UC Berkeley

Abstract

From Cobble to Canyon: inferring the effects of discharge, climate and trophic interactions

on primary productivity in a Northern California river, over reach to watershed to

coastal ocean and annual to multi-decadal scales.

by

John Blackburn Sculley

Doctor of Philosophy in Integrative Biology

University of California, Berkeley

Professor Mary E. Power, Chair

Climate change along the California North Coast is expected to have significant impacts on primary production by riverine algae in coastal watersheds through increased precipitation intensity and duration (Snyder and Sloan 2005), and regional changes in fog frequency (Johnstone and Dawson 2010, Lebassi et al. 2009), potentially altering solar irradiance and extending higher flows and spates later into the summer low flow period when algae accrue. Changes in flow velocity and irradiance may alter relationships between primary producers and affect biomass accrual (Dodds 1991a, Stevenson 1983). Primary production in the Eel River has been established by isotopic studies to feed local aquatic and terrestrial food webs (Finlay et al. 2002) before being exported by winter high flows to the continental shelf and submarine canyons by the following summer (Drexler et al. 2006).

I examined the combined and separate effects of two different radiation levels and flow velocity ranges on the growth and interaction of algae that dominate mainstem primary producer guilds in the food web of a northwestern California river during the biologically active summer baseflow season: Cladophora glomerata (L.) Kutz, (the dominant benthic macroalga) and Cocconeis placentula (Her), C. pediculus (Ehrenb.) Kütz., and Epithemia turgida (Ehrenb.) Kütz. and E. sorex Kütz. (the dominant genera of epiphytic diatoms on Cladophora during the middle and later summer months, respectively). Cladophora-epiphyte assemblages attached to cobbles were collected from the South Fork Eel River (39°43'47''N, 123°38'34''W), and placed in in-stream channels that were gated and shaded to provide two velocity regimes (slow (0-9 cm/s) and faster (10-20 cm/s)) and two light treatments (shaded (peak irradiance 150 µE/m2/s) and full sunlight (peak irradiance 1500 µE/m2/s)). Cladophora filaments grew 68% longer under the higher light level (26.5 vs 15.8 cm in high vs low light), but did not differ under different water velocities. Epiphyte densities on Cladophora were 65% higher (6141 vs 3726 cells/mm2 of host area) in slow velocity treatments. However, epiphyte biovolume was 67% higher (4.26 vs. 2.55 x 105 µm3/mm2) in faster velocity treatments. Total epiphyte biovolume per host filament increased 88% in higher light.

I identified and counted freshwater diatom frustules in datable annual layers (varves) from a sedimentary core recovered from the submarine Eel canyon, which receives up to 50% of discharged sediment from the nearby Eel River. These freshwater diatoms, mainly Epithemia, Cocconeis, and Rhoicosphenia spp., appear to provide an accurate paleo-productivity (bloom size) proxy of algae and algae-based food web dynamics over an 84 year period. With more than 47% of the variation in algal bloom height being captured by Epithemia spp. in offshore canyon cores, and more than 33% of the variation captured by total freshwater diatoms, I have discovered a useful proxy for algal biomass throughout the Eel River basin. I can thus extend temporal inferences about hydrologically mediated food web responses, e.g. the two-state algae bloom response to bankfull discharge regimes, back to 1917, and upscale these inferences to the entire 9540 km2 watershed.

Using this proxy record of freshwater diatom frustules for basin-wide algal productivity, I discovered a strong and significant correlation between Rhopalodiaceae and total freshwater frustules and two satellite records of marine chlorophyll in the coastal ocean adjacent to the Eel mouth. Records of mean annual chlorophyll from the SeaWiFS sensor over the period 1997-2001 (the period where the core and satellite records overlap) and from the CZCS sensor over the entire mission period of 1978-1986 are positively correlated with Rhopalodiaceae and total freshwater frustules, accounting for between 62 and 72% of the variation in mean annual chlorophyll a in a ~2400 km2 region of the continental shelf directly offshore of the Eel River mouth. I also found a significant and strongly positive correlation of mean annual marine chlorophyll with peak averaged modal Cladophora bloom height from upstream algal surveys during the period of overlap with the satellite record (1988-2011). River plumes have been posited as a possible source of nutrients for marine phytoplankton blooms off the California and Oregon coasts (Bruland et al. 2001, Wetz et al. 2006). These results imply a connection between riverine algae and the offshore marine ecosystem. While upwelling is clearly the major driver of marine phytoplankton blooms, my data support a role for freshwater-derived nutrients in winter/spring marine productivity.

Taken together, these results provide a more detailed picture of how environmental conditions can affect the biomass of hosts and epiphytes in riverine primary producer assemblages, and they enable the tracking of this biomass from production in river channels to burial in submarine canyons. Better knowledge of how environmental controls exert different effects on the relative growth and colonization rates of hosts and epiphytes will improve my understanding of how the energy flow and biogeochemical fluxes that they mediate may change with climate change. The ability to track primary production using the direct correlation between freshwater diatoms and bloom height allows us insight into algal food web dynamics far earlier than contemporary observations permit. These records have the potential to greatly increase the spatial and temporal scales of inferences linking food webs to environments, enhancing our understanding of how river webs may respond to future changes in climate, land cover, and other factors affecting riverine runoff and conditions, as well as their linkages to marine systems worldwide.