Surface microlayer in lakes of different trophic status: Nutrients concentration and accumulation

In humic, mesotrophic and eutrophic lakes (Masurian Lakeland, Poland) 100-200 samples of water were taken with the Larsson' plate from 0.5 mm surface microlayer (SM) and compared with the subsurface water (SSW). The concentrations of dissolved (in filtered water) and total (unfiltered) Kjeldahl nitrogen (organic and ammonium) (DKN, TKN) and phosphorus (DP TP) were measured and the concentration of their particulate (sestonic) forms (PON, PP) estimated. The enrichment (accumulation) factor (Ef) values were calculated as the ratio of nutrients concentration in SM versus SSW Accumulation of nutrients in SM was a common phenomena in the studied lakes. However, it occurred more frequently (close to 100% of samples) for TKN than for TP (60-70%) as well as generally more frequently in humic lake than in other lakes. Mean values of Ef for TKN were 2.6, 2.2 and 1.6 respectively for the humic, meso- and eutrophic lakes but the accumulation values for TP were lower 2.3, 2.1 and 1.5 respectively for above lakes (the differences between concentrations in both layers as well as between lakes are significant). It means that in average the concentration of N and P in SM is at least two times greater than in SSW in humic and mesotrophic lakes. Accumulation of particulate forms of nutrients (PP, PKN) in SM is less frequent but the respective values of Ef are higher comparing with the total content of these nutrients and again higher for nitrogen than phosphorus. The higher values of the weight ration NIT (33-76) in seston were found in SM in comparison with SSW (24-56) as well as higher contribution of particulate (sestonic) N (PON) to its total content in respective layer. Generally higher values of these two indices were found in both layers of humic lake than in other lakes. No significant differences were found for the Ef values calculated for spring and summer periods (expected to direr in organic matter production and algae biomass) as well as for different sites (open water. shore region) in the lakes. It was concluded that the humic substances of allochthonous origin (mostly refractory and of high molecular weight) possibly predominate in the surface microlayer of the humic lake and support the stability and specificity of this layer in comparison with deeper water layer as well as in comparison with the meso- and eutrophic lakes. In highly productive, eutrophic lake. the surface microlaver is less distinct and stable possibly because of relatively greater role of in situ autochthonous production of labile organic matter which show's low affinity to the air-water interface. The surface microlayer in mesotrophic lake displays somehow intermediate properties in comparison with humic and eutrophic lakes.