Helena Guasch

Biofilm Structure and Function and Possible Implications for Riverine DOC Dynamics

Biofilms are major sites of carbon cycling in streams and rivers. Here we elucidate the relationship between biofilm structure and function and river DOC dynamics. Metabolism (extracellular enzymatic activity) and structure (algae, bacteria, C/N content) of light-grown (in an open channel) and dark-grown (in a dark pipe) biofilms were studied over a year, and variations in dissolved organic carbon (DOC) and biodegradable DOC (BDOC) were also recorded. A laboratory experiment on 14C-glucose uptake and DOC dynamics was also performed by incubating natural biofilms in microcosms. On the basis of our field (annual DOC budget) and laboratory results, we conclude that light-grown biofilm is, on annual average, a net DOC consumer. This biofilm showed a high monthly variability in DOC uptake/release rates, but, on average, the annual uptake rate was greater than that of the dark-grown biofilm. The higher algal biomass and greater structure of the light-grown biofilm may enhance the development of the bacterial community (bacterial biomass and activity) and microbial heterotrophic activity. In addition, the light-grown biofilm may promote abiotic adsorption because of the development of a polysaccharide matrix. In contrast, the dark-grown biofilm is highly dependent on the amount and quality of organic matter that enters the system and is more efficient in the uptake of labile molecules (higher 14C-glucose uptake rate per mgC). The positive relationships between the extracellular enzymatic activity of biofilm and DOC and BDOC content in flowing water indicate that biofilm metabolism contributes to DOC dynamics in fluvial systems. Our results show that short-term fluvial DOC dynamics is mainly due to the use and recycling of the more labile molecules. At the river ecosystem level, the potential surface area for biofilm formation and the quantity and quality of available organic carbon might determine the effects of biofilm function on DOC dynamics.

Short-term toxicity of zinc to microbenthic algae and bacteria in a metal polluted stream

The response to different levels of metal pollution by assemblages of microbenthic algae and bacteria was investigated in a lowland stream. Colonized glass-discs and samples of natural assemblages on coarse sand were used to test zinc tolerance. Tolerance was characterized by measuring inhibition of 14C-incorporation in microalgae and inhibition of 3H-thymidine incorporation in bacteria. Bacterial assemblages on both glass and sand from a high zinc and cadmium site were resistant to zinc when compared to populations from a cleaner upstream site. Algae from the strongly polluted site were only slightly affected by the highest test concentrations of zinc, in contrast to other algal communities. A change towards metal tolerant bacteria and algae was indicated and seems to occur despite the strong binding of metals by precipitates of iron hydroxide and organic deposits in the biofilm. The observations were compared with published data on short-term and long term zinc toxicity in algae and bacteria. The gradual changes in micro- and macrobenthos under high metal loads is discussed.

LIGHT HISTORY INFLUENCES THE SENSITIVITY TO ATRAZINE IN PERIPHYTIC ALGAE

This study examines the relationship between light history (i.e. light conditions during colonization) and sensitivity to atrazine for periphyton communities grown on etched glass substrates (colonized for 2–3 weeks) at seven stream and river sites differing in water chemistry and light regime. For each site, taxonomic composition of the community (by microscopic examination and cell counts), pigment composition, and photosynthetic parameters were measured and related to sensitivity to the herbicide. Photosynthetic parameters were estimated from photosynthesis– irradiance (P‐I) curves; the responses of the algae to atrazine were analyzed using short‐term concentration–response tests. Periphyton communities colonizing shaded sites were diatom dominated, had higher percentages of accessory pigments, and were more susceptible to photoinhibition at high light intensities compared to periphyton communities colonizing open sites; communities in the more open sites were dominated by green algae or cyanobacteria. Periphyton communities colonizing open sites were more sensitive to atrazine; the concentration of atrazine needed to inhibit photosynthesis by 50% in the short‐term tests (EC50 values) was correlated with light conditions during colonization. The interaction between treatment factors (light conditions during colonization, irradiance of incubation, and atrazine concentration) was analyzed by three‐way ANOVA. The two‐way interactions were significant, indicating that the response of the periphyton to irradiance during incubation and to atrazine concentration both depend on light history. However, the magnitude of the interaction between light history and atrazine toxicity was attenuated by changes in irradiance during incubation (three‐way ANOVA). This investigation presents evidence that the apparent toxicity of atrazine to lotic periphyton in natural communities is greater for algae that are adapted to high‐light conditions than for those that are adapted to low‐light conditions.

Community composition and sensitivity of periphyton to atrazine in flowing waters : The role of environmental factors

The relationship between environmental variables, community composition and the sensitivity of periphyton on artificial substrata to the herbicide atrazine (EC50 values obtained by concentration-effect curves of photosynthesis to atrazine) was studied for 20 stream and river sites on a latitudinal across Europe (Sweden, The Netherlands, Spain). Sensitivity to atrazine was higher in Swedish than in the Spanish or Dutch sites. Direct gradient analyses were used to relate diatom taxa and algal groups with environmental variables. A first redundancy analysis (RDA) based on diatom taxa showed a pollution gradient (atrazine and nutrient concentration) associated to diatom taxa that are indicators of different degrees of pollution. A second RDA based on algal groups showed that diatom-dominated communities corresponded both to sites at higher altitudes and less industrialized areas and to sites with higher atrazine concentration; Cyanobacteria were the most common in industrial areas, whereas Chlorophyceae dominated in sites with high water temperature and alkalinity. Linear regression analyses were applied to find the relationship between the ordination axes obtained and the EC50 values. First axes of both RDA showed significant or marginally significant relationship with atrazine sensitivity. Regression analyses for the Spanish sites indicated that the sensitivity to atrazine was related with light conditions (EC50 was positively correlated with light) and the percentage of different algal groups (EC50 was positively correlated with the percentage of diatoms and negatively correlated with the percentage of green algae). The results indicating that differences in sensitivity are related to environmental variables such as light, nutrients or atrazine concentration, permitted us to identify biological indicators of sensitivity to atrazine in lotic systems: Bacillariophyceae-dominated periphyton communities were more tolerant than Chlorophyceae and Chrysophyceae-dominated communities. In addition, diatom taxa found to be tolerant to atrazine in this study have been considered in the literature to be tolerant to organic pollution.

Changes in atrazine toxicity throughout succession of stream periphyton communities

A study was made to describe atrazine toxicity and its changes throughout succession of periphyton communities of an undisturbed Mediterranean stream. Toxicity was assessed by short-term physiological tests (concentration-effect curves of photosynthesis to atrazine) in the laboratory using artificial substrates colonized in one stream site during winter, and two stream sites (one open and the other shaded) during summer. In the winter experiment, when environmental conditions were relatively steady and chlorophyll content was low, toxicity increased according to the increases in cell density and chlorophyll content throughout colonization. EC50 (concentration inhibiting photosynthesis by 50%) was above 0.8 µM atrazine until day 16 and below 0.4 µM atrazine after three weeks. In the summer experiment, under more variable environmental conditions, the differences between the EC50 at the beginning and the end of the colonization experiments were not significant (one factor ANOVA) at the two sites. EC50 was on average 0.89 µM atrazine in the shaded site and 0.29 µM atrazine in the open site. A significant negative correlation between irradiance and EC50 was observed all the experiments were considered together (r = 0.464, n = 20, p<0.05), suggesting that light history may have an important role in the response to atrazine. This investigation reveals that the response of stream periphyton to atrazine is likely to be influenced by colonization time and the corresponding changes in algal density and community composition as well as by environmental conditions (e.g. light regime) throughout succession.

EFFECT OF COPPER ON ALGAL COMMUNITIES FROM OLIGOTROPHIC CALCAREOUS STREAMS1

Two sets of experiments were done to quantify the effects of chronic copper exposure on natural peri‐ phyton in a nonpolluted calcareous river. The results of short‐term (up to 6 h exposure) experiments corroborated the significance of pH on copper toxicity. Copper toxicity increased when pH was reduced from 8.6 to 7.7, and this was related to the effect of pH on copper speciation (free copper concentration increased from 0.2% to 2.3% of total copper). Longer term experiments demonstrated that periphyton communities exposed to copper under pH variation (8.2–8.6) were already affected at 10 μg·L−1 (20–80 ng·L−1 Cu2+) after 12 days of exposure. Copper exposure caused stronger effects on structural (algal biomass and community structure) than on functional (photosynthetic efficiency) parameters of peri‐ phyton. Changes in community composition included the enhancement of some taxa (Gomphonema gracile), the inhibition of others (Fragilaria capucina and Phormidium sp.), and the appearance of filament malformations (Mougeotia sp.). The results of our study demonstrated that several weeks of exposure to copper (10–20 μg·L−1) were sufficient to cause chronic changes in the periphyton of oligotrophic calcareous rivers. This degree of copper pollution can be commonly found in the Mediterranean region as a result of agricultural practices and farming activities.

Effects of atrazine on periphyton under grazing pressure.

An experiment was carried out using indoor experimental channels to assess the long-term effect (18 days) of herbivores (Physella acuta, Gastropoda) on periphyton communities exposed to low levels of atrazine (14 microg l(-1)). We hypothesized that herbivorism modifies the response of periphyton to atrazine. Carbon incorporation, chlorophyll-a content, biovolume and algal taxonomic composition in the channels that contained atrazine were not significantly different from the control channels (not receiving atrazine). In channels with grazers and atrazine, there was a significant reduction of carbon incorporation and algal density. In this treatment, physiognomic forms and algal composition were significantly different from the others. The biomass of grazers (measured as change in dry mass) was not significantly affected by the addition of atrazine. Grazers maintained low levels of periphyton biomass, enhancing algal cell exposition to toxicant and inhibiting any adaptation of the algae to the toxic exposure. The increase in atrazine toxicity with grazing not only affected the metabolism, but also the structure of the algal community, which suggests that effects were not transient but permanent.

SEASONAL VARIATIONS IN PHOTOSYNTHESIS-IRRADIANCE RESPONSES BY BIOFILMS IN MEDITERRANEAN STREAMS

The relationships between primary production and irradiance were analyzed over an annual cycle in natural biofilms of two undisturbed streams: La Solana (LS), an open calcareous stream, and Riera Major (RM), a shaded siliceous stream. In LS, low photosynthetic efficiency (αchl and αarea) and high values of both the light saturation parameter (Ik) and the carotenoid/chlorophyll ratio indicated adaptation to high light regimes. On the other hand, higher photosynthetic efficiency and lower Ik as well as photoinhibition at high irradiance found in the biofilms of RM indicated shade adaptation. However, the lack of correlation between light availability in nature and the photosynthetic parameters studied in the laboratory suggested that light was not the most important factor in determining seasonal changes in the photosynthetic behavior in this stream. Both in the open and shaded streams, algal patches collected simultaneously exhibited different photosynthesis‐irradiance (P‐I) curues, showing that community composition influenced the P‐I parameters. In the open stream (LS), however, significant negative correlations between αarea and chlorophyll a and between Pmaxchland chlorophyll a (r=‐0.994, P < 0.001, and r =‐0.929, P < 0.05, respectively) suggested that photosynthesis was affected by self‐shading. Due to the absence of photoinhibition in the biofilms of LS, high photosynthetic rates were maintained at the ambient high light environment, thus compensating for low photosynthesis at low irradiance. In the shaded stream (RM), because photosynthesis was saturated at low irradiances, primary production was relatively high given the low light conditions.

Diurnal variation in dissolved oxygen and carbon dioxide in two low-order streams

Abstract This investigation aimed to establish the contribution of biological activity, physical and chemical processes, to the diurnal variation in dissolved oxygen and carbon dioxide in undisturbed streams. Changes in both gases were monitored during four surveys of 24xa0h duration in 1993 in two second-order Mediterranean streams: Riera Major, a siliceous shaded stream and La Solana, a calcareous open stream. In the latter, diurnal changes in carbon dioxide and dissolved oxygen showed similar, clear daily pattern when water discharge was low and algal activity high, indicating that although reaeration was high, changes in the two gases were greatly influenced by stream metabolism. In addition, diurnal changes in Ca 2+ and DIC (dissolved inorganic carbon) also showed a clear daily pattern, indicating that the precipitation of calcium carbonate was directly or indirectly affected by biotic processes. On the other hand, no relation between the two gases was observed in Riera Major, where turbulence was higher. In this stream, the concentration of dissolved oxygen was greatly affected by reaeration, but changes in carbon dioxide showed a clear daily pattern. The lack of agreement between the diurnal variation of the two gases observed in this stream may be explained by the difference in solubility, which led to marked differences in the exchange coefficient rate when turbulence is high.

The effect of copper exposure on a simple aquatic food chain

The effect of copper (44 microg l(-1)) on a simple food chain was studied using indoor experimental channels to identify the changes in periphyton community (metabolism, chlorophyll a content, abundance, composition and lipid and protein content) and in herbivore (Stagnicola vulnerata) growth rate and reproduction. The algal community was sensitive to copper at the beginning but differences between treatments were not significant during the experiment. However, copper affected growth rate, reproduction and embryo hatching on snails. These results indicate that the effects on snails are more sensitive endpoints in assessing sublethal copper toxicity than effects on periphyton.