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Validerat; 20101217 (root

Intraspecific Autochthonous and Allochthonous Resource Use by Zooplankton in a Humic Lake during the Transitions between Winter, Summer and Fall

Seasonal patterns in assimilation of externally produced, allochthonous, organic matter into aquatic food webs are poorly understood, especially in brown-water lakes. We studied the allochthony (share biomass of terrestrial origin) in cladoceran, calanoid and cyclopoid micro-crustacean zooplankton from late winter to fall during two years in a small humic lake (Sweden). The use of allochthonous resources was important for sustaining a small population of calanoids in the water column during late winter. However, in summer the calanoids shifted to 100% herbivory, increasing their biomass several-fold by making efficient use of the pelagic primary production. In contrast, the cyclopoids and cladocerans remained at high levels of allochthony throughout the seasons, both groups showing the mean allochthony of 0.56 (range in mean 0.17-0.79 and 0.34-0.75, for the respective group, depending on model parameters). Our study shows that terrestrial organic matter can be an important resource for cyclopoids and cladocerans on an annual basis, forming a significant link between terrestrial organic matter and the higher trophic levels of the food web, but it can also be important for sustaining otherwise herbivorous calanoids during periods of low primary production in late winter

Intraspecific Autochthonous and Allochthonous Resource Use by Zooplankton in a Humic Lake during the Transitions between Winter, Summer and Fall

Seasonal patterns in assimilation of externally produced, allochthonous, organic matter into aquatic food webs are poorly understood, especially in brown-water lakes. We studied the allochthony (share biomass of terrestrial origin) in cladoceran, calanoid and cyclopoid micro-crustacean zooplankton from late winter to fall during two years in a small humic lake (Sweden). The use of allochthonous resources was important for sustaining a small population of calanoids in the water column during late winter. However, in summer the calanoids shifted to 100% herbivory, increasing their biomass several-fold by making efficient use of the pelagic primary production. In contrast, the cyclopoids and cladocerans remained at high levels of allochthony throughout the seasons, both groups showing the mean allochthony of 0.56 (range in mean 0.17-0.79 and 0.34-0.75, for the respective group, depending on model parameters). Our study shows that terrestrial organic matter can be an important resource for cyclopoids and cladocerans on an annual basis, forming a significant link between terrestrial organic matter and the higher trophic levels of the food web, but it can also be important for sustaining otherwise herbivorous calanoids during periods of low primary production in late winter

Seasonal patterns in nutrient bioavailability in boreal headwater streams

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

Stacked zooplankton biomasses measured at the center of lake Övre Björntjärn.

<p>The biomass for all organisms is divided into (A) an autochthonous part and (B) an allochthonous part, based on a calculation using stable hydrogen isotope data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120575#pone.0120575.g001" target="_blank">Fig. 1b</a>) together with an algebraic mixing model as described in the text.</p

Allochthony of (A) calanoid copepods, (B) cyclopoid copepods, (C) total cladocerans, and (D) the biomass-weighted mean of the total crustacean zooplankton community, as functions of the ordinal date of lake Övre Björntjärnen 2009 and 2011.

<p>Curved solid regression lines show significant quadratic relationships (<i>p</i> < 0.01). Dashed lines show the corresponding relationships in high and low end allochthony scenarios, obtained by manipulating model parameters in all possible combinations as explained in the text. In lack of significant relationships, straight lines indicate means.</p

Pearson r correlations^a between organic matter pools, or the phytoplankton: bacterioplankton biomass ratio, and the allochthony in different zooplankton groups in the lake Övre Björntjärn, sampled during different parts of the year.

<p>Values in brackets show the possible range of correlation coefficients obtained for different allochthony scenarios. The biomass ratio is log-transformed because of strong (>2) skewness.</p><p>^aSignificance: not significant (denoted n.s.);</p><p>p < 0.10 (denoted ⁽*⁾);</p><p>p < 0.05 (denoted *)</p><p>Pearson r correlations<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120575#t001fn002" target="_blank">^a</a> between organic matter pools, or the phytoplankton: bacterioplankton biomass ratio, and the allochthony in different zooplankton groups in the lake Övre Björntjärn, sampled during different parts of the year.</p

Allochthony in the different zooplankton groups as functions of (A) the ratio between phytoplanktonic and bacterioplanktonic biomass^a and (B) amount dissolved organic carbon in lake Övre Björntjärn.

<p>Regression lines are drawn separately for each zooplankton group which represents significant (p < 0.05) correlations. Zooplankton groups without significant correlations are shown in faded (50% less ink) symbols. Note the logarithmic scale on the x axis in banner A. ^<b>a</b>Excluding one extreme low-end outlier on July 28, 2009, when the algal bloom was interrupted by a rain storm which suddenly flushed most of the phytoplankton biomass out of the lake (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120575#pone.0120575.g001" target="_blank">Fig. 1a</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120575#pone.0120575.s001" target="_blank">S1 Table</a>).</p

Seasonal patterns in lake Övre Björntjärn from spring to fall.

<p>(A) Phytoplankton net primary production (PP) and bacterioplankton production (BP) shown together with catchment water discharge and lake water temperature; (B) stable hydrogen isotope ratios (δ²H) of water, zooplankton groups and allochthonous and autochthonous organic matter. Dashed lines show ± 1 <i>SD</i> of allochthonous and autochthonous organic matter.</p

Toward an ecologically meaningful view of resource stoichiometry in DOM-dominated aquatic systems

Research on nutrient controls of planktonic productivity tends to focus on a few standard fractions of inorganic or total nitrogen (N) and phosphorus (P). However, there is a wide range in the degree to which land-derived dissolved organic nutrients can be assimilated by biota. Thus, in systems where such fractions form a majority of the macronutrient resource pool, including many boreal inland waters and estuaries, our understanding of bacterio-and phytoplankton production dynamics remains limited. To adequately predict aquatic productivity in a changing environment, improved standard methods are needed for determining the sizes of active (bioavailable) pools of N, P and organic carbon (C). A synthesis of current knowledge suggests that variation in the C:N:P stoichiometry of bioavailable resources is associated with diverse processes that differentially influence the individual elements across space and time. Due to a generally increasing organic nutrient bioavailability from C to N to P, we hypothesize that the C:N and N:P of bulk resources often vastly overestimates the corresponding ratios of bioavailable resources. It is further proposed that basal planktonic production is regulated by variation in the source, magnitude and timing of terrestrial runoff, through processes that have so far been poorly described