The distribution and population dynamics of O-group plaice (Pleuronectes platessa L.) on nursery grounds in the Firth of Forth

Midsummer (1 August) population estimates of about 2 million O-group plaice (Pleuronectes platessa L.) were derived for sandy bays around the Firth of Forth in 1979–1980. This is an order of magnitude less than similar estimates made for the Clyde Sea Area in 1973–1974. Autumn population estimates of 0·4–1·0 million fish were comparable to estimates by the Ministry of Agriculture, Fisheries and Food for the area between the Scottish border and Flamborough Head (2·3 million for 1970 and 1973) which represented 4·8% (1973) to 5·3% (1970) of the total number of O-group fish on the English east coast.Largo Bay was the most important nursery area holding 25% of the total population. It is particularly well situated to receive newly metamorphosed plaice carried in water currents along the north side of the Forth from the spawning ground off Fife Ness. Plaice in the Forth are mainly distributed on fine to medium sandy beaches (186–480 μm), the mean number per haul in midsummer (D) being correlated with the median diameter (m.d. in μm) of the low water sediments by the equation: D=−45·7666+0·2327 m.d. (n=11,r=0·68,P<0·02 but>0·01).The shallow inshore water in sandy bays in the outer Firth was well mixed and more marine than estuarine (27·7–35·0‰). The correlation coefficient between fish density and water temperature was low, while that with salinity (S‰) was: D=6·1618+0·2238S (n=23,r=0·62,P<0·005).Regression analysis demonstrated that the relationship between the instantaneous mortality rate (Z) and the initial population density (D_p) was: Z×100=0·7480+0·0546d_p (n=12,r=0·87,P<0·001).The mean mortality rate for the O-group plaice in the Forth nursery areas was 53% month⁻¹.     

Settleable and Non-Settleable Suspended Sediments in the Ogeechee River Estuary, Georgia, U.S.A.

Suspended particle dynamics were investigated in the Ogeechee River Estuary during neap tide in July 1996. Samples were operationally separated into ‘ truly suspended ’ (settling velocity <0·006 cm s⁻¹) and ‘ settleable ’ (settling velocity >0·006 cm s⁻¹) fractions over the course of a tidal cycle to determine whether these two fractions were comprised of particles with differing biological and chemical characteristics. Total suspended sediment, organic carbon and nitrogen, chlorophyll a and phaeopigment concentrations were measured in each fraction, as well as rates of bacterial hydrolytic enzyme activity [β-1,4-glucosidase (βGase) and β-xylosidase (βXase)]. The majority of the suspended sediment (by weight) was in the truly suspended fraction; all measured parameters were largely associated with this fraction as well. When compared to the settleable material, the truly suspended material was significantly higher in % POC (5·7±0·6 vs. 3·9±1·8), % chlorophyll (0·07±0·02 vs. 0·03±0·01), % phaeopigment (0·030±0·006 vs. 0·018±0·012), and weight-specific maximal uptake rates (V_maxper mg suspended sediment) of both enzymes (1·8±0·4 vs. 0·7± 0·2 nmol mg⁻¹ h⁻¹βGase and 1·1±0·3vs . 0·3±0·2 nmol mg⁻¹ h⁻¹βXase), providing clear evidence for a qualitative distinction between the two fractions. These results are interpreted to mean that the more organic-rich, biologically active material associated with the suspended fraction is likely to have a different fate in this Estuary, as ‘ truly suspended ’ sediments will be readily transported whereas ‘ settleable ’ sediments will settle and be resuspended with each tide. These types of qualitative differences should be incorporated into models of particle dynamics in estuaries.     

Sub-surface temperature oscillations and associated flow in the western Bay of Bengal

Unusual temperature variations at subsurface depths were noticed while making continuous observations (for 13 h) on temperature, salinity and currents at a point in the vicinity of river Krishna in the western Bay of Bengal. Sharp temperature oscillations recorded at four different levels between 14 and 20·1 m had an average periodicity of about 1·6 h. Similar periodicities were also observed in the case of flow components. Computed Brunt-Vaisälä frequency, vertical shear and Richardson number showed remote possibilities of vertical mixing occurring in this area. These oscillations showed features of internal waves propagating with a speed of about 13·3 cm s⁻¹ and wavelengths between 0·8 and 1·3 km and apparently generated as a result of flood current passing over a submarine obstacle in the presence of stratification.     

Behavior of underwater suction anchor in soil

To study the feasibility and efficiency of underwater suction anchors in soil, the performance of eight model anchors was evaluated in a 30 in. (76·2 cm) by 72 in. (183·0 cm) by 32 in. (81·3 cm) deep test tank. The soil studied were a medium fine sand, a silt and a clay. A gantry-type loading mechanisms was used to apply the vertical breakout force. A pullout rate of approximately 0·1 in/min (0·25 cm/min) was used for all tests.For the range of conditions studied, the test results indicate that the underwater suction anchor is feasible and effective. The breakout behavior of the suction anchors depends upon the anchor geometry including anchor diameter and skirt length, soil strength properties, soil-anchor friction and adhesion, and suction; i.e. the difference between the pressure underneath the porous plate and the ambient pressure. For the anchors and soils tested, the anchor capacity increases linearly with increasing suction provided the anchor skirt length to diameter ratio remains constant. An increase in the anchor capacity results with increasing suction and with increasing internal friction angle of the test soils. To study the results suggest that the underwater suction anchor is particularly useful for short-term anchorage.     

Studies on the population ecology of Upogebia deltaura (Leach) (Crustacea, Thalassinidea)

Upogebia deltaura was collected quantitatively and qualitatively between June 1980 and August 1982 from a level, sandy bottom at a 12 m depth in the archipelago of Lysekil (on Gullmarsfjorden) about 70 km north of Göteborg, in western Sweden. A total of 347 individuals were obtained. The deepest burrows reached a depth of more than 65 cm into the substrate. During the cold season, U. deltaura was situated much deeper in the substrate than it was during the summer, probably as a result of ‘hibernation’. The average density was 5·0 individuals 0·1 m⁻², and the maximum of 10 specimens was found in one 0·1 m² sample. Out of the 341 individuals identifiable as to sex, 186 (54·6%) were females and 155 (45·4%) were males. Berried females were found between May and August, and the average number of eggs carried was 4757. Ecdysis took place between May and August; most females moulted in the middle of June and most males about one month later, in mid July. The chelipeds of large males were proportionally bigger than were those of large females. Maximum total body length of the females was 65·7 mm, and that of the males was 65·3 mm.     

Eddy Diffusion Coefficients of Suspended Particulate Matter: Effects of Wind Energy Transfer and Stratification

Gross sedimentation rates (GSR) have been measured using sediment traps placed at nine different levels above the bed (0·3, 0·5, 0·8, 1·0, 2·0, 4·0, 6·0, 8·0 and 10·0 m). The sediment traps were deployed for 1·25 years and recovered 28 times during the study period. Low average GSR values of 5·5 g m^-2 day^-1 were obtained at 10·0 m, and high average GSR values of 114·8 g m^-2 day^-1 were obtained at 0·3 m. An expression for the eddy diffusion coefficient of suspended particulate matter (K_s), based on the measured GSR is given. The expression has been used for modelling of K_s at the different trap levels above the bed. High values (≈42 cm² s^-1) of K_s were obtained at the upper traps, whereas low values (≈2 cm² s^-1) were obtained near the bed. Comparison between level of turbulent energy in terms of shear stress at the boundaries of the water column, i.e. from the wind and the bed flow, showed that wind energy exceeded that of the bed flow by a factor 16. At 5·0 m K_s was positively correlated (r=0·66) to the eddy diffusion coefficient of momentum (K_m) derived from the wind energy transfer to the water, giving an average β of 0·5 for K_s =βK_m. The density difference between surface and bottom waters has been designated a parameter of stratification, and is discussed in relation to variations of K_s and K_m .     

Benthic microalgal biomass in sediments of Onslow Bay, North Carolina

Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g⁻¹ sediment (mean, 0·55), or 2·6–62·0 mg m². Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g⁻¹ sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface.Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.     

Depth Correlated Benthic Faunal Quantity and Infaunal Burrow Structures on the Slopes of a Marine Depression

In the northern part of the Kattegat, western Sweden, a series of marine depressions remain since the last glaciation. One of these, the well-oxygenated Alkor Deep, is about 3 km long and 800 m wide and with a depth of 138 m. Random depth-stratified sampling was made along four transects on the slopes including benthic macrofauna (0·1 m²grab samples) and sediment profile imaging. A significant positive correlation was found between depth and the faunal variables abundance and biomass. Deposit feeders such as Maldane sarsi, Heteromastus filiformis andAbra alba were among the dominants and may have been supported by down-slope advected organic material. In many images, pockets and extensive burrows were seen in the sediment that appeared to be constructed by the crustaceans Calocaris macandreae and Maera loveni. The ecological significance of their irrigation of the sediment is discussed. Due to the faunal activity deep down in the sediments of the slopes, the mean apparent redox potential discontinuity (RPD) was found as deep as between 8·0 and 11·3 cm depth, and RPD was significantly positively correlated with water depth. On the slopes there appears to be a balance between the input of organic material and the capacity of the benthic organisms to assimilate that carbon.     

The excretion of heavy metals by the salt marsh cord grass,Spartina alterniflora, and Spartina's role in mercury cycling

Excreted salts and leaves from the Salt Marsh Cordgrass Spartina alterniflora were collected from two different sites. One site, Piles Creek (PC), is near heavily industrialized Linden, New Jersey, USA. The other site, Big Sheepshead Creek (BSC), is located near non-industrialized Tuckerton, New Jersey, USA. PC soil concentrations of mercury were 18·17 ± 7·67 ppm, while BSC soil concentrations were 0·22 ± 0·04 ppm. Spartina leaves from PC contained 0·16 ± 0·07 ppm of mercury, and BSC leaves contained 0·02 ± 0·0 ppm. Laboratory studies showed that S. alterniflora from both sites was capable of excreting mercury. Field collected salts from PC Spartina plants contained 0·11 ± 0·02 ppm of mercury, 2·60 ± 0·52 ppm of cadmium and 11·98 ± 0·94 ppm of zinc. These levels of heavy metals were as much as five- and-a-half times the concentrations of these metals found in ambient sea salts.     

Seasonal Variations in Benthic Community Metabolism and Nitrogen Dynamics in a Shallow, Organic-Poor Danish Lagoon

The dynamics of benthic primary production and community respiration in a shallow oligotrophic, marine lagoon (Fællestrand, Denmark) was followed for 1·5 years. The shape of the annual primary production cycle was explained primarily by seasonal changes in temperature (r² = 0·67-0·72) and daylength (r² = 0·63), whereas temperature almost explained all variation in benthic community respiration (r² = 0·83-0·87). On a daily basis the benthic system was autotrophic during spring and summer supplied by 'new' and 'regenerated' nitrogen and predominantly heterotrophic during fall and winter caused by light and nutrient limitation. The linear depth-relationship between porewater alkalinity and ammonium indicated that the C:N ratio of mineralized organic matter is low in spring and summer (3-6) and high in fall and winter (9-16). This is inversely related to net primary production and thus the input of labile, nitrogen-rich algal cells. Accordingly, mineralization occurred predominantly in the upper 2-5 cm of the sediment. The pool of reactive material (microalgal cells) was estimated to account for 12% of total organic carbon in the upper 3 cm, and had an average turnover time of less than 1 month in summer. Assimilation of organic carbon by benthic animals was equivalent to about 30% of the annual gross primary production. Grazing reduced chlorophyll a concentration in the sediment during summer and spring to values 30-40% lower than in winter, but maintained a 3-4 times higher specific microalgal productivity. The rapid turnover of organic carbon and nitrogen, and important role of benthic microalgae showed that the benthic community in this oligotrophic lagoon is of a very dynamic nature.     

Estimation of the leeway drift of small craft

Small craft (<6·4 m) leeway is determined as a function of the wind speed in the range of 5–20 knots (3·6–10·3 m/sec). Leeway is calculated relative to the surface current by measurement of the separation distance of the small craft from a dyed patch of surface water at sea, using time-sequenced aerial photography. Leeway increases linearly with wind speed for small craft equipped with or without a sea anchor in the wind range studied. Leeway for small craft without sea anchor can be calculated from the equation U_L = 0.07 U_W + 0.04 where U_W is the wind speed at 2 m elevation. Leeway for small craft drifted off the be calculated from the equation U_LD = 0·05 U_W − 0·12. The small craft drifted off the downwind direction in about 80% of the experiments. The drift angle is variable and difficult to predict.     

The Significance of Diagenesis versus Riverine Input in Contributing to the Sediment Geochemical Matrix of Iron and Manganese in an Intertidal Region

Summer porewater and spring and summer surficial sediment samples were collected from 26 locations in the intertidal region of the Fraser River estuary. Porewaters were analysed for dissolved iron and manganese (as defined by species <0·2μm in diameter) to assess the contribution of diagenesis to concentrations of iron and manganese oxides at the sediment–water interface. Surficial sediment samples were geochemically characterized as: % organic matter (% LOI); reducible iron (RED Fe, iron oxides) and easily reducible manganese (ER Mn, manganese oxides). Grain size at each site was also determined. The sediment geochemical matrix, as defined by the above four parameters, was highly heterogeneous throughout the intertidal region (three-way ANOVA;P<0·0001). For RED Fe and ER Mn, this heterogeneity could be explained by either diagenetic processes (RED Fe) or by a combination of the proximity of the sample sites to the mouth of the Fraser River estuary plus diagenetic processes (ER Mn). Correlation (Spearman Rank Correlation Test (r_s), of dissolved iron within the subsurface sediments with amounts of RED Fe recovered from the associated surface sediments was highly significant (r_s=0·80, P<0·0001); high concentrations of RED Fe at the sediment–water interface co-occurred with high concentrations of dissolved iron, regardless of the proximity of the sample locations to riverine input. Compared with iron, the relationship between dissolved manganese and ER Mn from surface sediments was lower (r_s=0·58;P<0·0008). Locations most strongly influenced by the Fraser River contained greater concentrations of ER Mn at the sediment–water interface than that which would be expected based on the contribution from diagenesis alone. Sediment grain size and organic matter were also influenced by the proximity to riverine input. Surficial sediment of sites close to the river mouth were comprised primarily of percent silt (2·0μm–50μm) whereas sites not influenced by riverine input were primarily percent sand (grain size >50μm). Concentrations of organic matter declined from the mouth to the foreslope of the estuary. With the exception of RED Fe, temporal variation (May vs July) was insignificant (P>0·05, three-way ANOVA). Concentrations of RED Fe recovered from the surficial sediments were in general greater in the summer vs spring months, although spring and summer values were highly correlated (Pearson Product Moment Correlation Coefficient; PPCC; R=0·89;P<0·0001). As the bioavailability of metals is dependent on sediment geochemistry, availability throughout the intertidal region will also be spatially dependent. This heterogeneity needs to be taken into account in studies addressing the impact of metals on estuarine systems.     

Reproductive success, xenobiotic contaminants and hepatic mixed-function oxidase (MFO) activity in Platichthys stellatus populations from San Francisco Bay

To identify some specific effects of organic contaminants on fisheries in an urbanized estuary we compared the reproductive success of starry flounder from San Francisco Bay with concentrations of tissue contaminants and hepatic mixed-function oxidase (MFO) activity. We found significantly lower (P < 0·05) sediment concentrations of total identified polynuclear aromatic hydrocarbons (PAHs) in the less urbanized San Pablo Bay (SP) area (Fig. 1) than in the more urbanized central bay (CB) stations (Table 1). For flounder in early gametogenesis (August and September) the SP fish (n = 20) had significantly lower (P < 0·01) liver concentrations of Aroclor 1260 (0·34 ± 0·14 μg/g) than those at the CB stations: Berkeley (BK, n = 20, 1·6 ± 1·6 μg/g); Oakland (OK, n = 16, 2·3 ± 2·8 μg/g); and Alameda (AL, n = 4, 2·2 ± 1 μg/g). A similar pattern existed for DDT concentrations: SP = 0·2 ± 0·16 μg/g; BK = 0·1 ± 0·34 μg/g; OK = 0·4 ± 0·53 μg/g; and AL = 0·4 ± 0·33 μg/g. Total PAHs in livers were as follows: SP = 0·14 μg/g; BK = 2·6 μg/g; OK = 1·4 μg/g; and AL = 14 μg/g. Although gonad index, liver index, and presence of fin rot are inversely related to aryl hydrocarbon hydroxylase (AHH) activity, healthy fish in a similar reproductive state have lower AHH activities in the SP area. For example, in August and September, 1984, mean AHH activities were as follows: SP = 203 ± 89, and CB = 355 ± 200 pmol 3-OH-B[a]P mg microsomal protein min. We found a log-linear relationship for AHH activity and its percent inhibition by 7,8-benzoflavone (10⁻⁴m) and only a few fish from SP showed enhanced AHH activity after addition of 7,8-benzoflavone. This suggests that most of the starry flounder in San Francisco Bay are induced.     

The toxicity of effluents from a TiO2 plant to the harpacticoid copepod nitocra spinipes Boeck

The harpacticoid copepod, Nitocra spinipes Boeck, was used as an experimental animal in a series of tests where the toxicity of different fractions (solid and soluble) of TiO₂ industrial waste water was investigated. The tests were performed in brackish water (7). Water temperature was used as a variable in the fractionation of the waste water. Temperature was found to influence the acute toxicity of the waste water. LC₅₀ values for N. spinipes were 0·087 (± 4°C), 0·105 (+ 7°C)) and 0·12% (+ 21°C) of undiluted waste water. At room temperature the animal's fecundity was affected at two intervals, one in dilutions between 0·0003% and 0·001% and the other between 0·03% and 0·3%. An acute toxicity test with the dried solid metal fraction resuspended into clean brackish water had an acute LC₅₀ value of 7·6% of undiluted waste water. The results show that different fractions of the waste water have different harmful biological effects under different temperature conditions.     

Fishes in Mangrove Prop-root Habitats of Northeastern Florida Bay: Distinct Assemblages across an Estuarine Gradient

Seasonal changes in freshwater inflow and other environmental conditions may induce changes in density and species composition of mangrove fishes along estuarine gradients. Fishes within mangrove habitats in a subtropical estuary were sampled monthly from May 1989 to May 1990, using block nets with rotenone and visual censuses. At 18 stations, temperature ranged from 22 to 34°C, depth from 10 to 104cm and underwater visibility from 1 to 13m. Salinity ranged from 0 to 60 upstream, and 35 to 54 mid- and downstream. A total of 573191 individuals (76 species) was observed or collected, with an average density of 6·5 fish m⁻². Engraulidae, Atherinidae, Poeciliidae and Cyprinodontidae numerically dominated the assemblage. Distinct assemblages occurred up-, mid- and downstream and maintained coherent groups in these gradient positions over the seasons. Residents totalled 94·5% of the individuals, estuarine transients comprised 5·1% and occasional marine visitors were less than 0·4%. Densities of resident fishes peaked in winter as temperatures and water levels fell, uncorrelated with changes in salinity. These observations suggest that mangrove habitats may sustain diverse and abundant fish communities dominated by euryhaline residents. Although estuarine transients were consistently rare in upstream sub-basins, downstream were found numerous sub-adults of species occurring as adults on nearby reefs (Lutjanidae, Haemulidae). Thus, reef-associated estuarine transients may be abundant in mangrove habitats having near-marine salinities. Contrary to expectations, mangrove habitats in northeastern Florida bay did not function as a nursery as defined under the nursery-ground paradigm: young-of-the-year juveniles of estuarine transient species did not seek low salinity sub-basins. However, northeastern Florida Bay may not be representative of most mangrove estuaries as the area: (1) is without lunar tides and related circulation; (2) has low and variable amounts of submersed vegetation; and (3) experiences severe hypersaline conditions.     

The solubility of calcite in seawater at atmospheric pressure and 35%permil; salinity

The apparent (stoichiometric) solubility product of calcite in artificial seawater of salinity 35‰ was measured by a saturometer technique. The value of the apparent solubility product was found to be (4·59 ± 0·05) × 10⁻⁷ moles/(kilogram of seawater)² at 25°C with a temperature coefficient of −0·0108 × 10⁻⁷/°C between 2 and 25°C. These values are significantly smaller than those found by MacIntyre (1965) and other workers. The effect of these results on the saturation of the oceans with respect to calcite is examined.     

Scyphomedusae in surface waters near the Oregon coast, May–August 1981

The scyphomedusae of the surface waters off Oregon and southern Washington were collected with commercial purse seines from May–August 1981. Twelve east-west transects, located from north of the Columbia River to south of Coos Bay were sampled from the 37 m isobath to distances up to 48 km from shore. Chrysaora fuscescens was the dominant species collected in each month. Maximum sampled abundances reached 18001 of medusae per 10⁵ m³. Using an estimated carbon content of 0·280% of wet weight, this medusa density was calculated to contain approximately 50 mg Cm⁻³. Seven of the 263 samples contained so many medusae that they exceeded the capacity of the sampling gear. In all months but May, when medusa densities were relatively low, the density of C. fuscescens was greatest closest to shore and decreased rapidly offshore. Mean umbrella diameter increased from 8·6 cm in May to 18·5 cm in August, while the largest specimens increased from 19 cm in May to 37 cm in August. Aurelia aurtia, Cyanea capillata and Phacellophora camtschatica were also collected, but were much less abundant than C. fuscescens. The relative abundance of C. fuscescens was compared with the maximum abundance of copepods off the Oregon coast, and the hydrographic features influencing medusa distribution patterns are discussed.     

Nutrient flux in the Rhode River: Tidal transport of microorganisms in brackish marshes

Concentrations of bacteria, chlorophyll a, and several dissolved organic compounds were determined during 11 tidal cycles throughout the year in a high and a low elevation marsh of a brackish tidal estuary. Mean bacterial concentrations were slightly higher in flooding (7·1 × 10⁶ cells ml⁻¹) than in ebbing waters (6·5 × 10⁶ cells ml⁻¹), and there were no differences between marshes. Mean chlorophyll a concentrations were 36·7 μg l⁻¹ in the low marsh and 20·4 μg l⁻¹ in the high marsh. Flux calculations, based on tidal records and measured concentrations, suggested a small net import of bacterial and algal biomass into both marshes. Over the course of individual tidal cycles, concentrations of all parameters were variable and not related to tidal stage. Heterotrophic activity measured by the uptake of ³H-thymidine, was found predominantly in the smallest particle size fractions (< 1·0 μm). Thymidine uptake was correlated with temperature (r = 0·48, P < 0·01), and bacterial productivity was estimated to be 7 to 42 μg Cl⁻¹ day⁻¹.     

Reassessment of the Rates at which Oil from Natural Sources Enters the marine environment

Previous estimates of the world-wide input of oil to the marine environment by natural seeps ranged from 0·2 to 6·0 million (metric) tonnes per year with a ‘best estimate’ of 0·6 million tonnes per year. Based on considerations of the availability of oil for seepage from the world's known and assumed oil resources, we believe that the world-wide natural oil seepage over geological time should be revised to about 0·2 million tonnes per year with a range upward or downward of a factor of ten leading to estimates between 0·02 and 2 million tonnes per year. Our estimate of the amount of oil eroding from the land and being transported to the oceans is about 0·05 million tonnes per year with an order of magnitude uncertainty. Therefore, while the uncertainties are large, we estimate that the total amount of oil entering the marine environment by natural, geological processes, is about 0·25 million tonnes per year, and the estimate may range from about 0·25 to 2·5 million tonnes per year.     

Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary

Cathodic stripping voltammetry (CSV) is used to determine total (after UV-irradiation) and labile dissolved metal concentrations as well as complexing ligand concentrations in samples from the river Scheldt estuary. It was found that even at high added concentrations of catechol (1 m for copper and 0·4 m for iron) and of APDC (1 m for zinc) only part of the dissolved metal was labile (5–58% for copper, 34–69% for zinc, 10–38% for iron); this discrepancy could be explained by the low solubility of iron which is largely present as colloidal material, and by competition for dissolved copper and zinc by organic complexing ligands. Ligand concentrations varied between 28 and 206 n for copper and between 22 and 220 n for zinc; part of the copper complexing ligands could be sub-divided into strong complexing sites with concentrations between 23 and 121 n and weaker sites with concentrations between 44 and 131 n . Values for conditional stability constants varied between (logK′ values) 13·0 and 14·8 for strong and between 11·5 and 12·1 for weaker copper complexing ligands, whereas for zinc the values were between 8·6 and 10·6. The average products of ligand concentrations and conditional stability constants (a-coefficients) were 6 × 10² for zinc and 6 × 10⁶ for copper.The dissolved zinc concentration was found to co-vary with the zinc complexing ligand concentration throughout the estuary. It is argued that the zinc concentration is regulated, in this estuary at least, by interactions with dissolved organic complexing ligands. A similar relationship was apparent between the dissolved copper and the strong copper complexing ligand concentration. The total copper complexing ligand concentrations were much greater than the dissolved copper concentrations, suggesting that only strongly complexed copper is kept in solution.These results provide evidence for the first time that interactions of copper and zinc with dissolved organic complexing ligands determine the geochemical pathway of these metals.