Showing papers in "Limnology and Oceanography in 1990"
TL;DR: In this article, a simple spectral atmospheric radiative transfer model for oceanographic applications is proposed, which is based on spectral extraterrestrial solar u-radiance corrected for earth-sun orbital distance.
Abstract: A simple spectral atmospheric radiative transfer model specific for oceanographic applications begins with spectral extraterrestrial solar u-radiance corrected for earth-sun orbital distance. Irradiance is then attenuated in passing through the atmosphere by Rayleigh scattering, ozone, oxygen, and water vapor absorption, and marine aerosol scattering and absorption, and is finally reduced by reflectance at the air-sea interface. The model is an extension of the continental aerosol model of Bird and Riordan, modified to include maritime aerosol properties, irradiance transmittance through the air-sea interface, and atmospheric absorption at very high spectral resolution (1 nm). Atmospheric optical constituents and the surface reflectance are functions of the local meteorological conditions, imparting flexibility to the model to reproduce the spectral ii-radiance under a variety of maritime atmospheres. The model computes ii-radiance at or just below the ocean surface at high spectral resolution in the range 350-700 nm, i.e. within the range required for photosynthetically available radiation (PAR) calculations. It agrees spectrally with observed surface spectral irradiances to within f 6.6% (rms) and as integrated PAR to within + 5.1%. The computed spectral u-radiance is useful as an input to bio-optical models in the ocean, to phytoplankton growth and primary production models, and in remote-sensing applications. Increasingly sophisticated phytoplankton growth and primary production models (e.g. Platt 1986), ecosystem simulation models (e.g. Walsh et al. 1988), and bio-optical models (e.g. Carder and Steward 1985; Gordon et al. 1988b) have great potential for increasing the understanding of phytoplankton dynamics, the magnitude of oceanic primary production, and the fate of light in the sea. Use of these models to assess phytoplankton growth as a function of the I Present address: Research and Data Systems Corp., 7855 Walker Drive, Suite 460, Greenbelt, Maryland 20770. Acknowledgments We thank T. G. Peacock and R. G. Steward at the University of South Florida for help in obtaining the spectral irradiance observations. We also thank three anonymous reviewers for their helpful comments. This work was supported by NASA grant NAGW465 and ONR grant NO00 14-89-J109 1. availability of light at depth and the distribution of optical constituents requires information on light at the surface, which requires measurements or estimates of the surface light field. Furthermore, recent advances in understanding the spectral character of light have suggested its importance in the absorption of light by phytoplankton (Sathyendranath et al. 1987), its role in primary production (Laws et al. 1990), and its effect on incubation methods for determining in situ primary production (Grande et al. 1989), in contrast to the conventionally used photosynthetically available radiation (PAR). Laws et al. (1990) have demonstrated for the oligotrophic ocean that ignoring the spectral character of light available for absorption by phytoplankton can result in underestimates of primary production rates by more than a factor of two. Until recently,
623 citations
TL;DR: In this article, a 2-yr period was characterized by a midsummer maximum in NH, + efflux to the overlying water and a May peak in NO, removal from water by sediments.
Abstract: Contemporaneous measurements are reported for nitrification, denitrification, and net sedimentwater fluxes of NH,+ and N03- in the mesohaline region of Chesapeake Bay. Seasonal cycles over a 2-yr period were characterized by a midsummer maximum in NH, + efflux to the overlying water and a May peak in NO,-. removal from water by sediments. Coherent temporal patterns for nitrification and denitrification were observed, with relatively high values in spring and fall and virtual elimination of both processes in summer. Indirect measurements indicate that nitrification was limited by the shallow 0, penetration (< 1 mm) here compared to reports for other marine sediments (2-6 mm). In addition, a strong positive correlation between the two processes suggested that denitrification was generally controlled by nitrification. Comparisons of NO,- fluxes and net nitrification rates (nitrification minus N03- reduction to NH,+) revealed that measurements of denitrification with the acetylene block method systematically underestimated actual rates. Rates of N, loss in denitrification were similar to NH,+ recycling fluxes to the overlying water in spring and fall, but in summer negligible denitrification contributed to enhanced NH,+ recycling. These results suggest that inhibition of denitrification in eutrophic estuaries such as Chesapeake Bay may reinforce the effects of nutrient enrichment by allowing increased rates of NH,’ recycling.
476 citations
TL;DR: The average export of total organic carbon (TOC) was 36.1 Tg yr-i (8.5 g m-2 yr-I), of which 62% was DOC, 34% was FPOC, and 4% was CPOC.
Abstract: Depth-integrated, discharge-weighted water samples were collected over 1,800km ofthe Amazon River on eight cruises at different stages of the hydrograph, 1982-1984. Fine (FPOC, 163 pm) and coarse (CPOC, > 63 rm) particulate organic carbon as weight percentage of suspended sediment varied between 0.9-1.5% for FPOC and 0.5-3.49/o for CPOC. Concentrations of FPOC ranged from 5 mg liter-’ upriver to 2 mg liter-’ downriver in the mainstem and from 6 mg liter-’ inthe Rio Madeira to i 1 in the Rio Negro. CPOC had similar distribution patterns. but with concentrations 15.-30% those of FPOC. Dissolved organic carbon (DOC) averaged 4-6 mg liter-r in the mainstem and up to 12 ml; liter -I in the Rio Negro. Upriver dissolved inorganic carbon (DIC) concentrations of about 1,200 @LM were diluted by tributaries and floodplain drainage to 600 PM at the most downriver site. Evasion ofCO,, invasion of O,, and in situ oxidation were of comparable magnitude, 3-8 pmol m-2 s-r. The average export of total organic carbon (TOC) was 36.1 Tg yr-i (8.5 g m-2 yr-I), of which 62% was DOC, 34% was FPOC, and 4% was CPOC. TOC inputs were insufficient to support in situ oxidation by a factor of at least two. A relatively small, rapidly cycling pool of labile organic matter may coexist with a much larger pool of more refractory material.
446 citations
TL;DR: Iron uptake rates are controlled by the complex formation kinetics of the transport site with monomeric ferric hydroxide species and are consistent with a standard model for carrier-mediated transport.
Abstract: .4bstract Iron is taken up via specific sites on the cell surface in two coastal phytoplankters-the coccolithophorid Pleurochrysis carterae and the diatom Thalassiosira weissflogii. Direct uptake from the cell surface of - 1 O-l7 mol Fe cell-l was observed in pulse-chase experiments with cells grown under IFe limitation. This quantity corresponds to the theoretical number of transport sites needed by each cell given the slow coordination kinetics and the low seawater concentrations of Fe. The observed turnover time of Fe in the sites---20 min-the observed half-saturation constants for uptake-O.7 and 3.1 nM for P. carterae and T. weissflogii-and maximal Fe complexation rates are consistent with a standard model for carrier-mediated transport. Analysis of the reaction kinetics mdicates that the sites are not at equilibrium with dissolved Fe species. Iron uptake rates are thus controlled by the complex formation kinetics of the transport site with monomeric ferric hydroxide species. Fe-limited P. carterae and T. weissjlogii can take up Fe at rates up to 14-20% of the maximum rate at which dissolved inorganic Fe can diffuse to the cell. Since oceanic phytoplankters must also be subject to the same strongly size-dependent limit on uptake rates imposed by diffision, large species are more likely to experience growth rate limitation under low-Fe conditions in the ocean.
444 citations
TL;DR: Hydrogen sulfide suppressed the activity of alcohol dehydrogenase (ADH), the enzyme that catalyzes the terminal step in alcoholic fermentation, in the roots of two wetland macrophytes, lending support to the hypotheses that ADH activity, as a mcasurc of fermcntative metabolism, is important in maintaining the root energy status of wetland plants under hypoxic-anoxic conditions.
Abstract: Hydrogen sulfide, a phytotoxin that often accumulates in anoxic marine and freshwater marsh soils, suppressed the activity of alcohol dehydrogenase (ADH), the enzyme that catalyzes the terminal step in alcoholic fermentation, in the roots of two wetland macrophytes. This inhibition of root ADH activity with increasing sulfide concentration was associated with decreases in root total adenine nucleotide pool (ATP + ADP + AMP), the adenylate energy charge ratio (AEC), nitrogen uptake (percent recovery of rSNH,+-N) and growth (leaf elongation). These responses were species-specific with a greater negative impact in the freshwater marsh species that naturally inhabits low-sulfide environments. These findings lend support to the hypotheses that ADH activity, as a mcasurc of fermcntative metabolism, is important in maintaining the root energy status of wetland plants under hypoxic-anoxic conditions, that there is a significant negative effect of H,S on the anoxic production of energy in these roots, and that an important negative effect of H,S on plant growth is an inhibition of the energy-dependent process of N uptake.
382 citations
TL;DR: A distinct diel pattern in forward-angle light scatter was observed in cells in the mixed layer over vast regions, which suggests that mixing rates in these areas are slow relative to the abilities of the cells to photoacclimate.
Abstract: Dual-beam flow cytometry was used to analyze the distribution and optical characteristics of Synechococcus in the North Atlantic and Pacific Oceans. The depth range over which Synechococcus cells were abundant was related to the depth of the nitrite maximum and the chlorophyll maximum, but was not significantly correlated with the depth of the surface isothermal layer. Dual-beam analysis of chromophore pigment types revealed that the majority of the populations were of the high-urobilin type; low-urobilin types, similar to the isolate WH7803, were found only in coastal waters where they almost always co-occurred with high-urobilin strains. Phycoerythrin fluorescence intensity per cell increased dramatically with depth in the lower euphotic zone at all stations; at some open-ocean stations, very deep cells were as much as 100 times brighter than those at the surface. The maximal fluorescence intensity per cell was about the same at the coastal and oceanic stations, and the depth of maximal fluorescence was closely related to the depth of the nitrite maximum. At most stations, fluorescence per cell was constant throughout the mixed layer, but at some open-ocean stations it decreased continuously to the surface. The latter pattern suggests that mixing rates in these areas are slow relative to the abilities of the cells to photoacclimate. A distinct diel pattern in forward-angle light scatter was observed in cells in the mixed layer over vast regions, which we hypothesize to be coupled to growth of the cells during daylight hours.
346 citations
TL;DR: Surface topography increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.
Abstract: Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.
345 citations
TL;DR: POd3- depletion and subsequent sedimentation was most likely the prime cause for the termination of the spring bloom and increase of heterotrophic flagellate populations was prevented by grazing within the microbial loop.
Abstract: Abundance, biomass, production, and grazing loss rates of phytoplankton, free-living bacteria, and Protozoa were assessed during an intense spring phytoplankton bloom in prealpine Lake Constance (Bodensee). More than 50% of the primary production was channeled through the microbial loop. Bacteria and ciliates responded rapidly to increasing phytoplankton biomass and production. Maximal growth rates of bacteria and Protozoa were slightly lower than those of the dominating phytoplankton species. Averaged over the spring bloom, bacterial C amounted to 2 1% of phytoplankton C, bacterial production to 18% of particulate primary production. Increase of heterotrophic flagellate populations was prevented by grazing within the microbial loop, probably mainly by the feeding impact of ciliates. Although ciliates controlled flagellate production, they satisfied their food demand primarily by feeding on algae and consumed - 14% of primary pro- duction. Metazoan microzooplankton (copepod nauplii and rotifers) removed -7% of the phy- toplankton production. Herbivorous metazooplankton ingested slightly less. Therefore, the phy- toplankton bloom was not terminated by excess grazing. POd3- depletion and subsequent sedimentation was most likely the prime cause for the termination of the spring bloom.
276 citations
TL;DR: Three different types of iron oxo-hydroxo colloids with well-defined sizes and chemistries have been synthesized by hydrolysis of Fe(III) in acidic and alkaline medium, and thermal (dark) dissolution of the colloid appears sufficient to satisfy the iron requirement of the diatom.
Abstract: Three different types of iron oxo-hydroxo colloids with well-defined sizes and chemistries have been synthesized by hydrolysis of Fe(III) in acidic and alkaline medium as well as by oxidative hydrolysis of Fe(U), resulting in solutions of &FeOOH (30 nm), a hexanuclear complex (HN, 2 nm), and a polynuclear oxidation product (OP, 3 nm). These colloids were tested for their availability to the coastal diatom Thalussiosiru weissflogii. Iron-limited cells were grown in synthetic seawater containing O-O.5 PM of total iron in colloidal form as the only iron source. We found no evidence of direct uptake of colloids by the algae. The P-FeOOH colloids, which dissolve exceedingly slowly in the light or in the dark, did not support diatom growth. In contrast, maximal growth rates and cell yields were observed in the presence of either 50 nM OP or HN. Photoreduction accounted for the availability of HN. In the case of OP, thermal (dark) dissolution of the colloid appears sufficient to satisfy the iron requirement of the diatom.
270 citations
TL;DR: In this paper, a simple model for the depth-averaged suspended sediment concentration based on surface wave height was calibrated with about 10 h of data collected during one storm event and verified against 15 d of collected at the same site.
Abstract: Field experiments were conducted in Lake Balaton, a large (surface area, 600 km2) but shallow (mean depth, 3.2 m) lake in Hungary, to quantify the resuspension and deposition of bottom sediment due to episodic storm events. Measurements were made of windspeed and direction, surface waves, mean water velocity, and suspended sediment concentration. During significant wind events, the computed bottom stress due to surface waves dominated that due to the mean current, and therefore surface waves were assumed to be the major cause of sediment resuspension. A simple model for the depth-averaged suspended sediment concentration based on surface wave height was calibrated with about 10 h of data collected during one storm event and verified against 15 d of data collected at the same site. The success of the suspended sediment model, which assumes that the bottom sediment was noncohesive, is surprising since the bottom material was composed predominantly of sediment in the clay and fine-silt size ranges. This fit may indicate the presence of a thin surface layer of loosely bound sediment that is continuously involved in resuspension. The suspended sediment model could easily be integrated into a water quality model (e.g. to predict light attenuation), provided that lateral transport is negligible, or it could be used to provide the bottom boundary condition for a more general suspended sediment transport model in which advective transport is included. Due to their small fall velocities, finegrained particles (i.e. those in the silt and clay size ranges) are transported easily by flows. An understanding of the dynamic behavior of these particles is particularly important in shallow lakes and estuaries since there they may repeatedly settle to the bot
258 citations
TL;DR: In this paper, seasonal and die-1 changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH,+ on NO, assimilation, and the results showed that NO, was more important as a source of N in spring than in summer.
Abstract: Seasonal and die1 changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH,+ on NO,- assimilation. Surface-water NO,- concentrations ranged from 6 to 17 ah4 while NH.,+ concentrations ranged from 0 to 0.4 PM. Total N assimilation ranged from 84 to 732 nM d-r but showed no seasonal trend. NH,’ and urea concentrations were < 1% of total dissolved inorganic N, but use of this “regenerated” N still accounted for 44-89*/s of total N assimilation. Rates of NO,- assimilation were negatively correlated with ambient NH,+ concentrations, and concentrations of NH., + between 0.1 and 0.3 PM caused complete inhibition of NOsassimilation, NO,- was more important as a source of N in spring than in summer. We attribute this pattern to a summer increase in turnover rates for NH;‘. Turnover times for the dissolved NH,+ pool were half as long in August as in May. Grazing and recycling in the euphotic zone apparently both play significant roles in preventing depletion of NO,- in the oceanic subarctic Pacific.
TL;DR: In this article, the formation of horizontal temperature gradients in the surface waters of a sidearm of a small water supply reservoir at a time during summer when radiative heating and vertical stratification of the reservoir waters were both quite strong.
Abstract: We present observations of the diurnal formation of horizontal temperature gradients in the surface waters of a sidearm of a small water supply reservoir at a time during summer when radiative heating and vertical stratification of the reservoir’s waters were both quite strong. Our measurements show that because the closed end of the sidearm was relatively shallow, daytime heating and nighttime cooling created larger temperature changes there than in the body of the lake, resulting in large horizontal temperature gradients that drove strongly sheared, horizontal exchanges. Because of more vigorous turbulent mixing during cooling, cooling-driven flows were slower and of greater vertical extent than were heating-driven flows. The overall flow exhibited inertia in that it was not in phase with the daily heating cycle. Averaged over the daily cycle, there appeared to be a net residual flow, with surface waters flowing out and metalimnetic waters flowing in. This thermally driven flow, the “thermal siphon,” greatly enhanced the rate of horizontal exchange between the sidearm and the body of the reservoir such that the time required to replace water in the sidearm when the siphon is operating is substantially less than estimates with conventional formulae based on horizontal turbulent diffusion.
TL;DR: Large populations of Daphnia effectively “break” the microbial loop and funnel bacterial production to higher consumers in Upton Lake, New York, and most bacterial communities appear to have turnover times substantially > 1 d.
Abstract: We examined the fate of planktonic bacterial production and the balance between bacterial growth and grazing mortality in the surface waters of Upton Lake, New York. Growth rates were measured by the incorporation of t3H]thymidine into DNA. Grazing rates on bacteria were determined with small cells produced by a mutant strain ofEscherichia coli and made either fluorescent or radioactive to monitor feeding. Bacterial community turnover times calculated from either growth or grazing rates ranged from 1.5 to 16 d. On the basis of these data and results from 29 other studies, most bacterial communities appear to have turnover times substantially > 1 d. Our measurements of feeding rates on bacteria frequently exceeded estimates of growth. Limitations of precision and doubts about the accuracy of methods make attempts to balance measurements of bacterial growth and grazing with current techniques unrealistic. The fate of bacterial production depends on planktonic community structure. Flagellates were the primary consumers of bacteria in winter and fall. At other times, Daphnia galeata consumed most of the bacterial production. Ciliates and rotifers were never important bacterial grazers. In Upton Lake large populations of Daphnia effectively “break” the microbial loop and funnel bacterial production to higher consumers.
TL;DR: Planktonic bacteria may represent a substantial nutrient resource available to support the next higher trophic level in the microbial loop (heterotrophic flagellates) and the emphasis was to determine if prey-size selection was a function of predator size.
Abstract: Planktonic bacteria may represent a substantial nutrient resource available to support the next higher trophic level in the microbial loop (heterotrophic flagellates). In this work we examined the utilization of different size classes of bacteria by flagellated protozoan predators of various sizes. The emphasis was to determine if prey-size selection was a function of predator size. Pseudomonas sp. was grown in chemostats under conditions to yield “large” (mean size, 1.19 pm’) or “small” individuals (mean size, 0.36 pm3). Cells were fluorescently labeled (DTAF), mixed in various proportions, and fed to four protozoans ranging in size from 21 to 119 pm3. Bacteria in food vacuoles were enumerated and measured. Size distributions of both offered and taken prey were compared, and a selectivity index was calculated. Protozoans of all four sizes preferred large bacteria (between 0.8 and 1.2 pm’). Since the work of Brooks and Dodson
TL;DR: Fish production was more sensitive to variations in phytoplankton production in oligotrophic than in eutrophic marine environments as a result of a nonlinear decrease in f, the ratio of new to total production, as primary production decreased.
Abstract: In the trophic-dynamic hypothesis, biomass production was assumed limited by factors that control energy transfer. An analysis using trophic-dynamic models applied to ocean data, however, leads to a rejection of that hypothesis and supports a hypothesis that carnivorous fish production is controlled by the amount of new N annually incorporated into phytoplankton biomass and transferred through food webs. For an average 2.5 trophic transfers from phytoplankton to fish in environments ranging from oceanic to northeast North American coastal waters, N transfer efficiency was a constant 0.28 while C transfer efficiency increased nonlinearly from 0 to an asymptotic value of 0.16. Fish production was more sensitive to variations in phytoplankton production in oligotrophic than in eutrophic marine environments as a result of a nonlinear decrease in f, the ratio of new to total production, as primary production decreased.
TL;DR: Fractal dimensions of aggregates can potentially be used to classify aggregate morphology as well as to identify coagulation mechanisms as discussed by the authors, suggesting that these aggregates are formed through cluster-cluster co-agulation.
Abstract: Fractal dimensions of aggregates can potentially be used to classify aggregate morphology as well as to identify coagulation mechanisms. Microbial aggregates of Zoogloea ramigera have a cluster fractal dimension of 1.8±0.3 (± SD), suggesting that these aggregates are formed through cluster-cluster coagulation. An analysis of size-porosity correlations for two types of marine snow aggregates yielded fractal dimensions of 1.39±0.06 and 1.52±0.19, which were lower than values describing inorganic colloidal aggregation.
TL;DR: In this paper, the marginal ice zone of the Weddell Sea in Australia was studied and the relative preference indices for NH,+ were observed whenever the ambient NH, + concentration was > 0.3 PM, which was interpreted as evidence that Antarctic phytoplankton growing at NH.,+ concentrations CO.
Abstract: Rates of primary production, N03- uptake, and NH,l uptake by phytoplankton were measured in the marginal ice zone of the Weddell Sea in austral spring 1983 and autumn 1986. In spring Chl concentrations averaged 3 pg liter-‘, primary productivity 490 mg C m-2 d-l, and surface phytoplankton growth rates 0.30 doublings d-l. In autumn these rates were all much lower, averaging 0.14 pg liter- l, 126 mg C m-2 d-l, and 0.14 doublings d-l. During both seasons NH,+ was consistently the preferred source of N, but because of the much greater availability of N03- in the euphotic zone (21-28 PM NO,- vs. -0.4 yM NH,+ during both seasons), N03- uptake rates generally equaled or exceeded those of NH, +. Vertically integrated&ratios (the ratio of NO3- uptake to the sum of NO,- plus NH,+ uptake) averaged 0.52 (range from 0.35 to 0.70) in spring and 0.72 (range from 0.60 to 0.84) in autumn. In 1983 we observed sharply elevated relative preference indices for NH,+ whenever the ambient NH,+ concentration was ~0.3 PM, which we interpret as evidence that Antarctic phytoplankton growing at NH.,+ concentrations CO.3 PM have the ability to increase their rate of NH,+ uptake rapidly in response to increased availability. We estimate the new production of the marginal ice zone to be at least 49 g C m-2 yr-1 for the NovemberMarch period. This value supports previous findings that the ice edge is the main site of production in the Southern Ocean and that it is likely to dominate the annual cycle of organic matter flux from the euphotic zone.
TL;DR: The results point out that the ecological niches of oceanic prochlorophytes are probably more diverse than initially thought.
Abstract: Cells with characteristics similar to the recently discovered oceanic prochlorophytes have been detected with ship-board flow cytometry during winter in the surface waters of the northwestern Mediterranean Sea as well as in the low-salinity dilution zone of the Rhone River. Maximum abundances reached 50,000 cells ml−1 at the surface and were only slightly lower than those observed previously at the bottom of the euphotic zone in the Atlantic and Pacific Oceans. The concentration, light scatter, and pigment fluorescence of these cells correlated tightly with those of Synechococcus spp. cyanobacteria. These results point out that the ecological niches of oceanic prochlorophytes are probably more diverse than initially thought.
TL;DR: In this article, the authors quantified the small-scale energy dissipation rates required to disaggregate marine snow settling through a gradient of turbulent kinetic energy in a laboratory tank and found that only the highest shears associated with storm events or flows in tidal channels would be able to fragment even the most fragile organic aggregates in the upper ocean.
Abstract: Abiotic fragmentation of large, rapidly sinking aggregates into smaller, suspended particles by fluid shear has been suggested as an important process governing the particle size spectrum in the ocean and as one explanation for the exponential decrease of particulate flux with depth below the euphotic zone. We investigated this process by quantifying the small-scale energy dissipation rates required to disaggregate marine snow settling through a gradient of turbulent kinetic energy in a laboratory tank. Aggregates of detrital debris, gelatinous houses of larvacean tunicates, and aggregates of living bacteria did not break apart even at energy dissipation rates > 1 cm2 ss3. The rate of energy dissipation required to disaggregate fragile diatom floes up to 25 mm long ranged from 10m3 to > 1 cm2 se3 and increased exponentially with decreasing maximum aggregate diameter. Aged diatom aggregates were significantly stronger than otherwise identical but unaged particles. These results indicate that only the highest shears associated with storm events or flows in tidal channels would be able to fragment even the most fragile organic aggregates in the upper ocean. Biological processes of disaggregation, such as animal grazing, appear far more likely to mediate the size spectrum of aggregated particulate matter in the ocean than abiotic fragmentation due to fluid motion.
TL;DR: In four data sets investigating the variability of algae, bacteria, and P among lakes, bacterial abundance was more closely related to P concentration than to chlorophyll, indicating that algal-bacterial competition for P does not determine algal or bacterial abundance.
Abstract: The literature suggests two models describing the relationship between phyto- and bacterioplankton abundance in freshwater: that total P abundance determines algal abundance, which in turn determines bacterial abundance, or that algae and bacteria compete for P. In four data sets investigating the variability of algae, bacteria, and P among lakes, bacterial abundance was more closely related to P concentration than to chlorophyll. Bacterial abundance was strongly related to the residuals of the Chl-P relationship, explaining 18-65% of the residual variance. The partial correlation is positive, however, indicating that algal-bacterial competition for P does not determine algal or bacterial abundance. The data are most consistent with an alternative model postulating that P directly influences both algal and bacterial abundance, that algae and bacteria directly influence each other’s abundance, and that a third factor (temperature or perhaps bacterivore abundance) also influences both algal and bacterial abundance in the same manner. Abundances and activities of planktonic organisms vary enormously. This variation is evident on several different spatio-temporal scales: seasonal, interannual, spatial variation within individual lakes, and interlake variation. Undoubtedly, many factors influence this variation; yet at particular scales of time and space most of the variability in plankton abundance can usually be related to small number of factors. Consider, for example, the phytoplankton. Phytoplankton abundance may be influenced by nutrient availability, grazers, interactions with bacteria, sinking, temperature, light levels, parasites, allelopathy, and water-column physics (Kalff and Knoechel 1978; Harris 1986). Most of the variability in phytoplankton abundance among lakes, however, and some of the variability among years, has been clearly shown to be related to variations in total P (TP) concentration
TL;DR: In this paper, a biological dosimeter was developed for use in aquatic environments based on the sensitivity of a DNA repair-deficient strain of Escherichia coli to ultraviolet (UV) radiation.
Abstract: A biological dosimeter has been developed for use in aquatic environments. This method is based on the sensitivity of a DNA repair-deficient strain of Escherichia coli (CSR06) to ultraviolet (UV) radiation. The dosimeter permits evaluation of the penetration of biologically active UV radiation within a water column, reflecting the potential effect of exposure over selected time intervals. With the use of various filters, the biological dosimeter can discriminate between the effects of UV-B (280-320 nm) and UV-A (320-400 nm) or other selected portions of the solar UV spectrum. During springtime ozone depletion over the Antarctic in 1988, a general relationship was observed between stratospheric ozone concentration and the contribution of incident solar UV-B radiation to lethality of dosimeter cells. The use of dosimeters within the water column indicated that significant amounts of UV-B can be transmitted to a depth of 10 m and biological effects of UV could be detected to 20 and 30 m. Biological dosimeters may provide a means of standardizing in-water UV measurements across all types ofaquatic habitats and at any geographical location.
TL;DR: The model of Kirk was used to extract scattering and absorption coefficients from the measurements in waters considerably more turbid than those in which the model was previously applied as discussed by the authors, and the estimated scattering coefftcients were linearly related to mineral suspended solids.
Abstract: Spectral diffuse attenuation coefficients were measured in the Rhode River and Chesapeake Bay, Maryland, on 28 occasions in 1988 and 1989. The model of Kirk was used to extract scattering and absorption coefficients from the measurements in waters considerably more turbid than those in which the model was previously applied. Estimated scattering coefftcients were linearly related to mineral suspended solids. Estimates of total absorption coefficients were decomposed as the sum of contributions by water, dissolved organic matter, phytoplankton chlorophyll, and particulate detritus, each having a characteristic spectral shape. The 1988 data were used to develop a model of scattering and absorption coefficients based on the specific curves regressed against water-quality parameters. Diffise attenuation coefficients in the 1989 data ranging from 1 to 10 m-l and photic depths ranging from < 1 to 4.5 m were predicted with a C.V. of about 25%. The problem of estimating concentrations of water-quality constituents from optical measurements was indeterminate due to the similarity in shape of the specific curves of dissolved substances and depigmented particulates. Chlorophyll concentration could be estimated because it was strongly related to water-corrected absorption in the 670-nm waveband, but several outliers occurred due to biological variability in specific absorption of pigments.
TL;DR: A simple, one-dimensional, advection-diffusion model was developed by Roughgarden et al. as mentioned in this paper to predict rates of settlement by larvae transported passively within turbulent boundary layers onto bottoms of differing roughness.
Abstract: A simple, one-dimensional, advection-diffusion model was developed lhat predicts rates of settlement by larvae transported passively within turbulent boundary layers onto bottoms of differing roughness. The model describes depth-limited boundary layers and considers variation in boundary shear velocity, larval fall velocity, and the height and areal density of bottom-roughness features. Larvae were allowed to settle onto the bottom but not onto roughness features. The model predicts that rates of settlement should increase monotonically with areal density of roughness features. Relatively flat bottoms should experience rates of settlement 2-4 times higher than bottoms with low densities of roughness features, but l-2 orders of magnitude lower than bottoms with relatively high densities of roughness. Whereas roughness density defines the direction of differences in settlement expected, the degree of difference is also affected by the height of roughness features and the ratio of larval fall velocity to shear velocity. The dispersal of marine benthic organisms of many species occurs through the water column via larval transport or by the swimming or drift of adults. Factors affecting settlement on the bottom by potential colonists thus have been the focus of considerable study. Laboratory and field studies show that larvae and adults exhibit complex behaviors and are capable of distinguishing among potential settlement sites based on diverse cues (Meadows and Campbell 1972; Gray 1974; Crisp 1984; Woodin 1986). Hydrodynamic processes also exert important influences on dispersal and settlement. Patterns of oceanic circulation affect broad-scale zoogeographic distributions (Ekman 1967; Scheltema 1974), produce tidal to interannual fluctuations in larval recruitment (Shanks 1986; Roughgarden et al. 1988), and can accumulate lar
TL;DR: In this paper, the relative strength of the two factors varied markedly during the seasonal succession of phytoplankton in Lake Mendota, Wisconsin, and the seasonal changes in grazing and nutrient limitation were caused not only by seasonal changes of zooplanklet community structure and nutrient availability but also by changes in phyto-top-ankton community structure, which led to changes in edibility to zoopls.
Abstract: Observations and experiments on zooplankton grazing and nutrient (N and P) limitation of phytoplankton of Lake Mendota, Wisconsin, revealed that the relative strength of the two factors varied markedly during the seasonal succession of phytoplankton. Furthermore, seasonal changes in grazing and nutrient limitation were caused not only by seasonal changes in zooplankton community structure and nutrient availability but also by changes in phytoplankton community structure, which led to changes in edibility to zooplankton. The spring phytoplankton bloom of diatoms and small flagellates was highly vulnerable to grazing by both cyclopoid copepods and Duphnia, although Daphnia had a much stronger effect. Increased Duphnia grazing in late spring resulted in low phytoplankton biomass (the clear-water period) and a phytoplankton community dominated by colonial green algae- taxa vulnerable to grazing by Daphnia but not other zooplankton. Nutrients (N and P) did not limit phytoplankton growth during the spring bloom or the clear-water period. After the clear-water period, the summer phytoplankton community was dominated by blue-greens and Ceratium. Grazing effects by both Daphnia and copepods were low in summer, while nutrient limitation (both N and P) became severe. Reduced grazing impacts in summer may have resulted from prior intense grazing impacts, which led to dominance of grazing-resistant taxa. These results suggest that seasonal variation in the strength of grazing and nutrient controls in eutrophic lakes results from changes in zooplankton biomass and community structure, nutrient availability, and phytoplankton community structure interacting to determine phytoplankton seasonal succession.
TL;DR: Food size preferences were related to body sizes between strains but not within one strain (Brachionus calyciflorus), and food-size partitioning by differently sized congeners is believed to be an important phenomenon among herbivorous zooplankton.
TL;DR: Extreme seasonal variation in the degree to which planktivorous fish can regulate daphnid population dynamics may have allowed daphNids to persist in Lake Mendota during the past century in the presence of abundant zooplanktivores.
Abstract: The ability of planktivorous fish to regulate the population dynamics of Daphnia gale&a in Lake Mendota, Wisconsin, was examined during spring and summer 1987. Low rates of consumption by yellow perch (Percaflavescens) and cisco (Coregonus artedii) allowed populations of D. galeata to expand rapidly during spring, resulting in a distinct clear-water period that lasted from midMay until early June. A subsequent decline in D. guleatu abundance resulted from low food availability brought about by its exploitation of algal resources. Once populations of D. guleuta were reduced, predation by perch and cisco was sufficient to keep populations of D. g,lleata at low densities during July and August in spite of more abundant algal food resources, Extreme seasonal variation in the degree to which planktivorous fish can regulate daphnid population dynamics may have allowed daphnids to persist in Lake Mendota during the past century in the presence of abundant zooplanktivores.
TL;DR: In this paper, the effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO, in stream sediments was studied with whole-core techniques and with 0, and N,O microsensors.
Abstract: The effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO,in stream sediments was studied with whole-core techniques and with 0, and N,O microsensors. Photosynthetic oxygen production increased the thickness of the aerobic surface layer from 1.5 mm in the dark to -3.5 mm at a light intensity saturating photosynthesis. The 0, flux changed concurrently from net uptake to net release and the overall rate of denitrification was reduced by 70%. Denitrification was always restricted to a narrow zone immediately below the aerobio anaerobic interface. Calculated NO,- microprofiles showed that overall denitrification was primarily dependent on the thickness of the aerobic layer which acted as a barrier for diffusion of NO,- from the overlying water. In the light, algal NO,- assimilation could exceed NO,- consumption by denitrification when availability of NH,+ was low. Assimilation of N03-, however, had no influence on the flux of NO,- to the denitrification zone, since assimilation occurred relatively close to the sediment surface.