Kinetics of trace metal accumulation and excretion in the polychaete <Emphasis Type="Italic">Nereis diversicolor</Emphasis>

The comparative kinetics of the accumulation of the trace metals copper, zinc and cadmium have been measured in the estuarine burrowing polychaete worm Nereis (Hediste) diversicolor from two sites: (a) a metal-rich site, Restronguet Creek, Cornwall, UK, which hosts a copper- and zinc-tolerant population of worms, and (b) the Blackwater estuary, Essex, UK as a control site. A sediment transfer experiment showed that the Blackwater worms responded to the increased copper bioavailability in Restronguet Creek sediment by accumulating significantly increasing copper concentrations over 50 days. The Restronguet Creek worms showed no significant change in copper concentration over 50 days in sediment from either site or in sand. Nevertheless, electron microscopy showed that some Restronguet Creek worms do appear to excrete accumulated copper, probably in association with renewal of the cuticle over a long time scale. The Blackwater worms did not accumulate extra zinc from the zinc-rich Restronguet Creek sediment, in probable reflection of the regulation of body zinc concentration by N. diversicolor. Radiolabelled zinc and cadmium were accumulated from labelled sediment and labelled solution by worms from both sites. The rate of uptake of labelled zinc from sediment was significantly greater in the Restronguet Creek worms, as was the rate of uptake of labelled cadmium from 10 μg l⁻¹ dissolved exposure; other rates of uptake did not differ between populations. Mucus, which is secreted by Restronguet Creek worms in response to enhanced copper exposure, adsorbed very small proportions of zinc and cadmium present in solution, indicating that the mucus does not act as an adsorption barrier against excessive metal uptake by these worms.     

The Nutrient Status of Some Graminaceous Species in Britain. 1. Molinia Caerulea (L.) Moench

Above-ground biomass biomass of Molinia caerulea from 36 sites throughout Britain was analysed for concentrations of elements (N, P, K, Ca, Mg, Fe, Mn, Cu, Zn and Na), with matching analyses of soil nutrients, together with pH and LOI. Between-year and seasonal variation in the nutrients was also studied at one of the sites. the grass was found on acid sites ranging between pH 3.1 and 5.8. the soil humus content varied widely, and there were significant correlations between that and elements (N, P, K, Ca, Mg and Na) in the soil. Concentrations of calcium, magnesium, zinc and manganese in the plant material were significantly correlated with extractable levels in the soils. Between year variation in the plant nutrient concentrations at one site was of a similar order to variation between the sites. Generally there was less variation in nitrogen, phosphorus and potassium tissue levels than in calcium, magnesium, zinc and copper. Iron and sodium were the most variable. Tissue concentrations of nitrogen, phosphorus, potassium and copper declined throughout the growing season, whereas magnesium, calcium and iron built up until August/September after which a decline set in as the leaves senesced. the results are discussed in relation to the importance of standardizing the time of sampling, in comparison with concentrations of tissue levels in other plants and the growth strategy of this deciduous grass.     

Population structure of two northern hemisphere polychaetes,<Emphasis Type="Italic"> Neanthes virens</Emphasis> and<Emphasis Type="Italic"> Hediste diversicolor</Emphasis> (Nereididae), with different life-history traits

The present study compared population genetic structure of two northern hemisphere polychaetes, Neanthes virens (Sars, 1835) and Hediste diversicolor (O.F. Müller, 1776), with different life-history traits. N. virens has a pelagic larval phase that may last as long as 9 days, while H. diversicolor broods its offspring. It was predicted that N. virens populations, especially over relatively short geographic distances, would be more genetically similar than H. diversicolor populations. Sequence data were obtained for two mitochondrial genes, cytochrome c oxidase I (COI) and cytochrome b (cyt b). The COI gene showed no variation in 16 N. virens individuals from 12 locations, and it was not examined in H. diversicolor. The nucleotide sequence of the cyt b gene was examined in 93 N. virens individuals collected in 1992, 1999, and 2000 from 12 localities throughout its range (North America, Europe, Japan) and from 33 H. diversicolor individuals collected in 1999 and 2001 from one locality in eastern Canada and two localities in France (Roscoff and Marseille). In N. virens, one cyt b haplotype was shared by 89% (83 out of 93) and only two other haplotypes were found. Significant population structure was observed in H. diversicolor. In the 33 sequences compared, 18.9% of sites were polymorphic and 12 haplotypes were identified. There were two haplotypes in the Canadian population, three in the Marseille samples, and seven in the Roscoff samples, and no haplotype was found in common between any two sites. These results show that widely separated H. diversicolor populations are strongly differentiated at the cyt b locus. The most plausible interpretation for the low variation at the COI and cyt b loci in N. virens, and for the lack of variation at allozyme loci (based on an initial survey of a smaller subset of populations), is that this species has experienced a drastic bottleneck and that a few remaining lineages recolonized its entire range.     

Automated interpulse duration assessment (AIDA) in the shore crab Carcinus maenas in response to copper exposure

The effects of acute (24 h) and chronic (4␣weeks) copper exposure and subsequent recovery in “clean” conditions on cardiac activity were examined in shore crabs ( Carcinus maenas). Cardiac activity was monitored by automated interpulse duration assessment (AIDA) which permits detection of irregularities in cardiac rhythms in selected invertebrates. Mean interpulse durations, maximum interpulse durations and coefficients of variation were significantly affected by exposure to copper for 24 h and 4 weeks. Mean interpulse durations decreased reflecting an increase in heart rate, whereas maximum interpulse durations and coefficients of variation increased with increasing exposure concentration. Recovery of normal cardiac activity was evident following a 6-week recovery period in “clean” conditions. However, high levels of mortality were seen among crabs exposed to the two highest copper concentrations (0.6 and 0.8 mg l⁻¹ Cu). The extent of recovery was assessed by subjecting the surviving crabs to a physiological challenge. There were no apparent differences in circulatory response to physiological challenge between crabs previously exposed to copper and control crabs. Gill copper concentrations were significantly elevated in crabs that had previously been exposed to copper, despite their return to “clean” sea water for 6 weeks. Midgut gland wet weight/dry weight ratios, haemolymph total protein and midgut gland metallothionein concentrations did not reflect the previous copper exposure. The findings of the present study indicate that AIDA may be a valuable tool for use in toxicity testing. Received: 24 March 1997 / Accepted: 21 October 1997     

Uptake of 55Fe from contaminated sediments by the polychaete Nereis diversicolor

The polychaete Nereis diversicolor O. F. Müller was exposed to marine sediments which had been contaminated with ⁵⁵Fe either through the testing of nuclear devices or by the release of liquid waste effluent from a nuclear fuel reprocessing plant. The amount of ⁵⁵Fe taken up reached a concentration in the worms about 6% of the concentration in both types of sediment. Bioavailability of ⁵⁵Fe was shown to depend on its concentration in sediment and not on sediment type, stable iron concentration, or the source of contamination. N. diversicolor may not extract from sediment the same fraction of ⁵⁵Fe as does 0.1N HCl and caution should be exercised in using such measurements to represent the bioavailable fraction.     

Species-specific effect of macrobenthic assemblages on meiobenthos and nematode community structure in shallow sandy sediments

Three functionally different macrofaunal species (the filter- and/or surface deposit-feeding polychaete Hediste diversicolor, and the suspension-feeding bivalves Mya arenaria and Cerastoderma glaucum) were introduced as single- and two-species treatments into microcosms containing sandy sediment with a natural meiofaunal community. H. diversicolor is a burrowing species building a system of galleries, C. glaucum lives actively near the sediment surface acting as a biodiffuser and M. arenaria buries deeply and leads a sessile lifestyle. It is shown that H. diversicolor extended the vertical distribution of meiofauna into deeper sediment layers compared to the control and non-Hediste treatments. The response of the nematode community varied significantly among treatments and was dependant on the macrobenthic species composition but not on the species number. Nematode assemblages in all treatments with the polychaete, both in monoculture and with either bivalve, differed significantly from those recorded in other treatments and were more similar than replicates within any other single treatment. H. diversicolor also appeared to have stimulated nematode species diversity. The present study demonstrated that the impact of macrobenthic assemblages on meiofauna is not a simple summation of individual species effects but is species specific.     

Microbial reaction rates and bacterial communities in sediment surrounding burrows of two nereidid polychaetes (Nereis diversicolor and N. virens)

The effects of infaunal mode of life on sediment properties, microbial reaction rates, as well as abundance and composition of bacterial communities were studied in sediment surrounding burrows (mucus lining, oxidised wall, ambient anoxic and surface sediment) of two closely related, but behaviourally different, nereidid polychaete worms: the facultative suspension-feeder Nereis (Hediste) diversicolor and the obligate deposit-feeder Nereis (Neanthes) virens. Burrow sediment of the two species was collected from two adjacent (50 m distance) shallow sandy locations (Kertinge Nor, Denmark). The burrow lining and wall of both polychaete species were enriched in organic matter originating from mucous secretions by the inhabitants and phytoplankton trapped through irrigation. This was more evident for N. diversicolor that shows a significantly higher irrigation rate than N. virens. Both the organic matter mineralisation rates (based on anaerobic incubations) and bacterial abundance were higher along the burrow linings and walls. However, accumulation of porewater TCO₂ and dissolved organic carbon in sediments adjacent to burrows increased most rapidly in the presence of N. diversicolor, suggesting higher heterotrophic activity associated with this species. Surprisingly, bacterial abundance was lower around burrows of N. diversicolor than those from N. virens indicating that burrow environments from the first species harbour a more active bacterial community. Molecular fingerprints of the 16S rRNA gene from bacterial communities showed that the composition of the burrow linings and walls resembled the ambient anoxic sediment rather than the oxic sediment surface. On the other hand, the bacterial fingerprints of the sediment surrounding the burrows of the two polychaete species were markedly different suggesting either a site-specific difference in sediment parameters or a significant species-specific impact of the burrow inhabitants.     

Plutonium and Americium: Uptake from contaminated sediments by the polychaete Nereis diversicolor

The polychaete Nereis diversicolor O.F. Müller was exposed to marine sediments which had been contaminated with plutonium and americium either through the testing of nuclear devices or by the release of liquid waste effluent from a nuclear fuel reprocessing plant. The uptake of both plutonium and americium was small, about 0.5% of the concentration of these transuranics in the sediments. The degree of relative uptake of plutonium from both sediments was comparable; relative uptake of americium from sediments contaminated by waste effluent was greater than that from nuclear sediment and those previously determined from water-uptake experiments to environmentally determined plutonium concentrations in sediments and overlying waters leads to the tentative conclusion that water may be the predominant pathway for plutonium accumulation by deposit-feeding worms like N. diversicolor.     

High copper concentrations in squid livers in association with elevated levels of silver,cadmium, and zinc

Livers from 43 Loligo opalescens, 14 Ommastrephes bartrami, and 7 Symplectoteuthis oualaniensis were analyzed for their silver, cadmium, copper, zinc and iron contents. Copper concentrations of up to 15,000 μg/g dry weight were found in L. opalescens in conjunction with significant correlations between this element and Ag, Cd and Zn. The latter elements are known to affect Cu metabolism in terrestrial organisms; however, whether the correlations occurring in marine organisms represent casual, or cause and effect, relationships is as yet unknown.     

Heavy metal action on transintegumentary absorption of glycine in two annelid species

Absorption of ¹⁴C-glycine from ambient sea water across the body surface of the oligochaete Enchytraeus albidus Henle was, after 6 h preincubation, significantly reduced in the presence of 0.1 to 0.15 ppm mercury, 0.25 ppm copper, 1.5 ppm silver, and 2 ppm cadmium. Addition of heavy metal salts reduced maximum uptake rates (V _max ), without affecting transport constants (K _t ). Influx of ¹⁴C-glycine remained unchanged in the presence of up to 10 ppm aluminium, chromium, iron, lead, molybdenum, vanadium, and zinc. Effects of up to 150 ppm nickel, manganese, cobalt and selenium were negligible. Inhibition of glycine absorption by mercury and cadmium in the polychaete Nereis diversicolor Müller is almost identical with results presented for E. albidus. Transintegumentary solute absorption in soft-bodied marine invertebrates obviously represents a suitable biological function for studies on heavy metal toxicity. The close relationship between rates of heavy metal absorption, inhibition of transintegumentary glycine uptake, and acute toxicity is discussed.     

The Nutrient Status of Some Graminaceous Species in Britain. 1. Molinia Caerulea (L.) Moench

Above-ground biomass biomass of Molinia caerulea from 36 sites throughout Britain was analysed for concentrations of elements (N, P, K, Ca, Mg, Fe, Mn, Cu, Zn and Na), with matching analyses of soil nutrients, together with pH and LOI. Between-year and seasonal variation in the nutrients was also studied at one of the sites. the grass was found on acid sites ranging between pH 3.1 and 5.8. the soil humus content varied widely, and there were significant correlations between that and elements (N, P, K, Ca, Mg and Na) in the soil. Concentrations of calcium, magnesium, zinc and manganese in the plant material were significantly correlated with extractable levels in the soils. Between year variation in the plant nutrient concentrations at one site was of a similar order to variation between the sites. Generally there was less variation in nitrogen, phosphorus and potassium tissue levels than in calcium, magnesium, zinc and copper. Iron and sodium were the most variable. Tissue concentrations of nitrogen, phosphorus, potassium and copper declined throughout the growing season, whereas magnesium, calcium and iron built up until August/September after which a decline set in as the leaves senesced. the results are discussed in relation to the importance of standardizing the time of sampling, in comparison with concentrations of tissue levels in other plants and the growth strategy of this deciduous grass.     

Comparisons of genotype-tolerance responses in populations of <Emphasis Type="Italic">Hediste diversicolor</Emphasis> (Polychaeta: Nereididae) exposed to copper stress

The analysis of genotypic shifts in field-exposed populations has been proposed as a tool for monitoring the environmental impacts of contaminants. Previous laboratory experiments showed the occurrence of genotypic shifts induced by copper exposure in samples of Hediste diversicolor from the Pialassa estuary (North Adriatic Sea, Italy). In order to test if genotype-tolerance responses observed were consistent at larger spatial scales, populations of H. diversicolor were sampled in three haphazardly chosen estuaries along the Italian coast (Comacchio, Pialassa, Serchio) and exposed to 0.48 mg l⁻¹ Cu²⁺ under laboratory conditions. Survival analysis procedures were used to test for possible relationships between time-to-death and genotypes at three allozyme loci (ALD, FH, PGI). Genotype-tolerance responses observed at locus ALD were consistent among populations up to 2500 km distant, with genotype ALD ^102/102 associated with the shortest survival times. Comparisons with previous laboratory experiments showed that responses to copper stress at locus ALD were also consistent in time, with genotypes ALD ^102/102 and ALD ^100/102 associated with higher 96 h mortality ratios. Results suggested the occurrence of stable relationships between genotypes at locus ALD and tolerance responses of H. diversicolor to copper stress. Conversely, genotype-tolerance responses at loci PGI and FH were not consistent either in space (among populations from the three estuaries) or in time (between laboratory experiments), being possibly affected by site-specific features of populations or stochastic factors. Results show that genotype-tolerance responses should be carefully verified at different spatial and temporal scales, before considering genotypic shifts among the possible tools for monitoring the effects of environmental stressors.     

Simultaneous, long-term monitoring of valve and cardiac activity in the blue mussel Mytilus edulis exposed to copper

Valve and cardiac activity were simultaneously measured in the blue mussel (Mytilus edulis) in response to 10?d copper exposure. Valve movements, heart rates and heart-rate variability were obtained non-invasively using a Musselmonitor^® (valve activity) and a modified version of the Computer-Aided Physiological Monitoring system (CAPMON; cardiac activity). After 2?d exposure of mussels (4 individuals per treatment group) to a range of dissolved copper concentrations (0 to 12.5?M as CuCl₂) median valve positions (% open) and median heart rates (beats per minute) declined as a function of copper concentration. Heart-rate variability (coefficient of variation for interpulse durations) rose in a concentration-dependent manner. The 48?h EC₅₀ values (concentrations of copper causing 50% change) for valve positions, heart rates and heart-rate variability were 2.1, 0.8, and 0.06?M, respectively. Valve activity was weakly correlated with both heart rate (r?=?0.48?±?0.02) and heart-rate variability (r?=?0.32?±?0.06) for control individuals (0?M Cu²⁺). This resulted from a number of short enclosure events that did not coincide with a change in cardiac activity. Exposure of mussels to increasing copper concentrations (≥0.8?M) progressively reduced the correlation between valve activity and heart rates (r?=?0 for individuals dosed with ≥6.3?M Cu²⁺), while correlations between valve activity and heart-rate variability were unaffected. The poor correlations resulted from periods of valve flapping that were not mimicked by similar fluctuations in heart rate or heart-rate variability. The data suggest that the copper-induced bradycardia observed in mussels is not a consequence of prolonged valve closure.     

Measurement of the adenylate energy charge in Nereis diversicolor and Nephtys sp. (Polychaeta: Annelida):

ATP-content and adenylate energy charge (AEC) ratios were determined in two polychaete species (Neries diversicolor and Nephytys sp.), sampled in ten stations along the heavily polluted Western Scheldt estuary (N. Belgium, S. Holland). The samples were taken between 27 December 1982 and 6 January 1983. Nereis diversicolor was also sampled in an unpolluted brackish water pond, and subjected to artificial stress by drying the organisms on filter paper. Adenine nucleotide levels were determined using the firefly bioluminescence reaction. The light emission was measured with the integration mode. As the Constant Light Signal reagent used contained AMP and probably ADP itself, both myokinase and pyruvatekinase had to be inactivated before the assay. The use of pepsin in acidic conditions to denaturate these enzymes is discussed. For both N. diversicolor and Nephtys sp., a significant relation between ATP-content and fresh weight is found in most samples. Within each species, ATP-content per g fresh weight is dependent on the sampling method, but it is not significantly different between stations. It is higher in Nephtys sp. from the W. Scheldt (130. 8 10^-5 g ATP/g fresh weight) than in N. diversicolor from the Dievengat (83.9 10^-5 g ATP/g fresh weight). This value in turn is significantly higher than that for N. diversicolor in the W. Scheldt (64.0 10^-5 g ATP/g fresh weight). In the drying experiment, AEC gradually decreases with increasing drying time. In the field, AEC is generally high and not significantly different between the three populations. AEC ratios also do not differ between stations within each population. It is argued that field monitoring of AEC in invertebrates is not valuable as a pollution assessment tool for two reasons. In polluted stations, only pollutant-resistant species will be found, and there is evidence that the maintenance of a stable population is impossible when the individuals constantly have low AECs.     

Modification of sediment permeability and shear strength by two burrowing invertebrates

Laboratory experiments have been conducted on the effects of bioturbation by the amphipod Corophium volutator (Pallas) and the polychaete Nereis diversicolor (O.F. Müller) on the physical properties of estuarine muddy sand. Invertebrates and muddy sediment were collected from Langbank, Firth of Clyde, Scotland. Sediment columns were seeded with low, medium and high densities of invertebrates in single-and mixed-species experiments. Permeability decreased with increasing density of C. volutator, but increased with increasing density of N. diversicolor. Water content decreased with increasing density of both species. Shear strength increased with increasing density of both species, but more so with C. volutator than with N. diversicolor. Mixed-species effects on shear strength were additive, but some of the permeabilities were lower than predicted. Bivariate and multivariate correlation, and partial correlation analyses were applied to the data. Shear strength was negatively correlated with water content. The correlations between shear strength and permeability and between permeability and water content were affected differently by the two species. There were few partial correlations between the variables.     

Concentration and distribution of heavy metals in tissues of two cephalopods,Eledone cirrhosa and Sepia officinalis,from the French coast of the English Channel

The concentrations of 11 heavy metals (Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn) were measured in the tissues (digestive gland, branchial hearts, gills, digestive tract, kidney, genital tract, muscle, skin, shell) of the two cephalopods Eledone cirrhosa (d'Orb.) and Sepia officinalis (L.) collected from the French coast of the English Channel in October 1987. The tissues of both species displayed a similar pattern of heavy-metal accumulation: the digestive gland, branchial hearts and kidney were the major sites of concentration for all 11 metals; the digestive gland accumulated silver, cadmium, cobalt, copper, iron, lead and zinc, the branchial hearts high concentrations of copper, nickel and vanadium, and the kidney high concentrations of manganese, nickel and lead. The digestive gland, which constituted 6 to 10% of the whole-animal tissue, contained >80% of the total body burden of Ag, Cd and Co and from 40 to 80% of the total body burden of the other metals. The ratios between heavy metal concentrations in the digestive gland and those in the muscle separated the elements into three groups, those with a ratio 10 (Cr, Mn, Ni, Pb, V, Zn), those with a ratio >10 to <50 (Co, Cu, Fe), and those with a ratio 50 (Ag, Cd). The digestive gland of cephalopods (carnivorous molluscs whose age can be easily calculated with great accuracy) would seem to constitute a good potential indicator of heavy metal concentrations in the marine environment.     

Trace metal concentrations and contents in the tissues of the shore crab<Emphasis Type="Italic"> Carcinus maenas</Emphasis>: effects of size and tissue hydration

Water content in the tissues, tissue proportions, and content and concentrations of copper, zinc, iron, cadmium, manganese, and cobalt were determined in male shore crabs Carcinus maenas of different sizes. Average wet to dry weight ratios in midgut gland, gills, muscle, exoskeleton, stomach, heart, testes, hypodermis, and haemolymph were 5.3, 9.5, 4.1, 1.5, 4.8, 6.1, 3.8, 4.7, and 13, respectively. The water content in the soft tissues generally increased with increasing size of the crabs (except in testes). Dry midgut gland, gills, muscle, exoskeleton, stomach, heart, testes, hypodermis, and haemolymph constituted 4, 0.9, 20, 68, 0.5, 0.12, 0.5, 0.7 and 5.3%, respectively, of the body dry weight. Dry midgut gland, gills, stomach, hypodermis, and haemolymph constituted a larger proportion of the body weight in small than in large crabs, whereas the opposite was the case for exoskeleton. Protein concentrations in the haemolymph decreased with size whereas haemolymph volume increased. Seventy percent of the copper body burden was located in the haemolymph. The majority of zinc (61%) occurred in muscle, whereas the midgut gland was the major site of cadmium storage (89%). The exoskeleton contained the major part of the body burden of iron (71%) and manganese (95%). Most of the cobalt body burden was located in the exoskeleton (45%) and haemolymph (44%). As tissue hydration and tissue proportions changed with size, tissue metal concentrations did not necessarily vary identically with size when expressed on a dry weight compared to a wet weight basis. Tissue potassium concentrations were negatively correlated with tissue water contents, indicating that water in tissues with a high wet to dry weight ratio was of extracellular origin. However, comparison of tissue and haemolymph copper levels showed that this extracellular water could not be pure haemolymph as tissue copper concentrations would have been higher than the values actually found.     

Interindividual variability in metal status in the shore crab<Emphasis Type="Italic"> Carcinus maenas</Emphasis>: the role of physiological condition and genetic variation

The relations between genetic variation, physiological condition, size and metal status in the shore crab Carcinus maenas (L.) were investigated. Shore crabs were collected at three sites around the island of Funen, Denmark, and carapace width, colour, moulting stage and major ions and protein in the haemolymph were determined. Concentrations of water, cadmium, copper and zinc in midgut glands, muscles and gills were measured, and allozyme variation at 15 loci was studied. Generally, tissue water content and copper, zinc and cadmium concentrations decreased from the moulting stages C₃ over C₄ to D crabs, whereas the opposite was true for copper and zinc concentrations in gills and muscle. However, the water content of the tissues increased with the size of the crabs. Since tissue water contents changed consistently over moulting stages, conclusions on changes in metal concentrations sometimes diverged when expressed on a dry versus wet weight basis. Regarding allozymes, esterase, malate dehydrogenase and phosphoglucomutase were polymorphic. The crabs showed a low level of genetic variability for both polymorphism (0.14) and for heterozygosity (0.07) at the three sites. Allelic frequencies showed very low levels of differentiation among the samples, no evidence of inbreeding and no population subdivision. Multilocus heterozygosity was positively correlated with size. Concentrations of copper and zinc in midgut gland, muscle tissue and gill were not correlated with genotype, whereas a weak correlation between phosphoglucomutase genotypes and concentrations of cadmium in the midgut gland was found.Communicated by M. Kühl, Helsingør     

Short-term interaction between nitrate and ammonium uptake inThalassiosira pseudonana: Effect of preconditioning nitrogen source and growth rate

The effect of preconditioning nitrogen source and growth rate on the interaction between nitrate and ammonium uptake was determined inThalassiosira pseudonana (Clone 3H). A new method, using cells on a filter (Parslow et al. 1985), allowed continuous measurement of uptake from 0.5 to 9 min after the addition of nitrate, ammonium, or both, with no variation in concentration during the course of the experiment. For each preconditioning N source and growth rate, a series of uptake experiments was conducted, including controls with only nitrate or only ammonium, and others with different combinations of concentrations of nitrate and ammonium. For the first time, preference for ammonium was separated from inhibition of nitrate uptake by ammonium. Ammonium was the preferred N source, i.e. if nitrate and ammonium were presented separately, ammonium uptake rates exceeded nitrate uptake rates. Preference for ammonium varied with both preconditioning N source and growth rate. Inhibition of nitrate uptake by ammonium, determined by comparing nitrate uptake in the presence and absence of ammonium, was observed at ammonium concentrations > 1µM, but was rarely complete. Inhibition of nitrate uptake by ammonium was less in the ammonium-limited culture than in the cultures growing on nitrate, but invariant with growth rate in the nitrate-grown cultures. Below 1µM ammonium, nitrate uptake was often stimulated and rates exceeded those in the controls without ammonium. Ammonium uptake was not inhibited by the presence of nitrate.T. pseudonana fits the classical view of the interaction between nitrate and ammonium uptake in some respects, such as preference for ammonium, and inhibition of nitrate uptake by ammonium concentrations > 1µM. However, at ammonium concentrations typical of most marine environments, nitrate uptake occurs at rapid rates. In other respects, N uptake inT. pseudonana deviates from the classical view in the following ways: (1) stimulation of nitrate uptake by low concentrations of ammonium; (2) lack of inhibition of nitrate uptake by ammonium at low nitrate concentrations; and (3) variation in preference and inhibition with preconditioning, which is markedly different for other species. Because of the apparent enormous species variation in the interaction between nitrate and ammonium uptake and the lack of detailed information for a variety of species, it is difficult to generalize about the effect of ammonium on nitrate uptake, especially in the field, where prior N availability and species composition are not usually addressed.     

Biological and environmental influences on the trophic ecology of leatherback turtles in the northwest Atlantic Ocean

Understanding the causes and consequences of variability in trophic status is important for interpreting population dynamics and for identifying important habitats for protected species like marine turtles. In the northwest Atlantic Ocean, many leatherback turtles (Dermochelys coriacea) from distinct breeding stocks throughout the Wider Caribbean region migrate to Canadian waters seasonally to feed, but their trophic status during the migratory and breeding cycle and its implications have not yet been described. In this study, we used stable carbon and nitrogen isotope analyses of bulk skin to characterize the trophic status of leatherbacks in Atlantic Canadian waters by identifying trophic patterns among turtles and the factors influencing those patterns. δ¹⁵N values of adult males and females were significantly higher than those of turtles of unknown gender (i.e., presumed to be subadults), and δ¹⁵N increased significantly with body size. We found no significant differences among average stable isotope values of turtles according to breeding stock origin. Significant inter-annual variation in δ¹⁵N among cohorts probably reflects broad-scale oceanographic variability that drives fluctuations in stable isotope values of nutrient sources transferred through several trophic positions to leatherbacks, variation in baseline isotope values among different overwintering habitats used by leatherbacks, or a combination of both. Our results demonstrate that understanding effects of demographic and physiological factors, as well as oceanographic conditions, on trophic status is key to explaining observed patterns in population dynamics and for identifying important habitats for widely distributed, long-lived species like leatherbacks.