Accumulation of Zn,Cu and Cd by crabs and barnacles

The crab Carcinus maenas (L.) and the barnacle Elminius modestus Darwin were exposed to a range of dissolved concentrations of Zn, Cu and Cd for 21 days in artificial seawater. Accumulation of Zn and Cu by crabs has been interpreted in terms of the presence of a regulation mechanism to maintain constant body concentrations (83·2 ± 19·4 μg Zn g^?1 dry wt.; 39·8 ± 9·8 μg Cu g^?1 dry wt.) under varying external dissolved metal levels, until a threshold dissolved metal concentration (c. 400 μg Zn l^?1; c. 170 μg Cu l^?1) beyond which net accumulation of metal begins. Cadium appears to be accumulated by C. maenas at all exposures with no evidence for regulation of body cadmium concentrations. Exposure of E. modestus to Zn, Cu or Cd caused net accumulation of the respective metal in the bodies of the barnacles, with no evidence for regulation of body metal concentrations.     

The relationship between bacteria and micro-algae in the sediment of a Bay of Fundy mudflat

Bacterial abundance was studied from April to December 1979 in the sediments of a Bay of Fundy mudflat. Bacterial biomass ranged from 1·6 to 2·8 g C m⁻² in the top 5 cm and bacterial numbers normalized for the amount of organic carbon in the sediment were about 9 × 10¹¹ g⁻¹ C at the surface during most of the year. However, when a bloom of pennate diatoms occurred in the fall, bacterial numbers per gram carbon doubled. There was a correlation of r=0·83 between bacteria g⁻¹ C and chlorophyll a for all sampling dates at the high intertidal station. Sieved size fractions of surface sediment taken during a period of low micro-algal biomass and just after the micro-algal bloom showed a strong correlation (r=0·92) between the increases in bacterial and algal abundance for the fractions > 20 μm. We suggest that the apparent relation between benthic micro-algal and bacterial abundance on both a seasonal and within-sediment basis may be due to the release of extracellular material by the micro-algae.     

An electrochemical study of the speciation of copper,zinc and iron in two estuaries in England

The organic speciation of copper, iron and zinc in estuarine waters is studied using electrochemical techniques. Complexing capacities for copper and zinc were determined by cathodic stripping voltammetry (CSV) of their complexes with respectively catechol and amino pyrrolidine dithiocarbamate (APDC). Iron speciation was studied by CSV measurements of dissolved iron before (‘free iron’) and after acidification and UV-irradiation (‘total iron’) of the filtered samples. Complexing capacities of copper were found to vary between 1·4 and 5 × 10⁻⁷m with conditional stability constants, logK′_CuL, between 9·2 and 10·3 in the Tamar estuary. Complexing capacities of zinc were less at between 0·4 and 1·6 × 10⁻⁷m with values for logK′_ZnL between 8·1 and 9·4. Copper complexing capacities generally decreased with increasing salinity, and variations in the results were related to high concentrations of suspended material. Similar variations in the dissolved vanadium concentrations suggested that part of this element was associated with colloidal material. The total dissolved iron concentration in samples from the River Ribble decreased from 10⁻⁶m at low salinity to 10⁻⁷m at high salinity, but the free iron concentration was found to decrease from 8 to 3 × 10⁻⁸m over the same salinity range, which may be compared with the calculated solubility of iron in seawater of 2 × 10⁻⁸m. Comparative experiments showed that on average about 24% of the non-labile iron fraction was stabilized by organic material, the rest being composed of inorganic colloidal material.     

Coulometric total carbon dioxide analysis for marine studies: Automation and calibration

An automated instrument for the coulometric detection of total carbon dioxide (TCO₂) was developed from the manual prototype of Johnson et al. Pure CO₂ was used to calibrate the detector, the whole analytical procedure was controlled by a microcomputer, and the unit was compact enough for field and shipboard use. Some 67 samples from the Askö field station on the Baltic Sea (salinity = 6; TCO₂ ranging from 1224.3 to 1276.1 μmol l⁻¹ with a mean of 1245.6 μmol l⁻¹) and 31 samples from Swedish lakes (TCO₂ ranging from 19.5 to 252.3 μmol l⁻¹ with a mean of 72.9 μmol l⁻¹) were analyzed. The pooled standard deviation of these analyses was ± 1.0 μmol l⁻¹ (% CV of 0.08 and 1.3%, respectively). Precisions of < 0.1% were routine for waters exceeding 1000 μmol l⁻¹ TCO₂. At Askö on the Baltic Sea, the mean absolute difference between coulometric and pH-alkalinity determinations of TCO₂ on 22 parallel samples was 6.7 μmol kg⁻¹ or 0.5% of the mean. Community metabolism studies gave maximum rates of net production (TCO₂ uptake) and respiration (TCO₂ production) of 0.9-1.2 and 0.3-0.5 μmol l⁻¹ b⁻¹, respectively. The results of this study provide solutions to the problems observed with the earlier manual prototype.     

Trace metals in the Göta river estuary

The concentrations of the trace metals Cd, Cu, Fe, Ni, Pb and Zn in the Göta River estuary have been investigated. The following metal fractions have been determined: acid-leachable, dissolved, labile and particulate.The estuary represents a salt wedge type estuary and is situated in a densely populated region of Sweden. The metal concentrations found for the dissolved fraction is in the range of what can be considered as background levels for freshwater. It is difficult to evaluate any estuarine processes other than conservative mixing for Cd, Cu, Ni and Zn. The dissolved levels in the freshwater end member are Cd, 9–25 ngl^?1; Cu, 1·1–1·4 μgl^?1; Fe, 20–75 μg l^?1: Ni, 0·7–0·9 μg l^?1: Pb 0·09–0·2 μg l^?1; and Zn, 6–7 μg l^?1:The results from the acid-leachable fraction show that at high suspended load the particles sediment in the river mouth. The trace metal levels in this fraction are subject to large variations.     

Chemical determination of particulate nitrogen in San Francisco Bay. Nitrogen: chlorophyll a ratios in plankton

Particulate nitrogen (PN) and chlorophyll a (Chla) were measured in the northern reach of San Francisco Bay throughout 1980. The PN values were calculated as the differences between unfiltered and filtered (0·4 μm) samples analyzed using the UV-catalyzed peroxide digestion method. The Chla values were measured spectrophotometrically, with corrections made for phaeopigments. The plot of all $\text{PN}\text{Chl}\text{a}$ data was found to be non-linear, and the concentration of suspended particulate matter (SPM) was found to be the best selector for linear subsets of the data. The best-fit slopes of $\text{PN}\text{Chl}\text{a}$ plots, as determined by linear regression (model II), were interpreted to be the N: Chla ratios of phytoplankton. The Y-intercepts of the regression lines were considered to represent easily-oxidizable detrital nitrogen (EDN). In clear water ( < 10 mg l^?1 SPM), the N: Chla ratio was 1·07 μg-at N per μg Chla. It decreased to 0·60 in the 10–18 mg l^?1 range and averaged 0·31 in the remaining four ranges (18–35, 35–65, 65–155, and 155–470 mg l^?1). The EDN values were less than 1 μg-at N l^?1 in the clear water and increased monotonically to almost 12 μg-at N l^?1 in the highest SPM range. The N: Chla ratios for the four highest SPM ranges agree well with data for phytoplankton in light-limited cultures. In these ranges, phytoplankton-N averaged only 20% of the PN, while EDN averaged 39% and refractory-N 41%.     

Environmental tolerances of the beach-hopper Orchestia gammarellus (Pallas) (Crustacea:Amphipoda)

Orchestia gammarellus (Pallas) will tolerate temperatures from 0°C to c. 30°C but dies in < 1 h at 35°C. Both small and large animals survive > 96 h in water (aerated) between 1 and 60%(.) salinity. In distilled water and 100%(.) brine death occurred after 8–24 h. In fully saline water, pH reduction to c. 3·3 is tolerated. Highly alkaline (pH 11–13) conditions in low salinity media are rapidly fatal. Ammonium concentrations (in fully saline water) some five orders of magnitude higher than normal for seawater are tolerated, reflecting the enhanced ammonium concentration encountered in rotting weed beds. Cl⁻ ion concentration equivalent to that in normal seawater will not support O. gammarellus in the absence of Na⁺ ions. High NO₃⁻ concentrations (> 100 mg NO₃⁻ litre⁻¹) are fatal in < 24 h. A large range of NaCl concentrations is tolerated (one to two orders of magnitude dilution) by immersed animals. Animals will live longer in air, however, when fed low Na⁺ food than when immersed in seawater diluted to an equivalent ion strength. Ca²⁺ and K⁺ ions cannot substitute for Na⁺. Orchestia gammarellus will, however, tolerate wide variations in the ratio of Na⁺:K⁺ ions (i.e to a shift from 30:1 to 0·3.1) in hypo-osmotic media. At 10°C it will survive c. 5 h of anoxia. It will penetrate into strongly reducing conditions (− 318 mV) even with access to better oxygenated conditions. In nature, behavioural avoidance probably ensures that O. gammarellus does not encounter many of these parameters at acutely limiting values. That this species is so tolerant, however, bears on the evolutionary adaptability of talitrid amphipods to non-marine conditions. Desiccation sensitivity and Na⁺ ion dependency are the species' most important barriers to life on land.     

Diel dissolved carbohydrate accumulation in coastal water of South Florida,Bermuda and Oahu

Dissolved polysaccharide and monosaccharide concentrations were measured over 24 h periods in large volume in situ incubations at three subtropical coastal locations to determine their diel patterns and rates of net accumulation and bacterial utilization. Total dissolved carbohydrate consistently accumulated over 6 to 9 h periods, beginning in the late morning or early evening, at net rates of 3·2 to 7·9 μgC l⁻¹ h⁻¹. These carbohydrates were utilized by bacteria in 3 to 6 h during the late evening or early morning.     

A study of metal ion adsorption at low suspended-solid concentrations

A procedure for conducting adsorption studies at low suspended solid concentrations in natural waters (<50 mg l⁻¹) is described. Methodological complications previously associated with such experiments have been overcome. Adsorption of zinc ion onto synthetic colloidal titania (TiO₂) was studied as a function of pH, supporting electrolyte (NaCl) concentration (0·1-0·002 m) and particle concentration (2–50 mg l⁻¹). The lack of success of the Davis Leckie site bonding model in describing Zn(II) adsorption emphasizes the need for further studies of adsorption at low suspended-solid concentrations.     

Seasonal variability in the kinetics of Cu,Pb, Cd and Hg accumulation by macroalgae

Here it is demonstrated that both Porphyra spp. and Enteromorpha spp. of macro-algae display similar and very marked seasonal variations in their concentration factor (CF) of Cu, Pb, Cd and Hg in field conditions. The CF variations are specific for each metal and reproducible over several years. The way variations in the biological activity affect the equilibrium and kinetics of the interaction between trace metals and live algae was studied in vitro. Natural seawater was used as the culture medium. Voltammetry was used for the determination of natural organic ligands and trace metals except Hg, which was determined by mercury cold vapour after on-line pre-concentration. Titrations with the relevant metal demonstrated that the maximum binding capacity of the algae was not significantly dependent on the season for Pb (ca. 100 μmol g_{dry algae}⁻¹), Cd (ca. 50 μmol g⁻¹) and Hg (80–100 μmol g⁻¹). Marked seasonal variations were observed for Cu (ca. 40 μmol g⁻¹ in January; 70 μmol g⁻¹ in May; and 100 μmol g⁻¹ in August). The conditional stability constants of metal–algae complexation sites were seasonally independent and similar for both algae: logK_MS′=8.5±0.3 (Cu), 5.6±0.2 (Pb), 5.3±0.2 (Cd) and 18.0±0.3 (Hg). Exudates with a strong Cu complexing capacity (logK′_CuL=12.47±0.06) were determined in cultures with added Cu, Pb or Cd concentrations, and identified by cathodic stripping voltammetry (CSV) as cysteine or glutathione. All the tested metals promoted the liberation of exudates, both cysteine- and glutathione-like ligands were exuded in the presence of Cu, only cysteine-like ligands in the presence of Pb, and only glutathione-like ligands in the presence of Cd, the rise depending of the season of the year, particularly for Cu. Highest levels were produced in the presence of added Pb. When exposed to either 1- or 100-μM total dissolved metal concentrations, the metal uptake, and its rate, varied with the season and the algae.     

Disruption of magnesium regulation in the crab Cancer productus exposed to chlorinated seawater

Exposure of the crab Cancer productus to chlorinated seawater resulted in alterations in haemolymph sodium and magnesium concentrations. At the highest chlorination levels, which approached the 96-h LC₅₀, regulation of both ions was essentially abolished. Reciprocal changes in the sodium and magnesium levels suggested an effect on the activity of the bladder wall, which has been implicated in the regulation of haemolymph magnesium in crustaceans.Additionally, exposure to 1·19 μg/ml applied Cl (0·68 μg/ml TRO) for 96 h resulted in a fourfold increase in the ammonia excretion rate. Crabs also contributed considerable chlorine demand to the exposure seawater. Measurements of TRO in the inhalant seawater and water exiting the crabs' branchial chambers indicated a reduction in TRO equivalent to 57% of that initially present in the inhalant water (0·51 μg/ml TRO).Measurements of ammonia concentrations in the exposure water indicated that ammonia in seawater was consumed in reactions with oxidants. This was especially evident at higher levels of chlorination. At 0·58 μg/ml applied Cl and above, chlorination resulted in near disappearance of ammonia from the water.     

Contrasting food-web support bases for adjoining river-influenced and non-river influenced continental shelf ecosystems

Nutrient and chlorophyll a concentrations and distributions in two adjoining regions of the South Atlantic Bight (SAB), Onslow Bay and nearshore Long Bay, were investigated over a 3-year period. Onslow Bay represents the northernmost region of the SAB, and receives very limited riverine influx. In contrast, Long Bay, just to the south, receives discharge from the Cape Fear River, draining the largest watershed within the State of North Carolina, USA. Northern Long Bay is a continental shelf ecosystem that has a nearshore area dominated by nutrient, turbidity and water-color loading from inputs from the river's plume. Average planktonic chlorophyll a concentrations ranged from 4.2 μg l⁻¹ near the estuary mouth, to 3.1 μg l⁻¹ 7 km offshore in the plume's influence, to 1.9 μg l⁻¹ at a non-plume station 7 km offshore to the northeast. Average areal planktonic chlorophyll a was approximately 3X that of benthic chlorophyll a at plume-influenced stations in Long Bay. In contrast, planktonic chlorophyll a concentrations in Onslow Bay were normally <0.50 μg l⁻¹ at a nearshore (8 km) site, and <0.15 μg l⁻¹ at sites located 45 and 100 km offshore. However, high water clarity (K_PAR 0.10–0.25 m⁻¹) provides a favorable environment for benthic microalgae, which were abundant both nearshore (average 58.3 mg m⁻²) and to at least 45 km offshore in Onslow Bay (average 70.0 mg m⁻²) versus average concentrations of 10–12 mg m⁻² for river-influenced areas of Long Bay. This provides evidence that much of the inner shelf food web in Onslow Bay is based on benthic microalgal production, in contrast to a plankton-based food web in northern Long Bay and more southerly areas of the SAB.     

Chlorinated hydrocarbons in the seawater and surface sediments of Blanca Bay,Argentina

In order to characterize our study area and to provide reference values to be used in the future to measure the changes produced by an increase in contamination, the concentrations of chlorinated hydrocarbons have been investigated in fifty-one samples of seawater, taken at four different depths: air-sea interface, surface, one metre and bottom waters, and in twenty-three samples of surface sediments from Blanca Bay, Argentina. Of eleven organochlorine compounds we were looking for (α BHC, lindane, heptachlor, δ BHC. aldrin, heptachlor epoxide, dieldrin, o-p′DDD, p-p′DDD, o-p′DDT and p-p′DDT), seven could be detected in seawater and three in surface sediments with the following mean concentrations: α-BHC=48·2 ng l^?1; lindane=54·2 ng l^?1; heptachlor=45·0 ng l^?1; δ BHC=12·5 ng l^?1; aldrin=61·8 ng l^?1 and ΣDDT=67·0 ng l^?1; and δ BHC=3·2 ng g^?1; lindane=4·2 ng g^?1 and heptachlor=1·0 ng g^?1 for seawater, regarding the surface waters, and sediment samples, respectively.Concentration factors among the different water layers were also studied to see if there was any correlation between chlorinated hydrocarbon contents and the water depths from which the samples were taken. As a mean value, the air-sea interface water contains 18 times more of these compounds than that of the water near the bottom. A comparison of the values corresponding to seawater and surface sediments from our study area with those levels measured in samples from other geographic locations is also presented.With the purpose to detect a relationship between chlorinated hydrocarbon concentrations and the contents of particulate matter (PM) on the one hand, and particulate organic material (POM) on the other hand, four groups of samples containing different amounts of PM and POM, respectively were formed. From a comparison of the results obtained, lindane, heptachlor and δ BHC showed a tendency to lower concentrations in those samples containing little PM whereas α BHC and aldrin remained without important changes. No significant correlation was found between organochlorine levels and contents of POM.     

Influence of water column dynamics on sulfide oxidation and other major biogeochemical processes in the chemocline of Mariager Fjord (Denmark)

Major electron donors (H₂S, NH₄⁺, Mn²⁺, Fe²⁺) and acceptors (O₂, NO₃⁻, Mn(IV), Fe(III)), process rates (³⁵SO₄²⁻ reduction, dark ¹⁴CO₂ fixation) and vertical fluxes were investigated to quantify the dominant biogeochemical processes at the chemocline of a shallow brackish fjord. Under steady-state conditions, the upward fluxes of reductants and downward fluxes of oxidants in the water column were balanced. However, changes in the hydrographical conditions caused a transient nonsteady-state at the chemocline and had a great impact on process rates and the distribution of chemical species. Maxima of S⁰ (17.8 μmol l⁻¹), thiosulfate (5.2 μmol l⁻¹) and sulfite (1.1 μmol l⁻¹) occurred at the chemocline, but were hardly detectable in the sulfidic deep water. The distribution of S⁰ suggested that the high concentration of S⁰ was (a) more likely due to a low turnover than a high formation rate and (b) was only transient, caused by chemocline perturbations. Kinetic calculations of chemical sulfide oxidation based on actual conditions in the chemocline revealed that under steady-state conditions with a narrow chemocline and low reactant concentrations, biological sulfide oxidation may account for more than 88% of the total sulfide oxidation. Under nonsteady-state conditions, where oxic and sulfidic water masses were recently mixed, resulting in an expanded chemocline, the proportion of chemical sulfide oxidation increased. The sulfide oxidation rate determined by incubation experiments was 0.216 μmol l⁻¹ min⁻¹, one of the highest reported for stratified basins and about 15 times faster than the initial rate for chemical oxidation. The conclusion of primarily biological sulfide oxidation was consistent with the observation of high rates of dark ¹⁴CO₂ fixation (10.4 mmol m⁻² day⁻¹) in the lower part of the chemocline. However, rates of dark ¹⁴CO₂ fixation were too high to be explained only by lithoautotrophic processes. CO₂ fixation by growing populations of heterotrophic microorganisms may have additionally contributed to the observed rates.     

Sensitivity of estuarine phytoplankton to hexavalent chromium

Experiments were conducted in which we tested the effects of hexavalent chromium on both natural assemblages of phytoplankton and cultures of Thalassiosira pseudonana clone 3H. Water was collected from various locations in Yaquina Bay, Oregon, with salinities ranging from 32·5‰ to 0·03‰. A variety of nutrient regimes were tested by adding major nutrients and micro-nutrients, and/or stripping micro-nutrients with activated carbon. In one high salinity experiment, chromium was stimulatory, as were micro-nutrient additions. In other high and medium salinity experiments, chromium was neither stimulatory nor inhibitory, at levels up to 1·9 μ mole l^?1 Cr. There was, however, slight inhibition of growth at 19·0 μ mole l^?1 Cr, due specifically to inhibition of Skeletonema costatum. In our freshwater experiments, chromium was very inhibitory at 1·9 μ mole l^?1 Cr, and slightly inhibitory at 0·19 μ mole l^?1 Cr. Species inhibited by chromium were Surirella ovata, Detonula confervacea, and Cyclotella sp. Experiments were conducted with T. pseudonana grown over a wide range of salinities. Chromium was found to be very inhibitory in freshwater and became progressively less toxic as the salinity increased. Most inhibition was neutralized by a salinity of 2·1‰.     

The effect of salinity on the microbial mineralization of two polycyclic aromatic hydrocarbons in estuarine sediments

Sediments from the lower Hudson River estuary and two other coastal environments were examined experimentally for their ability to mineralize (convert to CO₂) the polycyclic aromatic hydrocarbons (PAHs) naphthalene and anthracene over a range of salinities. Routine assays employed 1:1 (vol fresh sed:vol water) sediment slurrys in order to overcome natural variability in mineralization rates among replicates. Mineralization rates were stimulated by about 2·5 fold, compared to unslurried controls, while the coefficient of variation fell from 13% to 3·5%.Rates of naphthalene mineralization in surface sediments from along the mainstem of the Hudson River (salinities from 2 to 27%) ranged from 0·011 to 1·5 nmol cm⁻³ day⁻¹ (pool turnover [T_n] from 60 to 2040 days) with no discernible trends along the estuarine gradient. For two stations examined experimentally (mile point 5, salinity 23%; mile point 26, salinity 5%), microbial assemblages appeared acclimated to broad salinity variations as alter rates of mineralization compared to controls.Sediments from two upstream marshes of the Hudson (mile points 36 and 45) showed rates of naphthalene mineralization from 0·007 to 0·15 nmol cm⁻³ day⁻¹ (T_n from 14 to 368 days), while sediments from a third marsh in freshwater (mile point 76) had high rates (66 nmol cm⁻³ day⁻¹; T_n 40 days). For the two upstream marsh stations which rarely experienced salt intrusion, there was a substantial decrease in mineralization of naphthalene and anthracene with increasing salinity.Consistently high rates of naphthalene mineralization (780 to 1600 nmol cm⁻³ day⁻¹; T_n 5 to 6 days) were observed in petroleum contaminated sediments from Port Jefferson Harbor (PJH) on the north shore of Long Island. PJH has a relatively constant salinity regime (about 27%) and imposed decreases in salinity effected decreases in rates of naphthalene and anthracene mineralization. Lowest rates of naphthalene mineralization (0·003 to 0·004 nmol cm⁻³ day⁻¹; T_n from 714 days to 833 days) were found in sediments from two stations in the relatively pristine Carmans River estuary on the south shore of Long Island.The ability of increases or decreases in salinity to affect the rate of model PAH mineralization appeared to be dependent on the natural variation in the salinity regime from which a sample was obtained. Data from all the environments studied indicated a strong positive correlation between PAH concentration and the rates of mineralization of naphthalene. Rates of PAH mineralization in all environments examined appear to be primarily controlled by the extent of pollutant loading and not by natural variations in the salinity regime.     

Phytoplankton spring bloom of the Gironde plume waters in the Bay of Biscay: early phosphorus limitation and food-web consequences

During the spring 1995 (2–25 May), a cruise was carried on the RV Poseidon (Germany) on the continental shelf of the south Bay of Biscay. The objective was a comprehensive study of the planktonic food web within the Gironde plume waters. In these waters phosphate was present at very low concentrations (undetectable to < 0.1 μmol.L⁻¹), whereas nitrate, silicate and ammonium concentrations were much higher (several μmol·L⁻¹ for nitrate and silicate and 0.5 to 1.0 μmol·L⁻¹ for ammonium). The size distribution of the phytoplankton biomass (estimated from chlorophyll a measurements by high performance liquid chromatography) and primary production (measured by ¹⁴C in situ method) showed a great proportion of small (40 to 70 % < 3 μm) and active autotrophic cells (growth rates estimated from 0.4 to 0.8 d⁻¹ for the entire euphotic layer). Considering the very high values of NO₃-N:PO₄-P ratios and the high C:P and N:P ratios for the particulate organic matter, it is suggested that an early phosphorus depletion limits the spring bloom phytoplankton and particularly the new production (nitrate uptake coming from the Gironde waters).From these results and other simultaneous observations on the heterotrophic processes (such as grazing of microzooplankton), we can conclude that the planktonic food web would be close to a maintenance system as defined by Platt et al. The possible generalisation of these results for each spring is discussed with respect to the scarcity of previous and reliable phosphate data.     

The release of reducing sugars and dissolved organic carbon from Spartina alterniflora Loisel in a Georgia salt marsh

Eight monosaccharides were found to be released from both tall and short forms of Spartina alterniflora during tidal submergence including: 2-d ribose, rhamnose, ribose, mannose, arabinose, fructose, galactose and xylose. Glucose was not detected in the leachate of either growth form. Two additional monosaccharides were found but were not identified. Losses of total reducing sugars (TRS) and total dissolved organic carbon (TDOC) ranged from 14–54 μgCg⁻¹ dry wth⁻¹ and 42 to 850 μgCg⁻¹ dry wth⁻¹, respectively. Losses of individual monosaccharides were generally <5μgCg⁻¹ dry wth⁻¹ and varied from 0·5–17 μgCg⁻¹ dry wth⁻¹. Differences were observed in seasonal patterns of losses between tall and short Spartina. Tall Spartina TRS losses peaked in midsummer, while in short Spartina TRS losses peaked in the spring and fall. TDOC losses in both tall and short Spartina followed similar patterns with peak losses occurring in the spring and fall. Periods of net uptake of TDOC were observed in both growth forms in midsummer. Uptake rates varied from 142–930 μgCg⁻¹ dry wth⁻¹. Estimated annual losses of TDOC from tall and short Spartina were between 100–150 and 5–10 gCm⁻² year⁻¹, respectively. The magnitude and seasonal pattern of TDOC losses reported here support Turner's conclusions that losses of labile DOM from Spartina are substantial in Georgia salt marshes and related to seasonal patterns of estuarine metabolism.     

Changes in biomass structure and trophic status of the plankton communities in a highly dynamic ecosystem (Gulf of Venice,Northern Adriatic Sea)

The changes in the plankton biomass structure in relation to nutrient inputs were studied in the Gulf of Venice (Northern Adriatic Sea), an area characterized by a very marked trophic state variability. The investigation was carried out at two stations, in March, May and July 2005 and 2006, considering the whole water column. The size structure (from picoplankton to mesozooplankton) of both autotrophs and heterotrophs was analysed. Signals of diluted waters and nutrient inputs were more marked in 2005 than in 2006. In 2005, the total plankton biomass was almost double (87 ± 37 μg·C·l^?1) that in 2006 (44 ± 26 μg·C·l^?1). The variations were determined mainly by phytoplankton, with a 70% decrease, and a shift from a community dominated by microphytoplankton (49 ± 12%) in 2005 to one dominated by bacteria (43 ± 11%) in 2006 was observed. The relationship between the heterotrophic (H) and autotrophic (A) biomass indicated a rapid decline of the H/A ratio with increasing phytoplankton biomass. This study, although temporally limited, is consistent with the results reported for other marine environments and it seems to confirm the importance of nutrient inputs in structuring the biomass of plankton community.     

Mercury in the Southern North Sea and Scheldt estuary

Dissolved and particulate mercury and methylmercury concentrations were determined in the Southern Bight of the North Sea and the Scheldt estuary in the period 1991–1999. Mercury and methylmercury concentrations are higher before 1995 than after 1995, especially in the fluvial part.The North Sea: In the offshore stations, dissolved Hg concentrations are generally higher in winter than in summer while the reverse is true for particulate Hg K_D values (K_D=the concentration of particulate Hg (Hg_P in pmol kg⁻¹) divided by the concentration of dissolved Hg (Hg_D in pmol l⁻¹)) range from 100,000 to 1000,000 l kg⁻¹. Dissolved methylmercury concentrations vary from 0.05 to 0.25 pmol l⁻¹ in summer and from d.l. to 0.23 pmol l⁻¹ in winter and particulate methylmercury concentrations from 1.8 to 36 pmol g⁻¹ in summer and from 0.9 to 21 pmol g⁻¹ in winter. The K_D ranges from 9,000 to 219,000 l kg⁻¹.The Scheldt estuary: In winter, dissolved Hg concentrations are elevated in the upper estuary, decrease exponentially in the low salinity range followed by a very slow decrease towards the mouth. In summer, they are low in the fluvial part, increase in the low salinity range or in the mid-estuary and sometimes show an increase in the lower estuary. Particulate Hg concentrations do not show any seasonal trend.Dissolved MMHg concentrations are much lower in winter, when maximum concentrations are found in the upper estuary, than in summer. In summer, the MMHg concentrations are low at low salinity, they show a first increase in the salinity range from 3 to 12, a decrease in the mid-estuary and a second increase in the lower estuary.The highest particulate MMHg concentrations are found in the upper estuary, while in the lower estuary generally lower and more constant values are observed. The ratio of dissolved MMHg to dissolved Hg (cruise averages between 1.3% and 20%), is higher than the ratio of particulate MMHg to particulate Hg (cruise averages of 0.27–0.90%). The K_D values for MMHg are lower in the summer (30,000–65,000) than in autumn and winter (77,000–114,000).The Scheldt river: In the fluvial part of the Scheldt, dissolved increases in the most upstream stations, while particulate Hg shows no particular pattern. Dissolved MMHg ranges from 0.1 to 0.39 pmol l⁻¹ and particulate MMHg from 3.1 to 43.5 pmol g⁻¹. The MMHg concentrations are comparable to those found in the estuary and no seasonal variations could be observed.