Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary (Belgium/The Netherlands)

In contrast with the marine reaches of estuaries, few studies have dealt with zooplankton grazing on phytoplankton in the upper estuarine reaches, where freshwater zooplankton species tend to dominate the zooplankton community. In spring and early summer 2003, grazing by micro- and mesozooplankton on phytoplankton was investigated at three sites in the upper Schelde estuary. Grazing by mesozooplankton was evaluated by monitoring growth of phytoplankton in 200 μm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoïd copepod Eurytemora affinis, the cyclopoid copepods Acanthocyclops robustus and Cyclops vicinus and the cladocera Chydorus sphaericus, Moina affinis and Daphnia magna/pulex. No significant grazing impact of mesozooplankton in any experiment was found despite the fact that mesozooplankton densities used in the experiments (20 or 40 ind. l⁻¹) were higher than densities in the field (0.1–6.9 ind. l⁻¹). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 μm filtered water. Microzooplankton in the 30–200 μm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. l⁻¹. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41–1.83 day⁻¹ and grazed up to 84% of initial phytoplankton standing stock per day. Rotifer clearance rates estimated from microzooplankton community grazing rates and rotifer abundances varied from 8.3 to 41.7 μl ind.⁻¹ h⁻¹. CHEMTAX analysis of accessory pigment data revealed a similar phytoplankton community composition after incubation with and without microzooplankton, indicating non-selective feeding by rotifers on phytoplankton.     

Inter-annual variability in phytoplankton summer blooms in the freshwater tidal reaches of the Schelde estuary (Belgium)

The inter-annual variability in phytoplankton summer blooms in the upper reaches of the Schelde estuary was investigated between 1996 and 2005 by monthly sampling at 10 stations. The large inter-annual variations of the chlorophyll a concentration in the freshwater tidal reaches were independent from variations in chlorophyll a in the tributary river Schelde. Summer mean chlorophyll a concentrations were significantly negatively correlated with flushing rate (Spearman correlation: r = −0.67, p = 0.05, n = 9) but not with temperature, irradiance and suspended particulate matter or dissolved silica (DSi) concentrations. During dry summers, low flushing rates permitted the development of dense phytoplankton populations in the upper part of the estuary, while during wet summers high flushing rates prevented the development of dense phytoplankton blooms. Flushing rate was also found to be important for the phytoplankton community composition. At low flushing rates, the community was dominated by diatoms that developed within the upper estuary. At high flushing rates, chlorophytes imported from the tributary river Schelde became more important in the phytoplankton community. The position of the chlorophyll a maximum shifted from the head of the estuary when flushing rates were low, to more downstream when flushing rates were high. Although DSi concentrations tended to be lower during years of high phytoplankton (mainly diatom) biomass, the relation with flushing rate was not significant.     

Changes in phytoplankton diversity and community composition along the salinity gradient of the Schelde estuary (Belgium/The Netherlands)

Phytoplankton diversity and community composition were studied along the Schelde river–estuary–coastal zone continuum during the summer of 2003. DCA analysis indicated a gradual compositional turnover of the phytoplankton community within the estuary. GAM modelling of species response curves along the estuarine gradient was used to identify taxa that had their population maximum in the river, the coastal zone or within the estuary. Taxa that had their population maximum within the estuary did not form a homogenous community but comprised species with different salinity optima and rather restricted salinity tolerances. The observed changes in community composition along the estuarine transition zone correspond more closely to an ecocline than to an ecotone model. Despite the fact that few taxa had their population maximum at or near the salinity gradient, alpha diversity did not display a minimum around the salinity gradient. This lack of a diversity minimum within the estuary was ascribed to an important contribution of taxa of riverine or coastal origin to alpha diversity within the estuary contributed. On average 55% of the alpha diversity in the estuarine samples was due to riverine or coastal taxa. Beta diversity displayed a clear maximum around the salinity gradient. For planktonic organisms which are subject to mass effects, beta diversity is probably a better indicator for the impact of the salinity gradient on diversity in estuaries than alpha diversity.     

Nutrient limitation of phytoplankton growth in the freshwater tidal zone of a turbid,Mediterranean estuary

Identification of the limiting nutrient(s) is a requirement for the rational management of eutrophication. Here, we present the first experimental analysis of nutrient limitation of phytoplankton growth and its seasonal variation in the Guadiana estuary (SE Portugal-SW Spain). Ten microcosm experiments were performed during 2005 and 2008, using water samples collected in the freshwater tidal zone of the Guadiana estuary. Nitrate, phosphate and silicate were added in a single pulse, alone and in combinations. Experimental treatments were incubated for 4 days under controlled laboratory conditions. Phytoplankton response to nutrient enrichment was evaluated through changes in biomass (Chla), and abundance of specific phytoplankton groups.     

Possible effects of climate change on estuarine nutrient fluxes: a case study in the highly nutrified Schelde estuary (Belgium, The Netherlands)

Global change models predict effects of climate change on hydrological regimes at the continental scale in Europe. The aim of this study was to gain a better understanding of the possible effect of changing external forcing conditions on the functioning of estuarine ecosystems. In densely populated areas, anthropogenic nutrient enrichment and consequent alteration of nutrient biogeochemical cycles have already had a big impact on these ecosystems. The average yearly discharge of the upper Schelde estuary increased nearly threefold over the period 1996–2000, from 28 m³ s⁻¹ in 1996 to 73 m³ s⁻¹ in 2000. The continuously rising discharge conditions over the five-year period were used as a reference situation for possible future effects of climate on ecological functioning through increase of discharge. At high discharges, nutrient (NH₄⁺, NO₃⁻, dissolved silica and PO₄³⁻) concentrations in the tidal fresh- and brackish water showed a decrease of up to 50% while total discharged nutrient loadings increased up to 100%. Opposite effects of increasing discharge on NH₄⁺, NO₃⁻ and dissolved silica concentrations in summer and winter, resulted in the flattening out of seasonal cycles for these nutrients. Under high discharge conditions, silica uptake by diatom communities was lowered. Dissolved silica loadings to the coastal area increased concurrently with total silica loadings upstream. Salt intrusion to the marine parts of the estuary decreased. This resulted in a downstream shift of the salinity gradient, with lower salinity observed near the mouth. As a result, TDIN, NO₃⁻ and dissolved silica concentrations doubled at the mouth of the estuary.     

Restoration of tidal freshwater vegetation using controlled reduced tide (CRT) along the Schelde Estuary (Belgium)

Throughout the world, tidal marshes are being restored to obtain natural protection against recurring storm surges and sea level rise and to preserve the resources these habitats provide. Managed realignment, or breaching of the seaward dikes, is a restoration technique increasingly used; yet unsuitable site elevation has been a major constraint to habitat development. Controlled reduced tide (CRT), a new technique to implement tidal regime on low elevated sites, could offer solutions for several problems associated with reconstruction or mitigation of tidal marshes.     

Zooplankton community structure in a highly turbid environment (Charente estuary,France): Spatio-temporal patterns and environmental control

Zooplankton assemblages were studied from January 2007 to January 2008 along the salinity gradient of the Charente estuary (France). A Lagrangian survey was performed monthly at five sampling stations defined by salinity (freshwater, 0.5, 5, 15 and 25) in order to collect zooplankton and measure the main environmental parameters (concentrations of suspended particulate matter, particulate organic carbon, chlorophyll a and phaeopigments). A combination of multivariate cluster analysis, species indicator index and canonical correspondence analysis was used to relate the spatio-temporal patterns of the zooplankton assemblages with environmental drivers. The estuary was divided into three different zones by means of environmental parameters while four zooplankton assemblages were identified along the salinity gradient. The Charente estuary appeared as one of the most turbid systems in Europe, with suspended particulate matter (SPM) concentration reaching 3.5 g l⁻¹ in the Maximum Turbidity Zone (MTZ). Algal heterotrophy and microphytobenthos resuspension from the wide mudflats could be responsible for the relatively high chlorophyll a concentrations measured within this MTZ. Salinity and SPM affected significantly the spatial distribution of zooplankton species while temperature and river flow seemed to control their temporal variations. From a zooplanktonic viewpoint, the highly turbid Charente estuary seemed to match an “ecotone–ecocline” model: the succession of species assemblages along the salinity gradient matched the concept of ecocline while the MTZ, which is a stressful narrow area, could be considered as an ecotone. Although such ecoclinal characteristics seemed to be a general feature of estuarine biocenoses, the ecotone could be more system-specific and biological compartment-specific.     

Seasonal phytoplankton composition, productivity and biomass in the Neuse River estuary, North Carolina

Phytoplankton community composition, productivity and biomass characteristics of the mesohaline lower Neuse River estuary were assessed monthly from May 1988 to February 1990. An incubation method which considered water-column mixing and variable light exposure was used to determine phytoplankton primary productivity. The summer productivity peaks in this shallow estuary were stimulated by increases in irradiance and temperature. However, dissolved inorganic nitrogen loading was the major factor controlling ultimate yearly production. Dynamic, unpredictable rainfall events determined magnitudes of seasonal production pulses through nitrogen loading, and helped determine phytoplankton species composition. Dinoflagellates occasionally bloomed but were otherwise present in moderate numbers; rainfall events produced large pulses of cryptomonads, and dry seasons and subsequent higher salinity led to dominance by small centric diatoms. Daily production was strongly correlated (r = 0·82) with nitrate concentration and inversely correlated (r = −0·73) with salinity, while nitrate and salinity were inversely correlated (r = −0·71), emphasizing the importance of freshwater input as a nutrient-loading source to the lower estuary. During 1989 mean daily areal phytoplankton production was 938 mgC m⁻², mean chlorophyll a was 11·8 mg m⁻³, and mean phytoplankton density was 1·56 × 10³ cells ml⁻¹. Estimated 1989 annual areal phytoplankton production for the lower estuary was 343 gC m⁻².     

Microphytobenthic biomass and species composition in intertidal flats of the Nakdong River estuary, Korea

Intertidal microphytobenthos (MPB) were investigated monthly from August 2006 to March 2008 at four different sites in the sand flats of Nakdong River estuary, Korea. Samples of surface sediment (ca. 1 cm) were collected, and chlorophyll a was extracted as biomass estimation. Species identification and enumeration were carried out by light microscopy, assisted where necessary by scanning electronic microscopy. Biomass varied between 0.47 and 16.58 μg cm⁻³, abundance changed from 5.25 to 414.75 × 10³ cells cm⁻³, while the Shannon diversity indexes ranged between 0.69 and 2.35 H′. Thirty-nine MPB taxa were identified, primarily composed of epipelic diatoms, among which Amphora and Navicula were the most abundant genera. Based on the biomass, abundance, species composition and their dynamics, MPB assemblages of sampling sites were grouped into three distinct communities corresponding to their sediment composition characteristics. Multivariate correlation analysis revealed that biomass was positively related to mud and very fine sand, negatively related to fine and medium sand, but not significantly related to environmental factors such as pore water nutrients, light intensity and salinity, which fluctuated rapidly during emersion period. Cluster analysis corroborated the division of MPB communities according to site types on seasonal scales, and also showed seasonality between sites by cluster of all summer groups. Principal component analysis identified that variability in species composition was significantly affected by mud, very fine sand, fine sand, light intensity, and sediment temperature. This study suggests that sediment composition plays an important role in the functioning of intertidal MPB communities in estuarine ecosystems.     

长江口海水盐度和悬浮体对中肋骨条藻生长的影响

2001年5月通过实验室培养的方法研究了长江口优势赤潮浮游植物种类——中肋骨条藻(Skeletonema costatum)在长江口海水中的生长情况。实验结果显示,在实验光强范围内该藻相对生长速率常数随光强的减小而减小,表明光照的可得性是影响其生长的重要因素;悬浮体的遮光效应会限制其生长。海水盐度超出其适宜生长的范围也会对其的生长产生不利影响。两个相互对比的实验说明在泥沙含量很高的长江口咸淡混合水中光照的可得性和盐度均影响中肋骨条藻的生长,综合这两个因素,在实验盐度范围内最适宜中肋骨条藻生长的盐度是约为19．2的咸淡水。适于中肋骨条藻适宜生长的盐度为14～23。     

In situ growth of Soletellina alba (Bivalvia: Psammobiidae) in response to detrital supply and mouth status in a seasonally-closed estuary

The supply of detritus is an important food source for many soft-sediment invertebrates, but its importance for their growth and condition is rarely, if ever, tested directly using manipulative field experiments. Therefore, we designed such a study to: (1) test the importance of fine particulate organic matter for the growth and condition of the infaunal bivalve Soletellina alba; (2) indirectly test the feeding mode of S. alba, which has been assumed to be a deposit feeder like other members of the same superfamily (Tellinoidea); (3) compare growth rates across two summers with contrasting patterns of estuary mouth opening/closing; and (4) compare the condition of individuals used in two field studies (i.e. present versus past) and a past laboratory study. Neither growth nor condition differed when organic content of the sediments was varied, which suggests that S. alba is either a suspension feeder or capable of switching modes of feeding. There was considerable interannual variation in growth with greater growth occurring during the summer with a longer period of mouth opening. This suggests that periods of mouth closure may reduce secondary production within seasonally-closed estuaries. Potential artefacts associated with laboratory trials were also identified, with laboratory bivalves exhibiting poorer condition than those used in two field trials. The present study provides no evidence that variable quantities and qualities of organic matter within the sediments influence the growth and condition of S. alba, but future studies should focus on food supplied via the water column when the estuary is open versus closed.     

Impact of freshwater influx on microzooplankton mediated food web in a tropical estuary (Cochin backwaters – India)

The diversity, abundance and biomass of microzooplankton in Cochin backwaters were studied for the first time during pre-summer monsoon to peak of summer monsoon (April–July 2003) to understand the impact of large freshwater influx. Microzooplankton abundance and biomass were highest during pre-summer monsoon (av. 3817 ind. L⁻¹ and 146 μg C L⁻¹) that declined with the onset (av. 2052 ind. L⁻¹ and 45 μg C L⁻¹) and peak (av. 409 ind. L⁻¹ and 10 μg C L⁻¹) summer monsoon. Species diversity, richness and evenness of microzooplankton also showed similar trends as that of abundance and biomass. Grazing experiment showed that microzooplankton consumes 43 ± 1% of the daily phytoplankton standing stock during the high saline condition (27.5). Low abundance of microzooplankton during summer monsoon period (1/8 of the pre-summer monsoon value) along with the concomitant occurrence of low mesozooplankton (1/8 times of pre-summer monsoon value) suggests that there could be a general lack of planktonic grazers. This would result in a weak transfer of primary and bacterial carbon to higher trophic levels, eventually leaving behind much unconsumed basic food in the estuary during summer monsoon. Thus a major portion of the primary carbon either settles down or gets transported to the coastal regions during monsoon. High flushing of Cochin backwaters also facilitates faster removal of primary producers to the coastal regions during monsoon.     

An analysis of the trophic network of a macrotidal estuary: the Seine Estuary (Eastern Channel, Normandy, France)

A steady state, mass balance, trophic network has been constructed to illustrate the flow of energy in the Seine Estuary by using Network Analysis and Ecopath methods. This ecosystem shows 15 compartments from primary producers to the top consumers (fish and birds). This study has been compared with other ecosystems of comparable nature located in North America (Narragansett, Chesapeake, Delaware Bays), Europe (Ems Estuary, Dublin Bay and Bay of Somme), and South Africa (Swartkops Estuary) in which analysis of trophic network has been applied with similar methods.The Seine Estuary shows values of some global parameters and indices either close to large North American bays and a South African estuary characterised by the complexity of their trophic network, or values near European bays and estuaries, or else remain typical of the Seine estuary. All of this reflects specific functioning of the Seine Estuary which can be explained by the analysis of the dominant food web. In the upstream sector an important pelagic food web was found correlated with highest primary production, especially planktonic, which is rapidly consumed by an abundant zooplankton and suprabenthos (Mysidacae and Decapoda Crustacea). This reveals the dominant consumer role of this sector. The external fluvial inputs (277.80 gC m⁻² y⁻¹) are transferred to the downstream sector which produces the major export (548.43 gC m⁻² y⁻¹), in parallel with the low consumption and efficiency of dominant benthos component from its bentho-pelagic food web. This reflects the dominant exporter role of this sector.In the Seine Estuary low values of detritivory index D/H (2.52), recycling index FCI (16.1%) and connectance (0.24) were found associated with high values of P/B ratio (38.2%), sum of exports (548.43 gC m⁻² y⁻¹) and the great difference between ratio of ascendency to capacity development A/C and internal ratio A_i/C_i. This shows the lack of a dominant resource as in Delaware Bay, that the state of development is different from a mature ecosystem, and the dependance on external connections similar to the Bay of Somme, another ecosystem of Eastern Channel, France.     

The nitrogen isotopic composition of dissolved nitrate in the Yangtze River (Changjiang) estuary,China

The nitrogen isotopic composition of dissolved nitrate (δ¹⁵N-NO₃⁻) in surface water of the Yangtze River estuary was determined in four seasons of 2006. δ¹⁵N-NO₃⁻ ranged from 0.4‰ to 6.5‰ and varied with seasons and geographic regions, reflecting the dynamics of nitrogen cycling in the estuarine ecosystem. δ¹⁵N-NO₃⁻ was markedly lower in February than in other seasons and exhibited conservative mixing, which was probably attributed to the NO₃⁻ being sourced from the atmospheric deposition and agricultural fertilizer. In the upper estuary, the influence of riverine inputs was important during all surveys. In the turbidity maximum zone, nitrification was found with nitrate depleted in ¹⁵N in May, whereas denitrification resulting in heavy δ¹⁵N-NO₃⁻ played an important role in August. More enriched δ¹⁵N-NO₃⁻ values coinciding with losses of nitrate concentrations based on the conservative mixing model were found in the adjacent marine area in May, and may reflect obvious phytoplankton assimilation of dissolved nitrate. In this manner, δ¹⁵N-NO₃⁻ may be a sensitive indicator of nitrogen sources and biogeochemical processing existing in this estuary in conjunction with the variations of dissolved nitrate and other environmental factors.     

Multi-bioindicator study of a small estuary in Vendée (France)

Surficial sediment (about 5 mm) was scraped off in 15 stations selected in the Vie Estuary, a small estuary on the French Atlantic coast. The samples, prepared using the suitable methods, were used for comparing the behaviour of periphytic algae, sporopollinic and other palynologic material, ostracoda and foraminifera for proposing a synthetic evaluation of the information provided by each of them. Dinoflagellate cysts, which are found in an almost monospecific assemblage in low-salinity water, are dispersed by tidal currents, but the location of tintinnids in only the lower reaches shows a limited inward transport. Sporopollinic material is clearly related to the adjacent vegetation, indicating a local origin, and then little influence of transport on the microbenthos, even if fragments of freshwater crustaceans and of Pediastrum alga are found down to the mouth in the sporopollinic deposits. This limited transport of microbenthos allows the use of periphytic algae, benthic ostracoda and benthic foraminifera as indicators of local environmental conditions. It was then possible to determine the extent of marine influence in the estuary, to discriminate between the main channel and adjacent basins, and between different stages of evolution of the basins, from a recently dug basin to a eutrophicated one.     

Temporal and spatial changes of dunes in the Changjiang (Yangtze) estuary,China

The response of subaqueous dunes to the variation of riverine sediment supply in an estuary is rarely reported. Five sets of field measurements of bed sediments and topography were made in the Changjiang estuary, China during dry and flood seasons for the past 9 years. Results show that dunes were mainly three-dimensional (3D) during flood seasons and two-dimensional (2D) dunes during dry seasons. Dunes were also much larger in flood seasons. Dunes mainly occurred within the main channels of the estuary although they extended 11.7 km seaward of their normal extent in the South Channel during dry seasons from 2002 to 2006. This extension of the dune field is likely due to a sharp decline of sediment discharge caused by large reservoirs such as Three Gorges.     

长江口邻近海域春季藻华期间浮游植物生物量与环境要素的拟合分析

基于2005年长江口邻近海域春季现场调查资料,本文采用非线性拟合方法,构建了该海域春季硅藻藻华和甲藻藻华期间浮游植物生物量与无机营养盐、温度和盐度之间的定量关系方程,进而分析了春季硅、甲藻藻华发生的最适环境条件。结果表明,硅藻藻华出现的最适环境条件为:溶解态无机氮浓度10.1μmol/L、磷酸盐浓度0.21μmol/L、硅酸盐浓度6.39μmol/L、温度11.9℃、盐度30.5;甲藻藻华暴发的最适环境条件为:溶解态无机氮浓度3.98μmol/L、磷酸盐浓度0.18μmol/L、温度22.0℃、盐度27.2。上述硅、甲藻藻华发生的最适环境条件可作为硅、甲藻藻华形成的关键环境参数,为开展长江口邻近海域藻华形成和演替过程的数值模拟和参数优化提供参考。     

Inshore-offshore and vertical patterns of phytoplankton biomass and community composition in Santa Monica Bay, CA (USA)

Spatial gradients in biomass and community composition have important consequences for ecosystem structure and function. In this study, small-scale inshore-offshore (1-10 km) and vertical (1-50 m) patterns of microphytoplankton biomass and community composition are described, and the environmental controls of microphytoplankton biomass are evaluated in a coastal ecosystem of the Southern California Bight (SCB). During a two-year period, persistent inshore-offshore gradients in phytoplankton biomass and occasional inshore-offshore gradients in community composition, coincident with regional precipitation, were found, although the strength of the gradients varied between sampling periods. The chlorophyll a maximum was generally present between 15 and 45 m, the cell abundance maximum occurred in surface waters, and there was little evidence of vertical gradients in community composition. Variability in chlorophyll a concentrations was linked to variability in environmental parameters only after some rain and upwelling events. This study demonstrates that inshore-offshore patterns in phytoplankton biomass previously documented at large spatial scales (100-700 km) in the SCB can also persist at smaller scales (1-10 km), although the mechanisms for the gradients are likely to be different at the different spatial scales. The results provide a baseline data set that can be used to focus monitoring and management efforts in the SCB. In particular, this work shows that a limited number of sampling stations are sufficient for phytoplankton monitoring in Santa Monica Bay.