Showing papers by "Stephen R. Carpenter published in 1997"

Influence of Food Web Structure on Carbon Exchange Between Lakes and the Atmosphere

Abstract: Top predators and nutrient loading in lakes were manipulated to assess the influence of food web structure on carbon flux between lakes and the atmosphere. Nutrient enrichment increased primary production, causing lakes to become net sinks for atmospheric carbon (C atm ). Changes in top predators caused shifts in grazers. At identical nutrient loading, C atm invasion was greater to a lake with low grazing than to one with high grazing. Carbon stable-isotope distributions corroborated the drawdown of lake carbon dioxide and traced C atm transfer from algae to top predators. Thus, top predators altered ecosystem carbon fixation and linkages to the atmosphere.

Resilience and Restoration of Lakes

Abstract: Lake water quality and ecosystem services are normally maintained by several feedbacks. Among these are nutrient retention and humic production by wetlands, nutrient retention and woody habitat production by riparian forests, food web structures that cha nnel phosphorus to consumers rather than phytoplankton, and biogeochemical mechanisms that inhibit phosphorus recycling from sediments. In degraded lakes, these resilience mechanisms are replaced by new ones that connect lakes to larger, regional economi c and social systems. New controls that maintain degraded lakes include runoff from agricultural and urban areas, absence of wetlands and riparian forests, and changes in lake food webs and biogeochemistry that channel phosphorus to blooms of nuisance al gae. Economic analyses show that degraded lakes are significantly less valuable than normal lakes. Because of this difference in value, the economic benefits of restoring lakes could be used to create incentives for lake restoration.

Dystrophy and eutrophy in lake ecosystems : implications of fluctuating inputs

Abstract: Eutrophic and dystrophic states of lake ecosystems are associated with distinct differences in phosphorus (P) input, refractory dissolved organic carbon (RDOC) input, and certain internal processes. Eutrophic lakes have high P input and high primary production. Dystrophic lakes have high RDOC input. In both types of lake, bacterial metabolism may help reduce RDOC levels. In dystrophic lakes, bacterial metabolism may be suppressed by low pH, and primary production is reduced due to light attenuation by RDOC. We analyzed several simple models to ask whether eutrophy and dystrophy are alternative stable states of lake ecosystems. In-lake processes could create alternative states under certain circumstances, but more likely watershed processes maintain eutrophy or dystrophy through contrasts in inputs of P and RDOC. Simulations suggest that pulses of RDOC result in dystrophic conditions that reverse very slowly. Land-use changes or climate fluctuations that change RDOC input rates may have long-lasting effects on trophic state of temperate and boreal lakes. Lack of information on microbial degradation of RDOC, and the dependency of degradation rate on RDOC levels, primary production, and pH, are major sources of uncertainty in our analysis and are suggested as priorities for further research.

Seasonal effects of variable recruitment of a dominant piscivore on pelagic food web structure

Abstract: Piscivorous fish play an important role in regulating lake food web structure. However, most ultimately piscivorous fish pass through a period of planktivory before becoming piscivorous. In 1993 and 1994, two large cohorts of young-of-the-year largemouth bass (Micropterus salmoides) were recorded in Paul Lake, Michigan. The 1993 and 1994 cohorts were the largest recorded in data extending back to 1984 and occurred during a period in which adult and juvenile bass biomass was low. In 1993, consumption of large-bodied zooplankton by the young-of-theyear cohort eliminated large-bodied cladocerans by the middle of August. As large cladocerans declined, small cladocerans, especially Bosminu Zongirostris, increased. By early Septcmbcr the biomass of B. Zongirostris was similar to that of the entire cladoceran community in previous years. Coincident with the shifts in the cladoceran community were increases in epilimnetic Chl a concentrations <35 pm. However, total cpilimnetic Chl a did not increase. In contrast, the 1994 cohort had no discernible effect on pelagic food web structure because production of large-bodied zooplankton rapidly exceeded predation. Our results show the potential for large cohorts of piscivorous fish to affect food web structure, at a seasonal scale, through intense planktivory.

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Abstract: Although both nutrient inputs and zooplankton grazing are important to phytoplankton and bacteria in lakes, controversy surrounds the relative importance of grazing pressure for these two groups of organisms. For phytoplankton, the controversy revolves around whether zooplankton grazers, especially large cladocerans like Daphnia, can effectively reduce phytoplankton populations regardless of nutrient conditions. For bacteria, little is known about the balance between possible direct and indirect effects of both nutrients and zooplankton grazing. However, there is evidence that bacteria may affect phytoplankton responses to nutrients or zooplankton grazing through direct or apparent competition. We performed a mesocosm experiment to evaluate the relative importance of the effects of nutrients and zooplankton grazing for phytoplankton and bacteria, and to determine whether bacteria mediate phytoplankton responses to these factors. The factorial design crossed two zooplankton treatments (unsieved and sieved) with four nutrient treatments (0, 0-5, 1.0 and 2.0 ug phosphorus (P) H day 1, together with nitrogen (N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300 um mesh reduced the average size of crustacean zooplankton in the mesocosms, decreased the numbers and biomass of Daphnia, and increased the biomass of adult copepods. Nutrient enrich- ment caused significant increases in phytoplankton chlorophyll a (4-5 X), bacterial abundance and production (1.3x and 1.6X, respectively), Daphnia (3x) and total zooplankton biomass (2x). Although both total phytoplankton chlorophyll a and chlorophyll a in the <35 um size fraction were significantly lower in unsieved mesocosms than in sieved mesocosms, sieving had no significant effect on bacterial abundance or production. There was no statistical interaction between nutrient and zoo- plankton treatments for total phytoplankton biomass or bacterial abundance, although there were marginally significant interactions for phytoplankton biomass <35 um and bacterial production. Our results do not support the hypothesis that large cladocerans become less effective grazers with enrich- ment; rather, the difference between phytoplankton biomass in sieved versus unsieved zooplankton treatments increased across the gradient of nutrient additions. Furthermore, there was no evidence that bacteria buffered phytoplankton responses to enrichment by either sequestering P or affecting the growth of zooplankton.

A Bayesian observation error model to predict cyanobacterial biovolume from spring total phosphorus in Lake Mendota, Wisconsin

Abstract: Nous avons elabore un modele logistique pour prevoir le biovolume d'algues bleu-vert en ete a partir de la concentration moyenne (log metrique) de phosphore total au printemps dans le lac Mendota, au Wisconsin. Le modele tient compte de l'incertitude dans les estimations, basees sur les echantillons, des valeurs moyennes reelles du phosphore total. Nous utilisons le theoreme de Bayes pour evaluer les parametres du modele et l'incertitude de prevision pour 19 annees de donnees. Lorsqu'on compare avec un modele naif qui ne prend pas en consideration l'incertitude liee au phosphore, le modele tenant compte de l'erreur d'observation presente une variance plus elevee des parametres, mais une incertitude de prevision plus faible. L'incertitude de prevision plus faible survient parce qu'une partie du bruit dans les donnees est traitee comme une incertitude liee au phosphore, reduisant ainsi la variance de du terme de perturbation du modele. Le modele tenant compte de l'erreur d'observation entraine des objectifs moins rigoureux en ce qui a trait a la concentration de phosphore pour obtenir des concentrations d'algues bleu-vert acceptables que ne le fait le modele naif, en raisons de cette incertitude de prevision plus faible.