Jesper Aagaard Christensen:
Monitoring aquatic environments with autonomous systems

Date: 28-02-2013    Supervisor: Kirsten S. Christoffersen

High frequency measurements from autonomous sensors have become a widely used tool among aquatic scientists. This report focus primarily on the use of ecosystem metabolism based on high frequency oxygen measurements and relates the calculations to spatial variation, biomass of the primary producers and environmental variables. The results can be formulated in three main conclusions.

1) Primary production and respiration in stratified lakes are not evenly distributed in the water column. Generally you can expect the net production to decreases with depth as gross primary production (GPP) decreases with depth, unless it is a very clear lake, while respiration is relatively stable. Metabolism estimates based on data from the epilimnion will only represent a minor proportion of the whole lake metabolism under conditions of strong stratification and high water transparency. At a low depth of the upper mixed layer (Zmix) and a euphotic zone (Zeu) which is deeper than Zmix (Zeu> Zmix) GPP in epilimnion will most likely represent less than 80% of GPP in the whole water column and respiration (R) in the epilimnion will most likely be less than 60% of respiration integrated over the whole water column.

2) Physical water movement across the thermocline can account for around 50% of the variation in the oxygen concentrations in metalimnion and up to 80% of the variation in oxygen concentration in hypolimnion in a clear water, mesotrophic, polymictic lake. In contrast the oxygen variations in epilimnion were primarily a result of metabolism and gas exchange with the atmosphere, while only 10% of the variation was due to physical movement of water across the thermocline.

3) Dense macrophyte populations in oligotrophic systems may have a higher GPP than expected based on nutrient conditions in the water phase and in shallow systems the macrophytes can completely dominate primary production. This was despite the fact that the plants in the studied system were light-saturated most of the light hours and occasionally carbon limited. It was also shown that the GPP and the total phytoplankton biomass in a nutrient-rich but deeper lake may be below the expected level based on nutrient conditions when algal succession was regularly interrupted due to repeated mixing events during an otherwise stratified period.