Adaptations in a hierarchical food web of southeastern Lake Michigan

Two issues in ecological network theory are: (1) how to construct an ecological network model and (2) how do entire networks (as opposed to individual species) adapt to changing conditions? We present a novel method for constructing an ecological network model for the food web of southeastern Lake Michigan (USA) and we identify changes in key system properties that are large relative to their uncertainty as this ecological network adapts fromone time point to a second time point in response to multiple perturbations. To construct our foodweb for southeastern Lake Michigan,we followed the list of seven recommendations outlined in Cohen et al. [Cohen, J.E., et al., 1993.Improving foodwebs. Ecology 74, 252–258] for improving food webs. We explored two inter-related extensions of hierarchical system theory with our food web; the first one was that subsystems react to perturbations independently in the short-term and the second onewas that a system’s properties change at a slower rate than its subsystems’ properties. We used Shannon’s equations to provide quantitative versions of the basic food web properties: number of prey, number of predators, number of feeding links, and connectance (or density).We then compared these properties between the two time-periods by developing distributions of each property for each time period that took uncertainty about the property into account.We compared these distributions, and concluded that non-overlapping distributions indicated changes in these properties that were large relative to their uncertainty. Two subsystems were identified within our food web system structure (p \u3c 0.001). One subsystem had more non-overlapping distributions in food web properties between Time 1 and Time 2 than the other subsystem. The overall system had all overlapping distributions in food web properties between Time 1 and Time 2. These results supported both extensions of hierarchical systems theory. Interestingly, the subsystemwithmore non-overlapping distributions in foodweb propertieswas the subsystemthat contained primarily benthic taxa, contrary to expectations that the identifiedmajor perturbations (lower phosphorous inputs and invasive species) would more greatly affect the subsystem containing primarily pelagic taxa. Future food-web research shouldemploy rigorous statistical analysis and incorporate uncertainty in food web properties for a better understanding of how ecological networks adapt

Changes in the nearshore and offshore zooplankton communities of southeastern Lake Michigan

Master of ScienceNatural Resources and EnvironmentUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/114497/1/39015052046854.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/114497/2/39015052046854.pd

Winter storms: Sequential sediment traps record Daphnia ephippial production, resuspension, and sediment interactions

For species that do not over-winter, an essential part of the life cycle is the production of diapausing eggs. We use sequential sediment traps in southern Lake Michigan (1997-2000) to capture settling Daphnia ephippia and sediments, characterizing ephippial production and sediment interactions during hazardous conditions. Each year, there was an enormous pulse of D. mendotae ephippia (ca. 7.2 × 1013 ephippia, 1.2 × 1014 diapausing eggs) that coincided with autumn population decline. Most ephippia settled through the water column, although a few were captured at the water surface and blown shoreward. The duration and amplitude of the autumn ephippial fluxes were similar among years. Ephippial production was positively correlated with water column depth, a consequence of Daphnia spatial abundance. In contrast, resuspension of ephippia was inversely related to water column depth and spatially complex, influenced by waves, coastal currents, and offshore gyre circulation. Large winter storms created nearshore sediment plumes, could resuspend vast numbers of ephippia (e.g., 10 March 1998 storm; 1.9 × 1012 ephippia and 3.1 × 1012 diapausing eggs), and were important in the formation of egg banks. Almost all newly produced and resuspended diapausing eggs came from D. mendotae, with very few from two other species (D. retrocurva, D. dentifera) that dominated waters 12 yr ago. These observations suggest a relatively short relaxation time for species cycling out of egg banks in Lake Michigan ( \u3c 10 yr), due in part to (1) differential resuspension of unconsolidated versus consolidated sediments and (2) the spatially restricted nature of high-sedimentation zones. Our study is the first use of sequential sediment traps to document diapause egg production

Adaptations in a hierarchical food web of southeastern Lake Michigan

Two issues in ecological network theory are: (1) how to construct an ecological network model and (2) how do entire networks (as opposed to individual species) adapt to changing conditions? We present a novel method for constructing an ecological network model for the food web of southeastern Lake Michigan (USA) and we identify changes in key system properties that are large relative to their uncertainty as this ecological network adapts fromone time point to a second time point in response to multiple perturbations. To construct our foodweb for southeastern Lake Michigan,we followed the list of seven recommendations outlined in Cohen et al. [Cohen, J.E., et al., 1993.Improving foodwebs. Ecology 74, 252–258] for improving food webs. We explored two inter-related extensions of hierarchical system theory with our food web; the first one was that subsystems react to perturbations independently in the short-term and the second onewas that a system’s properties change at a slower rate than its subsystems’ properties. We used Shannon’s equations to provide quantitative versions of the basic food web properties: number of prey, number of predators, number of feeding links, and connectance (or density).We then compared these properties between the two time-periods by developing distributions of each property for each time period that took uncertainty about the property into account.We compared these distributions, and concluded that non-overlapping distributions indicated changes in these properties that were large relative to their uncertainty. Two subsystems were identified within our food web system structure (p \u3c 0.001). One subsystem had more non-overlapping distributions in food web properties between Time 1 and Time 2 than the other subsystem. The overall system had all overlapping distributions in food web properties between Time 1 and Time 2. These results supported both extensions of hierarchical systems theory. Interestingly, the subsystemwithmore non-overlapping distributions in foodweb propertieswas the subsystemthat contained primarily benthic taxa, contrary to expectations that the identifiedmajor perturbations (lower phosphorous inputs and invasive species) would more greatly affect the subsystem containing primarily pelagic taxa. Future food-web research shouldemploy rigorous statistical analysis and incorporate uncertainty in food web properties for a better understanding of how ecological networks adapt

Zebra mussel (\u3ci\u3eDreissena polymorpha\u3c/i\u3e) selective filtration promoted toxic \u3ci\u3eMicrocystis\u3c/i\u3e blooms in Saginaw Bay (Lake Huron) and Lake Erie

Microcystis aeruginosa, a planktonic colonial cyanobacterium, was not abundant in the 2-year period before zebra mussel (Dreissena polymorpha) establishment in Saginaw Bay (Lake Huron) but became abundant in three of five summers subsequent of mussel establishment. Using novel methods, we determined clearance, capture, and assimilation rates for zebra mussels feeding on natural and laboratory M. aeruginosa strains offered alone or in combination with other algae. Results were consistent with the hypothesis that zebra mussels promoted blooms of toxic M. aeruginosa in Saginaw Bay, western Lake Erie, and other lakes through selective rejection in pseudofeces. Mussels exhibited high feeding rates similar to those seen for a highly desirable food alga (Cryptomonas) with both large (\u3e53 mm) and small (\u3c53 mm) colonies of a nontoxic and a toxic laboratory strain of M. aeruginosa known to cause blockage of feeding in zooplankton. In experiments with naturally occurring toxic M. aeruginosa from Saginaw Bay and Lake Erie and a toxic isolate from Lake Erie, mussels exhibited lowered or normal filtering rates with rejection of M. aeruginosa in pseudofeces. Selective rejection depended on “unpalatable” toxic strains of M. aeruginosa occurring as large colonies that could be rejected efficiently while small desirable algae were ingested