No abstract

The utilization of dissolved organic carbon (DOC) by bacteria in grazer-free cultures can be used to measure the concentration of labile DOC (DOCL). A database of 126 measurements was used to test whether or not the concentration of DOCL is positively related to total DOC. A positive and significant correlation between DOCL and DOC was found for the entire database as well as for lakes, rivers and seawater. An average response of about 0.17 pm01 DOCL pmol-' DOC could be calculated and 60% of the variance was explained by DOC. The DOCL concentration averaged 14, 19, and 19% of the total DOC pool in lakes, rlvers and seawater, respectively; however, the median value of 25% for rivers was about twice the values for lakes and seawater The high relative amount of DOCL in half the rivers was explained by anthropogenic influence. The control of DOCL concentrations was analyzed according to models of bacterial and substrate interactions applying Michaelis-Menten-Monod kinetics. It is suggested that the higher concentrations of DOCL in eutrophic as opposed to oligotrophic systems may be explained by a higher 'theoretical community half-saturation constant' (K,) in eutrophic systems. The consequence of a higher K, will be a higher steady-state concentration of labile substrate, as was found. Other possible explanations were analyzed, but rejected as major causes for the crosssystem difference.

Shallow lakes are likely to be strongly impacted by climate changes and, in particular, by increased temperatures. To enable realistic experimental studies of the effects of higher temperatures on in-lake processes and dynamics, technologically advanced systems are required. This paper presents design details, operating characteristics, and background information on a currently operating experimental flow-through mesocosm system that allows investigation of the interactions between simulated climate warming and eutrophication and their impacts on biological structure and ecosystem processes in shallow lakes. We use 24 mesocosms to combine three temperature scenarios (one unheated and two heated relative to the Intergovernmental Panel on Climate Change climate scenario A2 and A2 + 50%, respectively) and two nutrient levels (enriched and nonenriched). Planktivorous fish (male sticklebacks, Gasterosteus aculeatus) are stocked in accordance with the nutrient level. The water residence time is regulated by the semicontinuous addition of water and is approximately 2.5 mo in each mesocosm. For heating, we use electrically powered heating elements. The heating system has performed well over 16 mo of continuous heating, and seasonal and diurnal temperature variations of the unheated reference mesocosms were paralleled well by the heated mesocosms. The performance of the flow-through system and the heating technique are discussed with special emphasis on strengths, limitations, and potential improvements of the system. To illustrate the performance of the system and its potential, we present data for selected periods on total phosphorus retention in the mesocosms and system primary production and respiration.

During April to September, plankton community structure was monitored in large enclosures (60 m3) in the presence and absence of planktivorous fish. Weekly sampling included inorganic nutrients, oxygen, pH, and transparency as well as biomass of phytoplankton, bacteria, heterotrophic flagellates, rotifers, and macrozooplankton.In fishless enclosures, concentrations of inorganic nutrients and transparency were higher than in enclosures with fish, whereas oxygen, pH, and biomasses of phytoplankton, picoplankton and rotifers were lower. As an average for the entire period, Chl a was 14 pg liter-' in the enclosures without fish and 46 pg liter-l in enclosures with fish.

Marine mesocosms were manipulated with inorganic nutrients over a period of 22 d to investigate organic carbon partitioning under a variety of nutrient regimes. The chemical analyses and biotic measurements included inorganic nutrients, pigment signatures, particulate and dissolved organic species, bacterial production, and community respiration. The biodegradability of dissolved organic carbon (DOC) was investigated with in vitro decomposition experiments.The net particulate organic carbon (POC) production was 50% of the total organic production during the initial 6 d of nutrient-replete growth and during a major diatom bloom. In all other situations the carbon partitioning was strongly in favor of DOC, which accounted for 82 to 111% of the total production. The production of new DOC preceded new DON by about 1 week. Thus, the new dissolved organic matter (DOM) initially had an infinite C : N ratio, which fell to 11-20 when DON started to accumulate. The highest C : N ratio was measured during a nutrientreplete diatom bloom. Dissolved polysaccharides accounted for 50 to 70% of the new DOC, and the lowest relative amount was produced during a diatom bloom. The chemical analyses unequivocally demonstrated that carbon partitioning in favor of carbon-rich DOM can take place during an active diatom bloom and not only during the decay of a bloom. The DOC-producing mechanisms cannot be fully identified. However, during the different growth phases the DOC production varied, as did the speciation of DOM with respect to the C : N ratios. When net 1 Corresponding author (MSondergaard@zi.ku.dk).