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Current location: Home > Biological Effects Model > 1° Driver: Susceptibility > 2° Driver: Adverse Effects of Low DO on Food Web |
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Secondary Driver: Adverse Effects of Jump down this page to: Steelhead | Chinook Salmon | Delta Smelt | Longfin Smelt | Sacramento Splittail | White Sturgeon | Green Sturgeon | Striped Bass The effects of low DO concentrations on trophic transfer may be highly complex and vary between systems and between different predator/prey species assemblages. Links between predators and herbivores to plants are not always linear (Krebs 2001). This section focuses on general discussion about how low DO concentrations in the DWSC may affect prey resources for the fish found there but is not intended to be a tutorial on food web ecology. Additionally, we do not have detailed knowledge of the food chain present in the DWSC and all the potential linkages among the species found there. A more complete discussion of food web ecology can be found in most general ecology texts such as Krebs (2001). It is important to consider impacts on specific species based on species-specific biology and trophic position as each species may be affected differently depending on the effect of low DO concentrations on a particular prey item. Severe alterations in the composition and abundance of the primary producer and primary/secondary consumer assemblages in the Delta have been implicated in the decline of native fish species found there (U.S. Fish and Wildlife Service 1996; Kimmerer 2002). Low DO concentrations in the DWSC have the potential to alter both the abundance and the composition of food resources in this area. DO concentrations could fall low enough to limit the food supply to fish by directly reducing food availability or causing prey to be found only in habitats beyond the physiological tolerances of the fish that feed on such prey. Variation among species in ability to tolerate low DO concentrations and the effect of low DO concentrations on escape behavior can result in changes to predator-prey interactions (Breitburg et al. 1997). Different prey species (and life stages) probably have different minimum DO requirements. For example, conditions in the DWSC may affect zooplankton resources for planktivorous fish species or life stages. However, because zooplankton are generally more tolerant of low DO concentrations than their predators, low DO concentrations likely will affect planktivorous fish before affecting the zooplankton prey base. The following studies have reported 24-hour DO LC50 values for several zooplankton species:
Apart from causing outright mortality of prey species, low DO concentrations may make prey species unavailable to predators. If prey species (such as the zooplankton mentioned above) occupy habitats of lower DO concentrations than predators can tolerate, such prey may be functionally unavailable, even though physically still present. However, the combination of environmental conditions (e.g., increased temperatures, low DO concentrations) required to make prey items unavailable probably depends on the tolerances of individual species. Low DO concentrations may have a different effect on prey availability, depending on the prey species and the predators that feed on them. For example, compared to small prey fish species (such as juvenile salmon), adult striped bass could take advantage of their stronger ability to tolerate low DO concentrations to increase their predation rates. In this instance, impacts at lower trophic levels can have unpredictable impacts on species that feed at higher levels in the food chain. For this reason, it is difficult to speculate about trophic interactions in specific ecological systems without experimental evidence. In addition to pelagic food resources, low DO concentrations in the DWSC have the potential to alter both the abundance and the composition of benthic food resources in this area. The impact of extremely low DO concentrations on the benthic community may be long lasting. Many benthic organisms have short dispersal distances; thus, they require time to reoccupy environments that have experienced negative effects of low DO concentrations. Initial studies of the DWSC indicate that, within areas of the DWSC that have been affected by low DO concentrations, the benthic community is radically different in both composition and abundance from areas of the Delta beyond the extent of the plume of low DO concentrations (Mueller-Solger pers. comm.). As a result, prey availability throughout the year for benthic foraging fish could be influenced by the history of low DO concentrations there. The importance of this driver as a significant source of mortality for different species may depend also on whether the DWSC is used primarily as a migration corridor or as a feeding resource. Migratory species will be inherently less affected by reduced prey availability as a result of low DO concentrations found there, and species feeding and rearing in the DWSC are more likely to be affected. In general, a low DO concentration varies in its impact on trophic interactions and therefore has the potential to cause major alteration in the relative importance of different pathways of energy flow in aquatic ecosystems (Breitburg et al. 1997). Jump to "General Effects" discussion under other Secondary Drivers: Steelhead (Oncorhynchus mykiss) Hypothesis: Low DO concentrations affect the number and type of prey species available to trout. 1. How does this driver operate? No data are available on the foraging activities or diet of steelhead juveniles and smolts in the lower San Joaquin River and DWSC. Steelhead are opportunistic feeders that feed on many types of prey—drifting invertebrates, terrestrial insects, benthic invertebrates, zooplankton, and fish (Moyle 2002). Severe alterations in the composition and abundance of the primary producer and primary/secondary consumer assemblages in the Delta have been implicated in the recent decline of delta smelt and other native fish species (U.S. Fish and Wildlife Service 1996; Kimmerer 2002; Bennett 2005). The low DO concentrations in the DWSC have the potential to alter both the abundance and composition of food resources for steelhead. 2. Are there critical thresholds associated with this driver? Different steelhead prey species (and life stages) likely have different minimum DO requirements. The thresholds associated with those requirements are not known for most prey species. 3. How important is this driver? It is unlikely that the DWSC is an important foraging area for juvenile steelhead. Adult and juvenile steelhead likely use the ship channel as a migration corridor and move quickly through the area to reach suitable habitat upstream and downstream of the DWSC. 4. How well is this driver understood? No information is available to assess whether the availability of prey in the Delta is a limiting factor for juvenile steelhead or to evaluate the effect of low DO concentrations on the availability of prey in the DWSC. Jump to "Steelhead" discussion under other Secondary Drivers: Chinook Salmon (Oncorhynchus tshawytscha) Hypothesis: Low DO concentrations affect the number and type of prey species available to juvenile Chinook salmon. 1. How does this driver operate? Common prey items of emigrating juveniles in the Delta and upper estuary include Corophium spp., crab megalopae, and mysid shrimp (most prevalent to least prevalent) (MacFarlane and Norton 2002). As juveniles migrate from the western Delta to San Francisco Bay, the dominant food items change progressively from invertebrates to fish larvae. Based on the age, growth, condition, and body composition of proteins and lipids, MacFarlane and Norton concluded that Chinook salmon smolts exhibit little estuarine dependence, migrating rapidly through the estuary and not increasing much in size until they reach more productive marine waters in the Gulf of the Farallones. DO concentrations can affect the availability of food for juvenile Chinook salmon through direct effects on the survival and production of prey organisms or by eliminating access to food if prey organisms are concentrated in habitats where DO concentrations are too low for juvenile Chinook salmon. 2. Are there critical thresholds associated with this driver? Different salmon prey species (and life stages) probably have different minimum DO requirements. The thresholds associated with those requirements are not known for most salmon prey species. 3. How important is this driver? The importance of this potential driver is unknown although the limited residence time of smolts in the Delta suggests that a localized reduction in food supply in the DWSC would not have a significant effect on emigrating juveniles. 4. How well is this driver understood? The dynamics of food availability (in general and in relation to DO) and the relative importance of the DWSC and lower Delta as a rearing habitat for young (pre-smolt) salmon are unknown. Jump to "Chinook Salmon" discussion under other Secondary Drivers: Delta Smelt (Hypomesus transpacificus) Hypothesis: The delta smelt food web is negatively affected by DO concentrations below the regulatory minimum. 1. How does this driver operate? Delta smelt are pelagic foragers that feed extensively on copepods, cladocerans, amphipods, insect larvae, and shrimp (U.S. Fish and Wildlife Service 1996; Moyle 2002; Bennett 2005). Severe alterations in the composition and abundance of the primary producer and primary/secondary consumer assemblages in the Delta have been implicated in the recent decline of delta smelt and other native fish species (U.S. Fish and Wildlife Service 1996; Kimmerer 2002; Bennett 2005). Low DO concentrations in the DWSC have the potential to alter both the abundance and composition of food resources in this area. 2. Are there critical thresholds associated with this driver? Different delta smelt prey species (and life stages) probably have different minimum DO requirements. The thresholds associated with those requirements are not known for most delta smelt prey species. 3. How important is this driver? The effect of extremely low DO concentrations on pelagic food resources in this area may be severe but is not expected to be long lasting because pelagic organisms can migrate back into the DWSC after low DO concentrations subside. The negative effects of low DO concentrations in the DWSC may be partially mitigated by positive effects on aquatic productivity downstream (particularly in the area of the 2-ppt salinity isocline) as nutrient-rich water from the DWSC mixes with and enriches well-oxygenated water from other parts of the Delta (Breitburg 2002). 4. How well is this driver understood? The effect of diminished delta smelt food supplies that may result from low DO concentrations in the DWSC is not well understood. Because of recent declines in delta smelt populations, the effects of a suspected decline in the delta smelt prey base are a matter of considerable interest and study by agencies (Bennett 2005). No results of these food-web studies are yet available. Jump to "Delta Smelt" discussion under other Secondary Drivers: Longfin Smelt (Spirinchus thaleichthys) Hypothesis: The longfin smelt food web is negatively affected by DO concentrations below the regulatory minimum. 1. How does this driver operate? Longfin smelt are pelagic foragers that feed extensively on copepods, amphipods, and shrimp (U.S. Fish and Wildlife Service 1996; Moyle 2002). Severe alterations in the composition and abundance of the primary producer and primary/secondary consumer assemblages in the Delta have been implicated in the recent decline of longfin smelt and other native fish species (U.S. Fish and Wildlife Service 1996; Kimmerer 2002). Low DO concentrations in the DWSC have the potential to alter both the abundance and composition of food resources in this area. 2. Are there critical thresholds associated with this driver? Different longfin smelt prey species (and life stages) probably have different minimum DO requirements. Those limits are not known for most longfin smelt prey species. 3. How important is this driver? The impact of extremely low DO concentrations on pelagic food resources in this area may be severe but is not expected to be long lasting as pelagic organisms can migrate back into the DWSC after low DO concentrations subside. The negative effects of low DO in the DWSC may be partially mitigated by positive effects on aquatic productivity downstream (particularly in the area of the 2-ppt salinity isocline) as nutrient-rich water from the DWSC mixes with and enriches well-oxygenated water from other parts of the Delta (Breitburg 2002). 4. How well is this driver understood? The impact of diminished food supplies for longfin smelt that may result from low DO concentrations in the DWSC is not well understood. Because of recent native fish species population declines, the effects of a suspected decline in the Delta’s prey base are a matter of considerable interest and study by agencies and academics (Bennett 2005). No results of these food-web studies are yet available. Jump to "Longfin Smelt" discussion under other Secondary Drivers: Sacramento Splittail (Pogonichthys macrolepidotus) Hypothesis: The Sacramento splittail food web is negatively affected by DO concentrations below the regulatory minimum. 1. How does this driver operate? Sacramento splittail are benthic foragers that fed extensively on the opossum shrimp (Neomysis mercedis) prior to that organism’s recent decline (U.S. Fish and Wildlife Service 1996; Moyle et al. 2004). Severe alterations in the composition and abundance of the primary producer and primary-secondary consumer assemblages in the Delta have been implicated in the decline of Sacramento splittail and other native fish species (U.S. Fish and Wildlife Service 1996; Kimmerer 2002). Low DO concentrations in the DWSC have the potential to alter both the abundance and composition of benthic food resources in this area. The effect of extremely low DO concentrations on the benthic community may be long lasting. Many benthic organisms have short dispersal distances, and these organisms therefore require time to reoccupy environments that have experienced negative effects of low DO. Initial studies of the DWSC indicate that, within areas of the DWSC that have been affected by low DO concentrations, the benthic community is radically different in both composition and abundance from areas of the Delta beyond the extent of the low-DO plume (Mueller-Solger pers. comm.). As a result, foraging of Sacramento splittail in the DWSC throughout the year probably has been influenced by the history of low DO concentrations there, although the nature and extent of those effects are not known. 2. Are there critical thresholds associated with this driver? Each species and life stage of Sacramento splittail prey probably has a different minimum DO requirement. Those requirements are not known for most Sacramento splittail prey species. 3. How important is this driver? The impact of extremely low DO concentrations on benthic food resources in this area may be severe and long lasting because benthic organisms require time to reoccupy the DWSC after low DO concentrations subside. Sacramento splittail foraging in the DWSC (e.g., on their way to or from spawning locations farther up the San Joaquin River) may be adversely affected by limited benthic food supplies in the DWSC. The negative effects of low DO concentrations in the DWSC may be partially mitigated by positive effects on aquatic productivity downstream (particularly in the area of the 2-ppt salinity isocline) as nutrient-rich water from the DWSC mixes with and enriches well-oxygenated water from other parts of the Delta (Breitburg 2002). 4. How well is this driver understood? The effects of diminished Sacramento splittail food supplies that may result from low DO concentrations in the DWSC are not well understood. Because of recent population declines in native fish species, the effects of a suspected decline in the Delta’s prey base are a matter of considerable interest and study by agencies (Bennett 2005). No results of these food-web studies are currently available. Jump to "Sacramento Splittail" discussion under other Secondary Drivers: White Sturgeon (Acipenser transmontanus) Hypothesis: The white sturgeon food web is negatively affected by DO concentrations below the regulatory minimum. 1. How does this driver operate? White sturgeon are benthic foragers that have been reported to consume opossum shrimp, amphipods, small fish, clams, and crabs (Moyle 2002). The other sturgeon in the Delta, green sturgeon (A. medirostrus), have similar benthic feeding habits and diets (Radtke 1966; Wydoski and Whitney 1979; Moyle 2002). Severe alterations in the composition and abundance of the primary producer and primary/secondary consumer assemblages in the Delta have been implicated in the decline of other native fish species (U.S. Fish and Wildlife Service 1996; Kimmerer 2002). Low DO concentrations in the DWSC have the potential to alter both the abundance and composition of food resources in this area. Initial studies of the DWSC indicate that, within areas of the DWSC that have been affected by low DO concentrations, the benthic community is radically different in both composition and abundance from areas of the Delta beyond the extent of the low-DO plume (Mueller-Solger pers. comm.; CA Dept. of Water Resources unpublished data). As a result, white sturgeon foraging in the DWSC are probably affected by the history of low DO concentrations there, although the extent of those impacts is not known. 2. Are there critical thresholds associated with this driver? Different white sturgeon prey species (and life stages) likely have different minimum DO requirements. Those limits are not known for most white sturgeon prey species. 3. How important is this driver? The impact of extremely low DO concentrations on benthic food resources in this area may be severe and moderately long lasting. The area of the DWSC affected by low DO concentrations is small relative to the area required by individual white sturgeon. Although foraging in this area is probably adversely affected by the history of low DO concentrations, white sturgeon are capable of swimming to other areas of the Delta that have greater food availability. The negative effects of low DO in the DWSC may be partially mitigated by positive effects on aquatic productivity downstream (particularly in the area of the 2-ppt salinity isocline) as nutrient-rich water from the DWSC mixes with and enriches well-oxygenated water from other parts of the Delta (Breitburg 2002). 4. How well is this driver understood? The impact of diminished white sturgeon food supplies that may result from low DO concentrations in the DWSC is not well understood. Because of recent native fish species population declines, the effects of a suspected decline in the Delta’s prey base are a matter of considerable interest and study by agencies and academics (Bennett 2005). No results of these food-web studies are yet available. Jump to "White Sturgeon" discussion under other Secondary Drivers: Green Sturgeon (Acipenser medirostris) Green sturgeon are benthic foragers that have been reported to consume opossum shrimp, amphipods, small fish, clams, and crabs (Radtke 1966; Wydoski and Whitney 1979; Moyle 2002). Although little species-specific information is available for green sturgeon, it is likely that information for white sturgeon is generally applicable to green sturgeon. Jump to "Green Sturgeon" discussion under other Secondary Drivers: Striped Bass (Morone saxatilis) Hypothesis: The striped bass food web is adversely affected by DO concentrations below the regulatory minimum. 1. How does this driver operate? Striped bass are primarily opportunistic feeders that feed mainly on small fish and invertebrates. In the Delta, adult and subadult bass feed largely on threadfin shad, but other small fish, such as salmon fry, juvenile striped bass, various native fishes, and crayfish, are common in their diets, depending on local food availability (Moyle 2002). Larval and postlarval striped bass feed mainly on copepods, and the diet of young-of-the-year bass is dominated by opossum shrimp and occasionally amphipods, copepods, and small fish. DO concentrations may fall low enough to limit available food supply to striped bass by direct reduction of available food or by eliminating access to food if prey organisms are available only in habitats where DO concentrations are too low for striped bass. 2. Are there critical thresholds associated with this driver? Different striped bass prey species (and life stages) probably have different minimum DO requirements. The following studies have reported 24-hour DO LC50 (lethal concentration that results in 50% mortality) values for several zooplankton species.
3. How important is this driver? The effects of low DO concentrations and other environmental factors on food abundance and production and the resulting effects on predator populations may be highly complex and vary between systems. It is likely that zooplankton prey species have greater physiological tolerances for low DO concentrations than striped bass (Breitburg et al. 1997), resulting in no detectable changes in zooplankton community composition and structure (although juvenile striped bass may be less able to access prey in such areas). Alternately, low DO concentrations may decrease the abundance of small prey fish and affect the suitability of low DO areas as feeding habitat for adult striped bass. It is difficult to speculate about trophic interactions in specific ecological systems without empirical or experimental evidence. Effects may be greater on larval striped bass as they pass through the DWSC after hatching, or during juvenile rearing, when they have increased metabolic demands associated with early development. The effects of food supply on growth and survival likely will vary depending on the specific life stage (e.g., transition of larvae to external feeding) and duration of exposure to low prey densities in the DWSC. This driver is not likely to be a significant stressor for adult striped bass because the DWSC is used primarily as a migration corridor. 4. How well is this driver understood? The impacts of low DO concentrations on food supply and the importance of the DWSC as a feeding and rearing area for striped bass are not well understood. Jump to "Striped Bass" discussion under other Secondary Drivers: |