Bacterial productivity in Lake Superior, 2001: Implications for food web efficiencies in oligotrophic freshwater ecosystems

Abstract

A synoptic survey of Lake Superior from 16 to 24 August 2001 investigated heterotrophic bacterial abundance, productivity and role in phosphorus dynamics. We examined integrated surface samples (0–20 m) drawn from three nearshore (depth < 50 m) and four offshore (depth > 100 m) stations. Bacterial abundance ranged from 5.7 to 10.0 × 10 ⁵ cells ml⁻¹ . Heterotrophic bacterial productivity measured using ³ H-leucine and primary productivity measured using ¹⁴ C-bicarbonate at nearshore stations were generally greater than at offshore stations. Bacterial productivity generally represented a greater percentage of primary productivity at stations with higher phytoplankton productivity. Phosphate as soluble reactive phosphate (SRP) was low at all stations examined but greater at nearshore stations (ave. 110 nM) than offshore (ave. 75 nM). ³³ P-orthophosphate uptake by size-fractionated groups indicated that greater than 61% (and up to 93%) of the instantaneous phosphate uptake was by bacteria in both nearshore and offshore habitats. Bacterial phosphorus use efficiency (production per mole of P taken up) was greatest at stations where primary productivity exceeded 1.5 μ g C l⁻¹ h⁻¹ . Where bacterial phosphorus use efficiency is relatively low, the phosphorus-to-carbon ratio was relatively high, suggesting that bacterioplankton may be especially important in phosphorus-dynamics and that bacterivory may be an especially important process mobilizing phosphorus to higher trophic levels in Lake Superior and ultraoligotrophic aquatic communities in general. The implications for emerging issues of investigation in Lake Superior are discussed in the context of contemporary marine and freshwater research.