Guadiana Estuary Research Articles

Role of transient silicon limitation in the development of cyanobacteria blooms in the Guadiana estuary, south-western Iberia

The Guadiana estuary, located between Portugal and Spain, has the fourth largest drainage basin of Iberian river systems. Up to 75% of the catchment area has been regulated by dams since the early 1970s. During the 1980s and 1990s, an increasing occurrence of summer cyanobacte- ria blooms dominated by the potentially toxic Microcystis spp. was reported. In an effort to under- stand the causes of recurrent noxious blooms in the Guadiana estuary, nutrients (phosphorus, nitro- gen (nitrate, nitrite and ammonium), and silicon (silicic acid), chlorophyll a and phytoplankton abundance, specific composition and biomass were evaluated during field surveys, from April 1997 to March 1998. A pattern of successive blooms of different phytoplankton assemblages was observed throughout this period. Diatoms (nano-sized, chain-forming) dominated an early spring bloom (max. 18 × 10 3 cells ml -1 ) in March and April. Following the decrease in diatom abundance, a chlorophyte bloom (max. 11 × 10 3 cells ml -1 ) and then a cyanobacteria bloom (>6 × 10 4 cells ml -1 ) quickly followed during late spring through to early summer. From July to September, a major cyanobacteria bloom dominated by the potentially toxic Microcystis spp. (> 4 × 10 5 cells ml -1 ) developed in the freshwater zone. The results indicate that high winter loads of nitrogen and phosphorus led to the depletion of silicate (down to as low as 0.2 µM) during the early spring diatom bloom, which conditioned the suc- cessive phytoplankton assemblages during the remaining productive period. Low monthly discharge rates during spring and summer further provided an environment with low Si:N and N:P relative availability which, coupled with high water-column temperature (>21°C), seemed to favour the dominance of cyanobacteria over chlorophytes during the summer.

Postglacial sea-level rise and sedimentary response in the Guadiana Estuary, Portugal/Spain border

Abstract The study of sedimentological features, foraminifera and molluscan fauna in samples recovered from four rotary boreholes permitted, after 14 C dating, the first assessment of the post-glacial transgression in Guadiana River Estuary. The Holocene sequence is underlain by coarse sediments from delta fan enclosing continental swamp deposits dated 16 980 year BP. The Holocene sedimentary sequence was deposited either directly on this delta fan gravels or on top of fluvial sands deposited probably between 11 000 and 10 000 yr BP. An accelerated phase of the estuary infilling by clayey sediments containing the salt marsh foraminifer Trochammina , began ca. 9800 year BP, when sea level was about 39 m below present. Between 7500 and 7000 year BP, the central part of the estuary started to accommodate coarser sediments, partially introduced from the continental shelf. The first phase of Holocene sea-level rise at a rate of 0.85 m/century terminated ca. 6500 year BP. Since then, lagoonal sediments in the vicinity of the estuary have been enclosed behind sand spits and predominantly sandy sedimentation was initiated within the estuary. After a second phase of slower rise at the rate of 0.3 m/century, which lasted until ca. 5000 year BP, the sea approached the present level.

Trace Metals in Estuarine Sediments from the Southwestern Spanish Coast

The impact of river-transported metal pollution and industrial wastes on the metal distribution (Cr, Cu, Zn, Pb) in estuarine sediments was studied in the southwestern Spanish estuaries. Intertidal and subtidal surface sediments of the Tinto–Odiel Estuary are very highly polluted by heavy metals, with geoaccumulation indices up to 4 in the three sedimentary environments studied (channel, channel border and salt marsh). The single exception is the Punta Umbria channel, very protected from the point sources by salt marsh deposits and hydraulic processes. In the remaining two estuaries, pollution (Pb, Cu) was only significant near the harbour situated in the Piedras river mouth, whereas very low values were found in the Guadiana Estuary. In these last rivers, the enrichment factor increases from the channel to the salt marsh sediments.

Status of the Guadiana Estuary (south Portugal) during 1996-1998: An ecohydrological approach

Abstract The aims of this study were to monitor basic hydrological and ecological characteristics of the Guadiana Estuary (1996-1998) before the construction of the Alqueva dam. This work was carried out to determine how environmental factors affect seasonal and tidal variations of plankton populations in the estuary. The available information on the subcatchment of the estuary (e.g., urban, agricultural and forested areas) was integrated into a geographic information system-based software program. Mean monthly river flow varied markedly on a seasonal and yearly basis. River flow near Mértola (ca 50 km upstream from the mouth) reached 3400 2 10 6 m 3 in winter and decreased to 42 2 10 6 m 3 in summer. With respect to nutrients, nitrogen to phosphorus ratios indicated some limitation by phosphorus, except at the end of summer, when nitrogen limitation appeared. During this period, cyanobacterial blooms usually occurred in the upper/middle estuary. Estuarine Turbidity Maximum may significantly influence the retention of zooplankton in the estuary. The fish larval life cycle, especially sensitive to environmental alterations, showed high ratios of ribonucleic to desoxyribonucleic acids indicating good physiological condition. It was concluded that an ecohydrological approach, allowing integration of different elements from the cellular to the habitat level into a geographic information system, can contribute to a better understanding of the processes that influence the aquatic biota of the estuary. The approach will be a useful assessment tool for monitoring the estuary following dam completion.

The distribution of estuarine fish larvae: Nutritional condition andco-occurrence with predators and prey

Fish larvae were collected monthly between March and September 1997 in the Mira and Guadiana estuaries (southern Portugal). Hydrological parameters were registered and zooplankton samples were obtained simultaneously. Densities of fish larvae (ind·100 m -3 ) were calculated from 211 samples and larval nutritional condition measured as RNA/DNA ratios were obtained for 346 individuals, using a fluorimetric method for nucleic acid quantification. Correlating variables were further studied using multiple regression analysis in order to assess the relative importance of abiotic and biotic factors affecting within-year trends in abundance and nutritional condition of estuarine fish larvae. Results indicated that: 1) the abundance of fish larvae seems conditioned by temperature and predation; and 2) their nutritional condition is dependent on temperature and prey availability. Temperature is an important variable structuring estuaries and therefore conditions the behaviour and physiology of fish larvae. Furthermore, the co-occurrence of predators and larvae might be related to similar feeding patterns or comensalism. Whenever feeding conditions are suitable, they usually determine enhanced growth and nutritional condition. However, predation seems to control this latter relationship through its effect on larval mortality. © 2000 Editions scientifiques et medicales Elsevier SAS

Comparison of RNA/DNA ratios obtained with two methods for nucleic acid quantification in gobiid larvae

In this paper, RNA and DNA contents and RNA/DNA ratios obtained with two fluorometric methods for the extraction and quantification of nucleic acids in individual estuarine gobiid larvae are compared. A total of 141 gobiid larvae, collected in the Mira and Guadiana estuaries (Portugal), were analysed. This study indicated that the nucleic acids content and RNA/DNA ratios of individual larvae varied with the procedure used. The two methods yielded similar log DNA values but different log RNA and log (RNA/DNA). The differences could be due to a number of factors related to specific steps of tested protocols. Comparison of nucleic acids contents and calibration of the two methods by regression analysis provided a reasonable basis for comparison although differences between estimates were evident.

Distribución y ecologíade los foraminíferos y ostrácodos actuales del estuario mesomareal del río Guadiana (SO España)

In the Guadiana estuary, thirty-two recent samples were studied. The grain-size analysis differentiates between six distributions in the estuarine channel, four in the channel border and three in the salt marsh. The heavy metal concentration is low, with a maximum in the middle estuary, because of the floculation by water mixing. The three different estuarine zones (fluvial, middle and maríne) have been distinguished by the tidal-fluvial interaction and the microfaune (ostracoda and foraminifera). The biometry study of any species delimited the post-mortem transport of these organisms, mainly during the tidal reflux.

The Mississippi river improvements

The morphodynamic response of a mixed-energy ebb-tidal delta (Guadiana estuary, southern Portugal) and its down-drift barrier island (Canela Island) to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades. Morphological analysis indicates that the ebb delta is in an immature state, characterised by weak sediment bypassing. Landward-migrating shoals on the swash platform have been produced by the jetty-induced artificial bank-breaching and by the collapse of the eastern portion of the delta. The welding of these shoals has largely controlled the evolution of the coast, with local accretion and erosion lasting for years, and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down-drift. These observations provide insights into the potential response of a coast to very large, locally concentrated sand nourishment in the form of shoals. The main effects of the jetties on the coast are observed at the centre of Canela Island, with the production of an erosion hot-spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas. Significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals. After the depletion of this sediment source, and in the context of weak sediment bypassing, the most severe down-drift erosion induced by the jetties is predicted to occur some decades after their construction. This study demonstrates that the geomorphic response of an ebb-tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down-drift coast.