Kathryn Cottingham

Background/Question/Methods Cyanobacteria are generally thought to have strong negative effects on aquatic food webs due to their scums, toxins, poor food quality for zooplankton grazers, and post-bloom anoxia. In oligotrophic lakes, however, cyanobacteria may stimulate plankton food webs by increasing nutrient availability. Gloeotrichia echinulata is a large colonial cyanobacterium that is increasing in nutrient-limited lakes across the northeastern United States. We hypothesize that G. echinulata has the potential to substantially affect ecosystem functioning in oligotrophic systems because it both fixes nitrogen and translocates large quantities of stored phosphorus from the sediments to the water column annually during recruitment. We examined the effects of G. echinulata on nutrient concentrations and plankton by adding rinsed colonies to nutrient-limited ~100 L in situ mesocosms during the summer of 2008 and ~1000 L experimental ponds in the summer of 2010. In 2010, we also ma...

ABSTRACT We describe a collaboration between mathematicians and ecologists studying the cyanobacterium Gloeotrichia echinulata and its possible role in eutrophication of New England lakes. The mathematics includes compartmental modeling, differential equations, difference equations, and testing models against high-frequency data. The ecology includes observation, field sampling, and parameter estimation based on observed data and the related literature. Mathematically and ecologically, a collaboration like this progresses in ways it never would have if either group worked alone.

ABSTRACT Background/Question/Methods Ecologists have long been interested in understanding how, when and where single species have the potential to modulate ecosystem functioning. We hypothesize that the physiology and life history of the colonial cyanobacterium Gloeotrichia echinulata make this species a biotic key that unlocks otherwise inaccessible stores of atmospheric nitrogen (by N fixation) and sediment phosphorus (during the process of germination and recruitment from sediment resting stages). Because G. echinulata has the potential to alleviate both N and P limitation, we predict that the factors that drive G. echinulata blooms may also indirectly accelerate lake eutrophication by eroding the resilience of the oligotrophic state. We are examining the drivers of G. echinulata abundance and the impact of this species on eutrophication and resilience using field observations of key biotic and abiotic variables in combination with modeling and targeted field experiments. Specifically, we are using empirical models to understand the abiotic and biotic factors that drive fluctuations in G. echinulata density and simple differential equations models to explore how the changes in N and P cycling caused by G. echinulata might affect lake ecosystem stability and resilience. Results/Conclusions Modeling and empirical work to date suggest that water column densities of G. echinulata result from a complex interplay of biotic and abiotic factors affecting recruitment from sediment resting stages and subsequent growth in the water column. Light and temperature appear to predict recruitment from resting stages, while water column stability, grazing, and density-dependent growth influence water column populations. In our simulation models, changes in nutrient cycling due to G. echinulata affect stability and resilience. Simple one-box P cycling models suggest that internal loading by G. echinulata can create a second, eutrophic equilibrium and decrease the resilience of the oligotrophic equilibrium, particularly in lakes with oxic hypolimnia and low external inputs. Moreover, our coupled model of N and P cycling shows equilibria whose exact formulas are quite complicated, but whose close approximations also suggest that internal recycling raises equilibrium nutrient concentrations. Thus, the activities of G. echinulata are likely to be important in affecting the ability of oligotrophic lakes to maintain low-nutrient conditions. We look forward to exploring these findings further in a coupled population-nutrient model.

Limnologists are now reconsidering the role of the biota in the phosphorus (P) cycles of lakes. Changes in lake communities can have significant consequences for ecosystem P cycles. At seasonal timescales, the relative importance of nitrogen (N) and Pas limiting factors for primary production depends in part on zooplankton species composition. Phosphorus storage and recycling by fish and zooplankton can be large components of P budgets, and mobile consumers can be important vectors in P transport. Stability, resilience and resistance of lake P cycles may depend heavily on fluxes to and from upper trophic levels.

Page 1. Transactions of the American Fisheries Society 122:756-772, 1993 © Copyright by the American Fisheries Society 1993 Food Web Structure and Phosphorus Cycling in Lakes DANIEL E. SCHINDLER AND JAMES F. KITCHELL Center for Limnology. ...

Creative approaches at the interface of ecology, statistics, mathematics, informatics, and computational science are essential for improving our understanding of complex ecological systems. For example, new information technologies, including powerful computers, spatially ...

Page 1. Transactions of ihe American Fisheries Society 122:773-783, 1993 €» Copyright by the American Fisheries Society 1993 Food Web Structure and Long-Term Phosphorus Recycling: A Simulation Model Evaluation Xi HE Center for Limnology. ...

Invasions by non-native taxa can have severe consequences for native species. In the heavily invaded serpentine grasslands of central California, many native species appear to be restricted to isolated outcrops of shallow serpentine soil, or “hummocks,” although the extent to which these hummocks function as refuges for native vegetation has never been quantified. We tested whether native plant species were restricted to hummocks within a serpentine grassland at the University of California Sedgwick Reserve near Santa Barbara, California by sampling species along hummock-grassland gradients. We also examined the influence of soil parameters, hummock area, proximity to other hummocks, and spatial location on species composition across 16 hummocks at this site. Both the hummocks and the surrounding grassland had high Mg, low Ca, and low Ca to Mg ratios typical of serpentine systems. Hummocks appeared to be more stressful environments because of their shallower soils, lower cation exchange capacity, and greater percent sand. Of the 27 most common plant species sampled along hummock-grassland transects, we identified 8 hummock specialists, 7 edge specialists, 8 matrix specialists, and 4 generalists. Importantly, both the hummock and matrix specialist groups included native species. Plant community composition was correlated with spatial positioning of the hummocks and with soil Ca, Na, K, and N. The number of species increased and community composition changed with increasing hummock area. Species composition was most similar among hummocks in close proximity to each other, and decreased with increasing distance between hummocks. Our results suggest that the community structure of serpentine grasslands is spatially complex and an effective management or restoration plan must address this complexity.

FULL TEXT FREELY AVAILABLE FROM http://www.escholarship.org/uc/item/0718522k . . .  Community variability has a dual nature. On the one hand, there is compositional variability, changes in the relative abundance of component species. On the other hand, there is aggregate variability, changes in summary properties such as total abundance, biomass, or production. Although these two aspects of variability have received much individual attention, few studies have explicitly? related the compositional and aggregate variability of natural communities. In this paper, we show how simultaneous consideration of both aspects of community variability might advance our understanding of ecological communities.

We use the distinction between compositional and aggregate variability to develop an organizational framework for describing patterns of community variability. At their extremes, compositional and aggregate variability combine in four different ways: (I) stasis, low compositional and low aggregate variability; (2) synchrony, low compositional and high aggregate variability; (3) asynchrony, high compositional and high aggregate variability; and (4) compensation, high compositional and low aggregate variability. Each of these patterns has been observed in natural communities, and can be linked to a suite of abiotic and biotic mechanisms. We give examples of the potential relevance of variability patterns to applied ecology, and describe the methodological developments needed to make meaningful comparisons of aggregate and compositional variability across communities. Finally, we provide two numerical examples of how our approach can be applied to natural communities.