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A common perception in limnology is that shallow lakes are homogeneously mixed owing to their small water volume. However, this perception is largely gained by downscaling knowledge from large lakes to their smaller counterparts. Here we show that shallow vegetated lakes (less than 0.6 m), in fact, undergo recurring daytime stratification and nocturnal mixing accompanied by extreme chemical variations during summer. Dense submerged vegetation effectively attenuates light and turbulence generating separation between warm surface waters and much colder bottom waters. Photosynthesis in surface waters produces oxygen accumulation and CO depletion, whereas respiration in dark bottom waters causes anoxia and CO accumulation. High daytime pH in surface waters promotes precipitation of CaCO which is re-dissolved in bottom waters. Nocturnal convective mixing re-introduces oxygen into bottom waters for aerobic respiration and regenerated inorganic carbon into surface waters, which supports intense photosynthesis. Our results reconfigure the basic understanding of local environmental gradients in shallow lakes, one of the most abundant freshwater habitats globally.

We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11-m 3 marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 mg L 21 , an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (2Si). Addition of inorganic nutrients in a N : P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom-dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N-limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was 1 Corresponding author (msondergaard@bi.ku.dk). AcknowledgmentsWe thank Anne J. Jacobsen, Louise Oriol, and Winnie Martinsen for their technical assistance. The thoughtful comments by two reviewers improved the final product. Bergen LSF is thanked for support (HPRI-CT-2002-00181) as is the European Union (EVK3-CT-2000-00034) and the Niels Bohr Foundation (TK).

Studies indicate that bacterial taxa utilize different fractions of the dissolved organic carbon (DOC) pool, while others suggest functional redundancy among constituents of bacterioplankton, implying only a weak coupling between community structure and function. We examined bacterial compositional and functional [ectoenzymatic activities and growth efficiency; bacterial growth efficiency (BGE)] responses to a gradient in bioavailable DOC (bDOC). This was achieved over 10 days in DOC utilization assays containing Baltic Sea water with variable amounts of natural bDOC. Measurements of bacterial growth, O2 and DOC consumption in the assays using non-invasive sampling showed that BGE changed over time and that the bDOC utilized accounted for 4-13% of the DOC pool. Pyrosequencing of 16S rRNA genes demonstrated minor differences at the phylum level between samples, whereas larger successional differences were discernible at lower phylogenetic levels. Our study suggests that changes in concentrations of bDOC affect bacterioplankton BGE and community structure by selecting for some taxa while the relative abundance of most taxa remained unaffected. Ectoenzymes activities suggested preferential degradation of protein-rich compounds by bacteria, switching to carbohydrate-rich DOC when proteins were depleted. Hence, there was a fairly weak linkage between bacterial community composition and DOC utilization suggesting that overall bacterioplankton community structure only to some extent has predictive power for processing of the DOC pool.

A phytoplankton bloom and decay sequence was created in 2 laboratory containers and mesozooplankton was added to one container before the peak of algal biomass. Each day for 22 d, the net production of autochthonous dissolved organic carbon (DOC) was measured and on 5 occasions the degradation kinetics and the total pool of biodegradable DOC (BDOC) were assayed in experiments lasting 230 d. Net accumulation of new DOC was 235 and 280 µM in the containers with and without zooplankton, respectively. The best description of microbial DOC degradation was a 2-pool model and 1st order exponential decay. Without mesozooplankton present, the degradation experiments showed accumulation of a large pool of labile BDOC characterised by decay coefficients > 0. ). The amount of newly produced recalcitrant DOC (RDOC) accounted for about 12% of new DOC. The differences observed with respect to degradation kinetics and net DOC production are explained by food web interactions and nutrient limitation. The presence of mesozooplankton resulted in high bacterial production keeping labile BDOC at low concentrations. In the container without mesozooplankton, the bacterial uptake capacity was reduced, probably by a combination of protist grazing and nutrient limitation. Consequently, about 75 µM BDOC with a half-life of less than 3 d accumulated during the experiment. Mineralisation of the accumulated dissolved organic matter (DOM) during microbial degradation in a nutrient replete environment was measured as the decrease in DOC and net mineralisation/immobilisation of inorganic N and P. The mineralisation of DOC was accompanied by low mineralisation of N and P and even immobilisation of phosphate during degradation of DOM produced in the container with mesozooplankton present. Bacterial production of DON and DOP is believed to result in a recalcitrant DOM pool enriched in N and P, and the activity of mesozooplankton seems to enhance this scenario.KEY WORDS: Dissolved organic carbon · Dissolved organic matter · Autochthonous DOC · Mesozooplankton · Biodegradability · Mineralisation · N-rich recalcitrant DOM · P-rich recalcitrant DOM Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 36: [61][62][63][64][65][66][67][68][69][70][71][72] 2004 can be a major DOC source (Fogg 1966, Baines & Pace 1991, Søndergaard et al. 2000b. Cell lysis during 'natural' mortality, e.g. caused by viral attack, contributes to the continuous production of DOC and zooplankton grazing has also been suggested as a prominent DOC and dissolved organic nitrogen (DON) producing process (Lampert 1978, Olsen et al. 1986, Nagata 2000. With respect to grazing, it seems that the produced DOC is very labile, fuels instant bacterial production and does not accumulate to high concentrations (Olsen et al. 1986, Strom et al. 1997.If DOC production and removal are in steady state, the concentration of each degradable compound or class of compounds is constant and inversely proportional to its lability. However, aquatic ...

This study reports on the interacting effect of photochemical conditioning of dissolved organic matter and inorganic phosphorus on the metabolic activity of bacteria in freshwater. Batch cultures with lake-water bacteria and dissolved organic carbon (DOC) extracted from a humic boreal river were arranged in an experimental matrix of three levels of exposure to simulated sunlight and three levels of phosphorus concentration. We measured an increase in bacterial biomass, a decrease in DOC and bacterial respiration as CO(2) production and O(2) consumption over 450 h. These measurements were used to calculate bacterial growth efficiency (BGE). Bacterial degradation of DOC increased with increasing exposure to simulated sunlight and availability of phosphorus and no detectable growth occurred on DOC that was not pre-exposed to simulated sunlight. The outcome of photochemical degradation of DOC changed with increasing availability of phosphorus, resulting in an increase in BGE from about 5% to 30%. Thus, the availability of phosphorus has major implications for the quantitative transfer of carbon in microbial food webs.

Elevated levels of biomass and productivity are often associated with ocean frontal systems. The Subtropical Convergence Zone (STCZ) in the southern Sargasso Sea shows pronounced and stable thermal fronts, but little is known about the ecological consequences of these hydrographic features. With the aim of improving the understanding of physical and lower trophic level processes in the area, we carried out field studies of physical, chemical and biological characteristics along 3 transects crossing thermal fronts associated with the STCZ in March to April 2007. Thermal and chemical stratification were evident at all stations within the STCZ, and a depletion of nitrate was measured in the upper ~150 m. The phytoplankton biomass was dominated by Prochlorococcus spp. with highest abundance, up to > 400 000 cells ml -1, at the chlorophyll a (chl a) maximum at 100 to 150 m depth. Synechococcus spp. were generally located higher in the water column. Picoalgae were less abundant, up to 8000 cells ml -1 , but explained most of the variation in chl a. Even though diatoms and dinoflagellates were few, the biomass of larger phytoplankton equalled or exceeded that of picoplankton at a few stations. Bacterial biomass was roughly equal to half of the phytoplankton biomass. We did not find elevated levels of primary production or biomass of specific phytoplankton groups associated with the STCZ, probably due to a pronounced variability between stations along transects. Nevertheless, distinct increases in chl a were associated with the zone and thermal fronts bordering the STCZ.

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