Basal resources for collector-gatherer invertebrates in the Fall River, Shasta County

Background/Question/Methods

Baetid mayflies (Ephemeroptera, Baetidae) are one of the most abundant insect families found in the aquatic food web of the Fall River, Shasta County and are a key food resource for rainbow trout (Oncorhynchus mykiss) -- which play a large role in the ecosystem, culture and economy of the Fall River valley. Baetids generally feed as collector-gatherers and rely on fine particulate organic matter (FPOM) as a nutritive resource. Given their importance in the Fall River food web, we ask: What are the dominant sources of FPOM available to Baetid mayflies and other collector-gatherers? How do these sources vary seasonally? We predict that the proportion of detritus versus algal or other resources present in FPOM will vary on a seasonal basis, with the greatest proportion of algae/periphyton and phytoplankton occurring in the summer and fall and the greatest proportion of detritus and microbes occurring in the winter and spring. The seasonal availability of resources can influence management decisions on the Fall River. For example, 

Results/Conclusions

Preliminary findings indicate that there are some basal resources for which unique stable isotope signatures exist. For example, macrophytes and algae/periphyton are easily distinguished from one another using stable isotope signatures. However, some overlap of dN¹⁵ and dC¹³signatures exists for selected riparian species and some macrophytes making it difficult to discern the source materials based on stable isotope signatures alone. For example, one cannot easily distinguish the difference between willow leaves (a riparian source) and Zannichellia palustris (an aquatic macrophyte) using only  dN¹⁵ and dC¹³signatures. FPOM samples form distinct clusters on a dN¹⁵ and dC¹³biplot (Figure 3), and indicate that the FPOM dN¹⁵ and dC¹³signatures assessed in July 2013 most resemble the signatures of periphyton and algae. Similarly, the dN¹⁵ and dC¹³signatures of selected macroinvertebrates (Corixidae, Amphipoda, Dytiscidae, Baetidae, Brachycentridae, etc.) indicate that their primary food source is isotopically similar to periphyton and algae.