Metabolic adaptation of Zostera marina (eelgrass) to diurnal periods of root anoxia

The temperate seagrass Zostera marina L. is common in coastal marine habitats characterized by the presence of reducing sediments. The roots of this seagrass grow in these anoxic sediments, yet eelgrass is highly productive. Through photosynthesis-dependent oxygen transport from leaves to roots, aerobic respiration is supported in eelgrass roots only during daylight; consequently, roots are subjected to diurnal periods of anoxia. Under anoxic root conditions, the amino acids alanine and -amino butyric acid accumulate within a few hours to account for 70% of the total amino acid pool, while glutamate and glutamine decline. Little ethanol is produced, and the pool size of the organic acid malate changes little or declines slowly. Upon the resumption of shoot photosynthesis and oxygen transport to the roots, the accumulated -amino butyric acid declines rapidly, glutamate and glutamine pools increase, and alanine declines over a 16-h period. These adaptive metabolic responses by eelgrass to diurnal root anoxia must contribute to the successful exploitation of shallow-water marine sediments that have excluded nearly all vascular plant groups. A metabolic scheme is presented that accounts for the observed changes in organic and amino acid pool sizes in response to anoxia.     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Adaptation of the polychaete worm Scoloplos armiger to hypoxic conditions

The anaerobic metabolism of the intertidal polychaete Scoloplos armiger, its recovery from anaerobiosis and the importance of anaerobic energy production during low tide in the field were investigated. Under anaerobic conditions S. armiger produces energy in the same manner as Arenicola marina, a prototype of an euryoxic invertebrate from the intertidal. Energy is produced from the phosphagen stores and from the breakdown of glycogen to volatile fatty acids, mainly propionate and to a lesser extend acetate. However, S. armiger cannot reduce its energy demand to the same degree as A. marina. This and the relatively small pool of glycogen may be the reason for its only moderate resistance to anoxia. The recovery from anaerobiosis proceeds in S. armiger significantly slower than in A. marina. S. armiger is able to maintain a fully aerobic metabolism down to a Pw_{O 2}of ca. 20 torr and even at a Pw_{O 2}of 10 torr a partly aerobic metabolism was retained. In the field during low tide S. armiger ascends into the oxidative layer, where it is able to maintain an aerobic metabolism even at parts without remaining puddels on the surface.     

Nitrogen accretion,and the nature and possible significance of N2 fixation (Acetylene reduction) in a Nova Scotian Spartina alterniflora stand

Total aboveground nitrogen accretion through the 1975 growing season at a short Spartina alterniflora stand was estimated as 78 kg N ha^-1, compared to estimated N₂ fixation on the mud surface of 22 kg N ha^-1 and subsurface N₂ fixation of 93 kg N ha^-1. Subsurface N accretion was estimated to be of the order of 77 kg N ha^-1. Mudsurface ARA (acetylene-reducing activity) exhibited a pronounced mid-season maximum, while subsurface ARA exhibited a general trend of increase from May to September, and then a decline as a function of falling temperature. Various experiments suggested that mud-surface ARA was associated largely with non-heterocystous blue-green algae and photosynthetic bacteria, while subsurface ARA was associated mainly with vital activity of S. alterniflora. Counts of various groups of bacteria indicated an enrichment of anaerobic (glucose-utilizing) and microaerophilic (malateutilizing) N₂-fixing bacteria in the rhizosphere in comparison to non-rhizosphere soil. Treatment of roots with chloramine-t for 2 h reduced total numbers (plate count), anaerobic N₂ fixers, and microaerophilic N₂ fixers by factors of 357, 172, and 22, respectively, suggesting a relative enrichment of microaerophiles in the interior or endorhizosphere of the roots. ARA of excised roots was correlated with ¹⁴C-activity for roots from a plant previously exposed to ¹⁴CO₂, and with branching and age of the roots.     

Nitrate reductase activity in Zostera marina

Eelgrass (Zostera marina L.) has access to nutrient pools in both the water column and sediments. We investigated the potential for eelgrass to utilize nitrate nitrogen by measuring nitrate reductase (NR) activity with an in vivo tissue assay. Optimal incubation media contained 60 mM nitrate, 100 mM phosphate, and 0.5% 1-propanol at pH 7.0. Leaves had significantly higher NR activity than roots (350 vs 50 nmoles NO ₂ ^– produced g FW^–1 h^–1). The effects of growing depth (0.8 m MLW, 1.2 m, 3.0 m, 5.0 m) and location within the eelgrass meadow (patch edge vs middle) on NR activity were examined using plants collected from three locations in the Woods Hole area, Massachusetts, USA, in July 1987. Neither depth nor position within the meadow appear to affect NR activity. Nitrate enrichment experiments (200 M NO ₃ ^– for 6 d) were conducted in the laboratory to determine if NR activity could be induced. Certain plants from shallow depth (1.2 m) showed a significant response to enrichment, with NR activity increasing from >100 up to 950 nmoles NO ₂ ^– g FW^–1 h^–1 over 6 d. It appears that Z. marina growing in very shallow water (0.8 m) near a shoreline may be affected by ground water or surface run-off enrichments, since plants from this area exhibited rates up to 1 600 nmol NO ₂ ^– g FW^–1 h^–1. Water samples from this location consistently had slightly higher NO ₃ ^– concentrations (1.4 M) than all other collection sites (0.7 M). Thus, it is possible that chronic run-off or localized groundwater inputs can create sufficient NO ₃ ^– enrichment in the water column to induce nitrate reductase activity in Zostera leaves.     

The association of N2-fixing bacteria with sea urchins

Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C₂H₂ reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6g N fixed g wet wt^-1 d^-1, with a yearly mean activity of 1.36 g N fixed g wet wt^-1 d^-1. Studies with ¹⁵N confirmed the C₂H₂ reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N₂ fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity.     

Metabolic and blood characteristics of the hydrothermal vent tube-worm Riftia pachyptila

Specimens of the hydrothermal vent pogonophoran Riftia pachyptila Jones were collected by submersible at a depth of 2 600 m at the 21°N hydrothermal vent site on the East Pacific Rise (20°50N, 109°06W) in April and May of 1982. The worms were maintained in pressurized aquaria for up to 45 d for metabolic studies. Consumption of O₂ was regulated down to low P_{O 2} (oxygen partial pressure) values; O₂ consumption rates were 0.63 and 1.12 mol g^-1 wet wt h^-1 at 2.5° and 8°C, respectively; such rates were comparable to those previously measured for other pogonophorans. Intact specimens of R. pachyptila (including bacterial symbionts) did not consume significant amounts of CH₄ from the environment. The respiratory quotients, in the absence of added sulfide, indicated that metabolism was mainly heterotrophic. High rates of uptake of dissolved amino acids were recorded for one specimen. The total [CO₂] in the vascular blood and the Hb-containing coelomic fluid were high. Under anaerobic conditions, there were equilibrium distributions of pH, total [CO₂] and sulfide concentrations between the vascular blood and the coelomic fluid, apparently because these metabolites were readily exchanged between the two compartments. The vascular blood bound neither CH₄ nor H₂. However, sulfide was reversibly bound by both the vascular blood and coelomic fluid; because this binding depended strongly on pH (with a maximum at about 7.5), HS^- was probably the molecular species bound. Under anaerobic, but not aerobic conditions, the trophosome bound substantial amount of sulfide; thus, the high concentrations of sulfide in the trophosome may have resulted mainly from sulfide bound to sulfide oxidases under anaerobic conditions. The coelomic fluid had a relatively low buffering capacity (2.2 mmol CO₂pH^-1).     

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 g g^-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.     

The composition and biosynthesis of lipids in Thysanoessa raschi from the Clyde Estuary,Scotland

The lipid composition and biosynthesising activity of Thysanoessa raschi collected from the Clyde Estuary, Scotland, in May 1981 were examined. Triacylglycerols were the major lipid class present, although 16.7% of the total lipid were wax esters in which phytol was the dominant fatty alcohol. The thoracic contents (hepatopancreas) of the krill were capable of biosynthesising lipids in vitro from various labelled substrates. Radioactivity from [1-¹⁴C] palmitic acid was incorporated into lipids in the order phospholipids>triacylglycerols>wax esters; the bulk of the radioactivity was present in all cases in the fatty acyl moieties of the lipids. [U-¹⁴C] glucose labelled lipids in the order phospholipids>triacylglycerols>free fatty acids> was esters; in the first two lipids the radioactivity was mainly in the glycerol moieties, whereas in was esters it was solely in the fatty acyl moieties. The extent of labelling of these lipids from [U-¹⁴C] alanine was less than that from [U-¹⁴C] glucose, but the pattern of labelling was generally similar. More than 90% of the radioactivity incorporated into total lipid from ³H₂O was present in free fatty acids from which it was calculated that the hepatopancreas of T. raschi can synthesise 2.5 g of fatty acid per hour at 15°C. This value is approximately three times lower than that previously determined for T. inermis from Balsfjorden, northern Norway. The results are discussed in terms of the sources of the dietary lipids of krill and the role of endogenous biosynthesis in contributing to its lipid reserves.     

Microbial control of organic carbon in marine sediments: Coupled chemoautotrophy and heterotrophy

The chemoautotrophic fixation of carbon dioxide is responsible for an appreciable component of the organic matter apparent as a carbon-rick peak at 40 cm sub-bottom in the marine muds of Halifax Harbour, Nova Scotia, Canada. Dissolved oxygen and sulfide profiles show that the 40 cm horizon represents a transitional environment from aerobic to underlying anaerobic conditions. A dissolved sulfate maximum and pH minimum at 40 cm indicate that sulfur-oxidizing chemoautotrophs are associated with the organic carbon production at this horizon. The stimulation of CO₂-fixation by thiosulfate and inhibition by anaerobic conditions, ammonia, nitrate and nitrite further support the contention that sulfur-oxidizers are primary producers at this horizon. Heterotrophic activity data show that both aerobic and anaerobic heterotrophs are active in the sediments. These data, in conjunction with the measurement of CO₂-fixation and the calculation of organic carbon accumulation over time, show that the peak of organic carbon observed is residual carbon that is not heterotrophically recycled to CO₂ at the 40 cm horizon.     

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities. Received: 23 June 1998 / Accepted: 19 November 1998     

Isolation,characterization and glucose metabolism of glycogen cells (=vesicular connective-tissue cells) from the labial palps of the marine mussel Mytilus edulis

A method has been developed to isolate glycogen cells (=vesicular connective tissue cells) from the labial palps of the marine mussel Mytilus edulis L. These cells have a modal density of 1.14 g ml^-1 and this property has been exploited to separate them from pronase-dissociated cell dispersions by density-gradient centrifugation. The isolated cells were mainly spherical, with a small peripheral nucleus. The quasitotality of the cell was filled with -like glycogen particles with cellular organelles being limited to the cell periphery. The major recipients of [U-¹⁴C] glucose-derived carbon were glycogen and amino acids. Only small quantities of ¹⁴C radiolabel were recovered from lipids, protein and CO₂. Glucose incorporation into glycogen increased linearly over 6 h and showed saturation kinetics with respect to exogenous glucose concentration.     

Genetic structure and gene flow of eelgrass <Emphasis Type="Italic">Zostera marina</Emphasis> populations in Tokyo Bay,Japan: implications for their restoration

Massive losses of eelgrass Zostera marina beds in Japan have occurred over the past 100 years. Toward their restoration, transplantation of eelgrass has been attempted in some areas, including Tokyo Bay. This study examined population genetic structures and gene flow in eelgrass in Tokyo Bay to establish guidelines for conducting restoration. Genotypes of a total of 360 individuals from 12 beds were determined using five microsatellite markers. The eelgrass beds in inner bay had above-average genetic diversity. A neighbor-joining tree based on F _ST values among beds revealed that a strong gene flow had occurred among six beds in the inner bay. Genetic assignment testing of drifting shoots indicated that those with seeds migrate in both directions between the inner and outer bay. We suggested that the restoration of eelgrass in the innermost part of Tokyo Bay, where natural habitats have been lost, should be conducted using the inner bay beds.     

Nitrogen fixation (acetylene reduction) associated with macroalgae in a coral-reef community in the Bahamas

N₂ fixation (C₂H₂ reduction) was associated with several species of macroalgae on a coral reef near Grand Bahama Island. The highest rates were associated with Microdictyon sp. (Chlorophyceae) and Dictyota sp. (Phaeophyceae). Extensive mats of filamentous blue-green algae, not heterotrophic bacteria, were the N₂ fixing agents: in experiments with samples of Microdictyon sp., the activity was lightdependent and not stimulated by organic compounds under either aerobic or anaerobic conditions. Assays in situ, at 20 m depth, and on shipboard, gave similar rates of N₂ fixation; the cyanophytes presumably have pigment adaptations to function in blue light. The maximum rate of N₂ fixation, associated with Microdictyon sp., was 3.8 g N fixed g dry weight^-1 h^-1. Coral-reef communities flourish in nutrientimpoverished waters, and therefore any input of nitrogen is probably important in stabilizing such ecosystems.     

Aerobic nitrogenase activity measured as acetylene reduction in the marine non-heterocystous cyanobacterium Trichodesmium spp. grown under artificial conditions

Aerobic nitrogenase activity in the marine non-heterocystous cyanobacterium Trichodesmium spp. NIBB 1067, isolated off the Izu Peninsula, Japan in 1983 and grown under artificial conditions, was assayed by the acetylene reduction method. This strain exhibited acetylene reduction activity under aerobic conditions when cells had been grown in the medium free of combined nitrogen. Activity was markedly enhanced by light, and dependent on the growth phase being higher during the exponential growth phase and lower during the late linear and stationary growth phases. Since typical colony formation occurred during the last growth phase, the present results contradict the idea that N₂-fixation depends on colony formation. The photosynthesis inhibitor DCMU at 10^-6 M inhibited light-dependent acetylene reduction completely. Acetylene reduction by Trichodesmium spp. was tolerant of O₂ as strongly as that in the heterocystous cyanobacteria. Even at a partial pressure of oxygen (p_{O 2}) of 3 atm, the activity still remained as high as half of the maximum. It was almost under anaerobic conditions. Maximum activity was obtained at pO₂ of ca. 0.1 atm.     

Nitrogen fixation in the rhizosphere of marine angiosperms

High rates of acetylene reduction were observed in systems containing excised rhizomes of the Caribbean marine angiosperms Thalassia testudinum, Syringodium filiforme and Diplanthera wrightii, and the temperate marine angiosperm Zostera marina. For 4 plant and plant-sediment systems the ratio of acetylene reduced/N₂ fixed varied from 2.6 to 4.6. For T. testudinum the estimated rates of nitrogen fixation are in agreement with estimated requirements of the plant for nitrogen. For a typical T. testudinum stand, N₂ fixation is estimated to be 100 to 500 kg N/hectare per year. Numbers of N₂-fixing bacteria in the rhizosphere sediments were roughly 50 to 300 times more abundant than those in the nonrhizosphere sediments, and in both types of sediments were of the same orders as the estimated numbers of heterotrophic aerobes.Canadian IBP Contribution No. 137.     

Inhibitory effect of the iron chelator desferrioxamine (Desferal) on the generation of activated oxygen species by Chattonella marina

Recent studies demonstrated that the toxic red tide phytoplankton Chattonella spp. produce activated oxygen species such as superoxide anion (O ₂ ^- ), hydrogen peroxide (H₂O₂), and hydroxyl radicals (·OH), which may be responsible for the toxicity of this flagellate. However, the mechanism behind the production of these oxygen radicals and H₂O₂ by Chattonella spp. is largely unknown, and the physiological significance of activated oxygen species for Chattonella spp. is also unclear. In the present study, we investigated the involvement of iron in the generation of O ₂ ^- and H₂O₂ by C. marina. The generation of O ₂ ^- by C. marina was related to the growth phase; the highest rate of O ₂ ^- production was observed during the exponential growth phase. However, no such increase during the exponential growth phase was observed in C. marina growing in an iron-deficient medium, even though the growth of C. marina was not significantly affected by iron-deficiency during the first 4 d. In addition, the iron chelator desferrioxamine (Desferal) strongly inhibited the generation of both O ₂ ^- and H₂O₂ by C. marina in a concentration-dependent manner. The growth of C. marina was also inhibited by Desferal. Furthermore, in the presence of 500 M Desferal, C. marina-induced growth inhibition of the marine bacteria Vibrio alginolyticus was almost completely abolished. These results suggest that iron is required for the generation of activated oxygen species by C. marina, as well as for its own growth.     

Use of an inert radioactive particle for measuring particle accumulation by filter-feeding bivalve molluscs

Because the rate of microbial degradation differs for the various sources contributing to the detrital pool in marine systems, their availability to detritivores might also vary. Carbon-14 tracer experiments were used to compare differences in the oxidation and net incorporation by the polychaete Capitella capitata of a nitrogenrich, easily-decomposable detritus derived from the red macrophytic algae Gracilaria sp. versus a nitrogen-poor, decay-resistant detritus derived from the eelgrass Zostera marina. The net incorporation of Gracilaria sp. detritus by C. capitata reached a maximum (91 g dry weight of detritus/mg dry weight of worm/day) after only 14 days of decomposition, whereas that of z. marina detritus equaled this level after 30 days of aging, but continued to increase to 375 g at 180 days. The oxidation rate of Gracilaria sp. detritus was consistently higher (peak of 61 mg dry weight of detritus/day at 30 day-aging) than z. marina detritus, which reached this level only after 180 days of aging. The presence or absence of C. capitata did not significantly alter the oxidation rate. The above difference might be attributed to a rapid exploitation and mineralization by bacteria of the more available Gracilaria sp. detritus, but a slow, controlled utilization by the microbes of the less available Z. marina detritus, especially during the early stage of decomposition. This would allow maximum exploitation of the substrate by macroconsumers, resulting in more of the detrital resource being tied up in detritivore biomass rather than being rapidly mineralized to CO₂. Difference in the length of aging at which various detrital sources become available to detritivores could result in a temporal partitioning of food resources and should be considered in attempting to understand the dynamics of detrital-based food chains.     

Disturbance by ice and life-history adaptations of the seagrassZostera marina

The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass     

13C label identifies eelgrass (Zostera marina) Carbon in an Alaskan estuarine food web

The food web of Izembek Lagoon, Alaska draws most of its carbon from eelgrass (Zostera marina) and phytoplankton. The¹³C:¹²C ratios of these primary producers are sufficiently different to enable their contributions to consumers to be estimated from consumer¹³C:¹²C ratios. Although the technique is conceptually simple, carbon inputs from other sources and isotope fractionations occurring in the food web limit its precision. Isotopic data nevertheless helps to establish the major carbon fluxes through the community and to assess the importance of eelgrass carbon to individual animals. It is particularly useful when dealing with detritus food chains, where direct observations of animal feeding habits are difficult to make. The Izembek community draws much of its carbon from eelgrass. Detritus food chains provide the major pathway for assimilation of eelgrass carbon by the community, but grazers are also important. Eelgrass carbon is more important to benthic animals than to the eelgrass epibiota and the fishes, which depend mainly on phytoplankton carbon.Publication No. 381 of the Institute of Marine Science, University of Alaska.