Littoral macroinvertebrate communities and environmental parameters of alpine lakes along an elevational gradient (Hohe Tauern National Park, Austria) in July/August 2018

Alpine lakes support unique communities which may respond with great sensitivity to climate change. To understand the drivers of benthic macroinvertebrate community structure, samples were collected in the littoral of 28 lakes within Hohe Tauern National Park, Austria. Sampling took place from early July to early August 2018 between altitudes of 2,000 and 2,700 m a.s.l. The extent of habitat types in the lake littoral was estimated. Habitat types were classified into sediment (maximum grain size of 2 mm), small rocks (up to 20 cm x 15 cm x 5 cm), and large boulders/sheer rock faces. The extent of rocky habitats was calculated as the sum of areas covered by small rocks and boulders/sheer rock faces. A total area of 1 m² was sampled in each lake, using a hand net with a sharp frame (25 cm in width) and 500 µm mesh-size. Mixed samples were taken, covering each habitat type proportional to its extent in the lake (100% corresponding to 1 m²). For habitats covering up to 10% of the lake, a standardized area of 0.1 m² was sampled. In sediment, the uppermost 5 cm of the ground were scooped into the net by sweeping it swiftly through the sediment. When sampling large boulders or rock faces, a metal spatula was used to scrape macroinvertebrates off the surface and collect them in the net. Macroinvertebrates were brushed off small rocks using a toothbrush over water-filled trays. The dimensions of those small rocks were measured, and total surface area was calculated, assuming a suitable geometric form (ellipsoid or cuboid). Samples were presorted in the field and preserved in 4% formalin. After 3-4 weeks, all samples were rinsed in tap water and transferred to 70% ethanol for further storage. Identification was performed using a stereomicroscope (OLYMPUS SZX16, 11.2x-184x) to the lowest taxon possible. Lake size was determined by aerial photograph in Google Earth Pro. To do so, the outlines of the lakes were traced, and the area of the polygon then calculated. Physical and chemical water parameters were measured with a multi-parameter sonde (EXO2 YSI) (for lakes 1-18 from a boat, otherwise from a rock or by wading into the lake): water temperature (°C), dissolved oxygen (% saturation), conductivity (µS/m), pH, nitrate (mg/l), turbidity (FNU), blue-green algae phycocyanin (µg/l) and chlorophyll-a (µg/l). Maximum depth (m) was measured with a sonar by rowing up to 10 transects across lakes. Maximum depth was not measured for lakes 19-28. Two data loggers had been planted per lake in lakes 1-18 in the previous year and were recovered in 2018. Data loggers measured water temperature at about half a meter depth in six-hour intervals over an entire year. Ice-free days were deduced from available logger data, assuming an ice-cover at water temperatures below 2 °C (daily maximum temperature). Additionally, zoo- and phytoplankton samples were taken from the first 18 lakes. Zooplankton was sampled with vertical tows from the hypolimnion to the surface in deeper lakes, and with oblique tows in shallow lakes using a 29 cm diameter net with a 30 µm mesh size. Samples were then fixed in sucrose-formalin and counted under an Olympus SZX16 stereomicroscope equipped with a 0.7 – 11.5 zoom objective. Phytoplankton samples from lakes 1-18 were taken with a 1.2 L water sampler from the middle of the epilimnion, and when one was present, also from the deep chlorophyll maximum. Samples were fixed with Lugol's iodine and counted in sampling chambers with a Nikon TE2000 inverted microscope using a 20x objective.