Estimation of food limitation in Daphnia Pulex from Boulder Basin, Lake Mead

In February, 1982 I began a year-long study to determine if growth and reproduction in Daphnia Pulex were limited by the amount of food available in Boulder Basin, Lake Mead. To determine this, I made monthly collections of Daphnia Pulex and natural lake seston from an already established station in Boulder Basin. I cultured the Daphnia Pulex under simulated field conditions in a flow-through feeding apparatus using four different food regimes; 1-natural lake seston filtered through 80 um mesh to remove other zooplankton, 2-lake seston (as above) with an enrichment of 103 cells-ml-1 of Chlamydomonas reinhardti, 3-lake seston (as above) with an enrichment of 105 cells-ml-1 of Chlamydomonas relnhardti, 4-0.45 um (HA Millipore) filtered lake water with 105 cells-mi-1 of Chlamydomonas relnhardti. The last two food regimes contain amounts of algae in excess of the incipient limiting concentration and therefore, growth and reproduction should not be limited by lack of food. For each of the 8 months that I was able to run successful experiments, I found that animals fed only lake seston grew more slowly and reproduced less than the animals fed enriched food regimes. There were some months (February, May 1982 and January 1983) when the amount of extra food in the lowest enrichment (103 cells-mi-1 of Chlamydomonas relnhardti) was not enough to improve growth or reproduction significantly. These were the months when phytoplankton was most abundant in Boulder Basin and 103 cells-mi-1 provided relatively less enrichment than in other months. The months for which data could not be obtained (July-October) coincide with the lowest abundance of Daphnia pulex in Boulder Basin. The low levels of Daphnia pulex and moderate levels of chlorophyll-a present in Boulder Basin, indicate that the animals may have been already food limited during those months. From this, I concluded that during the entire year, Daphnia pulex is food limited in Boulder Basin, Lake Mead. Comparison of this study with other studies confirms that food limitation of Daphnia pulex is primarily due to low phytoplankton abundance (as indicated by low concentrations of chlorophyll-a) although the food quality of the seston may also contribute to the food limitation