Energetic role of the mycoloop
In the investigated mycoloop food web, there are two alternative energy pathways: one from edible phytoplankton directly to zooplankton and the other from inedible phytoplankton\(\ \)via parasitic fungi to zooplankton (mycoloop). A comparison between three sets of assumptions: (M1) linear food uptake rates for all species with zooplankton without prey preference (\(p_{Z}\) = 0.5), (M2) saturating food uptake rates for phytoplankton and zooplankton without prey preference, and (M2+) in addition to M2, zooplankton with adaptive prey preference (Fig. 2), shows that predictions on the dominance of energy flow between both pathways is independent of the zooplankton feeding strategy. A strong dominance of energy flow along the direct phytoplankton pathway is predicted at low nutrient availabilities. An increasing importance of energy flow along the mycoloop is predicted with nutrient enrichment (Fig. 2e,f). This reflects the increase in fungi biomass and the decrease in edible phytoplankton biomass along the nutrient gradient (Fig. 2a,b,c). For the adaptive preference case, the increasing importance of the mycoloop along the nutrient gradient is much more pronounced, with almost exclusive preference of zooplankton for edible phytoplankton in Regime I to an equal importance of energy flow between both energy pathways in Regime II (Fig. 2d,e,f). Energy flow along the mycoloop would even dominate for nutrient enrichment levels beyond the investigated values (see Appendix S4).
Comparing the shift in the distribution of energy flow between both pathways along the nutrient gradient, reveals significant differences between scenarios M1, M2 and M2+ (Fig. 2e,f). At low nutrient availability, predictions on net energy gain of zooplankton from fungi are highest under the assumption of linear food uptake terms (Fig. 2e). However, at high nutrients, under the assumption of saturating food uptake terms, zooplankton is predicted to gain up to 50-55% of its energy from fungi, while net energy gain stays well below 40% for the linear case (Fig. 2e). The difference in predictions is even more pronounced for the transfer efficiency along the mycoloop, which reaches 30% under the assumption of saturating functional responses while it remains below 5% under the assumption of linear food uptake rates (Fig. 2f). At lower nutrient availabilities, transfer efficiency is highest under the assumption of saturating food uptake rates and fixed preference (pZ = 0.5), however, the adaptive preference case reaches similarly high values under high nutrient availabilities (Regime II) (Fig. 2f).