Nutrient composition, microbial biomass and activity at the air-water interface of small boreal forest lakes

Abstract

Nutrient contents, microbial biomass and microbial activities were studied at the air-water interface in the surface microlayer (SM) and subsurface waters (10 cm depth) in small boreal forest lakes. Two different sampling techniques were used to evaluate differences in the nutrient composition in SM- and subsurface waters and to study their effects on microbial biomass and activities of neustonic and planktonic microbial communities. Inorganic nutrients were only slightly enriched in the SM compared to the subsurface water samples. P-PO4 varied between 6–10 mg P-PO4 m−3 in the SM and between 1–2 mg P-PO4 m−3 in the subsurface waters. Dissolved inorganic nitrogen (NO 2 - +NO 3 − +NH 4 + ) varied between 12–20 mg N m−3 in the SM and between 3–12 mg N m−3 in subsurface waters. Polymeric organic bound phosphorus and nitrogen were about 10 times enriched in the SM compared to the subsurface samples. However, microbial biomass like chlorophyll was by a factor of 8 to 280 times enriched in the SM of meso-polyhumic lakes and 2–25 time enriched in an acidified lake. Bacterioneuston biomass was by a factor of 1.5 to 2 times enriched in the SM compared to that of bacterioplankton. However, fungal biomass was 30 to 40 times higher in the SM than in the subsurface samples. Microbial activities was measured as [14C]-UL-α-D-Glucose uptake and as microbial biopolymer processing measured with 4-methylumbelliferyl-α-D-Glucopyranoside as model substrate for tracing the ;enzymatic cleaving rate of a-glycosidic polymer bound glucose via microbial α-glucosidase (αGlucAse). [14C] UL-α-D-Glucose uptake was about 4–5 times higher in bacterioneuston compared to bacterioplankton and varied between 4–22 μm m-3 h-1 in bacterioneuston communities and between 1.5–2.5 μm m−3 h−1 in bacterioplankton. aGlucAse was about 1.5 to 8 times higher in SM samples and varied between 38–108 μm m−3 h−1 in the SM microbial communities compared to 12–35 μm m−3 h−1 in the subsurface water microbial communities. The ratios between aGlucAse activities and [14C]-UL-α-D-Glucose uptake was about 3–5 times lower in the bacterioneuston than in the bacterioplankton communities which means a tighter metabolic coupling between biopolymer processing and substrate uptake in bacterioneuston than in bacterioplankton. Biofilms in surface microlayers at the air-water interface in small humic forest lakes can provide favourable microhabitats for the growth of neuston communities which may act as important sinks for allochthonous nutrient resources and may then generate new nutrient pools and prey for planktonic microbial food webs.