Mussel isotope signature as indicator of nutrient pollution in a freshwater eutrophic lake: species, spatial, and seasonal variability

Stable nitrogen isotope ratios of five mussel species from littoral and pelagic areas were investigated with different trophic states in the eutrophic Lake Taihu, the third largest lake in China. Interpopulation variability for these mussels was relatively small in foot tissues because of the slow turnover time. Seasonal and spatial variations among the delta N-15 values of mussels might be due in part to the natural variation in delta N-15 values of potential food sources and the variation in the amount of human pollutions discharged into various locations of the lake. Although the increase of mussel delta N-15 values was accompanied by the increase of nutrient concentrations in most situations in this study, statistically significant correlations were only 22% of the total correlations in this survey, which might be attributed to the different time-scale variations in nutrient concentrations and isotope signatures and the unknown details of the trophic pathways and metabolism for incorporation of these nutrients

Icefish (Salangidae) as an indicator of anthropogenic pollution in freshwater systems using nitrogen isotope analysis

We investigated differences in delta N-15 of seston and icefishes from seven freshwater ecosystems with different trophic states in China. An increase of seston delta N-15 values was accompanied by an increase of total nitrogen and phosphorus concentrations. Significantly positive correlations were observed between delta N-15 of icefishes and delta N-15 of seston, total nitrogen and phosphorus concentrations. This study demonstrated that icefishes could be preferred indicators of anthropogenic contamination in test systems because they integrated waste inputs over long time periods and reflected the movement of waste through the pelagic food chain.We investigated differences in delta N-15 of seston and icefishes from seven freshwater ecosystems with different trophic states in China. An increase of seston delta N-15 values was accompanied by an increase of total nitrogen and phosphorus concentrations. Significantly positive correlations were observed between delta N-15 of icefishes and delta N-15 of seston, total nitrogen and phosphorus concentrations. This study demonstrated that icefishes could be preferred indicators of anthropogenic contamination in test systems because they integrated waste inputs over long time periods and reflected the movement of waste through the pelagic food chain

Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish

Few studies have examined how foraging niche shift of a predator over time cascade down to local prey communities. Here we examine patterns of temporal foraging niche shifts of a generalist predator (yellow catfish, Pelteobagrus fulvidraco) and the abundance of prey communities in a subtropical lake. We predicted that the nature of these interactions would have implications for patterns in diet shifts and growth of the predator. Our results show significant decreases in planktivory and benthivory from late spring to summer and autumn, whereas piscivory increased significantly from mid-summer until late autumn and also increased steadily with predator body length. The temporal dynamics in predator/prey ratios indicate that the predation pressure on zooplankton and zoobenthos decreased when the predation pressure on the prey fish and shrimps was high. Yellow catfish adjusted their foraging strategies to temporal changes in food availability, which is in agreement with optimal foraging theory. Meanwhile the decrease in planktivory and benthivory of yellow catfish enabled primary consumers, such as zooplankton and benthic invertebrates, to develop under low grazing pressure via trophic cascading effects in the local food web. Thus, yellow catfish shifts its foraging niche to intermediate consumers in the food web to benefit the energetic demand on growth and reproduction during summer, which in turn indirectly facilitate the primary consumers. In complex food webs, trophic interactions are usually expected to reduce the strength and penetrance of trophic cascades. However, our study demonstrates strong associations between foraging niche of piscivorous fish and abundance of prey. This relationship appeared to be an important factor in producing top-down effects on both benthic and planktonic food webs. Citation: Xu J, Wen Z, Gong Z, Zhang M, Xie P, et al. (2012) Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish. PLoS ONE 7(12): e49691. doi:10.1371/journal.pone.004969

Contrasting energy pathways at the community level as a consequence of regime shifts

Ecological regime shifts typically result in abrupt changes in ecosystem structure through several trophic levels, which leads to rapid ecosystem reconfiguration between regimes. An interesting aspect of the impact of regime shift is that alternative regimes may induce distinct shifts in energy pathways; these have been less tested than structural changes. This paper addresses this by using stable isotopes to establish the energy pathways in fish communities. We specifically focus on the impact of regime shift on changes of the energy pathways, and how the magnitude and direction of these changes affect the local community. We found that energy pathways significantly varied among the planktivorous, benthivorous, and piscivorous trophic guilds as a result of the alternative regimes. The regime shift from a clear to a turbid state altered the food web towards planktonic energy pathways and truncated food chain length, which is indicative of less ecological efficiency. This was confirmed by the adaptive foraging strategies of prevalent omnivores in the current communities. These structural and functional characteristics of trophic interactions might not facilitate classic trophic cascading effects in such a turbid regime and suppress the system’s response to environmental changes, e.g., nutrient loading, and restoration efforts in turbid to clear water regime shifts. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00442-013-2878-2) contains supplementary material, which is available to authorized users

Box-and-whisker plots of seasonal variation in predator/prey ratios between the predator, yellow catfish, and its prey, zooplankton (PPR_z, A), benthic invertebrates (PPR_b, B), forage fishes and shrimps (PPR_f, C), respectively.

<p>Upper and lower ends of boxes represent 75^th and 25^th percentiles. Whiskers represent 75^th and 25^th percentiles and open circles, outliners. The mean is depicted with a solid line. ANOVA indicated there were significant seasonal changes in PPR_z (<i>F</i>_7,16 = 2.9826, <i>P</i><0.05) and PPR_f (<i>F</i>_7,125 = 7.4025, <i>P</i><0.001), but not PPR_b (<i>F</i>_7,125 = 0.9975, <i>P</i> = 0.467). Different letters show the statistically significant differences indicated by a post hoc Tukey's test.</p

Box-and-whisker plots of seasonal variation in foraging niches of yellow catfish, including planktivory (A), benthivory (B) and piscivory (C).

<p>Upper and lower ends of boxes represent 75^th and 25^th percentiles. Whiskers represent 75^th and 25^th percentiles and open circles, outliners. The mean is depicted with a solid line. ANOVA indicated there were significant seasonal changes in planktivory (<i>F</i>_7,125 = 13.055, <i>P</i><0.001), benthivory (<i>F</i>_7,125 = 3.7291, <i>P</i><0.005) and piscivory (<i>F</i>_7,125 = 12.307, <i>P</i><0.005). Different letters show the statistically significant differences indicated by a post hoc Tukey's test.</p

Trends in foraging niches of yellow catfish along size gradient, including planktivory, benthivory and piscivory.

<p>LOWESS smoothing curves (the smoother span was set at 1) have been fitted to size series of yellow catfish. On a percentage basis, planktonivory of yellow catfish remained stable before the size of approximate 120 mm and decreased to a low level, and benthivory decreased and reached to a steady low level after the size of approximate 120 cm, while piscivory has been increasing steadily over the range of body length.</p

Seasonal abundance of prey of yellow catfish.

<p>Mean values are plotted with standard errors integrated over all sampling sizes.</p

Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish

Few studies have examined how foraging niche shift of a predator over time cascade down to local prey communities. Here we examine patterns of temporal foraging niche shifts of a generalist predator (yellow catfish, Pelteobagrus fulvidraco) and the abundance of prey communities in a subtropical lake. We predicted that the nature of these interactions would have implications for patterns in diet shifts and growth of the predator. Our results show significant decreases in planktivory and benthivory from late spring to summer and autumn, whereas piscivory increased significantly from mid-summer until late autumn and also increased steadily with predator body length. The temporal dynamics in predator/prey ratios indicate that the predation pressure on zooplankton and zoobenthos decreased when the predation pressure on the prey fish and shrimps was high. Yellow catfish adjusted their foraging strategies to temporal changes in food availability, which is in agreement with optimal foraging theory. Meanwhile the decrease in planktivory and benthivory of yellow catfish enabled primary consumers, such as zooplankton and benthic invertebrates, to develop under low grazing pressure via trophic cascading effects in the local food web. Thus, yellow catfish shifts its foraging niche to intermediate consumers in the food web to benefit the energetic demand on growth and reproduction during summer, which in turn indirectly facilitate the primary consumers. In complex food webs, trophic interactions are usually expected to reduce the strength and penetrance of trophic cascades. However, our study demonstrates strong associations between foraging niche of piscivorous fish and abundance of prey. This relationship appeared to be an important factor in producing top-down effects on both benthic and planktonic food webs. Citation: Xu J, Wen Z, Gong Z, Zhang M, Xie P, et al. (2012) Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish. PLoS ONE 7(12): e49691. doi:10.1371/journal.pone.0049691Few studies have examined how foraging niche shift of a predator over time cascade down to local prey communities. Here we examine patterns of temporal foraging niche shifts of a generalist predator (yellow catfish, Pelteobagrus fulvidraco) and the abundance of prey communities in a subtropical lake. We predicted that the nature of these interactions would have implications for patterns in diet shifts and growth of the predator. Our results show significant decreases in planktivory and benthivory from late spring to summer and autumn, whereas piscivory increased significantly from mid-summer until late autumn and also increased steadily with predator body length. The temporal dynamics in predator/prey ratios indicate that the predation pressure on zooplankton and zoobenthos decreased when the predation pressure on the prey fish and shrimps was high. Yellow catfish adjusted their foraging strategies to temporal changes in food availability, which is in agreement with optimal foraging theory. Meanwhile the decrease in planktivory and benthivory of yellow catfish enabled primary consumers, such as zooplankton and benthic invertebrates, to develop under low grazing pressure via trophic cascading effects in the local food web. Thus, yellow catfish shifts its foraging niche to intermediate consumers in the food web to benefit the energetic demand on growth and reproduction during summer, which in turn indirectly facilitate the primary consumers. In complex food webs, trophic interactions are usually expected to reduce the strength and penetrance of trophic cascades. However, our study demonstrates strong associations between foraging niche of piscivorous fish and abundance of prey. This relationship appeared to be an important factor in producing top-down effects on both benthic and planktonic food webs. Citation: Xu J, Wen Z, Gong Z, Zhang M, Xie P, et al. (2012) Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish. PLoS ONE 7(12): e49691. doi:10.1371/journal.pone.004969