An Ecological analysis of relic diatoms in sediments of Las Vegas Bay, Lake Mead

Relic diatoms in sediments of the inner Las Vegas Bay, near the Las Vegas Wash sewage inflow, were examined in order to assess historic trophic conditions in this area of Lake Mead. Diatom sedimentation rates and ratios of Araphidineae/Centrales (A/C) diatom groups were determined from sediment cores collected in the old wash channel 1.5 km from the sewage inflow (station 2), in a small cove 1.5 km further downstream (station 3) and in an adjacent embayment off Gypsum Wash (station 4). Diatom sedimentation rates generally increased from the bottom to the top of each core, but pronounced minima existed at various sediment intervals. Siltation from floods that occurred during 1975 appeared to cause these minima and may also have masked true rates of diatom sedimentation and A/C ratios at stations 2 and 4 which were located in areas most affected by flooding. A/C ratios in sediments at station 3 revealed that the inner bay has been primarily mesotrophiceutrophic since 1971, but a sharp decrease in annual diatom sedimentation rates after 1975 indicates there has been a decline in diatom production. This could, in part, account for the recent decrease in chlorophyll-a concentrations observed in past monitoring of the inner bay