Zooplankton allochthony is spatially heterogeneous in a boreal lake

1. The proportion of consumer biomass from terrestrial origin (i.e. allochthony) has been shown to vary greatly among lakes and also seasonally, but has been assumed to be spatially homogeneous within a lake. Given that the distribution of different organic carbon (C) sources tends to be spatially patchy in most lakes, this assumption may not be warranted. 2. We tested this hypothesis using a spatially intensive sampling designed to capture the in-lake heterogeneity in terrestrial inputs, phytoplankton, benthic algae and a dominant aquatic macrophyte (Brasenia schreberi: Cabombaceae) in a medium-sized boreal lake, and used a dual-isotope Bayesian mixing approach (delta C-13, delta H-2) to establish the degree of allochthony of the dominant copepod Leptodiaptomus minutus (Diaptomididae) across these sites. Samples were collected in spring when tributaries had high flow rates and aquatic primary producers (phytoplankton, macrophytes) had rapid growth rates, and in mid-summer when tributary flows were at the lowest. 3. There was substantial spatial variability in the stable-isotope composition of the copepod and consequently in its levels of allochthony in both seasons. Allochthony in L. minutus varied from 34 to 50% in spring and from 45 to 65% in summer, and this range was linked to the spatial variability in the main sources of organic C (terrestrial inputs via tributaries, B. schreberi and phytoplankton). Allochthony in L. minutus was lowest in areas dominated by macrophytes, and further influenced by the distribution of tributary-derived terrestrial C across the lake. Macrophyte and phytoplankton carbon contributed, respectively, up to 28 and 38% during growing season (spring) to the diet of the L. minutus, while benthic algae contribution was negligible. 4. Our results clearly show that the reliance of zooplankton on terrestrial C may be spatially heterogeneous even in a relatively small lake and, in particular, that macrophytes, whose distribution is typically patchier than that of phytoplankton, may play a major role in shaping the spatial patterns of zooplankton allochthony in lakes.