Indices derived from structural food web theory (A–B) and from thermodynamic and information theory (C–F).

<p>(A) Weighted connectance <i>C_w</i> increased during succession. <i>C_w</i> was positively correlated with the flow diversity of the trophic flows between the 8 major groups and the detritus pool (Fig. S8 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090404#pone.0090404.s001" target="_blank">Text S1</a>). (B) The small-world properties weighted average cluster coefficient <i>Q_norm</i> and characteristic path length <i>D_norm</i>. <i>Q_norm</i>. <i>Q_norm</i> (primary axis) was maximal during times when the keystone consumer (daphnids) dominated the community. Changes in <i>D_norm</i> (secondary axis) were small and opposite to <i>Q_norm</i>. (C) Ascendancy (<i>Asc</i>) and Total system throughput (<i>TST</i>) peaked during early stages of succession. (D) Relative ascendancy (<i>Asc_rel</i>) peaked in early spring when the average mutual information (<i>AMI</i>) (c) was maximal. Development capacity (<i>K_dev  =  TST * H_flow</i>) was dominated by <i>TST</i> and also maximal in spring. (E) <i>AMI</i> decreased during succession while its upper bound, the flow diversity (<i>H_flow</i>) calculated from all flows considered in <i>TST</i> (including detrital flows, external in- and outputs, biomass storage flows and respiration, see Methods), increased during succession and exceeded <i>AMI</i> during late succession. The difference <i>H_flow</i> − <i>AMI</i> is the system's relative (normalized by <i>TST</i>) overhead which measures the residual uncertainty in the energy flow pattern (see Methods). (F) Total exergy (<i>Ex</i> in g detritus equivalents/m², cf. Methods) and specific exergy (<i>Ex_sp</i> in units of <i>Ex</i> per unit of biomass in gC/m²) peaked during the CWP due to the high biomass of herbivorous crustaceans.</p

Body mass and metabolic indices.

<p>(A–C) Body mass, predator-prey body mass ratio (<i>PPMR</i>), and trophic position increased during succession. (A) The biomass-weighted, average body mass of dietary groups changed seasonally due to species shifts within groups. The body mass of large carnivorous crustaceans (<i>Leptodora & Bythothrephes</i>) remained constant. Lines in (A) and (B) were drawn only if the group's biomass was >5% of its annual maximum. (B) The increase in herbivore-phytoplankton <i>PPMR</i> led to an increase in average <i>PPMR</i> by an order of magnitude from spring to summer. <i>PPMR</i> was maximal during the CWP due to herbivorous crustaceans' dominance. (C) The average consumers' trophic position established from fractional dietary flows for each group increased with body mass and more carnivorous diets in summer. Bac remained at trophic position 1 and HNF at trophic position 2. (D) Mass-specific metabolic activity (<i>P_tot/B_tot</i>) of major functional groups and at system level in comparison with total biomass (in µgCm⁻²) and total production (in µgCm⁻²d⁻¹). Total production and metabolic activity peaked in early stages of succession before total biomass. (E) The trophic transfer efficiency (<i>TE</i>) in units of C within the grazing chain (avg. across trophic level 1–3, cf. Methods) correlated positively (Table S2 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090404#pone.0090404.s001" target="_blank">Text S1</a>) with the plankton community's size spectrum slope (<i>SSS</i>) and was maximal during the CWP.</p

Seasonal changes in trophic structure.

<p>(A) The biomass pyramids of the grazing chain and (B) the detritus chain on ascending trophic levels for the 8 major functional groups in units of carbon. Summer and autumn data were pooled to summarize similar distributions. (C) The production pyramids of the grazing chain and d) the detritus chain. Autotrophic biomass and primary production (<i>PP</i>) in (A, C) and bacterial biomass and production (<i>BP</i>) in (B, D) was set to 100% in each phase. Without this standardization, the ratio between <i>PP</i> and <i>BP</i> is approximately 9:1 (cf. Fig. 1D). Seasons and groups in (B–D) same as in (A–B). The detritus chain only shows two trophic levels because consumers partly feeding on bacterivores were assigned to the grazing chain. Arrows indicate that fish biomass and production were underestimated because fish biomass is reduced by commercial fisheries in LC (cf. Methods).</p

The LC food web model in the 8-groups resolution as the basis of the mass-balanced flow networks.

1<p>The 7 detrital flows link the dead organic matter of each functional group except of the bacteria back to PDOM which is then taken up by bacteria.</p

Conceptual scheme linking indices of successional progress.

<p>Several drivers of successional progress induce changes in community composition during succession in LC. Higher average body size entails lower system metabolic activity and respiration, while the feeding activities of the more diverse and more specialized consumer community combined with lower non-grazing mortality result in a more efficient exploitation of food resources. These changes in size, diet, and trophic structure enhance the efficiency of the energy transfer towards higher trophic levels and along the size gradient. With more energy reaching larger consumers and the higher trophic levels, biomass becomes more evenly distributed along the size gradient in a functionally more diverse and more complex food web with more closed energy and nutrient cycles. Four key indices (i.e. the transfer efficiency across trophic levels <i>TE,</i> the system metabolic activity <i>P_tot/B_tot</i>, the functional diversity <i>H_bio</i>, and the weighted connectance <i>C_w</i>) are marked in red and combined to a composite index of successional progress (Fig. 9).</p

Composite index of successional progress.

<p>The composite index based on the average of the four normalized indices <i>TE, P_tot/B_tot</i>, <i>H_bio</i>, and <i>C_w</i> (black line) increases approximately linearly during the growing season from phase 2–6. The result is similar if <i>C_w</i> is excluded from the calculation (gray line). Higher values during the winter phases (dashed/dotted part of the lines) are caused by the very low <i>P_tot/B_tot</i> values which are due to the influence of abiotic forcing rather than biotic processes.</p

System-level indices used to test H1–H3.

<p>System-level indices used to test H1–H3.</p

Biomass (A–B) and production development (C–D) during succession.

<p>(A) Absolute biomass of the 7 major plankton groups in reference to concentrations of Soluble Reactive Phosphorus (SRP, dashed line, data from 1995, see Methods) and cellular levels of polyunsaturated fatty acids (PUFA, dotted line, avg. 2008–2009, see Methods) within the plankton of size fraction <140 μm in µg/l. (B) Relative biomass of all 20 planktonic guilds (cf. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090404#pone-0090404-t001" target="_blank">Table 1</a>). (C) Temporal course of the absolute and (D) relative production.</p

Functional diversity (A), succession rate (B), food quality (C), and system residence times (D).

<p>(A) Functional diversity within four major plankton groups: phytoplankton (Phy), ciliates (Cil), rotifers (Rot), and all crustaceans (HerbCru + CarnCru), and system functional diversity <i>H_bio</i> of all 20 plankton guilds (cf. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090404#pone-0090404-t001" target="_blank">Table 1</a>). Functional diversity of Rot is only shown when Rot biomass exceeded 1% of total biomass. (B) Succession rate <i>σ</i> of the 20 functional plankton guilds peaked twice shortly before and after the CWP. (C) C:P ratios of algal and bacterial biomass and food quality of the food ingested by different consumer groups (average across 1987–1993) in relation to phosphorus concentrations (SRP from 1995, dashed line) and cellular levels of polyunsaturated fatty acids (PUFA average 2008–2009, dotted line) within the sestonic size fraction <140 µm <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090404#pone.0090404-Hartwich1" target="_blank">[92]</a>. Food quality for herbivores decreased with increasing C:P ratios during succession. (D) System residence times for carbon (<i>SRT_C</i>) and phosphorus (<i>SRT_P</i>). <i>SRT_C</i> and <i>SRT_P</i> were maximal during the CWP due to the dominance of larger crustaceans with slower metabolism and in autumn-winter due to decreasing temperature and on average lower metabolic activity (Fig. 5D).</p

The LC food web model comprises 24 functional guilds aggregated to 8 major functional groups.

<p>The 24 functional guilds and 8 groups are: Phytoplankton (guild ID: 1–6), Bacteria (ID: 7), Heterotrophic Nanoflagellates (ID: 8), Ciliates (ID: 9–13), Rotifers (ID: 14–17), Herbivorous Crustaceans (ID: 18), Carnivorous Crustaceans (ID: 19–20), Fish (ID: 21–24). ¹Guild ID. ²Size class is log2 (avg. body mass in pgC). ³ID of prey guilds. ⁴edibility (++: well-edible, +: less edible, –: edible only for specialists). ⁵Dead particulate and dissolved organic matter. ⁶Links 18→19 and 19→19 describe adult Cyclopoids feeding on juvenile herbivorous Cladocerans (18) and juvenile Cyclopoids (19), respectively. ⁷general diet description (B  =  bacterivorous, H  =  herbivorous, C  =  carnivorous, O  =  omnivorous). For details, please refer to the Methods section.</p