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5/22/2016  |   09:15 - 10:00   |  Hall DE

DROUGHT, FLOODS, AND ALTERNATE STATES OF ALGAL-BASED RIVER FOOD WEBS IN THE THIRSTY EEL Winter scour and summer base flow control algal-based food webs that assemble during the biologically active summer in the Eel and similar rivers under the Mediterranean seasonality along the California North Coast. Large algal blooms of the macroalga Cladophora follow winters with one or more bed-mobilizing flood, which scour away over-wintering armored grazers. Inedible armored grazers abound after winters without flood scour, and sequester early summer growths of Cladophora and diatoms, reducing supplies for higher trophic levels. If winter floods release spring algae from large grazers, however, Cladophora proliferates over the summer low flow season, and becomes overgrown with highly edible epiphytic diatoms. Salmonids and other predators are then supported by prey built of epilithic and epiphytic algal carbon, but only if scouring winter floods are followed by sustained summer base flow. If prolonged drought (or human water extraction) reduce flows enough to disconnect and warm mainstem pools, cyanobacteria, some toxic, can proliferate and cover both rocky substrates and the Cladophora-diatom assemblages. Twenty-five years of observation, 5 field experiments, and diatom frustule counts in an 84-year stratigraphic record from the Eel’s marine canyon help link year-to-year variation in hydrology to the three contrasting summer food web states: with scouring floods and sufficient summer baseflow leading to 3-4 link chains that support salmon; flood-free winters leading to 2-link food chains with algal energy sequestered by predator resistant grazers if base flows keep mainstem reaches hydraulically connected; and the recently seen, concerning development that under severely reduced summer baseflows, a one-link food chain may develop, capped by cyanobacteria, some of which are toxic to vertebrates. Agency, academic, and citizen scientists are now collaborating to monitor and study conditions that flip river food webs from salmon-supporting to cyanobacterially-dominated states, and to learn more about the fates of algal production in river and coastal food webs. More Eyes on the Eel and other rivers are needed to track critical flow-driven ecological changes, and to steward watersheds through the altered hydrologic regimes that lie ahead.

Mary Power (Primary Presenter), University of California, Berkeley, mepower@berkeley.edu;
Dr. Mary E. Power is Professor in the Department of Integrative Biology at the University of California, Berkeley. She was awarded an honorary doctorate by Umea University, the Kempe Medal for distinguished ecologists, and the Hutchinson Award from the American Society of Limnologists and Oceanographers. She is a member of the California Academy of Science, the American Academy of Arts and Sciences, and National Academy of Sciences, USA. She has served on the Editorial Board of PNAS (2014 to present) and Science (2006-2009). Mary also served as President of the American Society of Naturalists, and of the Ecological Society of America. Since 1988, she has been the Faculty Director of the Angelo Coast Range Reserve, (one of the UC Natural Reserve System sites, a 3500 ha reserve protected for university teaching and research). She has studied food webs in temperate and tropical rivers, as well as linkages of rivers, watersheds and near-shore environments. Focal organisms include cyanobacteria, algae, invertebrates, fish, estuarine crustaceans and terrestrial grasshoppers, spiders, lizards, birds and bats. By studying how key ecological interactions depend on landscape and temporal contexts, her group hopes to learn how river-structured ecosystems will respond to changes over space and time in climate, land use, and biota. Her group also collaborates closely with Earth and atmospheric scientists in site-based research to investigate linkages among riverine, upland, and near-shore ocean ecosystems.