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

Abstract

Abstract. 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 "total pools", 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 µg C L −1 for BDOC (2 % of initial total DOC), 24-288 µg N L −1 for BDN (31 % of initial total dissolved nitrogen), and 0.2-17 µg P L −1 for BDP (49 % 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 % of BDN and 61 % of BDP. In addition, we demonstrate that total C : N and C : P ratios are as much as 13-fold higher than C : N and C : 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.

Bioavailability of Dissolved Organic Phosphorus in Temperate Lakes

“…In a Baltic Sea study, the DOP pool was 75% bioavailable (Stepanauskas et al, 2002) and a similar value (33.2-60%) was reported for 3 stations in the central Baltic as part of a different study (Nausch and Nausch, 2007). A more recent analysis suggested that ∼40% of the DOP in four boreal lakes was bioavailable (Soares et al, 2017). Our study of 27 unique systems supports the idea of very labile BDOP with a median value of ∼78%, but also highlights the large amount of variability in BDOP across systems.…”

“…However, a fundamental problem associated with understanding these imbalances is our capacity to know what the resource availability is that organisms actually experience. Our ability to accurately describe the resource imbalance experienced by bacterial communities in situ is hindered by our lack of simultaneous measurements of the bioavailability of multiple elements (Berggren et al, 2014;Soares et al, 2017). Here, we observed that the experienced BDOC:BDOP resource ratios of aquatic bacterial communities were typically less than measured DOC:DOP pools (Figure 11).…”

“…Specifically, there is a growing appreciation for the role of nutrients bound in complex organic forms (such as dissolved organic phosphorus, DOP) in acting as important resources for aquatic organisms (Jackson and Williams, 1985;Cotner and Wetzel, 1992;Björkman and Karl, 2003;Nausch and Nausch, 2007;Soares et al, 2017). Therefore, it is imperative to understand the bioavailability of DOP in natural systems in order to better predict its capacity to serve as a resource in the absence of (or supplementary to) inorganic phosphorus.…”

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

“…Although protozoa were not removed, based on Daniel et al (2005) it can be assumed that bacteria vastly dominate the biomass (∼ 90 %) when dark bioassays are performed on natural humic water. We assumed that there was no nitrogen or phosphorus limitation due to the high concentrations of DOC (∼ 15 mg L −1 ) known to be associated with high organic nutrient bioavailability in lakes (Soares et al, 2017) and streams of the region. Our assumption is supported by Jansson et al (2001), who showed that bacterial metabolism in humic lakes is generally not nutrient limited when DOC is higher than ∼ 15 mg L −1 .…”

Bacterioplankton Responses to Increased Organic Carbon and Nutrient Loading in a Boreal Estuary—Separate and Interactive Effects on Growth and Respiration

“…Empirically, BP has been shown to increase with P concentrations in a wide range of low- and high-productive surface waters [50, 51], but results from correlational studies are not consistent [52]. Often, the identification of P as main regulator of BP is hindered by the fact that the export of terrestrially derived DOC co-occurs with that of inorganic and organic P [9, 53], both of which are known to be highly bioavailable at Baltic Sea river mouths [54]. A large number of studies have addressed the direct effects of increasing DOC on aquatic ecosystems, but increased P fluxes driven by the increase in riverine DOM exports are often neglected [55].…”