Importance of allochthonous matter for profundal macrozoobenthic communities in a deep oligotrophic lake

This study evaluates the nutritional importance of allochthonous matter supply for profundal macrozoobenthic communities. It determines chironomid and oligochaete abundances and carbon sources at two sampling sites with different allochthonous contributions in the deep oligotrophic waters of Lake Constance. Site AL, characterized by sedimenting organic matter (POM sed ) with a more allochthonous origin was dominated by oligochaetes, whereas site AU characterized by POM sed of predominantly autochthonous origin, was dominated by chironomids. Total macroinvertebrate abundances were 10–20 times higher at site AL compared to site AU. At site AL microbial biomass per gram dry sediment (indicated by ATP content and bacterial abundances) was lower, whereas microbial activity (incorporation of leucine, respiration of glucose) was higher than at site AU. A higher metabolic activity at site AL was also indicated by steeper oxygen gradients in sediment microprofiles. Mean δ 13 C stable isotope signatures of oligochaetes ( δ 13 C = −33.24 ± 1.27‰) and, to an even greater extent, of the dominant chironomid Micropsectra sp. ( δ 13 C = −40.08 ± 1.65‰) at site AL were depleted compared to site AU for oligochaetes ( δ 13 C = −30.19 ± 0.60‰) and chironomids ( δ 13 C = −30.40 ± 0.75‰). At site AL, mean δ 13 C values of chironomids and oligochaetes were 11‰ and 4‰ lower, respectively, than mean POM sed δ 13 C. At site AU, benthic macroinvertebrates and POM sed were of comparable magnitude. The lower invertebrate δ 13 C values at site AL suggest an increased contribution of methane oxidizing bacteria (MOB) to the diet of the benthic organisms. A mixing model based on stable carbon isotopes suggests that at site AL 21 to 44% of chironomid diet and 9 to 19% of oligochaete diet stems from MOB consumption, whereas calculated MOB consumption was zero at site AU. Mean δ 15 N values of chironomids and oligochaetes were higher at both sites compared to POM sed, but at site AL (oligochaetes = 7.07 ± 0.61‰, “ Micropsectra sp.” = 8.62 ± 0.30‰) the difference was less pronounced than at site AU (oligochaetes = 13.71 ± 0.30‰, “ Micropsectra sp.” = 14.00 ± 0.95‰). These differences are considered to reflect mainly the degree of biotic processing of POM sed before consumption by invertebrates. Given that autochthonous POM sed , i.e. chlorophyll sedimentation, was comparable at both sites, but differences were observed with respect to total POM sed sedimentation rates, δ 13 C and δ 15 N isotope signatures of POM sed , and microbial activities in the sediments, we suggest that there is a strong contribution of allochthonous POM sed supply to the nutrition of macrobenthic communities at site AL. This suggests that allochthonous carbon supply is at least partly bioavailable and stimulates microbial growth and metabolic activities in lake sediments, which in turn provides additional nutritional sources for benthic organisms. Furthermore, the example of Lake Constance shows that, even in a well‐oxygenated oligotrophic lake, at least part of the allochthonous POM sed is made available to higher trophic levels via methane production and MOB biomass.