Daphnia versus copepod impact on summer phytoplankton: functional compensation at both trophic levels.

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalinaxgaleata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 microm3), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.

Heterotrophic microbial activity and organic matter degradation in coastal lagoons of Colombia.

In this study we measured the community respiration and the bacterial respiration as part of the overall degradation process of organic material. Additionally, the turnover rates of the pools of dissolved free glucose and acetate as representatives of the fraction of easily degradable low molecular organic solutes were determined. The study was performed in several coastal lagoons of the "Outer Delta of the Río Magdalena" in northern Colombia. The lagoons can be separated into two groups: The first group contains highly productive brackish lagoons with chl a concentrations ranging from 62-130 micrograms/l. The second group consists of less productive freshwater lagoons with chl a between 5.5-19 micrograms/l. Turnover rates of glucose and acetate were very fast in the highly productive lagoons resulting in turnover times of less than 20 min for both compounds. In the less productive systems the cycling of glucose and acetate was much slower. Here the mean values of the turnover times were 2 hr for glucose and 1.5 hr for acetate. The rates of bacterial DNA-formation measured as thymidine incorporation differed significantly between both groups of lagoons, being very high (1.86-2.76 nmol/l/hr) in the highly productive and relatively low (0.073-0.55 nmol/l/hr) in the less productive group. Water column community respiration ranged between 122 and 16 micrograms C/l/hr with means of 88 micrograms C/l/hr in the highly and 19 micrograms C/l/hr in the less productive group. In the first group the mean values of the bacterial contribution to community respiration amounted to 37% and in the second group to 18%. The bacterial respiration was determined in an indirect way via bacterial biomass production and assuming a growth efficiency of 50%. It is discussed whether this relatively high growth efficiency allows reasonable results in both groups of lagoons.

Bacterial growth and primary production along a north-south transect of the Atlantic Ocean.

The oceanic carbon cycle is mainly determined by the combined activities of bacteria and phytoplankton, but the interdependence of climate, the carbon cycle and the microbes is not well understood. To elucidate this interdependence, we performed high-frequency sampling of sea water along a north-south transect of the Atlantic Ocean. Here we report that the interaction of bacteria and phytoplankton is closely related to the meridional profile of water temperature, a variable directly dependent on climate. Water temperature was positively correlated with the ratio of bacterial production to primary production, and, more strongly, with the ratio of bacterial carbon demand to primary production. In warm latitudes (25 degrees N to 30 degrees S), we observed alternating patches of predominantly heterotrophic and autotrophic community metabolism. The calculated regression lines (for data north and south of the Equator) between temperature and the ratio of bacterial production to primary production give a maximum value for this ratio of 40% in the oligotrophic equatorial regions. Taking into account a bacterial growth efficiency of 30%, the resulting area of net heterotrophy (where the bacterial carbon demand for growth plus respiration exceeds phytoplankton carbon fixation) expands from 8 degrees N (27 degrees C) to 20 degrees S (23 degrees C). This suggests an output of CO2 from parts of the ocean to the atmosphere.

Planktonic primary production in a tidally influenced mangrove forest on the Pacific coast of Costa Rica

The seasonal variation of planktonic primary productivity was measured during one year in the main channel in the interior part of the mangrove forest of the Estero de Morales (Estero de Punta Morales), a mangrove system located in the Golfo de Nicoya at the Pacific coast of Costa Rica. Samples were incubated at the surface, 0.5 m and 1.0 m depth and the [quot ]light and dark bottle technique[quot ] was employed. The annual gross primary productivity (PPg) was 457 and the net primary productivity (PPn) was 278 g C m(-2) a(-1). Daily PPg ranged from 0.29 to 3.88 and PPn from 0.12 to 2.76 g C m(-2) d(-1). The highest rates observed in May and September were due to red tide blooms. The seasonal variation of primary productivity inside the mangrove forest depends closely on the PP in the adjacent area of the upper Golfo de Nicoya. Obviously the PP was light-limited since the compensation depth in the ebb current was found at only 1 m depth. In the flood current it was somewhat deeper. The planktonic primary productivity inside the mangrove forest was completely restricted to the open channels. A simultaneous measurement demonstrated that PPn of the phytoplankton could not take place under the canopy of the mangroves. Additional studies on the time course of the oxygen concentration in the mouth of the main channel over 24 hrs demonstrated a relation between the O2 and the tidal curves. The ebb current had always lower O2 concentrations than the flood current, regardless of the time of the day. The difference to the foregoing high tide, however, was much smaller when the low tide occurred during the day. This indicates that under the canopy the net primary production and hence O2 liberation of the attached macro- and microalgae, together with the high PPn of the phytoplankton in the channels, helped the oxygen concentration not to decrease as far as during the night. Nevertheless it shows that the consumtion of organic material in the submersed part of the...

Microbial Decomposition in Aquatic Environments: Combined Process of Extracellular Enzyme Activity and Substrate Uptake.

The aim of this study was to define a model for the coupling between extracellular enzyme activity and substrate uptake by bacterial populations in natural waters. The balance between uptake of leucine and extracellular hydrolytic production of leucine from a peptide model substrate was investigated in a combined fluorescence-radiotracer experiment with [H]leucine as a marker for the leucine pool and l-leucine-4 methyl-7-coumarinylamide (Leu-MCA) as a marker for the pool of dissolved peptide substrates. Results show that at low concentrations of the model substrate the input and uptake processes of leucine are nearly balanced, whereas at high concentrations of the model substrate much more leucine is liberated than taken up. In addition, samples from one polluted and one less polluted station in the Kiel Fjord were investigated for their extracellular enzymatic and uptake properties in an annual cycle. It was found that turnover rates of leucine (T(r), percent per hour) and hydrolysis rates of Leu-MCA (H(r), percent per hour), as well as the quotient T(r)/H(r), reflect the impact of environmental conditions on decomposition processes at both sampling sites. The quotient T(r)/H(r) is interpreted as an indirect measurement of the pool size ratio (polymers/monomers), which may serve as an index of hydrolysis-uptake coupling in bacterial utilization of dissolved protein. Calculated on an annual average basis, turnover rates are ca. nine times higher than hydrolysis rates at the polluted station and ca. five times higher at the less polluted station. From the described model, this would mean that the relative fraction of polymers within the total dissolved organic carbon pool (with regard to the substrate combination dissolved protein-leucine) is about twice that at the polluted than at the less polluted station.