Biomass Of Lower Trophic Levels Research Articles

Effects of vertical mixing on the Lake Michigan food web: an application of a linked end-to-end earth system model framework

AbstractPhysical processes may affect ecosystem structure and function through the accumulation, transport, and dispersal of organic and inorganic materials, nutrients, and organisms; they structure physical habitat and can influence predator–prey interactions and trophic production. In the Laurentian Great Lakes, horizontal currents generally dominate, but little is known about the effects of vertical mixing on lake food webs. We developed a linked earth system model and used it to explore how vertical mixing affects the productivity of Lake Michigan (LM), the world’s fifth-largest lake, whose food web and fisheries have been adversely affected by invasive Dreissena mussels. We hypothesized that higher vertical mixing would result in higher food web biomass by making phosphorus more available to the lower food web, and that filtration by invasive mussels would counter the effects of mixing and decrease food web biomass. Using linked climate, hydrodynamics, and ecosystem models, we projected the response of LM’s food web to scenarios of different levels of vertical mixing, with and without invasive mussels. Biomass of most functional food web groups increased with increases in vertical mixing, with the greatest increases in phytoplankton and zooplankton. Increased biomass was due to the replenishment of nutrients into the euphotic zone, which enhanced growth and biomass of lower trophic levels through bottom-up effects. However, filtration by invasive mussels reduced the positive effects of mixing for most species. Future applications of the linked earth system framework will explore the effects of climate warming and nutrient reduction on fisheries production to inform fisheries managers.

Biogeographic context dependence of trophic cascade strength in bromeliad food webs.

Ecosystem functions and the biomass of lower trophic levels are frequently controlled by predators. The strength of top-down control in these trophic cascades can be affected by the identity and diversity of predators, prey, and resources, as well as environmental conditions such as temperature, moisture, and nutrient loading, which can all impact interaction strength between trophic levels. Few studies have been able to replicate a complete community over a large geographic area to compare the full trophic cascade in a manipulative experiment. Here, we identify geographic dependency in trophic cascade strength, and the driving factors and specific mechanisms behind it, by combining geographically replicated experiments with a novel approach of community analogues of common garden and transplant experiments. We studied a predator-detritivore-detritus food web in bromeliads in Puerto Rico, Costa Rica, and Brazil. We found that interaction strengths between resources, consumers, and predators were strongly site-specific, but the exact mechanism differed between trophic levels. Large bodied predators created strong interaction strengths between predator and consumer trophic levels, reducing consumer abundance regardless of the geographic location, whereas small-bodied predators created weak interactions with no impact on consumer abundances in any site. In contrast, the interaction strength between consumers and resources varied among sites, depending on the dominant species of leaf detritus. More labile leaf species in Costa Rica created a strong consumer-resource interaction and therefore strong trophic cascade, whereas tougher leaf species in Brazil created a weak consumer-resource interaction, and an overall weaker trophic cascade. Our study highlights the importance of replicating experiments over geographic scales to understand general patterns of ecological processes.

Probopyrinella latreuticola parasite infestation frequencies in pelagic Sargassum-associated shrimp, Latreutes fucorum

Abstract Bopyrid isopod parasitic infestation of a variety of decapod definitive hosts is common worldwide. We report frequencies of a parasite infestation in the shrimp Latreutes fucorum associated with the pelagic macroalgae Sargassum in the Gulf of Mexico, Sargasso Sea and Eastern Caribbean. Average Probopyrinella latreuticola infestation frequency was 6.7% and did not significantly vary between regions. The presence of the ectoparasite appeared to impact fertility with only one infested individual found carrying eggs. In contrast, across all three regions, 13% (of n = 4001) of the non-infested shrimp were carrying eggs. With L. fucorum accounting for three quarters of lower trophic level biomass in the pelagic Sargassum-associated faunal community, parasite infestation may have negative consequences for ecologically and commercially important populations that rely directly or indirectly on the host as a food source.

Variation in phenology and density differentially affects predator-prey interactions between salamanders.

Variation in the timing of breeding (i.e., phenological variation) can affect species interactions and community structure, in part by shifting body size differences between species. Body size differences can be further altered by density-dependent competition, though synergistic effects of density and phenology on species interactions are rarely evaluated. We tested how field-realistic variation in phenology and density affected ringed salamander (Ambystoma annulatum) predation on spotted salamanders (Ambystoma maculatum), and whether these altered salamander dynamics resulted in trophic cascades. In outdoor mesocosms, we experimentally manipulated ringed salamander density (low/high) and breeding phenology (early/late) of both species. Ringed salamander body size at metamorphosis, development, and growth were reduced at higher densities, while delayed phenology increased hatchling size and larval development, but reduced relative growth rates. Survival of ringed salamanders was affected by the interactive effects of phenology and density. In contrast, spotted salamander growth, size at metamorphosis, and survival, as well as the biomass of lower trophic levels, were negatively affected primarily by ringed salamander density. In an additional mesocosm experiment, we isolated whether ringed salamanders could deplete shared resources prior to their interactions with spotted salamanders, but instead found direct interactions (e.g., predation) were the more likely mechanism by which ringed salamanders limited spotted salamanders. Overall, our results indicate the effects of phenological variability on fitness-related traits can be modified or superseded by differences in density dependence. Identifying such context dependencies will lead to greater insight into when phenological variation will likely alter species interactions.

The trophic structure of Spongosorites coralliophaga-coral rubble communities at two northeast Atlantic cold water coral reefs

ABSTRACTWe examined the isotopic signatures (δ13C, δ15N) of fauna living in association with the sponge Spongosorites coralliophaga colonizing coral rubble on cold-water coral reefs in the northeast Atlantic – the shallow inshore (122–131 m collection depth) Mingulay 01 area and the deep offshore (683–800 m) Logachev 02 mound. The δ15N signatures of suspended particulate organic matter and three primary consumers, i.e. Spongosorites coralliophaga, Reteporella beaniana and Parazoanthus anguicomus were used as trophic baselines and the resulting trophic structure was compared. In both regions four trophic levels were distinguished. However, the use of S. coralliophaga or R. beaniana as baselines resulted in a skewed trophic structure due to the enriched δ15Ν signatures of these two species on the Logachev 02 mound. Using suspended particulate organic matter and P. anguicomus as baselines, the Mingulay 01 area communities were characterized by elevated relative biomass of lower trophic levels compared to the Logachev 02 mound. Relative biomass of suspension/filter feeders was also higher at the Mingulay 01 area. The two regions differed significantly with regard to the prevailing environmental conditions: apart from the difference in depth and distance from shore, the Mingulay 01 area was characterized by higher primary production in surface waters, tight pelagic–benthic coupling and higher velocity of bottom currents, and it is hypothesized that these characteristics were the main drivers of the observed differences. This study highlighted that multiple trophic baselines can provide a better interpretation of food-web structure and that the use of sponges or bryozoans as baselines across bathymetric gradients should be avoided.

Population size-structure-dependent fitness and ecosystem consequences in Trinidadian guppies.

Decades of theory and recent empirical results have shown that evolutionary, population, community and ecosystem properties are the result of feedbacks between ecological and evolutionary processes. The vast majority of theory and empirical research on these eco-evolutionary feedbacks has focused on interactions among population size and mean traits of populations. However, numbers and mean traits represent only a fraction of the possible feedback dimensions. Populations of many organisms consist of different size classes that differ in their impact on the environment and each other. Moreover, rarely do we know the map of ecological pathways through which changes in numbers or size structure cause evolutionary change. The goal of this study was to test the role of size structure in eco-evolutionary feedbacks of Trinidadian guppies and to begin to build an eco-evolutionary map along this unexplored dimension. We used a factorial experiment in mesocosms wherein we crossed high- and low-predation guppy phenotypes with population size structure. We tested the ability of changes in size structure to generate selection on the demographic rates of guppies using an integral projection model (IPM). To understand how fitness differences among high- and low-predation phenotypes may be generated, we measured the response of the biomass of lower trophic levels and nutrient cycling to the different phenotype and size structure treatments. We found a significant interaction between guppy phenotype and the size structure treatments for absolute fitness. Size structure had a very large effect on invertebrate biomass in the mesocosms, but there was little or no effect of the phenotype. The effect of size structure on algal biomass depended on guppy phenotype, with no difference in algal biomass in populations with more, smaller guppies, but a large decrease in algal biomass in mesocosms with phenotypes adapted to low-predation risk. These results indicate an important role for size structure partially driving eco-evolutionary feedbacks in guppies. The changes in the ecosystem suggest that the absence of a steep decline in guppy fitness of the low-predation risk populations is likely due to higher consumption of algae when invertebrates are comparatively rare. Overall, these results demonstrate size structure as a possible dimension through which eco-evolutionary feedbacks may occur in natural populations.

Fading out of the trophic cascade at the base of the microbial food web caused by changes in the grazing community in mesocosm experiments

Abstract Biomanipulation of consumer populations can have strong top-down impacts on the composition and biomass of lower trophic levels. In this paper, we assess how changes in crustaceans’ biomass influence classical grazing and the microbial food web in an oligo-mesohumic, low-pH lake (Mazurian Lake District, Poland). Removal of mesozooplankton from the experimental mesocosms created a gradient of crustacean biomass resulting in the biomass increase of rotifers, phytoplankton and protozooplankton, while autotrophic eukaryotic picoplankton (eu-APP) and bacteria were not affected. The strongest modifications concerned the rotifer biomass and phytoplankton community structure. Our results imply that the trophic cascade generated in the experiment did not extend to bacteria and eu-APP.

Carbon-based balanced trophic structure and flows in the offshore Lake Ontario food web before (1987–1991) and after (2001–2005) invasion-induced ecosystem change

Replicate mass-balanced solutions to Ecopath models describing carbon-based trophic structures and flows were developed for the Lake Ontario offshore food web before and after invasion-induced disruption. The food webs link two pathways of energy and matter flow: the grazing chain (phytoplankton–zooplankton-fish) and the microbial loop (bacteria–protozoans) and include 19 species-groups and three detrital groups. Mass-balance was achieved by using constrained optimization techniques to randomly vary initial estimates of biomass and diet composition. After the invasion, production declined for all trophic levels and species-groups except Chinook salmon. The trophic level (TL) increased for smelt, adult sculpin, adult alewife and Chinook salmon. Changes to ecotrophic efficiencies indicate a reduction in phytoplankton grazing, increased predation pressure on Mysis, adult smelt and alewife and decreased predation pressure on protozoans. Specific resource to consumer TTE changed; increasing for protozoans (8.0–11.5%), Mysis (0.6–1.0%), and Chinook salmon (1.0–2.3%) and other salmonines (0.4–0.5%) and decreasing for zooplankton (20.2–15.1%), prey-fish (9.7–8.8%), and benthos (1.7–0.6%). Direct trophic influences of recent invasive species were low. The synchrony of the decline in PP and species-group production indicates strong bottom-up influence. Mass balance required an increase of two to threefold in lower trophic level biomass and production, confirming a previously observed paradoxical deficit in lower trophic level production. Analysis of food web changes suggest hypotheses that may apply to other similar large pelagic systems including, (1) as pelagic primary productivity declines, overgrazing of zooplankton results in an increase in protozoan production and a loss of trophic transfer efficiency, (2) habitat and food web changes increased Mysis predation on Diporeia and contributed to their recent decline, and (3) production of Chinook salmon, the primary piscivore, was uncoupled from pelagic production processes. This study demonstrates the value of food web models to better understand the impact of invasive species and to develop novel hypotheses concerning trophic influences.

Planktonic dispersal dampens temporal trophic cascades in pond metacommunities

Trophic cascades, in which changes in predation affect the biomass of lower trophic levels, vary substantially in strength and incidence. Most work to explain this variation has focused on local factors and has ignored larger regional effects. To study how metacommunity dynamics can alter trophic cascades, we constructed mesocosm metacommunities consisting of three pond communities with heterogeneous levels of fish predation and examined how planktonic dispersal rate (5-140% per week) affected biomass partitioning. Two of the three communities differed continually in the occurrence of fish and supported different but constant environments in a 'spatial trophic cascade,' while the third community supported temporally variable fish occurrence in a 'temporal trophic cascade.' We find that the presence, but the not the magnitude, of dispersal dampens temporal trophic cascades through an increase in grazer biomass. In contrast, dispersal has no effect on the strength of spatial cascades due to strong sorting pressures in the communities with constant presence or absence of fish as top predators.

Effect of Cadmium on a Stable, Large Volume, Laboratory Ecosystem Containing Daphnia and Phytoplankton

Stable 3400-L indoor Daphnia-phytoplankton ecosystems were exposed to cadmium to test the hypothesis that sublethal concentrations of toxicants cause an increase in the biomass of lower trophic levels and a decrease in the biomass of higher trophic levels. At 5 µg∙L−1 the Daphnia population collapsed after approximately 9 wk and chlorophyll levels increased to levels substantially greater than those observed in the control. At 15 µg∙L−1 the Daphnia population declined after approximately 5 wk. Chlorophyll levels remained low even after the Daphnia collapse. At concentrations of 1 µg∙L−1 and below, the ecosystem appeared to remain stable. The nature of the response of the ecosystem to 5 µg Cd∙L−1 was predicted correctly, but the magnitude of the response was not. A synergistic effect between cadmium toxicity and Daphnia population growth inhibition by high algal concentrations resulted in a much more dramatic response than predicted. A standard benchtop Daphnia bioassay could be used to accurately predict the concentration of cadmium below which no detrimental ecosystem effects are likely to occur; however, the degree of response cannot be predicted.

Current-topography interactions at mid-ocean seamounts and the impact on pelagic ecosystems

Ocean currents impinging on topographic obstacles such as seamounts create a high level of variability in mesoscale physical oceanography. In the N Pacific, for example, the structure of the Kuroshio and its extension differ significantly E and W of the Emperor Seamount chain, and eddy fields detected downstream may be attributed to seamount effects. Nearfield effects of seamounts have been theoretically predicted for several decades but only recently has theory been confirmed by observation. Taylor columns, quasi-stationary eddies over seamounts, alter flow patterns and thus have impacts on both benthos on seamounts and on the biota in water overlying the seamount. SE Hancock Seamount, located at the N end of the Hawaiian Ridge (29°47′N; 179°04′E), has a summit depth of 265 m. This seamount is located near the subtropical front and is at the southerly extent of productive seamounts where trawl fisheries have existed in the past. The pelagic ecosystem in the upper 200 m over the seamount clearly differs from waters at control stations at distances of 10's of kilometers away as shown by plankton and midwater trawl hauls and hydroacoustic transects conducted during 1984 and 1985. Over the seamount, hydroacoustic transects show a significantly higher biomass of scatterers as compared to control stations. Sampling these scattering layers with small midwater trawls demonstrates high densities of a resident micronekton fauna dominated by the sternoptychid fish “Maurolicus muelleri” and the mysid “Gnathophausia longispina”; these taxa were virtually absent from the control stations, were oceanic micronekton, particularly larger forms, were generally in higher abundance than at the seamount stations. Similarly, ichthyoplankton abundance differs above the seamount and at reference stations. In summer sampling, larval fishes were less abundant over the seamount whereas in winter the abundance was greater there. The differences in distribution and abundance of both micronekton and ichthyoplankton are significant and consistently observed, suggesting that physical or biological processes at the seamount have important effects on the pelagic ecosystem. Hypotheses concerning current — topography interactions, exclusion of vertical migrators, and predation by resident micronekton and fishes can be used to explain the observed effects. Seamounts and other areas of complex topography are frequently sites of highly productive ecosystems; the S Emperor and N Hawaiian Ridge seamounts provide a good example, with a catch of approximately one million tons of boarfish in ten years. The interaction of ocean currents and complex topography may play an important role in this high productivity, as demonstrated in the high biomass of lower trophic levels in the seamount ecosystem. Interannual variability in the latitudinal position of the subtropical front and the strength of current flow over these seamounts may result in significant differences in mesoscale physical oceanography and therefore in the productivity of these ecosystems.