Showing papers in "Estuarine Coastal and Shelf Science in 2004"

Macrobenthic community structure of soft-bottom sediments at the Belgian Continental Shelf

Abstract: Within the frame of different research projects, a large number of sites at the Belgian Continental Shelf (BCS) have been sampled for the macrobenthos between 1994 and 2000. These samples cover a diverse range of habitats: from the sandy beaches to the open sea, from the gullies between the sandbanks to the tops of the sandbanks, and from clay to coarse sandy sediments. To investigate the large-scale spatial distribution of the macrobenthos of the Belgian Continental Shelf, the data of all these research projects—728 samples—were combined and analysed. By means of several multivariate techniques, 10 sample groups with similar macrobenthic assemblage structure were distinguished. Each sample group is found in a particular physico-chemical environment and has a specific species composition. Four sample groups differ drastically, both in habitat and species composition, and are considered to represent four macrobenthic communities: (1) the muddy fine sand Abra alba–Mysella bidentata community is characterized by high densities and diversity; (2) the Nephtys cirrosa community occurs in well-sorted sandy sediments and is characterized by low densities and diversity; (3) very low densities and diversity typify the Ophelia limacina–Glycera lapidum community, which is found in coarse sandy sediments and (4) the Eurydice pulchra–Scolelepis squamata community is typical for the upper intertidal zone of sandy beaches. These macrobenthic communities are not isolated from each other, but are linked through six transitional species assemblages. The transition between the A. alba–M. bidentata community and the N. cirrosa community is characterized by a reduction in the mud content and is dominated by Magelona johnstoni. The transition between the N. cirrosa and the O. limacina–G. lapidum community is distinctive by decreasing densities and coincides with a gradual transition between medium and coarse sandy sediments. From the N. cirrosa to the E. pulchra–S. squamata community, transitional species assemblages related to the transition from the subtidal to the intertidal environment were found. Each community or transitional species assemblages was found over a specific range along the onshore–offshore gradient, four types can be discerned: (1) almost restricted to the near-shore area, but possible wider distribution; (2) distributed over the full onshore–offshore gradient; (3) restricted to the near-shore area and (4) restricted to the sandy beach environment. The diversity pattern on the BCS follows this division, with species rich and poor assemblages in the near-shore area to only species poor assemblages more offshore. The distribution and diversity patterns are linked to the habitat type, distinguished by median grain size and mud content.

Tracing organic matter sources and carbon burial in mangrove sediments over the past 160 years

Abstract: Mangrove ecosystems may be a source of organic carbon and nutrients to adjacent coastal systems on one hand and provide a sedimentary sink for organic carbon on the other. The balance between these two functions may be sensitive to both natural and anthropogenically induced variability, yet these effects have not been thoroughly evaluated in mangrove ecosystems. We determine organic matter sources and carbon burial rates over the past 160 years in three lagoons on the Yucatan Peninsula, Mexico. Carbon isotopes and C/N elemental ratios are utilized to trace the three sources contributing to sedimentary organic matter, mangroves, seagrasses and phytoplankton, while nitrogen isotopes are used to elucidate potential post-depositional biogeochemical transformations in mangrove lagoon sediments. All three organic matter sources contribute to organic carbon burial. Phytoplankton and mangroves are the dominant sources of organic matter in lagoon bank sediments and seagrasses are a significant source to central lagoon sediments. Organic carbon burial rates are higher at the lagoon fringes, where mangrove vegetation dominates, than in seagrass-dominated mid-lagoon areas. A reduction in mangrove contribution to the sedimentary organic matter pool concurrent with reduced total organic carbon burial rates is observed in the recent past at all three lagoons studied. Natural cycles in sediment organic matter source over the past 160 years are observed in a high-resolution core. These fluctuations correspond to climatic variability in this region, as recorded in deep-sea foraminiferal assemblages. Additional work is required in order to differentiate between recent anthropogenic perturbations and natural variability in organic carbon sources and burial rates within these ecosystems.

Calculating age and residence time in the tidal York River using three-dimensional model experiments

Abstract: The age concept is applied to an estuary to estimate time scales quantifying transport processes of dissolved substance for a spatially varying case. A real-time three-dimensional numerical model was applied to the tidal York River estuary, a western branch of the Chesapeake Bay, to calculate the age distribution for a substance released from the headwaters of tributaries of the estuary under different hydrological conditions. The mean age, residence time, and turnover time were computed through a series of numerical model experiments. Both mean age and residence time are functions of freshwater discharge. The model results show that substantial time is required for a substance discharged into the system at the headwaters of the tributaries to be transported to the downstream estuary. It takes about 2 months and 3 months for the substance discharged into the headwaters to be transported to the mouth of the estuary under high flow and mean flow conditions, respectively. The results provide useful information for understanding the transport processes in the estuary and can be used to estimate dissimilative capacity of the estuary for environmental assessment.

The relationship of reef fish densities to the proximity of mangrove and seagrass nurseries

Abstract: Visual census surveys were used to study the distribution of coral reef fishes that are associated with seagrass beds and mangroves in their juvenile phase, on various coral reef sites along the coast of the Caribbean island of Curacao (Netherlands Antilles). The hypothesis tested was that various reef fish species occur in higher densities on coral reefs adjacent to nursery habitats than on reefs located at some distance to these habitats. Of 17 coral reef fish species that are known to use bays with seagrass beds and mangroves as nurseries (nursery species), 15 were observed in quadrats on the reef. Four nursery species, Haemulon sciurus, Lutjanus apodus, Ocyurus chrysurus and Scarus coeruleus occurred in significantly higher densities on coral reefs adjacent to bays with seagrass beds and mangroves. Lutjanus analis, Lutjanus mahogoni and Sphyraena barracuda also had their highest densities on reefs adjacent to these bays, although differences between the distinguished reef categories were not always significant. It is suggested that these seven species are highly dependent on the presence of bays with seagrass beds and mangroves as nurseries on an island scale. Eight other species that are known to use seagrass beds and mangroves as nurseries did not have their highest densities on reefs adjacent to bays with seagrass beds and mangroves. For six of these species, juveniles were also observed on the reef. It is suggested that these species are able to use the reef as an alternative nursery and do not depend strictly on the presence of bays with seagrass beds and mangroves as nurseries.

Use of satellite imagery for water quality studies in New York Harbor

Abstract: The utility of satellite imagery for water quality studies in New York Harbor is investigated. Ground data from a routine sampling program (New York Harbor Water Quality Survey) are compared to imagery from the Landsat Thematic Mapper (TM) and Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors. New York Harbor is a challenging environment for remote sensing because of the complex hydrography and strong tidal influence. Using a time-averaged spatial analysis it is shown that turbidity as determined from Secchi depth correlates with Landsat TM red reflectance in regions affected by the Hudson River sediments (N = 21, R 2 = 0.85). Based on this correlation the estuarine turbidity maximum of the Hudson River is mapped. Landsat TM red reflectance is also used to identify and map plumes of increased turbidity caused by rainfall runoff and/or spring tide resuspension in Newark Bay. Chlorophyll a concentration correlates with the ratio of Landsat TM green to red reflectance in the eutrophic East River and Long Island Sound (N = 16, R 2 = 0.78). Terra MODIS estimates of chlorophyll a show no correlation

Low concentration of heavy metals in the Yangtze estuarine sediments, China: a diluting setting

Abstract: An examination of the spatial and temporal distribution of 17 major heavy metals, i.e., Fe, Zn, Ni, Mg, Co, Mn, etc., was made in the Yangtze estuarine sediments revealed by a number of vibrocores taken from the different sediment zones, including delta front, prodelta, and delta–shelf transition. The results obtained from the numerous core samples, which were also measured for Pb-210 and Cs-137, show that: (1) the silty clay comprising the prodelta facies serves as a depositional sink attracting high concentrations of heavy metals delivered from the river mouth; (2) after being normalized to aluminum (as a proxy for grain size), most heavy metals presented in the prodelta facies have lower concentrations than in the other adjacent sediment zones; (3) also after normalization, concentrations of most heavy metals in the vibrocore sediments tend to increase up-core; and (4) sedimentation rates in the study area range from ∼2.0 to 6.0 cm/a; hence, the vibrocores contain a sedimentary record of metal deposition covering more than 50 years. It has previously been assumed that sediments off the river mouth were heavily polluted due to industrialization of the Shanghai metropolitan area, which peaked about 50 years ago. However, the low concentrations of heavy metals in the study area before and after normalization do not support this assumption. The Yangtze estuary is characterized largely by the tremendous runoff and ‘unpolluted’ sediments derived from the upper drainage basin to constitute a unique diluted setting, in which the dispersal behavior of heavy metals from the adjacent industrialized coast is influenced substantially. As a result, the heavy metals in the study area are obviously lower than those previously determined along the coast.

Phosphorus speciation and availability in intertidal sediments of a temperate estuary: relation to eutrophication and annual P-fluxes

Abstract: For a better understanding of the phosphorus dynamics and bioavailability in temperate climates, sequential chemical extraction techniques were used to study sediment P-pools distribution and relative importance in a eutrophicated estuary. Results indicate a shift in importance from the iron-bound P fraction to the CaCO3-bound P fraction with increasing salinity, which suggests a decrease in bioavailable P from the most inner part of the estuary seaward, as the CaCO3 associated P is believed to be less available for organisms. Salt marsh plants also influence sediment P speciation, through higher organic P fractions (mainly humic acid associated P) when compared to non-vegetated sediments. The ratio of iron-bound P to iron-oxyhydroxides may be used as an indicator of P availability, since it reflects the number of available P sorption sites in the metal oxides. This ratio was lowest in the intermediate zone of the estuary (13.7) when compared to the outer site (29.6) and the upper site (26.1), suggesting saturation of sorption sites. Co-incident with this, the intermediate site is characterized by the occurrence of macroalgal blooms during summer. In the present case, superficial sediments in the Mondego estuary do not show a clear seasonal trend, either in total amounts of exchangeable P or P speciation. Pools of exchangeable P in the surface sediment were ∼30 times higher than the annual net-export of P (14 ton year−1) from the estuary. The annual P release from the sediment appears diminutive in comparison with the pool-size. Management measures to reduce nutrient input into coastal water systems should therefore consider that phosphorus availability may persist as a result of sediment supply, even after reducing the external point sources.

The European coastal zone: characterization and first assessment of ecosystem metabolism

Abstract: The geomorphic, oceanographic, terrestrial and anthropogenic attributes of the European coastal zone are described and published data on ecosystem function (primary production and respiration) are reviewed. Four regions are considered: the Baltic Sea, Mediterranean Sea, Black Sea and the European Atlantic coast including the North Sea. The metabolic database (194 papers) suffers from a non-homogeneous geographical coverage with no usable data for the Black Sea which was therefore excluded from this part of our study. Pelagic gross primary production in European open shelves is, by far, the most documented parameter with an estimated mean of 41 mmol C m −2 d −1 , the lowest value is reported in the Mediterranean Sea (21 mmol C m −2 d −1 ) and the highest one in the Atlantic/North Sea area (51 mmol C m −2 d −1 ). Microphytobenthic primary production, mostly measured in shallow areas, is extrapolated to the entire 0–200 m depth range. Its contribution to total primary production is low in all regions (mean: 1.5 mmol C m −2 d −1 ). Although macrophyte beds are very productive, a regional production estimate is not provided in this study because their geographical distribution along the European coastline remains unknown. Measurements of pelagic community respiration are clearly too sparse, especially below the euphotic zone, to yield an accurate picture of the fate of organic matter produced in the water column. With a mean value of 17 mmol C m −2 d −1 , benthic community respiration consumes approximately 40% of the pelagic organic matter production. Estuaries generally exhibit high metabolic rates and a large range of variation in all parameters, except microphytobenthic primary production. Finally, the problem of eutrophication in Europe is discussed and the metabolic data obtained in the framework of the Land–Ocean Interactions in the Coastal Zone (LOICZ) project are compared with available direct measurements of net ecosystem production.

Flood related loss and recovery of intertidal seagrass meadows in southern Queensland, Australia

Abstract: The loss and recovery of intertidal seagrass meadows were assessed following the flood related catastrophic loss of seagrass meadows in February 1999 in the Sandy Strait, Queensland. Region wide recovery rates of intertidal meadows following the catastrophic disturbance were assessed by mapping seagrass abundance in the northern Great Sandy Strait region prior to and on 3 occasions after widespread loss of seagrass. Meadow-scale assessments of seagrass loss and recovery focussed on two existing Zostera capricorni monitoring meadows in the region. Mapping surveys showed that approximately 90% of intertidal seagrasses in the northern Great Sandy Strait disappeared after the February 1999 flooding of the Mary River. Full recovery of all seagrass meadows took 3 years. At the two study sites (Urangan and Wanggoolba Creek) the onset of Z. capricorni germination following the loss of seagrass occurred 14 months post-flood at Wanggoolba Creek, and at Urangan it took 20 months for germination to occur. By February 2001 (24 months post-flood) seagrass abundance at Wanggoolba Creek sites was comparable to pre-flood abundance levels and full recovery at Urangan sites was complete in August 2001 (31 months post-flood). Reduced water quality characterised by 2–3 fold increases in turbidity and nutrient concentrations during the 6 months following the flood was followed by a 95% loss of seagrass meadows in the region. Reductions in available light due to increased flood associated turbidity in February 1999 were the likely cause of seagrass loss in the Great Sandy Strait region, southern Queensland. Although seasonal cues influence the germination of Z. capricorni, the temporal variation in the onset of seed germination between sites suggests that germination following seagrass loss may be dependent on other factors (eg. physical and chemical characteristics of sediments and water). Elevated dissolved nitrogen concentrations during 1999 at Wanggoolba Creek suggest that this site received higher loads of sediments and nutrients from flood waters than Urangan. The germination of seeds at Wanggoolba Creek one year prior to Urangan coincides with relatively low suspended sediment concentrations in Wanggoolba Creek waters. The absence of organic rich sediments at Urangan for many months following their removal during the 1999 flood may also have inhibited seed germination. Data from population cohort analyses and population growth rates showed that rhizome weight and rhizome elongation rates increased over time, consistent with rapid growth during increases in temperature and light availability from May to October

Sterols as markers of sewage contamination in a tropical urban estuary (Guanabara Bay, Brazil): space–time variations

Abstract: Guanabara Bay, located adjacent to Rio de Janeiro in Brazil, is highly contaminated by substances derived from domestic and industrial effluents as well as from agricultural runoff. In the present work, sedimentary coprostanol and other faecal sterols were used to investigate historical trends in sewage contamination. Sediment cores were collected from eight different (and characterized) locations in the bay and were sectioned into dated segments. Organic carbon was determined by dry combustion and sterols were separated and quantified by GC/MS. The space-distribution of organic carbon and faecal sterol concentrations generally coincided with the presence of known pollution sources. Concentrations of coprostanol as high as 40 μg g −1 were found, indicating areas of severe sewage contamination. Faecal sterol ratios, commonly used as tools to investigate contamination in temperate environments, however, were found to have limited applicability to this tropical estuarine system, probably due to the influence of intensive primary production and microbial processes.

Residence time in a semi-enclosed domain from the solution of an adjoint problem

Abstract: The residence time measures the time spent by a water parcel or a pollutant in a given water body and is therefore a widely used concept in environmental studies. While many previous studies rely on severe hypotheses (assuming stationarity of the flow and/or neglecting diffusion) to evaluate the residence time, the paper introduces a general method for computing the residence time and/or the mean residence time without such simplifying hypotheses. The method is based on the resolution of an adjoint advection-diffusion problem and is therefore primarily meant to be used with numerical models. The method and its implications are first introduced using a simplified one-dimensional analytical model. The approach is then applied to the diagnostic of the three-dimensional circulation on the Northwest European Continental Shelf. (C) 2004 Elsevier Ltd. All rights reserved.

Biogeochemistry of a tropical river affected by human activities in its catchment: Brantas River estuary and coastal waters of Madura Strait, Java, Indonesia

Abstract: On a global scale tropical SE Asia is critical in terms of fluvial nutrient and sediment input into the ocean as well as in terms of human modifications of the coastal zone altering these inputs. In order to obtain information on the biogeochemistry of the Brantas River estuary and adjacent coastal waters of the Madura Strait in eastern Java, Indonesia, a densely-populated area which is strongly affected by human activities in the river catchment, we investigated water, suspended matter and sediments from estuarine and coastal waters and plants and soils from the catchment collected during the dry season. Water samples were analyzed for dissolved nutrients and phytoplankton abundance and composition. Suspended matter, sediment, plant and soil samples were analyzed for carbon, nitrogen, amino acids and stable carbon and nitrogen isotopes. Nutrient concentrations were high in the river and then decreased rapidly seaward. Runoff from agricultural soils may be a major nutrient source. Phytoplankton was dominated by diatoms and most abundant in the estuarine mixing zone. A fairly high phytoplankton abundance despite nitrate-depletion and an N/P ratio <4 in coastal waters indicates additional nitrogen sources. Biogeochemical characteristics and stable isotopes indicate the autochthonous origin of suspended organic matter (OM) during the dry season. Major part of the fluvial input appears to be trapped in the inner estuary. Similar information obtained from the analysis of sediments suggests seasonal differences in the quantity and origin of suspended sediments and OM transported by the river with high inputs into coastal waters during the rainy season, the period of peak river discharge. The amino acid composition in combination with stable isotopes indicates agricultural soils to be a major OM source during that time. A high proportion of amino acid-bound nitrogen in suspended matter and sediments and the presumed sources of dissolved nitrogen other than nitrate to sustain primary productivity in the coastal zone suggest that organic nitrogen may play a more important role for coastal food webs and the nitrogen cycle in tropical regions than previously thought.

A numerical study of storm surge and inundation in the Croatan–Albemarle–Pamlico Estuary System

Abstract: An integrated storm surge and inundation modeling system is used to simulate the storm surge and inundation in the CroataneAlbemarleePamlico Estuary System in eastern North Carolina under the influence of 10 hypothetical Category 2 and 3 hurricanes representing typical historical hurricane scenarios in the study region. The integrated storm surge and inundation modeling system is numerically stable in the complex and shallow CAPES environment under hurricane forcing conditions. For an assumed northward or northeastward moving Category 3 hurricane with a translation speed of 25 km/h, the peak storm surge occurs along the western Pamlico Sound and western Albemarle Sound. The most severe flooding as measured by inundation area is in the Pamlico River mouth region where the flooding area reached 500 km 2 . In general, a more intense or larger hurricane (lower minimum central pressure, MCP or larger radius of maximum wind, RMW) produces higher storm surge and a larger inundation area in the entire region. For the cases considered in this study, the storm surge height and inundation area are more sensitive to MCP than to RMW. Slower translation speed produces higher storm surge, and thus larger inundation area, but the sensitivity of storm surge to storm translation speed can be vastly different for different storms. 2003 Elsevier Ltd. All rights reserved.

Large-scale spatial patterns of benthic assemblages in the SW Atlantic: the Río de la Plata estuary and adjacent shelf waters

Abstract: This study deals with the spatial distribution of macrobenthic communities (biomass and abundance) in the Rio de la Plata estuary, Argentina–Uruguay, and the adjacent shelf waters. The benthic invertebrates were caught with an epibenthic dredge (41 samples). Multivariate analysis (cluster, MDS), SIMPER and BIO-ENV analyses were applied to analyze benthic community structure and their relationships with environmental variables. A consistent large-scale diversity pattern was found: a distinctive estuarine zone could be distinguished, with muddy sediments and a wide range of salinity, characterized by higher abundance of fauna and lower diversity, dominated by the deposit-feeding bivalve Mactra isabelleana; and a marine zone, with sandy-shell debris sediments and higher and less variable salinity values, with higher values of diversity. Major presence of deposit feeders was related to higher particulate organic matter in the estuarine area. Bottom type, salinity and the presence of a turbidity front are considered the main physical variables in structuring benthic communities of the Rio de la Plata estuary.

Residence time of the Danshuei River estuary, Taiwan

Abstract: The residence time of an estuary is defined in this study as the average time the initially existing water parcels reside in the system before they are flushed out. The residence time of the Danshuei River is calculated through a series of numerical experiments using a laterally integrated two-dimensional hydrodynamic eutrophication model (HEM-2D). The results show that the residence time is on the order of 1e2 days under the mean river flow to zero river flow condition, which is very short compared with most time scales of biogeochemical processes. A procedure is developed to quantify individual contribution to flushing by each of the three major physical transport mechanisms: tide, river discharge, and the density induced circulation. The results indicated that, in general conditions, tidal flushing exerts the greatest influence to the flushing of the Danshuei River estuary, while the density induced circulation hardly contributes any. Tidal transport contributes more than 50% of the flushing when river discharge is below its longterm mean. The suitability of applying the tidal prism method, the modified tidal prism methods, and the fraction of freshwater method in this estuary is also investigated. The relatively short residence time is likely to be one of the limiting factors that result in low phytoplankton biomass in spite of extremely high nutrient concentrations, and causes a significant fraction of pollutants to exert their effects in the coastal waters outside the estuary. 2004 Elsevier Ltd. All rights reserved.

Simulation of water exchange in Jiaozhou Bay by average residence time approach

Abstract: A dispersion model coupled with the Princeton Ocean Model was used to estimate the average residence time of the water in Jiaozhou Bay. The tidal simulation agreed quite well with drift experiments and water elevation observations at the Dagang tide station in the east coast of the bay. In particular, in situ measurements of 228Ra and salinity were carried out to calibrate the dispersion model. The modelled average residence time was about 52 days, ranging from less than 20 days in the deep part near the bay channel, the only passage connecting the bay to the Yellow Sea, to over 100 days in the shallow area in the northwest. The spatial difference of average residence time was controlled by tidal residual currents and the distance to the bay channel. The modelled tidal exchange rate was uneven in the bay, and consistent with 228Ra observations. The temporal evolution of the passive tracer accords with the evolution of the rain fraction after the rainstorm in August 2001.

Organic carbon sources to SE Asian coastal sediments

Abstract: The carbon, nitrogen and the stable carbon isotopic composition, δ13C, of organic matter has been used to elucidate the source(s) of this material to coastal sediments. Sediments were collected at 15 coastal locations in the Philippines and Vietnam, which broadly represented different depositional environments ranging from seagrass meadows, through seagrass meadows located near mangroves and to mangrove stands. In addition, short-term sediment traps were deployed at 12 of the sites where seagrass was present. Mean sediment organic carbon concentration and C/N increased and δ13C of organic matter decreased from seagrass to mangrove dominated settings. The organic carbon flux measured by the sediment trap deployments was very variable (32±3 to ∼700 mmol m−2 d−1) and represented only a small fraction of the total particle flux. The importance of seagrass as a source of organic matter to the sediments was assessed by using a simple mixing model and the average δ13C values for seagrass, their epiphytic community and surface water particulate matter (seston). A positive correlation between seagrass leaf biomass and sediment δ13C in seagrass dominated settings suggests that these macrophytes do significantly influence the composition of sedimentary organic matter. Seagrass was however rarely found to be the dominant source of organic matter to the underlying sediments. Both sediment trap and sediment data suggest that material of planktonic origin was the dominant source of sedimentary organic matter in these settings. At the sites dominated by mangroves the concentration of organic matter ( ∼1–13 ×10 3 mmol g −1 ) in the sediment is generally higher than at seagrass dominated sites due to the outwelling of organic matter from the mangrove stands. Mangrove organic matter often dominates the sedimentary input but other sources of organic matter must contribute to cause the observed range in sediment δ13C.

Impacts of inorganic nutrient enrichment on phytoplankton community structure and function in Pamlico Sound, NC, USA

Abstract: Human activities in the watersheds of the tributaries of Pamlico Sound (PS) in North Carolina have resulted in increased riverine loading of nutrients. Pamlico Sound is a regionally important aquatic resource, and provides crucial foraging and nursery habitats for Southeast Atlantic fisheries. Changes in phytoplankton community composition that may result from increased frequency and quantity of inorganic nutrient inputs could have negative ecological effects on PS. In this study we conducted a series of nutrient bioassays to assess the relationship between increased inorganic nutrient concentration and phytoplankton community structure and function. Experiments were conducted on the native phytoplankton community of the southwest basin of PS. We utilized nutrient addition treatments and all-but-one nutrient treatments in bioassays. This allowed the comparison of the effect of adding one potentially growth-limiting nutrient (e.g. nitrogen) to adding all potentially limiting nutrients except one (e.g. all except nitrogen). Data from these bioassays indicated that the phytoplankton community in PS is primarily nitrogen (N) limited. Dissolved inorganic N concentrations in PS were relatively stable during this study. The biology of its estuarine tributaries, as has been shown for the Neuse River Estuary (NRE), acted as an effective filter for most of the nutrients transported from upstream. We found stoichiometric predictors of phytoplankton community nutrient limitations to be reliable in some instances, but inaccurate in others. Some taxa-specific responses to nutrient additions were observed, however there were no consistent patterns throughout the experiments. Results indicated that changes in the PS phytoplankton community could result from changes in nutrient regime, and changes may not be consistent across phytoplankton taxonomic groups. Unlike the NRE where pulses of riverine N have significant effects on phytoplankton community structure, the PS phytoplankton community did not appear to be subjected to these periodic N enrichments.

Ascendency as an ecological indicator: a case study of estuarine pulse eutrophication

Abstract: Increasingly, management agencies require that the remediation of eutrophic waters be addressed at the level of the whole ecosystem. One whole-system approach to quantify ecosystems is called ecological network analysis. Ascendency theory, the branch of the field that deals with the quantification of whole-system status, specifically addresses the definition of eutrophication. This definition has been applied to data taken over a gradient of eutrophication. Three separate areas were observed: a non-eutrophic area (with Zostera noltii meadows), an intermediate eutrophic area (Z. noltii absent and macroalgae abundant at times) and a strongly eutrophic area (where Enteromorpha spp. blooms occur with regularity). Pulse eutrophication was considered as the major driving force behind a gradual shift in primary producers from a community dominated by rooted macrophytes (Z. noltii) to a community dominated by green macroalgae. The measures associated with the intermediate eutrophic region turned out not to be intermediate to those at the gradient extremes. The most likely explanation appears to be the highly unstable nature of this area. Conditions along the spatial gradient are discussed as representing various stages in the temporal evolution of the system, and analysed in the framework of the Intermediate Disturbance Hypothesis, Bifurcation, Chaos, and Catastrophe theories.

Heterogeneity of macrozoobenthic assemblages within a Zostera noltii seagrass bed: diversity, abundance, biomass and structuring factors

Abstract: The macrobenthic fauna community of a 70-km 2 Zostera noltii seagrass bed (Arcachon bay, France) was studied by sampling 49 stations systematically. A total of 126 taxa were identified. Cluster Analysis based on χ 2 distance showed that in this apparently homogeneous habitat, four distinct macrobenthic communities could be identified. Multiple Discriminant Analysis highlighted the major contribution of the overlying water mass as a forcing variable, and, to a lesser extent, of tidal level and Z. noltii 's below-ground parts. Seven stations did not constitute any conspicuous group, and were characterized by a low biomass of leaf ( −2 ), considered as the lowest value to constitute a Z. noltii community. Less than 24% of the seagrass bed was situated in more oceanic waters and at a quite low tidal level. In this relatively stable environment, the macrofauna community was characterized by a high species richness (mean = 39) and a moderate density and high biomass (12 638 individuals m −2 and 25 g AFDW m −2 , respectively). Annelids dominated, particularly the oligochaetes. When physical constraints increased (emersion or brackish water conditions), diversity decreased, abundance and biomass increased. The seagrass bed (55%) was flooded with highly fluctuating waters in term of temperature and salinity, here species richness was low (mean = 27) but abundance and biomass were high (24 384 individuals m −2 and 28 g AFDW m −2 , respectively), with a dominance of molluscs. The meadow (7%) was in external waters but at a higher tidal level (2.4 m vs 1.8 m above medium low tide level). This community was characterized by the particularly high density (41 826 individuals m −2 ) and dominance of oligochaetes (79% of total abundance). Species richness was high (mean = 37) here. A fourth community, extending over 12% of the meadow was dominated by the gastropod Hydrobia ulvae but could not be linked to a specific forcing variable. This study confirmed the almost complete replacement of the native clam Ruditapes decussatus by the Manila clam Ruditapes philippinarum since its introduction in the 1980s.

Fish community structure and dynamics in a coastal hypersaline lagoon: Rio Lagartos, Yucatan, Mexico

Abstract: Rio Lagartos, a tropical coastal lagoon in northern Yucatan Peninsula of Mexico, is characterized by high salinity during most of the year (55 psu annual average). Even though the area has been designated as a wetland of international importance because of its great biodiversity, fish species composition and distribution are unknown. To determine whether the salinity gradient was influencing fish assemblages or not, fish populations were sampled seasonally by seine and trawl from 1992 to 1993 and bimonthly during 1997. We identified 81 fish species, eight of which accounted for 53.1% considering the Importance Value Index (Floridichthys polyommus, Sphoeroides testudineus, Eucinostomus argenteus, Eucinostomus gula, Fundulus majalis, Strongylura notata, Cyprinodon artifrons and Elops saurus). Species richness and density declined from the mouth to the inner zone where extreme salinity conditions are prominent (>80) and competitive interactions decreased. However, in Coloradas basin (53 average sanity) and in the inlet of the lagoon, the highest fish density and number of species were observed. Greater habitat heterogeneity and fish immigration were considered as the best explanation. Multivariate analysis found three zones distinguished by fish occurrence, abundance and distribution. Ichthyofaunal spatial differences were attributed to selective recruitment from the Gulf of Mexico due to salinity gradient and to changing climatic periods. Estuarine and euryhaline marine species are abundant, with estuarine dependent ones entering the system according to environmental preferences. This knowledge will contribute to the management of the Special Biosphere Reserve through baseline data to evaluate environmental and anthropogenic changes.

Predicting sea level variations with artificial neural networks at Hillarys Boat Harbour, Western Australia

Abstract: In the present study, the artificial intelligence meshless methodology of neural networks was used to predict hourly sea level variations for the following 24 h, as well as for half-daily, daily, 5-daily and 10-daily mean sea levels. The methodology is site specific; therefore, as an example, the measurements from a single tide gauge at Hillarys Boat Harbour, Western Australia, for the period December 1991–December 2002 were used to train and to validate the employed neural networks. The results obtained show the feasibility of the neural sea level forecasts in terms of the correlation coefficient (0.7–0.9), root mean square error (about 10% of tidal range) and scatter index (0.1–0.2).

Undular tidal bore dynamics in the Daly Estuary, Northern Australia

Abstract: Measurements in the macro-tidal Daly Estuary show that the presence of an undular tidal bore contributed negligibly to the dissipation of tidal energy. No recirculation bubble was observed between a trough and the following wave crest in the lee waves following the undular bore. This differs to stationary undular bores in laboratory experiments at larger Froude numbers where a recirculation bubble exists. Secondary motions and the turbulence generated by the undular bore had no measurable influence on the sediment transport. This situation contrasts with the intense sediment resuspension observed in breaking tidal bores. The tidally averaged sediment budget in the Daly Estuary was controlled by the asymmetry of tidal currents. The undular bore may widen the river by breaking along the banks that it undercuts, leading to bank slippage. A patch of river-wide macro-turbulence of 3-min duration occurred about 20 min after the passage of the bore during accelerating tidal currents.

Spatial and temporal structure of fish assemblages in an “inverse estuary”, the Sine Saloum system (Senegal)

Abstract: As a consequence of the Sahelian drought, the Sine Saloum, a large estuarine system located in Senegal (West Africa), has become an ‘‘inverse estuary’’ since the late sixties, i.e. salinity increases upstream and reaches 100 in some places. To study the fish assemblages of such a modified system, a survey was conducted in 1992, collecting fish every two months with a purse seine at eight sites spread over the three main branches of the estuary. A total of 73 species belonging to 35 families were identified. Eight species comprised 97% of the total numbers of fish. The predominant species was a small clupeid, Sardinella maderensis, representing more than half of the total biomass and nearly 70% of the total number of fish. The spatio-temporal structure of the fish assemblages was studied using the STATIS-CoA method, which combines the multitable approach with the correspondence analysis method. Whatever the season, a strong spatial organization of fish assemblages was observed, mainly related to depth and salinity. Three types of assemblages were identified. In shallow water areas, fish assemblages were dominated by Mugilidae, Gerreidae and Cichlidae and were stable with time. In open water areas, large fluctuations in the species composition were observed, due to the occasional presence of large schools of pelagic species: in the southern area, where salinity and water transparency were the lowest, the main species were Ilisha africana, Brachydeuterus auritus and Chloroscombrus chrysurus, associated with a few Sciaenidae and Tetraodontidae, while the poorest areas were characterized by only two dominant species, S. maderensis and Scomberomorus tritor. 2003 Elsevier Ltd. All rights reserved.

Interpreting the colour of an estuary.

Abstract: This paper explores the possibility of using water colour to quantify the concentration of coloured dissolved organic matter (CDOM), and through it, dissolved organic carbon (DOC) and salinity in a turbid estuary in which suspended sediments also influence water colour. The motivation of the work is that the method could be applied to water colour measurements made remotely from an aircraft (or, in larger estuaries, a satellite) enabling near-synoptic mapping of surface salinity and DOC distributions. The paper describes observations at 29 stations distributed along the salinity gradient of the Conwy estuary in North Wales. At each station, surface water samples were collected and analysed for salinity, concentrations of DOC, chlorophyll and suspended particles and absorption spectra of CDOM, or yellow substance. Profiles were made of both upwelling and downwelling irradiance in four narrow band channels, and these were used to calculate irradiance reflection and attenuation coefficients. Results show that spectrally averaged light absorption in the estuary is caused principally and equally by mineral suspended solids and yellow substance, with water and chlorophyll in third and fourth place. The CDOM is strongly correlated (R2=0.99) in a negative sense with salinity, and more weakly correlated with DOC. There is a linear relationship between CDOM and the ratio of reflection coefficients in the red (670 nm) and blue-green (490 nm) parts of the spectrum, which could be applied to remote sensing; the slope and intercept of the relationship are however different to those found in less turbid water bodies. It is shown that the change in slope and intercept are consistent with the presence, in the Conwy estuary, of suspended particles which influence the water colour. A method is described and tested for inverting water colour measurements in a turbid estuary to give estimates of CDOM in the presence of suspended particles. The solution, which has not been adjusted to fit the data, produces profiles of CDOM, DOC and salinity, which behave reasonably but which currently have a limited accuracy. RMS differences between measured and optically derived parameters for the entire data set are 0.27 m−1 (CDOM), 4 PSU (salinity) and 67 μM (DOC) although better accuracy is obtained on individual surveys. The fact that there is little bias between predicted and observed parameters indicates that much of the scatter is caused by random measurement error and that the approach is fundamentally sound.

Changes in stoichiometric Si, N and P ratios of Mississippi River water diverted through coastal wetlands to the Gulf of Mexico

Abstract: During the spring of 2001, we monitored nutrient concentrations and stoichiometry of diverted Mississippi River water as it flowed through the Breton Sound estuary, Louisiana, USA. River water was discharged through a diversion structure at Caernarvon as a two-week pulse that peaked at 220 m 3 s −1 . There were reductions in observed concentrations of TN, TP, DIN, DIP and DSi, of up to 44%, 62%, 57%, 23%, and 38%, respectively, as water flowed through the estuary. TN, TP, DIN, DIP and DSi concentrations in the river were 137–140, 5.0–5.1, 104–153, 1.1–1.3 and 114–121 μM, respectively, and 36–122, 1.8–3.6, 13–119, 0.3–1.8 and 29–110 μM, respectively, at the Gulfward end member stations. The DSi:DIN ratio rose from 0.9 at the Caernarvon diversion to 2.6 at the Gulf end member station, while the DIN:DIP ratio fell from 107 to 26. This study shows that freshwater diversions can significantly alter riverine nutrient concentrations and ratios and reduce the overall amount of exported nitrogen.

Reconstructing 150 years of eutrophication in the north-western Adriatic Sea (Italy) using dinoflagellate cysts, pollen and spores

Abstract: Recent trends in the trophic conditions of the north-western Adriatic Sea have been evaluated using organic-walled dinoflagellate cysts (dinocysts), pollen and spores. Palynological analyses performed on a sediment core covering the period AD 1830e1990 have revealed a progressive increase in eutrophication beginning in the 20th century. The first signal of a change seems to occur earlier in the terrestrial ecosystem and later in the marine realm. Pollen data indicate that the source of enhanced nutrient loading to the North Adriatic Sea, which partly resulted from increased wetland reclamation and forest clearance, began at about AD 1910. The clear shift in the relative abundance of the dinocyst Lingulodinium machaerophorum at AD 1930 suggests an increase in eutrophication. Dinocyst assemblages point to stressful conditions from AD 1960, reaching a maximum at about AD 1978. Subsequently eutrophication levels decrease, although dinocyst diversity suggests that the ecosystem has not completely recovered. � 2004 Elsevier Ltd. All rights reserved.

Phosphorus cycling in sediments of the Bohai and Yellow Seas

Abstract: Using a box-sampler, surface and core sediment samples were collected from the Bohai and Yellow Seas in 1998–1999. The sediments were sequentially extracted to determine six phosphorus fractions, and non-sequentially extracted for total P (TP) and inorganic P (IP), where the difference between TP and IP was assumed to be organic P (OP). It was found that more than 70% of the total P is inorganic P. The concentrations of phosphorus were high near the river mouth, reflecting the influence from the Huanghe and Changjiang sediment loads. The phosphorus levels in the Bohai Sea are higher than those in the Yellow Sea, which is related to the sediment sources and distribution of chlorophyll a in these areas. Concentrations of TP in the Bohai and Yellow Seas are comparable to the other worldwide coastal areas. Among the six forms of phosphorus, phosphorus distribution ranges were 0.8–2% for loosely sorbed P, 3–10% for iron-bound inorganic P, 4–19% for leachable organic P, 3–15% for authigenic apatite, 35–66% for detrital apatite and 14–36% for refractory organic P. Potentially bioavailable phosphorus accounts for a small amount of TP, while phosphorus load associated with eroded soil may be larger than the anthropogenic load. The regeneration of dissolved P from sediments is a slow process, which could play a role in maintaining low phosphate levels in the seas. Most of the phosphorus was regenerated in a water column, and almost all of the remaining phosphorus was buried in the sediments after its accumulation. The high values of P burial efficiencies in the study areas were related to low benthic phosphate flux, which is due to the high percent of detrital apatite that is not bioavailable and the high sediment accumulation rate.

Zooplankton community structure and biomass in a southern African temporarily open/closed estuary

Abstract: The zooplankton community structure and biomass in the upper, middle and lower reaches of the temporarily open/closed Kasouga estuary were investigated monthly over a period of one year. Multiple range tests performed after ANOVA indicated that there were no distinct spatial patterns in the physico-chemical and biological variables during the study ( P >0.05). Total chlorophyll- a (chl- a ) concentration and zooplankton biomass during the investigation ranged between 0.91 and 5.92 mg chl- a m −3 and between 19.51 and 103.50 mg Dwt m −3 , respectively. The highest levels (>3 mg chl- a m −3 ; >45 mg Dwt m −3 ) were consistently recorded during summer and the lowest ( a m −3 ; −3 ) during winter and during those periods when the estuary breached. Zooplankton community structure was strongly linked to the presence/absence of a link to the marine environment. In the absence of any link to the sea, the zooplankton community was almost entirely dominated by copepods (mainly Pseudodiaptomus hessei , Acartia longipatella and Halicyclops spp.), which comprised between 72 and 97% of the total zooplankton biomass. The inflow of seawater into the estuary following overtopping or breaching coincided with an increase in the contribution of mysids, amphipods and fish larvae to the total zooplankton biomass. Hierarchical cluster analysis identified three distinct zooplankton groupings during the study. These groupings corresponded to the summer and winter communities and a community associated with overtopping and breaching events. SIMPER analysis indicated that the differences between the summer and winter communities reflect changes in the relative contributions of the dominants as opposed to the presence/absence of species. The overtopping/breaching group was characterized by the presence of larvae of marine breeding invertebrates and fish. These data indicate that the zooplankton community structure and biomass in the temporarily open/closed Kasouga estuary are determined by the interactive effects of freshwater inflow, water temperature and mouth status.

Effects of boating activities on aquatic vegetation in the Stockholm archipelago, Baltic Sea.

Abstract: The aquatic vegetation in 44 similar shallow and sheltered inlets exposed to different kinds of disturbances by boating was studied in a rocky archipelago in the Baltic Sea. The results indicate that both recreational boating activities and traffic by medium sized ferryboats may cause significant changes in community composition and have significant negative effects on species richness and the development of the macrophytic vegetation at greater depth. Changes in inlet morphology by dredging and increases in resuspension and turbidity by wave-action from boats were most probably the major factors contributing to the demonstrated differences in the vegetation between inlets. In inlets used as harbours for private boats (marinas) or adjacent to ferryboat routes, vegetation cover and species richness declined significantly more with depth than in reference inlets not exposed to disturbance by boating activities. In marinas, turbidity was significantly higher than in reference inlets. Accordingly, a canonical correspondence analysis showed that the abundance of species sensitive to poor light conditions, such as Chara spp. and Ruppia spp. were negatively correlated with marinas while Myriophyllum spicatum and Ceratophyllum demersum that are common in nutrient rich turbid habitats were positively correlated with marinas. Mechanical disturbance by propellers may also have contributed to the results, disfavouring Potamogeton pectinatus compared to similar caulescent species in marinas. Chara tomentosa and Najas marina that are exposure sensitive mud thriving species were negatively correlated with inlets adjacent to ferryboat routes, while the macroalga Fucus vesiculosus that is dependent on clean substrate for successful recruitment was positively correlated with inlets adjacent to ferryboat routes. It is important to explore further the effects of boating activities in these habitat types, since the studied inlets sustain a high diversity of both plants and invertebrates and provide highly significant recruitment areas for coastal fish in the Baltic Sea.