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

Abstract

Estuarine food webs are highly variable and complex, making identification of their trophic pathways difficult. Energy for the food web of the San Francisco Estuary is thought to be based largely on in situ phytoplankton production, but little attention has been paid to littoral habitats, where other energy sources may be important. We analyzed the stomach contents of over 960 juvenile fishes and the stable carbon and nitrogen isotope ratios of these fishes and their potential food resources in pelagic and littoral habitats from the tidal freshwater area of the estuary. The mixing model IsoSource was used to examine energy sources important to consumers. Our results show evidence of two predominant food web pathways. Pelagic fishes and some littoral fishes showed strong dependence on a zooplankton–phytoplankton trophic pathway. However, the majority of fishes in littoral habitats had diets and carbon isotope ratios consistent with energy arising from submerged aquatic vegetation and epiphytic macroalgae. IsoSource revealed that the overall majority of nutrition of littoral fishes originated from consumption of grazer amphipods. Examining both stable isotopes and stomach contents allowed us to identify a food web with contributions to resident fishes that had been previously underestimated in the estuary. This study provides insight to how estuarine food webs have changed over the last few decades and highlights why the functions of habitats must be understood for effective restoration planning.

The Aquatic Trophic Ecology of Suisun Marsh, San Francisco Estuary, California, During Autumn in a Wet Year

“…Feeding of the shrimps on G. daiberi is likely the reason why SAV probabilities were relatively high for striped bass ( Figure 3) even though MixSIAR identified Siberian prawn and mysids as dominant foods (Figure 4): striped bass were probably feeding indiscriminately on all three shrimp species, including oriental and California bay shrimp (Ganssle 1966), despite the presence of only Siberian prawn in the guts of our fish. Thus, G. daiberi is likely a major route through which δ 13 C-enriched carbon can be incorporated by higher-level consumers in Suisun Marsh, similar to the results of Grimaldo et al (2009) and consistent with the diet study of Bryant and Arnold (2007). The high SAV probabilities for striped bass also may have resulted from their heavier feeding on G. daiberi before our sampling in conjunction with inclusion of the larger Siberian prawn in their diets as the fish grew.…”

“…In sum, there were two broad groups of primary producers along the carbon isotope axis: a δ 13 C-depleted group comprised of phytoplankton, terrestrial vegetation, and emergent aquatic vegetation; and a δ 13 Cenriched group comprised of phytobenthos and SAV. This isotopic arrangement of carbon sources corresponded well with results of Grimaldo et al (2009) and with the results of the fresher regions of Cloern et al (2002) and Howe and Simenstad (2011). That the Suisun Marsh primary producer isotope values matched up with the fresher regions of both and Howe and Simenstad (2011) .…”

“…Relatively new alien species such as G. daiberi (and possibly S. laticauda; Figure 3) are likely a major route through which carbon from alien SAV enters the food web to higher-level consumers, as appeared to be the case in this study with the shrimps and, to a lesser extent, striped bass, in addition to these alien invertebrates being eaten directly by many native and alien fishes in other regions in the estuary (Feyrer et al 2003;Bryant and Arnold 2007;Grimaldo et al 2009). Significant isotopic differences between native and alien species in three taxonomic groups (e.g., polychaetes, decapod shrimp, and amphipods; Table 1) indicate that alien species often occupy slightly different niches, thereby elevating the food web's complexity.…”

“…Howe and Simenstad (2011) examined aquatic food webs of wetlands along the estuary's salinity gradient using sulfur, carbon, and nitrogen stable isotopes. In contrast to Grimaldo et al (2009), most of the organisms in Howe and Simenstad's study ultimately derived their food from wetland-produced vegetation, reflecting differences in the dominant vegetation types present. To extend the geographic scope of food-web knowledge of the estuary, we performed a study in Suisun Marsh, a region of the estuary slated for large-scale tidal restoration for the benefit of native fishes.…”

“…Grimaldo et al (2009) used both stable isotopes and gut-content analyses to delineate the food web of flooded agricultural islands in the freshwater portion of the estuary. They found that some fishes, primarily pelagic species, depended heavily on pelagic food sources, but the majority of species mostly ate food supported by submerged aquatic vegetation (SAV) and associated periphyton.…”