Food Web Carbon Research Articles

Mammal communities of primeval forests as sentinels of global change.

Understanding the drivers and consequences of global environmental change is crucial to inform predictions of effects on ecosystems. We used the mammal community of Białowieża Forest, the last lowland near-primeval forest in temperate Europe, as a sentinel of global change. We analyzed changes in stable carbon (δ13 C) and nitrogen (δ15 N) isotope values of hair in 687 specimens from 50 mammal species across seven decades (1946-2011). We classified mammals into four taxonomic-dietary groups (herbivores, carnivores, insectivores, and bats). We found a significant negative trend in hair δ15 N for the mammal community, particularly strong for herbivores. This trend is consistent with temporal patterns in nitrogen deposition from (15 N depleted) industrial fertilizers and fossil fuel emissions. It is also in line with global-scale declines in δ15 N reported in forests and other unfertilized, non-urban terrestrial ecosystems and with local decreases in N foliar concentrations. The global depletion of 13 C content in atmospheric CO2 due to fossil fuel burning (Suess effect) was detected in all groups. After correcting for this effect, the hair δ13 C trend became non-significant for both community and groups, except for bats, which showed a strong decline in δ13 C. This could be related to an increase in the relative abundance of freshwater insects taken by bats or increased use of methane-derived carbon in food webs used by bats. This work is the first broad-scale and long-term mammal isotope ecology study in a near-primeval forest in temperate Europe. Mammal communities from natural forests represent a unique benchmark in global change research; investigating their isotopic temporal variation can help identify patterns and early detections of ecosystem changes and provide more comprehensive and integrative assessments than single species approaches.

Chemosynthetic and photosynthetic trophic support from cold seeps in Arctic benthic communities

Benthic communities below the photic zone are largely reliant on the export of surface-water primary production and the flux of partially degraded organic matter to the seabed, i.e. pelagic−benthic coupling. Over the past decades, however, the role of chemosynthetically produced carbon in food webs has been recognized in various habitats. Cold seeps are now known to be widespread across circumpolar Arctic shelves where natural release of hydrocarbons occurs at the seabed. Here, we investigated to what extent chemosynthesis-based carbon (CBC) enters the food web in a high latitude shelf-system. Specifically, we estimated the contributions of chemosynthesis-based carbon to primarily benthic invertebrate taxa from seeps at both shallow and deeper shelves and comparative non-seep areas in the Svalbard-Barents Sea region using bulk stable isotope-analysis of carbon and nitrogen. Our results show low δ13C values (-51.3 to -32.7 ‰) in chemosymbiotic siboglinids and several species of benthic, higher-trophic level, invertebrates (mainly polychaetes and echinoderms; -35.0 to -26.1‰) collected at cold seeps, consistent with assimilation of chemosynthesis-based carbon into the Arctic benthic food web. Using a two-component mixing equation, we demonstrate that certain species could derive more than 50% of their carbon from chemosynthesis-based carbon. These findings show that autochthonous chemosynthetic energy sources can contribute to supporting distinct groups of ‘background’ benthic taxa at these Arctic seep-habitats beyond microbial associations and chemosymbiotic species. Furthermore, we found a higher degree of chemosynthesis-based carbon in benthos at the deeper Barents Sea shelf seeps (>330 m) compared to seeps at the Western Svalbard shelf (<150 m water-depth), and we suggest this result reflects the differences in depth range, surface production and pelagic-benthic coupling. We detected large intra-species variations in carbon signatures within and across geographical locations and, combined with isotopic niche-analysis, our results show that certain taxa that inhabits seeps, have wider trophic niches in comparison to taxa inhabiting non-seeps. The increasing number of discovered natural seeps in the Arctic suggests that chemosynthetic production from seeps could play a more critical role in Arctic trophic structure than previously thought. Seep-derived carbon should, hence, be accounted for as an additional carbon source and included in food-web and energy-flow models in future work.

Trophic significance of microzooplankton to commercially important small pelagic fishes along the southwest coast of India.

Microzooplankton is an important component in the plankton food web transferring microbial food web carbon to the higher trophic levels, including fishes. This study investigates the role and significance of microzooplankton in the diet of three economically important small pelagic fishes along the southwest coast of India: Indian oil sardine (Sardinella longiceps), Indian mackerel (Rastrelliger kanagurta) and Commerson's anchovy (Stolephorous commersoni). The diet content of oil sardine was dominated by diatoms throughout the year with an increase in the presence of microzooplankton and copepods during the Northeast Monsoon. Phytoplankton, microzooplankton and copepods represented the diet of Indian mackerel throughout the year. Commerson's anchovy differed considerably in their diet composition from both Indian oil sardine and Indian mackerel as they chiefly depended on a carnivorous diet feeding on copepods, fish eggs, ostracods, lucifers and microzooplankton. The present study concludes that organic carbon from the microbial food web also, through microzooplankton, provides nutritional support to small pelagic fishes along the southwest coast of India but in varying degrees depending on the fish species as well the seasons.

Food Web Functions and Interactions During Spring and Summer in the Arctic Water Inflow Region: Investigated Through Inverse Modeling

We used inverse modelling to reconstruct major planktonic food web carbon flows in the Atlantic Water inflow, east and north of Svalbard during spring (18-25 May) and summer (9-13 August), 2014. The model was based on three intensively sampled stations during both periods, corresponding to early, peak, and decline phases of a Phaeocystis and diatom dominated bloom (May), and flagellates dominated post bloom stages (August). The food web carbon flows were driven by primary production (290 to 2850 mg C m-2 d-1), which was channeled through a network of planktonic compartments, and ultimately respired (180 to 1200 mg C m2 d-1), settled out of the euphotic zone as organic particles (145 to 530 mg C m-2 d-1), or accumulated in the water column in various organic pools. The accumulation of dissolved organic carbon was intense (1070 mg C m-2 d-1) during the early bloom stage, slowed down during the bloom peak (400 mg C m-2 d-1), and remained low during the rest of the season. The heterotrophic bacteria responded swiftly to the massive release of new DOC by high, but decreasing carbon assimilation rates (from 534 to 330 mg C m-2 d-1) in May. The net bacterial production was low during the early and peak bloom (26 to 31 mg C m-2 d-1) but increased in the late and post bloom phases (> 50 mg C m-2 d-1). The heterotrophic nanoflagellates did not respond predictably to the different bloom phases, with relatively modest carbon uptake, 30 to 170 mg C m2 d-1. In contrast, microzooplankton increased food intake from 160 to 380 mg C m2 d-1 during the buildup and decline phases, and highly variable carbon intake 46 to 624 mg C m2 d-1, during post bloom phases. Mesozooplankton had an initially high, but decreasing carbon uptake in May (220 to 48 mg C m-2 d-1), followed by highly variable carbon consumption during the post bloom stages (40 to 190 mg C m-2 d-1). Zooplankton shifted from herbivory (92-97% of total food intake) during the early bloom phase to a herbivorous, detritovorous and carnivorous mixed diet as the season progressed.

Exploring effects of dredging and organic waste on the functioning and the quantitative biomass structure of the Ems estuary food web by applying Input Method balancing in Ecological Network Analysis

Detailed food webs of the three main Ems estuary reaches (Lower Reaches/LR, Middle Reaches/MR and Dollard/DO) representing the 1975–1980 period, were used for explorations. The effects of turbidity and organic waste were explored for ∼1955 (waste loads and low suspended matter) and ∼2005 (no waste loads and high suspended matter) by applying the Input Method-based balancing as part of the Ecological Network Analysis (ENA) package to analyse food web carbon flows. The total phytoplankton primary production (sum of true phytoplankton and resuspended microphytobenthos) of the LR in 2005 is reduced by 56% compared to 1955. For MR and DO the primary production reduction over the same period is ∼70%. Expressed in tons carbon the 2005 total primary production (including microphytobenthos) of the entire estuary is 59% of that in 1955. In 2005 the total (all living compartments) estuarine biomass is 63% of that in 1955. The data suggest a further fast future biomass decline. Compared to 1955 the relative biomass changes of the living compartments are in 2005 −71.1% for LR, −66.7% for MR and −70% for the Dollard. Results are in line with independent monitoring. The biomass changes per species range for LR from −1.3% (microzooplankton) and −1.5% (intertidal bivalves) to −71.7% (pelagic bacteria), for MR from −15.3% (mesozooplankton, Sprattus sprattus, Clupea harengus) to −90.5% (pelagic bacteria) and for DO from −6.6% (Chelon labrosus) to −97.4% (pelagic bacteria). The relative mean total biomass changes caused by the sanitation of the organic waste loads are −1.5% for LR, −5.0% for MR and −12.5% for DO. The IM-based balancing results in an increasing ratio between detritivory and herbivory and increase of the carbon cycling. The effects are strongest for the LR. The described changes are driven by i) the combination of changed system boundary conditions and the species specific diets and ii) the absence of international political collaboration because of a disputed border that results in the absence of an internationally agreed integral solution via one master plan Ems estuary. The varying changes at the species level and the strong connection of pelagic and benthic systems in intertidal flat estuaries demand an integral approach for assessments and management.

Effects of experimental nitrogen fertilization on planktonic metabolism and CO2 flux in a hypereutrophic hardwater lake.

Hardwater lakes are common in human-dominated regions of the world and often experience pollution due to agricultural and urban effluent inputs of inorganic and organic nitrogen (N). Although these lakes are landscape hotspots for CO2 exchange and food web carbon (C) cycling, the effect of N enrichment on hardwater lake food web functioning and C cycling patterns remains unclear. Specifically, it is unknown if different eutrophication scenarios (e.g., modest non point vs. extreme point sources) yield consistent effects on auto- and heterotrophic C cycling, or how biotic responses interact with the inorganic C system to shape responses of air-water CO2 exchange. To address this uncertainty, we induced large metabolic gradients in the plankton community of a hypereutrophic hardwater Canadian prairie lake by adding N as urea (the most widely applied agricultural fertilizer) at loading rates of 0, 1, 3, 8 or 18 mg N L-1 week-1 to 3240-L, in-situ mesocosms. Over three separate 21-day experiments, all treatments of N dramatically increased phytoplankton biomass and gross primary production (GPP) two- to six-fold, but the effects of N on autotrophs plateaued at ~3 mg N L-1. Conversely, heterotrophic metabolism increased linearly with N fertilization over the full treatment range. In nearly all cases, N enhanced net planktonic uptake of dissolved inorganic carbon (DIC), and increased the rate of CO2 influx, while planktonic heterotrophy and CO2 production only occurred in the highest N treatments late in each experiment, and even in these cases, enclosures continued to in-gas CO2. Chemical effects on CO2 through calcite precipitation were also observed, but similarly did not change the direction of net CO2 flux. Taken together, these results demonstrate that atmospheric exchange of CO2 in eutrophic hardwater lakes remains sensitive to increasing N loading and eutrophication, and that even modest levels of N pollution are capable of enhancing autotrophy and CO2 in-gassing in P-rich lake ecosystems.

Trophic state changes can affect the importance of methane-derived carbon in aquatic food webs.

Methane-derived carbon, incorporated by methane-oxidizing bacteria, has been identified as a significant source of carbon in food webs of many lakes. By measuring the stable carbon isotopic composition (δ13C values) of particulate organic matter, Chironomidae and Daphnia spp. and their resting eggs (ephippia), we show that methane-derived carbon presently plays a relevant role in the food web of hypertrophic Lake De Waay, The Netherlands. Sediment geochemistry, diatom analyses and δ13C measurements of chironomid and Daphnia remains in the lake sediments indicate that oligotrophication and re-eutrophication of the lake during the twentieth century had a strong impact on in-lake oxygen availability. This, in turn, influenced the relevance of methane-derived carbon in the diet of aquatic invertebrates. Our results show that, contrary to expectations, methane-derived relative to photosynthetically produced organic carbon became more relevant for at least some invertebrates during periods with higher nutrient availability for algal growth, indicating a proportionally higher use of methane-derived carbon in the lake's food web during peak eutrophication phases. Contributions of methane-derived carbon to the diet of the investigated invertebrates are estimated to have ranged from 0-11% during the phase with the lowest nutrient availability to 13-20% during the peak eutrophication phase.

Copepod summer grazing and fecal pellet production along the Western Antarctic Peninsula

Copepods are important grazers on phytoplankton and contributors to carbon export, but their role is poorly understood in the Western Antarctic Peninsula (WAP), a region of high productivity and rapid climate warming. We conducted grazing and egestion experiments with large, dominant copepods each January from 2012 to 2014. We found higher gut evacuation rates (k), initial gut pigment and ingestion rates (I) for Calanus propinquus and Rhincalanus gigas compared with Calanoides acutus. Since k and I linearly increased with chlorophyll a for most species, ingestion rates were 4–70 times greater in more productive coastal regions than offshore, slope waters. Copepods have a low grazing impact on phytoplankton biomass (<1%) and productivity (1%, up to 11%) compared with the dominant WAP macro- and microzooplankton. Egestion rates were high (0.8–37.3 µgC ind.−1 day−1); however, ∼58% of fecal pellets are retained in the upper water column. Daily carbon rations of ∼1% indicated feeding on other carbon sources (protozoans and metazoans) to meet metabolic demands. However, during a coastal phytoplankton bloom, daily C rations increased to up to 13%, indicating increased reliance on phytoplankton. Future changes in the WAP plankton community may affect food web carbon flow and export.

Differences in carbon isotope signatures of polyunsaturated fatty acids of two microalgal species.

183 The study of fluxes of organic carbon in the bio sphere is one of the most relevant problems of modern ecology [1]. The paths of carbon in food webs in natu ral ecosystems are tracked by various methods based on the use of biochemical markers and stable isotopes as well as their combination, namely, the compound specific isotopic analysis (CSIA) that are biomarkers [2, 3]. However, the use of isotopic analysis of the key Differences in Carbon Isotope Signatures of Polyunsaturated Fatty Acids of Two Microalgal Species E. S. Kravchuka, N. N. Sushchika, O. N. Makhutovaa, M. Yu. Trusovaa, G. S. Kalachevaa, and M. I. Gladysheva, b Presented by Academician I.I. Gitel’zon June 2, 2014

Aquatic Food Web Structure and the Flow of Carbon

Abstract Carbon cycling is a cornerstone concept of ecosystem ecology, which has implications for climate change, ecosystem health, and human activities. This review investigates pathways of carbon within freshwater ecosystems, the role of terrestrial carbon in food webs, and the effects of food web structure on C emissions. Carbon may co-limit primary production even in waters super-saturated with CO2. Allochthonous carbon-subsidies make most lakes and rivers net heterotrophic; however, the use of carbon-subsidies by the food web (FW) may be limited by low nutritional quality of terrestrial C-compounds and the inability of bacteria to synthesise polyunsaturated fatty acids (PUFA), which are essential for metazoan growth. Bacterivorous nanoflagellates which can synthesise PUFA are likely to create a channel connecting allochthonous C with metazoan production in some water bodies. Published studies suggest that FW structure may affect: carbon fluxes in and out of lake ecosystems; carbon accumulation and di...

Temporal and spatial variation in ecosystem metabolism and food web carbon transfer in a wet‐dry tropical river

Summary1. High light availability and stable base flow during the dry season promote primary production in perennial rivers of the wet–dry tropics, in contrast to production during the wet season which is often limited by turbidity and scouring. The Mitchell River of northern Queensland (Australia) was studied to understand controls on aquatic production and respiration in the dry season in relation to spatial and temporal gradients of light and temperature.2. At three sites along the river, whole‐ecosystem gross primary production (GPP) and respiration (ER) were measured from diel changes of dissolved oxygen using the open‐channel single station method. Using stable carbon and nitrogen isotope analysis, aquatic consumers and their potential basal food resources were also assessed to determine food web relationships at the beginning and end of the dry season.3. Nutrient limitation of aquatic net primary production was implied from the oligotrophic conditions and high algal C:N ratios. Rates of GPP were comparable with other tropical and temperate rivers and were regulated by light availability.4. Respiration rates were high and similar to other tropical and subtropical rivers. Up to 52% of temporal variation of ER was explained by temperature, while P/R was lowest at the downstream site.5. Benthic algae were the major carbon source for primary and secondary benthic consumers (insects) in the dry season but not for higher consumers (fish and crustaceans). Despite high rates of ER, which were probably supported by decaying terrestrial C3 plant material, this carbon source was not identified as contributing to animal consumer biomass.6. While benthic algal production in the dry season sustained benthic invertebrates, the importance of external subsidies of carbon along the river, probably from the floodplain, was emphasised for fish and large invertebrates, which evidently were feeding on carbon sources not present in channel waterholes during the dry season.

The Use of Carbon and Nitrogen Stable Isotope Analysis to Characterize Food Web Changes in Aquatic Systems for Reclamation of Oil Sands Process-Affected Materials

Abstract One strategy for reclamation of oil sands leases in northern Alberta is the construction of lakes and wetlands by capping oil sands process-affected material (OSPM) with water. To assess this approach, experimental sites containing a range of OSPM have been constructed to monitor the evolution of the resulting aquatic habitats. Stable isotopes of carbon and nitrogen were used to assess the effects of OSPM on aquatic food webs. Carbon and nitrogen isotopic signatures of sediment, dissolved inorganic and organic carbon, particulate organic matter, periphyton, plants, plankton, aquatic invertebrates, and fish were used to assess differences related to the naphthenic acid (NA) concentration in OSPM and reference sites. NAs are a principal contaminant of concern in OSPM. Sites were grouped into low (0 to 4 mg/L), medium (4 to 15 mg/L), and high (&amp;gt;15 mg/L) NA concentrations. There were no significant differences in food web area or length among the three NA groupings. In most cases, carbon isotope analyses of samples from low, medium, and high NA concentration sites were not significantly different, suggesting that OSPM is not a significant contributor to food web carbon sources. Significant differences were found in nitrogen isotope signatures between low, medium, and high NA sites. Ammonia from OSPM is suggested as the main contributor to δ15N enrichment.

Invasive species impacts on ecosystem structure and function: A comparison of Oneida Lake, New York, USA, before and after zebra mussel invasion

Exotic species invasion is widely considered to affect ecosystem structure and function. Yet, few contemporary approaches can assess the effects of exotic species invasion at such an inclusive level. Our research presents one of the first attempts to examine the effects of an exotic species at the ecosystem level in a quantifiable manner. We used ecological network analysis (ENA) and a social network analysis (SNA) method called cohesion analysis to examine the effect of zebra mussel ( Dreissena polymorpha) invasion on the Oneida Lake, New York, USA, food web. We used ENA to quantify ecosystem function through an analysis of food web carbon transfer that explicitly incorporated flow over all food web paths (direct and indirect). The cohesion analysis assessed ecosystem structure through an organization of food web members into subgroups of strongly interacting predators and prey. Our analysis detected effects of zebra mussel invasion throughout the entire Oneida Lake food web, including changes in trophic flow efficiency (i.e., carbon flow among trophic levels) and alterations of food web organization (i.e., paths of carbon flow) and ecosystem activity (i.e., total carbon flow). ENA indicated that zebra mussels altered food web function by shunting carbon from pelagic to benthic pathways, increasing dissipative flow loss, and decreasing ecosystem activity. SNA revealed the strength of zebra mussel perturbation as evidenced by a reorganization of food web subgroup structure, with a decrease in importance of pelagic pathways, a concomitant rise of benthic pathways, and a reorganization of interactions between top predator fish. Together, these analyses allowed for a holistic understanding of the effects of zebra mussel invasion on the Oneida Lake food web.

Isotopic detection of recent photosynthate carbon flow into grassland rhizosphere fauna

In this study, we measured the incorporation of recent photosynthate-C inputs into active rhizosphere fauna (earthworms, enchytraeids, mites and collembolans) in an upland grassland soil under natural environmental conditions. This was achieved by means of a 13CO 2 pulse-chase experiment made during the growing season, followed by a 20-day dynamic sampling of soil fauna for 12C/ 13C analysis by IRMS. The effect of post- 13C labelling defoliation (cutting) on fauna 12C/ 13C ratios was also examined. Results showed that earthworms made up over 93% of the extracted fauna biomass, while mites, collembolans and enchytraeids together accounted for less than 7%. All fauna groups showed evidence of tracer 13C in their tissues within a week of 13CO 2 pulse labelling in both control and cut treatments. Cutting significantly increased the amount of tracer 13C entering the organisms ( P = 0.0002 ). Similarly, the fauna group also had a significant effect ( P = 0.0001 ). Time did not have any effect on fauna 13C content between groups as differences were only significant at the last sampling occasion. The interaction time×animal group, however, had a significant effect ( P = 0.0054 ). Collembolans accounted for most of the tracer 13C measured within the fauna biomass, i.e. mean±standard deviation of 44.78±12.75% and 44.29±14.69% of fauna 13C in control and cut treatments, respectively. Mites and earthworms contained between 22.13% and 28.45%, and enchytraeids less than 6% of the tracer 13C. We conclude that, during the growing season, there was a rapid incorporation of recent photosynthate-C into rhizosphere mesofauna. This carbon transfer was most significantly increased by defoliation in mites and collembolans ( P < 0.01 ). These results provide evidence that soil foodweb carbon dynamics are not solely underpinned by detrital decomposition but are also affected by short-term plant rhizodeposition patterns.

A preliminary analysis of the correlation of food-web characteristics with hydrology and nutrient gradients in the southern Everglades

We estimated trophic position and carbon source for three consumers (Florida gar, Lepisosteus platyrhincus; eastern mosquitofish, Gambusia holbrooki; and riverine grass shrimp, Palaemonetes paludosus) from 20 sites representing gradients of productivity and hydrological disturbance in the southern Florida Everglades, U.S.A. We characterized gross primary productivity at each site using light/dark bottle incubation and stem density of emergent vascular plants. We also documented nutrient availability as total phosphorus (TP) in floc and periphyton, and the density of small fishes. Hydrological disturbance was characterized as the time since a site was last dried and the average number of days per year the sites were inundated for the previous 10 years. Food-web attributes were estimated in both the wet and dry seasons by analysis of δ15N (trophic position) and δ13C (food-web carbon source) from 702 samples of aquatic consumers. An index of carbon source was derived from a two-member mixing model with Seminole ramshorn snails (Planorbella duryi) as a basal grazing consumer and scuds (amphipods Hyallela azteca) as a basal detritivore. Snails yielded carbon isotopic values similar to green algae and diatoms, while carbon values of scuds were similar to bulk periphyton and floc; carbon isotopic values of cyanobacteria were enriched in C13 compared to all consumers examined. A carbon source similar to scuds dominated at all but one study site, and though the relative contribution of scud-like and snail-like carbon sources was variable, there was no evidence that these contributions were a function of abiotic factors or season. Gar consistently displayed the highest estimated trophic position of the consumers studied, with mosquitofish feeding at a slightly lower level, and grass shrimp feeding at the lowest level. Trophic position was not correlated with any nutrient or productivity parameter, but did increase for grass shrimp and mosquitofish as the time following droughts increased. Trophic position of Florida gar was positively correlated with emergent plant stem density.

Food web structure and biogeochemical processes during oceanic phytoplankton blooms: An inverse model analysis

Abstract The relationship between food web structure and function across two ocean biomes was investigated using an inverse method to recover solutions of food web carbon flows. We estimated the carbon exchanges between major assemblages within plankton food webs in the North Atlantic, using the JGOFS NABE data set (1989) and near the Western Antarctic Peninsula (WAP), using the Palmer Station LTER data set, two areas exhibiting strong seasonal phytoplankton blooms. The recovery of all the potential flows of carbon allowed a system-level analysis, providing insight to processes that are seldom measured in the field and a means of comparing food webs from different regions. In the NABE food web, the dominant carbon flows involved the microorganisms including bacterial carbon demand and grazing by microzooplankton and protozoans. In the WAP food web, krill had the most significant carbon flows including grazing of large phytoplankton and respiration. A comparison between the NABE and the WAP carbon-based food webs showed key differences. Recycling and the activity of the microbial food web were much greater in the NABE food web than in the WAP. However in the WAP inverse solution, the microbial food web was just as important as the classical food web (diatoms to krill to penguins) that is traditionally believed to dominate carbon flows. Carbon flows through the NABE and WAP regions were more highly dependent on recycling than would be anticipated from the size structure of the primary producers, when analyzed using a classification scheme of Legendre, L., Rassoulzadegan, F [1996. Food-web mediated export of biogenic carbon in oceans: hydrodynamic control. Marine Ecology Progress Series 145, 179–193].

The influence of climate on mechanistic pathways that affect lower food web production in Northern San Francisco Bay estuary

Significant coherence among time series of environmental and biological production variables suggested mechanistic pathways through which climate contributed to the downward shift in estuarine production (biomass) in northern San Francisco Bay estuary, 1975–1993. Climate directly and indirectly affected physical processes in the estuary through precipitation and its subsequent impact on streamflow and physical variables affected by streamflow. Climate also directly influenced air temperature and wind velocity. The influence if climate was evaluated through a climate index based on sea level pressure. A shift in this climate index in the early 1980s coincided with changes in many environmental variables including water transparency, water temperature, wind velocity, and rainfall. These physical changes were accompanied by a decrease in diatom, total zooplankton, andNeomysis mercedis carbon at the base of the food web throughout the estuary. Box-Jenkins time series coherence analysis was used to quantify associations among these physical, chemical, and biological time series for nine regions of the estuary. These associations were used to develop a conceptual model of mechanistic pathways that directly linked food web carbon production to climate. Strong coherence among diatom, zooplankton, andN. mercedis carbon time series suggested climate also had an indirect impact on food web production through trophic cascade. Differing mechanistic pathways among the nine regions of the estuary suggested climate was an important contributor to the spatial variability in total food web production and trophic structure.

Dominance of autochthonous autotrophic carbon in food webs of heterotrophic rivers

This paper addresses the river heterotrophy paradox, “How can animal biomass within riverine food webs be fueled primarily by autochthonous autotrophic production if the ecosystem as a whole is heterotrophic?”. Reviewed, stable isotope data from tropical, temperate, and arctic rivers provide evidence consistent with the revised riverine productivity model (RPM): “The primary, annual energy source supporting overall metazoan production and species diversity in mid‐ to higher‐trophic levels of most rivers (≥4th order) is autochthonous primary production entering food webs via algal‐grazer and decomposer pathways”. The revised RPM does not conflict with the heterotrophy paradox because: (a) the decomposer (microbial loop) food pathway processes most of the transported, allochthonous and autochthonous carbon and, with algal respiration in some cases, is primarily responsible for a river's heterotrophic state (P/R&lt;1); but (b) biomass production of mid‐ to higher‐trophic levels is principally supported by an algal‐grazer (phytoplankton and benthic microalgae) pathway that is only weakly linked to the decomposer pathway. The reason the algal‐grazer pathway supports the majority of metazoan biomass is that allochthonous carbon is mostly recalcitrant, whereas carbon from autochthonous primary production, though much less plentiful, is commonly more labile (easier to assimilate), contains more energy per unit mass, and is typically preferred by metazoa.

Impact of allochthonous organic carbon on microbial food web carbon dynamics and structure in Lake Örträsket

Bacterial utilization of allochthonous organic carbon and the subsequent fate of the bacterial production in the pelagic food web were estimated in Lake Ortrasket, a large humic lake in northern Sw ...

Organic Matter Dynamics in the Ogeechee River, a Blackwater River in Georgia, USA

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