Seasonal patterns in nutrient bioavailability in boreal headwater streams

Abstract

Changes in nutrient bioavailability due to increased loading of dissolved organic matter (DOM) may impact boreal freshwaters. Yet, the relative bioavailability of carbon (C), nitrogen (N), and phosphorus (P) associated with terrestrial DOM remains poorly understood. We applied short-term bioassays with natural bacterial inocula to determine seasonal variation in bioavailable organic nutrient pools from four boreal headwater streams in northern Sweden. Experiments were designed to exhaust bioavailable nutrients associated with DOM by inducing limiting conditions when all required resources except for the targeted nutrient (C, N, or P) are provided in excess. We hypothesized that the supply of different bioavailable nutrients to streams would reflect seasonal variations in terrestrial demand, hydrology, and temperature. The delivery of bioavailable DOM-associated resources from the four streams were, on average, 2%, 11%, and 38% of the total dissolved organic C, N, and P, respectively, emphasizing the relatively low C bioavailability in these DOM-rich waters. Bioavailable N : P ratios peaked in autumn for all sites, with lower values in winter and spring. Both in terms of relative (% of total) and absolute bioavailable organic nutrient concentrations, the seasonal pattern was characterized by systematically high values for the autumn period. Furthermore, links between bioavailable resources and temperature and hydrology varied across sites, time periods, and the different elements. Thus, elevated concentrations of bioavailable organic resources in autumn suggest the potential for leaf fall, as well as late season storms that rewet dry soils, to serve as considerable sources of C, N, and P to boreal aquatic ecosystems.

The emerging role of drought as a regulator of dissolved organic carbon in boreal landscapes

“…0.2–0.5 units in the Abs ratio) could correspond to as much as a 50% increase in microbial growth efficiency in receiving streams. Consequently, elevated energy mobilization through increased autumn pulses of high-quality DOC is likely to promote aquatic ecosystem respiration 51 , 52 ; indeed, particularly large DOC pulses following drought may create acute periods of anoxia in lakes that cause fish mortality 53 . In addition to these direct biological effects, such DOC pulses may also promote inputs of pollutants to aquatic systems, including a variety of toxic metals that form complexes with organic matte r 8 , 11 .…”

Coastal darkening exacerbates eutrophication symptoms through bottom‐up and top‐down control modification

“…We sampled pelagic bacteria from the water sample after filtration (< 70 μ m, see above) and stored them with 2% glutaraldehyde at 4°C until processing. Bacteria densities were estimated using a flow cytometer, as described in Rulli et al (2022).…”

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes

“…7,18−20 This may be because microbes show different hydrolytic abilities for DOM of varying molecular weights and sources, which thus directly affect the distribution of bioavailable nutrients. 20,21 As an important support for secondary production in lakes, the distribution of DOM has important implications for the food web. 4,22−25 In 15 lakes in northern Sweden, both allochthonous and autochthonous carbon (C) contributed significantly to the growth of bacterioplankton, phytoplankton, and zooplankton, even at different C levels.…”

“…Lakes, where active chemical reactions occur, incur massive deposits of organic matter from aquatic and surrounding terrestrial organisms, greatly influencing the composition of DOM in the water. − Mineralization of DOM derived from phytoplankton facilitates the rerelease of phosphorus (P), supporting the growth of algal blooms. , In addition, labile algal detritus is more beneficial to dissimilatory nitrate reduction to ammonium (DNRA), while macrophyte detritus promotes nitrate accumulation and an elevated abundance of denitrification genes . In addition to endogenous DOM, terrestrial DOM directly changes the composition of DOM in lakes, which then changes the nutrient status of these water bodies. ,− This may be because microbes show different hydrolytic abilities for DOM of varying molecular weights and sources, which thus directly affect the distribution of bioavailable nutrients. , …”