The relationship between surface water chemistry and geology in the North Branch of the Moose River

The chemistry of lakes and streams within the North Branch of the Moose River is strongly correlated with the nature and distrubution of geologic materials in the watershed. The dominance of thin glacial till and granitic gneiss bedrock in the region north and east of Big Moose Lake results in a geologically sensitive terrain that is characterized by surface water with low alkalinity and chemical compositions only slightly modified from ambient precipitation. In contrast, extensive deposits of thick glacial till and stratified drift in the lower part of the system (e.g. Moss-Cascade valley) allow for much infiltration of precipitation to the groundwater system where weathering reactions increase alkalinity and significantly alter water chemistry.The hypothesis that surficial geology controls the chemistry of surface waters in the Adirondacks holds true for 70 percent of the Moose River watershed. Exceptions include the Windfall Pond subcatchment which is predominantly covered by thin till, yet has a high surface water alkalinity due to the presence of carbonate-bearing bedrock. The rapid reaction rates of carbonate minerals allow for complete acid neutralization to occur despite the short residence time of water moving through the system. Another important source of alkalinity in at least one of the subcatchments is sulfate reduction. This process appears to be most important in systems containing extensive peat deposits.An analysis of only those subcatchments controlled by the thickness of surficial sediments indicates that under current atmospheric loadings watersheds containing less than 3 percent thick surficial sediments will be acidic while those with up to 12 percent will be extremely sensitive to acidification and only those with over 50 percent will have a low sensitivity.     

Paleolimnological reconstruction of the effects of atmospheric deposition of acids and heavy metals on the chemistry and biology of lakes in New England and Norway

Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by ¹³⁷Cs, ²¹⁰Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. ¹³⁷Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3–0.8 pH units start between I890 and I930 in N lakes already acidic (pH 5.0–5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.     

Chemical characteristics of lakes in the High Tatra Mountains,Slovakia

The chemistry of 53 lakes at various stages of acidification and inhabited (at the presence and/or in the past) by pelagic Crustacea was studied in September 1984. Ten of these lakes were investigated in detail biannually (July and October 1987–1990). The July results reflect the influence of snowmelt and were compared with the October ones. The most important anion was sulphate with the average values of 98 and 104 μeq 1^-1 in 1984 and 1987–1990, respectively. High concentrations of nitrate (21–56 μeq 1^-1) were observed in lakes above the treeline. Mean relative composition of cations does not differ between July and October; small changes are in the mean relative composition of anions. Acidification of lakes, expressed as a decrease in alkalinity, is 100 μeq l^-1, and is equal to the increase in the sum of sulphate and nitrate. The values of total phosphorus and COD are the lowest in the range of pH 5–6.5. Alkalinity, sulphate, nitrate and pH do not show any trend with time over the last ten years.     

Concentration of nutrients in selected lakes in the High Tatra Mountains, Slovakia: effect of season and watershed

Concentrations of total phosphorus (TP), inorganic and organic nitrogen, organic matter, and chlorophyll-a were studied in ten mountain lakes at various stages of acidification, trophy, and type of watershed during each July and October from 1987 to 1990. Concentrations of TP and total organic matter were higher in July than in October. Concentrations of NH4₄ ⁺-N decreased and NO₃ ^–-N increased from July to October. The relative composition of total nitrogen (TN) and its concentration were strongly dependent on the type of watershed: the lowest TN concentrations were observed in lakes with forested watersheds, increasing above the timberline and reaching maximum values in acidified lakes with rocky watersheds. In the pool of TN, nitrate was most important in lakes above the timberline (70–86% of TN), and organic nitrogen in forest lakes (> 90% of TN). Lakes with rocky watersheds were characterized by high ratios of TN:TP (> 250 by mass). The concentration of chlorophyll-a varied widely, from 0.01 to 22.6 µg l^–1, without any consistent change between July and October, and were P limited.     

Relationships between lakewater pH and sedimentary diatoms in the Matamek Watershed,Northeastern Quebec,Canada

Twenty lakes in the Matamek and nearby watersheds were sampled in the summer of 1983 for water chemistry and surficial sediments. Thirty-two physical and chemical variables, including pH, alkalinity, cations and metal concentrations were measured on samples from the epilimnion and hypolimnion of each lake. In three lakes, two to four replicate cores were collected to estimate spatial variability of the sediment flora.All lakes were acidic (pH 4.59 to 5.80), highly colored and poorly buffered. Aluminum and magnesium concentrations reached 494 and 70 µg l^-1, respectively. The pH of the lakes appears to be declining, as indicated by a comparison of our results with those from a survey done in 1970. An empirical chemical model based on the alkalinity/sulfate ratio and the regression of pH on calcium also indicated that these lakes may be undergoing acidification.Analyses of the diatom flora of the surficial sediments showed strong dominance of 6 species that cooccurred with a large number of rare species (in all, 229 taxa were found). Variability among samples within a lake was as high as among lakes. Calculation of Nygaard's alpha index for each sample and the regression of its log-transformed value on surface pH yielded a relationship that was significantly from other published models. The regression model was applied to a down-core analysis of the diatom flora of the sediments of two lakes. Although the large confidence intervals on pH values predicted by the model obscured any evidence of pH change with sediment depth, there was a significant increase of acidophilous and a decrease of circumneutral species over time, suggesting that a change in the flora, possibly correlated with pH, is taking place.In making comparisons among lakes, the surficial-sediment flora did not provide clear evidence of a relationship with the pH of surface water. Indices computed from the surficial-sediment flora are apparently insensitive to differences in pH over a narrow range, particularly when the lakes being compared are similar chemically. Nygaard's alpha index is shown to be unduly sensitive to outliers. The currently accepted assumption of a progressive linear change in communities may be inadequate for the quantification of acidification processes. Other models, derived from catastrophe theory, may prove more fruitful.     

Hydrogeologic controls of surface-water chemistry in the Adirondack region of New York State

Relationships between surface-water discharge, water chemistry, and watershed geology were investigated to evaluate factors affecting the sensitivity of drainage waters in the Adirondack region of New York to acidification by atmospheric deposition. Instantaneous discharge per unit area was derived from relationships between flow and staff-gage readings at 10 drainage basins throughout the region. The average chemical composition of the waters was assessed from monthly samples collected from July 1982 through July 1984. The ratio of flow at the 50-percent exceedence level to the flow at the 95-percent exceedence level of flow duration was negatively correlated with mean values of alkalinity or acid-neutralizing capacity (ANC), sum of basic cations (SBC), and dissolved silica, for basins containing predominantly aluminosilicate minerals and little or no carbonate-bearing minerals. Low ratios are indicative of systems in which flow is predominately derived from surface- and ground-water storage, whereas high ratios are characteristic of watersheds with variable flow that is largely derived from surface runoff.In an evaluation of two representative surface-water sites, concentrations of ANC, SBC, and dissolved silica, derived primarily from soil mineral weathering reactions. decreased with increasing flow. Furthermore, the ANC was highest at low flow when the percentage of streamflow derived from ground water was maximum. As flow increased, the ANC decreased because the contribution of dilute surface runoff and lateral flow through the shallow acidic soil horizons to total flow increased. Basins having relatively high ground-water contributions to total flow, in general, have large deposits of thick till or stratified drift. A major factor controlling the sensitivity of these streams and lakes to acidification is the relative contribution of ground water to total discharge.     

Alkalinity and pH of tarns and streams in the English Lake District (Cumbria)

SUMMARY. 1. The distribution of softwater and acid tarns and streams in central Cumbria is briefly summarized and compared for the periods 1983-85 and 1949-56. 2. Fifty-three upland tarns (altitude >350m) and forty lowland tarns (altitude <320m) were sampled on Skiddaw Slates, the Borrowdale Volcanic Series, and igneous intrusions. On these bedrocks, tarns and streams would be highly sensitive to further acidification if acid deposition increases above current levels. Twenty-six upland and seven lowland tarns are permanently acid with zero or negative alkalinity values throughout the year; mean pH ranged from 5.3 to 4.2. The other sixty upland and lowland tarns had positive mean Alk in the summers of 1983-85, ranging from 7 to 1227μequiv. I^?1 Alk and mean pH5.7 to >7.0. Thirty-one of the sixty tarns had mean Alk < 100μequiv. I^?1 in summer (May-September); sixteen became temporarily acid (negative Alk) for varying periods in winter. Acid episodes also occurred in the tarns in 1949-56. 3. Acid and very low alkalinity tarns and streams commonly occur on the high western, central and northern fells. Few occur on the high eastern felts, where there are veins of calcite in exposed rocks. 4. Seventy-five lowland tarns were sampled on Silurian Slates in southern Lakeland. On these bedrocks, tarns and streams are not highly sensitive to further acidification. Only six tarns had summer mean Alk <100μequiv. 1^?1. No permanently acid tarns or streams were found. 5. Over a mean span of about 30 years. Eel Tarn appears to have become slightly more acid and Harrop Tarn slightly less acid. The remainder of all tarns surveyed in 1983-85 have altered little, although there is a heavy load of acidic deposition on to the catchments and some tarns contain very low concentrations of Alk. Rapid acidification in recent years has been prevented by the neutralizing capacity of volcanic and sedimentary rocks. These rocks produce relatively high concentrations of Alk, up to 650μequiv.l^?1 in some spring-fed streams on the central fells and even more at lower altitudes. Absence of coniferous     

Glasgow Lake: an early-warning sentinel of lake acidification in Cape Breton Highlands National Park (Nova Scotia,Canada)

In contrast to other lakes studied in Cape Breton Highlands National Park (Nova Scotia, Canada), our paleolimnological results indicated that Glasgow Lake has been impacted by acidic deposition starting in the early 1900s. Based on analysis of diatom assemblages, the lake experienced a decrease in diatom-inferred lakewater pH from a pre-industrial pH of ~5.8 to a current pH of 5.3 (2000–2002 measured mean pH = 5.0) as well as a decrease in diatom-inferred Gran-alkalinity. In this study, diatom-based paleolimnological techniques were used in conjunction with a dynamic biogeochemical model (MAGIC) to assess both the timing and extent of the acidification trend, as well as determine a probable explanation as to why this lake, and none of the other 15 Cape Breton Highlands lakes studied for paleolimnology thus far, acidified under a peak non-marine sulphate deposition load of 43.6 mmol_c m⁻² year⁻¹ in the mid-1970s. Steady-state models estimate that Glasgow Lake had the lowest buffering capacity of six study lakes and estimated critical sulphate loading of <1 mmol_c m⁻² year⁻¹. MAGIC also estimated a loss of charge balance alkalinity from a pre-1850 value of 38 μmol_c l⁻¹ to a low of 12 μmol_c l⁻¹. While no evidence of biological recovery has been recorded, MAGIC estimates an increase in charge balance alkalinity to 27 μmol_c l⁻¹ in 2002 in response to decreased SO₂ emissions. Of the five other lakes that were modelled, all showed trends towards more acidic states and subsequent increases in charge balance alkalinity; however, the empirical paleo-diatom approach applied to these lakes showed no evidence of acidification. Thus, Glasgow Lake has the lowest buffering capacity among the Cape Breton Highland study lakes and serves as a sentinel of potential acidification trends and recovery in this region. Handling editor: J. Saros     

A combination of biological activity and the nitrate form of nitrogen can be used to ameliorate subsurface soil acidity under dryland wheat farming

The management of subsurface soil acidity remains a challenge. We tested the ability of calcium nitrate fertilization to force net anion uptake by wheat within acidic subsurface layers and hence its ability to increase, or at least maintain, soil pH within the acidic layer. The trials were conducted at two field sites: the moderately acidic site ran for 3 years (2006?C08) while the trial at the most acidic site was conducted for 2 years (2006?C07). Five treatments (nitrogen form and application method) were compared. Uptake efficiency of nitrogen (N) as measured by ¹⁵N was similar (at ~40 to 70%) for both urea and nitrate sources at both sites. The urea source acidified the soils (up to 0.2 pH in 3 years) whilst the nitrate form resulted in increases in soil pH (up to 0.3 pH). The increases in pH were of a similar magnitude to the acidification rate measured in a nearby long-term trial. The dry matter production and grain yield in our trials were compromised by the decade long drought in our region, so the work should be repeated during wetter seasons. Calcium nitrate fertilization is a useful tool for at least maintaining soil pH, and even reversing soil acidification, in acidic subsurface soil layers.     

Whitefish stocking in acidified lakes: ecological and physiological responses

In autumn 1986, six small lakes at different stages of acidification were stocked with one-summer-old whitefish, Coregonus pallasi Valenciennes 1848, in order to see whether whitefish stocking would be a suitable method for the mitigation of acidification effects. In two of the lakes the introduction was a complete failure: the whitefish did not survive, evidently due to high acidity and high aluminium concentrations of the lake waters. In one of the most acidified lakes (pH 4.3–4.8, Al_lab 29–125 g 1^–1) and in two less acidic lakes (pH 5.0–5.2 and 5.4–6.4), introduction was successful. Three years after the introduction, the mean weights of the fish in those three lakes were 580, 250 and 360 g respectively, with the weight and also the condition factor of stocked whitefish being highest in the most acidified lake. In that lake there were few or no fish present during the introduction, whereas in the less acid lakes there were dense populations of perch and therefore a potential interspecific competition for food. Different availability of food in the lakes was presumed to be the main reason for the growth differences. Plasma Na⁺ and Cl^– concentrations of whitefish were lower in the acidic lakes than in the lake with pH around 6 three years after stocking. This suggests that, despite the good growth and highest condition factor of whitefish in the most acid lake, the fish still experienced some acid stress.     

Long-term lake acidification trends in high- and low-sulphate deposition regions from Nova Scotia,Canada

Diatom-based paleolimnological techniques were used to study 14 lakes from two regions of Nova Scotia which represent regions of high and low sulphate deposition. Using decadal scale intervals, changes in diatom assemblages and diatom-inferred pH were tracked in relation to deposition of anthropogenic-sourced strong acids. Eight study lakes were located in Kejimkujik National Park in the southwestern part of the province, which receives an annual sulphate deposition (2000–2002) of ~10.5 kg ha⁻¹ yr⁻¹. These lakes showed significant changes in diatom assemblages with overall diatom-inferred acidification of ~0.5 pH units starting between 1925 and 1940, with the timing of acidification related to pre-industrial (or pre−1850) lakewater pH. Six study lakes were located in Cape Breton Highlands National Park, in northern Nova Scotia, a region of lower sulphate deposition. These lakes did not show any consistent trends in diatom assemblages or inferred pH values consistent with recent acidic deposition, but rather variations that may be related to climatic influences. Nova Scotia lakes that have been most impacted by acidic deposition had the lowest pre-industrial lakewater pH values and were in an area of relatively high sulphate deposition. Handling editor: K. Martens     

Effects of benthic filamentous algae on the sediment–water interface in an acidic mining lake

Natural alkalinity generation by microbial sulphate reduction in acidic lakes is usually inhibited by the low pH and a low primary production which results in a lack of suitable organic carbon sources. In some acidic mining lakes mass developments of filamentous benthic algae occur. The effects of this periphyton layer on the biogeochemistry of the sediment–water interface were investigated by in situ microsensor measurements and laboratory incubations in Mining Lake Grünewalder Lauch (Germany). Microsensor measurements showed that the oxic–anoxic boundary was located in the periphyton layer and was moving up and down depending on light triggered photosynthesis. The sediment itself was permanently anoxic. The diurnal redox shift and the maintenance of neutral conditions in the periphyton layer lead to an effective precipitation of iron and phosphorus. Under the periphyton layer very high sulphate reduction rates up to 265 nmol cm⁻³ d⁻¹ were measured in the sediment. These are the highest rates reported for mining lakes so far. The microbial activity was high enough to keep the pH in the surface sediment neutral and contributed to natural alkalinity production. Handling editor: L. Naselli-Flores     

Long-Term Trends in Stream Nitrate Concentrations and Losses Across Watersheds Undergoing Recovery from Acidification in the Czech Republic

Stream nitrogen (N) export and nitrate concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha⁻¹ y⁻¹ at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha⁻¹ y⁻¹. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition (R ² = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils (R ² = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N (R ² = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power (R ² = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output.     

Enhanced Calcite Dissolution in the Presence of the Aerobic Methanotroph Methylosinus trichosporium

Microbial aerobic methane oxidation (MOx) is intrinsically coupled to the production of carbon dioxide, favoring carbonate dissolution. Recently, microbial organic polymers were shown to be able to induce carbonate dissolution. To discriminate between different mechanisms causing calcite dissolution, experiments were conducted in the presence of solid calcite with (1) actively growing cells (2) starving cells, and (3) dead cells of the methanotrophic bacterium Methylosinus trichosporium under brackish conditions (salinity 10) near calcite saturation (saturation state (Ω) 1.76 to 2.22). Total alkalinity and the amount of dissolved calcium markedly increased in all experiments containing M. trichosporium cells. After initial system equilibration, similar calcite dissolution rates, ranging between 20.16 (dead cells) and 25.68 μmol L^?1 d^?1 (actively growing cells), were observed. Although concentrations of transparent exopolymer particles declined with time in the presence of actively growing and starving cells, they increased in experiments with dead cells. Scanning electron microscopy images of calcite crystals revealed visible surface corrosion after exposure to live and dead M. trichosporium cells. The results of this study indicate a strong potential for microbial MOx to affect calcite stability negatively, facilitating calcite dissolution. In addition to CO₂ production by methanotrophically active cells, we suggest that the release of acidic or Ca²⁺-chelating organic carbon compounds from dead cells could also enhance calcite dissolution.     

The evolution of saline lake waters: gradual and rapid biogeochemical pathways in the Basotu Lake District,Tanzania

The biogeochemical evolution of solutes markedly alters the chemistry in the closed-basin maar lakes that comprise the Basotu Lake District (Tanzania, East Africa). Examination of 11 (out of 13) lakes in the Basotu Lake District identified two distinct evolutionary pathways: a gradual path and a rapid path. During the course of biogeochemical evolution these waters follow either the gradual path alone or a combination of the gradual and rapid paths. Solute evolution along the gradual path is determined by all of the biogeochemical processes that for these waters appear to be tightly coupled to evaporative concentration (e.g. mineral precipitation, sorption and ion exchange, C0₂ degassing, and sulfate reduction). Rapid evolution occurs when mixing events suddenly permit H₂S to be lost to the atmosphere. The chemistry of waters undergoing rapid evolution is changed abruptly because loss of every equivalent of sulfide produces an equivalent permanent alkalinity.The Basotu Lake District in north central Tanzania is comprised of 13 maar lakes. They range in surface water conductivity from 592 to 24 000 µ S cm ^–1 (at 20°). Within these lake basins only a few of the variety of geo- and biogeochemical processes known to occur in lakes of this type are actually responsible for the gain and/or loss of individual solutes. For example, potassium appears to be taken up in the formation of illite. Calcium is precipitated as calcite. Magnesium interacts with alumino-silicate precursors to form a variety of clay minerals that contain magnesium (e.g. stevensite). This process is also known as reverse weathering. Sulfate is reduced to sulfide and subsequently lost as H₂S and/or metal sulfides. Alkalinity is lost owing to calcite precipitation and as a consequence of reverse weathering. Alkalinity is gained in the form of extra permanent alkalinity when sulfide is lost from these waters (via metal sulfide precipitation or gaseous emission to the atmosphere). Rapid (punctuated) evolution can occur in any lake containing anoxic waters providing that mixing events take place which cause H₂S to be lost to the atmosphere.Peter Kilham died on March 20, 1989, in Kisumu, Kenya, while working as part of a research team on Lake Victoria.     

Recovery from acidification in boreal lakes inferred from macroinvertebrates and subfossil chironomids

Acidification of waters and soils caused by emissions and the long-range transport of air pollutants has been a serious worldwide problem during the last decades. The extent of the acidification problem in Finnish acid-sensitive forest lakes was examined in the Acidification Research Project (HAPRO) in the mid-1980s. The recent decline in the emissions of air pollutants has resulted in the chemical recovery of watersheds in many regions, and the present work on the recovery processes in acidified Finnish headwater lakes (REPRO) was launched to examine whether the chemical recovery has already been accompanied by biological recovery. The patterns of recovery were studied by re-sampling littoral macrozoobenthos in a subset of the previously sampled HAPRO lakes. Paleolimnological samples were taken in order to assess the possible dependence of lacustrine chironomid communities on the changing degree of acidification. Acid sensitive and moderately acid sensitive benthic species revealed slight recovery in the formerly most acidic (pH 5.5) but recently recovered lakes. The most significant factors affecting the response of benthic communities were increased mean lake pH and decreased labile aluminium concentration. Paleolimnological chironomid analysis revealed a slight response along the pH gradient, but also significant structural similarity between the present and pristine chironomid assemblages. This implies that no major changes in chironomid communities of these acidic lakes have occurred during the past centuries. The alternative future trends and threats to biological recovery in small headwater lakes are discussed.     

Nitrification and nitrate uptake: Leaching balance in a declined forest ecosystem in eastern France

Nitrate uptake and leaching were measured during one year in a declined fir forest on the Vosges highlands (eastern France), in order to investigate whether excess nitrification could be responsible for a deleterious acidification of the ecosystem. Nitrate uptake by the vegetation was active mainly from spring to early fall, and then reached about 66 kg N ha^-1. No significant leaching loss occurred during the growth period of the vegetation. Significant nitrate leaching occurred in winter (about 17 kg N ha^-1). During fall and winter the nitrification rate was of the same magnitude as values reported for other ecosystems, and, thus, was not considered to be abnormaly strong. No abnormal temporal discoupling of nitrate production and nitrate uptake occurred in the ecosystem, and forest decline must therefore have some other cause.     

Temporal coherence of two alpine lake basins of the Colorado Front Range, U.S.A.

1. Knowledge of synchrony in trends is important to determining regional responses of lakes to disturbances such as atmospheric deposition and climate change. We explored the temporal coherence of physical and chemical characteristics of two series of mostly alpine lakes in nearby basins of the Colorado Rocky Mountains. Using year‐to‐year variation over a 10‐year period, we asked whether lakes more similar in exposure to the atmosphere be‐haved more similarly than those with greater influence of catchment or in‐lake processes. 2. The Green Lakes Valley and Loch Vale Watershed are steeply incised basins with strong altitudinal gradients. There are glaciers at the heads of each catchment. The eight lakes studied are small, shallow and typically ice‐covered for more than half the year. Snowmelt is the dominant hydrological event each year, flushing about 70% of the annual discharge from each lake between April and mid‐July. The lakes do not thermally stratify during the period of open water. Data from these lakes included surface water temper‐ature, sulphate, nitrate, calcium, silica, bicarbonate alkalinity and conductivity. 3. Coherence was estimated by Pearson's correlation coefficient between lake pairs for each of the different variables. Despite close geographical proximity, there was not a strong direct signal from climatic or atmospheric conditions across all lakes in the study. Individual lake characteristics overwhelmed regional responses. Temporal coherence was higher for lakes within each basin than between basins and was highest for nearest neighbours. 4. Among the Green Lakes, conductivity, alkalinity and temperature were temporally coherent, suggesting that these lakes were sensitive to climate fluctuations. Water tem‐perature is indicative of air temperature, and conductivity and alkalinity concentrations are indicative of dilution from the amount of precipitation flushed through by snowmelt. 5. In Loch Vale, calcium, conductivity, nitrate, sulphate and alkalinity were temporally coherent, while silica and temperature were not. This suggests that external influences are attenuated by internal catchment and lake processes in Loch Vale lakes. Calcium and sulphate are primarily weathering products, but sulphate derives both from deposition and from mineral weathering. Different proportions of snowmelt versus groundwater in different years could influence summer lake concentrations. Nitrate is elevated in lake waters from atmospheric deposition, but the internal dynamics of nitrate and silica may be controlled by lake food webs. Temperature is attenuated by inconsistently different climates across altitude and glacial meltwaters. 6. It appears that, while the lakes in the two basins are topographically close, geologically and morphologically similar, and often connected by streams, only some attributes are temporally coherent. Catchment and in‐lake processes influenced temporal patterns, especially for temperature, alkalinity and silica. Montane lakes with high altitudinal gradients may be particularly prone to local controls compared to systems where coherence is more obvious.     

Gas exchange responses of a mangrove species, Avicennia marina, to waterlogged and drained conditions

This study was undertaken in summer on fully expanded leaves of Avicennia marina trees in the Beachwood Mangroves Nature Reserve, Durban,South Africa. Data sets were obtained over 5–7 days of relatively dry conditions and over two periods of 5 days during which the swamp was continuously inundated with dilute seawater (< 150 mol m⁻³NaCl). Gas exchange responses were strongly influenced by photosynthetic photon flux density (PPFD), leaf temperature and leaf to air vapour pressure deficit (Δw). Carbon dioxide exchange was saturated at a PPFD of about 800 μmol m⁻² s⁻¹. Maximal CO₂ exchange rates ranged from 8.5 to 9.9 μmol m⁻² s⁻¹ with no differences between drained and waterlogged conditions. Under drained conditions, leaf conductance,transpiration and internal CO₂ concentrations were generally lower, and water use efficiencies higher, than during waterlogging. Continuous waterlogging for 5 days had no adverse effect on CO₂ exchange. Xylem water potentials ranged from −1.32to −3.53 MPa during drained and from −1.02 to −2.65 Mpa during waterlogged conditions. These results are discussed in relation to anatomical and metabolic adaptations of A. marina to waterlogging stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of ¹⁴C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of ¹⁴C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.