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Abstract:
Two microbial extracellular enzyme activities (MEEA) were studied in HUMEX Lake Skjervatjern: acid phosphatase (APHA) and leucine aminopeptidase (LeuAMPA). Both enzyme activities varied in the vertical and horizontal scale in both lake sites. APHA varied in the acidfied Basin A between 945-1706 nmol L(-1) h(-1) and LeuAMPA between 3.7-25 nmol L(-1) h(-1). Both MEEA reached maxima in 0.5 m depth. In the control site (Basin B), APHA was lower by a factor of two, and varied between 156-669 nmol L(-1) h(-1). LouAMPA reached similar values as in Basin A and varied between 7.8-34.8 nmol L(-1) h(-1). Maxima of APHA were found in the upper layer (0-2 m), while LeuAMPA had only one distinct maxima at 2-2.5 m depth. The number of bacteria (AFDC) varied between 4.4-8.8 10^(6) cells mL(-1) and was not significantly different in either side, but both had maxima in the thermocline. Highest specific LeuAMPA activities were found in the thermocline (3.2-4.5 fmol L(-1) h(-1) cell(-1)) in both sides and varied between 0.4-4.5 fmol L(-1) h(-1) cell(-1) in both water columns. The main contributor (60-70%) to LeuAMPA was found in the microplankton fraction, retained on Nuclepore filters with pore sizes between 2.0-0.2 µm. APHA was retained less even on a filter with pore size smaller than 0.2 µm. About 50-70% of APHA passed through 0.2 µm-0.1 µm Nuclepore filters and could be found in the dissolved organic matter (DOM) fraction. APHA and bacteria counts (AFDC) showed a distinct gradient from the littoral zone to the pelagial in the surface water samples (0.2 m depth). APHA and LeuAMPA are regarded as important regulators for nutrient availabilty to microplankton. However, all data from vertical and horizontal samples showed that Lake Skjervatjern is a strongly gradient-dominated aquatic ecosystem. Watershed-littoral effects are more pronounced in the shallow, acidified Basin A than in the control side, Basin B.
