Maurice A. Lock

Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels

Peatlands represent a vast store of global carbon. Observations of rapidly rising dissolved organic carbon concentrations in rivers draining peatlands have created concerns that those stores are beginning to destabilize. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives—warming and increased river discharge—cannot offer satisfactory explanations. Here we show that the third proposed mechanism, namely shifting trends in the proportion of annual rainfall arriving in summer, is similarly unable to account for the trend. Instead we infer that a previously unrecognized mechanism—carbon dioxide mediated stimulation of primary productivity—is responsible. Under elevated carbon dioxide levels, the proportion of dissolved organic carbon derived from recently assimilated carbon dioxide was ten times higher than that of the control cases. Concentrations of dissolved organic carbon appear far more sensitive to environmental drivers that affect net primary productivity than those affecting decomposition alone.

Longitudinal Patterns of Ecosystem Processes and Community Structure in a Subarctic River Continuum

Ecosystem processes and community structure in running waters of the boreal forests of Quebec, Canada, are strongly influenced by climate and channel geomorphology. Here we present an overview of a project examining longitudinal trends as small streams gradually coalesce into large rivers, summarizing our results in a series of budgets and predictive equations describing changes in organic carbon dynamics and community structure. There were significant trends with stream order for 70% of the 73 components, processes, and ratios examined. Of 46 independent components examined, 63% showed a significant trend with stream order. As stream size increased from 1st to 9th order there was a decrease in total carbon inputs (i.e., precipitation, throughfall, primary production, and allochthonous materials) followed by a gradual increase due to greater primary production in streams >6th order. The standing stock of carbon decreased exponentially downstream, and total carbon outputs (i.e., respiration, leaching, methane evasion, and insect emergence) increased slightly downstream. Nevertheless, some ecosystem—level processes, as well as community structure, showed equivocal trends, which were apparently due to the hierarchical scale of examination and the relative degree of physicochemical vs. biological control, of the processes and communities. The data, when placed in a watershed perspective, showed that total carbon inputs were evenly distributed by stream order throughout the 19 871—km2 Moisie River drainage network. Most carbon was stored in the small 1st to 3rd order streams, whereas the majority of organic carbon was metabolized in the 7th to 9th order rivers. Fluvial transport of organic carbon to the Gulf of St. Lawrence was nearly three times that of the measured total annual input, suggesting that inputs of dissolved organic carbon in groundwater were more important than previously expected. Ecosystem—level measurements of carbon retention and utilization also showed significant trends with stream order. The spiraling length for carbon increased exponentially from 8—15 km in small streams to 426 km in the 9th order river. There was a concomitant decrease in reach retention with stream order, while the rate coefficient of respiration and rate of downstream movement increased with order. The stream metabolism index, a measure of ecosystem efficiency, increased from 1st to 7th order, thereafter decreasing as streams became larger. These trends with stream order were related to physical gradients in channel dimensions, hydrology, riparian influences, and sunlight. We conclude that these subarctic lotic ecosystems have numerous strong relationships with stream order and that the dynamics can be described by a relatively small set of predictive equations.

Fluxes of CO2, CH4 and N2O from a Welsh peatland following simulation of water table draw-down: Potential feedback to climatic change

A potential effect of climatic change was simulated by manipulating the water table height within intact peat monoliths. The treatment decreased methane flux (maximum −80%) and increased both carbon dioxide flux (maximum 146%) and nitrous oxide flux maximum 936%). Returning the water table height to its original level caused both nitrous oxide and carbon dioxide flux to rapidly return to control levels. However, methane flux remained at its experimentally induced low levels.

Transformation of a Tundra River from Heterotrophy to Autotrophy by Addition of Phosphorus

Continuous enrichment of an arctic river with only 10 parts per billion phosphate-phosphorus caused an immediate growth of attached algae for more than 10 kilometers downstream, showing that phosphorus alone limited photosynthesis. As a result of the increased photosynthesis, there was an increase in bacterial activity in films on rocks on the bottom of the stream. The major source of energy became the photosynthetic carbon fixed in the stream rather than the organic material entering from the surrounding tundra, and the overall metabolism of the stream shifted from heterotrophy to autotrophy. An increase in the size and developmental stage of some of the dominant aquatic insects illustrates the food limitation in this nutrient-poor habitat.

The use of fluorogenic substrates for measuring enzyme activity in peatlands

The fluorogenic model substrates, methylumbelliferyl [MUF]-β-D-glucoside, MUF-phosphate and MUF-sulphate, were used to investigate the activities of β-glucosidase, phosphatase and sulphatase, respectively, in Welsh peatland soils. The method was used to investigate depth dependent variations in enzyme activity in a riparian wetland, and flush channel wetland. The highest activities were found at depths of less than 10 cm, thus confirming other studies which suggest this upper depth to be the site of greatest microbial activity. The most serious limitation to the technique was found to be the (fluorescence) quenching effects of the phenolic materials that dominate peatland dissolved organic carbon. The problem necessitates the adoption of a time consuming quench correction procedure with every sample. Fluorogenic substrates have led to a greater understanding of the role of enzymes in other aquatic systems. It seems likely that they will prove of equal value in elucidating their role in nutrient cycling and the biogeochemistry of peatlands.

Enzymic degradation of phenolic materials in peatlands : measurement of phenol oxidase activity

The model substrate L-dihydroxy phenylalanine (L-DOPA) was used to measure the activity of phenol-oxidase (PO) in peat from a Welsh riparian wetland. The sensitive and relatively simple technique measured the rate of formation of the red coloured compound 2-carboxy-2,3-dihydroindole-5,6-quinone from the enzymic oxidation of L-dopa. The method was used to test the hypothesis that the large exports of phenolic materials from peatlands into aquatic systems were caused by low phenolic-degrading enzyme activities within the peat matrix. The low oxygen availability and acidic pH of the peat soil were found to be sub-optimal for PO activity. Furthermore, a depth-dependent decline in PO activity was inversely correlated with phenolic concentrations. Thus, the findings supported the above hypothesis.

Microbial activity and enzymic decomposition processes following peatland water table drawdown

Microbial activity and enzymic decomposition processes were followed during a field-based experimental lowering of the water table in a Welsh peatland. Respiration was not significantly affected by the treatment. However, the enzymes sulphatase, β-glucosidase and phosphatase were stimulated by between 31 and 67% upon water table drawdown. A further enzyme, phenol oxidase, was not significantly affected. The observation of elevated enzyme activities without an associated increase in microbial respiratory activity suggests that drought conditions influence peatland mineralisation rates through a direct stimulation of existing enzymes, rather than through a generalised stimulation of microbial metabolism (with associated de-novo enzyme synthesis). Hydrochemical data suggest that the stimulation may have been caused by a reduction in the inhibitory action of iron and phenolics in the peat pore waters. Overall, the findings support the recent hypothesis that drier conditions associated with climate change could stimulate mineralisation within wetlands. ei]R Merckx

Nutrient Incluence on a Stream Grazer: Orthocladius Microcommunities Respond to Nutrient Input

A whole—stream enrichment experiment of phosphorus and, further down—stream, of phosphorus and nitrogen, allowed us to examine the growth and density responses of the tube—building larval chironomid Orthocladius rivulorum to nutrient enrichment of the Kuparuk River in arctic Alaska, and to evaluate nutrient effects on the tube microbial community. The larva feeds by grazing a diatom monoculture of Hannaea arcus from the tube exterior, thus direct nutrient effects on the tube microbiota may translate into indirect nutrient effects on the larva. Electron microscopy indicated that tube silk was formed into a sheet, with a filamentous substructure that repeated at 50—nm intervals. Bacterial micro—colonies occurred at the points where the erect diatoms were attached to the silk. Microbial activity of Orthocladius tubes in the P—fertilized section was 2—3 times that of the control section of the river, and total microbial biomass in the P—fertilized section was 3—4 times that of the control section. Chlorophyll a was also higher on Orthocladius tubes downstream of both P and N + P fertilization sites. However, the rate of biomass accumulation on tubes was more rapid downstream of N + P addition, suggesting primary P and secondary N limitation of the rate of primary production in the river. Chlorophyll a was higher on tubes than on rocks or experimental tiles, which indicated that tubes were a more favorable algal habitat for Hannaea. Pupal tubes had less chlorophyll a than larval tubes, suggesting that larval activity may have contributed to the higher algal biomass on tubes. Orthocladius benefitted from the enhanced tube flora; larvae grew larger in the fertilized sections of the stream than in the upstream sections. The results suggest that Orthocladius with its tube and associated biota function as microcommunities that respond directly and indirectly to the surrounding nutrient regime, but have considerable trophic independence from surrounding portions of the epilithon. They may constitute 12—43% of total epilithic algal biomass.

Enzymes and biogeochemical cycling in wetlands during a simulated drought

Possible interactions between soil enzymes and thebiogeochemistry of wetlands were investigated duringa field-based drought simulation. Under control(waterlogged) conditions, correlations were foundbetween the activity of the enzyme B-glucosidase andtwo properties associated with carbon cycling, namelyi) CH4 release r = 0.79,p lt 0.01) and ii) dissolvedorganic carbon concentration (r= -0.81, p lt 0.01). In contrast,the transition to drought conditions resulted in correlations betweenB-glucosidase activity and certain mineralisationprocesses, namely the release of mg and Ca(r = 0.72, p lt 0.05). Sulphataseactivity correlated with changes in sulphate concentration during the droughtsimulation (r = 0.73, p lt 0.05).Further support for the suggested enzymic involvement in biogeochemicalprocesses was found in laboratory studies. Theseexperiments indicated that increasing the abundance ofB-glucosidase could stimulate trace gas emissions(p lt 0.001) and increase the concentration ofmagnesium and calcium (p lt 0.05). Increasedsulphatase abundance caused a suppression of methane emissions(p = 0.053).

Seasonal determinations of extracellular hydrolytic activities in heterotrophic and mixed heterotrophic/autotrophic biofilms from two contrasting rivers

The temporal changes in extracellular enzyme activities in freshwater microbial biofilms were examined in two contrasting river sites in North Wales over a 12 month period. Sites were a first order, unshaded oligotrophic upland stream (Nant Waen) and a fourth order, mildly eutrophic river with riparian tree cover (River Clywedog). When algal populations were low, biofilms of the more eutrophic site supported greater enzyme activities and higher population densities than the oligotrophic site. Composition, concentration and origin of substrates available to the respective biofilm communities influenced extracellular processing patterns. Reduction in algal populations depressed total and extracellular activities in biofilms from the first order site, suggesting that biofilm communities here were maintained by in situ primary production. Biofilms from Nant Waen were often found to contain higher extracellular activities per cell than the more eutrophic River Clywedog biofilms, which might represent the enhanced ability of an oligotrophic biofilm to accumulate extracellular enzymes. In contrast, light and darkgrown River Clywedog biofilms were not enzymatically distinct, inferring a less important role for biofilm phototrophs. Some evidence was found for increased reliance on allochthonous substrates in the River Clywedog for biofilm maintenance.