Herbivorous Pathways Research Articles

A new type of plankton food web functioning in coastal waters revealed by coupling Monte Carlo Markov chain linear inverse method and ecological network analysis

Abstract Plankton food webs (PFW) typology is based on different categories of functioning, according to the dominant processes and the role played by heterotrophic bacteria, small vs large phytoplankton, and small vs large zooplankton. Investigating the structure and the function of planktonic food webs in two SW Mediterranean waters (inshore and marine sites) at four seasons, using inverse (LIM-MCMC) and ecological network (ENA) analyses, we identified a new type of food web, called the “bacterial multivorous food web”. This food web adds to the conventional trophic continuum as previously reported. The “bacterial multivorous food web” present in winter showed the lowest primary production among seasons, but highest bacterial production. Several food web ratios characterized this new typology e.g. picophytoplankton net primary production to total primary production varied from 0.20 to 0.28; bacterial to primary production ratio is higher than values reported in global scale (≅1); bacterial net production to the potential protozoan prey net production was high (>0.2). In this special food web, carbon was mostly recycled, with a moderate fraction channeled to deep waters, which lead to a higher retention of carbon inside the ecosystem. This winter PFW also seemed to be the most organized, specialized, stable and mature, as related to common interpretations of ENA. The spring was characterized by herbivorous food web, with highest activity coinciding with low stability. Although less usual, the herbivorous pathway was also observed during summer, in inshore waters. The autumn food webs, which functioned as multivorous or microbial food webs, appeared to be stable and mature. Finally, our study demonstrates the usefulness of food web models derived ratios combined with ecological network analysis indices to conduct evaluation of the structure and functioning of ecosystems and potentially to support management decisions in marine environment.

Plankton food-web functioning in anthropogenically impacted coastal waters (SW Mediterranean Sea): An ecological network analysis

The study is the first attempt to (i) model spring food webs in three SW Mediterranean ecosystems which are under different anthropogenic pressures and (ii) to project the consequence of this stress on their function. Linear inverse models were built using the Monte Carlo method coupled with Markov Chains to characterize the food-web status of the Lagoon, the Channel (inshore waters under high eutrophication and chemical contamination) and the Bay of Bizerte (offshore waters under less anthropogenic pressure). Ecological network analysis was used for the description of structural and functional properties of each food web and for inter-ecosystem comparisons. Our results showed that more carbon was produced by phytoplankton in the inshore waters (966–1234 mg C m−2 d−1) compared to the Bay (727 mg C m−2 d−1). The total ecosystem carbon inputs into the three food webs was supported by high primary production, which was mainly due to >10 µm algae. However, the three carbon pathways were characterized by low detritivory and a high herbivory which was mainly assigned to protozooplankton. This latter was efficient in channelling biogenic carbon. In the Lagoon and the Channel, foods webs acted almost as a multivorous structure with a tendency towards herbivorous one, whereas in the Bay the herbivorous pathway was more dominant. Ecological indices revealed that the Lagoon and the Channel food webs/systems had high total system throughput and thus were more active than the Bay. The Bay food web, which had a high relative ascendency value, was more organized and specialized. This inter–ecosystem difference could be due to the varying levels of anthropogenic impact among sites. Indeed, the low value of Finn’s cycling index indicated that the three systems are disturbed, but the Lagoon and the Channel, with low average path lengths, appeared to be more stressed, as both sites have undergone higher chemical pollution and nutrient loading. This study shows that ecosystem models combined with ecological indices provide a powerful approach to detect change in environmental status and anthropogenic impacts.

Impact of biofilm resuspension on mesozooplankton in a shallow coastal ecosystem characterized by a bare intertidal mudflat

A prey–predator experimental setup was conducted in a shallow coastal ecosystem characterized by a bare intertidal mudflat to test if benthic biofilm resuspension causing microalgae inputs and carbon export toward nanoflagellates would favour the highest planktonic trophic level (i.e. mesozooplankton) when nutrient concentrations are high in the water column. Mesozooplankton predation and somatic production were studied by comparing the evolution of the prey assemblage (diversity and abundances) in the presence and absence of these predators during 24 h experiments. The results were then statistically analysed according to the cross-calculation method. Biofilm resuspension caused (i) a direct input of benthic microorganisms that had changed prey structure in term of diversity and/or size and (ii) a differential growth ability between prey taxa. Both reasons implied a bottom-up control on both micro- and mesozooplankton. The carbon export toward heterotrophic nanoflagellates favoured pelagic ciliate growth while mesozooplankton benefited from largest diatoms with high growth rates, both benthic and R-strategist pelagic species. Even if these microbial and herbivorous pathways are controlled by benthic inputs, they seemed to be totally disconnected since ciliates represented only a small part of mesozooplankton diet. The sensitivity of mesozooplankton production appeared species-dependent with the most tolerant taxa dominating the zooplankton assemblages. This suggests a role of the intensities and the frequencies of biofilm resuspension on the spatio-temporal structuring of mesozooplankton in macrotidal coastal ecosystems.

Soil energy pathways of different ecosystems using nematode trophic group analysis: a meta analysis

We analysed 67 raw data sets of nematode genera from three types of ecosystems (grassland, cropland, and forest) to compare relative magnitude of energy pathways through the soil food web. Bacterial-, fungal- and herbivorous-based energy pathways were compared by percentages (in either abundances or biomass) of three soil nematode trophic groups (i.e., bacterivore, fungivore and herbivore). The patterns of soil energy pathways were similar whether expressed as relative abundance or relative biomass. However, the percentage values of bacterivorous biomass in each type of ecosystem exceeded the percentage values of their abundance. Specifically, relative abundance of bacterivorous nematodes was similar among ecosystems but mean values of biomass were greatest in grassland and similarly less in cropland and forest ecosystems. By contrast, both relative abundance and biomass of fungivorous nematodes decreased progressively from forest to cropland and grassland ecosystems. The opposite pattern across ecosystems was observed for both relative abundance and biomass of herbivorous nematodes. We conclude that energy pathways are bacterial-dominated in all of the ecosystems whether expressed as abundance or biomass. Fungal and herbivorous pathways are second in dominance in forest and grassland ecosystems, respectively. The relative size of the fungal-based energy pathway suggests a gradient of resource quality among ecosystems. We suggest that herbivorous-based energy pathways are more important in grassland ecosystems than reported previously.

Seasonal variation of microzooplankton (20–200 μm) and its possible implications on the vertical carbon flux in the western Bay of Bengal

Stratification (throughout the year) and low solar radiation (during monsoon periods) have caused low chlorophyll a and primary production (seasonal average 13–18 mg m −2 and 242–265 mg C m −2 d −1, respectively) in the western Bay of Bengal (BoB). The microzooplankton (MZP) community of BoB was numerically dominated by heterotrophic dinoflagellates (HDS) followed by ciliates (CTS). The highest MZP abundance (average 665±226×10 4 m −2), biomass (average 260±145 mg C m −2) and species diversity (Shannon weaver index 2.8±0.42 for CTS and 2.6±0.35 for HDS) have occurred during the spring intermonsoon (SIM). This might be due to high abundance of smaller phytoplankton in the western BoB during SIM as a consequence of intense stratification and nitrate limitation (nitracline at 60 m depth). The strong stratification during SIM was biologically evidenced by intense blooms of Trichodesmium erythraeum and frequent Synechococcus–HDS associations. The high abundance of smaller phytoplankton favors microbial food webs where photosynthetic carbon is channeled to higher trophic levels through MZP. This causes less efficient transfer of primary organic carbon to higher trophic levels than through the traditional food web. The microbial food web dominant in the western BoB during SIM might be responsible for the lowest mesozooplankton biomass observed (average 223 mg C m −2). The long residence time of the organic carbon in the surface waters due to the active herbivorous pathways of the microbial food web could be a causative factor for the low vertical flux of biogenic carbon during SIM.

Carbon flows through a benthic food web: Integrating biomass, isotope and tracer data

The herbivorous, detrital and microbial pathways are major components of marine food webs. Although it is commonly recognized that these pathways can be linked in several ways, elucidating carbon transfers between or within these pathways remains a challenge. Intertidal flat communities are driven by a wide spectrum of organic matter sources that support these different pathways within the food web. Here we reconstruct carbon pathways using inverse analysis based on mass balancing, stable isotope signatures and tracer data. Data were available on biomass, total carbon production and processing, integrated diet information from stable isotope signatures and the transfer of recently produced carbon through the food web from an isotope tracer study. The integration of these data improved the quality of the inverse food web reconstruction considerably, as demonstrated explicitly by an uncertainty analysis. Deposition of detritus (detrital pathway) from the water column and subsequent assimilation and respiration by bacteria and to a lesser extent by microbenthos (microbial pathway) dominated the food web. Secondary production was dominated by bacteria (600 mg C m −2 d −1 ), but transfer to higher trophic levels was limited to 9% and most bacterial carbon was recycled back to dissolved organic carbon (DOC) and detritus. Microbenthos secondary production (77 mg C m −2 d −1 ) was supported by DOC (73%) and detritus (26%) and was entirely transferred up the food web. The higher trophic levels consisting of nematodes, meiobenthos (copepods, ostracods and foraminifera) and macrobenthos fed highly selectively and relied primarily on microphytobenthos and pelagic primary production (herbivorous pathway). Deposit feeding is a common feeding mode among these sediment dwelling fauna, but detritivory was negligible due to this selective feeding. This strong resource selectivity implies that the herbivorous and detrital-microbial pathways function more or less autonomously, with limited interaction.

Sensitivity of a pelagic ecosystem model to variations of process parameters within a realistic range

A one-dimensional physical–biological coupled model is constructed with reference to an oceanic area with a seasonal thermocline and to a pelagic trophic network dominated by a herbivorous pathway. The sensitivity of the model is then tested for the effect of parameter variations within the range actually encountered in the literature. In the reference simulation, the physical component of the model reproduces reasonably well the annual cycle of the hydrographic structure. The biological component simulates a cycle where the spring bloom is dependent on the critical depth criterion and yields, for the state variables and the fluxes, values which are consistent with available data. A comparison of the sensitivity tests within a realistic range (absolute sensitivity) is effected with the more commonly performed tests over a fixed proportion of variation in parameter values (relative sensitivity). The latter appear to essentially reflect the internal behaviour of the model and overestimate the sensitivity of the system to a number of parameters, especially those involved in complex relationships. The absolute sensitivity tests identify a restricted set of parameters with a major influence. Phytoplankton biomass and production, as well as the nitrate stock, are highly dependent on a small number of parameters, among which the most important are the coefficients of the photosynthetic curve and the sinking velocity of one class of particles. Zooplankton biomass and production, as well as the ammonium stock, show a more complex sensitivity to a larger number of parameters, among which the same as above, plus grazing parameters and excretion and remineralization rates. Overall, the fluxes within and from the system appear mostly dependent on the first step in the food web, and the latter, in turn, is mainly dependent on the physical forcing.

Simulated phytoplankton bloom input in top-down manipulated microcosms: comparative effect of zooflagellates, ciliates and copepods

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 09:137-147 (1995) - DOI: https://doi.org/10.3354/ame009137 Simulated phytoplankton bloom input in top-down manipulated microcosms: comparative effect of zooflagellates, ciliates and copepods Christaki U, Van Wambeke F Succession of microbial populations and carbon flow were studied experimentally in a 2-stage linked system where phytoplankton growth was separated from decay or consumption. Three phytoplankton loss processes were considered: bacterial lysis, protozoan grazing and mesozooplankton grazing. An axenic culture of the diatom Phaeodactylum tricornutum was transferred to 3 second-stage microcosms (150 l) kept in the dark. The first vessel (L: lysis batch) contained no herbivores and was top-limited by bacterivorous flagellates, the second (G: grazing batch) by ciliates and the third (Z: zooplankton batch) by copepods. In the L container there was no evidence of grazing on phytoplankton; the particle-associated protozoa were abundant (3.7 x 103 cell l-1). In the G vessel, ciliates (up to 96 x 103 cell l-1) controlled both the phytoplankton and nanoflagellate populations and ciliates ingested phytoplankton at rates from 1.7 to 16 phytoplankton cell ciliate-1 h-1 (average, 7.5 cell ciliate-1 h-1). The average growth yields for bacterivorous flagellates and ciliates were 35% and 45%, respectively. In the Z treatment, the concentration of &GT10 μm protozoa (able to ingest P. tricornutum) was apparently kept low due to copepod grazing pressure. Consequently, a considerable fraction of the phytoplankton was channelled through the detrital pathway instead of the classical herbivorous pathway. The general trend of the parameters describing microbial activity was Z &GT G &GT L. An influence of the complexity of the system on the turnover rate of organic matter was evident with the ratio 'cumulative biomass/POC' in the different vessels showing a lower level of detritus when predators of phytoplankton were present. Phytoplankton loss . Microbial food web . Top-down control . Carbon flow Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 09, No. 2. Publication date: August 31, 1995 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1995 Inter-Research.