| 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. LeuAMPA 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 4.4–8.8 106 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 availability to microplankton. However, all data from vertical as important regulators for nutrient availability 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. |
