Abstract
Heterotrophic bacteria are, in many aquatic systems, reliant on autochthonous organic carbon as their energy source. One exception is low-productive humic lakes, where allochthonous dissolved organic matter (ADOM) is the major driver. We hypothesized that bacterial production (BP) is similarly regulated in subarctic estuaries that receive large amounts of riverine material. BP and potential explanatory factors were measured during May–August 2011 in the subarctic Råne Estuary, northern Sweden. The highest BP was observed in spring, concomitant with the spring river-flush and the lowest rates occurred during summer when primary production (PP) peaked. PLS correlations showed that ∼60 % of the BP variation was explained by different ADOM components, measured as humic substances, dissolved organic carbon (DOC) and coloured dissolved organic matter (CDOM). On average, BP was threefold higher than PP. The bioavailability of allochthonous dissolved organic carbon (ADOC) exhibited large spatial and temporal variation; however, the average value was low, ∼2 %. Bioassay analysis showed that BP in the near-shore area was potentially carbon limited early in the season, while BP at seaward stations was more commonly limited by nitrogen-phosphorus. Nevertheless, the bioassay indicated that ADOC could contribute significantly to the in situ BP, ∼60 %. We conclude that ADOM is a regulator of BP in the studied estuary. Thus, projected climate-induced increases in river discharge suggest that BP will increase in subarctic coastal areas during the coming century.Electronic supplementary materialThe online version of this article (doi:10.1007/s00248-015-0714-4) contains supplementary material, which is available to authorized users.
Highlights
Produced organic carbon has been shown to be a major driver of bacterial production (BP) in many aquatic systems [16]
In systems greatly influenced by allochthonous organic carbon, such as low-productive humic lakes, bacteria are decoupled from autotrophic phytoplankton due to the high availability of allochthonous dissolved organic matter (ADOM) [38]
Total phosphorus (TotP) concentrations were markedly lower than total nitrogen (TotN) concentrations (Table 1), no increase was recorded due to elevated river flow in May and mean concentrations remained similar at all sampling events (Table 1 and Fig. 4c)
Summary
Produced organic carbon has been shown to be a major driver of bacterial production (BP) in many aquatic systems [16]. In systems greatly influenced by allochthonous organic carbon, such as low-productive humic lakes, bacteria are decoupled from autotrophic phytoplankton due to the high availability of allochthonous dissolved organic matter (ADOM) [38]. Subarctic estuaries are highly influenced by ADOM, especially during the spring river-flush [50]. The terrestrial inflow of organic matter may promote BP, since the availability of food substrates is often a growth-limiting factor in natural aquatic systems [17, 30, 61]. BP may be driven by autochthonous production
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