Organic matter sources and rehabilitation of the Sacramento-San Joaquin Delta (California, USA)

Abstract

1. The Sacramento-San Joaquin River Delta, a complex mosaic of tidal freshwater habitats in California, is the focus of a major ecosystem rehabilitation effort because of significant long-term changes in critical ecosystem functions. One of these functions is the production, transport and transformation of organic matter that constitutes the primary food supply, which may be sub-optimal at trophic levels supporting fish recruitment. A long historical data set is used to define the most important organic matter sources, the factors underlying their variability, and the implications of ecosystem rehabilitation actions for these sources.2. Tributary-borne loading is the largest organic carbon source on an average annual Delta-wide basis; phytoplankton production and agricultural drainage are secondary; wastewater treatment plant discharge, tidal marsh drainage and possibly aquatic macrophyte production are tertiary; and benthic microalgal production, urban run-off and other sources are negligible.3. Allochthonous dissolved organic carbon must be converted to particulate form -with losses due to hydraulic flushing and to heterotroph growth inefficiency -before it becomes available to the metazoan food web. When these losses are accounted for, phytoplankton production plays a much larger role than is evident from a simple accounting of bulk organic carbon sources, especially in seasons critical for larval development and recruitment success. Phytoplankton-derived organic matter is also an important component of particulate loading to the Delta. 4. The Delta is a net producer of organic matter in critically dry years but, because of water diversion from the Delta, transport of organic matter from the Delta to important, downstream nursery areas in San Francisco Bay is always less than transport into the Delta from upstream sources.5. Of proposed rehabilitation measures, increased use of floodplains probably offers the biggest increase in organic matter sources.6. An isolated diversion facility-channelling water from the Sacramento River around the Delta to the water projects-would result in substantial loading increases during winter and autumn, but little change in spring and summer when food availability probably matters most to developing organisms.7. Flow and fish barriers in the channel could have significant effects, especially on phytoplankton sources and in dry years, by eliminating 'short-circuits' in the transport of organic matter to diversion points.8. Finally, productivity of intentionally flooded islands probably would exceed that of adjacent channels because of lower turbidity and shallower mean depth, although vascular plants rather than phytoplankton could dominate if depths were too shallow.

Dietary Segregation of Pelagic and Littoral Fish Assemblages in a Highly Modified Tidal Freshwater Estuary

“…However, experimental work shows that the detrital-derived energy is of minor significance to pelagic pathways compared to phytoplankton, which is far more bioavailable (Sobczak et al 2002(Sobczak et al , 2005 and nutritious for zooplankton (Mueller-Solger et al 2002). This study and others in the estuary have demonstrated, using different approaches, that phytoplankton is the dominant energy source for pelagic consumers (Canuel et al 1995;Jassby and Cloern 2000;Mueller-Solger et al 2002;Sobczak et al 2002Sobczak et al , 2005. Contributions of detrital material to pelagic consumers cannot be ruled out entirely, but for the purposes of our study phytoplankton is assigned as the basal source of food for pelagic food webs (i.e., consumers with depleted d 13 C signatures) based on its likely dominance in assimilation to upper trophic levels.…”

“…These studies have demonstrated the importance of in situ phytoplankton production as the primary source of available organic carbon for pelagic consumers in the tidal freshwater region of the estuary (Canuel et al 1995;Jassby and Cloern 2000;Mueller-Solger et al 2002;Sobczak et al 2002Sobczak et al , 2005. However, the relative importance of phytoplankton as an energy source for consumers in other habitats is unclear.…”

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

“…In contrast, the food webs of lotic systems are driven primarily by riparian-derived allochthonous production. Estuarine aquatic food webs are a complex mix of both pelagic and intertidal marsh production (Jassby et al 1993;Jassby and Cloern 2000;Sobczak et al 2002Sobczak et al , 2005.…”

Designing a high-frequency nutrient and biogeochemical monitoring network for the Sacramento–San Joaquin Delta, northern California

“…Because most water enters the Delta from the Sacramento River (about 84 percent annually) and San Joaquin River (about 13 percent annually) (Jassby and Cloern, 2000), and exits the Delta through Suisun Bay, establishment of monitoring stations at the periphery of the Delta at approximately these three locations would allow a first approximation of nutrient loads entering and exiting the Delta. The Sacramento and San Joaquin Rivers both receive nutrients from natural, agricultural, and urban sources.…”