Spatiotemporal variation in pollinator taxa on the santa ana river wooly star Eriastrum densifolium ssp. Sanctorum (Milliken) mason (Polemoniaceae)

Flood control, via the construction of the Seven Oaks Dam in the Santa Ana River in southern California, has altered habitat in the downstream alluvial wash community and jeopardized the persistence of pioneer plant species that rely on periodic flood-scouring and sand recharge. One species, Eriastrum densifolium ssp. sanctorum (“Eriastrum”), an endangered perennial, has been greatly affected and persists in spatially separated populations on successional vegetation terraces. We made “dawn to dusk” observations of pollinators in three phenological stages at four sites, representing young and old seral stages, to identify primary pollinators and to elicit daily, site, and seasonal patterns of visitors. Data were compared to previous observations and correlated with annual rainfall to determine long-term trends. Shifts in pollinator taxa have occurred, with some consistency through time and space, during nine years. The sites with the highest pollinator abundance (older sites) are least suited to Eriastrum. Hummingbirds (prevalent in early season) and the Acton giant flower-loving fly (prevalent in late season) have been consistently present across years, whereas other taxa have varied. A shift from native bees to non-native bees has occurred, although native bees in the families Halictidae and Apidae (Micranthophora and Melissodes) remain important. In general, there was no correlation between taxa abundance and rainfall. Eriastrum appears to be a generalist able to take advantage of the behaviour, cycling, and availability of diverse pollinators daily, seasonally, and annually.

Tracing Artificially Recharged Groundwater using Water and Carbon Isotopes

We conducted an isotopic analysis of groundwater in Orange County, California, USA, around the Talbert Seawater Injection Barrier to determine if recycled water, used to artificially recharge local aquifers, carries a unique isotopic signature that can be used as a tracer. From September 2014 to April 2015, we collected groundwater from six privately owned wells within the coastal groundwater basin, along with various surface waters. All water samples were analyzed for their stable isotopic composition (δ18O, δD), the δ13C and 14C signature of the dissolved inorganic carbon (DIC) pool, DIC concentration, pH, and salinity. The DIC of groundwater mixing with recycled water is enriched in 14C above natural background levels, with varying signal strength through time, depleted in δ13C, and low in DIC concentration. Water isotopes further suggest that recycled water is a mixture of Colorado River water and regional groundwater. In contrast, groundwater found further away from the injection barrier has carbon and water isotope composition consistent with regional groundwater and Santa Ana River water. Our findings imply that recycled water injected through the Talbert Barrier is isotopically unique, and that 14C enrichment may be used as an intrinsic tracer of artificial recharge within the basin.

The Santa Ana

The Santa Ana River, running through three of the largest counties in America, has a long, often overlooked history from Native American settlement to the contemporary industrial corridor, but always riparian and beautiful. This essay and accompanying photographs explore the middle section of the river, which runs through San Bernardino and Riverside Counties, the Native American settlement of Agua Mansa, the exploration of de Anza, and the current landscape of homeless residents in an emerging post-industrial landscape.