Bennett Research Seminar: Top down, bottom up, and cross-habitat controls in algal-based river food webs

When algae are the root of the food web, weather events leave lasting impacts.

From headwaters to the mouth of the Eel River, isotopic surveys indicate that in channels that drain > 10 km², river consumers derive most of their carbon from algae. Large algal blooms follow winter floods that scour away over-wintering armored grazers. Blooms are initially dominated by filamentous green macroalgae (Cladophora glomerata) that becomes overgrown with highly edible epiphytic diatoms (Epithemia spp.). 

Emerging insects that derive their carbon from algae feed consumers in terrestrial uplands and in cool, dark headwaters. The second biological backflow may help sustain salmonids and other cold-water predators in warming rivers. River Cladophora and its diatom epiphytes are also preferred over seaweeds and voraciously grazed by estuarine crustaceans. 

Riverine and likely marine salmonids are nourished in food chains of 3-4 levels by prey built of algal carbon, if scouring winter floods are followed by sustained summer base flow. In drought winters without flood scour, inedible armored grazers deplete early summer growths of Cladophora and diatoms, depriving predators of algal energy. These inferences, from 25 years of observation and 5 field experiments, are supported by diatom frustule counts in an 84-year stratigraphic record from the Eel’s marine canyon, which suggest that top-down effects of floods on algal blooms (grazer release) dominate bottom-up control (e.g. nutrient inputs).

Dr. Mary E. Power is Professor in the Department of Integrative Biology at the University of California, Berkeley. She is also the Faculty Director of the Angelo Coast Range Reserve, an 8000-acre natural reserve protected for university teaching, research, and outreach. Power studies food webs in temperate and tropical rivers. Her research is often framed as "predictive mapping": by studying how controls over key ecological interactions change over space and time, she seeks insights that will help forecast how river-structured ecosystems will respond to watershed or regional scale changes in climate, land use, or biota.