Fuel Food Webs Research Articles

Bottom-up processes drive isotopic variation in the South American sea lion Otaria flavescens across a 2300 km latitudinal gradient

Spatial differences in the isotope values of widely distributed marine apex consumers may reflect geographical differences in the isotopic composition of basal resources (e.g., phytoplankton) fueling food webs (bottom-up effects) or spatial differences in the trophic ecology of the taxon of interest (top-down effects). We examined spatial variation in δ13C and δ15N values from 264 South American sea lions (SASL, Otaria flavescens) of different age classes (adults, subadults and juveniles), their putative prey consisting of pelagic and benthic coastal fishes, and particulate organic matter (POM) measured from locations situated across >2300 km of the Chilean coast (between 18°42′ and 39°17′ S). We used generalized least squares (GLS) models to compare the form of the relationship between δ13C and δ15N and latitude between the three functional groups. Our results show that SASL from northern, central, and southern areas were isotopically distinct, with individuals from the north having lower δ13C and higher δ15N values in comparison to individuals from the south. When the relationship for each functional group was modelled individually using GLS, results indicated that for each degree of increasing latitude δ15N decreased on average by 0.12‰ (POM), 0.15‰ (prey), and 0.14‰ (SASL), while δ13C increased by 0.06‰ (POM) and 0.05‰ in both prey and SASL. We suggest that the latitudinal differences observed in SASL δ13C and δ15N values reflect baseline isotopic variation rather than marked differences in trophic ecology of these widely distributed consumers.

Stream food webs in tropical mountains rely on allochthonous carbon regardless of land use

The relative importance of allochthonous and autochthonous carbon (C) as sources of energy for tropical stream food webs remains an open question. Allochthonous C might be the main energy source for small and shaded forest streams, while autochthonous C is more likely to fuel food webs draining land uses with less dense vegetation. We studied food webs in cloud forest streams draining watersheds with forests, coffee plantations, and pastures. Our goal was to assess the effects of those land uses on the C source and structure of stream food webs. The study took place in tropical montane streams in La Antigua Watershed, in eastern Mexico. We selected three streams per land use and sampled biofilm and leaf litter as the main food resources, and macroinvertebrates and aquatic vertebrates from different trophic guilds. Samples were analyzed for δ13C and δ15N isotopes. Using a Bayesian mixing model, we estimated the proportional assimilation of autochthonous and allochthonous carbon by each guild. We found that consumers were mostly using allochthonous C in all streams, regardless of watershed land use. Our findings indicate that montane cloud forest streams are dominated by allochthony even in watersheds dominated by pastures. Abundant precipitation in this life zone might facilitate the movement of allochthonous C into streams. While food webs of streams from coffee plantations and pastures also rely on allochthonous resources, other impacts do result in important changes in stream functioning.

Emigration patterns of motile cryptofauna and their implications for trophic functioning in coral reefs.

Patterns of movement of marine species can reflect strategies of reproduction and dispersal, species' interactions, trophodynamics, and susceptibility to change, and thus critically inform how we manage populations and ecosystems. On coral reefs, the density and diversity of metazoan taxa are greatest in dead coral and rubble, which are suggested to fuel food webs from the bottom up. Yet, biomass and secondary productivity in rubble is predominantly available in some of the smallest individuals, limiting how accessible this energy is to higher trophic levels. We address the bioavailability of motile coral reef cryptofauna based on small-scale patterns of emigration in rubble. We deployed modified RUbble Biodiversity Samplers (RUBS) and emergence traps in a shallow rubble patch at Heron Island, Great Barrier Reef, to detect community-level differences in the directional influx of motile cryptofauna under five habitat accessibility regimes. The mean density (0.13-4.5 ind cm-3) and biomass (0.14-5.2 mg cm-3) of cryptofauna were high and varied depending on microhabitat accessibility. Emergent zooplankton represented a distinct community (dominated by the Appendicularia and Calanoida) with the lowest density and biomass, indicating constraints on nocturnal resource availability. Mean cryptofauna density and biomass were greatest when interstitial access within rubble was blocked, driven by the rapid proliferation of small harpacticoid copepods from the rubble surface, leading to trophic simplification. Individuals with high biomass (e.g., decapods, gobies, and echinoderms) were greatest when interstitial access within rubble was unrestricted. Treatments with a closed rubble surface did not differ from those completely open, suggesting that top-down predation does not diminish rubble-derived resources. Our results show that conspecific cues and species' interactions (e.g., competition and predation) within rubble are most critical in shaping ecological outcomes within the cryptobiome. These findings have implications for prey accessibility through trophic and community size structuring in rubble, which may become increasingly relevant as benthic reef complexity shifts in the Anthropocene.

Primary production by phytoplankton in the territorial seas of the Republic of Korea

The primary production (PP) by phytoplankton in marine ecosystems is essential for carbon cycling and fueling food webs. Hence, estimating the PP in the territorial sea of each country is a necessary step to achieving carbon neutrality. To estimate the PP in the territorial sea of the Republic of Korea from 2005 to 2021, we analyzed various physiochemical parameters, such as sea surface temperature (SST), Secchi depth, and concentrations of chlorophyll-a and nutrients in the seas of five regions, including the East Sea, West Sea, western South Sea, eastern South Sea, and the waters off Jeju Island. During the 17-year study period, the SST tended to increase, while the nutrient concentrations declined, except in the Jeju area. Overall, the PP did not show a specific temporal trend, but daily PP in the western South Sea was the highest among the five regions. Moreover, the maximum PP in the Korean territorial waters (76,450 km2 ) was estimated at 11,227 Gg C y-1, which accounts for 0.03% of the global PP. The results may give insights into a better understanding of the PP, further resource utilization, and environmental sustainability in the studied region.

Geochemistry of Coastal Permafrost and Erosion-Driven Organic Matter Fluxes to the Beaufort Sea Near Drew Point, Alaska

Accelerating erosion of the Alaska Beaufort Sea coast is increasing inputs of organic matter from land to the Arctic Ocean, and improved estimates of organic matter stocks in eroding coastal permafrost are needed to assess their mobilization rates under contemporary conditions. We collected three permafrost cores (4.5–7.5 m long) along a geomorphic gradient near Drew Point, Alaska, where recent erosion rates average 17.2 m year−1. Down-core patterns indicate that organic-rich soils and lacustrine sediments (12–45% total organic carbon; TOC) in the active layer and upper permafrost accumulated during the Holocene. Deeper permafrost (below 3 m elevation) mainly consists of Late Pleistocene marine sediments with lower organic matter content (∼1% TOC), lower C:N ratios, and higher δ13C values. Radiocarbon-based estimates of organic carbon accumulation rates were 11.3 ± 3.6 g TOC m−2 year−1 during the Holocene and 0.5 ± 0.1 g TOC m−2 year−1 during the Late Pleistocene (12–38 kyr BP). Within relict marine sediments, porewater salinities increased with depth. Elevated salinity near sea level (∼20–37 in thawed samples) inhibited freezing despite year-round temperatures below 0°C. We used organic matter stock estimates from the cores in combination with remote sensing time-series data to estimate carbon fluxes for a 9 km stretch of coastline near Drew Point. Erosional fluxes of TOC averaged 1,369 kg C m−1 year−1 during the 21st century (2002–2018), nearly doubling the average flux of the previous half-century (1955–2002). Our estimate of the 21st century erosional TOC flux year−1 from this 9 km coastline (12,318 metric tons C year−1) is similar to the annual TOC flux from the Kuparuk River, which drains a 8,107 km2 area east of Drew Point and ranks as the third largest river on the North Slope of Alaska. Total nitrogen fluxes via coastal erosion at Drew Point were also quantified, and were similar to those from the Kuparuk River. This study emphasizes that coastal erosion represents a significant pathway for carbon and nitrogen trapped in permafrost to enter modern biogeochemical cycles, where it may fuel food webs and greenhouse gas emissions in the marine environment.

Assessing the Effect of Rubber (Hevea brasiliensis (Willd. ex A. Juss.) Muell. Arg.) Leaf Chemical Composition on Some Soil Properties of Differently Aged Rubber Tree Plantations

Leaf litter plays a major role in carbon and nutrient cycling, as well as in fueling food webs. The chemical composition of a leaf may directly and indirectly influence decomposition rates by influencing rates of biological reactions and by influencing the accumulation of soil organic carbon content, respectively. This study aimed to assess the impact of the chemical composition of rubber (Hevea brasiliensis (Willd. ex A. Juss.) Muell. Arg.) leaves on various soil properties of different ages of rubber (4–5, 11–12, and 22–23 year-old). Synchrotron-based Fourier transform infrared microspectroscopy (Sr-FTIR) was utilized for analyzing the chemical composition of plant leaves. The Sr-FTIR bands illustrated that the epidermis of rubber leaves from 4–5-year-old trees was found to contain a high quantity of polysaccharides while mesophyll from 22–23-year-old trees had a large number of polysaccharides. The change in soil properties in the older rubber plantation could be attributed to its chemical composition. The change in soil properties across all tree ages, i.e., increased litter and organic carbon content, was a relatively strong driver of soil biota evolution. The aliphatic of C-H in the leaves showed high correlation with soil organic carbon (SOC) and permanganate-oxidizable C (POXC) from 22–23 year-old trees. This study shows the differences in the organic chemical composition of leaves that are consequential to soil organic carbon.

Trophic basis of production in tropical headwater streams, Puerto Rico: an assessment of the importance of allochthonous resources in fueling food webs

The relative importance of allochthonous and autochthonous resources in fueling tropical headwater streams remains an open topic. We combined estimates of secondary production and assessment of its trophic basis to determine which resources were responsible for animal production. We studied benthic insect assemblages in two streams in the Luquillo Experimental Forest, Puerto Rico. Habitat-weighted production estimates were similar in both streams (528.5 and 591.5 mg m−2 year−1), but production was over twice as high in pool versus riffle habitats. The mayfly Neohagenulus (Leptophlebiidae) was a major contributor to total production (259.1 and 352.2 mg m−2 year−1). All taxa relied heavily on amorphous detritus and plant tissue. Aquatic insect production was similar to that reported for shrimp assemblages in the same study area, but low relative to temperate region estimates. The trophic basis of production appears to be allochthonous organic matter, which agrees with the small size and closed canopy cover over the study streams. This is the first study quantifying the production and trophic basis of the non-shrimp macroinvertebrate assemblage in tropical island streams. We also provide support for the importance of riparian vegetation as the main energy sources for stream tropical stream food webs.

Diversity of Viruses Infecting Eukaryotic Algae.

Algae are photosynthetic organisms that drive aquatic ecosystems, e.g. fuelling food webs or forming harmful blooms. The discovery of viruses that infect eukaryotic algae has raised many questions about their influence on aquatic primary production and their role in algal ecology and evolution. Although the full extent of algal virus diversity is still being discovered, this review summarizes current knowledge of this topic. Where possible, formal taxonomic classifications are referenced from the International Committee on Taxonomy of Viruses (ICTV); since the pace of virus discovery has far surpassed the rate of formal classification, however, numerous unclassified viruses are discussed along with their classified relatives. In total, we recognized 61 distinct algal virus taxa with highly variable morphologies that include dsDNA, ssDNA, dsRNA, and ssRNA genomes ranging from approximately 4.4 to 560 kb, with virion sizes from approximately 20 to 210nm in diameter. These viruses infect a broad range of algae and, although there are a few exceptions, they are generally lytic and highly species or strain specific. Dedicated research efforts have led to the appreciation of algal viruses as diverse, dynamic, and ecologically important members of the biosphere, and future investigations will continue to reveal the full extent of their diversity and impact.

Organic matter resources fuelling food webs in a human-modified lowland river: importance of habitat and season

The role of hydromorphological degradation and temporal variation for food webs in human-modified rivers is still not fully evaluated. We tested the hypothesis that man-made engineering structures alter macroinvertebrate resource use in the Elbe River (Germany) in relation to seasonal variation. Stable isotopes (δ13C, δ15N) and mixing models revealed that dietary contributions of benthic organic matter (BOM) and phytoplankton were driven by engineering structure. Contributions of biofilm were driven by season, while contributions of terrestrial particulate organic matter (t-POM) were driven by both engineering structure and season. Contributions of t-POM were larger than those of phytoplankton in spring and summer, but not in autumn, which adds to the debate about the sources of organic matter fuelling riverine benthic food webs. Resource availability was not systematically related to resource use, indicating that factors other than resource limitation were responsible for the observed results. By demonstrating that human alterations determine consumer resource use independently from resource availability, our study links hydromorphological modifications to fluxes of matter in riverine food webs. Future studies should quantify organic matter fluxes from various autochthonous and allochthonous pathways in human-modified and natural rivers to allow for a robust synthesis of how hydromorphological modifications alter benthic food webs.

Facilitation by ecosystem engineers enhances nutrient effects in an intertidal system

AbstractEcosystem engineering research has recently demonstrated the fundamental importance of non‐trophic interactions for food‐web structure. Particularly, by creating benign conditions in stressful environments, ecosystem engineers create hot beds of elevated levels of recruitment, growth, and survival of associated organisms; this should fuel food webs and promote production on the ecosystem scale. However, there is still limited empirical evidence of the influence of non‐trophic interactions on the classical food‐web processes that determine energy transfer, that is, consumer–resource interactions. On the basis of a biomanipulation experiment covering 600 m2 of an intertidal flat, we show that ecosystem engineers influence resource uptake efficiency and the accumulation of algae following nutrient enrichment in a soft‐sediment food web. Nutrient additions increased chlorophyll a concentrations in the sediment by 90%, but only in plots where we also introduced high densities (2000 per m2) of a burrowing bivalve, the common cockle Cerastoderma edule. The artificial cockle beds increased the nutrient uptake efficiency of the biofilm and promoted sediment accumulation, which suggests that the cockles facilitated the sediment‐living algae by increasing sediment stability. This indicates that ecological interactions, rather than the availability of nutrients per se, set the limits for production in this coastal ecosystem. Our results emphasize the need to include facilitation theory and recognize that positive interactions between species are key to understand, manage, and restore ecosystems under human influence.

Riparian forest modifies fuelling sources for stream food webs but not food-chain length in lowland streams of Denmark

Several studies have shown that the origin of carbon fuelling food webs in streams depends on riparian cover type. In forested stream sites allochthonous resources fuel food webs, whereas autochthonous resources support biomass in grassland (open-canopy) stream sites. However, some studies suggest that autochthonous carbon (of highest quality) is preferentially assimilated regardless of riparian cover and that the food-chain length (FCL) may be larger in grassland than in forested sites. We used stable isotopes of carbon and nitrogen in adjacent grassland and forested reaches to compare the contribution of autochthonous vs. allochthonous resources to the biomass of the whole macroinvertebrate assemblage and to the most abundant taxa. Moreover, we compared the FCL between forested and grassland sites by estimating the trophic position of brown trout, Salmo trutta. Autochthonous support to macroinvertebrate biomass was higher in grassland than in forested sites, often changing from a dominantly autochthonous to an allochthonous-generated biomass from grassland to forested. This held true for the whole macroinvertebrate assemblage and for specific species. FCL remained similar between reach types. Our study suggests that autochthonous resources are assimilated to a higher extent when their availability increases with canopy openness but allochthonous carbon sustain macroinvertebrate biomass in forested reaches.

Relationships between estuarine modification and leaf litter decomposition vary with latitude

Decomposition of leaf litter plays a major role in carbon and nutrient cycling and in fuelling food webs. Environmental conditions may influence decomposition rates directly by influencing rates of biological reactions, and indirectly by influencing consumer communities. This study assessed how climate influences the relationship between estuarine nutrient enrichment and decomposition. Eelgrass (Zostera muelleri) and mangrove (Avicennia marina) leaves from a common source were deployed within two estuaries with anthropogenically enhanced and two with largely unmodified nutrient loadings, at each of two temperate latitudes of New South Wales, Australia. The latitudes were separated by ∼1000 km and differed in mean water temperature by ∼2 °C. Mass loss of litter was assessed after 13, 27 and 65 days, and the invertebrate communities colonizing the bags were censused at 13 and 65 days. At the end of the experiment, mass loss of seagrass blades was greater in nutrient enriched estuaries of the lower latitude than in higher latitude estuaries or those that were relatively unmodified. Mass loss of mangrove leaves was greater in the low latitude or nutrient enriched estuaries than in the largely unmodified estuaries of the higher latitude. Aspects of the abiotic environment, in particular temperature and phosphorous loading, were key correlates of litter mass loss, with only weak relationships between mass loss and macroinvertebrate communities. These results suggest that the sensitivity of decomposition processes to estuarine modification may be dependent on climatic setting, and the dominant litter resources present. Manipulative experiments are now needed to confirm the environmental conditions under which nutrient impacts on decomposition are exacerbated.

Isotopic variability in a stream longitudinal gradient: implications for trophic ecology

Stable isotope analysis (SIA) constitutes a powerful tool for evaluating trophic ecology. However, natural spatio-temporal isotopic variability may complicate the application of this approach in rivers. In this study, we analysed different sources of variability and potential limitations in the use of SIA along the Tea Stream (NW Iberian Peninsula). Gut content analyses (GCA), δ13C and δ15N ratios for major food web components were evaluated in replicate samples collected at six sites along the mid-downstream gradient of the stream in 2010, 2006 and 2007. In this study, we tested (1) isotopic changes on each basal resource and consumer across sites (spatial variability), (2) specific isotopic patterns on the different food web components within each studied site (within-site variability), (3) isotopic consistency through years (temporal variability), and (4) isotopic differences due to fish size (ontogenetic variability). We found higher variability for δ13C than for δ15N ratios in major food web components along the studied gradient. Additionally, autochthonous basal sources exhibited higher variability than allochthonous ones. A clear decreasing pattern in the δ13C ratios of biofilm was observed towards the stream mouth, which was also reflected in both primary and secondary consumers. The high parallelism between biofilm isotopic patterns and those of higher trophic levels stress the importance of exploring the relevance of this resource fuelling food webs along the Tea Stream. Moreover, the importance of combining GCA and SIA was confirmed, allowing a better interpretation of fish trophic interactions in the stream. Our results proved that a site-specific study perspective (considering longitudinal variability) would be more appropriate when evaluating food web issues in greater detail for the Tea Stream.

Diet‐specific biomarkers show that high‐quality phytoplankton fuels herbivorous zooplankton in large boreal lakes

Summary The zooplankton is a key link in the transfer of energy from primary producers up through aquatic food webs. Previous efforts to quantify the importance of basal resources to aquatic consumers have used stable isotopes (SI) and simple ternary models, including only ‘bulk’ phytoplankton, bacteria or terrestrial particulate organic matter (t‐POM). We used a novel Bayesian mixing model based on fatty acids (FA) to quantify the dietary assimilation of seven basal resources, including five phytoplankton groups, pelagic bacteria and t‐POM, to Cladocera in large boreal lakes in Finland. To account for trophic enrichment of FA from the diet to consumers, we parameterised the model with a resource library, from many feeding trials, consisting of Daphnia magna fed 22 diverse basal taxa. The results of the feeding trials show that the distinctive FA profiles of algal groups are transferred to consumers. Moreover, the large number of FA variables (n = 22) used in the model avoids the limitations of underdetermined mixing problems, common to SI modelling, in cases when the number of resources outnumbers the tracer variables. We show that cladocerans were generally supported by phytoplankton (86–94%), with little use of t‐POM (1–9%) and bacteria (1–3%). Cladocerans used primarily high‐quality phytoplankton (cryptophytes, diatoms and dinoflagellates) in both summer (51 ± 22%) and autumn (79 ± 12%), and the relative importance of medium‐quality resources (cyanobacteria, chlorophytes and chrysophytes) declined from 37 ± 23% in the summer to 8 ± 2% in the autumn. High‐quality resources, rich in essential biochemical compounds, are critical in fuelling food webs in large lakes, even those with high concentrations of allochthonous organic matter.

Multiple fish predators: effects of identity, density, and nutrients on lower trophic levels

The ongoing loss of predators is chang- ing the composition of food webs, with largely un - known consequences. In particular, the effects of multiple fish predators on food webs are difficult to predict due to the prevalence of omnivory and intra - guild predation. At the same time, many ecosystems experience high nutrient loads that fuel food webs from the bottom up. To test the combined effects of both multiple predators and nutrient enrichment on food web composition, we created monocultures of perch, roach, and stickleback as well as mixed assem- blages of all 3 species at 3 different density levels using an additive design, and induced nutrient en - richment in half of the cages. The biomass of inverte- brate herbivores and algae was measured. Stomach analyses of predators were used to detect prey switching. Herbivore biomass depended mainly on predator identity, while top-down effects on algae were mediated by predator density and nutrient en - richment. Specifically, perch strongly reduced amphi - pods and isopods, and roach mainly reduced gas- tropods but also isopods, while stickleback had weak overall effects on herbivores. These species-specific effects were attenuated in the mixed fish assem- blages, probably due to prey switching. Algal growth strongly increased under high fish density and nutri- ent enrichment, but was not affected by predator- induced changes in the herbivore composition. This study shows that identity effects from predators in isolation are attenuated in multiple predator assem- blages, probably due to increasing interference among the predators. Algal biomass was enhanced by nutrient enrichment and high predator density, which affected algae probably through non-lethal effects, such as reduced activity of the herbivores.

Dynamic stability in dry season food webs within tropical floodplain rivers

Debate exists about the effects of hydrological variation on food web dynamics and the relative importance of different sources of organic carbon fuelling food webs in floodplain rivers. Stable carbon isotope analyses and ecological stoichiometry were used to determine the basal sources in dry season macroinvertebrate food webs in two floodplain river systems of Australia’s wet–dry tropics that have contrasting flow regimes. Algae, associated with phytoplankton and biofilm, were the primary food source, potentially contributing >55% organic carbon to the biomass of a wide range of primary and secondary consumers. However, many consumers assimilated other sources in addition to algae, e.g. detritus from local C3 riparian vegetation. Food webs were characterised by substantial flexibility in the number and types of sources identified as important, which was indicative of generalist feeding strategies. These findings suggest ‘dynamic stability’ in the food webs, which imparts resilience against natural disturbances like flow regime seasonality and variation in hydrological connectivity. This adaptation may be characteristic of macroinvertebrate assemblages in highly seasonal river systems or in those with high levels of flow variability.

Evidence for limited microbial transfer of methane in a planktonic food web

Methane-derived carbon may be an important, but overlooked source of energy fueling food webs in a variety of aquatic ecosystems. Although it is commonly assumed that the flow of methane-derived carbon is regulated by aquatic invertebrate consumption of methane-oxidizing bacteria (MOB), few studies have characterized this trophic interaction. We used stable isotope analysis, bioassay experiments, and PCR-based molecular techniques to investigate the interactions between Daphnia and MOB in the pelagic region of a humic lake located in southwestern Michigan, USA. We observed moderate depletion of 13 C in the plankton, but these data alone could not provide evidence for the consumption of MOB by Daphnia. Using quantitative PCR, we determined that MOB attained relatively high densities in the water column (3% of the bacterial community), but we found no evidence that they were grazed upon by Daphnia. Moreover, our results do not support the hypothesis that Daphnia harbored symbiotic MOB. Therefore, the isotopic composition of Daphnia could not be explained by direct trophic interactions with MOB, suggesting the potential importance of indirect trophic interactions (e.g. consumption of MOB-feeding protists) or other processes that alter the isotopic composition of zooplankton resources (e.g. CO2 recycling).

Impact of the rhizosphere on soil microarthropods in agroecosystems on the Georgia piedmont

We pulse-labeled corn ( Zea mays) and weed plants with photosynthetically fixed Carbon-14 to investigate the importance of the rhizosphere as a food source for soil microarthropod food webs. In field samples, we followed the movement of 14C from plant shoots to roots to microarthropods. In conventionally tilled (CT) agroecosystems, the soil microarthropods accumulated radioactive tracer and reached concentrations as high as those in roots. In no-tillage (NT) agroecosystems, radioisotope concentrations in microarthropods were not as high. The CT systems lack surface organic litter, an alternate food base for microarthropods. Results suggest that feeding in the rhizosphere is more important in CT systems. Weeds transferred higher concentrations of tracer into the rhizosphere than did crop plants, suggesting that weeds in CT systems may be important in fueling food webs.

Primary production and macro-detritus dynamics in a European salt marsh: carbon and nitrogen budgets

Primary production and the detritus pathway (i.e., detritus production, litter fall, export to coastal waters, and decomposition) were studied over a 1-year period in three salt marsh levels (low, middle and high marshes). Carbon and nitrogen content in biomass and macro-detritus were estimated at each step of the detrital pathway. Several lines of evidence suggested that low, middle and high marshes had different properties which affected their interaction with adjoining waters. More than species diversity, the salt marsh zonation across a 1.8 km transect resulted in a spatial and temporal heterogeneity in the distribution of organic matter and nutrients within the salt marsh. The low marsh was the less productive level (1080 g dry weight m −2 per year) and was a source of organic matter for the two other marsh levels. Almost 89% of the organic matter produced in the low marsh was flushed away by the tide and redistributed inside the marsh, whereas the remainder decomposed quickly (0.023 per day) at the production site. The export of macro-detritus to coastal waters was probably insignificant to the carbon and nitrogen balance of the salt marsh. In the middle and high marshes, most of the production fell as litter directly at the production site. In the high marsh, vegetation stands were very productive (1990 g dry weight m −2 per year) and litter decomposed slowly (0.0047 per day). The middle marsh was as productive as the high marsh (1910 g dry weight m −2 per day), but the decay rate (0.028 per day) was similar to that estimated for the low marsh. This study demonstrates that the mineralization of halophyte-derived organic matter takes place almost completely within the marsh itself. Data on carbon and nitrogen decay indicated that only a small part of the high marsh production was rapidly available to fuel food webs of the ecosystem or to be exported to coastal waters. In contrast, the high amount of biomass produced in the middle marsh was rapidly available for food webs. Export of organic matter may be controlled by macro-detritus production (amount and timing), intensity and time distribution of tide events, and, mostly, decay rates. Because for most of the year the marsh is exposed to the atmosphere, we suggest that exchanges of gaseous carbon and nitrogen with the atmosphere should be investigated.