Experimental Detritus Manipulations Unite Surface and Cave Stream Ecosystems Along a Common Energy Gradient

Distinctive Connectivities of Near-Stream and Watershed-Wide Land Uses Differentially Degrade Rural Aquatic Ecosystems

The water-quality effects of low-density rural land-use activities are understudied but important because of large rural land coverage. We review and synthesize spatially extensive studies of oligotrophic mountain streams in the rural Southern Appalachian Mountains, concluding that rural land-use activities significantly degrade water quality through altered and mostly enhanced landscape–stream connections, despite high forest retention. Some connections (insolation, organic inputs, root–channel interactions, stream–field connectivity, individual landowner discharges) are controlled by near-stream land-use activities, whereas others (reduced nitrogen uptake and cycling, enhanced biological nitrogen fixation, nutrient subsidy, runoff from compacted soils, road runoff delivery) are controlled by basin-wide land use. These connections merge to alter basal resources and shift fish, salamander, and invertebrate assemblages toward species tolerant of higher turbidity and summer temperatures and those more competitive in mesotrophic systems. Rural water quality problems could be mitigated substantially with well-known best management practices, raising socioecological governance questions about best management practice adoption.

Variability in macrozoobenthic assemblages along a gradient of environmental conditions in the stream water of karst caves (Lower Shakuranskaya Cave, western Caucasus)

The fauna of the stream water in the Lower Shakuranskaya Cave in central Abkhazia, western Caucasus, was studied. This cave has a large inlet and an extended entrance ecotone area of approximately 60 m, which makes it a convenient area for studying macrozoobenthic assemblages across a gradient of environmental factors. The cave has 13 species of stygobionts, 10 species of stygophiles and 18 species of stygoxenes. The number of species and the abundance and biomass of stygobionts per station were the highest near the boundary of the photic zone, at a distance of 50–60 m from the entrance to the cave, and gradually decreased toward both the remote parts of the cavity and the cave exit. The most abundant stygobionts were gastropod mollusks of the Hydrobiidae family, and Xiphocaridinella shrimp comprised the main part of the biomass. It has been shown that the main environmental factors determining the distribution of macrozoobenthos are luminosity and distance from the entrance to a cave. According to the differences in their reactions to these environmental factors, several groups of species were identified. In addition, three main assemblages of macrozoobenthic species were described: (1) an assemblage of epigean species near the cave entrance area; (2) stygobionts in remote parts of the cave outside the photic zone; and (3) a mixed assemblage in the cave ecotone, where a faint light penetrates. The specific details related to the faunal structure in the ecotone of the cave are discussed, as well as active and passive methods by which stygoxenes invade underground cavities.

Postojna-Planina Cave System in Slovenia, a Hotspot of Subterranean Biodiversity and a Cradle of Speleobiology

The Postojna-Planina Cave System (PPCS) in central Slovenia is a globally exceptional site of subterranean biodiversity, comprised of many interconnected caves with cumulative passage length exceeding 34 km. Two rivers sink into the caves of the PPCS, called the Pivka and Rak, and join underground into Unica River, which emerges to the surface. The studies of fauna of PPCS began in the 19th century with the first scientific descriptions of specialized cave animals in the world, making it “the cradle of speleobiology”. Currently, the species list of PPCS contains 116 troglobiotic animal species belonging to eight phyla, confirming its status as the richest in the world. Of these, 47 species have been scientifically described from the PPCS, and more than 10 await formal taxonomic descriptions. We expect that further sampling, detailed analyses of less studied taxa, and the use of molecular methods may reveal more species. To keep the cave animals’ checklist in PPCS up-to-date, we have supplemented the printed checklist with an online interface. As the revised checklist is a necessary first step for further activities, we discuss the importance of PPCS in terms of future research and conservation.

Stream Macroinvertebrates and Carbon Cycling in Tangled Food Webs

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year. Dissolved organic matter (DOM) flows through the food web to macroinvertebrates, but little is known about the effect of DOM increase on stream food webs and how much macroinvertebrates may contribute to the regulation of carbon fluxes in rivers. Using a before and after control impact (BACI) experimental design, we increased by 12% (+ 0.52 mg C L−1) the concentration of DOM in a stream for three weeks by adding sucrose, with a distinctive δ13C signature, to simulate a pulse of natural DOM supply from soils. We partitioned the diet of macroinvertebrates from carbon sources according to the green pathway (autotrophs) and detrital pathways (bacteria and terrestrial organic matter). Our flow food web approach based on C fluxes, with bacteria as a key node, showed the dominant contribution of the detrital pathways for macroinvertebrates in the reference stream. DOM addition induced changes in the diets of individual taxa, but did not have any strong effects on the relative overall contribution of the detrital pathways versus the green pathway. Autotrophic uptake of CO2 respired by bacteria was much larger than bacterial C flux to invertebrates (that is, the classic microbial loop) and allowed a significant fraction of natural allochthonous organic carbon to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria. Overall macroinvertebrates did not regulate directly to any great extent the flux of stream DOM towards downstream ecosystems.