Abstract
Branchinecta lynchi (vernal pool fairy shrimp) is a federally listed large Branchiopoda species endemic to ephemeral waters of the California Central Valley and Southern Oregon. Little is known about this species’ ecology and population dynamics, and no published studies are available regarding their presence in rock pools; however, other aquatic invertebrates are known to be regulated by both abiotic and biotic factors within this habitat subclass of ephemeral waters. This study analyzed the effect of rainfall, pool size and community structure on B. lynchi occurrence within a cluster of rock pools during the 2016-2017 wet season. Rock pool basin dimensions were measured, and pools were sampled multiple times from November to March for depth, vertebrate and macro-invertebrate species, water temperature, and dissolved oxygen. The presence of B. lynchi during the light rain and cool water temperature of the November-December surveys (Early period) was compared to their occurrence during the more frequent rain and warm water temperature of the February-March surveys (Late period). Generalized logistic models based on summarized data for each time-period were evaluated separately. SIMPER, two-way PERMANOVA, Mann-Whitney U and Fisher’s Exact Test assessed patterns and significant differences in the data between pools grouped by sampling period and B. lynchi presence-absence. Occurrence was detected in 69% of Early pools, and B. lynchi occurrence declined to 29% detection in Late pools. Early B. lynchi occurrence was positively associated with pool volume, maximum hydroperiod and elevation; however, volume and maximum hydroperiod interacted to have a significant negative effect. Late B. lynchi occurrence was negatively associated with pool volume and predator richness, and positively associated with consumer richness and dissolved oxygen concentration, however, there was an additional negative interaction between predator richness and percent oxygen saturation. Greater volume pools with longer hydroperiods supported B. lynchi populations initially, but it was the lower volume pools that supported their populations for the entire study. Lower volume pools also had lower predator richness, and some had 2 suitable hydroperiods for this species which increases overall B. lynchi density within the pool. Furthermore, increasing pool size may negatively affect B. lynchi pool occurrence by allowing greater predator richness within a pool during later, warmer conditions. This study concluded that hydroperiod duration and timing, and the influence of abiotic and biotic factors are all important for B. lynchi population permanence. Further studies would benefit from investigating the pool shape more accurately and the influence of shade from pool recession within the parent rock. The population dynamics found in this rock-pool cluster supported previous studies of B. lynchi populations in mesocosms which had greatest densities immediately following the first rains and, in this study, responded negatively to increased competition, sustained predation, and changing water chemistry. To conserve and restore B. lynchi populations, habitat variability is a key factor because annually the species reproduced in a variety of pool sizes, but seasonally, smaller pools sustained the population as refuge from biotic interaction.