New insights on resource stoichiometry: assessing availability of carbon, nitrogen, and phosphorus to bacterioplankton

Abstract

<p><strong>Abstract.</strong> Boreal lake and river ecosystems receive large quantities of organic nutrients and carbon (C) from their catchments. How bacterioplankton respond to these inputs is not well understood, in part because we base our understanding and predictions on <q>total pools</q>, yet we know little about the stoichiometry of bioavailable elements within organic matter. We designed bioassays with the purpose of exhausting the pools of readily bioavailable dissolved organic carbon (BDOC), bioavailable dissolved nitrogen (BDN), and bioavailable dissolved phosphorus (BDP) as fast as possible. Applying the method in four boreal lakes at base-flow conditions yielded concentrations of bioavailable resources in the range 105–693<span class="thinspace"></span>µg C L⁻¹ for BDOC (2<span class="thinspace"></span>% of initial total DOC), 24–288<span class="thinspace"></span>µg N L⁻¹ for BDN (31<span class="thinspace"></span>% of initial total dissolved nitrogen), and 0.2–17<span class="thinspace"></span>µg P L⁻¹ for BDP (49<span class="thinspace"></span>% of initial total dissolved phosphorus). Thus, relative bioavailability increased from carbon (C) to nitrogen (N) to phosphorus (P). We show that the main fraction of bioavailable nutrients is organic, representing 80<span class="thinspace"></span>% of BDN and 61<span class="thinspace"></span>% of BDP. In addition, we demonstrate that total C<span class="thinspace"></span>:<span class="thinspace"></span>N and C<span class="thinspace"></span>:<span class="thinspace"></span>P ratios are as much as 13-fold higher than C<span class="thinspace"></span>:<span class="thinspace"></span>N and C<span class="thinspace"></span>:<span class="thinspace"></span>P ratios for bioavailable resource fractions. Further, by applying additional bioavailability measurements to seven widely distributed rivers, we provide support for a general pattern of relatively high bioavailability of P and N in relation to C. Altogether, our findings underscore the poor availability of C for support of bacterial metabolism in boreal C-rich freshwaters, and suggest that these ecosystems are very sensitive to increased input of bioavailable DOC.</p>