Nutrient-enhanced productivity in the northern Gulf of Mexico: past,present and future

Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg l^–1 O₂) in 1985–1992 averaged 8000 to 9000 km² but increased to up to 16000 to 20700 km² in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions.     

Nutrient cycling and productivity in a desert saline lake: observations from a dry,low-productivity year

Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1^–1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m^–2 day^–1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of > 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (< 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

Ecology and fishery management of reservoirs in India

India has 19 370 small reservoirs with a total water surface area of 3 153 366 ha. At least 100 of them have been subjected to scientific studies. Habitat variables responsible for a reservoir's productivity can be summed up into climatic, morphometric and hydro-edaphic factors. The peninsular reservoirs are characterized by a narrow range of fluctuations in water and air temperature across seasons, a phenomenon which prevents the formation of thermal stratification. Many reservoirs in the upper peninsula show thermal stratification during summer. Wind-induced turbulence facilitates the return of nutrients to the trophogenic zone. Most reservoirs on the mountain slopes of Western Ghats, Himalayas and the other highlands are deeper, with steeper basin walls, compared to irrigation impoundments. Mean depth does not show any direct correlation with productivity, either at primary or fish level. A high shoreline development index gives a better indication of productivity. Plankton, benthos and periphyton pulses of Indian reservoirs coincide with the months of least level fluctuations. Oligotrophic tendencies shown by some reservoirs are mainly due to poor nutrient status and other chemical deficiencies. In most cases, poor water quality is accountable to poor catchment soil. Low levels of phosphate and nitrate are not indicative of low productivity due to quick recycling of these nutrients. Specific conductivity reflects the production propensities of reservoirs satisfactorily. Almost all productive reservoirs have a klinograde oxygen curve and a vertical stratification of chemical variables such as pH, carbon dioxide, total alkalinity and specific conductivity. High seasonal rainfall and discharge of water during monsoon result in high flushing rates, which do not favour colonization by macrophytic communities. Similarly, inadequate availability of suitable substrata retards the growth of periphyton. Plankton constitutes the major link in the trophic structure. A rich plankton community with well-marked succession is the hallmark of Indian reservoirs with blue-green algae as the major component. The main factors that retard the growth of benthos are a rocky bottom, frequent water level fluctuation and rapid deposition of silt and other suspended particles. Large reservoirs, on average, harbour 60 species of fishes, of which at least 40 contribute to the commercial fisheries. Fast-growing Indian major carps are the prominent commercial fishes. Dam construction has adversely affected populations of many other species such as Tenualosa ilisha, Torspp. and Cirrhinus spp. Formulae for estimating fish yield potential and stocking density are described. While culture-based fisheries have been successfully practiced in many small reservoirs, the management norm followed in medium and large reservoirs is primarily on capture fishery. In large and medium reservoirs, stocking was successful only when stocked fishes bred. Indian experience on species enhancement and introductions is described. Environmental enhancement of small reservoirs has been attempted in some reservoirs of Tamil Nadu. Modeling, using standard population parameters, such as the density-dependent growth, size dependent mortalityand weight–length relationshipis discussed. Two exotic fishes viz., Oreochromis mossambicus and Cyprinus carpio have been introduced into Indian reservoir with discouraging results. Hypophthalmichthys molitrix, after an accidental introduction, has performed well in Gobindsagar, a reservoir with a distinct cold water regime. Reservoir fisheries in India are well poised for development, provided scientific management norms are adopted.     

Paleoenvironmental changes during sapropel 19 (i-cycle 90) deposition: Evidences from geochemical,mineralogical and micropaleontological proxies in the mid-Pleistocene Montalbano Jonico land section (southern Italy)

An integrated micropaleontological, geochemical and mineralogical study has been performed across the mid-Pleistocene sapropel 19 (i-cycle 90) from the Montalbano Jonico land section (southern Italy), to reconstruct the paleoenvironmental conditions at time of its formation. The sapropel interval is characterized by two oxygen depletion phases (phase A and C) interrupted by a temporary re-oxygenation interval (phase B). The beginning and the end of sapropel deposition are dated at 957 ± 0.81 kyr and 950 ±0.86 kyr respectively. The duration of the interruption is estimated to 0.350 ± 0.32 kyr. The multiproxy approach highlights that deposition of sapropel 19 reflects a period of enhanced freshwater runoff induced by a wetter climate. As a consequence of a more efficient fluvial erosion, a higher terrigenous input, mostly ascribable to a southern Apennines source, and an increased turbidity of surface waters accompanied most of sapropel deposition. Biotic and abiotic proxies document that different paleoenvironmental conditions occur through phases A–C. The beginning of phase A is characterized by warm on-land paleoclimate as well as warm and oligotrophic surface water conditions. During the upper part of phase A temperature starts decreasing and surface waters appear more productive. This change probably represents the prelude to cooler and drier conditions characterizing phase B, which displays a river supply reduction and an eolian input increase (Sahara dust). During phase C the restored depleted oxygen environment at the bottom sediments is clearly coupled with the re-establishment of humid conditions and increased river supply. At the same time, enhanced mixing of water column, a cooler paleoclimate, and increased productivity of surface waters are recorded, the latter likely favored by the enhanced mixing of water column and also increased delivery of land-derived nutrients. The end of phase C is marked by a restored “normal” run-off. Enhanced productivity in surface waters and low oxygen conditions at the bottom sediments persist slightly above phase C. The overall results suggest that the onset of sapropel deposition is related to water stratification that caused low oxygen exchanges with the sea-bottom. Although enhanced productivity characterizes most of the sapropel deposition it was not the primary factor triggering sapropel deposition.     

A Vigorous Specialized Microbial Food Web in the Suboxic Waters of a Shallow Subtropical Coastal Lagoon

To examine the extent of the microbial food web in suboxic waters of a shallow subtropical coastal lagoon, the density and biomass of bacteria and protozooplankton were quantified under different dissolved oxygen (DO) levels. In addition, bottom waters of a stratified site were compared with bottom waters of a homogeneous site under periods of high and low biological oxygen production/consumption in the lagoon. At the stratified site, microbial biomass decreased with oxygen decline, from oxia to suboxia, with a recovery of the initial total biomass after a 20-day period of persistent suboxia. A peak in density and biomass of purple sulfur bacteria (PSB) (90 μg C L(-1)) occurred in the suboxic waters 20 days prior to the peak in biomass of ciliates >50 μm (Loxophyllum sp. of 150 μm) (160 μg C L(-1)), demonstrating a top down biomass control. Ciliates >50 μm were positively correlated with PSB and bacteriochlorophyll a (photosynthetic pigment of PSB). Total protozoan biomass reached 430 μg C L(-1) in the suboxic waters of the stratified site, with ciliates >50 μm accounting for 90% of the total ciliate biomass and of 55 % of biomass of protozoa. At the homogeneous site, total protozoan biomass was only 66 μg C L(-1), where flagellates and ciliates <25 μm were the dominant microorganisms. Therefore, as light is available for primary producers in the bottom waters of shallow stratified coastal lagoons or estuaries, one can expect that high primary production of PSB may favor a specialized microbial food web composed by larger microorganisms, accessible to zooplankton that tolerate low DO levels.     

Production by Hexagenia limbata in a warm-water reservoir and its association with chlorophyll content of the water column

We investigated the productivity of nymphs of the mayfly Hexagenia limbata in Lake Waco, a central Texas reservoir, and assessed its association with chlorophyll content of the water. We hypothesized that food availability measured as chlorophyll content of the water may directly associate with growth of Hexagenia and predict population productivity. To test this, we compared production by mayfly populations at two stations in the same reservoir; a northern station receiving water input with high chlorophyll content, and a southern station receiving water with low chlorophyll content. Both stations had similar substrate type and abundant mayflies. Benthic samples were collected from October 1984 through September 1985, and dissolved oxygen and temperature of the water were monitored.Annual production (size-frequency method) was 1270 mg m^–2 (P/B = 7.5) at the northern station and 1990 mg m^–2 (P/B = 6.1) at the southern station. The mean standing crop was 323 mg m ^{– 2} at the southern station and 169mg m^–2 at the southern station. Densities of mayflies at the two stations were not significantly different.Mean chlorophyll concentration (total mg pigment) during the sampling period was 23.5 mg m^–3 at the northern station and 16.7 mg m ^{– 3} at the southern station. Therefore, the station with lower mean chlorophyll content had higher secondary productivity by Hexagenia. Conversely, the station with higher mean chlorophyll content had lower mayfly productivity. The productivity of the mayfly populations did not positively associate with the chlorophyll content of the water, and chlorophyll content did not predict the success of the population of Hexagenia. Variation in mayfly growth success was associated with differences in temperature and dissolved oxygen. The northern station with higher chlorophyll content and lower productivity had low dissolved oxygen and temperatures higher than optimum for growth.     

Depth variations in the chemistry of Oguta Lake in Southeastern Nigeria

Monthly changes in the depth profiles of some chemical characteristics were studied for Oguta Lake, the largest natural lake in the southeastern region of Nigeria. The investigations were conducted from May, 1982, to April, 1983. Hydrogen ion concentration, salinity and alkalinity did not show significant variations with depth. Nitrate-nitrogen showed no definite pattern but in February, October and November, distinct metalimnetic nitrate maxima occurred. Silica was approximately homogeneously distributed with slight negative heterograde curves in February and October. Total phosphorus levels were relatively lower in the bottom water but the most distinct discontinuity occurred in November when a metalimnetic maximum co-occurred with a nitrate-nitrogen maximum. Dissolved oxygen concentration showed a consistent clinograde profile typical of thermally stratified eutrophic waters (WETZEL, 1975). Biochemical oxygen demand values followed the dissolved oxygen pattern very closely.     

On the ionic composition of five tropical fish-ponds of Aligarh (U.P.), India

Summary The ionic composition offour very productive and one unproductive fishpond has been investigated. The concentration of various ions is typical of carbonate waters except in two ponds for reasons of pollution. Calcium and bicarbonate are the major cation and anion while inspite of high productivity potassium is not very abundant. Because of the geographical position the area enjoys, the carbonate content seems to be important in productivity evaluation. For the same, its equivalent ratio to calcium and bicarbonate is very significant.     

Application of photorespiration concepts to whole stream productivity

We conducted two-station diel surveys of dissolved oxygen content to estimate whole-stream productivity in the experimental streams of the Monticello Ecological Research Station for two years following channel reconstruction. Community productivity measurements compare well to previous measurements in these streams, but apparent hysteresis in the P/I relation was measured in over two-thirds of the diel surveys. Apparent hysteresis in photosynthesis with solar irradiance is a characteristic of photorespiration, and modeling the effect of light on whole-stream respiratory rates reduced the magnitude of P/I curve hysteresis and improved the predictions of dissolved oxygen content (DO) in the stream. Stream productivity models normally assume respiratory rates measured at night are constant throughout the day, but when this assumption yields apparent hysteresis in the P/I curve, the inclusion of a photorespiration model in the analyses of whole-stream productivity facilitates the comparison of photosynthesis and respiratory rates between different streams. The computed total daily consumption of oxygen by photorespiratory processes is proportional to the total daily photosynthetic production of oxygen in the streams. We also found that the diel DO curves occurring in the experimental streams are best described by a photorespiration model that utilizes a four hour moving average of irradiance. Accounting for photorespiration in the streams increases the apparent efficiency of photosynthesis, improves the accuracy of DO predictions, and reduces uncertainty in photosynthesis and respiratory rate estimates. This revised version was published online in July 2006 with corrections to the Cover Date.     

In situ measurements of net primary production in a Colorado mountain stream

Six stations were established on a Colorado mountain stream, and net primary productivity was measured in situ during all seasons. For 24-hour periods the dissolved oxygen and temperature of the water were electronically monitored over an undisturbed 1 x 1 m section of rubble bottom enclosed by a large plastic dome tightly fitted to the substrate. A submerged pump maintained a current within the dome, and the whole apparatus was submerged below the stream level. The bottom community net metabolism varied between heterotrophy and autotrophy with no correlations with altitude, season, light, water chemistry, and temperature. Readings were all very low and ranged from -27.38 to 35.59 grams of carbon fixed per square meter per year. There were no correlations between biomass of the bottom fauna and net community productivity.Contribution No. 71, University of Colorado Limnology Laboratory     

Bacterioplankton and Organic Carbon Dynamics in the Lower Mesohaline Chesapeake Bay

The mesohaline portion of the Chesapeake Bay is subject to annual summertime hypoxia and anoxia in waters beneath the pycnocline. This dissolved oxygen deficit is directly related to salinity-based stratification of the water column in combination with high levels of autochthonously produced organic matter and a very high abundance of metabolically active bacteria. Throughout the water column in the lower, mesohaline part of the bay, between the Potomac and Rappahannock rivers, near the southern limit of the mainstem anoxia, bacterial abundance often exceeded 10 × 10⁶ cells per ml and bacterial production exceeded 7 × 10⁹ cells per liter per day during summer. Bacterial biomass averaged 34% (range, 16 to 126%) of the phytoplankton biomass in summer. These values are equal to or greater than those found farther north in the bay, where the oxygen deficit is more severe. Seasonal variations in bacterial abundance and production were correlated with phytoplankton biomass (lag time, 7 to 14 days), particulate organic carbon and nitrogen, and particulate biochemical oxygen demand in spring; but during summer, they were significantly correlated only with dissolved biochemical oxygen demand. During summer, dissolved biochemical oxygen demand can account for 50 to 60% of the total biochemical oxygen demand throughout the water column and 80% in the bottom waters. There is a clear spring-summer seasonal shift in the production of organic matter and in the coupling of bacteria and autochthonous organic matter. The measurement of dissolved, microbially labile organic matter concentrations is crucial in understanding the trophic dynamics of the lower mesohaline part of the bay. The absolute levels of organic matter in the water column and the bacterial-organic carbon relationships suggest that a lower bay source of organic matter fuels the upper mesohaline bay oxygen deficits.     

Upwelling-like bottom intrusion enhances the pelagic–benthic coupling by a fish predator in a coastal food web

Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ¹³C and δ¹⁵N for a demersal fish predator, Acropoma japonicum. This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ¹³C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ¹³C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.     

Nile water as a cause of Eastern Mediterranean sapropel formation: Evidence for and against

During the late Pleistocene, sapropels (layers of organic-carbon rich sediment) formed throughout the entire Eastern Mediterranean Basin in close association with glacial/interglacial transitions. The current theory for the mechanism of sapropel formation involves a density stratification of the water column, due to the invasion of a large quantity of low-saline water, which resulted in oxygen depletion of the bottom waters. Most workers believe that this low-salinity water was glacial meltwater that entered the Mediterranean via the Black Sea and a series of interconnected glacial lakes, but the suggestion also has been made that the freshwater originated from the Nile River. In this study the oxygen isotope values of planktonic foraminifera,Globigerinoides ruber, have been examined in six gravity cores and one piston core from the southern Levantine Basin, and compared with the oxygen isotope records ofG. ruber from other areas of the Eastern Mediterranean. This study deals mainly with the latest sapropel which was deposited approximately 7000 to 9000 years ago. Results indicate that Nile discharge probably does reduce salinities somewhat in the immediate area surrounding the mouth of the Nile, but this water is rapidly mixed with the highly saline waters of the easternmost Mediterranean.Using a mixing equation and surface water salinity limitations, an approximate oxygen isotope balance of surface waters was calculated for the time of latest sapropel deposition. This calculation shows that neither Nile River discharge nor Black Sea input (nor both together) are large enough to account for the large-scale oxygen isotope depletion associated with latest sapropel deposition in the Eastern Mediterranean. This suggests that part of the isotopic change at Termination I is probably due to increased surface water salinities during the last glacial maximum. In addition, evidence from the timing of sapropel 1 deposition and the dissolved oxygen balance indicates that deposition of the latest sapropel is associated with increased surface water production of biogenic material, as much as three times higher than that of present day.     

Hydrobiological studies with reference to sudden fish mortality

Summary Hydrobiological investigation of Gandhisagar (Nagpur) was undertaken with a view to finding out the causes of sudden fish mortality. Extensive observations carried out for over a period of several weeks, prior to, during and after mortality, have led us to the conclusion that depletion of dissolved oxygen was the main cause of sudden fish mortality.The depletion of Oxygen was brought about by several factors, viz., the depletion of phytoplankton resulting in the lesser evolution of oxygen, the gradual increase in the zooplankton which reached the maximum on the day of the fish mortality, resulting in the increased consumption of Oxygen for respiratory purposes, and the decomposition of the bottom sludge which also consumed oxygen to a large measure for the purposes of Chemical oxidation.The changes that are normally correlated with the decrease in the phytoplankton, such as, (i) lowering of the pH (from 8.9 to 7.5), (ii) the decrease in the carbonate contents of the water and (iii) the increase in the bottom turbidity were also noticed. Causes of the depletion of phytoplankton have not been ascertained but may be due to the presence ofDaphnia in very large numbers. Other aspect of the problem is under investigation.Published with the permission of the Director.     

Limnological Features of a Tropical Impoundment,Bhavanisagar Reservoir (Tamil Nadu), India

In the Bhavanisagar Reservoir a stable thermal stratification did not occur, but the thermal stability was fairly high. Oxygen deficits occurred frequently as well as complete anoxic conditions. Influx of flood waters improved the bottom oxygen conditions. This together with other factors indicates a high level of metabolic activity and productivity. Primary production rates were fairly high. There has been a change in fish population; some introduced species (like Catla and Labeo calbasu) have increased in the catches. It is felt that more fish could be exploited from this reservoir.     

Some aspects of photosynthetic characteristics in a set of perennial irrigation reservoirs located in five river basins in Sri Lanka

Phytoplankton primary productivity of eleven irrigation reservoirs located in five river basins in Sri Lanka was determined on a single occasion together with light climate and nutrient concentrations. Although area-based gross primary productivity (1.43–11.65 g O₂ m^–2 d^–1) falls within the range already established for tropical water bodies, net daily rate was negative in three water bodies. Light-saturated optimum rates were found in water bodies, with relatively high algal biomass, but photosynthetic efficiency or specific rates were higher in water bodies with low algal biomass, indicating nutrient limitation or physiological adaptation of phytoplankton. Concentrations of micronutrients and algal biomass in the reservoirs are largely altered by high flushing rate resulting from irrigation release. Underwater light climate and nutrient availability control the rate of photosynthesis and subsequent area-based primary production to a great extent. However, morpho-edephic index or euphotic algal biomass in the most productive stratum of the water column is not a good predictor of photosynthetic capacity or daily rate of primary production of these shallow tropical irrigation reservoirs.     

Methane and oxygen dynamics in a shallow floodplain lake: the significance of periodic stratification

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to 6 °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l^–1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l^–1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m^–3 h^–1, respectively. These values correspond to net and gross primary production rates of 0.7–1.2 and 1.0–1.9 g C m^–3 day^–1, respectively.     

Limnological features of glacier-fed rivers in the Southern Tibetan Plateau, China

We describe the limnological features of six glacier-fed rivers located at an altitude of over 4,000?m in Southern Tibetan Plateau, China, based on diurnal observations of the water environment and biota in the summer of 2010. The rivers showed significant diurnal changes in water level, temperature, and turbidity; clear and cool streams in the morning changed to cloudy rapid flow in the afternoon, and water temperature was elevated under strong light conditions where riparian forest did not cover the surface of the water. On the other hand, the pH and dissolved oxygen saturation did not show any diurnal fluctuations, which indicates low periphytic algal productivity. The aquatic insect communities were poor both in diversity and density, and the dominant life forms (from a functional feeding group perspective) were collectors, which depend on allochthonous organic matter; scrapers, which depend on autochthonous algal production, were scarce. We therefore suggest that diurnal or seasonal high waters transport products from marginal pools along dried riverbeds or from ponds on flood plains, and may serve as the source of organic matter in glacier-fed rivers.     

Effects of changes in discharge level on temperature and oxygen regimes in a new reservoir and downstream

Temperature and oxygen regimes were monitored weekly in a four-year old reservoir and downstream for ten weeks during the summer (May 15 to July 30, 1979), and monthly in the fall and winter. During the summer discharge levels were changed from top to bottom in the eighth week, back to top in the ninth, all at 0.71 m³/s (= 25 cfs), and returned to the bottom in the tenth, but at 1.14 m³/s (= 40 cfs). Changing the discharge level had little or no impact on the thermal regime within the lake or downstream at 1.14 m³/s and only a moderate impact on water in the lake near the dam at 0.71 m³/s bottom discharge by generating double thermoclines. Oxygen depletion rates were greatest near the surface of the lake, mainly due to temperature effects, but increased greatly at the 3 m depth, the level of the top discharge port, when discharge was changed from bottom to top. Discharge of anoxic waters through the bottom of the dam caused a drop in oxygen content immediately downstream but oxygen content returned to inflow values within 1 km downstream of the dam.