Seasonal and spatial variation in primary productivity in Boulder Basin, Lake Mead, Clark County, Nevada

The 14C light and dark bottle technique for measurement of primary production was utilized as a means of assessing the amount of inorganic carbon being converted Into organic form by the photosynthesis of phytoplankton populations In the Boulder Basin of Lake Mead. Spatial and time series changes of productivity levels observed at eight sampling locations within Boulder Basin Indicate that the Influence of treated municipal arts\u27 industrial effluent flowing into Les Vegas Bay is contributing high levels of available nutrients at Las Vegas Wash Inlet to cause productivity to approximate those levels associated with polluted waters. Productivity levels at the Inlet to Las Vegas Bay showed a rapid increase from July to September, attaining levels associated with artificially eutrophic lakes and a rapid decrease to mesotrophic levels Into the winter months. Unlike the outlying stations, the photosynthetic depth curves at Station 2, Las Vegas Wash Inlet, during the peak production periods indicated maximum production occurring at the one meter depth of the euphotic column. Inhibition of photosynthesis at the surface together with the shading effect by the dense phytoplankton layer near the surface resulted in greater than 70 to 80% of total primary productivity occurring near the surface within the euphotic column. The photosynthetic variability with depth showed a more gradual decline at outlying stations where the Influence of nutrient enriched waters appeared negligible within the euphotic column. Peak production levels were noted at around the 5 meter depth. No drastic shading effects by phytoplankton were noted at the outlying stations. The compensation depth below the trophogenic zone, the depth at which there remains one percent of the light Incident at the water surface, ranged from 2 to 8 meters at Station 2 and from 4 to 15 meters at other stations. Greater light penetration correlated with periods of low production and the presence of dense phytoplankton or turbidity layers resulted in measurable decreases In light transmittance with depth. Total alkalinity showed decreases in value when there were corresponding Increases In primary productivity rates. Under these conditions, pH values increased. Physico-chemical parameters which were measured at each sampling station within the euphotic column Indicate that the waters of Boulder Basin, Lake Mead, are characterized by high specific conductance of 950 to 110 umho/cm and as high as 2400 umho/cm at the bottom where the effluent stream enters Las Vegas Bay. Alkalinity values ranged from 67 to 193 mg/l and as high as 282 mg/l where the effluent stream crosses the North Shore Road. Values for pH ranged from 7.2 to 8.4; all anomalous values greater than 8.4 were checked by titration with phenolphthalein indicator, Dissolved oxygen levels were usually greater than 100% saturation within the euphotic zone at all stations and all seasons. Oxygen depletion at about 10 meters was noted In early June and resulted In a negative heterograde oxygen curve. This depletion correlated with the expected formation of a metalimnetic layer and an associated cold water hypolimnion below the trophogenic zone. Water surface temperatures ranged from a high 29.5°C in mid-summer to a low of 10.5°C In winter. Phytoplankton cell numbers at the surface were generally low from March to early July. A bloom of Cyciotelia spp. occurred at the inner stations of Las Vegas Bay in mid and late July and into August. Anabaena spp. then succeeded as the dominant phytoplankter at all but the innermost station in Las Vegas Bay. Dominant phytoplankters from mid-July Into October were Cyclotelia spp., Anabaena spp., Carteria spp., Navicula spp. and Chiamydomonas spp., whereas Glenodinium spp. was dominant during the spring when cell numbers were low. Other genera of phytoplankton noted but in lower numbers were Cymbelia, Epithemia, Gymnodinium, Ceratium, Oocystis, Synedra, Scenedesmus, Dinobryon. Cosmarium, Pinnularia and Phacotus. Primary productivity levels from the surface waters of all locations did not correlate well with surface cell numbers. Numerical counts of zooplankters were not included in this study. Some common zooplankters noted however, were the protozooan, Difflugia spp., and rotifers, Keratella spp. and Asplanchna spp. Based on seasonal rates of primary productivity, the inner and middle portions of Las Vegas Bay exhibit artificially eutrophic or polluted conditions during peak production periods from July through September and rapidly decrease to mesotrophic levels in late fall and winter. Stations located in the basin reflect naturally eutrophic conditions during the peak growing period followed by a decrease to oligotrophic levels In the winter months when extinction of the metalimnetic layer occurs