These removal value trends for fipronil and cyfluthrin correspond to their seasonal concentration values in water and sediment. This suggests that the highest removal of these two analytes took place when their mass inputs were at their peak. Removal of the remaining compounds was likely less influenced by this phenomenon since they were present at much lower levels. Importantly, none of the compounds displayed a clear removal trend based on seasonality and weather. Southern California is relatively warm year-round, with cooler temperatures occurring during the rainy winter seasons. If temperature and weather patterns contributed significantly to the removal of fiproles andpyrethroids, we would have expected to observe higher removal of all analytes of interest in the summer and lower values in the fall and winter. Overall, these results suggest that removal was more dependent on the availability of sediment binding sites and microbial degradation, which was also observed for a Prado Wetlands vegetated CW. The mass influx, mass efflux, and change in mass flux of each analyte of interest through the UPOW-CW are also reported in Table 4.1 to provide additional vital information regarding removal of fiproles and pyrethroids. Fipronil, bifenthrin, and cyfluthrin were imported into the cell at the greatest rates,planting gutter with mean mass influx values of 37.6-123 mg d-1, 0-45.3 mg d-1, and 20.3-218 mg d-1, respectively. Lower mean mass influx values were observed for fipronil desulfinyl , fipronil sulfide , and fipronil sulfone. 

Changes in mass flux represent the net import or net export of a given analyte to or from the UPOW-CW. Fipronil , bifenthrin , and cyfluthrin experienced the greatest changes in mass flux, several of which were statistically significant. The lowest changes in mass flux were reported for fipronil desulfinyl, fipronil sulfide, and fipronil sulfone, with values of 0-1.16 mg d-1, 0-1.22 mg d-1, and -0.230-14.9 mg d-1, respectively. Several statistically significant differences were also reported for these analytes. These changes in mass flux were much lower than those reported in a vegetated cell at the Prado Wetlands, but this seems to be a result of much lower mass influxes of fiproles and pyrethroids in the UPOW-CW. Negative changes in mass flux were reported for fipronil sulfone and cyfluthrin in July 2018. However, neither of these changes in mass flux was statistically significant. Furthermore, concentration-based removal values were positive despite net mass export. It is likely that the higher flow values at the UPOW-CW outlet relative to the inlet that were calculated for most of the sampling months resulted in resuspension of some contaminated sediment particles and led to release of transiently stored residues in these two instances. This is supported by the finding that all sedimentation rates calculated during the study were negative , indicating net export of TSS. Predominantly positive changes in mass flux and positive concentration based removal values indicate that fiproles and pyrethroids were effectively treated by the UPOW-CW despite this loss of TSS. As discussed elsewhere , flow through the Prado wetlands is regulated to optimize water quality and quantity throughout the entire pond complex, which results in occasional net outflow from isolated wetland cells. Furthermore, a small number of negative changes in mass flux occurred at the Prado Wetlands in a vegetated cell when water flow was higher at the outlet than the inlet. To further emphasize the importance of fiprole and pyrethroid sorption during treatment, the percent of analytes detected on TSS in UPOW-CW whole water samples was calculated. Where applicable, values are reported for inlet, midpoint, and outlet samples for comparison.

Statistically significant differences were measured between inlet and outlet values for fipronil in June, August, and December 2018; for fipronil sulfone in December 2018; and for cyfluthrin in December 2018. Every one of these statistically significant differences represents an increase in the relative amount of analyte adsorbed to TSS from inlet to outlet. This further supports the consensus of the data detailed thus far: that contaminants entering the UPOW-CW become enriched in the sediment compartment via settling of previously contaminated solids and sorption of dissolved fiproles and pyrethroids. Since whole water and sediment concentrations were lower at the outlet than the inlet , it is clear that degradation of these sediment-bound residues is taking place which offsets the loss of analyte via particle resuspension. All other inlet-outlet comparisons were statistically similar, meaning that there was insufficient evidence to conclude that there were any differences between the reported mean values. Some of the % on TSS values for fipronil and cyfluthrin in June and July 2018 were anomalously low. It is possible that conditions in the wetland water altered the phase partitioning of these insecticides. Alternatively, residues that were apparently dissolved in the aqueous phase may have been present in suspended organic colloids, distorting analyte partitioning. Previous research at the Prado Wetlands has also suggested that this occurs. The remainder of the values ranged from 63.2-100%, which is more in line with the expected partitioning of these HOCs. Several linear regression analyses were performed in an effort to identify additional conditions that facilitated fiprole and pyrethroid removal from the UPOW-CW. Two dependent variables—concentration-based removal and change in mass flux—were each analyzed with three independent variables—sedimentation rate, water pH, and water temperature. None of the performed regressions revealed any statistically significant linear relationships, indicating that sedimentation rate, water pH,gutter berries and water temperature did not significantly contribute to the concentration-based removal values or changes in mass flux that are reported in this study. Sedimentation rate was expected to share a significant relationship with at least one of the dependent variables due to the importance of analyte sorption in wetland removal and based on past results at the Prado Wetlands. 

However, the negative sedimentation rates calculated in this study already seemed to contradict this hypothesis. Therefore, it is likely that the net loss of TSS from the UPOW-CW caused by efforts to regulate the flow of the Prado Wetlands complex precluded additional confirmation of the importance of sedimentation in the removal of fiproles and pyrethroids in the UPOW-CW based on the measurements taken. Future research should consider laboratory experiments to identify the contribution of other mechanisms to the removal of these compounds. Sublethal and lethal toxicity values derived for the amphipod Hyalella azteca were used to calculate TUs for bifenthrin and cyfluthrin. Previous research has identified this organism as being particularly sensitive to pyrethroids. Sublethal and lethal outlet TUs were always lower than corresponding inlet TUs for bifenthrin and cyfluthrin, and the majority of these reductions were statistically significant. Sublethal bifenthrin TUs were reduced from 0-20.2 at the inlet to 0-7.70 at the outlet while lethal values decreased from 0-8.65 to 0-3.30. Cyfluthrin sublethal TUs changed from 27.5-204 to 9.21-32.5 at the inlet and outlet, respectively. Lethal TUs decreased from 22.7-168 at the inlet to 7.61-26.8 at the outlet for cyfluthrin. These sublethal and lethal TU values are similar to those reported in a previous Prado Wetlands study. TUs for fipronil sulfide, fipronil, and fipronil sulfone were calculated for the midge Chironomus dilutus since it is the most sensitive organism reported in the literature. As was the case for pyrethroids, UPOW-CW treatment reduced fiprole TU values in every instance, and these decreases were statistically significant in the majority of comparisons Table 4.4). Fipronil sulfide sublethal TUs were reduced from 0-0.550 at the inlet to 0-0.0731 at the outlet. Lethal fipronil sulfide TUs were 0-0.0788 and 0-0.0105 at the inlet and outlet, respectively. Fipronil TUs decreased from 0.804-9.74 to 0.121-3.41 and from 0.320-3.88 to 0.0482-1.36 as a result of CW treatment. Sublethal TUs for fipronil sulfone were reduced from 0-6.43 at the inlet to 0-1.46 at the outlet. Similarly, fipronil sulfone lethal TUs were 0-0.476 at the inlet and decreased to 0-0.108 at the outlet. Similar TU values were observed in a vegetated CW at the Prado Wetlands. The results of the TU analysis reveal that the UPOW-CW always reduced fiprole and pyrethroid TUs for sensitive aquatic invertebrates. It is important to note that the TU values calculated in this study represent worst-case scenarios, given that they utilize the most sensitive known organisms and whole water concentrations. If bioavailability were considered, actual TUs would be much lower since these analytes would be predominantly sorbed to solids in the water column. In addition, the UPOW-CW cell studied does not represent the treatment efficacy and toxicity reduction of the entire Prado Wetlands complex. However, the values reported in this study are useful to demonstrate effective mitigation of toxicity via comparison of the inlet and outlet values. Similarly effective TU reductions have been reported in previous research at the Prado Wetlands. The ecology of Phytophthora, a genus of fungal-like oomycetes historically erected and known for plant pathogenic species primarily associated with destructive diseases in agriculture, has undergone substantial reconsideration in recent years.

The recent emergence of a number of Phytophthora-caused plant epidemics in forests and other non-agricultural ecosystems has clearly shown that many members of the genus have potential as invasive species that can threaten natural ecosystems. As a consequence of research in non-agricultural environments, a surprising diversity and abundance of Phytophthora species have been discovered, many previously undescribed. Incidental to this research has been the discovery that many species of Phytophthora are abundant in natural surface waters, especially in streams. Many such species are so widespread and regularly encountered that they are now considered resident, if not endemic, and characteristic of such environments. Nevertheless, isolates of well-known plant pathogenic species or species complexes are also regularly recovered, often without discernible symptoms or signs of disease on the vegetation. Though the prevalence of Phytophthora in surface waters is now well established, the ecology underpinning this phenomenon is largely speculative. Because these organisms are known primarilyas causes of often devastating plant diseases, the nature of their presence in these environments and its implications for the persistence and spread of pathogenic species are important considerations for disease prevention and management. There is also a growing interest to understand the role of Phytophthora, among other Peronosporales, in decomposition of vegetative matter in aquatic environments. The biology of Phytophthora, a genus of well adapted plant pathogens with a necrotrophic phase, suggests that their ecological role in leaf decomposition should be early colonization and breakdown of relatively fresh, live vegetative tissue. As they colonize leaves newly exposed in streams, they can open the integral tissues for colonization by saprotrophic organisms less able to penetrate the leaf cuticle, in a process analogous to ‘conditioning’ of leaf litter for palatability to shredder organisms. The co-occurrence of both known plant pathogens and primarily stream-associated Phytophthora in aquatic environments also raises the question of whether these taxa have similar or divergent modes of life and whether they compete for resources in these environments. In streams, vegetative litter is the primary source of nutrients for microorganisms, but the quality of vegetative tissues available varies with respect to senescence and degree of decomposition. Coastal forests of northern California largely consist of evergreen trees and shrubs and so green leaves are a regular component of leaf litter introduced into streams, especially in winter and spring when, based on the region’s climate, most rainstorms occur. Nevertheless, much vegetative litter is in the form of senesced leaves. California bay Nutt. is a common, broadleaf evergreen component of northern California’s coastal forests and a frequently occurring tree species in riparian zones. It is also a primary source of P. ramorum inoculum in California forests affected by sudden oak death, epidemic mortality of certain species in the beech family resulting from P. ramorum infection of the vascular cambium of the main trunk. California bay leaves are highly conducive to sporulation by P. ramorum which, despite causing localized necrotic lesions and spots on leaves, nevertheless causes little damage to the tree species itself. Additionally, bay leaves are sclerophyllous, as is typical for broad leaf evergreen plants in this Mediterranean climate, and so they decompose slowly. Bay leaves are therefore both very common as leaf litter in northern California forest streams and a highly suitable substrate for P. ramorum. Leaf senescence in California bay increases in the hot and dry summer months, peaking in late summer. Thus, though green leaves often enter streams during winter and spring storms, as summer progresses, most of the bay leaves shed into streams are either dropped directly upon senescence from trees or are blown in from accumulated litter on the forest floor, nearby.