Lepidium latifolium is an aggressive plant species that is invading both wetlands and uplands across a wide range of salinities. This study examined how salinity and moisture gradients influence the potential for invasion by L. latifolium. Three sites in the San Francisco Bay Delta with varying salinity and moisture levels were chosen as research sites. These sites corresponded to a dry (18.32%) freshwater (3.88‰) site, a wet (40.53%) brackish (23.16‰) site, and a moderately wet (38.33%) saline (32.33‰) site. Our results showed that inflorescence number and height were unaffected by salinity or soil moisture. Seed production was significantly affected by salinity (P = 0.0297) and moisture levels (P = 0.0004). Seed production at the high salinity site was reduced by 29% from the freshwater site. Seed production at the wettest site had an 87% reduction from the driest site. Seed viability was also reduced by both salinity (P < 0.0001) and soil moisture (P < 0.0001). Viability at the highest salinity site was reduced by 49% from freshwater sites and was reduced by 8% from the wettest to driest sites. Mean seed dispersal distance was 0.23 m greater at the freshwater site, which was not statistically significant (P = 0.1815). The deleterious effects of salinity and moisture resulted in reduced L. latifolium densities in high salinity and moisture locations, but only at the highest salinity site. With increased seed production and viability, drier freshwater sites experience greater propagule pressure, resulting in an increased invasion potential. Therefore, variability along salinity and soil moisture gradients serve as useful metrics for prioritizing control and eradication efforts of L. latifolium.

The Sacramento splittail (Pogonichthys macrolepidotus) is composed of two genetically distinct populations endemic to the San Francisco Estuary (SFE). The allopatric upstream spawning habitat of the Central Valley (CV) population connects with the sympatric rearing grounds via relatively low salinity waters, whereas the San Pablo (SP) population must pass through the relatively high-salinity Upper SFE to reach its allopatric downstream spawning habitat. We hypothesize that if migration through SFE salinities to SP spawning grounds is more challenging for adult CV than SP splittail, then salinity tolerance, osmoregulatory capacity, and metabolic responses to salinity will differ between populations. Osmoregulatory disturbances, assessed by measuring plasma osmolality and ions, muscle moisture and Na⁺-K⁺-ATPase activity after 168 to 336 h at 11‰ salinity, showed evidence for a more robust osmoregulatory capacity in adult SP relative to CV splittail. While both resting and maximum metabolic rates were elevated in SP splittail in response to increased salinity, CV splittail metabolic rates were unaffected by salinity. Further, the calculated difference between resting and maximum metabolic values, aerobic scope, did not differ significantly between populations. Therefore, improved osmoregulation came at a metabolic cost for SP splittail but was not associated with negative impacts on scope for aerobic metabolism. These results suggest that SP splittail may be physiologically adjusted to allow for migration through higher-salinity waters. The trends in interpopulation variation in osmoregulatory and metabolic responses to salinity exposures support our hypothesis of greater salinity-related challenges to adult CV than SP splittail migration and are consistent with our previous findings for juvenile splittail populations, further supporting our recommendation of population-specific management.

The Sacramento splittail (Pogonichthys macrolepidotus) is a minnow endemic to the highly modified San Francisco Estuary of California, USA and its associated rivers and tributaries. This species is composed of two genetically distinct populations, which, according to field observations and otolith strontium signatures, show largely allopatric distribution patterns as recently hatched juveniles. Juvenile Central Valley splittail are found primarily in the nearly fresh waters of the Sacramento and San Joaquin rivers and their tributaries, whereas San Pablo juveniles are found in the typically higher-salinity waters (i.e. up to 10‰) of the Napa and Petaluma Rivers. As the large salinity differences between young-of-year habitats may indicate population-specific differences in salinity tolerance, we hypothesized that juvenile San Pablo and Central Valley splittail populations differ in their response to salinity. In hatchery-born and wild-caught juvenile San Pablo splittail, we found upper salinity tolerances, where mortalities occurred within 336 h of exposure to 16‰ or higher, which was higher than the upper salinity tolerance of 14‰ for wild-caught juvenile Central Valley splittail. This, in conjunction with slower recovery of plasma osmolality, but not ion levels, muscle moisture or gill Na(+),K(+)-ATPase activity, in Central Valley relative to San Pablo splittail during osmoregulatory disturbance provides some support for our hypothesis of inter-population variation in salinity tolerance and osmoregulation. The modestly improved salinity tolerance of San Pablo splittail is consistent with its use of higher-salinity habitats. Although confirmation of the putative adaptive difference through further studies is recommended, this may highlight the need for population-specific management considerations.