Pelagic Sites Research Articles

Do Littoral Macrophytes Influence Crustacean Zooplankton Distribution?

In a meso-eutrophic reservoir, cladoceran and copepod assemblages were characterised in two sampling sites: One located in the pelagic zone (site 1) and the other in the shallow littoral zone (site 2), the latter colonised by emergent macrophytes. Samples were collected biweekly from June to July 2001 and from May to July 2002 at the two sites. At site 1, crustacean zooplankton samples were obtained by vertical hauls using a Wisconsin type net of 64 mm mesh size. At site 2 several random samples were obtained using a van Dorn bottle. Those samples were pooled together and the total sample was sieved through a 64 mm mesh size. Macrophyte relative abundance for each species was visually estimated. Macrophyte community was composed of Glyceria declinata, Eleocharis palustris and Carex sp. A Mann-Whitney U-test was carried out to test for statistically significant differences between sites, for environmental parameters and crustacean zooplankton species densities. Alona rectangula, Alona costata, Alona quadrangularis and Chydorus sphaericus were only found in littoral samples. Daphnia longispina, Daphnia pulex, Ceriodaphnia pulchella, Bosmina longirostris, Diaphanosoma brachyurum, Copidodiaptomus numidicus, and Acanthocyclops robustus were found at both sites. However, Daphnia and Bosmina densities did not differ significantly between the pelagic and littoral sites. Ceriodaphnia, Diaphanosoma, and C. numidicus densities were significantly higher in the pelagic site, whereas A. robustus densities showed the opposite pattern. Key-words: Zooplankton, Cladocera, Copepoda, reservoir, macrophytes, pelagic zone, littoral zone

Partitioning of dissolved thallium by seston in Lakes Erie and Ontario

The fate of thallium (Tl) was traced in the epipelagic and coastal waters of Lake Erie (in 2000) and Lake Ontario (in 2001) sampled during the summer period of thermal stratification. Fractionation of 204Tl bound to particulate matter was conducted to trace the scavenging potential of various seston size ranges (0.2–2 µm, 2–20 µm, and 20–210 µm,). Added [204Tl]-Tl(I) scavenged by seston over 1–3 days was 0.34–4.05 % in Lake Erie and 0.09–0.21 % in Lake Ontario, where scavenging was correlated with suspended particulate matter content and chlorophyll a concentration. There was no consistent trend with respect to Tl scavenging by a particular size fraction, except at coastal sites in Lake Erie where most Tl was scavenged by the 2–20 µm seston. Partition coefficients (Kd) of Tl scavenging by seston in Lake Ontario were 938–1483 L·kg–1. At the same pelagic site in Lake Ontario, [204Tl]-dimethylthallium partitioned into seston (Kd = 807 ± 258) to a lesser degree than added Tl(I) (Kd = 998 ± 297), a finding consistent with laboratory assays of these two Tl species. These results suggest that the plankton food web plays a role in the geochemical cycle of this toxic metal in the water column of the lower Great Lakes.

On the harmonic constants of tides and tidal currents of the South-eastern brazilian shelf

Series of sea-level heights from three permanent coastal stations, and sea-surface heights and currents from three non-permanent pelagic sites, in the South-eastern Brazilian shelf, were subjected to tidal analysis by the harmonic and response methods, in order to determine their tidal constants. Semi-diurnal heights constituents M 2 and S 2 have a clockwise propagation in the southern part of the area and a counterclockwise one in the northern part. Their amplitudes show a general increase from east to west, being higher near the port of Paranaguá, and then decrease further south. Among the diurnals, Q 1 and O 1, propagate from west to east in a clockwise propagation, while the K 1 and P 1 propagations approximately follow the semi-diurnals components and also have higher amplitudes near Paranaguá Cananéia, propagating clockwise in the southern part of the area and counterclockwise in the northernmost one. The M 3 has higher amplitudes, as the semi-diurnals, along the Paranaguá shelf compared to the other stations. The phases of M 3 from the shelf values also seem to indicate a double sense of propagation in the area. The diurnal current components O 1 and K 1 have greater amplitudes at the surface layers (major axes of tidal ellipses with 7.5 and 11.2 cm/s in the northern part of the shelf) and tend to become smaller with depth, with their ellipses axes rotating counterclockwise from surface to the bottom. The semi-diurnal current components M 2 and S 2 have, in each station, approximately similar amplitudes and nearly the same ellipses orientation, along the vertical profiles (major axes at the surface with 7.3 and 4.1 cm/s in the southern part of the shelf); these features are also followed by M 3 (this current constituent was quite uniform in the whole area and only the ellipses orientation varied significantly). Most of the tidal currents are counterclockwise from surface to the bottom and are seen to be affected by the bathymetry and displacements of water masses present in the area–shelf water (SW) at the surface, tropical water (TW) at mid column and sub-tropical water (STW) at the bottom layer. Prevailing winds and inertial motions also affect the constituents of tides and tidal currents in the study area.

Seasonal and spatial variation in algal growth potential and growth‐limiting nutrients in a shallow endorheic lake: Lake Pátzcuaro (Mexico)

AbstractThe present research focused on the functional role of the phytoplankton of an economically important endorheic tropical lake from the perspective of algal growth bioassays. The algal growth potential of the lake water was compared for littoral and pelagic sites during the wet and dry seasons. Algal growth potentials at open waters reached minimum and maximum values following the seasonal alternation of dilution (by rain) and concentration (by evaporation) respectively. Conversely, at southern littoral stations high algal growth responses related to the availability of nutrients from point contamination sites. There was no such effect at nearby offshore sites or elsewhere in the lake which suggests filtration and competitive interactions for these nutrients between phytoplankton and littoral macrophytes. Nitrogen and phosphorus both acted consistently as limiting nutrients at open waters by colimitation. Nitrogen to phosphorus ratios seemed to approach equilibrium where limitation easily shifted to one element or the other temporally and spatially.

Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous–Tertiary boundary: constraints on Re-PGE transport in the marine environment

The nature of Re–platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous–Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ∼1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ∼95% relative to chondritic Ir proportions. A similar depletion in Os (∼90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ∼1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ∼65 Ma, the effective diffusivities are ∼10 −13 cm 2/s, much smaller than that of soluble cations in pore waters (∼10 −6 cm 2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios ≥1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ∼25% of the K-T impactor’s Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.

Effects of zooplankton on the sinking flux of organic carbon in Lake Biwa

To clarify the roles of zooplankton in the sedimentation of seston from the epilimnion, the sinking flux of particulate carbon was measured along with primary production rate and zooplankton biomass from July 1996 to October 1997 at a pelagic site in the north basin of Lake Biwa. During the study period, the flux varied seasonally from 66 to 510 mg C m−2 day−1 and was low in summer when zooplankton, composed mainly of Eodiaptomus japonicus and Daphnia galeata, were abundant. Simple correlation analysis revealed that the sinking flux correlated neither with the primary production rate nor with the amount of sestonic carbon above the sediment trap. However, the particle elimination rate, estimated as the difference between the primary production rate and the sinking flux, correlated positively with the zooplankton biomass. These results suggest that zooplankton play a substantial role in decreasing the sinking flux in Lake Biwa.

Relative importance of biotic and abiotic factors affecting bacterial abundance in Lake Biwa: an empirical analysis

To determine the relative importance of factors affecting bacterial abundance in Lake Biwa, correlation and multiple regression analyses were performed with relevant biotic and abiotic variables. Data used in the analyses were collected weekly from April 1997 to June 1998 at a pelagic site in the north basin. The bacterial abundance ranged from 1 to 7 × 106 cells ml−1, and its spatio-temporal pattern was virtually identical to that in previous studies conducted 12–15 years ago. In the surface layer (0–12.5 m), bacterial abundance was significantly correlated with water temperature and with protozoan and metazoan grazers, but not with chlorophyll a and nutrient concentrations. The results suggest that loss factors rather than growth factors are more important in determining bacterial abundance in this lake. However, grazing effects on bacterial abundance differed among zooplankton. Bacterial abundance correlated negatively with phagotrophic nanoflagellates (PNF) and Daphnia, but positively with Eodiaptomus. Thus, PNF and Daphnia act to reduce the bacterial abundance, while Eodiaptomus acts to stimulate. In contrast, these biotic factors did not explain changes in bacterial abundance in the middle (12.5–25 m) and deep (>25 m) layers. Instead, the bacterial abundance in the deep layer was highly correlated with vertical mixing regimes, suggesting that bacterial abundance was directly or indirectly affected by abiotic factors. These results indicate that bacterial abundance in Lake Biwa was regulated by different factors at different depths.

Isotope analyses of molecular and total organic carbon from miocene sediments

Carbon-isotope compositions of n-alkanes, pristane and phytane, and total organic carbon were measured and compared against isotopic trends of coeval alkadienones from Miocene sediments containing very low organic-carbon contents. Compound-specific isotope analysis of n-alkanes and isoprenoid lipids, in conjunction with abundance distributions of n-alkanes reveal the influence of terrestrially derived organic carbon at all sites analyzed. In general, n-alkanes are derived from allochthonous sources with the exception of n-C 37 from site 516, which appears genetically related to coeval alkadienones. Further, pristane and phytane from pelagic sites 608 and 516 apparently derive from terrestrial sources as well, although a marine origin cannot be excluded. δ TOC values lack a coherent relationship to %TOC and δ 13C 37:2. Differential alteration and mixing of diverse isotopic signals most likely contribute to temporal variation and spatial differences in δ TOC. Therefore, when working with sediments from oligotrophic settings, we do not recommend δ TOC as an indicator of phytoplankton δ 13C values.

Pelagic productivity changes in the equatorial area of the northwest Indian Ocean during the last 400,000 years

Major and selected trace element (Al, Fe, Mg, K, Ba, P, Mn, V, Cu, Ni, and Ti) records were analyzed in two cores recovered from the northwestern Indian Ocean (core MD 90940 on the Madingley Rise and core MD 85668 in the Somali Basin) in order to monitor the sedimentary response to pelagic productivity in this area during the last 400,000 years. Geochemical proxies such as bulk mass accumulation rate (MAR), Al-normalized major and trace element contents, major and trace element MARs were compared to biologic markers of palaeoproductivity: a new radiolarian index (rad ratio), and the variations of foraminiferal and radiolarian assemblages. Special attention being paid to the barium record, a sequential leaching procedure determined the main Ba-carrier fraction in the sediment, and, thus, the amount of biogenic barium. The `blue ocean' sedimentation on the Madingley Rise (core MD 90940) is characterized by a low MAR and an insignificant terrigenous input. The bulk MAR variations are directly related to the variations of the rad ratio, reflecting a direct link between surface productivity and pelagic sedimentation at this site. In this sediment, barium is mainly found as barite and can be considered as a palaeoproductivity proxy. In the Somali Basin (core MD 85668), the terrigenous inputs are relatively higher, but the bulk MAR variations are still related to the variations of surface productivity. No barite was found and barium is mainly linked to the terrigenous input. A comparison of the two pelagic sites of the Madingley Rise and the Somali Basin points out the differences between the barium records at both sites. Despite the location of both cores in the pelagic domain, the geochemical record, and especially the barium one, is different from one core to the other. The comparison of the variations of the bulk MAR, which reflects the variations of the surface productivity, presents significant discrepancies between both sites. In the `blue ocean' (core MD 90940), the productivity peak values are mainly recorded during glacial intervals, but their maximum values are observed during interglacial isotope Stage 9. In the Somali Basin (core MD 85668), high bulk MAR periods occurred during glacial isotope stages. The interglacial episode of high productivity recorded on the Madingley Rise, the absence of significant variations of productivity at the 8/7 transition in the Somali basin, as well as the absence of a time lag between both records indicate that the surface productivity is, in this area, controlled more by the local variations of the palaeoceanographic patterns than the global climatic changes, e.g. the glacial/interglacial alternations.

Light availability as a possible regulator of cyanobacteria species composition in a shallow subtropical lake

1. Variations in the relative biovolumes of dominant cyanobacterial taxa were evaluated in the context of environmental conditions using canonical correlation analysis (CCorrA) and Redundancy Analysis (RDA). The objective was to test a conceptual model in which underwater irradiance determines dominance by bloom‐forming (high light adapted) or non‐blooming (low light adapted) taxa.2. The data set consisted of 404 contiguous observations, collected over a 3‐year period at eight pelagic sites, in shallow Lake Okeechobee, Florida, U.S.A. Data included species biovolumes, total phosphorus (TP), total nitrogen (TN), dissolved oxygen (DO) and chlorophyll a concentrations, as well as two indices: underwater irradiance (Secchi depth) and the ratio of Secchi:total depth.3. The first environmental canonical variable was strongly correlated with the two light‐related indices, and negatively correlated with TP. This reflects the predominant role of resuspended P‐rich lake sediments in controlling underwater irradiance in the shallow lake. The first species canonical variable displayed a strong negative correlation with Lyngbya limnetica and L. contorta, and positive correlations with Anabaena circinalis, Aphanizomenon flos aquae and Microcystis spp. The results support the conceptual model; the first pair of canonical variables explained 55% of the variation in the species–environmental data set. RDA results provided further support for the hypothesis that irradiance was the major force controlling community structure.4. One unexpected result was a positive association between Oscillatoria spp. dominance and indicators of high irradiance. This conflicts with past research indicating that Oscillatoria is a low light adapted taxon, and the finding that it is the most abundant taxon in Lake Okeechobee. This may reflect the fact that the two Lyngbya taxa were more strongly associated with low light conditions than Oscillatoria. CCorrA results indicated that Oscillatoria densities are strongly controlled by water temperature. There is a need for more detailed studies of cyanobacteria ecophysiology in order to explain fully the seasonality of phytoplankton in this and other shallow subtropical lakes.

Size Structure and Energetics in a Plankton Food Web

Studies were conducted to quantify how the size distribution of plankton affects ecological transfer efficiency in the pelagic food web. Simultaneous measurements of phytoplankton and zooplankton size (equivalent spherical diameter, ESD), biomass, and productivity were determined from bimonthly samples collected over a 13-month period at four pelagic sites in a large subtropical lake. Plankton ESD values were compared statistically with ratios of zooplankton:phytoplankton biomass and productivity, which served as two indices of ecological transfer efficiency. Communities dominated by large phytoplankton (generally filaments or colonies of cyanobacteria) and/or small-bodied zooplankton (rotifers and nauplii) displayed lower ratios of zooplankton:phytoplankton biomass than communities with smaller phytoplankton (nanoflagellates) and/or larger zooplankton (copepods and cladocerans). This pattern was robust, and was apparent both on inspection of data from individual sampling sites, and when all data were combined. Productivity ratios displayed the same general relationships with phyto- and zooplankton ESD, although those patterns were evident only when data were examined on a site-by-site basis. Overall, the results supported the hypothesis that when the pelagic community is dominated by large phytoplankton and small zooplankton, only a small fraction of the carbon fixed by photosynthesis is passed directly to the herbivore trophic level. At such times, alternative routes for carbon flow may predominate, including bacterial-based pathways with protozoa as intermediate consumers, or direct links from phytoplankton to fish.

Formation of macroscopic organic aggregates (lake snow) in a large lake: The significance of transparent exopolymer particles, plankton, and zooplankton

Macroscopic organic aggregates (lake snow) were collected and their abundance quantified by scuba divers at a distinct pelagic site in Lake Constance (Germany) at least twice a week throughout the growing season in 1993. Furthermore, concentrations of transparent exopolymer particles (TEP), chlorophyll, particulate organic carbon (POC), and the species composition of phytoplankton and zooplankton in the ambient water were determined. In addition, the formation of aggregates was studied in laboratory experiments by incubating water samples in rolling tanks. The abundance and composition of aggregates showed a pronounced seasonal and vertical pattern in close relation to phytoplankton and zooplankton dynamics and wind conditions. Numbers of aggregates ranged between &lt;1 and 50 liter−1, with maximum concentrations at 6 m depth in summer and fall. In spring, the peak of TEP concentration occurred simultaneously with that of lake snow, whereas the peaks in summer and fall followed those of aggregate abundance. More detailed analyses indicated that the role of TEP in aggregate formation varies throughout the season. The laboratory experiments revealed an inverse correlation between time of aggregate formation and the concentration of POC. Two different correlations, one for the spring bloom and one for the rest of the year, were established. The results show that multiple mechanisms of aggregate formation occurred throughout the seasons due to variations in source particles and physical conditions.

Seasonal incidence of some heterotrophic aerobic bacteria in Bahrain Pelagic and Nearshore waters and oysters

Concentrations of four bacterial groups namely coliforms, Vibrio cholerae, V. parahaemolyticus and Faecal streptococci in sea water and oyster (Pinctada radiata) were investigated from March 1991 to March 1992 at nearshore site and a pelagic site. In seawater, coliforms were isolated in high numbers during September‐October at both sites, while in oyster coliforms were higher than those in water all through the study period, with no definite pattern. Both V. cholerae and V. parahaemolyticus were present in seawater and oysters during the warmer months (June, July, September and October). In oysters, V. cholerae was more prominent than V. parahaemolyticus. Faecal streptococci were detected mainly in waters of both sites during September and October. Peak counts in oysters at both sites occurred in July; and they were five times greater at the pelagic site. In general, most bacteria were isolated from the pelagic waters and oyster samples, and they were highest during the warmer months. Some disappeared totally during February and March. Water temperature was found to have greater influence on distribution of different bacteria in water and oysters, compared with other environmental factors.

Ocean tides under the Filchner-Ronne Ice Shelf, Antarctica

A depth-averaged finite-difference numerical model has been used to make a preliminary study of the tides under the Filchner–Ronne Ice Shelf. Open boundary conditions were specified using the global ocean model of Schwiderski. Tidal constituents for the two principal semi-diurnal constituents M2 and S2, and the two principal diurnal constituents Ο1 and K1 were extracted from computed sea-surface elevations by harmonic analysis. Measured values near to the grounding line could only be reproduced satisfactorily by increasing the bottom friction coefficient under the ice to 50 times the open-ocean value. This destroys any agreement near the ice front or at pelagic sites. It is thought that a friction coefficient which varies with distance under the ice would be able to reproduce better all the available measurements. More tidal measurements are required to validate any model of the region with model experiments being used to help pinpoint possible sites for instrument deployment.

Ocean tides under the Filchner-Ronne Ice Shelf, Antarctica

A depth-averaged finite-difference numerical model has been used to make a preliminary study of the tides under the Filchner–Ronne Ice Shelf. Open boundary conditions were specified using the global ocean model of Schwiderski. Tidal constituents for the two principal semi-diurnal constituents M2 and S2, and the two principal diurnal constituents Ο1 and K1 were extracted from computed sea-surface elevations by harmonic analysis. Measured values near to the grounding line could only be reproduced satisfactorily by increasing the bottom friction coefficient under the ice to 50 times the open-ocean value. This destroys any agreement near the ice front or at pelagic sites. It is thought that a friction coefficient which varies with distance under the ice would be able to reproduce better all the available measurements. More tidal measurements are required to validate any model of the region with model experiments being used to help pinpoint possible sites for instrument deployment.

Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography

Fine sediment size (&lt;63 µm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 µm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10‐µm grain size, and noncohesive above that size. Thus silt coarser than 10 µm displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 µm behaves in the same way as clay (&lt;2 µm). Useful parameters of the distribution are the 10–63 µm mean size and the percentage 10–63 µm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, KM), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface KE of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar Current, and Brazil/Falkland currents in the Argentine Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both coccoliths and foraminiferal fragments can be recognized and analyzed, with the boundary between them again at about 10 µm. The flux of less than 10 µm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.

Sulfate reduction and diffusion in sediments of Little Rock Lake, Wisconsin

Rates of sulfate diffusion and reduction were measured in sediments of Little Rock Lake, an oligotrophic, soft‐water lake in northern Wisconsin. Laboratory measurements of kinetics of sulfate reduction found half‐saturation constants (20–30 µmol liter−1) and Q10 values (2.6) similar to values reported in the literature. Sulfate reduction under in situ conditions in sediment cores was limited by sulfate and followed similar uptake kinetics as in laboratory experiments. Some variation in kinetic parameters was evident as a function of location in the lake. No seasonal variation was observed in sulfate reduction rates in the lake sediments, and littoral and pelagic sites exhibited similar rates. Rates of sulfate reduction were much higher than fluxes of sulfate calculated from pore‐water profiles. Pore‐water profiles also indicated little difference in diffusive fluxes among pelagic and littoral sites and among seasons. The discrepancy between diffusive fluxes and sulfate reduction rates is ascribed to high rates of oxidation of reduced sulfur. Nonlinear rates of sulfate reduction and calculated turnover times of sediment sulfide pools support the hypothesis that sulfide oxidation occurs nearly as rapidly as sulfate reduction.

Induction of Anabaena Bloom by Nutrient Enrichment to the Lake Water Collected from the South Basin of Lake Biwa.

Lake water collected in mid-August 1991 at a pelagic site in the south basin of Lake Biwa was poured into 2 1 flasks, enriched with various nutrients (control, phosphate, phosphate plus Fe-EDTA, and phosphate plus nitrate), and incubated at 27°C under continuous light (ca. 2, 500 lux) for 26 days in the laboratory.In all treatments except P+N, Anabaena affinis, which was scarce (50 cells-ml⋅-1) in the original lake water, became predominant, but final cell densities of A. affinis in P and P+Fe-EDTA were several times higher than that in the control. In contrast, several green algae such as Scenedesmus spp., Coelastrum cambricum, C. microporum, Pediastrum duplex and Gloeocystis gigas became predominant in the N+P treatment (at a N: P supply ratio of 20: 1, by weight). These results support the hypothesis that a low nitrogen: phosphorus ratio of lake water is an important factor causing the appearance of Anabaena bloom in the south basin of Lake Biwa.

Changes in Bacterioplankton Production and Dominant Algal Species in the North Basin of Lake Biwa.

Production of bacterioplankton was studied together with phytoplankton composition at a pelagic site in the north basin of Lake Biwa from 19 February to 7 June 1990. During the investigation period, two species of phytoplankton predominated: Asterionella formosa (Bacillariophyceae) from 19 February to 18 April (the early period) and Uroglena americana (Chrysophyceae) from 9 May to 7 June (the late period). Bacterioplankton production estimated by the frequency of dividing cells method remained at a high level during the early period, ranging from 7.4 to 33μgC⋅l-1⋅d-1. During the late period, bacterioplankton production was low, ranging from 4.1 to 18μgC⋅l-1⋅d-1. The change in bacterioplankton production appeared to relate to the change in phytoplankton composition.

Early diagenesis in differing depositional environments: The response of transition metals in pore water

The cycling of Fe, Mn, Ni, Co, Cu, Cr, V, and Mo during early diagenesis was investigated in sediments from five different depositional environments in the California Borderland. Dissolved O 2, NO 3 −, NO 2 −, and SO 4 − were also measured at each site to establish the position of redox boundaries pertinent to this study. Sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals: bottom water O 2 concentration; sediment redox conditions; cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux. At 10 μM oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments, differing only in scale. Under these conditions the recycling of Mn oxides resulted in an enriched layer near the interface. At bottom water 0 2 concentrations < 5 μM Mn oxides are not recycled. Under these conditions Fe oxides and SO 4 2− are important oxidants. The depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides. Trapping and recycling of Ni and Co with Mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments. Scavenging of these metals is favored by a deep oxic zone in the sediments. In contrast, Cr, V, and Mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation. The source of the reduced species is assumed to be biogenic material. The accumulation of these metals is favored by reducing conditions in the sediments. Cu is enriched in the sediments by transport with detrital biogenic material, followed by adsorption onto sediment solids. Cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site, at the base of the slope. The magnitude of Cu released in the slowly accumulating pelagic sediment exceeds the Cu binding capacity of the solids, resulting in pore water Cu concentrations in excess of 100 nM. In general, metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study.