George H. Lauster

Nutrient Limitation of Aquatic Bacteria, and the Importance of Bacteria and Algae to Colloid Dynamics in Lake Michigan

Three separate studies were conducted on lake microbial ecology. Nutrient limitation was determined in eight study lakes chosen as examples of the three major lake types: eutrophic, humic and oligotrophic. Regardless of lake type, bacteria are most often limited by a combination of carbon and phosphorus-nitrogen, either concurrently or sequentially. The specific type of nutrient limitation for each system was better explained by total phosphorus and chlorophyll a concentrations than by lake type. When the experimental results were used to estimate bioavailable phosphorus and carbon, the differences in nutrient limitation among experiments were best explained, including the subtle differences between colimitation and sequential limitation by phosphorus, nitrogen and carbon.

Creation and utilization of organic colloids during growth and decomposition by bacteria and algae was determined using radiotracers in Lake Michigan. A substantial amount of primary production (average 21%) and bacterial production (average 73%) was found in the colloidal size range. Results indicate that both bacterial and algal extracellular production were important sources of colloidal and dissolved organic matter. Decomposition of algal and bacterial cells also created colloids, generally less than 0.05 micro-m in size. Removal of bacterial activity resulted in a net increase in less than 0.05 micro-m material, while removal of bacteriovores resulted in loss of less than 0.05 micro-m material, consistent with bacteriovores either creating this small material directly or suppressing bacterial utilization of this material under normal conditions.

Seasonal importance of algae to colloid material was also studied in Lake Michigan. Two regions of the Lake Michigan water column had both high colloid mass and high chlorophyll a concentrations in colloid size materials during stratification: the photic zone (>1% surface light) and the lower hypolimnion (130 to 150 meters). Increases in degradation products, pheophytin and pheophorbide, relative to chlorophyll a indicate that this lower hypolimnion zone of higher colloid concentration was due to colloids created during decomposition. Analysis of carotenoid pigments, diagnostic of different algal groups, indicate the diatoms are an important source of colloids despite their large cells. Algae, especially diatoms, are important to organic colloids dynamics throughout the water column.