Influence Of River Discharges Research Articles

Absence of hypoxia events in the adjacent coastal waters of Grijalva-Usumacinta river, Southern Gulf of Mexico

Globally, oxygen concentration in many coastal areas is depleting. River nutrient discharges may produce hypoxia events. The Southern Gulf of Mexico receives the discharges of the Grijalva-Usumacinta River System, the second largest in the Gulf of Mexico. To evaluate the influence of river discharges on dissolved oxygen concentrations in the receiving coastal ecosystem, we studied the variation of physicochemical variables in the water column. During the dry season, the influence of the river waters to the coastal area is scarce, but during the rainy season the river plume reached ~9 km offshore. The lowest concentration of dissolved oxygen (3.6 mg L−1) was observed within the river plume. We concluded that, in the studied area, hypoxia events (oxygen concentrations ≤ 2 mg L−1) would occur during the rainy season, low winds and in deeper waters (>80 m depth).

Monthly surface hydrographical variability in a coral reef system under the influence of river discharges

The annual variation of total nitrogen, dissolved oxygen, salinity, temperature, Chlorophyll-a and density time series in the surface layer (1 and 5 m depth) were analyzed in a tropical coral reef system, under the influence of city drainage and river discharges. It is important to fully understand the temporal variation of such parameters throughout the world since they are related to biological processes. Furthermore, most tropical coral reef systems, such as the Veracruz Reef System (VRS, Mexico), developed in areas where the reefs are under stress due to the influence of fresh water input during the rainy season, and sediments coverage (vertical mixing) during the northern winds (cold front) season. In addition to the atmospheric temporal variability at mesoscales in the western Gulf of Mexico (dry, rainy and northern wind seasons), the VRS exhibited two transition months between seasons, in contrast to the three months transition period expected from the atmospheric variability. Finally the system was found to be more productive than its surrounding areas due to nutrient input from rivers and city drainage.

Ecological typologies of large areas. An application in the Mediterranean Sea

One approach to identifying and mapping the state of marine biophysical conditions is the identification of large-scale ecological units for which conditions are similar and the strategies of management may also be similar. Because biological processes are difficult to directly record over large areas, abiotic characteristics are used as surrogate parameters. In this work, the Mediterranean Sea was classified into homogeneous spatial areas based on abiotic variables. Eight parameters were selected based on salinity, sea surface temperature, photosynthetically active radiation, sea-wave heights and depth variables. The parameters were gathered in grid points of 0.5° spatial resolution in the open sea and 0.125° in coastal areas. The typologies were obtained by data mining the eight parameters throughout the Mediterranean and combining two clustering techniques: self-organizing maps and the k-means algorithm. The result is a division of the Mediterranean Sea into seven typologies. For these typologies, the classification recognizes differences in temperature, salinity and radiation. In addition, it separates coastal from deep areas. The influence of river discharges and the entrance of water from other seas are also reflected. These results are consistent with the ecological requirements of the five studied seagrasses (Posidonia oceanica, Zostera marina, Zostera noltei, Cymodocea nodosa, Halophila stipulacea), supporting the suitability of the resulting classification and the proposed methodology. The approach thus provides a tool for the sustainable management of large marine areas and the ability to address not only present threats but also future conditions, such as climate change.

Impact of climate change on Gironde estuary

Within the THESEUS European project, a simplified mathematical model for storm surge levels in the Bay of Biscay was adjusted on 10 events at Le Verdon using wind and pressure fields from CLM/SGA, so that the water levels at Le Verdon have the same statistic quantiles as observed tide records for the period [1960-2000].A numerical model of the Gironde Estuary was used to evaluate future water levels at 6 locations of the estuary from Le Verdon to Bordeaux and to assess the changes in the quantiles of water levels during the XXIst century using ONERC's pessimistic scenario for sea level rise (60 cm).The analysis of future storm surge levels shows a decrease in their quantiles at Le Verdon,, whereas there is an increase of the quantiles of total water levels. This increase is smaller than the sea level rise and gets even smaller as one enters farther upstream in the estuary.A series of flood maps for different return periods between 2 and 100 years and for four time periods ([1960-1999], [2010-2039], [2040-2069] and [2070-2099]) have been built for the region of Bordeaux. Quantiles of water levels in the floodplain have also been calculated. The impact of climate change on the evolution of flooded areas in the Gironde Estuary and on quantiles of water levels in the floodplain mainly depends on the sea level rise. Areas which are not currently flooded for low return periods will be inundated in 2100. The influence of river discharges and dike breaching should also be taken into account for more accurate results.

Distribution, growth and survival of anchovy larvae (Engraulis encrasicolus L.) in relation to hydrodynamic and trophic environment in the Bay of Biscay

European anchovy larvae were sampled in June 2004 in the south-eastern part of the Bay of Biscay (limits: 4°W, 46°N). Larval distribution was analysed and compared with spawning distribution in previous weeks. Patches of old larvae were found around the French and Cantabrian shelf (CS) edges and even in oceanic waters, far from the main spawning areas, whereas the youngest larvae appeared in the middle French shelf, associated with adult spawning locations. The analysis of environmental conditions showed significant differences from coastal to oceanic stations as well as in areas under the influence of river discharges. A principal component analysis carried out on temperature, stratification, chlorophyll a and plankton concentration distinguished five areas: Gironde, Northern French Shelf, Southern French Shelf, CS and Oceanic area. Growth and mortality rates were estimated for anchovy larvae in these contrasting areas. Advective processes due to dominant north-easterly winds during summer influence larval distribution in the Bay of Biscay after spawning. However higher food concentration on the shelf did not result in significant differences in the growth rate. Mortality rates seem to be negatively related to larval size, although this effect may be partially due to advection. This is the first attempt to study the effect of environmental factors on anchovy larval distribution and mortality in the Bay of Biscay.

Organic matter sources and their use by bacteria in the sediments of the Altamaha estuary during high and low discharge periods

To understand the influence of river discharges on the biogeochemical cycling of organic matter (OM) in estuarine sediments, we conducted chemical and isotopic analyses of surface sediment samples collected from the Altamaha estuary (Georgia, USA) in March 2002 (high discharge period) and October 2002 (low discharge period). Chemical and isotopic analyses included bulk parameters (TOC, TN, δ 13C TOC and δ 15N TN), chlorophyll- a (Chl- a), lipids (fatty acids, alcohols, and sterols) and lipid carbon isotopic compositions. Bulk parameters were apportioned with a three-end-member mixing model and biomarkers were processed using principal component analysis (PCA). The modeling indicated that OM from C3 terrestrial and marine sources dominate (>80%) at most sediment sites during the two periods while C4 salt marsh plants contribute a large fraction (∼40%) of OM at one site during the low discharge period. The PCA demonstrated that more allochthonous OM deposits in the high discharge period while more autochthonous OM accumulates in the low discharge period; OM in the high discharge period is relatively fresh while that in the low discharge period is highly degraded. Distributions of bacteria-specific fatty acids in the estuarine sediments followed the same patterns as Chl- a, algal lipids, and mixed lipids but were not correlated with terrestrial lipids, suggesting that the microbial community in the sediments depends largely on OM from phytoplankton rather than from terrestrial higher plants. When salt marsh plants became an important input into the sediments, bacteria efficiently used this OM, indicated by the coincident changes between molecular isotope ratios of bacteria-specific fatty acids and bulk δ 13C TOC.

Modelling water discharges and nitrogen inputs into a Mediterranean lagoon: Impact on the primary production

The Soil and Water Assessment Tool (SWAT model, 2001 Version) has been applied to the Thau lagoon catchment area in order to simulate water discharges and nutrient inputs into the lagoon over a 10 years period (1989–1999), and to provide routing inflows to a previously developed lagoon ecosystem model. The watershed model has been calibrated and validated using measured data available for the two main rivers. The results indicate that the mean annual nitrogen inputs into the Thau lagoon is 117 ± 57 tons y −1, with the two main rivers, contributing for 80% of total annual nitrogen export. The variations of outputs to the lagoon are nonetheless important from 1 year to another. Due to the local agricultural practices and a reduced in-stream natural depuration, point sources seem to be the main factor affecting the fresh water quality. The coupling with the lagoon model allowed to estimate the impact of those terrestrial inputs on the lagoon nitrogen cycling and primary productivity. Influence of river discharges makes itself felt essentially near the river outlets. The northern bordure of the lagoon is then characterised by highly variable dissolved inorganic nitrogen concentrations, especially during flood events, while more stable and lower concentrations were simulated in the southern part of the lagoon. Simulated chlorophyll a ranged 1–15 μg l −1, with maximums in March. Mean annual phytoplankton production was 364 ± 142 gC m −2. The simulations showed that maximum annual productions are due to macrophytes (up to 1300 gC m −2 y −1), but at the whole lagoon scale, annual phytoplankton production resulted greater. From our results it also appeared that the greatest part of primary producers nitrogen requirements is satisfied by nutrient regeneration within the lagoon.

Trace-element Al composition of seston and plankton along the Portuguese coast

Abstract Surface water, seston, and material collected by 20 μm plankton nets mainly composed of phytoplankton were collected in February and July 2000, 1 mile from the mouth of major estuarine systems in Portugal and at two sites in the SW coast where freshwater inputs are negligible. Copper and Cd concentrations were higher in the NW coastal waters, evidencing the influence of river discharges on the water quality of this region. The levels of Cd reached extremely high values (19 nM) in the rainy season (February). The high values of seston (3.3–9.3 mg l –1 ), and Al content (1.7–3.0 mmol g –1 ) and low Fe/Al, Zn/Al, As/Al, Cu/Al and Cd/Al ratios confirm the presence of riverine-derived particles with a large lithogenic fraction. Under low-river flow conditions, Cu/Al and Cd/Al ratios were higher (reaching 2.4 × 10 –3 and 27 × 10 –3 , respectively), suggesting that the biogenic fraction of seston plays a major role in metal retention during this season. Comparison of metal distribution in seston and plankton-net material indicates that accumulation of As and Zn by phytoplankton along the coast rarely exceeds the levels existing in the lithogenic fraction. Otherwise, Cu and Cd accumulation appears as the dominant process in its distribution pattern along the coast. The distribution coefficients (Kd) of Cu and Cd were one order of magnitude higher in the SW coast, indicating a more effective transfer of these metals from water to the suspended phase, mainly to phytoplankton.

Contrast in Biological Responses to Tidally-induced Vertical Mixing for Two Macrotidal Ecosystems of Western Europe

The dynamics of phytoplankton blooms were studied during spring 1992 in two typical coastal ecosystems of Western Europe, which differ in the influence of river discharges on the vertical stratification of the water column. The Bay of Brest is a semi-enclosed ecosystem, which is connected with the adjacent ocean and entered by two nutrient-rich rivers. The Western English Channel is an open ocean situation and the studied area was remote from any significant riverine influence during spring. Both areas are macrotidal environments and are usually considered as well-mixed. The beginning of the annual diatom bloom is delayed until late May in the Channel, in comparison with the Bay of Brest where the bloom starts by early April. Water column stability induced by freshwater runoff, and local topography are responsible for the earlier start of the phytoplankton bloom in the Bay of Brest. In both areas, the spring period is marked by a succession of diatom blooms which is strongly dependent upon the spring–neap tidal cycle. However, blooms develop under opposite mixing regimes: they occur during neap tides in the Bay of Brest and during spring tides in the Channel. In the Channel where light is not a limiting factor, increased mixing during spring tides enables nutrient replenishment from the water–sediment interface and phytoplankton responds immediately after nutrients have been renewed in the water column. In the more turbid waters of the Bay of Brest, relaxation of vertical mixing during neap tides is required before phytoplankton is able to utilize nutrients originating from freshwater inputs or in situregeneration later in the season.