Description: 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.

Description: 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.

Description: Nanoplankton and picoplankton abundance and community grazing on picoplankton were determined in summer and autumn at several stations in a productive coastal environment (Georges Bank, NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 x 103 cells ml-1 on Georges Bank, and 2.2 to 6.8 x 102 cells ml-1 in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 x 103 and 1.3 to 4.7 x 102, respectively. Mixotrophic nanoplankton (MNAN), operationally defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30% in the Sargasso Sea. Mixotrophs represented up to 39% of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and 〈\3 µm algae, FLA). Absolute grazing rates (total picoplankton cells removed d-1) on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d-1 in the coastal ocean and 0.05 to 0.24 d-1 in the oligotrophic ocean). Rates of bacterivory were in approximate balance with rates of bacterial production (3H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the summer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.

Description: Zooplankton grazing on bacterio- and phytoplankton was studied in the Gulf of Aqaba and the Northern Red Sea during Meteor Cruise Me 44-2 in February-March 1999. Protozoan grazing on bacterioplankton and autotrophic ultraplankton was studied by the Landry dilution method. Microzooplankton grazing on phytoplankton 〉6 µm was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual zooplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae 〈6 µm were high (bacteria: 0.7 to 1.1 d-1, ultraphytoplankton: 0.7 to 1.3 d-1), while grazing rates on Synechococcus spp. were surprisingly low and undetectable in some experiments. Mesozooplankton grazing was weak, cumulative grazing rates being ca. 2 orders of magnitude smaller than the grazing rates by protozoans. Among mesozooplankton, appendicularians specialised on smaller food items and calanoid copepods on larger ones.

Description: Experiments were carried out on Georges Bank, a productive coastal region in the northwestern sector of the North Atlantic Ocean, and in the oligotrophic western Sargasso Sea to examine the effects of nutrient (inorganic nitrogen and phosphorus) and organic carbon (glucose) additions on bacterial and phytoplankton growth. Four experiments were conducted in each environment. Phytoplankton growth was monitored over a 36 h period by following changes in the concentration of chlorophyll in unfiltered seawater and in seawater prefiltered through 5 μm screening to reduce grazing pressure. Bacterial production was estimated initially and after 24 h using the 3H-thymidine (TdR) method in unfiltered seawater and in 1 μm filtrate. Phytoplankton biomass increased significantly in response to nutrient additions in all but 1 experiment, whereas chlorophyll concentrations remained unchanged or decreased in all of the unamended (control) treatments or treatments supplemented with glucose. Responses of the phytoplankton community were similar for the 〈5 μm and unfiltered treatments. Bacterial production increased after 24 h in all of the treatments on Georges Bank, and there was little effect of nutrient or glucose addition in unfiltered seawater relative to unamended controls. However, glucose addition to the 〈1 μm filtrate caused substantial increases in bacterial production relative to controls and N/P-amended treatments in 2 of the experiments from this environment. Glucose had no stimulatory effect (relative to unamended treatments) in 3 of the 4 Sargasso Sea experiments, and only a marginal effect in the fourth. However, the addition of inorganic nitrogen and phosphorus in the latter ecosystem resulted in higher bacterial production (relative to unamended treatments or glucose addition) in 2 of the experiments with unfiltered seawater, and very large increases in 3 of the experiments with 1 μm filtrate. The magnitude of the changes in bacterial production differed greatly between unfiltered and filtered seawater in both ecosystems, indicating an important role for bacterial grazers in controlling bacterial population growth. The results of this study indicate different nutritional restraints on bacterial production in these contrasting environments.

Description: Diversity within distinct trophic groups is proposed to increase ecosystem functions such as the productivity of this group and the efficiency of resource use. This proposition has mainly been tested with plant communities, consumer assemblages, and multitrophic microbial assemblages. Very few studies tested how this diversity-productivity relationship varies under different environmental regimes such as disturbances. Coastal benthic assemblages are strongly affected by temporal instability of abiotic conditions. Therefore, we manipulated benthic ciliate species richness in three laboratory experiments with three diversity levels each and analyzed biomass production over time in the presence or absence of a single application of a disturbance (ultraviolet-B [UVB] radiation). In two out of three experiments, a clear positive relationship between diversity and productivity was found, and also the remaining experiment showed a small but nonsignificant effect of diversity. Disturbance significantly reduced the total ciliate biomass, but did not alter the relation between species richness and biomass production. Significant overyielding (i.e., higher production at high diversity) was observed, and additive partitioning indicated that this was caused by niche complementarity between ciliate species. Species-specific contribution to the total biomass varied idiosyncratically with species richness, disturbance, and composition of the community. We thus present evidence for a significant effect of consumer diversity on consumer biomass in a coastal ciliate assemblage, which remained consistent at different disturbance regimes.

Description: Nitrification in aquatic sediments is catalyzed by bacteria. While many autecological studies on these bacteria have been published, few have regarded them as part of the benthic microbial food web. Ciliates are important as grazers on bacteria, but also for remineralization of organic matter. We tested the hypothesis that ciliates can affect nitrification. Experiments with Baltic Sea sediments in laboratory flumes, with or without the addition of cultured ciliates, were conducted. We found indication of a higher nitrification potential (ammonium oxidation) in one experiment and increased abundances of nitrifying bacteria in treatments with ciliates. This is likely due to higher nitrogen availability caused by excretion by ciliates and enhanced transport processes in the sediment.