Plankton distribution and vertical flux of biogenic matter during high summer stratification in the Krka estuary (Eastern Adriatic)

The aim of this study was to investigate phytoplankton abundance, composition and vertical export in the highly stratified Krka estuary, Croatia. The estuary is stratified throughout the year, and an interface between fresh- and brackish water plays an important role in production and degradation of biogenic matter. Vertical export of particulate organic carbon (POC), phytoplankton carbon (PPC) and faecal pellet carbon (FPC) was studied by deploying sediment traps in the middle and lower reach of the estuary and in the adjacent coastal zone. Zooplankton faecal pellet (FP) production experiments were conducted to provide additional information on the potential contribution of FP to the total carbon flux. High suspended concentrations of POC, chlorophyll a and phytoplankton was found in the lower reaches of the Krka estuary, adjacent to a source of anthropogenic eutrophication. The fraction of organic detritus to the total POC flux was 61–69% inside the estuary but only 7% at the marine station. This indicates that the primary producers in the surface layer of the Krka estuary are decomposed in and below the interface and then settle as detritus to the bottom. Low sedimentation rates in the coastal zone outside the estuary revealed that the eutrophication does not spread out of the estuary. Mesozooplankton played a modest role in vertical flux regulation, due to their low abundance and dominance of smaller forms as well as low faecal pellet production rates. It is concluded that processes taking place at the freshwater-seawater interface are of major importance for the vertical carbon flux in the investigated area.     

三峡工程一期蓄水后长江口悬浮体形态及物质组成

2003年6月1—10日为长江三峡工程一期蓄水期,为了研究长江三峡工程蓄水对长江口环境的影响,2003年6月15—25日组织了长江口及其邻近海域环境综合调查,对该次调查所取得的悬浮体样品进行了显微观察和X射线衍射测定,分析了不同水体中悬浮体颗粒的形态特征和物质组成。结果表明:从徐六泾到长江口邻近海域,悬浮体颗粒形态特征不同,徐六泾至南支主槽,悬浮体颗粒以单颗粒为主;拦门沙最大浑浊带,悬浮体颗粒以大絮团、团状絮团和絮网为主;长江口邻近海域,悬浮体颗粒以有机质絮团和单颗粒并存为主。长江口区悬浮体矿物颗粒主要以粒状或片状形式存在。悬浮体矿物颗粒主要由石英、长石等碎屑矿物和黏土矿物组成。从河道到长江口外的邻近海域,石英和长石含量逐渐降低,伊利石和绿泥石含量逐渐增加,高岭石含量先降低后增加。与蓄水前相比,蓄水后徐六泾附近小絮团数量有所增加,蓄水前后拦门沙和长江口邻近海域悬浮体形态特征和长江口地区悬浮体物质组成并未发生明显变化。     

River discharge controls phytoplankton dynamics in the northern San Francisco Bay estuary

Phytoplankton dynamics in the upper reach of the northern San Francisco Bay estuary are usually characterized by low biomass dominated by microflagellates or freshwater diatoms in winter, and high biomass dominated by neritic diatoms in summer. During two successive years of very low river discharge (the drought of 1976-77), the summer diatom bloom was absent. This is consistent with the hypothesis that formation of the diatom population maximum is a consequence of the same physical mechanisms that create local maxima of suspended sediments in partially-mixed estuaries: density-selective retention of particles within an estuarine circulation cell. Because the estuary is turbid, calculated phytoplankton growth rates are small in the central deep channel but are relatively large in lateral shallow embayments where light limination is less severe. When river discharge falls within a critical range (100–350 m³ s^?1) that positions the suspended particulate maximum adjacent to the productive shallow bays, the population of neritic diatoms increases. However, during periods of high discharge (winter) or during periods of very low discharge (drought), the suspended particulate maximum is less well-defined and is uncoupled (positioned downstream or upstream) from the shallow bays of the upper estuary, and the population of neritic diatoms declines. Hence, the biomass and community composition of phytoplankton in this estuary are controlled by river discharge.     

Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions

Water column concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured at three different depths in four different locations bracketing the estuarine turbidity maximum (ETM) along the main channel of a temperate riverine estuary (Winyah Bay, South Carolina, USA). Measurements were carried out over full tidal cycle (over 24 h). Salinity, temperature, current magnitude and direction were also monitored at the same time throughout the water column. Tidally averaged net fluxes of salt, TSS, POC and PN were calculated by combining the current measurements with the concentration data. Under the extreme low river discharge conditions that characterized the study period, net landward fluxes of salt were measured in the lower part of the study area, suggesting that the landward transport through the main channel of the estuary was probably balanced by export out through the sides. In contrast, the net fluxes of salt in the upper reaches of the study area were near zero, indicating a closed salt balance in this part of the estuary. In contrast to salt, the net fluxes of TSS, POC and PN in the deeper parts of the water column were consistently landward at all four sites in Winyah Bay indicating the non-conservative behavior of particulate components and their active transport up the estuary in the region around the ETM.The carbon contents (%POC), carbon:nitrogen ratios (org[C:N]a) and stable carbon isotopic compositions (δ¹³C_POC) of the suspended particles varied significantly with depth, location and tidal stage. Tidally averaged compositions showed a significant increase up the estuary in the %POC and org[C:N]a values of suspended particles consistent with the preferential landward transport of carbon-rich particles with higher vascular plant debris content. The combination of tidal resuspension and flood-dominated flow appeared to be responsible for the hydrodynamic sorting of particles along the estuary that resulted in denser, organic-poor particles being transported landward less efficiently. The elemental and isotopic compositions indicated that vascular C₃ plants and estuarine algae were the major sources of the particulate organic matter of all the samples, without any significant contributions from salt marsh C₄ vegetation (Spartina alterniflora) and/or marine phytoplankton.     

The carbohydrates in relation to mineralogic and granulometric composition of surface sediments in the karst estuary (River Krka estuary,Yugoslavia)

The investigation of mineral, granulometric and chemical composition of sediments of the River Krka estuary (Yugoslavia) were performed in order to elucidate the origin of the sediments and the pattern of sedimentation. Estuarine surface sediments were found to be fine-grained with a bimodal distribution. Environmental conditions in estuarine sediments favour conservation of the organic matter (anoxic conditions). The carbohydrates in the sediments were investigated to determine whether they are of terrigenous or authigenous origin. Glucose, galactose, mannose, xylose, rhamnose, glucosamine and glucuronic acid were detected in the sediments. Their mutual relationship indicates a preferentially terrigenous source of sedimented organic material in estuarine sediments.     

Organic matter and surface properties of solid particles in the estuarine mixing zone

Two physicochemical parameters: specific surface area (SSA) and heat-of-wetting (Δ_wH) were measured in order to characterize the surface of sediments from river and marine environments. The data for the sediment samples of the Krka River Estuary were compared with polluted river and shelf marine sediment samples. It was found that organic free samples from the polluted river sediments show two to three times larger SSA compared to native samples. At the same time, Δ_wH was two to three times higher for native samples. The inverse of this trend was found for marine sediment samples. In the Krka River Estuary a narrow transition zone is evident, where such an inversion occurs. This inversion is attributed to significant compositional changes of organic matter within the estuary.     

Some characteristic features of large amorphous particles (NUTA) in the seto Inland Sea,Japan

Large amorphous aggregates (NUTA) observed in coastal areas after spring and autumn phytoplankton blooms and red tide outbreaks were collected using NUTA traps which we devised. These particles become thickly attached to mooring ropes and/or fishing nets. The variation of the collection of materials by NUTA traps was about 23% in organic carbon and 25% in organic nitrogen (n=29). The chemical nature and vertical distribution pattern of NUTA were significantly different from those of suspended particles collected by water samplers. Since C/ATP and C/Chlα ratios in NUTA were very high compared with those of suspended particles, NUTA seems to consist of a small amount of phytoplankton but much detritus. These values suggest that NUTA has characteristics intermediate between suspended particles and sinking particles. The standing stock of NUTA was also estimated to be no more than 10% suspended particles from a calculation of the trapping efficiency of the NUTA trap.     

Suspended particulate matter in the St. Lawrence estuary and Gulf surface layer and development of a remote sensing algorithm

Remote sensing of chlorophyll concentration is potentially affected by the presence of inorganic matter in the water column. Seasonal variability of total suspended particulate matter (SPM) concentration and its partition into organic and inorganic fractions was thus measured in the estuary and Gulf of St. Lawrence during five cruises. These measures were made in the surface layer down to the depth of the 0.1% light level. Results indicate that vertical variability was small for the entire study area. Data analysis lead to the definition of two main regions having different SPM characteristics: 1) the estuary zone characterized by a strong spatial variability, intermediate SPM concentrations and a clear spring phytoplankton bloom that is combined to an increased inorganic matter load; 2) the gulf region characterized by a relatively low SPM concentration and phytoplankton blooms in the spring and fall periods. Combined with in situ measurements of remote sensing reflectances, the database was used to validate existing inorganic matter retrieval algorithms and develop a new one better adapted to the low concentrations encountered in the St. Lawrence estuary and Gulf.     

Estuarine transformation of organic matter: single coalescence events of estuarine surface active particles

Individual surface-active particles (SAP) in aqueous environmental samples can be characterized through their coalescence at a model hydrophobic interface. Each coalescence event is recorded as an electrical signal. As the electrical signal is a current transient (time on a millisecond scale), the sequence of fast processes leading to the formation of a condensed film can be studied from the moment of first attachment at the interface.In the stratified Krka Estuary, SAP are present in maximum concentrations (10⁷−5 × 10⁸ particles l⁻¹ min the size range > 1 μm) at the halocline (freshwater-seawater interface). This highly surface-active fraction of particulate organic matter can be viewed as oily droplets. SAP are fluid, and are readily transformed at interfaces to films of molecular thickness (surface area per particle is in the range 10⁻⁵–10⁻² cm²).Comparison of response of individual SAP with model particles, and the frequency of their appearance in estuarine samples, shows that a majority of estuarine SAP has, in terms of viscosity and film permeability, the properties of oleic acid. A significant fraction of SAP have more rigid structures, comparable to moleic acid + cholesterol model particles. A small fraction ( > 1%) behaves as phytoplankton cells with fluid outer membranes.We have also presented evidence for a strong interaction between elemental mercury and SAP, which is not sensitive to the molecular structure of the organic phase. The puzzling accumulation of mercury at the halocline of the Krka Estuary may be regarded as the result of interaction of elemental mercury with SAP.     

黄河口磷酸盐缓冲机制的研究Ⅱ.黄河口磷—钙矿物的离子平衡

1994年5月—1995年6月,利用黄河口悬浮颗粒物,在实验室内于不同温度、盐度、磷酸盐浓度、pH值及悬浮物粒度条件下进行了悬浮物对磷酸盐的吸附和解吸模拟实验,通过水体中几种磷—钙矿物的离子平衡计算,对黄河口磷酸盐的缓冲机制进行了探讨。结果表明,黄河口现场条件下水体中钙与磷之间存在某些离子平衡,黄河口水体中磷—钙矿物的离子积受温度、悬浮物种类及粒度影响较小,而与水体盐度、磷酸盐及钙含量有关,但不足以形成磷—钙矿物沉淀;磷—钙矿物的沉淀和溶解在黄河口磷酸盐浓度的控制机制上所起的作用较小,而悬浮颗粒物表面对磷酸盐的吸附和解吸起了较大的作用。     

东海陆架北部泥质区悬浮体的絮凝沉积作用

1997年2月在东海北部取得悬浮体和底质样品，通过粒度分析和扫描电镜及能谱分析，探讨了陆架北部泥质区悬浮体的絮凝沉积作用，结果表明，生物因素是泥区悬浮体的发生絮凝的机制之一，可影响悬浮体在海水中的行为，生物活动产生的软组织，分泌物和粘膜等有机中将矿物矿屑和生物骨屑结构，吸附和捕集在一起形成絮凝体而迅速沉积，在悬浮体含量较低，水化学环境相对稳定的东海陆架，絮凝作用是泥质区得以形成的关键过程之一。     

A spectral model of underwater irradiance in the Black Sea

We develop a semiempirical spectral model of penetrating irradiance taking into account the biooptical characteristics of the Black Sea. The evaluation of the contributions of the principal optically active components to the total absorption of light in the sea shows that, in the short-wave range (400–500 nm), light is mainly absorbed by the dissolved organic matter (41–77%). The contribution of phytoplankton to the total absorption attains its maximum values (26–37%) in the abyssal part of the sea in the period of spring blooming of diatoms. In the coastal waters, the absorption of light by suspended nonalgae particles in summer is almost twice as intense (20–30%) as in the open sea (8–13%). The analysis of the sensitivity of our model shows that the absorption of light by dissolved organic matter is more significant for the estimation of the photosynthetically active radiation in the Black Sea than the concentration of pigments and backscattering of light by suspended particles. The comparison of the results of model computations with the data of measurements of the underwater irradiance reveals high accuracy of the proposed model.     

Budgets of Mn,Cd and Cu in the macrotidal Gironde estuary (SW France)

The behavior and budget of Mn, Cd and Cu in the Gironde estuary were investigated through data from both the water column (WC) and sediment depth profiles. In the estuarine freshwater reaches, Mn and Cd removal from and Cu addition to the dissolved phase occurs with a magnitude equivalent to 10%, 30% and 25% of their respective annual fluvial gross dissolved input, respectively. In the saline estuary, diffusive benthic outflow is the main source of dissolved Mn (74% of the total gross dissolved input within the estuary) to the WC. In contrast, Cd (96%) and Cu (89%) are mainly released into the dissolved phase of the WC from fluvial, estuarine and dredging-related particles through complexation (Cd) and organic carbon mineralization (Cu). Anthropogenic activities (sediment dredging) induce pore water inputs, particulate sulfide oxidation and sediment resuspension, significantly contributing to the metal budget of the WC. The related amounts of metals released could be equivalent to 20–50% (Cd) and up to 70% (Cu) of their respective net dissolved addition. Mass balances suggest that a large part of the metals previously released into the dissolved phase from processes within the estuary are removed by suspended particles due to (co-)precipitation of Fe/Mn (oxy)hydroxides and scavenging on autochthonous organic matter. On an annual basis, the Gironde estuary acts as a net sink of dissolved Mn, removing 60% of the dissolved fluvial inputs, and as a net source of dissolved Cd and Cu, contributing ∼ 85% and 20–45% to the dissolved Cd and Cu fluxes to the ocean.     

Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde,France): Observations from field and MODIS satellite data

Over a 1-year period, field and satellite measurements of surface water turbidity were combined in order to study the dynamics of the turbidity maximum zone (TM) in a macrotidal estuary (the Gironde, France). Four fixed platforms equipped with turbidity sensors calibrated to give the suspended particulate matter (SPM) concentration provided continuous information in the upper estuary. Full resolution data recorded by the moderate resolution imaging spectroradiometer (MODIS) sensors onboard the Terra and Aqua satellite platforms provided information in the central and lower estuary twice a day (depending on cloud cover). Field data were used to validate a recently developed SPM quantification algorithm applied to the MODIS ‘surface reflectance’ product. The algorithm is based on a relationship between the SPM concentration and a reflectance ratio of MODIS bands 2 (near-infrared) and 1 (red). Based on 62 and 75 match-ups identified in 2005 with MODIS Terra and Aqua data, the relative uncertainty of the algorithm applied to these sensors was found to be 22 and 18%, respectively.Field measurements showed the tidal variations of turbidity in the upper estuary, while monthly-averaged MODIS satellite data complemented by field data allowed observing the monthly movements of the TM in the whole estuary. The trapping of fine sediments occurred in the upper estuary during the period of low river flow. This resulted in the formation of a highly concentrated TM during a 4-month period. With increasing river flow, the TM moved rapidly to the central estuary. A part of the TM detached, moved progressively in the lower estuary and was finally either massively exported to the ocean during peak floods or temporary trapped (settled) on intertidal mudflats. The massive export to the ocean was apparently the result of combined favorable environmental conditions: presence of fluid mud near the mouth, high river flow, high tides and limited wind speeds. The mean SPM concentration within surface waters of the whole estuary showed strong seasonal variations but remained almost unchanged on a 1-year-basis. These observations suggest that the masses of suspended sediments exported toward the ocean and supplied by the rivers were almost equivalent during the year investigated (2005). Results show the usefulness of information extracted from combined field and current ocean color satellite data in order to monitor the transport of suspended particles in coastal and estuarine waters.     

Composition and fluxes of particulate organic matter in a temperate estuary (Winyah Bay, South Carolina, USA) under contrasting physical forcings

To understand the role that physical processes play on the biogeochemical cycles of estuaries, we conducted intense field studies of the turbidity maximum region within a partially mixed estuary (Winyah Bay, SC, USA) under contrasting conditions of river discharge, tides and wind. Water samples and hydrographic data were collected at different depths and locations along the main channel over several tidal cycles during several cruises to Winyah Bay. Tidal variations in current speed, salinity, total suspended solid concentrations were measured within each cruise and were consistent with estuarine circulation processes. Salinity and total suspended solid concentrations ranged from 0 to 32 and from 20 to over 500 mg L⁻¹, respectively, with the highest salinity and total suspended solid values measured during periods of low river discharge. In fact, comparison of tidally averaged salinity and total suspended solid concentrations revealed marked differences among cruises that were negatively correlated to river discharge and SW wind speed. Moreover, significant contrasts in the chemical compositions of suspended particles were evident among periods of contrasting river discharge and wind regime. For example, the weight percent organic carbon content of suspended particles ranged from 1 to over 6% and displayed a positive correlation with river discharge. Similarly, both the molar carbon to nitrogen ratios (10 to 20 mol:mol) and stable carbon isotopic compositions (−25 to −29%) of the suspended organic matter varied significantly as a function of discharge and wind. Such trends indicate that in Winyah Bay low river discharge and steady SW winds promote resuspension of bed sediments from shallow regions of the estuary. These materials contain highly altered organic matter and their incorporation into the water column leads to the observed trends in suspended particle concentrations and compositions. Furthermore, these conditions result in net landward fluxes of salt, sediment and particulate organic matter throughout most of the water column, promoting efficient trapping of materials within the estuary. Our results illustrate the fundamental connection between physical forcings, such as discharge and wind, sediment transport processes and the cycling of biogeochemical materials in estuarine environments.     

A comparative study of mercury contamination in the Tagus estuary (Portugal) and major French estuaries (Gironde,Loire, Rhône)

Concentrations of mercury were determined for the waters, suspended matter and sediments of the Tagus and of major French estuaries.The Tagus estuary is one of the most contaminated by mercury derived from the outfalls of a chloralkali plant and from other industrial sources. In deposited sediments the median level, 1·0 μg Hg g^?1, is twenty times higher than the natural background and Hg contents depend on the sediment grain-size, age and the distance from waste-outfalls. Suspended matter is more regularly and highly contaminated (median value: 4·5 μg Hg g^?1). In the French estuaries Hg levels in the suspended material decrease with salinity due to dilution and/or remobilization processes. In June 1982, in the Loire estuary, high values of Hg are observed in the middle estuary and attributed to urban and industrial sources.In the Tagus estuary, the general distribution of total dissolved Hg confirms the contamination: it increases seaward from 10 ng 1^?1 in the river to 80 ng 1^?1 in the estuary outlet. The dissolved Hg is almost totally organic in the river, inorganic in the middle estuary due to inorganic Hg effluents and again organic in the lower estuary. This variation is related to the dissolved organic carbon values. The dissolved Hg levels in the Loire Estuary (5–300 ng 1^?1) are much higher than in the Gironde estuary (3–6 ng 1^?1) and of the same order as those observed in the Tagus estuary.     

Sediment transport in the mouth of the Eden estuary

Observations of water movement and sediment transport were made from a fixed platform at an intertidal location in the mouth of the Eden estuary. The effects of wave activity on water movement and suspended sediment fluxes were evaluated, and an empirical relationship between suspended sediment concentrations and near bed water movement is described. The circulation of water in St Andrews Bay indicates a residual movement onshore near to the bed. Onshore movement of sediment in the mouth of the estuary is indicated by suspended sediment fluxes and bedload transport (determined from bedform migration). The relative roles of suspended sediment and bedload transport are discussed. Morphological changes in the estuary mouth over the three-year survey period are relatively small and indicate a stable environment.     

Suspended particle organic composition and cycling in surface and midwaters of the equatorial Pacific Ocean

In this study we relate spatial and temporal variation in the organic composition of suspended particles to current conceptual models of open-ocean particle cycling. Suspended particles in surface (0–200 m) and midwaters (200–1000 m) of the equatorial Pacific Ocean were collected during the 1992 US JGOFS Equatorial Pacific (EqPac) program. Samples collected during El Niño (Survey I) and normal conditions (Survey II) were analyzed for pigment, amino acid, fatty acid, and neutral lipid concentrations and compositions. Principal Components Analysis (PCA) and other statistical methods were used to assess changes in particulate organic composition between Surveys I and II, over 24° of latitude, from 15 to 850 m depth, and to compare our compositional data with previously published data from EqPac sinking particles. These analyses indicated that surface suspended particles (0–200 m) were similar in composition to surface ocean phytoplankton and were less degraded than particles sinking out of the euphotic zone (105 m). The organic composition of suspended particles in surface waters varied with latitudinal and El-Niño-induced changes in phytoplankton assemblages. Midwater suspended particles (200–1000 m) contained labile phytodetrital material derived from particles exiting the euphotic zone (105 m). However, labile organic constituents of midwater suspended particles were increasingly degraded by microbes or consumed by midwater metazoans with depth. The increase in degradation state observed for midwater suspended particles may also have been caused by dilution of deeper (450–850 m) suspended particle pools with more refractory material originating from fast-sinking particles, e.g., fecal pellets. However, the mechanism controlling midwater particle degradation state varied with flux regime; dilution of midwater suspended particles dominated only in the higher flux regime found at equatorial latitudes (5°N–5°S) during Survey II (normal conditions). In summary, it is apparent that organic matter alteration in midwaters, and not cycling within the euphotic zone, has the larger effect on organic composition of suspended particles in the deep equatorial Pacific Ocean.     

福建九龙江口入海泥沙的扩散和河口湾的现代沉积

本文阐述了40年来九龙江入海泥沙量增加了两倍,入海泥沙在海门岛—鸡屿一带的浅滩区,其年平均淤积8—10cm,一次洪水流域搬来悬浮泥沙,使河口湾新淤积层厚2—3cm.并指出、进入河口湾的三条汉河各有独立的扇形粗粒冲积浅滩;悬移质的粉砂、粘土粒扩散至鼓浪屿以南海域;构成三角洲沉积主体的河口浅滩区的沉积速度由入海泥沙倍增而相应地加快进行.     

Particle-bound phosphorus along an urbanized coastal plain estuary

The distribution of particle-bound phosphorus in the suspended sediment of the Delaware Estuary was examined with a sequential chemical leaching technique. The phosphorus content of particles was highest in the tidal river (140–250 μmol g⁻¹) near major anthropogenic inputs. Despite this enrichment of river particles with phosphorus, suspended particles within the salinity gradient had a phosphorus content more similar to the world's average. Sequential chemical leaches revealed that particulate phosphorus was associated with organic matter, aluminum oxides, iron oxides, and apatite in all areas of the estuary. However, ‘excess’ particle-bound phosphorus in the tidal river was associated mainly with iron oxides (27%), aluminum oxides (23%), and organic matter (50%). Within the salinity gradient, particulate phosphorus associated with iron oxides, aluminum oxides, and apatite all decreased with increasing salinity. Estuarine mixing was simulated to determine whether the observed decreases in particle-bound phosphorus pools in field samples were due to release into solution. During simulated mixing, particulate phosphorus associated with iron and aluminum oxides decreased, but no change was observed in apatite-bound phosphorus. The results of the mixing study combined with the observed particle-bound phosphorus distributions suggest that phosphate concentrations along the Delaware Estuary may be partially ‘buffered’ by aluminum and iron oxide phases.