , the decrease in river discharge around 121.6 m3/s , is the most effective factors for the reduction in acidity(from 7.9 in 1988 to 8.1 in 2002), an increment in salinity ( from 1.6 mg/L in 1988 to 3.6 mg/L in 2002 as average amount of SAR in all sub-regions of watershed) and the high density of the anions (from 8.1 mg/Lin 1988 to 16.4 mg/L in 2002) and cations (from 8.8 mg/L to 16.5 mg/L) in Karkhe River.

Climate Change Impact on Water Resources at the Transboundary Level in West Africa: The Cases of the Senegal, Niger and Volta Basins

The series of droughts of the past three decades that affected the Sudano-Sahelian zone of West Africa, particu- larly Niger, Volta and Senegal basins impacted the water resources, ecosystems and the fragile economies of at least 13 of the 16 countries of the region. Declining rainfall, highly variable in time and space has resulted in more than proportionate decrease in river discharges, and declining level of freshwater bodies. This situation translated into falling groundwater levels and accelerated desertification process, hence major crises of food insecurity and massive migration, which often leads to conflicts. The challenges and uncertainties associated with the impacts of future climate changes on water re- sources in West Africa are further compounded by many other factors, including regional demographic factors, non- existing or inadequate water policies, inefficient management strategies and lack of reliable and adequate data. This paper synthesizes the state of art research regarding climate change impact on water resources in West Africa economies of Senegal, Niger and Volta Basins while also making prescriptions on what can be done in terms of adaptation strategies, rather than mitigation, to reduce the severity of the expected impacts on the already stressed and poverty-stricken econo- mies of West Africa,

Population dynamics of the migratory fish Prochilodus lineatus in a neotropical river: the relationships with river discharge, flood pulse, El Niño and fluvial megafan behaviour

The relative importance of flood pulse dynamics and megafan behaviour for the Sábalo (Prochilodus lineatus) catches in the neotropical Pilcomayo River is studied. The Sábalo catches can mainly be explained by decreased river discharges in the preceding years resulting in smaller inundated areas during rainy season floods and thereby in a decreased area of feeding grounds for the fishes. The decreased river discharges and the related decline of Sábalo catches in the 1990's can be linked to the 90-95 El Niño event. In 2007 the Sábalo catches were comparable to the catches before the "El Niño" event. The connectivity (continuity) between the main river and flood plain areas, which is influenced by sedimentation processes, is also of great importance and very probably plays a more important role since the late 1990's.

Reconstructing Past Seawater Mg/Ca and Sr/Ca from Mid-Ocean Ridge Flank Calcium Carbonate Veins

Proxies for past seawater chemistry, such as Mg/Ca and Sr/Ca ratios, provide a record of the dynamic exchanges of elements between the solid Earth, the atmosphere, and the hydrosphere and the evolving influence of life. We estimated past oceanic Mg/Ca and Sr/Ca ratios from suites of 1.6- to 170-million-year-old calcium carbonate veins that had precipitated from seawater-derived fluids in ocean ridge flank basalts. Our data indicate that before the Neogene, oceanic Mg/Ca and Sr/Ca ratios were lower than in the modern ocean. Decreased ocean spreading since the Cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early Tertiary may explain the recent rise in these ratios.

Aportaciones hídricas en la cabecera fluvial del río Duero: relación con la variabilidad climática y cambios en la cubierta vegetal

En este trabajo se analizan los cambios ocurridos en la evolución temporal de los caudales y del régimen hídrico en la cabecera fluvial del río Duero y su relación con la tendencia de las temperaturas, las precipitaciones y la superficie ocupada por el bosque. Los resultados muestran un decrecimiento significativo en las aportaciones hídricas anuales debido a la marcada reducción de las aportaciones entre los meses de febrero y septiembre, con un aumento sustancial del número de días al año con un caudal inferior a la tercera parte del caudal diário medio anual (“régimen de aguas bajas”). La buena correlación entre precipitaciones y aportaciones anuales (R = 0,72, valor-p < 0,001) y la ausencia de tendencia alguna en el análisis de los residuales prueban que el moderado avance de la superficie forestal durante las últimas décadas de momento no es suficiente para influir de manera determinante en la reducción de las aportaciones registrada. No obstante, la falta de correspondência entre las tendencias mensuales de precipitaciones y aportaciones y la destacable subida de las temperaturas sugieren un aumento del consumo de agua por parte del bosque que muy probablemente afecte de forma claramente perceptible a las aportaciones anuales de la cuenca a corto-medio plazo. Finalmente, la disminución de las precipitaciones y los cambios ocurridos en su reparto intraanual han afectado al régimen del río, que há pasado de tener un máximo invernal de carácter pluvial a un régimen bimodal con un pico principal en invierno y otro secundario en primavera. Esta circunstancia sin duda debe considerarse en la gestión de los embalses situados aguas abajo.

Recent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska

Export of nitrate and dissolved organic carbon (DOC) from the upper Kuparuk River between the late 1970s and early 2000s was evaluated using long‐term ecological research (LTER) data in combination with solute flux and catchment hydrology models. The USGS Load Estimator (LOADEST) was used to calculate June–August export from 1978 forward. LOADEST was then coupled with a catchment‐based land surface model (CLSM) to estimate total annual export from 1991 to 2001. Simulations using the LOADEST/CLSM combination indicate that annual nitrate export from the upper Kuparuk River increased by ∼5 fold and annual DOC export decreased by about one half from 1991 to 2001. The decrease in DOC export was focused in May and was primarily attributed to a decrease in river discharge. In contrast, increased nitrate export was evident from May to September and was primarily attributed to increased nitrate concentrations. Increased nitrate concentrations are evident across a wide range of discharge conditions, indicating that higher values do not simply reflect lower discharge in recent years but a significant shift to higher concentration per unit discharge. Nitrate concentrations remained elevated after 2001. However, extraordinarily low discharge during June 2004 and June–August 2005 outweighed the influence of higher concentrations in determining export during these years. The mechanism responsible for the recent increase in nitrate concentrations is uncertain but may relate to changes in soils and vegetation associated with regional warming. While changes in nitrate and DOC export from arctic rivers reflect changes in terrestrial ecosystems, they also have significant implications for Arctic Ocean ecosystems.

Structure and photosynthetic properties of phytoplankton assemblages in a highly dynamic system, the Northern Adriatic Sea

The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters. Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m −3). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m −3, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high P max B, 20.11 mg C (mg chl a) −1 h −1 was recorded in the Po River plume area and average P max B was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a) −1 h −1 and 3.22 mg C (mg chl a) −1 h −1, respectively. The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.

Anthropogenic impacts on continental surface water fluxes

Impacts of reservoirs and irrigation water withdrawals on continental surface water fluxes are studied within the framework of the Variable Infiltration Capacity (VIC) model for a part of North America, and for Asia. A reservoir model, designed for continental‐scale simulations, is developed and implemented in the VIC model. The model successfully simulates irrigation water requirements, and captures the main effects of reservoir operations and irrigation water withdrawals on surface water fluxes, although consumptive irrigation water use is somewhat underestimated. For the North American region, simulated irrigation water requirements and consumptive irrigation water uses are 191 and 98 km3year−1, while the corresponding numbers for the Asian region are 810 and 509 km3year−1, respectively. The consumptive uses represent a decrease in river discharge of 4.2 percent for the North American region, and 2.8 percent for the Asian region. The largest monthly decrease is about 30 percent, for the area draining the Western USA in June. The maximum monthly increase in streamflow (28 percent) is in March for the Asian Arctic region.

Relationship between suspended sediment load, channel geometry and land area increment in the Yellow River Delta

The annual river discharges and suspended sediment loads into the Yellow River Delta show a declining tendency with some distinct fluctuations over the last 50 years. The decrease of river discharge and suspended sediment load and the change in the river channel must influence the evolution of the Yellow River Delta. During this period several new river mouths formed via channel switch at the river delta, and the old watercourse was gradually abandoned. Recently, in years with very low annual suspended load, erosion of land area has been recorded for the delta. The aims of this work are (1) to determine what is the critical suspended sediment load needed in order to maintain the land balance of the Yellow River Delta for two periods before and after 1976 (the last time the channel shifted), and (2) to examine the variation in the channel geometry and gradient in response to changes in suspended sediment load and delta area at the river mouth. In order to estimate these critical values, we used statistical method to analyze the relationships between land area increment, and suspended sediment load and channel geometry. In order to examine the variation in the channel geometry, the channel cross-sections of the Q1 and Q6 were compared. The results show that to maintain the land area balance between 1953 and 1973, when the river mouth was the Diaokouhe, the critical annual suspended sediment load entering the delta was 4.21 ⁎ 10 8 tonnes/a. After the main channel switched to Qingshuigou in 1976, the critical value to maintain the Qingshuigou mouth between 1976 and 1997 was 1.51 ⁎ 10 8 tonnes/a. To maintain the land area balance for the entire Yellow River Delta between 1976 and 1997 the critical suspended sediment load was 3.18 ⁎ 10 8 tonnes/a. The annual mean channel thalweg elevation and channel gradient at the river delta increase with increasing land area increment at the Qingshuigou mouth. The critical channel gradient at the channel reach between cross sections Q1 and Q6 is 0.000095. The channel has narrowed during the time period from 1976 to 1997. Also, lateral channel migration has decreased remarkably, resulting in enhanced hydraulic efficiency of the deltaic channel and artificial levees. This channel geometry evolution was influenced by river adjustment and human activities. These results are of importance for the management of the lower Yellow River channel and the delta. Future water diversion or river damming should consider the balance between suspended sediment delivery and delta growth.

Decreasing river discharge in northern Canada

Freshwater discharge to high‐latitude oceans in 64 Canadian rivers is investigated. The mean annual discharge rate attains 1252 km3 yr−1 for an area of 5.6 × 106 km2, equating to a sink of 225 mm yr−1 in the surface water budget of northern Canada (excluding the Arctic Archipelago where insufficient data exist). Application of the Mann‐Kendall test to the data reveals a 10% decrease (−125 km3 yr−1 or −22 mm yr−1) in the total annual river discharge to the Arctic and North Atlantic Oceans from 1964 to 2003. This trend in river runoff is consistent with a 21 mm yr−1 decline in observed precipitation over northern Canada between 1964 and 2000. We find evidence of statistically‐significant links between the Arctic Oscillation, El Niño/Southern Oscillation, and the Pacific Decadal Oscillation to the total annual freshwater discharge in northern Canada's rivers at interannual‐to‐decadal timescales.

The smolt run and postsmolt survival of Atlantic salmon, Salmo salar L., in relation to early summer water temperatures in the northern Baltic Sea

Abstract – The timing of the smolt migration of Atlantic salmon, Salmo salar L., was investigated during 1972–2002 in the Simojoki, a river flowing into the northern Baltic Sea. The onset of the smolt run was positively correlated with the river water temperature; a rise in water temperature above 10 °C being the main proximate environmental triggering factor. There was also a weaker correlation between the decreasing river discharge in the spring and the onset of the smolt migration. The duration of the main run was shorter in the years when the onset of the smolt run was delayed. No differences were found in the onset timing or in the duration of the smolt run between wild smolts and semi‐wild smolts released into the river as parr. A polynomial equation fitted to the annual data on the survival of Carlin‐tagged wild smolts and the sea surface temperature (SST) in June off the river mouth appeared to follow a dome‐shaped pattern. Survival was lower in cold early summers (SST <9 °C) than in those with an average SST (9–11.9 °C), and lower again, although not significantly, in warm early summers (SST ≥12 °C). Too low and probably also too a high water temperature in early summer could thus be one of the underlying reasons for the fluctuations observed in postsmolt survival in the Baltic Sea.

A transient coastal wetland: from estuarine to supratidal conditions in less than 2000 years—Boca do Rio, Algarve, Portugal

AbstractPalaeoecological analysis of a 3 m sediment core from a coastal site in the western Algarve, near Lagos, Portugal, reveals the changing ecological characteristics of the wetland environment during the late Holocene. The dinoflagellate record and sedimentary characteristics show that approximately 2000 years ago the modern Boca do Rio wetlands were an estuary. By about 1200 14C years BP, transition to a lower energy environment was taking place. The pollen record at this time indicates the existence of typical salt marsh vegetation. There is a likely hiatus in the sedimentary record caused by the tsunami associated with the Lisbon earthquake of ad 1755. A barrier lake, impounded by tsunami deposits, existed for approximately 60 years before being breached, followed by a reoccurrence of saline conditions. A comparison of the mineral magnetic signature of the sediments with that of soil samples from the catchment suggests a relatively local provenance for some of the sediments, rather than the highly erodible Carboniferous rocks of the Algarve's Serra uplands. However, it is also suggested that there is a biogenic component to the magnetic mineralogy. Coastal wetlands are among the most vulnerable Mediterranean environments. The history and future of the Boca do Rio wetlands is examined in the context of alluviation history, rising sea levels, both past and future, and catchment land use. Changing patterns of cultivation, irrigation and land abandonment during the last 50 years are likely to have increased water demands within the catchment and therefore decreased river discharge to the wetlands. Consequently, the continued existence of the wetlands in their present form is in doubt, despite being part of a protected natural park. Copyright © 2003 John Wiley & Sons, Ltd.

El Niño–Southern oscillation and the climate, ecosystems and rivers of Amazonia

The El Niño–Southern Oscillation (ENSO) phenomenon is one of the dominant drivers of environmental variability in the tropics. In this study, we examine the connections between ENSO and the climate, ecosystem carbon balance, surface water balance, and river hydrology of the Amazon and Tocantins river basins in South America. First we examine the climatic variability associated with ENSO. We analyze long‐term historical climate records to document the “average” climatic signature of the El Niño and La Niña phases of the ENSO cycle. Generally speaking, the “average El Niño” is drier and warmer than normal in Amazonia, while the “average La Niña” is wetter and cooler. While temperature changes are mostly uniform through the whole year and are spatially homogeneous, precipitation changes are stronger during the wet season (January‐February‐March) and are concentrated in the northern and southeastern portions of the basin. Next we use a land surface/ecosystem model (IBIS), coupled to a hydrological routing algorithm (HYDRA), to examine how ENSO affects land surface water and carbon fluxes, as well as changes in river discharge and flooding. The model results suggest several responses to ENSO: (1) During the average El Niño, there is an anomalous source of CO2 from terrestrial ecosystems, mainly due to a decreased net primary production (NPP) in the north of the basin. There is also a decrease in river discharge along many of the rivers in the basin, which causes a decrease in flooded area along the main stem of the Amazon. (2) During the average La Niña, there is an anomalous sink of CO2 into terrestrial ecosystems, largely due to an increase in NPP in the northern portion of the basin. In addition, there is a large increase in river discharge in the Amazon basin, especially from the northern and western tributaries. There is a corresponding increase in flooded area, largely in the northern rivers. These results illustrate that changes in water and carbon balance associated with ENSO have complex, spatially heterogeneous features across the basin. This underscores the need for comprehensive analyses, using long‐term observational data and model simulations, of regional environmental systems and their response to climatic variability.