Seasonal variation in stable C and N isotope ratios of the Rhone River inputs to the Mediterranean Sea (2004–2005)

The temporal variation in the stable carbon and nitrogen isotope ratios of particulate organic matter (POM) in the Rhone River was investigated on a monthly basis during a 2-year period (2004–2005). In spite of high month-to-month variation, interannually consistent seasonal trends appeared, with significantly lower δ¹³C (<?28.2‰) in spring than in the other seasons. In contrast, δ¹⁵N did not exhibit significant temporal variation. In spring and summer, high chlorophyll a and b concentrations were associated with low C/N values (<8) and a high percentage of organic carbon (%C) and organic nitrogen (%N), testifying to high development of autochthonous riverine phytoplankton (mainly diatoms and chlorophytes). In fall and winter, higher δ¹³C (>?27.2‰) and C/N (>8) values, and lower %C, %N, and chlorophylls concentrations indicated the predominance of allochthonous terrestrial detritus material in the river POM. The lower δ¹³C values recorded in spring–summer, when the phytoplankton biomass was high, were related to the lower carbon isotopic signatures of freshwater diatoms and chlorophytes compared to those of terrestrial plants. Overall, Rhone River POM was mainly composed of terrestrially derived material (90%), with autochthonous phytoplankton representing only 10% as a mean, in spite of a higher mean contribution of phytoplankton (27%) to river POM in summer.     

The Nutrient Status of Mgazana,a Warm Temperate Mangrove Estuary in the Transkei,Eastern Cape,South Africa

Mgazana, a rural southern African mangrove system, was visited monthly from August, 1995 to February, 1997 to collect water samples for nutrient analysis. Surface and bottom samples were taken during spring low tide at seven stations along the estuary and the following physico-chemical parameters measured: river flow, temperature, salinity, oxygen, transparency, ammonia, nitrite, nitrate, phosphate, inorganic carbon (IC), organic carbon (OC), total carbon (TC), soluble nitrogen (SN), particulate nitrogen (PN) and total nitrogen (TN). Using correlation matrix analysis and ANOVA, river flow was found to affect estuarine salinity, transparency and stratification, which influenced nutrient dynamics. Significant seasonal (winter and summer) differences were found for temperature, river flow, nitrate, SN, TN, IC and OC. Most nutrients were significantly correlated with river flow showing gradients down the estuary, indicating allochthonous input from the catchment. OC levels within the estuary were high, probably due to autochthonous mangrove leaf-fall processing by the various in-fauna, but high levels measured at the head of the estuary during high river flow suggested additional allochthonous input from coastal forest litter. Conversely, IC was negatively correlated with river flow suggesting that autochthonous faunal and microbial mineralisation of organic matter occurs within creeks, which is then diluted by increased stream-flow. An N:P ratio of 2.7:1 was obtained for this rural mangrove system, which was low compared with Spartina-based East Cape estuaries subject to urban, industrial and agricultural pollution.     

The significance of side-arm connectivity for carbon dynamics of the River Danube, Austria

1. Side‐arms connected to the main stem of the river are key areas for biogeochemical cycling in fluvial landscapes, exhibiting high rates of carbon processing. 2. This work focused on quantifying autochthonous and allochthonous carbon pools and, thereby, on comparing transport and transformation processes in a restored side‐arm system of the River Danube (Regelsbrunn). We established a carbon budget and quantified carbon processing from March to September 2003. In addition, data from previous studies during 1997 to 1999 were assessed. 3. Gross primary production (GPP) and community respiration were estimated by diel oxygen time curves and an oxygen mass balance. Plankton primary production was determined to estimate its contribution to GPP under different hydrological conditions. 4. Based on the degree of connectivity, three hydrological phases were differentiated. Most of the organic matter, dominated by allochthonous carbon, was transported in the main channel and through the side‐arm during floods, while at intermediate and low flows (and thus connectivity), transformation processes became more important and autochthonous carbon dominated the carbon pool. The side‐arm system functioned as a sink for particulate matter [total suspended solids and particulate organic carbon (POC)] and a source of dissolved organic carbon (DOC) and chlorophyll‐a. 5. Autochthonous primary production of 4.2 t C day^?1 in the side‐arm was equivalent to about 20% of the allochthonous inputs of 20 t C day^?1 (POC and DOC) entering the area at mean flow (1% of the discharge of the main channel). Pelagic photosynthesis was generally high at mean flow (1.3–3.8 g C m^?2 day^?1), and contributed up to 90% of system productivity. During long stagnant periods at low discharge, the side‐arm was controlled by biological processes and a shift from planktonic to benthic activity occurred (benthic primary production of 0.4–14 g C m^?2 day^?1). 6. The transformation of the organic matter that passes through the side‐arm under different hydrological conditions, points to the importance of these subsystems in contributing autochthonous carbon to the food web of the main channel.     

Sources of dissolved and particulate organic material in Loch Vale Watershed,Rocky Mountain National Park,Colorado, USA

The sources of both dissolved organic carbon (DOC) and particulate organic carbon (POC) to an alpine (Sky Pond) and a subalpine lake (The Loch) in Rocky Mountain National Park were explored for four years. The importance of both autochthonous and allochthonous sources of organic matter differ, not only between alpine and subalpine locations, but also seasonally. Overall, autochthonous sources dominate the organic carbon of the alpine lake, while allochthonous sources are a more significant source of organic carbon to the subalpine lake. In the alpine lake, Sky Pond, POC makes up greater than one third of the total organic matter content of the water column, and is related to phytoplankton abundance. Dissolved organic carbon is a product of within-lake activity in Sky Pond except during spring snowmelt and early summer (May–July), when stable carbon isotope ratios suggest a terrestrial source. In the subalpine lake, The Loch, DOC is a much more important constituent of water column organic material than POC, comprising greater than 90% of the spring snowmelt organic matter, and greater than 75% of the organic matter over the rest of the year. Stable carbon isotope ratios and a very strong relation of DOC with soluble Al_(tot) indicate DOC concentrations are almost entirely related to flushing of soil water from the surrounding watershed during spring snowmelt. Stable carbon isotope ratios indicate that, for both lakes, phytoplankton is an important source of DOC in the winter, while terrestrial material of plant or microbial origin contributes DOC during snowmelt and summer.     

Bacterial Growth on Allochthonous Carbon in Humic and Nutrient-enriched Lakes: Results from Whole-Lake 13C Addition Experiments

Organic carbon (C) in lakes originates from two distinct sources—primary production from within the lake itself (autochthonous supply) and importation of organic matter from the terrestrial watershed (allochthonous supply). By manipulating the ¹³C of dissolved inorganic C, thereby labeling within-lake primary production, we examined the relative importance of autochthonous and allochthonous C in supporting bacterial production. For 35 days, NaH¹³CO₃ was added daily to two small, forested lakes. One of the lakes (Peter) was fertilized so that primary production exceeded total respiration in the epilimnion. The other lake (Tuesday), in contrast, was low in productivity and had high levels of colored dissolved organic C (DOC). To obtain bacterial C isotopes, bacteria were regrown in situ in particle-free lake water in dialysis tubes. The contribution of allochthonous C to bacterial biomass was calculated by applying a two-member mixing model. In the absence of a direct measurement, the isotopic signature of the autochthonous end-member was estimated indirectly by three different approaches. Although there was excess primary production in Peter Lake, bacterial biomass consisted of 43–46% allochthonous C. In Tuesday Lake more than 75% of bacterial growth was supported by allochthonous C. Although bacteria used autochthonous C preferentially over allochthonous C, DOC from the watershed contributed significantly to bacterial production. In combination with results from similar experiments in different lakes, our findings suggest that the contribution of allochthonous C to bacterial production can be predicted from ratios of chromophoric dissolved organic matter (a surrogate for allochthonous supply) and chlorophyll a (a surrogate for autochthonous supply).     

Characteristics of settling matter and its role in nutrient cycles in a deep oligotrophic lake

The settling flux of seston (dry weight, DW), chlorophyll a (Chl a), particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate phosphorus (PP) was measured monthly in 1981–1983 at 10 different depths in Lake Chuzenji, Japan; an oligotrophic lake with a maximum depth of 163 m. The Ti concentration in entrapped matter was used to separate the sedimentation flux into allochthonous and autochthonous components. Inflow loads of dissolved nutrients (DN: 4.5, DP: 0.48 g m^-2a^-1) were almost sufficient to supply the autochthonous fluxes at 30 m (PON: 2.9, PP: 0.51 g m^-2a^-1 ), and this flux of POC (26.6 g m^-2a ^-1) was about one-third of primary production (84 g C M^-2a^-1). Sedimentation of particulate matter was the main path of losing nutrients from lake water, explaining more than 80% removal of inflow loads (TN, TP). Decomposition rates during sedimentation which were calculated from the vertical difference in the autochthonous flux agreed very closely with the results obtained by laboratory experiments of a 100-day incubation (content ratios from field observations were: POC 0.67, PON 0.65, PP 0.85; and from laboratory experiments they were: POC 0.68, PON 0.70, PP 0.94). These decomposition rates and those near the sediment interface were used to explain dissolved oxygen depletion and nitrate increase in the hypolimnion during stratification. The average sinking velocities were 1.82m d^-1 for seston and 1.16 m d^-1 for Chl a at 30m, they were influenced by Chl a content of seston.     

Primary production under hypertrophic conditions and its relationship with bacterial production

In shallow hypertrophic lakes where light availability restricts the growth of macrophytes and benthic phytoplankton, pelagic phytoplankton modulates importantly ecosystem production and the energy transfer to heterotrophic bacteria. Diel and seasonal variations in primary production (PP) were studied in the hypertrophic Albufera de Valencia (Spain). Additionally, the relationship between PP and heterotrophic bacterial production (BP) was assessed. PP was extremely high, exceeding most values reported for hypertrophic lakes to date. PP displayed marked diurnal variations defined by the solar radiation curve. Likewise, PP changed importantly across seasons. Minimum PP coincided with maximum water transparency and short water residence times in winter, whereas maximum PP was observed in late spring associated with high chlorophyll a. The spring PP maximum contrasted with the summer maximum often observed in hypertrophic lakes. When compared to spring PP values, summer PP values were lower as a result of strong nitrogen limitation. In contrast to PP, BP remained fairly constant across seasons. Nonetheless, there was a joint diminution during increased water transparency followed by an increase in early spring. Phytoplankton was always the most relevant input to particulate carbon production, but the BP/PP ratio showed clear seasonal variations. The BP/PP ratio was minimum in spring, low in summer and highest in winter. The extracellular dissolved organic carbon released by phytoplankton was sufficient to meet bacterial carbon demand in all experimental dates, suggesting that allochthonous carbon sources play a minor role in sustaining BP, though they cannot be excluded. However, we hypothesize that high availability of dissolved organic carbon might explain the lack of coupling observed between BP and PP.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources oforganic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > β-glucosidase > α-glucosidase > exo-1,4-β-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and β-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/β-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). Therelative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2–3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.     

Growth, production and losses of phytoplankton in the lowland River Spree: carbon balance

1. Phytoplankton carbon assimilation and losses (exudation, dark carbon losses) as well as oxygen release and dark community respiration were measured regularly for 2 years at four stations along the lower Spree (Germany). Carbon balance of river phytoplankton was estimated using measured assimilation, metabolic losses and variations in algal carbon along a stretch of river. 2. The light/dark bottle method was modified to simulate vertical mixing. 3. Waxing and waning of phytoplankton populations dominated the load of particulate organic carbon as well as the oxygen budget of the river. 4. Phytoplankton assimilated 310–358 g C m^?2 yr^?1. A mean value of 586 mg C m^?3 day^?1 was fixed in photosynthesis, with 16.7 mg C being exuded during the day and 20.1 mg lost at night. The measured dark respiration was equivalent to only 28% of the daily gross oxygen production of the plankton community. Phytoplankton washed from upstream lakes and reservoirs was not measurably damaged by turbulent transport. 5. In spring, 18–22% of assimilated carbon was used for net biosynthesis of phytoplankton along the river course. At this time, the carbon balance of this part of the Spree was dominated by autochthonous net production. During summer, however, total carbon losses exceeded the intensive carbon assimilation. The decline of algal biomass along the river course in summer was not explicable by measurable physiological losses. The importance of sedimentation and grazing losses is discussed.     

The freshwater tidal wetland Liberty Island,CA was both a source and sink of inorganic and organic material to the San Francisco Estuary

It is hypothesized that perennial freshwater tidal wetland habitat exports inorganic and organic material needed to support the estuarine food web and to create favorable habitat for aquatic organisms in San Francisco Estuary. It is also hypothesized that most of the material flux in this river-dominated region is controlled by river flow. The production and export of material by Liberty Island were measured and compared using discrete monthly and continuous (15 min) measurements of a suite of inorganic and organic materials and flow between 2004 and 2005. Seasonal material flux was estimated from monthly discrete data for inorganic nutrients, suspended solids and salts, organic carbon and nitrogen and phytoplankton and zooplankton group carbon and chlorophyll a and pheophytin pigment. Estimates of material flux from monthly values were compared with measured daily material flux values for chlorophyll a concentration, salt and suspended solids obtained from continuous measurements (15 min) using YSI water quality sondes. Phytoplankton carbon produced within the wetland was estimated by in situ primary productivity. Most inorganic and organic materials were exported from the wetland on an annual basis, but the magnitude and direction varied seasonally. Dissolved inorganic nutrients such as nitrate, soluble phosphorus, total phosphorus and silica as well as total suspended solids were exported in the summer while total and dissolved organic carbon were exported in the winter. Salts like chloride and bromide were exported in the fall. Chlorophyll a and pheophytin were exported in the fall and associated with diatom and cyanobacteria carbon. Mesozooplankton carbon was dominated by calanoid copepods and exported most of the year except summer. Continuous sampling revealed high hourly and daily variation in chlorophyll a, salt and total suspended solids flux due to high frequency changes in concentration and tidal flow. In fact, tidal flow rather than river discharge was responsible for 90% or more of the material flux of the wetland. These studies indicate that freshwater tidal wetlands can be a source of inorganic and organic material but the export of material is highly variable spatially and temporally, varies most closely with tidal flow and requires high frequency measurements of both tidal flow and material concentration for accurate estimates.     

A study of absorption characteristics of chromophoric dissolved organic matter and particles in Lake Taihu,China

Absorptions by non-phytoplankton particles and phytoplankton, and chromophoric dissolved organic matter (CDOM) were measured at 50 sites in large, shallow, Lake Taihu in winter and summer 2006 to study their seasonal and spatial variations, and their relative contributions to total absorption. The CDOM absorption was significantly higher in winter than in summer, due to degradation and release of fixed carbon in phytoplankton and submerged aquatic vegetation (SAV). The hyperbolic model was used to model the spectral absorption of CDOM, and the mean spectral slope of 6.38 nm⁻¹ was obtained. At most sites, the spectral absorption of non-phytoplankton particles was similar to that of the total particles, demonstrating that the absorption of the total particles is dominated by the absorption of non-phytoplankton particles. In summer, phytoplankton absorption increased markedly, due to frequent algal blooms especially in Meiliang Bay. In winter, the significant increase in non-phytoplankton particle absorption resulted from the increase of inorganic particulate matter caused by sediment resuspension. Strong linear relationships were found between a _d(440) and total suspended matter (TSM), organic suspended matter (OSM), and inorganic suspended matter (ISM). Strong linear relationships were also found between a _ph(440), a _ph(675) and chlorophyll a (Chl-a) concentration. The total relative contributions of non-phytoplankton particles over the range of photosynthetically active radiation (PAR) (400–700 nm) were 48.4 and 79.9% in summer and winter respectively. Non-phytoplankton particle absorption dominated the total absorption, especially in winter, in Lake Taihu, due to frequent sediment resuspension in the large shallow lake as a result of strong windy conditions. The results indicate that strong absorption by CDOM and non-phytoplankton particles at the blue wavelength has an impact on the spectral availability, and acts as a selection factor for the composition of the phytoplankton community, with cyanobacteria being the dominate species in Lake Taihu. Handling editor: L. Naselli-Flores     

Reproductive cycle of the European clam Ruditapes decussatus from Óbidos Lagoon,Leiria, Portugal

ABSTRACT

In Portugal, the European clam (Ruditapes decussatus) is an important commercial resource. Óbidos Lagoon is a strong candidate as a cultivation area to increase European clam exploitation. However, the reproductive biology of this population has not been described. In this work, the reproductive cycle of R. decussatus was characterized by determining gonadal development stages, gonad index, condition index, and biochemical composition. The relationship between reproduction and environmental parameters (sea surface temperature, chlorophyll a, and particulate organic matter) was assessed. Ruditapes decussatus had an annual reproductive cycle. The gametogenic cycle started in late winter, and the ripe stage in spring was followed by spawning that began at the end of spring/early summer and extended until early autumn. The subsequent period of sexual rest occurred during the winter. Condition index showed seasonal variations related to food availability (chlorophyll a). The European clams in Óbidos Lagoon recovered rapidly after their reproductive period, most likely owing to the availability of food. This study will help to improve sustainable management of this wild stock and is important for future aquaculture development of this species.     

Predicting the spring algal biomass in Lough Neagh using time series analysis

1. Lough Neagh is a large eutrophic lake covering 387 km² with a mean depth of 8.9 m. It is an important natural resource, being the largest single source of potable water for Belfast, Northern Ireland. 2. This report examines the causes of the year-to-year variation in the April–June (spring) algal biomass, measured as chlorophyll a, for the period 1974–92. 3. The spring chlorophyll a declined following the introduction of a phosphorus (P) reduction programme at major sewage treatment works in 1981. However, since 1990 the chlorophyll a concentrations in the spring have increased. 4. Time series methodology was employed to develop a model which explained 76% of the annual variation in spring chlorophyll a concentrations. 5. The independent variables used in the multiple regression model were the previous year’s spring chlorophyll a concentration, soluble reactive P inputs for April–June and the particulate P concentration in the Lough during the previous summer.     

Physical and chemical limnology of 204 lakes from the Canadian Arctic Archipelago

The physical and chemical limnology of 204 lakes from across the Canadian Arctic Archipelago was examined. Mean summer air temperature did not correlate well with lake chlorophyll levels due to the predominance of ultra-oligotrophic hard-water lakes located in a polar climate. Local geology influences ion budgets and is an important factor in determining pelagic phosphorus availability, carbon cycling and metal concentrations. Ratios of particulate carbon, particulate nitrogen and chlorophyll a indicate that planktonic microorganisms are not always the major producers of organic carbon in arctic lakes. Allochthonous particulate matter contributes significantly to the carbon and phosphorus budgets of small and mid-sized lakes across the Arctic, although the availability of these elements is controlled by many interacting geochemical and biological factors. Phosphorus is generally limiting, however, increases in available phosphorus, nitrogen and carbon are all required to make significant long-term differences in lake productivity. Particulate phosphorus levels can be high in lakes where phosphorus-rich shales or carbonatite bedrock are present. These phosphorus-enriched lakes are found in several areas across the mid-arctic islands, however, only small amounts of this nutrient are available as soluble reactive phosphorus. Although lakes throughout the Arctic are typically ultra-oligotrophic, they still represent an important sink for allochthonous nutrient deposition.     

Chemical investigations of Loch Etive,Scotland. III. Participate organic carbon and particulate organic nitrogen

The concentration of the suspended matter in Loch Etive varies with river discharge, biological production, and water movement. The contribution from phytoplankton production shows a seasonal trend, with the highest values in the spring and the lowest in winter. The carbon: chlorophyll a and nitrogen: chlorophyll a ratios in the photic zone (1,4–5, and 10 m) during the period of high production ranged from 54.0 to 400.0 and 6.0 to 38.0, respectively, the autumn bloom having higher ratios than the spring bloom. The increase of these ratios in the landward direction indicates the influence of terrestrial material. The C/N ratios in the surface (1 m) of the loch throughout the year fluctuated between 6.5–20.0, being the highest in winter. The C/N ratios in the bottom layers of the Lower Basin (40–50 m), Deep Basin (115–140 m) and the Head of the Loch (25–35 m) during the year ranged between 2.25–14.0, 5.0–38.0, and 3.0–28.5, respectively. The lowest ratios were mainly found in early autumn and associated with high levels of dissolved organic nitrogen and amonium. The accumulation of suspended matter in the deeper layers of the Deep Basin was caused by the input of organic detritus from the Lower Basin and eroded sediment from the basin slope. The lateral distribution of the C/N ratios may be related to the hydrographical features of the loch.     

Shifts in allochthonous input and autochthonous production in streams along an agricultural land-use gradient

Relative contributions of allochthonous inputs and autochthonous production vary depending on terrestrial land use and biome. Terrestrially derived organic matter and in-stream primary production were measured in 12 headwater streams along an agricultural land-use gradient. Streams were examined to see how carbon (C) supply shifts from forested streams receiving primarily terrestrially derived C to agricultural streams, which may rely primarily on C derived from algal productivity. We measured allochthonous input, chlorophyll a concentration, and periphyton biomass in each stream, and whole-stream metabolism in six streams. Our results suggest a threshold between moderate- and heavy-agriculture land uses in which terrestrially derived C is replaced by in-stream algal productivity as the primary C source for aquatic consumers. A shift from allochthonous to autochthonous production was not evident in all heavy-agriculture streams, and only occurred in heavy-agriculture streams not impacted by livestock grazing. We then compared our findings to rates of allochthonous input and GPP in streams with minimal human influences in multiple biomes to assess how land-use practices influence C sources to stream ecosystems. The proportion of C derived from allochthonous versus autochthonous sources to heavy-agriculture streams was most similar to grassland and desert streams, while C sources to forested, light-, and moderate-agriculture streams were more similar to deciduous and montane coniferous forest streams. We show that C source to streams is dependent on land use, terrestrial biome, and degradation of in-stream conditions. Further, we suggest that within a biome there seems to be a compensation such that total C input is nearly equal whether it is from allochthonous or autochthonous sources.     

Allochthonous organic matter supplements and sediment transport in a polymictic reservoir determined using elemental and isotopic ratios

Because allochthonous organic matter (OM) loading supplements autochthonous OM in supporting lake and reservoir food webs, C and N elemental and isotopic ratios of sedimenting particulate OM were measured during an annual cycle in a polymictic, eutrophic reservoir. Particulate organic C and N deposition rates were greatest during winter and lowest during spring. C:N ratios decreased through our study indicating that OM largely originated from allochthonous sources in winter and autochthonous sources thereafter. δ¹³C were influenced by C₄ plant signatures and became increasingly light from winter through autumn. δ¹⁵N indirectly recorded the OM source shift through nitrate utilization degree with maximum values occurring in May as nitrate concentrations decreased. Unlike relationships from stratified systems, δ¹³C decreased with increasing algal biomass. This relationship suggests that minimal inorganic C fixation relative to supplies maintained photosynthetic isotopic discrimination during productive periods. Water column mixing likely maintained adequate inorganic C concentrations in the photic zone. Alternatively, OM isotopic composition may have been influenced by changing dissolved inorganic nutrient pools in this rapidly flushed system. δ¹⁵N also recorded increased N₂ fixation as nitrate concentrations declined through autumn. Secondary sediment transport mechanisms strongly influenced OM delivery. Particulate organic C and N deposition rates were 3× greater near the sediment-water interface. Isotopic ratio mixing models suggested that river plume sedimentation, sediment resuspension, and horizontal advection influenced excess sediment deposition with individual mechanisms being more important seasonally. Our findings suggest that allochthonous OM loading and secondarily-transported OM seasonally supplement phytoplankton production in productive reservoirs.     

The influence of floodplain habitat on the quantity and quality of riverine phytoplankton carbon produced during the flood season in San Francisco Estuary

Primary productivity, community respiration, chlorophyll a concentration, phytoplankton species composition, and environmental factors were compared in the Yolo Bypass floodplain and adjacent Sacramento River in order to determine if passage of Sacramento River through floodplain habitat enhanced the quantity and quality of phytoplankton carbon available to the aquatic food web and how primary productivity and phytoplankton species composition in these habitats were affected by environmental conditions during the flood season. Greater net primary productivity of Sacramento River water in the floodplain than the main river channel was associated with more frequent autotrophy and a higher P:R ratio, chlorophyll a concentration, and phytoplankton growth efficiency (α^B). Total irradiance and water temperature in the euphotic zone were positively correlated with net primary productivity in winter and early spring but negatively correlated with net primary productivity in the late spring and early summer in the floodplain. In contrast, net primary productivity was correlated with chlorophyll a concentration and streamflow in the Sacramento River. The flood pulse cycle was important for floodplain production because it facilitated the accumulation of chlorophyll a and wide diameter diatom and green algal cells during the drain phase. High chlorophyll a concentration and diatom and green algal biomass enabled the floodplain to export 14–37% of the combined floodplain plus river load of total, diatom and green algal biomass and wide diameter cells to the estuary downstream, even though it had only 3% of the river streamflow. The study suggested the quantity and quality of riverine phytoplankton biomass available to the aquatic food web could be enhanced by passing river water through a floodplain during the flood season.     

Dynamic status of primary production in lake yunoko,a small eutrophic subalpine lake in central Japan

Investigation was made of the primary productivity of Lake Yunoko. The concentrations of PO₄-P and NH₃-N were highest in summer. The total amount of chlorophylla in the entire water column varied from 45 mg/m² to 405 mg/m² during the year with the maximum values occurring in late spring and in December. These values are very high compared with those reported for other lakes in the same district. The standing crop of phytoplankton as organic carbon was approximately 50∼70% of the total particulate organic carbon in the lake with values as high as 80∼90% being obtained in the spring and autumn. The rate of phytosynthesis under saturated light conditions was 3.7 mg C/mg chla/hr in the case of phytoplankton of the sun type and 1.2∼1.6 mg C/mg chla/hr in those of the shade type. The daily gross primary production in the lake varied from a low of 134 mg C/m² to a high of 1,003 mg C/m² during the year, the mean value being 372 mg C/m². The total amount of daily microbial respiration varied from 177 mg C/m² in winter to 1,476 mg C/m² in summer. Thus, the daily balance of production and consumption of organic matter in the lake itself was concluded to be negative. The main factor contributing to the rectification of this imbalance of material budget seems to be the leaf litter coming from the forest surrounding the lake. In conclusion, Lake Yunoko is a very productive and at the same time, fairly heterotrophic lake. Special Project Research supported by the Ministry of Education on Dynamic State of Biosphere. This study was carried out as part of JIBP-PF.