GROWTH AND PHOSPHORUS UPTAKE BY THE TOXIC DINOFLAGELLATE ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN RESPONSE TO PHOSPHATE LIMITATION1

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO₄^3?) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO₄^3?‐limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P‐limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of ³³PO₄^3? uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (Q_P and Q_C). Shifts in trends of Q_P/Q_C and Q_P per cell (Q_P·cell?1) along the DR gradient allowed the definition of successive P‐stress thresholds for A. catenella cells. The maximal uptake rate of ³³PO₄^3? increased strongly with the decrease in DR and the decrease in Q_P/Q_C, displaying physiological acclimations to PO₄^3? limitation. Concerning maximal APA per cell, the observation of an all‐or‐nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO₄^3? limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO₄^3? threshold between 0.4 and 1 μM. As lower PO₄^3? concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.     

Biological response to lake remediation by phosphate stripping: control of Cladophora

• 1 The North and South Basins of Windermere, Cumbria, have experienced a large increase in concentrations of nutrients, particularly phosphate, since 1945 when detailed measurements began. Over‐winter concentrations have increased from 1 to 3 mg PO₄‐P m^‐3 in the 1940s, up to 30 mg PO₄‐P m^‐3 in the South Basin of Windermere in the early 1990s where nutrient enrichment has been most marked. A visible manifestation of this ‘eutrophication’ in recent years has been the production of a large biomass by the green filamentous macroalga, Cladophora.
• 2 Since April 1992, tertiary chemical stripping of phosphate at the two sewage treatment plants on Windermere has reduced direct sources of phosphate to both basins. In the South Basin, over‐winter concentrations of phosphate have fallen to values similar to those in the early 1970s.
• 3 The biomass of Cladophora has declined markedly in response to the reduced phosphate availability. Significant relationships were found between the annual maximum biomass of Cladophora and two measures of phosphate availability: the over‐winter concentration and, more strongly, the day of year when the concentration fell below 1 mg m^‐3.
• 4 The annual biomass maxima of Cladophora since 1945, estimated from the regressions, showed a gradual increased potential for biomass production after 1965 as phosphate concentrations increased, followed by a striking and rapid biological response to lake remediation by phosphate stripping.

     

A metagenome for lacustrine Cladophora (Cladophorales) reveals remarkable diversity of eukaryotic epibionts and genes relevant to materials cycling

Periphyton dominated by the cellulose‐rich filamentous green alga Cladophora forms conspicuous growths along rocky marine and freshwater shorelines worldwide, providing habitat for diverse epibionts. Bacterial epibionts have been inferred to display diverse functions of biogeochemical significance: N‐fixation and other redox reactions, phosphorus accumulation, and organic degradation. Here, we report taxonomic diversity of eukaryotic and prokaryotic epibionts and diversity of genes associated with materials cycling in a Cladophora metagenome sampled from Lake Mendota, Dane Co., WI, USA, during the growing season of 2012. A total of 1,060 distinct 16S, 173 18S, and 351 28S rRNA operational taxonomic units, from which >220 genera or species of bacteria (~60), protists (~80), fungi (6), and microscopic metazoa (~80), were distinguished with the use of reference databases. We inferred the presence of several algal taxa generally associated with marine systems and detected Jaoa, a freshwater periphytic ulvophyte previously thought endemic to China. We identified six distinct nifH gene sequences marking nitrogen fixation, >25 bacterial and eukaryotic cellulases relevant to sedimentary C‐cycling and technological applications, and genes encoding enzymes in aerobic and anaerobic pathways for vitamin B₁₂ biosynthesis. These results emphasize the importance of Cladophora in providing habitat for microscopic metazoa, fungi, protists, and bacteria that are often inconspicuous, yet play important roles in ecosystem biogeochemistry.     

Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore

The partial pressure of carbon dioxide (pCO₂) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO₂ extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO₂ with low light and phosphorus supplies. The experiment showed that increased pCO₂ stimulated algal growth but reduced algal P : C ratio. When feeding on algae grown under high pCO₂, herbivore growth decreased regardless of algal abundance. Thus, high CO₂‐raised algae were poor food for Daphnia. Short‐term experimental supplementation of PO₄ raised the P content of the high CO₂‐raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO₂ conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural processes as well as anthropogenic perturbations resulting in increased pCO₂ enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.     

Phosphorus and nitrogen relationships of Cladophora glomerata in two lake basins of different trophic status

• 1 Cladophora has increased in abundance and cover in the South Basin of Windermere, English Lake District, in recent years. During the growing seasons of 1992 and 1993, the maximum biomass of Cladophora in the South Basin, at nearly 200 g dry weight m^-2, was nearly six times greater and the percentage cover, at 95%, was nearly ten times greater than in the North Basin.
• 2 Nutrient analysis showed that Cladophora from the South Basin had significantly higher average contents of N than that from the North Basin. The suggested rate-limiting tissue N content, 1.3% of dry weight, was never reached in the South Basin but was reached in 20% of samples from the North Basin. In contrast, the average P content was not significantly different, at about 0.24% in both basins. The suggested rate-limiting P content, 0.16% of dry weight, was reached in 31% of the samples from the South Basin and 37% from the North Basin.
• 3 On average, the standing stock of P in the South Basin was 0.12 g m^-2, twelvefold that in the North Basin at 0.01 g m^-2 and the standing stock of N in the South Basin was 1.81 g m^-2, elevenfold that in the North Basin at 0.16 g m^-2. Maximum standing stocks of P were 0.38 g m^-2 in the South and 0.05 g m^-2 in the North Basin. Maximum standing stocks of N were 5.02 g m^-2 in the South and 0.60 g m^-2 in the North Basin.
• 4 The maximal rate of PO₄–P uptake ranged between 298 and 2949 μg P g^-1 DW h^-1 and the specific affinity varied between 2.9 and 24.3 μg P g^-1 DW h^-1 (mg PO₄–P m^-3)^-1. The higher values of both characteristics are within the range which is believed to indicate severe P limitation.
• 5 Using the nutrient uptake characteristics, rates of uptake at pelagic and littoral concentrations of PO₄–P were calculated to be between 0.1 and 1.7% of the maximal rates apart from one site where rates were up to 5.6% of V_maxi and 40% of V_maxi based on pelagic and littoral concentrations of PO₄–P, respectively. Despite these generally low rates of uptake, calculations suggest they were just sufficient to account for the measured rate of increase of P standing stock, assuming no biomass loss. Similar calculations suggested that pelagic concentrations of NO₃–N could support between 21 and 30% of maximal rates and these were between four and six times (North Basin) and twenty-five times (South Basin) the observed rates of increase in N standing stock. Littoral concentrations of NO₃–N were similar and so would have allowed similar rates.
• 6 Evidence from nutrient tissue contents, standing stocks and rates of uptake suggests that the growth of Cladophora, in both basins of Windermere, is controlled primarily by the availability of P.

     

The collapse of benthic macroalgal blooms in response to self‐shading

1. Our goal was to use physiological indicators [photosynthesis–irradiance (P–I) response, nutrient status], population level feedbacks (self‐shading) and ambient environmental conditions (dissolved nutrients, light, temperature) to improve our understanding of the seasonal and spatial population dynamics of Cladophora. 2. Cladophora grew in three distinct phases, rapid growth early in the season (May–July), a mid‐season population collapse (July–August) and autumn re‐growth. Across all sites and dates, mean net maximal photosynthesis [P_M (NET)] was 6.9 ± 3.9 mg O₂ g DM^?1 h^?1, and α was 0.055 ± 0.025 mg O₂ g DM^?1 μm photons^?1 m^?2. Mean values for critical irradiance (I_CR) and the half‐saturation light intensity (I_K), were 42.9 ± 32.1 and 189.3 ± 123.8 μm photons^?1 m^?2 s^?1 respectively. 3. At most sites growth was phosphorus‐limited. Values of α were significantly higher at a site influenced by a nutrient enriched river plume, where algal growth was phosphorus‐sufficient. 4. Photoinhibition was not apparent in any of our P–I experiments. Even if photoinhibition had been apparent during in vitro P–I experiments, population level photosynthetic rates in the field would be little affected because intense self‐shading restricts inhibiting irradiances to the upper few mm–cm of the algal canopy. 5. Our physiological (P–I response) experiments contradicted previous assertions that high ambient temperatures, or nutrient deficiency, were primary causes of mid‐summer sloughing. In our study, sloughing occurred simultaneously at nutrient enriched and nutrient deficient sites, at temperatures well below critical values found during in vitro experiments, and our indicator of physiological condition (P–I response) remained unchanged leading up to, or immediately after, the sloughing event. 6. Self‐shading can reduce the convexity of the P–I response within in vitro incubations, even when the amount of algal material is low. Our experiments used 0.08 g DM of algal material that formed clumps c. 1 cm thick. Under these conditions, we estimated negligible (<1%) effects on P_M, a 12% reduction in apparent values of α, and 14% and 17% increases in values of the α‐dependent terms I_CR and I_K, respectively. 7. Our results are consistent with the hypothesis that a population‐level negative feedback (self‐shading) is responsible for sloughing in dense macroalgal beds. Sloughing was probably inevitable once macroalgal bed density and thickness surpassed a critical threshold. Cells towards the base of the bed received insufficient light to maintain metabolic balance, began to decay and weaken, and became increasingly susceptible to physical detachment from shear stress.     

Changes in algal assemblages along observed and experimental phosphorus gradients in a subtropical wetland, U.S.A.

• 1 We wanted to determine if changes in algae in the Everglades were due to increased phosphorus (P) loading. Epiphytic algae, water chemistry, and surface sediment chemistry were characterized from 32 sloughs along a P gradient in the Everglades and changes in the algal assemblages along the P gradient were compared with those along an experimental P gradient of in situ mesocosms. The sloughs are the wettest open water habitats characterized by floating and submerged aquatic plants in the Everglades.
• 2 Algal species composition was much more sensitive to P concentration than algal biomass. The diatom species variance among sloughs, captured by 1st ordination axis, was more highly correlated with total P (TP) in surface sediments (r = ‐ 0.79), than soluble reactive P (SRP) (r = ‐ 0.08) and TP (r = ‐ 0.48) in the water column. Algal biomass (µg chl a cm^‐2) was not significantly correlated with P (SRP: r = 0.22, TP: r = 0.19, sediment TP: r = 0.07) along the P gradient in the Everglades. Cluster analysis classified diatom species assemblages in 32 sloughs into three groups (TWIN I, II, III), which corresponded to three zones along the P gradient. Dominant diatom species shifted from Mastogloia smithii (40.3%), Cymbella scotica (22.3%), and Fragilaria synegrotesca (21.8%) in TWIN I to Nitzschia amphibia (22.4%) and C. microcephala (12.4%) in TWIN III. TP in surface sediments and TP in epiphyton assemblages increased 4‐ and 5‐fold from TWIN I to TWIN III, respectively.
• 3 Patterns in epiphytic assemblages along the experimental P gradient in the mesocosms were very similar to those along the Everglades P gradient. Shannon diversity indices and species richness significantly increased along both P gradients. TN : TP ratio in epiphyton assemblages significantly decreased as sediment TP increased along both P gradient. Ordination analysis showed that diatom assemblages in the impacted zone (TWIN III) were ordinated closely to the assemblages from the highest P treatments in the mesocosms. The assemblages from the less impacted zone (TWIN I) were ordinated closely to the assemblages from controls in the mesocosms.
• 4 Concurrence between results of our survey and experiments suggest that changes in epiphytic assemblages along the P gradient in the Everglades are caused by increases in P concentrations.

     

Seston quality drives feeding,stoichiometry and excretion of zebra mussels

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Vertical distribution of biological and geochemical phosphorus subcycles in two southern Appalachian forest soils

We measured Al, Fe, and P fractions by horizon in two southern Appalachian forest soil profiles, and compared solution PO₄ ^–1 removal in chloroform-sterilized and non-sterilized soils, to determine whether biological and geochemical P subcycles were vertically stratified in these soils. Because organic matter can inhibit Al and Fe oxide crystallization, we hypothesized that concentrations of non-crystalline (oxalate-extractable) Al (Al₀) and Fe (Fe₀), and concomitantly P sorption, would be greatest in near-surface mineral (A) horizons of these soils.Al₀ and Fe₀ reached maximum concentrations in forest floor and near-surface mineral horizons, declined significantly with depth in the mineral soil, and were highly correlated with P sorption capacity. Small pools of readily acid-soluble (AF-extractable) and readily-desorbable P suggested that PO₄ ^3– was tightly bound to Al and Fe hydroxide surfaces. P sorption in CHCl₃-sterilized mineral soils did not differ significantly from P sorption in non-sterilized soils, but CHCl₃ sterilization reduced P sorption 40–80% in the forest floor. CHCl₃ labile (microbial) P also reached maximum concentrations in forest floor and near-surface mineral horizons, comprising 31–35% of forest floor organic P. Combined with previous estimates of plant root distributions, data suggest that biological and geochemical P subcycles are not distinctly vertically stratified in these soils. Plant roots, soil microorganisms, and P sorbing minerals all reach maximum relative concentrations in near-surface mineral horizons, where they are likely to compete strongly for PO₄ ^3– available in solution.     

Kinetics of alkaline phosphatase activity and phosphorus availability for phytoplankton and bacterioplankton in lake plu\see (North German Eutrophic Lake)

Annual studies of kinetics of alkaline phosphatase (APA) activity and phosphorus availability for microplankton in the photic zone of an eutrophic lake are reported. The total APA activity of microplankton varied strongly. V_max was highest during summer P depletion, and in autumn and winter total APA activity was low. The total APA specific activity of the microplankton was also highest (average 3.55 pmole PO₄ ^3– ng ATP^–1 min^–1) when ambient orthophosphate concentrations were very low. Both V_max and specific APA activity were not dependent on the biomass of microplankton; they were strongly affected by P available for microplankton. A differential filtration technique was used for separation of microplankton into two size classes, i.e., algal, larger than 3m, and bacterial fraction with size 0.2–3.0m. The algal size fraction had lower specific APA activity (average 1.224 pmole PO₄ ^3– ng ATP^–1 min^–1) and higher K_M values (38.8mole × liter^–1) than microorganisms which were smaller than 3m (2.011 pmole PO₄ ^3– ng ATP^–1 min^–1 and 25.4mole liter^–1, respectively). The K_M values of free, dissolved APA (36.8mole liter^–1) indicated that free APA was probably released by algae. Phytoplankton were major APA activity producers in the photic zone of the lake from March to November, and their activity constituted, on the average, 48.6% of the total APA activity in the water. Bacteria were the dominant APA activity producers in winter (41.3–44.9%); however, during other periods they contributed significantly (average 21.7%) to total APA activity. When surplus constituted less than 10% of particulate P in seston, phytoplankton produced high specific APA activity, and when surplus P was higher than 15%, the specific APA activity of phytoplankton size fraction rapidly decreased. APA of the bacterial size fraction of the seston was not affected by P concentrations. Orthophosphate was a competitive inhibitor of APA produced by microorganisms of the size fraction larger than 3.0m, and increasing concentrations of inorganic phosphate caused an increase in K_M values. The hypothetical metabolic-coupling between phytoplankton and bacterioplankton in the phosphorus cycle in conjunction with carbon metabolism in the lake is discussed.     

Microbial enzyme activity,nutrient uptake and nutrient limitation in forested streams

1. We measured NH₄⁺ and PO₄^?3 uptake length (S_w), uptake velocity (V_f), uptake rate (U), biofilm respiration and enzyme activity and channel geomorphology in streams draining forested catchments in the northwestern (Northern California Coast Range and Cascade Mountains) and southeastern (Appalachian and Ouachita mountains) regions of the United States. Our goal was to use measures of biofilm enzyme activity and nutrient uptake to assess nutrient limitation in forested streams across broad regional scales. 2. Geomorphological attributes, biofilm enzyme activity and NH₄⁺ uptake were significantly different among streams in the four study units. There was no study unit effect on PO₄^?3 uptake. The proportion of the stream channel in pools, % woody debris, % canopy closure, median substrate size (d₅₀), stream width (w), stream velocity (v), discharge (Q), dispersion coefficient (D) and transient storage (A_s/A) were correlated with biofilm enzyme activity and nutrient uptake in some study units. 3. Canonical correlation analyses across study units revealed significant correlations of NH₄‐V_f and PO₄‐V_f with geomorphological attributes (w, d₅₀, D, % woody debris, channel slope and % pools) and biofilm phosphatase activity. 4. The results did not support our expectation that carbon processing rates by biofilm microbial assemblages would be governed by stream nutrient availability or that resulting biofilm enzyme activity would be an indicator of nutrient uptake. However, the relative abundances of peptidases, phosphatase and glycosidases did yield insight into potential N‐, P‐ and C‐limitation of stream biofilm assemblages, and our use of biofilm enzyme activity represents a novel application for understanding nutrient limitations in forested streams. 5. Regressions of V_f and U against ambient NH₄⁺ and PO₄^?3 indicated that none of our study streams was either NH₄⁺ or PO₄^?3 saturated. The Appalachian, Ouachita and Coastal streams showed evidence of NH₄⁺ limitation; the Ouachita and Coastal streams were PO₄^?3 limited. As a correlate of nutrient limitation and saturation in streams, ratios of total aminopeptidase and phosphatase activities and the ratio of NH₄‐U to PO₄‐U indicate these forested streams are predominantly N‐limited, with only the streams draining Ouachita and Coastal catchments demonstrating appreciable levels of P‐limitation. 6. Our results comparing the stoichiometry of microbial enzyme activity with nutrient uptake ratios and with the molar ratios N and P in stream waters suggest that biological limitations are not strictly the result of stream chemistry and that the assessments of nutrient limitations in stream ecosystems should not be based on chemistry alone. 7. Our present study, along with previous work in streams, rivers and wetlands, suggests that microbial enzyme activities, especially the ratios of total peptidases to phosphatase, are useful indicators of nutrient limitations in aquatic ecosystems.     

UPTAKE AND RELEASE OF METHYLMERCURY-203 BY CLADOPHORA GLOMERATA1

Cladophora glomerata was exposed to CH₃²⁰³HgCl at concentrations of 10, 50, and 100 μg/liter of water. Formalin-killed alga was exposed to a concentration of 50 μg CH₃²⁰³HgCl/liter. Uptake was monitored at 2 and 12 hr, and days 1, 2, 4, 8, and 16. At the end of this period, the Cladophora was placed in uncontaminated water, and release of methylmercury was monitored at 1, 2, 4, 8, and 16 days. Sorption occurred at all concentrations, and the live algal material accumulated more methylmercury than the dead alga, at equal exposure concentrations. Accumulation of methylmercury by the live Cladophora peaked on or near the second day for all exposure concentrations, suggesting that the uptake rate was independent of methylmercury concentration in the water. Uptake was greatest at the 50 μg/liter exposure. Desorption was nominal during the 16-day release period. The mechanisms of methylmercury uptake by Cladophora are discussed.     

Retention of Inorganic Nitrogen by Epiphytic Bryophytes in a Tropical Montane Forest1

We developed and evaluated a model of the canopy of a tropical montane forest at Monteverde, Costa Rica, to estimate inorganic nitrogen (N) retention by epiphytes from atmospheric deposition. We first estimated net retention of inorganic N by samples of epiphytic bryophytes, epiphyte assemblages, vascular epiphyte foliage, and host tree foliage that we exposed to cloud water and precipitation solutions. Results were then scaled up to the ecosystem level using a multilayered model of the canopy derived from measurements of forest structure and epiphyte mass. The model was driven with hourly meteorological and event‐based atmospheric deposition data, and model predictions were evaluated against measurements of throughfall collected at the site. Model predictions were similar to field measurements for both event‐based and annual hydrologic and inorganic N fluxes in throughfall. Simulation of individual events indicated that epiphytic bryophytes and epiphyte assemblages retained 33–67 percent of the inorganic N deposited in cloud water and precipitation. On an annual basis, the model predicted that epiphytic components retained 3.4 kg N ha/yr, equivalent to 50 percent of the inorganic N in atmospheric deposition (6.8 kg N ha/yr). Our results indicate that epiphytic bryophytes play a major role in N retention and cycling in this canopy by transforming highly mobile inorganic N (ca. 50% of atmospheric deposition is NO^?₃) to less mobile (exchangeable NH⁺₄) and recalcitrant forms in biomass and remaining litter and humus.     

Factors influencing phosphate exchange across the sediment-water interface of eutrophic reservoirs

The results of a survey of the sediment chemistry of 7 East Anglian reservoirs are presented as part of a regional study on the assessment and control of eutrophication. The influence of water quality (dissolved oxygen, pH, temperature) on phosphate (PO₄) adsorption by sediment from hypertrophic Ardleigh Reservoir is also examined. Extractable phosphate-P (extr.-P) varied between 92 and 383 mg kg^–1 dry matter. Extractable P varied between 5.3 and 16.6% of the total phosphate-P (Tot. P) content and increased with the concentration of dissolved reactive phosphate-P (DRP) in the overlying water column. Organically complexed iron (organic Fe) was the determinand which correlated most closely with phosphate adsorption capacity, PAC (r = 0.8). Organic Fe was also related inversely to Extr. P. The rate and extent of PO₄ adsorption by Ardleigh Reservoir sediment increased with the initial concentration of DRP and adsorption equilibria were reached after 24 h. The equilibrium DRP concentration, [DRP], was 0.7 mg P 1^–1 under aerobic conditions indicative of a high potential for PO₄ exchange. The rate and extent of PO₄ adsorption was greater at 7 °C than at 22 °C PO₄ adsorption increased markedly with dissolved oxygen status. Ardleigh sediment exhibited a marked buffering capacity to a change in pH; however, PO₄ adsorption was greatest at an equilibrium pH of 5.6 and decreased progressively either side of this pH value.Options for the artificial control of sediment PO₄ release are discussed in relation to the seasonal variation in sediment PO₄ exchange observed for Ardleigh Reservoir.     

Disruption of planktonic phosphorus cycling by ultraviolet radiation

Ultraviolet radiation (UVR) may alter phosphorous (P) cycling by plankton through changes in the acquisition and/or regeneration of dissolved P. However, to date an effect of UVR on the uptake of P has not been observed at ambient phosphate (PO₄ ³⁻) concentrations. This has lead to the conclusion that the uptake of P by plankton may be insensitive to UVR. Past research has been limited to a few individual systems, prolonged incubations in bags, or lab cultures. We suspect that experimentation with natural plankton assemblages across broader environmental and/or chemical gradients is required to appreciably understand how UVR may alter P kinetics. Therefore, our study aimed to determine the effect of UVR on the turnover time of the dissolved PO₄ ³⁻ pool, the regeneration of dissolved P, the turnover rate of particulate P, and on PO₄ ³⁻ concentrations in natural plankton assemblages across broad environmental and chemical gradients. Second we aimed to assess how UVR may alter phosphatase activity and, determine if a change in phosphatase activity under UVR irradiance is correlated with a change in P uptake as proposed in the literature. Studies were conducted on 18 thermally stratified or polymictic lakes located in Ontario and Saskatchewan, Canada. Lake water samples were exposed to one of three experimental treatments: control, photosynthetically active radiation (PAR), or photosynthetically active radiation plus ultraviolet radiation (PAR + UVR). Our study is the first to demonstrate that UVR exposure has the potential to alter P cycling at ambient (picomolar) PO₄ ³⁻ concentrations. We have demonstrated that the turnover time of the PO₄ ³⁻ pool increases under UVR irradiance (i.e., P uptake decreases), while the regeneration rate of dissolved P and turnover rate of planktonic P are generally not affected; with the net effect being an increase in steady state PO₄ ³⁻ concentration (ssPO₄ ³⁻). Alkaline phosphatase activity (APA) in the dissolved and particulate fractions was significantly reduced in PAR + UVR treatments, but unrelated to changes in P uptake. In summary, we have demonstrated that the cycling of P may be disrupted by UVR, with a decrease in the uptake of P and the accumulation of PO₄ ³⁻ in the dissolved pool. This, in turn may exacerbate planktonic P limitation, alter the nutrient stoichiometry of plankton and/or indirectly alter rates of primary production in limnetic systems.     

Herbivorous caddisflies,macroalgae, and epilithic microalgae: dynamic interactions in a stream grazing system

Summary 1. During the low-flow period (April–October) in sunlit pools of Big Sulphur Creek (northern coastal California), the attached algal community predictably changes from an assemblage dominated by lush, upright Cladophora glomerata filaments in spring and early summer to one dominated by epilithic diatoms and blue-green algae (together=microalgae) in late summer through early autumn. Previous studies in this stream indicated that grazing by the caddisflies Helicopsyche borealis and Gumaga nigricula maintain low algal biomass during the latter part of this period. We used a combination of in situ exclusion/enclosure experiments to examine (1) the separate and combined effects of these grazers on Cladophora and microalgal assemblages, and (2) food preferences, growth, and microdistribution patterns of grazers when offered these different algal foods. 2. Grazers exerted strong but divergent effects on algal assemblages. Selective grazing on Cladophora by G. nigricula greatly accelerated the transition from upright Cladophora to epilithic microalgae, whereas selective grazing on microalgae by H. borealis dramatically reduced biomass of these forms. Grazers were largely ineffective at reducing the non-preferred algal food source (i.e. Cladophora by H. borealis, microalgae by G. nigricula). In the case of each grazer, growth was highest on the preferred algal food. Together, the activity of these grazers produced a low-biomass assemblage dominated by microalgal cells. 3. Removal of the Cladophora overstory by G. nigricula resulted in a three-fold increase in the abundance of epilithic microalgae, the preferred food of H. borealis. Elimination of Cladophora by G. nigricula can increase food availability for H. borealis and, in so doing, can indirectly facilitate the growth of this grazer during food-limited conditions. However, microdistribution of G. nigricula shifts from high overlap with H. borealis in spring and early summer when Cladophora is abundant to low overlap in late summer after Cladophora has been eliminated. This may indicate intense competition between these species for limited epilithic algae, and a concomitant movement by G. nigricula to areas in the stream where food resources are more available.     

Nutrient limitation of epilithic and epixylic biofilms in ten North American streams

1. Nutrient diffusing substrata were used to determine the effect of added inorganic nitrogen (N) and phosphorus (P) on the development of epilithic and epixylic biofilms in 10 North American streams. Four treatments of diffusing substrata were used: Control (agar only), N addition (0.5 m NaNO₃), P addition (0.5 m KH₂PO₄), and N + P combined (0.5 m NaNO₃ + 0.5 m KH₂PO₄). Agar surfaces were covered with glass fibre filters (for epilithon) or discs of untreated white oak wood veneer (for epixylon). 2. We found that if algae showed significant response to nutrient addition, N limitation (either N alone or N with P) was the most frequent response both on GF/F filters and on wood. Despite the low dissolved nutrient concentrations in our study streams, more than a third of the streams did not show any response to N or P addition. In fact, P was never the sole limiting nutrient for algal biofilms in this study. 3. Nutrient addition influenced algal colonisation of inorganic versus organic substrata in different ways. The presence of other biofilm constituents (e.g. fungi or bacteria) may influence whether algal biomass on wood increased in response to nutrient addition. Algae on organic and inorganic substrata responded similarly to nutrient addition in only one stream. 4. Fungal biomass on wood was nutrient limited in six of 10 study streams. N limitation of fungal biomass (with or without secondary P limitation) was most frequent, but P limitation did occur in two streams. 5. Our results show that biomass responses to nutrient addition by the heterotrophic and autotrophic components of the epixylic biofilm were different, though both experienced the same stream nutrient conditions. For algae and fungi growing on wood, limiting nutrients were rarely similar. Only three of nine streams showed the same biomass response to nutrient addition, including two that showed no significant change in biomass despite added nutrients.     

Variation in cell composition and utilization of N and P for growth of Selenastrum capricornutum

Cell material from different phases of growth for the green alga Selenastrum capricornutum Printz was analysed for major elements and chlorophyll a content. The ranges in percentage of the dry weight were 50–80%, 4.4–8.8%, 0.24–3.0%, and 0.20–3.23% for C, N, P and chlorophyll a, respectively. The dry weight per cell varied 2.3 times between different growth phases. A N/P-quotient of about 10 (by weight) was found to be a critical level for Selenastrum when these elements were the most limiting nutrients for growth. 31 different organic N-compounds were tested as single N sources in growth experiments with Selenastrum. At least 19 of them could be utilized to varying extent for algal growth. Three organic P-compounds of six tested, were utilized for algal growth. Yield coefficients (mm³ cell volume/mg limiting nutrient element) for Selenastrum were determined with NO₃, -N (y_N= 134). NH₄-N (Y_N= 111) and PO₄-P (Y_P= 2580).     

Molecular response of Anabaena flos-aquae to differing concentrations of phosphorus: A combined Fourier transform infrared and X-ray microanalytical study

The response of Anabaena flos‐aquae to varying levels of P availability was determined using the combined techniques of Fourier transform infrared (FTIR) microspectroscopy and energy‐dispersive X‐ray microanalysis (EDXRMA). Batch cultures of Anabaena were grown at initial P concentrations of 50 μg (low‐P culture), 500 μg (intermediate‐P) and 5000 μg (high‐P) (PO₄‐P L^?1) and were monitored for total biovolume, chlorophyll a, media nutrients (PO₄‐P and NO₃‐N), cellular P quota (EDXRMA) and carbon allocation (FTIR spectroscopy). The high‐P culture showed a sixfold increase in total biovolume over the sampling period. Phosphorus in the media remained at more than ～4000 μg L^?1 and intracellular P concentration (P quota) as determined by EDXRMA showed no decline, remaining at more than 0.20% dry weight (DW). The intermediate‐P culture showed a similar increase in total biovolume, but P concentrations in the media fell to ～20 μg L^?1, and this was reflected in a decline in the cellular P quota from 0.24% to 0.06% DW. Although the high‐ and intermediate‐P treatments differed markedly in both internal and external P, analysis of the FTIR spectra from the two treatments, using both hierarchical cluster analysis (HCA) and principal component analysis (PCA), indicated no difference in carbon allocation. Cells from the low‐P treatment showed strong evidence for P deficiency. P concentrations in the media were undetectable (<5 μg L^?1), total biovolume was much reduced, and P quotas were low, falling from 0.08% to 0.01% DW. HCA and PCA clearly separated FTIR spectra from low‐P cells from those of intermediate‐ and high‐P cultures, with low‐P cells having increased absorbance in the region of the spectra associated with carbohydrate. Both FTIR spectroscopy and EDXRMA have the resolution to study the elemental and macromolecular composition of single species within mixed phytoplankton populations and the combined use of these techniques has considerable potential for the study of cyanobacterial responses to environmental stress.     

Shifts in the relative availability of phosphorus and nitrogen along estuarine salinity gradients

Phosphorus (P) availability in estuaries may increase with increasing salinity because sulfate from sea salt supports production of sulfide in sediments, which combines with iron (Fe) making it less available to sequester P. Increased P availability with increased salinity may promote the generally observed switch from P limitation of primary production in freshwater ecosystems to nitrogen (N) limitation in coastal marine waters. To investigate this hypothesis, we analyzed pore water from sediment cores collected along the salinity gradients of four Chesapeake Bay estuaries (the Patuxent, Potomac, Choptank, and Bush Rivers) with watersheds differing in land cover and physiography. At salinities of 1–4 in each estuary, abrupt decreases in pore water Fe²⁺ concentrations coincided with increases in sulfate depletion and PO₄ ^3? concentrations. Peaks in water column PO₄ ^3? concentrations also occur at about the same position along the salinity gradient of each estuary. Increases in pore water PO₄ ^3? concentration with increasing salinity led to distinct shifts in molar NH₄ ⁺:PO₄ ^3? ratios from >16 (the Redfield ratio characteristic of phytoplankton N:P) in the freshwater cores to <16 in the cores with salinities >1 to 4, suggesting that release of PO₄ ^3? from Fe where sediments are first deposited in sulfate-rich waters could promote the commonly observed switch from P limitation in freshwater to N limitation in mesohaline waters. Finding this pattern at similar salinities in four estuaries with such different watersheds suggests that it may be a fundamental characteristic of estuaries generally.