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Pelagic Food Web (pelagic + food_web)
Selected AbstractsImpact of the fish Garra on the ecology of reservoirs and the occurrence of Microcystis blooms in semi-arid tropical highlands: an experimental assessment using enclosuresFRESHWATER BIOLOGY, Issue 8 2009TADESSE DEJENIE Summary 1.,Many man-made reservoirs in the semi-arid highlands of Northern Ethiopia (Tigray) are characterised by the occurrence of intensive blooms of cyanobacteria and a dominance of small riverine fishes belonging to the genus Garra. 2.,We carried out enclosure experiments to test for the effect of these small fish on abiotic characteristics, phytoplankton biomass and zooplankton community structure in the pelagic of two reservoirs (Gereb Awso and Tsinkanet). Two experiments were carried out in each of the reservoirs, one at the end of the rainy season (highest water level) and one at the end of the dry season (lowest water level). 3.,The presence of Garra in general increased the amount of suspended matter, nutrient concentrations (total nitrogen and total phosphorus), phytoplankton and Microcystis biomass (including the proportion of Microcystis in the phytoplankton community), and reduced water transparency. The positive effect of the presence of Garra on nutrient concentrations and phytoplankton productivity indicate that Garra has the potential to affect food web functioning indirectly through bottom-up effects, by enhancing nutrient concentrations through sediment resuspension and excretion of nutrients. Indeed, population densities of the cladoceran zooplankton taxa Ceriodaphnia and Diaphanosoma also showed an overall increase in enclosures with Garra. 4.,However, our data also provide some evidence for a potential of Garra to exert top-down control on large bodied daphnids (Daphnia carinata, D. barbata), although such effect varied among experiments. The limited capability of Garra to control zooplankton communities mainly reflects the low efficiency of these small, riverine and benthos-oriented fish in foraging on zooplankton and suggests the existence of an unoccupied niche for zooplanktivorous fish in the majority of the reservoirs. 5.,Although the main effects of Garra on the pelagic food web seemed to be mediated by bottom-up mechanisms, our results also indicate that one of the key variables, the relative abundance of Microcystis, was impacted by Daphnia -mediated trophic cascade effects. [source] Population Density of the Crayfish, Orconectes limosus, in Relation to Fish and Macroinvertebrate Densities in a Small Mesotrophic Lake , Implications for the Lake's Food WebINTERNATIONAL REVIEW OF HYDROBIOLOGY, Issue 5-6 2005Susanne S. Haertel-Borer Abstract The population density of Orconectes limosus in a mesotrophic lake was assessed in the context of fish and macroinvertebrate biomasses, and crayfish consumption by fish. The average O.limosus (,6 cm total length) abundance and biomass in the littoral zone was 2200 ind ha,1 and 32.2 kg ha1, respectively. O.limosus biomass accounted for a large percentage (49%) of the lake's macroinvertebrate biomass. O.limosus was equal to 35% of the non-predatory fish biomass and to 81% of the predatory fish biomass. O.limosus comprised 15 and 48% of the annual consumption of pike and predatory perch, respectively. Altogether, O.limosus was identified as quantitatively important for the lake's littoral food web, and might also subsidize the pelagic food web. This strengthens the need for an integrated view on lake food webs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Growth and food availability of silver and bighead carps: evidence from stable isotope and gut content analysisAQUACULTURE RESEARCH, Issue 14 2009Qiong Zhou Abstract A 2-year investigation of growth and food availability of silver carp and bighead was carried out using stable isotope and gut content analysis in a large pen in Meiliang Bay of Lake Taihu, China. Both silver carp and bighead exhibited significantly higher ,13C in 2005 than in 2004, which can probably be attributed to two factors: (i) the difference between isotopic compositions at the base of the pelagic food web and (ii) the difference between the compositions of prey items and stable isotopes. The significantly positive correlations between body length, body weight and stable isotope ratios indicated that isotopic changes in silver carp and bighead resulted from the accumulation of biomass concomitant with rapid growth. Because of the drastic decrease in zooplankton in the diet in 2005, silver carp and bighead grew faster in 2004 than in 2005. Bighead carp showed a lower trophic level than silver carp in 2005 as indicated by stable nitrogen isotope ratios, which was possibly explained by the interspecific difference between the prey species and the food quality of silver carp and bighead. [source] On the different nature of top-down and bottom-up effects in pelagic food websFRESHWATER BIOLOGY, Issue 12 2002Z. Maciej Gliwicz SUMMARY 1.,Each individual planktonic plant or animal is exposed to the hazards of starvation and risk of predation, and each planktonic population is under the control of resource limitation from the bottom up (growth and reproduction) and by predation from the top down (mortality). While the bottom-up and top-down impacts are traditionally conceived as compatible with each other, field population-density data on two coexisting Daphnia species suggest that the nature of the two impacts is different. Rates of change, such as the rate of individual body growth, rate of reproduction, and each species' population growth rate, are controlled from the bottom up. State variables, such as biomass, individual body size and population density, are controlled from the top down and are fixed at a specific level regardless of the rate at which they are produced. 2.,According to the theory of functional responses, carnivorous and herbivorous predators react to prey density rather than to the rate at which prey are produced or reproduced. The predator's feeding rate (and thus the magnitude of its effect on prey density) should hence be regarded as a functional response to increasing resource concentration. 3.,The disparity between the bottom-up and top-down effects is also apparent in individual decision making, where a choice must be made between accepting the hazards of hunger and the risks of predation (lost calories versus loss of life). 4.,As long as top-down forces are effective, the disparity with bottom-up effects seems evident. In the absence of predation, however, all efforts of an individual become subordinate to the competition for resources. Biomass becomes limited from the bottom up as soon as the density of a superior competitor has increased to the carrying capacity of a given habitat. Such a shift in the importance of bottom-up control can be seen in zooplankton in habitats from which fish have been excluded. [source] THE MESOZOIC RADIATION OF EUKARYOTIC ALGAE: THE PORTABLE PLASTID HYPOTHESIS,JOURNAL OF PHYCOLOGY, Issue 2 2003Daniel Grzebyk Although all chloroplasts appear to have been derived from a common ancestor, a major schism occurred early in the evolution of eukaryotic algae that gave rise to red and green photoautotrophic lineages. In Paleozoic and earlier times, the fossil record suggests that oceanic eukaryotic phytoplankton were dominated by the green (chl b -containing) algal line. However, following the end-Permian extinction, a diverse group of eukaryotic phytoplankton evolved from secondary symbiotic associations in the red (chl c -containing) line and subsequently rose to ecological prominence. In the contemporary oceans, red eukaryotic phytoplankton taxa continue to dominate marine pelagic food webs, whereas the green line is relegated to comparatively minor ecological and biogeochemical roles. To help elucidate why the oceans are not dominated by green taxa, we analyzed and compared whole plastid genomes in both the red and green lineages. Our results suggest that whereas all algal plastids retain a core set of genes, red plastids retain a complementary set of genes that potentially confer more capacity to autonomously express proteins regulating oxygenic photosynthetic and energy transduction pathways. We hypothesize that specific gene losses in the primary endosymbiotic green plastid reduced its portability for subsequent symbiotic associations. This corollary of the plastid "enslavement" hypothesis may have limited subsequent evolutionary advances in the green lineage while simultaneously providing a competitive advantage to the red lineage. [source] | ||||||||||