The role of bacterioplankton in Lake Erie ecosystem processes: Phosphorus dynamics and bacterial bioenergetics

Pelagic, heterotrophic bacteria are essential to lake ecosystem processes, serving as decomposers, nutrient re-mineralizers, and food web links to higher trophic levels. Understanding how phosphate is apportioned to plankton is important since phosphorus is often a limiting nutrient in lakes. The microbial shunt hypothesis (MSH) proposes that available carbon, as well as phosphate, may influence the apportionment of phosphate to plankton communities. The effects of labile dissolved organic carbon (LDOC) on phosphorus dynamics were examined at stations with diverse trophic states and LDOC regimes in Lake Erie to test the assertions of the microbial shunt hypothesis (MSH). More phosphate was apportioned to bacterioplankton at stations with the lowest trophic state index (TSI) and more phosphate was apportioned to phytoplankton at stations with the highest TSI. The highest bacterial P-quotas and phosphate uptake velocities (PUV) were observed at stations with the lowest LDOC concentrations. The lowest P-quotas and PUV were observed at stations with the highest LDOC concentrations. These results were consistent with the MSH. LDOC concentration was not related to TSI. Low P-quotas and low PUV were observed at many stations with low LDOC. These observations were inconsistent with the MSH. An inactive metabolic state in bacteria may explain these inconsistencies. A change in LDOC utilization was observed amongst bacterial assemblages from different stations suggesting that LDOC availability is dependent upon the local bacterial community composition.; The biotic and abiotic factors controlling bacterial bioenergetic processes---bacterial productivity (BP), bacterial respiration (BR), and bacterial growth efficiency (BGE) were examined at diverse trophic states in Lake Erie. Bacterial abundance and the condition of the algal community, based on chlorophyll a concentration, most influenced BP and BGE. LDOC concentration most influenced BR. These results suggest that algal-bacterial coupling may most influence bacterial bioenergetic processes in Lake Erie. Equal or greater abundance and activity (BP, BR, BGE) was observed by the microbial community at stations with hypoxia relative to stations with oxic conditions.