Microbiology and chemistry of acid lakes in Florida: I. Effects of drought and post-drought conditions

Short term changes in acid loading and dissolved organic carbon (DOC) content were studied in relation to water column bacteria of ten acid lakes on the Katharine Ordway Preserve, Florida. Five clear oligotrophic lakes and five dark dystrophic lakes were sampled during and after a drought period in July and September, 1985. Water column bacterial densities, light extinction, chlorophyll a, DOC, pH, dissolved oxygen, nutrients, and other chemical variables were measured. Significant positive correlations existed among DOC, chlorophyll a, pH, and water column bacterial densities during the drought period.There were no significant changes in water column bacterial densities or pH of clear lakes in the post-drought period, despite a 4.6 fold increase in acid loading from rainfall. A 3 fold increase of DOC, a decline in pH, and decreased bacterial densities in dark lakes suggested inhibition of bacteria by DOC and pH. A decrease in the relationship of DOC to bacterial numbers in all lakes was also noted. The correlations among DOC, chlorophyll a, and pH were no longer significant.Using data from both time periods significant polynomial regressions were observed between DOC and bacterial density and DOC and chlorophyll a. Maximum bacterial numbers occurred at 20 mg C 1^–1 of DOC. Above this bacterial numbers decreased also suggesting an inhibitory effect of DOC. Because pH was lower after DOC had increased in the dark lakes, the increase in acid conditions may have enhanced this inhibitory effect. The short term effects of DOC on the dark-lake bacteria greatly exceeded the influence of acid loading on clear-lake bacteria.     

Limitation of lowland riverine bacterioplankton by dissolved organic carbon and inorganic nutrients

Flow regulation in lowland rivers has reduced the amount of allochthonous dissolved organic carbon (DOC) entering main channels through less frequent wetting of benches, flood runners and floodplains. The hypothesis tested was that lowland riverine bacterioplankton are DOC limited when flow events are absent and simulating an increase in assimilable DOC similar to that expected during an environmental flow will lead to heterotrophic dominance. Experiments took place in the Namoi River, a highly regulated lowland river in Australia. Specifically, in situ microcosms were used to examine the responses of bacterioplankton and phytoplankton to various additions of DOC as glucose or leaf leachate, with and without additions of inorganic nutrients. The results indicated that ambient DOC availability limited the bacterioplankton for the three seasons over which we conducted the experiments. When DOC was added alone, dissolved oxygen concentrations decreased primarily because of increased bacterial respiration and bacterioplankton growth generally increased relative to controls. Additions of DOC alone led to a pattern of decreased chlorophyll a concentration relative to controls, except for willow leachate. Additions of inorganic nutrients alone increased chlorophyll a concentrations above controls, indicating limitation of phytoplankton. These findings support our hypothesis. Based on the present results, environmental flows should increase the duration of allochthonously driven heterotrophic dominance, thus shifting regulated lowland rivers to more natural (pre-regulation) conditions for greater periods.     

Virus dynamics in a large epishelf lake (Beaver Lake,Antarctica)

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Relating Phytoplankton Dynamics to Environmental Forcing in the Lower St. Johns River Estuary: A Multivariate Assessment

The St. Johns River, a 300‐mile riverine and estuarine system located along the north‐eastern coastline of Florida, has undergone extensive eutrophication through point and nonpoint source nutrient inputs. Moreover, recent reports of sudden fish kills and a high incidence of fish with lesions suggest the potential for harmful algal blooms. As part of a NOAA‐funded project involving the characterization of water quality parameters in relation to water inflows and nutrient inputs and the development of species‐specific markers/probes for instrumental‐based monitoring efforts, we used primer‐based multivariate analyses to examine the relationship between abiotic variables and both total and phylogenetic‐group chlorophyll a concentrations (derived from photopigments and ChemTax matrix factorization of diagnostic carotenoids) during 2001. Seven sampling sites (identified through principle components ordination of physical/chemical parameters as ranging from oligo‐ to mesohaline) were sampled intensively over 2‐week periods on a seasonal basis. Chlorophyll a concentrations typically ranged from 5 to 35 µg/L with the greatest concentrations occurring at the oligohaline sites. Phytoplankton assemblages were dominated by diatoms, cryptophytes, and cyanobacteria, and together typically comprised up to 90% of the total chlorophyll a. Temporal variability in phytoplankton assemblages followed seasonal trends impacted by meteorological and hydrological forcing. Spatial variability in phylogenetic‐group abundance (illustrated through multidimensional scalar ordination of sample dissimilarity) was dramatic and associated with differences in abiotic variables along the estuarine gradient.     

Seasonal relationships between planktonic microorganisms and dissolved organic material in an alpine stream

The relationships between the abundance and activity of planktonic, heterotrophic microorganisms and the quantity and characteristics of dissolved organic carbon (DOC) in a Rocky Mountain stream were evaluated. Peak values of glucose uptake, 2.1 nmol L⁻¹ hr⁻¹, and glucose concentration, 333 nM, occurred during spring snowmelt when the water temperature was 4.0°C and the DOC concentration was greatest. The turnover time of thein situ glucose pool ranged seasonally from 40–1110 hours, with a mean of 272 hr. Seasonal uptake of³H-glucose, particulate ATP concentrations, and direct counts of microbial biomass were independent of temperature, but were positively correlated with DOC concentrations and negatively correlated with stream discharge. Heterotrophic activity in melted snow was generally low, but patchy. In the summer, planktonic heterotrophic activity and microbial biomass exhibited small-scale diel cycles which did not appear to be related to fluctuations in discharge or DOC, but could be related to the activity of benthic invertebrates. Leaf-packs placed under the snow progressively lost weight and leachable organic material during the winter, indicating that the annual litterfall in the watershed may be one source of the spring flush of DOC. These results indicate that the availability of labile DOC to the stream ecosystem is the primary control on seasonal variation in heterotrophic activity of planktonic microbial populations.     

A comparative analysis of the relationship between phytoplankton standing crops and environmental parameters in four eutrophic prairie reservoirs

An analysis was performed with multivariate statistical methods of the relationship between chlorophyll a concentrations and eighteen physico-chemical parameters measured over a six year period in four eutrophic Nebraska reservoirs. In the reservoirs with relatively clear water early in the growing season, physical factors (Secchi depth, turbidity, temperature) and non-nutrient chemical factors (alkalinity, hardness, C. O. D.) were significantly related to chlorophyll a concentrations, but macronutrients (nitrogen and phosphorus) were not. In the reservoir with persistent abiogenic turbidity, chemical factors including nitrogen and phosphorus were significant but physical factors were not. Six models based upon intercorrelations between measured parameters and chlorophyll a are evaluated for their usefulness in accounting for chlorophyll a variance. The best model accounts for 67–70 percent of the total variation in chlorophyll a in the four reservoirs.     

Effects of Dissolved and Complexed Copper on Heterotrophic Bacterial Production in San Diego Bay

Bacterial abundance and production, free (uncomplexed) copper ion concentration, total dissolved copper concentration, dissolved organic carbon (DOC), total suspended solids (TSS), and chlorophyll a were measured over the course of 1 year in a series of 27 sample “Boxes” established within San Diego Bay. Water was collected through a trace metal-clean system so that each Box’s sample was a composite of all the surface water in that Box. Bacterial production, chlorophyll a, TSS, DOC, and dissolved copper all generally increased from Box 1 at the mouth of the Bay to Box 27 in the South or back Bay. Free copper ion concentration generally decreased from Box 1 to Box 27 presumably due to increasing complexation capacity within natural waters. Based on correlations between TSS, chlorophyll a, bacterial production or DOC and the ratio of dissolved to free Cu ion, both DOC and particulate (bacteria and algae) fractions were potentially responsible for copper complexation, each at different times of the year. CuCl₂ was added to bacterial production assays from 0 to 10 μg L⁻¹ to assess acute copper toxicity to the natural microbial assemblage. Interestingly, copper toxicity appeared to increase with decreases in free copper from the mouth of the Bay to the back Bay. This contrasts the free-ion activity model in which higher complexation capacity should afford greater copper protection. When cell-specific growth rates were calculated, faster growing bacteria (i.e. toward the back Bay) appeared to be more susceptible to free copper toxicity. The protecting effect of natural dissolved organic material (DOM) concentrated by tangential flow ultrafiltration (>1 kDa), illite and kaolinite minerals, and glutathione (a metal chelator excreted by algae under copper stress) was assessed in bacterial production assays. Only DOM concentrate offered any significant protection to bacterial production under increased copper concentrations. Although the potential copper protecting agents were allowed to interact with added copper before natural bacteria were added to production assays, there may be a temporal dose–response relationship that accounts for higher toxicity in short production assays. Regardless, it appears that effective natural complexation of copper in the back portions of San Diego Bay limits exposure of native bacterial assemblages to free copper ion, resulting in higher bacterial production.     

Oligotrophication outweighs effects of global warming in a large,deep, stratified lake ecosystem

Between 1951 and 1979, total phosphorous concentrations in Lake Constance increased from 7 to 87 μg L^?1. Following wastewater treatment, phosphorus levels were brought under control, returning to 7.6 μg L^?1 by spring 2007. The biological and chemical data from 1980 to 2004 were first modelled by seasonal time series analyses and then used to create a general model. Excluding collinear variables allowed the data set to be condensed to six variables that could be fitted into a general linear model that explained ～75% of the observed annual variation in chlorophyll a. A clear seasonal influence was apparent, with chlorophyll a tracking trends in temperature and the progress of spring. A nonseasonal influence was also observed in the interaction of two biological components, the proportion of phytoplankton biomass available to Daphnia (i.e. the percentage of ingestible size <30 μm) and the grazing intensity. In combination, these biotic variables had a negative impact on chlorophyll a levels. In contrast, the concentration of soluble reactive phosphorus (SRP) correlated positively with chlorophyll a. The effect of SRP showed a significant seasonal component, as it was more abundant in spring than at other times of year. In general, the model predicts a negative exponential response of chlorophyll a to further depletion of SRP in Lake Constance, while the temperature trends predicted by current global warming scenarios will result in a moderate increase in productivity. Data from 2005 to 2007 were used to verify the model. The modelled chlorophyll a values were nonbiased and showed a close match to the measured values (r²: 75%). Thus the applicability, reliability, and informative value of the model for pelagic Lake Constance was confirmed. The approach might easily be applied to other waters.     

Abundance and Community Structure of Picoplankton and Protists in the Microbial Food Web of Barguzin Bay,Lake Baikal

We examined the vertical abundance of bacteria, phytoplankton and protists along a transect of six stations from near-shore (Stn. 1) to off-shore (Stn. 6) in Barguzin Bay of Lake Baikal, in the summer of 2002. Chlorophyll concentrations at Stn. 1 were higher (>10μg l⁻¹) than at the other five stations (<3μg l⁻¹). Planktonic and sessile diatoms dominated at Stn. 1, while pico-phytoplankon was dominant at other stations. Densities of heterotrophic bacteria were high in both the epilimnion and the thermocline at all stations. Nanoflagellates were abundant in the epilimnion, and ciliates in the thermocline, but no horizontal trend could be found for these heterotrophs. At Stn. 1, not only filter feeding (Strombidium and Strobilidium) and raptorial (Balanion) ciliates but also predatory ciliates (Prorodon and Spathidiosus) dominated, while at other stations only the filter feeding and raptorial ciliates were dominant. In off-shore stations (Stns. 5 and 6), significant correlations were detected between concentrations of chlorophyll a and density of filter feeding or raptorial ciliates, suggesting tight food linkages between phytoplankton and these ciliates. The concentration of dissolved organic carbon (DOC) was significantly correlated with chlorophyll a concentration, although there was no significant correlation between DOC concentration and bacterial density. We suggest that there is a shift of the dominant food linkage from a herbivorous food chain in near-shore areas to a microbial food web in off-shore areas in Barguzin Bay of Lake Baikal.     

Effects of nutrient enrichment on the release of dissolved organic carbon and nitrogen by the scleractinian coral Montipora digitata

The effects of nutrient enrichment on the release of dissolved organic carbon and nitrogen (DOC and DON, respectively) from the coral Montipora digitata were investigated in the laboratory. Nitrate (NO₃ ⁻) and phosphate (PO₄ ³⁻) were supplied to the aquarium to get the final concentrations of 10 and 0.5 μmol l⁻¹, respectively, and the corals were incubated for 8 days. The release rate of DON per unit coral surface area significantly decreased after the nutrient enrichment, while the release rate of DOC was constant. Because the chlorophyll a (chl a) content of zooxanthellae per unit surface area increased, the release rate of DOC significantly decreased when normalized to unit chl a. These results suggested that the incorporation of NO₃ ⁻ and PO₄ ³⁻ stimulated the synthesis of new cellular components in the coral colonies and consequently, reduced extracellular release of DOC and DON. Actually, significant increase in N and P contents relative to C content was observed in the coral’s tissue after the nutrient enrichment. The present study has concluded that inorganic nutrient enrichment not only affects coral-algal metabolism inside the colony but also affects a microbial community around the coral because the organic matter released from corals functions as energy carrier in the coral reef ecosystem.     

Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll‐a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.     

Community structure of rotifers in two environments of the upper River Paraná floodplain (MS) - Brazil

We evaluate the influence of abiotic and biotic factors on the community structure of rotifers across a regional hydrological cycle in lotic and lentic environments of the upper River Paraná. Depth, transparency, temperature, pH, electrical conductivity, dissolved oxygen, chlorophyll a and densities of rotifers were measured at two stations in Lake Guaraná (littoral and open water regions) and at one station in the River Baía (open water region). Highest densities of rotifers were found at the lake littoral. Canonical correlation analysis related environmental variables with the densities of the most abundant rotifers. The strongest relationship was with chlorophyll a, dissolved oxygen, hydrological level and water temperature. Diversity of rotifers at each station was mainly explained by fluctuations in hydrological level. Results of grouping analysis suggested the formation of groups according to phases of the hydrological cycle.     

Long-term phytoplankton changes in an artificially divided,top-down manipulated humic lake

Lake Große Fuchskuhle (Brandenburg, Germany) is a naturally acidic bog lake that was artificially divided into four basins by large plastic curtains for biomanipulation experiments in 1990. Different numbers of perch were added to each compartment beginning in the spring of 1993. The species composition and abundance of phytoplankton, pH, nutrient concentrations, dissolved organic carbon (DOC) and chlorophyll a content were analyzed at regular intervals during 1991 and 1998. The division of the lake resulted in divergent developments in the physical and chemical environment of the compartments. This study compared the phytoplankton assemblages of the Northeast- (NE) and Southwest- (SW) basins which differed strongly in chemistry during the investigation period. Divergent developments in phytoplankton species composition in both basins can be explained by changes in physical and chemical conditions (bottom-up effects). Increased pH values and DOC concentrations probably favoured mass developments of the dinoflagellate Gymnodinium uberrimum since 1993, while increased nutrients (dissolved inorganic carbon, total nitrogen and especially total phosphorus) as well as further changes in pH and DOC led to the dominance of the raphidophyte Gonyostomum semen in 1998. This bloom was characterized by extreme biomasses of up to 143 mg l^–1 wet weight, corresponding with high chlorophyll a concentrations of up to 413 g l^–1 at the same time. In contrast, no significant relationship between experimental manipulations by piscivorous fish stocking (top-down effects) and phytoplankton biomass were observed.     

Estuary–ocean connectivity: fast physics,slow biology

Estuaries are connected to both land and ocean so their physical, chemical, and biological dynamics are influenced by climate patterns over watersheds and ocean basins. We explored climate‐driven oceanic variability as a source of estuarine variability by comparing monthly time series of temperature and chlorophyll‐a inside San Francisco Bay with those in adjacent shelf waters of the California Current System (CCS) that are strongly responsive to wind‐driven upwelling. Monthly temperature fluctuations inside and outside the Bay were synchronous, but their correlations weakened with distance from the ocean. These results illustrate how variability of coastal water temperature (and associated properties such as nitrate and oxygen) propagates into estuaries through fast water exchanges that dissipate along the estuary. Unexpectedly, there was no correlation between monthly chlorophyll‐a variability inside and outside the Bay. However, at the annual scale Bay chlorophyll‐a was significantly correlated with the Spring Transition Index (STI) that sets biological production supporting fish recruitment in the CCS. Wind forcing of the CCS shifted in the late 1990s when the STI advanced 40 days. This shift was followed, with lags of 1–3 years, by 3‐ to 19‐fold increased abundances of five ocean‐produced demersal fish and crustaceans and 2.5‐fold increase of summer chlorophyll‐a in the Bay. These changes reflect a slow biological process of estuary–ocean connectivity operating through the immigration of fish and crustaceans that prey on bivalves, reduce their grazing pressure, and allow phytoplankton biomass to build. We identified clear signals of climate‐mediated oceanic variability in this estuary and discovered that the response patterns vary with the process of connectivity and the timescale of ocean variability. This result has important implications for managing nutrient inputs to estuaries connected to upwelling systems, and for assessing their responses to changing patterns of upwelling timing and intensity as the planet continues to warm.     

Seasonal and spatial variability of virio-, bacterio-, and picophytoplanktonic abundances in three peri-alpine lakes

Flow cytometry (FCM) was used to assess microbial community abundances and patterns in three natural, large and deep peri-alpine hydrosystems, i.e., lakes Annecy (oligotrophic), Bourget, and Geneva (mesotrophic). Picocyanobacteria, small eukaryotic autotrophs, heterotrophic prokaryotes, and viruses were studied in the 0–50 m surface layers to highlight the impact of both physical and chemical parameters as well as possible biotic interactions on the functioning of microbial communities. Some specificities were recorded according to the trophic status of each ecosystem such as the higher number of viruses and heterotrophic bacteria in mesotrophic environments (i.e., Lakes Geneva and Bourget) or the higher abundance of picocyanobacteria in the oligotrophic Lake Annecy. However, both seasonal (temperature) and spatial (depth) variations were comparatively more important than the trophic status in driving the microbial communities’ abundances in these three lakes, as revealed by principal component analysis (PCA). A strong viral termination of the heterotrophic bacterial blooms could be observed in autumn for each lake, in parallel to the mixing of the upper lit layers. As virus to bacteria ratio (VBR) was indeed very high at this period with values varying between 87 and 114, such important relationships between viruses and bacteria were likely. The magnitudes of seasonal variations in VBR, with the highest values ever reported so far, were largely greater than the magnitude of theoretical variations due to the trophic status, suggesting also a strong seasonality in virioplankton production associated to prokaryotic dynamics. FCM analyses allowed discriminating several viral groups. Virus-Like Particles group 1 (VLP1) and group 2 (VLP2) were always observed and significantly correlated to bacteria for the former and chlorophyll a and picocyanobacteria for the latter, suggesting that most of VLP1 and VLP2 could be bacteriophages and cyanophages, respectively. On the basis of these results, new ways of investigation emerge concerning the study of relationships between specific picoplanktonic groups; and overall these results provide new evidence of the necessity to consider further viruses for a better understanding of lake plankton ecology. Handling editor: Luigi Naselli-Flores     

Relationship between bacterial and primary production in a newly filled reservoir: temporal variability over 2 consecutive years

Seasonal and spatial variations in bacterial abundance, biomass and production in a recently flooded reservoir were followed for 2 consecutive years, in conjunction with phytoplankton biomass (chlorophyll a) and activity (primary production). Between the 2 years of the study, the mean value of primary production remained constant, while those of the chlorophyll a concentration, bacterial abundance (BA), bacterial biomass (BB) and bacterial production (BP) decreased. The observed trends of the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter. Moreover, this factor would explain discrepancies observed between the slope of the model II regression equations established from results of the present study and those of the predictive models from the literature, relating to bacterial and phytoplankton variables. An estimate of the carbon budget indicated that 22 and 5% of the ambient primary production in the Sep Reservoir might be channeled through the microbial loop via BP during the 1st and 2nd year of the study, respectively. We conclude that heterotrophic BP in the Sep Reservoir may, on occasion, represent a significant source of carbon for higher order consumers.     

Inter-annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

1. Pacific salmon (Oncorhynchus spp.) deliver salmon‐derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash‐free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 μg L⁻¹ on average, 41× over background), SRP (by 6 μg L⁻¹, 14×), epilithon chl a (by 35 mg m⁻², 16×), and epilithon AFDM (by 3 g m⁻², 8×). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather‐driven hydrology as well as spawner biomass. 5. Salmon‐derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.     

Difficulty in Discerning Drivers of Lake Ecosystem Metabolism with High-Frequency Data

High-frequency measurements are increasingly available and used to model ecosystem processes. This growing capability provides the opportunity to resolve key drivers of ecosystem processes at a variety of scales. We use a unique series of high-frequency measures of potential predictors to analyze daily variation in rates of gross primary production (GPP), respiration (R), and net ecosystem production (NEP = GPP − R) for two north temperate lakes. Wind speed, temperature, light, precipitation, mixed layer depth, water column stability, chlorophyll a, chromophoric dissolved organic matter (CDOM), and zooplankton biomass were measured at daily or higher-frequency intervals over two summer seasons. We hypothesized that light, chlorophyll a, and zooplankton biomass would be strongly related to variability in GPP. We also hypothesized that chlorophyll a, CDOM, and temperature would be most strongly related to variability in R, whereas NEP would be related to variation in chlorophyll a and CDOM. Consistent with our hypotheses, chlorophyll a was among the most important drivers of GPP, R, and NEP in these systems. However, multiple regression models did not necessarily include the other variables we hypothesized as most important. Despite the large number of potential predictor variables, substantial variance remained unexplained and models were inconsistent between years and between lakes. Drivers of GPP, R, and NEP were difficult to resolve at daily time scales where strong seasonal dynamics were absent. More complex models with greater integration of physical processes are needed to better identify the underlying drivers of short-term variability of ecosystem processes in lakes and other systems.     

Alternation of Factors Affecting Bacterioplankton Abundance in the Danube River Floodplain (Kopački Rit, Croatia)

The purpose of this investigation was to determine the influence of different phases of the hydrological cycle on the abundance of colony-forming units (CFU) of bacteria in the Kopački Rit floodplain and to assess temporal and spatial variations in the proportion of eutrophic and oligotrophic bacteria (r- and K-strategists) as a biological indicator of dissolved organic carbon (DOC) availability. Determination of bacterial abundance was performed with abiotic variables (water temperature, depth, Secchi disc transparency, dissolved oxygen, pH, electrical conductivity, ammonia, nitrates+nitrites, total nitrogen and total phosphorus) and one biotic (chl-a) variable, and dimensionality reduced with principal component analysis (PCA). The component scores were used as independent variables in a multiple regression and the relationship between scores (indirect variables) and bacterial abundance was examined. An elevated bacterial abundance was observed during the limnophase (floodplain isolated from the river) and potamophase (flood). During the limnophase also the highest chl-a concentrations were found and in Sakadaš Lake during September 2003 significantly higher numbers of eutrophic than oligotrophic bacteria emerged (p = 0.026). During potamophase the opposite state was established. Elevated bacterial abundance occurred in parallel with minimal chl-a concentrations and during July 2004 in the Čonakut Channel a significantly higher number of oligotrophic than eutrophic bacteria emerged (p < 0.001). Differentiation in the number of eutrophic and oligotrophic bacteria suggests the existence of a higher concentration of labile DOC during September 2003 than during July 2004. Multiple regression analysis explains 13.6% of the variation in abundance of eutrophic bacteria. The regression model for oligotrophic bacteria is not significant. The results suggest two different mechanisms control bacterioplankton numbers to some extent in the Kopački Rit floodplain. In addition, two sources of DOC differentiating the quality of organic matter predominate under totally different hydrological regimes. Under these conditions, eutrophic bacteria may be partially bottom-up controlled, whereas it is not clear how the abundance of oligotrophs is controlled.