Potential contributions of vertically migrating Rhizosolenia to nutrient cycling and new production in the open ocean

We present a numerical model of nutrient uptake and photosynthesisduring migrations of the marine diatom Rhizosolenia that wasdeveloped to estimate fluxes of carbon and nitrogen due to thesemigrations in the open ocean. The predicted specific rate ofincrease of Rhizosolenia was 0.11–0.15 day¹, whereas thetotal time for one migration cycle ranged between 3 and 5 days.Using published estimates of Rhizosolenia abundance, we estimatethat new primary production due to Rhizosolenia migrations rangesbetween 0.018 and 0.033 mmol N m^–2 day^–1. Thesevalues represent up to 17% of new production due to turbulentdiffusive fluxes of nitrate into the euphotic zone and are ofthe same order of magnitude as new production due to nitrogenfixation in tropical oceans. Large-scale contributions of Rhizosoleniato oceanic new production are limited by their relatively lowstanding crop. Variations in the formulation of losses withdepth greatly affected gross and net fluxes of carbon and nitrogen.Better characterization of losses of Rhizosolenia and improvedestimates of its abundance will help determine more accuratelythe contributions of Rhizosolenia to global biogeochemical cycles. ¹Present address: Department of Agricultural and EnvironmentalScience, The Queen's University of Belfast New forge Lane, BelfastBT95PX, UK     

Nitrogen budget in the euphotic zone of Lake Biwa from spring to summer, 1986

The production rate of participate nitrogen (PN) in Lake Biwafrom March to June 1986 was calculated by monthly measurementsof the proteinaceous nitrogen production rates. The ‘new’production rate was estimated from the decrease in nitrate inthe euphotic zone; during this period (91 days) it was estimatedas 93 mg atoms m^–2, and accounted for 20% of the ‘total’PN production (460 mg atoms m^–2). This implies that {smalltilde}80% of the PN produced might be recycled in the euphoticzone. The increase in PN in the euphotic zone during this period(27 mg atoms m^–2) accounted for 5.9% of the ‘total’PN production. This indicates that the remaining 14% of PN producedin the euphotic zone was vertically transported. Sediment trapexperiments at 30 m depth indicate that 8.1% of the PN producedin the euphotic zone was measured as downward flux at 30 m depthduring the stagnation period. Decompositional loss of PN between12.5 (bottom of the euphotic zone) and 30 m depth was estimatedas 17 mg atoms m^–2. This was calculated on the basis ofaccumulation of ammonium, which accounted for 3.7% of the ‘total’PN production. The flux from the euphotic zone thus accountedfor 12% of the ‘total’ PN production, suggestingthe validity of the production model.     

Urea degradation by picophytoplankton in the euphotic zone of Lake Biwa

The ability of photoautotrophic picoplankton Synechococcus to degrade urea was examined in the euphotic zone of Lake Biwa. Samples were divided into pico (0.2–2.0 μm) and larger (>2.0 μm) size fractions by filtration. The rates of urea degradation (the sum of the rates of incorporation of carbon into phytoplankton cells and of liberation of CO₂ into water) measured by radiocarbon urea were 8 and 17 μmol urea m⁻³ day⁻¹ in June and July, respectively, for the picophytoplankton in the surface water, and 196 and 96 μmol urea m⁻³ day⁻¹, respectively for the larger phytoplankton. The rates decreased with depth, somewhat similar to the vertical profiles of the photosynthetic rate. The urea degradation rates were obviously high under light conditions. In daylight, urea was degraded into two phases, carbon incorporation and CO₂ liberation, whereas in the dark it was degraded only into the CO₂ liberation phase. The contribution of picophytoplankton to total phytoplankton in urea degradation was high in the subsurface to lower euphotic layer. Urea degradation activity was higher in the picophytoplankton fraction than in the larger phytoplankton fraction. Shorter residence times of urea were obtained in the upper euphotic zone. The contribution of picophytoplankton to urea cycling was 4% to 35%. The present results suggest that the picophytoplankton Synechococcus is able to degrade urea and effectively makes use of regenerated urea as a nitrogen source in the euphotic layer, and that picophytoplankton play an important role in the biogeochemical nitrogen cycle in Lake Biwa. Received: June 25, 1998 / Accepted: February 10, 1999     

Biological assessment of the euphotic zone in a turbid man-made lake

The underwater light climate in the turbid waters of the Hendrik Verwoerd Dam (South Africa) was assessed using an underwater lightmeter with glass colour filters, a Secchi disc and a biostimulatory response technique. Secchi readings varied between 17 to 25% of the light penetration as measured with the underwater lightmeter. Active ¹⁴C assimilation took place at depths 530% deeper than the 1% light intensity depth. Red light was transmitted most in the waters.     

Vertical migration of Rhizosolenia mats and their significance to NO3- fluxes in the central North Pacific gyre

Rhizosolenia mat abundance, distribution and chemical compositionwere studied on two cruises in the central North Pacific gyrein order to determine large-scale distribution patterns andcontribution to upward nitrogen (N) flux. These macroscopicdiatom mats are composed of multiple species of Rhizosoleniathat exploit subsurface nitrate pools by vertically migratingbelow the euphotic zone. Although numerically dominated by thesmall-diameter species, R.fallax (73–95% of total numbers),mat biovolume was dominated by large-diameter (>50 µmdiameter) Rhizosolenia spp. (85–99% of total volume).Integrated mat abundance was substantially higher when matsaccumulated at the surface during calm weather (     

Microbial biomass on particulate organic matter in seawater of the euphotic zone

Microbial biomass on suspended organic matter in seawater of the euphotic zone of Saanich Inlet was investigated. The viable microorganisms were measured by the glucose-uptake method. Microbial carbon on particulate organic matter in seawater was determined to be, on the average, 9.9 μg of C/liter, and there was a regression relationship as y = 0.0062 x − 1.79 with an unbiased variance V_yx^1/2 = 0.38, where x = particulate organic carbon in seawater (micrograms of C/liter) and y = logarithm of microbial carbon (micrograms of C/liter).     

Particulate carbohydrates in relation to phytoplankton in the euphotic zone of the Bransfield Strait

Summary The composition and depth distribution of particulate carbohydrates for 5 stations in the Bransfield Strait is discussed in relation to phytoplankton species composition. Glucose being the most prominent sugar at diatom dominated stations is reduced by 70% from the surface to 100 m depth whereas total carbohydrates show a 40% degradation. Values for a haptophyceae dominated station are 95 and 80%, respectively.     

Increased cyanophage infection at the bottom of the euphotic zone,especially in the fall

Picocyanobacteria contribute greatly to offshore primary production with cells extending through the deep euphotic zone. Literature indicates high viral infection of cyanobacteria in ocean transition zones. We postulate that the bottom of the euphotic zone is a transition zone, where communities transition from phototrophic to aphotic processes. We use single-copy core genes to examine cyanophage to cyanobacteria ratios in cellular metagenomes in the subtropical North Atlantic and Pacific. Cyanophage to cyanobacteria terL/rpoB ratios generally increase to >10 in the deep euphotic zone. As light levels decrease in the fall, Prochlorococcus in the deep euphotic zone experience reduced light levels. We find clear differences between spring (Geotraces GA02) and fall (GA03) in the North Atlantic, with terL/rpoB ratios increasing to >40 in the fall. When examining 23 months of the North Pacific Hawaii Ocean Timeseries, the depth of elevated cyanophage to cyanobacteria ratios in cellular metagenomes negatively correlated with surface photosynthetic radiation (PAR), particularly with the change in PAR, which reflected the season. In fall, all picocyanobacteria ecotypes were found at depths enriched with viruses, while in summer, only low light ecotypes were affected. Thus, we find high cyanophage infection both in the deep euphotic zone and during seasonal transitions.     

Diel variation in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi

Diel variations in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi were measured under fixed light intensity. The decomposition rate of urea was 17 to 44 μ mol urea m⁻³ h⁻¹ in the light and 10 to 27 μ mol urea m⁻³ h⁻¹ in the dark. Higher decomposition rates were obtained in the upper euphotic zone. A clear diel periodicity in the urea decomposition rate was observed, with high rates from 1200 to 1600 and low rates from 0000 to 0400. Chlorophyll a specific decomposing activity ranged from 12 to 21 μg urea C mg chl.a ⁻¹ h⁻¹ in the light and 7 to 13 μg urea C mg chl.a ⁻¹ h⁻¹ in the dark. In the light, high values were obtained from 1600 to 2000 and low values from 0400 to 0800. The diel change in specific decomposing activity exhibited a similar pattern to that of the photosynthetic assimilation number, following the diel change in photosynthetic activity. Received: March 10, 1999 / Accepted: October 22, 1999     

Groundwater and pore water inputs to the coastal zone

Both terrestrial and marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows further out on the shelf from confined aquifers. Marine processes such as tidal pumping and current-induced pressure gradients may induce interfacial fluid flow anywhere on the shelf where permeable sediments are present. The terrestrial and oceanic forces overlap spatially so measured fluid advection through coastal sediments may be a result of composite forcing. We thus define “submarine groundwater discharge” (SGD) as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. SGD is typically characterized by low specific flow rates that make detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may increase substantially the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coastal ocean. Terrestrially-derived fluids represent a pathway for new material fluxes to the coastal zone. This may result in diffuse pollution in areas where contaminated groundwaters occur. This paper presents an historical context of SGD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. We conclude that to fully characterize marine geochemical budgets, one must give due consideration to SGD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGD and to evaluate these fluxes more precisely.     

A talin-dependent LFA-1 focal zone is formed by rapidly migrating T lymphocytes

Cells such as fibroblasts and endothelial cells migrate through the coordinated responses of discrete integrin-containing focal adhesions and complexes. In contrast, little is known about the organization of integrins on the highly motile T lymphocyte. We have investigated the distribution, activity, and cytoskeletal linkage of the integrin lymphocyte function associated antigen-1 (LFA-1) on human T lymphocytes migrating on endothelial cells and on ligand intercellular adhesion molecule-1 (ICAM-1). The pattern of total LFA-1 varies from low expression in the lamellipodia to high expression in the uropod. However, high affinity, clustered LFA-1 is restricted to a mid-cell zone that remains stable over time and over a range of ICAM-1 densities. Talin is essential for the stability and formation of the LFA-1 zone. Disruption of the talin-integrin link leads to loss of zone integrity and a substantial decrease in speed of migration on ICAM-1. This adhesive structure, which differs from the previously described integrin-containing attachments displayed by many other cell types, we have termed the "focal zone."     

Photosynthetic production and exoenzymatic degradation of organic matter in the euphotic zone of a eutrophic lake

Photosynthetic production of organic matter, and its exoenzymaticdecomposition were studied in the euphotic zone of a naturallyeutrophic lake during early spring phytoplankton bloom, andafter its breakdown. Phytoplankton were the major biomass producerswhen algae were actively growing, and the algal fraction (>3.0µm) contributed on average 75–80% to the total biomassof microplankton. When the phytoplankton bloom began to declinebacterial biomass increased rapidly and, at the end of the bloom,bacteria contributed 48.7–69.98% to the total biomassof microplankton. The high bacterial abundance during phytoplanktonbloom breakdown followed the highest rates of glucose uptake,and the highest rates of alkaline phosphatase, leucine-amino-peptidase,ß-galactosidase and ß-glucosidase activities.The majority of enzyme activity was associated with the bacterialsize fraction of seston. The activities of free (dissolved inwater) exoenzymes were negligible. The synthesis of bacterialexoenzymes was under control of an induction/derepression mechanism,and depended on the amount of easily assimilable substrates,and/or the presence of polymeric organic compounds in the water,which served as substrates for exoenzymatic hydrolysis. Thetight metabolic coupling between bacterial exoenzymatic hydrolysisand uptake of low molecular weight substrates, and its ecologicalsignificance is discussed.     

Phytoplankton succession in the euphotic zone of Lake Corlo (Northern Italy) during summer stratification

Abstract

Lake Corlo is a dammed reservoir for hydro-electrical purposes in Northern Italy. Weekly samplings were made from May to August 1985 in the euphotic layer. Two major categories, flagellates and diatoms, alternatively dominated the algal community. Flagellates prevailed with stratified conditions. The strong influence of hydraulic conditions on phytoplakton succession is emphasized.     

Temporal variability and coherence of euphotic zone bacterial communities over a decade in the Southern California Bight

Time-series are critical to understanding long-term natural variability in the oceans. Bacterial communities in the euphotic zone were investigated for over a decade at the San Pedro Ocean Time-series station (SPOT) off southern California. Community composition was assessed by Automated Ribosomal Intergenic Spacer Analysis (ARISA) and coupled with measurements of oceanographic parameters for the surface ocean (0–5 m) and deep chlorophyll maximum (DCM, average depth ∼30 m). SAR11 and cyanobacterial ecotypes comprised typically more than one-third of the measured community; diversity within both was temporally variable, although a few operational taxonomic units (OTUs) were consistently more abundant. Persistent OTUs, mostly Alphaproteobacteria (SAR11 clade), Actinobacteria and Flavobacteria, tended to be abundant, in contrast to many rarer yet intermittent and ephemeral OTUs. Association networks revealed potential niches for key OTUs from SAR11, cyanobacteria, SAR86 and other common clades on the basis of robust correlations. Resilience was evident by the average communities drifting only slightly as years passed. Average Bray-Curtis similarity between any pair of dates was ∼40%, with a slight decrease over the decade and obvious near-surface seasonality; communities 8–10 years apart were slightly more different than those 1–4 years apart with the highest rate of change at 0–5 m between communities <4 years apart. The surface exhibited more pronounced seasonality than the DCM. Inter-depth Bray-Curtis similarities repeatedly decreased as the water column stratified each summer. Environmental factors were better predictors of shifts in community composition than months or elapsed time alone; yet, the best predictor was community composition at the other depth (that is, 0–5 m versus DCM).     

Pathways of carbon cycling in the euphotic zone: the fate of largesized phytoplankton in the Northeast Water Polynya

The fate of large-sized phytoplankton and pathways of carboncycling in surface waters, i.e. recycling within or export outof the euphotic zone, were investigated in the Northeast Water(NEW) Polynya (77–81N) from 23 May to 17 August 1993.Sampling represented a wide range of ice, hydrographic and nutrientconditions. Phytoplankton and zooplankton abundances, and phytoplanktonproduction rates were determined in the field, whereas potentialrates of grazing by copepods, dino flagellates and appendicularianswere calculated from abundances and temperature, using assumptionsfrom the literature. The potential downward and lateral exportof phytoplankton was also calculated by resolving a carbon budgetfor the euphotic zone. The present study suggests that, in theNEW, different pathways for the cycling of carbon existed inseasonally ice-free (in the polynya) and continuously ice-coveredareas (outside the polynya). Outside the polynya, the fate oflarge-sized phytoplankton could not be assessed because theheterotrophic community presumably grazed on a variety of fooditems, including ice algae, microzooplankton and large-sizedphytoplankton. In the polynya, the fate of large-sized phytoplanktonproduction was to be mostly recycled at the beginning of samplingand to be mostly exported downward or laterally as the bloomof large-sized phyto plankton developed. Generally, copepodsmostly contributed to recycling, but sometimes dinoflagellatesor appendicularians alone recycled most of the large-sized phytoplanktonproduction.     

Nitrogen inputs to a marine embayment: the importance of groundwater

We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods. Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m²/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove. Nitrogen inputs from groundwater were approximately 300 mmol-N/m³/y of Cove water. Road runoff contributed an additional 60 mmol/m³/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.     

Nitrogen retention in the riparian zone of catchments underlain by discontinuous permafrost

1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. 2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end‐member mixing model. 3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 μm overall. Nitrogen retention, measured using the end‐member mixing model, averaged 0.75 and 0.22 mmol N m^?2 day^?1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow. 4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone.     

Nitrogen inputs to a marine embayment: the importance of groundwater

We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods. Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m²/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove. Nitrogen inputs from groundwater were approximately 300 mmol-N/m³/y of Cove water. Road runoff contributed an additional 60 mmol/m³/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.     

Biosynthesis of unusual monocyclic alkenes by the diatom Rhizosolenia setigera (Brightwell)

Novel, polyunsaturated monocyclic sester- and triterpenes isolated from the diatom Rhizosolenia setigera (Brightwell), are biosynthesised mainly via the mevalonate pathway. The experiments involved incubation of the alga with [1-(13)C]acetate, isolation of the alkenes by extraction and silver ion HPLC, followed by determination of the labelling pattern of one of the monocyclic triterpenes by (13)C-NMR spectroscopy. In addition, the extent of (13)C incorporation was also measured by mass spectrometry which revealed that the involvement of the mevalonate route in the biosynthesis of these cyclic compounds was less than for the co-occurring acyclic highly branched isoprenoid alkenes.