Nutritional quality of food resources for zooplankton (Daphnia) in a tidal freshwater system (Sacramento‐San Joaquin River Delta)

Abstract

We examined the relative nutritional values of natural phytoplankton and particulate detritus for zooplankton growth in a detritus‐rich environment. Seston was collected seasonally from four different habitat types in a tidal freshwater system and fed to juvenile Daphnia magna under controlled culture conditions by use of a flow‐through design. Seston particulate organic carbon (POC) and chlorophyll a contents ranged from ~330 to 3,800 µg L-1 POC and 1.4 to 45 µg L-1 Chl a. A partial residual analysis revealed that detrital carbon concentrations were only weakly related to Daphnia growth, whereas Chl a proved to be highly predictive of Daphnia growth rates across all investigated habitat types. Overall, habitat type had a strong effect on growth rates, whereas season of seston collection did not, but differences among habitats could be attributed to differing Chl a concentrations. The results from this study imply that, even in systems with overwhelming amounts of detrital carbon from a variety of sources, nutritional factors associated with phytoplankton can be dominant in regulating zooplankton growth.

Reevaluating the Generality of an Empirical Model for Light-Limited Primary Production in the San Francisco Estuary

“…Like many other urbanized estuaries around the world, the SFE has experienced perturbations in key ecosystem drivers, including sediment and nutrient loads (Schoellhamer 2011;Jassby 2008), quantity and diversity of toxic pollutants (Brooks et al 2012), decreased phytoplankton biomass, and a loss of diatoms (e.g., Alpine and Cloern 1992;Kimmerer 2004;Dugdale et al 2007), and changes in zooplankton and fish (Mueller-Solger et al 2002;Kimmerer 2005). Although the original authors of the light-utilization model for the SFE (Cole and Cloern 1984) predicted that the empirical efficiency factor y would likely respond to such changes, this parameter has been previously recalculated for the northern estuary only twice (Alpine and Cloern 1992;Jassby et al 2002).…”

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

“…However, in order for emergent, terrestrial, or floating aquatic vegetation to be incorporated by zooplankton and other suspension feeders, it would have to be routed through a detrital microbial loop. 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.…”

“…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.…”

“…In addition, long-term declines in phytoplankton production may be increasing the relative importance of other energy sources . The answers to these questions remain outstanding because long-term monitoring and special studies have focused on pelagic habitats (Orsi and Mecum 1996;Kimmerer 2002aKimmerer , 2006Mueller-Solger et al 2002;Sobczak et al 2002).…”