James R. Hodgson

Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs

Ecosystems are supported by organic carbon from two distinct sources. Endogenous carbon is produced by photosynthesis within an ecosystem by autotrophic organisms. Exogenous carbon is produced elsewhere and transported into ecosystems. Consumers may use exogenous carbon with consequent influences on population dynamics, predator–prey relationships and ecosystem processes. For example, exogenous inputs provide resources that may enhance consumer abundance beyond levels supported by within-system primary production. Exogenous fluxes of organic carbon to ecosystems are often large, but this material is recalcitrant and difficult to assimilate, in contrast to endogenously produced organic matter, which is used more easily. Here we show, by the experimental manipulation of dissolved inorganic 13C in two lakes, that internal primary production is insufficient to support the food webs of these ecosystems. Additions of NaH13CO3 enriched the 13C content of dissolved inorganic carbon, particulate organic carbon, zooplankton and fish. Dynamics of 13C indicate that 40–55% of particulate organic carbon and 22–50% of zooplankton carbon are derived from terrestrial sources, showing that there is significant subsidy of these ecosystems by organic carbon produced outside their boundaries.

Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment

High-frequency monitoring of manipulated and reference lakes enabled early detection of subsequent catastrophic regime shift. Catastrophic ecological regime shifts may be announced in advance by statistical early warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich, but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early warning signals for an experimentally induced regime shift in an aquatic food web. We gradually added top predators to a lake over 3 years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts.

TROPHIC CASCADES, NUTRIENTS, AND LAKE PRODUCTIVITY: WHOLE‐LAKE EXPERIMENTS

Responses of zooplankton, pelagic primary producers, planktonic bacteria, and CO2 exchange with the atmosphere were measured in four lakes with contrasting food webs under a range of nutrient enrichments during a seven-year period. Prior to enrichment, food webs were manipulated to create contrasts between piscivore dominance and planktivore dominance. Nutrient enrichments of inorganic nitrogen and phosphorus exhibited ratios of N:P > 17:1, by atoms, to maintain P limitation. An unmanipulated reference lake, Paul Lake, revealed baseline variability but showed no trends that could confound the interpretation of changes in the nearby manipulated lakes. Herbivorous zooplankton of West Long Lake (piscivorous fishes) were large-bodied Daphnia spp., in contrast to the small-bodied grazers that predominated in Peter Lake (planktivorous fishes). At comparable levels of nutrient enrichment, Peter Lakes areal chlorophyll and areal primary production rates exceeded those of West Long Lake by factors of approximatel...

ECOSYSTEM SUBSIDIES: TERRESTRIAL SUPPORT OF AQUATIC FOOD WEBS FROM 13C ADDITION TO CONTRASTING LAKES

Whole-lake additions of dissolved inorganic C-13 were used to measure allochthony (the terrestrial contribution of organic carbon to aquatic consumers) in two unproductive lakes (Paul and Peter Lakes in 2001), a nutrient-enriched lake (Peter Lake in 2002), and a dystrophic lake (Tuesday Lake in 2002). Three kinds of dynamic models were used to estimate allochthony: a process-rich, dual-isotope flow model based on mass balances of two carbon isotopes in 12 carbon pools; simple univariate time-series models driven by observed time courses of delta(13)CO(2); and multivariate autoregression models that combined information from time series of delta(13)C in several interacting carbon pools. All three models gave similar estimates of allochthony. In the three experiments without nutrient enrichment, flows of terrestrial carbon to dissolved and particulate organic carbon, zooplankton, Chaoborus, and fishes were substantial. For example, terrestrial sources accounted for more than half the carbon flow to juvenile and adult largemouth bass, pumpkinseed sunfish, golden shiners, brook sticklebacks, and fathead minnows in the unenriched experiments. Allochthony was highest in the dystrophic lake and lowest in the nutrient-enriched lake. Nutrient enrichment of Peter Lake decreased allochthony of zooplankton from 0.34-0.48 to 0-0.12, and of fishes from 0.51-0.80 to 0.25-0.55. These experiments show that lake ecosystem carbon cycles, including carbon flows to consumers, are heavily subsidized by organic carbon from the surrounding landscape.

Food Web Structure and Phosphorus Cycling in Lakes

Abstract We assessed the effects of trophic structure on phosphorus (P) recycling by fishes, Chaoborus, and zooplankton in lakes with different food web configurations. Fishes in a piscivoredominated system recycled little P compared to that regenerated by zooplankton and Chaoborus. In a plan ktivore-dominated lake, excretion by fishes (especially young of year) dominated P recycling to algae; Chaoborus and zooplankton recycling was decreased due to intense predation by fishes. The sum of P incorporated into primary production and lost to sedimentation in the piscivore-dominated lake was only 77% of that in the planktivore-dominated lake, Our analyses suggest that the 30% greater primary production in the planktivore-dominated system may be accounted for by the greater than 40% increase in estimated P recycling that was directly due to differences in food web structure, Diel vertical migrations by Chaoborus concentrated P excretion in the epilimnion during night hours. In the plan ktivore-dominated system...

Resilience and Resistance of a Lake Phosphorus Cycle Before and After Food Web Manipulation

Parameters of a phosphorus cycling model were estimated for two configurations of a lake ecosystem. The piscivore-dominated configuration had one more trophic level than the planktivore-dominated configuration. We derived four main conclusions from analysis of the model. (1) Results support the argument of DeAngelis et al. that turnover rate of a limiting nutrient is directly related to ecosystem resilience. (2) Results support the hypothesis of Pimm and Lawton that longer food chains are less resilient. (3) Inputs of phosphorus to the pelagic system derived from inshore feeding by fishes were a large flux, which is comparable to inputs from physical-chemical fluxes. (4) Algal (seston) standing crops, unlike all other compartments, were less sensitive to phosphorus inputs in the piscivore-dominated system. Consistent with the trophic cascade hypothesis, the piscivore-dominated system had higher herbivore standing crops and lower algal standing crops than the planktivore-dominated system. Changes in trophic structure that derive from trophic cascades can be viewed as changes in the phosphorus cycle driven by fishes.

Density-dependent changes in individual foraging specialization of largemouth bass

Abstract Individual foraging specializations are an important source of intraspecific variability in feeding strategies, but little is known about what ecological factors affect their intensity or development. We evaluated stomach contents in marked individual largemouth bass (Micropterus salmoides) and tested the hypothesis that diet specialization is most pronounced during periods with high conspecific densities. We collected diet data over 10 years from an unexploited population of largemouth bass that displayed a greater than threefold variation in density. Although diet composition of the aggregate bass population did not change during the study, bass body condition was inversely correlated with population size. Individual marked bass exhibited high diet consistency (diet overlap between successive captures) during years with high population densities. Diet overlap between randomly assigned pairs of bass was not correlated with population size. We did not detect the expected positive relationship between diet breadth and population size. Our analyses demonstrate that population responses to density changes may represent the sum of many unique individual foraging responses and would be obscured by pooled sampling programs. Behavioral flexibility of individuals may contribute to the ability of largemouth bass to function as a keystone predator in many aquatic communities.

Food Web Structure and Littoral Zone Coupling to Pelagic Trophic Cascades

Lake productivity ultimately depends on phosphorus (P) supply rates (Schindler, 1977). Community interactions, especially size-selective predation by fishes and size-dependent rates of grazing and nutrient recycling by zooplankton, determine the efficiency and rate with which P inputs are translated to ecosystem productivity (Carpenter and Kitchell, 1993). Limnologists are now trying to determine how trophic cascades interact with the P cycle to influence lake productivity (McQueen et al., 1986; Elser and Goldman, 1991; Carpenter et al., 1991). One informative approach analyzes food web interactions in P currency, thereby integrating the direct and indirect effects of fishes on lake P cycles.

GRADIENT OF FISH PREDATION ALTERS BODY SIZE DISTRIBUTIONSOF LAKE BENTHOS

We used normalized size spectra analysis (originally developed in the study of pelagic communities) and a fish bioenergetics model to examine whether predation by fishes affects the size structure of macrobenthos. We conducted the study over five years in three north temperate lakes in which fish were experimentally manipulated while allowing for natural recruitment and mortality in lakes with fish. The resulting variation in predator populations allowed us to address how prey community size structure responds to a gradient of predation pressure. The slopes of normalized size spectra were used to characterize whether community biomass is relatively distributed as smaller or larger individuals with regard to mass. We ask whether predator effects on prey are manifested through fish pres- ence/absence or if a gradient of predation pressure is important. In addition, we examined whether consumption rate of prey by fish is a better measure of predation effects on prey size structure than fish biomass alone. Our results suggest that benthos size structure responds to a gradient of fish consumption, rather than a qualitative distinction of fish presence or absence. Consumption rates by fish on benthos explained more of the variation in slopes of normalized size spectra of benthos than fish biomass alone. Slope values were more variable at lower consumption rates, which included no predation by fish. Conversely, high consumption rates by fish produced con- sistent, predictable body size distributions of benthos in which large individuals were relatively underrepresented. However, these results may not be apparent through more traditional analyses based on predator or prey presence/absence, abundance, or taxonomic- based measures. Generalizations addressing patterns of body size distributions within and among ecosystems may be advanced by examining predation and other underlying size- structuring mechanisms.

Evaluating Alternative Explanations in Ecosystem Experiments

ABSTRACT Unreplicated ecosystem experiments can be analyzed by diverse statistical methods. Most of these methods focus on the null hypothesis that there is no response of a given ecosystem to a manipulation. We suggest that it is often more productive to compare diverse alternative explanations (models) for the observations. An example is presented using whole-lake experiments. When a single experimental lake was examined, we could not detect effects of phosphorus (P) input rate, dissolved organic carbon (DOC), and grazing on chlorophyll. When three experimental lakes with contrasting DOC and food webs were subjected to the same schedule of P input manipulations, all three impacts and their interactions were measurable. Focus on multiple alternatives has important implications for design of ecosystem experiments. If a limited number of experimental ecosystems are available, it may be more informative to manipulate each ecosystem differently to test alternatives, rather than attempt to replicate the experiment.