Characterizing macroinvertebrate community composition and abundance in freshwater tidal wetlands of the Sacramento-San Joaquin Delta.

Rosemary Hartman,Ryan Kok,Stacy Sherman,Dave Contreras,Alison Furler,Carina Zittra

doi:10.1371/journal.pone.0215421

Abstract

Restored tidal wetlands may provide important food web support for at-risk fish species in the Sacramento-San Joaquin Delta (Delta) of California, including Delta Smelt (Hypomesus transpacificus) and Chinook Salmon (Oncorhynchus tshawytscha). Since many tidal wetland restoration projects are planned or have recently been constructed in the Delta, understanding the diversity and variability of wetland invertebrates that are fish prey items is of increasing importance. During this study, two different invertebrate sampling techniques were tested (leaf packs and sweep nets) in four habitat types within three different wetland areas to evaluate which sampling technique provided the most reliable metric of invertebrate abundance and community composition. Sweep nets provided a better measure of fish food availability than leaf packs and were better able to differentiate between habitat types. Generalized linear models showed submerged and floating vegetation had higher abundance and taxa richness than channel habitats or emergent vegetation. Permutational multivariate analysis of variance showed significantly different communities of invertebrates in different habitat types and in different wetlands, and point-biserial correlation coefficients found a greater number of mobile taxa associated with sweep nets. There were more taxa associated with vegetated habitats than channel habitats, and one area had more taxa associated with it than the other two areas. These results suggest that restoration sites that contain multiple habitat types may enhance fish invertebrate prey diversity and resilience. However, the effect of habitat diversity must be monitored as restoration sites develop to assess actual benefits to at-risk fish species.

Highlights

The variance in total catch for sweep nets was higher than leaf packs (Bartlett’s K-squared: 32.39, P value
We found that sweep nets were more effective and efficient than leaf packs in sampling a variety of shallow water habitats in the Sacramento-San Joaquin Delta for macroinvertebrate taxa valuable in diets of key fish species
We found a wide variety of taxa in emergent aquatic vegetation (EAV), including Diptera, Hemiptera, and Amphipoda (Fig 4)

Summary

Methods

Sample location and timingWe conducted two intensive bouts of sampling, one on March 16–17, 2016, and one on May 2–3, 2016. Spring (February-May) is the period of upstream migration of Delta Smelt and the peak period for residence of juvenile salmonids [42,43,44] This is not the period of highest amphipod and insect abundance [2, 28], but the salmon and smelt that consume these are present at their highest densities, and are most able to take advantage of the available invertebrates. All samples were collected in the Cache Slough Complex, a region in the north-east Delta with high freshwater tidal wetland restoration potential because of high native fish density and appropriate intertidal elevations (Fig 1B, Table 1)[44, 45]. Preserved invertebrates were counted and identified to varying taxonomic levels, according to their importance in fish diets, grouped into larger taxonomic groupings (Order or Class) for analysis. We counted at least 400 organisms to achieve a precision of +/- 10%, as suggested by Harris et al [52]

Results

Discussion

Conclusion