Drought response and anthropogenic impacts as recorded in a high desert terminal lake, northern Great Basin USA, during the last millennium

Summit Lake, an alkaline mesotrophic terminal desert lake in the Great Basin province of northwestern Nevada, holds significant ecological and cultural importance to the Summit Lake Paiute Tribe, historically known as Agai Panina Ticutta (Summit Lake Trout Eaters). We present diatom and geochemical (δ13C, C:N wt.%) data from the upper 128 cm of sediment from cores taken in a deep part of Summit Lake. The sediment from 128-115 cm depth below lake floor is composed of stiff opaline mud with a pelleted texture interpreted to be the result of desiccation. Above 115 cm there is a sharp change to lower density homogeneous opaline mud interpreted as the return of lacustrine conditions. An age model developed using 210Pb artificial radionuclides and 14C dating techniques indicates the sedimentologic change at 115 cm corresponds to the end of the Medieval Climate Anomaly (MCA), ∼1250 Common Era (CE). Hierarchical cluster analysis reveals four diatom zones that are interpreted to be driven by shifts in trophic state, lake depth, and benthic macrophyte cover. The shift between zones II and III occurs at 108 cm (∼1380 CE) and is characterized by a transition from a shallow benthic community dominated by Amphora, Nitzschia, Gomphonema, Epithemia to an assemblage dominated by the epiphyte Cocconeis placentula and the phytoplankter Stephanodiscus minutulus. This shift occurs above the top of the desiccated sediments at 115 cm and indicates that the shallow benthic assemblage persisted for ∼175 years after the lake's return. A second major diatom shift is seen at 14.5 cm (∼1986 CE) and is coincident with a period of land disturbance from livestock in the watershed. This second major diatom shift is characterized by a rapid increase in a large species of Stephanodiscus (Stephanodiscus SUM sp. 1) in Zone IV that is morphologically similar to Stephanodiscus yellowstonensis. Stephanodiscus SUM sp. 1 is the dominant phytoplankter in the lake today and shows peak abundance below the mixed layer at 5 m lake depth during the summer. Sediment mass accumulation rates (MAR), calculated from the 210Pb age model, increase over the last century, and perturbations in MAR during the mid-1900s can be attributed to grazing-induced degradation of riparian areas in the 1950s through the mid-1980s. Sedimentologic and diatom data show the strong sensitivity of Summit Lake to prolonged drought that lowered lake levels to the point of desiccation during the MCA, record recent disturbances on the landscape, and a reorientation to a phytoplankton-dominated system during the last ∼40 years. This work may serve to inform watershed management strategies to help preserve this resource and create resilience to future drought threats facing this ecosystem.