Email: m.bracken@neu.edu

Phone: 781.581.7370, ext. 304
Fax: 781.581.6076

Location: Marine Science Center
Mail: NU/Marine Science Center

430 Nahant Road

Nahant, MA 01908

Research Description

My research, based primarily at Northeastern University’s Marine Science Center in Nahant, uses an interdisciplinary approach to evaluate the linkages between marine communities and ecosystems. Recently, ecologists have begun to realize that large-scale, ecosystem-level processes are important mediators of community structure and dynamics and that local-scale phenomena, such as the ecological and physiological attributes of species, influence ecosystem functioning. In particular, my research focuses on (1) the impacts of ecosystem functioning (the transformation and flux of energy and materials) on patterns of distribution, abundance, and community structure and (2) the roles that organisms play in mediating ecosystem functions. In evaluating these linkages between communities and ecosystems, I draw from a variety of ecological sub-disciplines, including community ecology, physiological ecology, and ecosystem ecology. Specific examples of my research are discussed below.

What are the consequences of realistic species losses for marine ecosystem functioning?

Because the diversity of species in an ecosystem is influenced by a variety of factors, it is important to understand both (a) how diversity is altered by these factors and (b) how these changes in diversity influence the transformation and flux of energy and matter in that ecosystem. Most research linking diversity and ecosystem-level processes has used random assemblages of species to experimentally assess diversity’s influences on ecosystem functioning. While these studies suggest that diversity influences processes such as productivity, nutrient cycling, and energy flow, they fail to evaluate the consequences of non-random changes in biodiversity associated with factors such as nutrient availability, consumers, biological invasions, abiotic stressors, local extinction, and habitat loss. My research indicates that while ammonium uptake by seaweeds increases with nitrogen-mediated increases in seaweed diversity, uptake does not increase with diversity when assemblages are randomly selected from a local species pool. I am currently conducting observations and experimental manipulations to determine the impacts of factors such as abiotic stress, consumers, and productivity on the diversity of rocky intertidal seaweeds. This work includes complementary field, laboratory, and mesocosm experiments to assess the influences of those specific biodiversity changes on ecosystem functions such as primary production and nutrient cycling.

How does seaweed diversity influence ecosystem-level processes?

Emerging evidence suggests that the relationship between biodiversity and ecosystem functioning is reciprocal: not only do ecosystem functions (such as chemical transformation) influence diversity, but an ecosystem’s diversity influences ecosystem functioning. My research in intertidal systems suggests that tide pools containing more seaweed species are characterized by higher nutrient-use efficiency. This occurs because species are complementary with respect to their utilization of nitrate and ammonium: some species are better at ammonium uptake, while others are more efficient at nitrate uptake. When both ammonium and nitrate are available, a more diverse algal assemblage is therefore more efficient at nitrogen uptake than predicted based on the component species’ uptake abilities.

How do invertebrate-mediated nitrogen fluxes influence seaweed diversity and growth?

This aspect of my work evaluates the influence of a key ecosystem function—nitrogen transformation by invertebrates—on the growth and diversity of intertidal primary producers. I have shown, for example, that local-scale nitrogen excretion by invertebrates can ameliorate nitrogen limitation, increasing seaweed growth and diversity. This research involves a combination of field measurements and manipulations and laboratory experiments in which I quantify nitrogen limitation in rocky intertidal habitats, excretion of ammonium by invertebrates, utilization of excreted nitrogen as a nutrient source by seaweeds, and consequent association between ammonium loading and the diversity and growth of macroalgae.

What factors influence species’ roles in mediating ecosystem-level processes?

Organisms mediate a variety of key ecosystem functions. These include their roles as consumers (influencing energy or material flow through ecosystems) and chemical transformers (changing the structure and availability of nutrients). My research evaluates the influences of variation in the nearshore environment on species’ roles in processes such as material (e.g., carbon and nitrogen) exchange and chemical transformation. My research suggests that a species’ effectiveness as a mediator of ecosystem functioning is context-dependent, varying with changes in the environment

Selected Publications

Bracken, M.E.S., S.E. Friberg, C.A. Gonzalez-Dorantes, and S.L. Williams. 2008. Functional consequences of realistic biodiversity changes in a marine ecosystem. Proceedings of the National Academy of Sciences, USA 105:924-928. PDF

Bracken, M.E.S., B.E. Bracken, and L. Rogers-Bennett. 2007. Species diversity and foundation species: potential indicators of fisheries yields and marine ecosystem functioning. California Cooperative Oceanic Fisheries Investigations Reports. 48:82-91. PDF

Elser, J.J., M.E.S. Bracken, E.E. Cleland, D.S. Gruner, W.S. Harpole, H. Hillebrand, J.T. Ngai, E.W. Seabloom, J.B. Shurin, and J.E. Smith. 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems. Ecology Letters 10: 1135-1142. PDF

Bracken, M.E.S., C.A. Gonzalez-Dorantes, and J.J. Stachowicz. 2007. Whole-community mutualism: associated invertebrates facilitate a dominant habitat-forming seaweed. Ecology 88: 2211-2219. PDF

Hillebrand, H., D.S. Gruner, E.T. Borer, M.E.S. Bracken, E.E. Cleland, J.J. Elser, W.S. Harpole, J.T. Ngai, E.W. Seabloom, J.B. Shurin, and J.E. Smith. 2007. Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure. Proceedings of the National Academy of Sciences, USA 104: 10904-10909. PDF

Bracken, M.E.S. and J.J. Stachowicz. 2007. Top-down modification of bottom-up processes: selective grazing reduces macroalgal nitrogen uptake. Marine Ecology Progress Series 330: 75-82. PDF

Bracken, M.E.S., and J.J. Stachowicz. 2006. Seaweed diversity enhances nitrogen uptake via complementary use of nitrate and ammonium. Ecology 87: 2397-2403. PDF

Bracken, M.E.S. 2004. Invertebrate-mediated nutrient loading increases growth of an intertidal macroalga. Journal of Phycology 40: 1032-1041. PDF

Bracken, M.E.S., and K.J. Nielsen. 2004. Diversity of intertidal macroalgae increases with nitrogen loading by invertebrates. Ecology 85: 2828-2836. PDF

Menge, B.A., J. Lubchenco, M.E.S. Bracken, F. Chan, M.M. Foley, T.L. Freidenburg, S.D. Gaines, G. Hudson, C. Krenz, H. Leslie, D.N.L. Menge, R. Russell, and M.S. Webster. 2003. Coastal oceanography sets the pace of rocky intertidal community dynamics. Proceedings of the National Academy of Sciences, USA 100:12229-12234. PDF

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