Primary productivity and sizes of pools of organic carbon in the mixed layer of the ocean

We estimated primary productivity and distributions of carbon in the phytoplankton, micro-zooplankton, and suspended and dissolved matter in various areas of the World Ocean to increase our information about the organic carbon cycle in the surface layer of the sea. Primary productivity ranged from about 0.1 gC m^–2 day^–1 in the Gulf of Mexico to 9 gC m^–2 day^–1 in nutrient-rich water off Peru. Phytoplankton carbon ranged from less than 10 g/l in the former to 750 g/l in the latter and in nutrient-rich water off southwest Africa. Micro-zooplankton carbon usually was less than 50 g/l in all waters, and was dominated by ciliates, copepodids, and copepod nauplii in all areas. Concentrations of particulate carbon ranged from 12 g/l off the east coast of South America to 850 g/l off southwest Africa. Concentrations of dissolved organic carbon varied between 0.5 and 1.5 mg/l in all areas except off Peru, where maximum values of 4.5 mg/l were observed. Turnover rates of carbon by small standing crops of micro-flagellates (1 to 5 longest dimension) and dinoflagellates in nutrient-poor waters were lower than those by large standing crops of diatoms and micro-flagellates in nutrient-rich waters. Concentrations of phytoplankton usually accounted for 20 to 55% and micro-zooplankton for 2 to 30% of the particulate carbon in the surface layer of the sea. Concentrations of dissolved organic carbon were not related to concentrations of particulate carbon in most waters except off Peru, where they appear to be directly related.     

Detritus-Bacteria-Meiofauna interactions in a seagrass bed (Posidonia oceanica) of the NW Mediterranean

The biochemical composition of the sediment organic matter, and bacterial and meiofaunal dynamics, were monitored over an annual cycle in aPosidonia oceanica bed of the NW Mediterranean to test the response of the meiofauna assemblage to fluctuations in food availability. Primary production cycles of the seagrass and its epiphytes were responsible for relatively high (compared to other Mediterranean systems) standing stocks of organic carbon in sediments (from 1.98 to 6.16 mg Cg^–1 sediment dry weight). The biopolymeric fraction of the organic matter (measured as lipids, carbohydrates, and proteins) accounted for only a small fraction (18%) of the total sedimentary organic carbon. About 25% of the biopolymeric fraction was of microphytobenthic origin. Sedimentary organic carbon was mostly refractory (56 to 84%) and probably largely not utilizable for benthic consumers. The biopolymeric fraction of the organic matter was characterized by high carbohydrate concentrations (from 0.27 to 5.31 mg g^–1 sediment dry weight in the top 2 cm) and a very low protein content (from 0.07 to 0.80 mg g^–1 sediment dry weight), which may be a limiting factor for heterotrophic metabolism in seagrass sediments. RNA and DNA concentrations of the Sediments varied significantly during the year. High RNA and DNA values occurred during the microphytobenthic bloom and in correspondence with peaks of bacterial abundance. Bacteria accounted for a small fraction of the total organic carbon (0.65%) and of the biopolymeric organic carbon (4.64%), whilst microphytobenthos accounted for 3.79% of total organic carbon and for 25.08% of the biopolymeric carbon. Bacterial abundance (from 0.8 to 5.8 × 10⁸ g^–1 sediment dry weight) responded significantly to seasonal changes of organic matter content and composition and was significantly correlated with carbohydrate concentrations. Bacteria might be, in the seagrass system, an important N storage for higher trophic levels as il accounted for 25% of the easily soluble protein. pool and contributed significantly to the total DNA pool (on average 12%). Total meiofaunal density ranged from 236 to 1858 ind. 10 cm^–2 and was significantly related, with a time lag, to changes in bacterial standing stocks indicating that microbes might represent an important resource. Bacterial abundance and biomass were also significantly related to nematode abundance. These results indicate that bacteria may play a key role in the benthic trophic     

Primary productivity and particulate organic matter flow in an estuarine mangrove-wetland in Hong Kong

Net primary productivity and organic matter flow of a mangrove-dominated wetland was estimated by following production and detritus dynamics in a tidal pond in north west Hong Kong in 1986–1988 (9.1 ha). Total productivity was 12.47 t dry wt ha^–1 yr^–1, of which >90% was from emergent macrophytes (the mangroveKandelia candel and the reedPhragmites communis). High turbidity and high summer temperatures probably limited respective production by phytoplankton and benthic macroalgae (dominated byEnteromorpha crinata). Despite the high total productivity, little detritus was exported from the emergent macrophyte stands, due to the low inundation frequency. This created a net water column carbon deficit which was provided for by the high organic matter import (mean = 4.42 g ash free dry wt m^–2 d^–1) from the incoming water. This same sediment and particulate organic carbon input giving a high accretion rate of 1.7 cm yr^–1 was probably also the force behind progressive dis-coupling of emergent macrophyte production from water column consumers. This resulted in a tendency to retain production in the emergent macrophyte stands while the water column community increasingly relied on allochthonous carbon. This shift from a net exporter to a net importer of carbon in landward wetlands is probably characteristic of the transition into nutrient-conservative terrestrial systems.     

Defecation by Salpa thompsoni and its contribution to vertical flux in the Southern Ocean

Measurements of the defecation rate of Salpa thompsoni were made at several stations during two cruises west of the Antarctic Peninsula in 2004 and 2006. Rates were quantified in terms of number of pellets, pigment, carbon and nitrogen for a wide size range of both aggregate and solitary salps. Measured defecation rates were constant over several hours when salps were held at near-surface conditions from which they had been collected. The defecation rate per salp increased with both salp size and the ambient level of particulate organic matter (POM) in the upper water column. The weight-specific defecation rate ranged between 0.5 and 6% day⁻¹ of salp body carbon, depending on the concentration of available particulate matter in the water. Carbon defecation rates were applied to biomass estimates of S. thompsoni to calculate daily carbon defecation rates for the populations sampled during the two cruises. Dense salp populations of over 400 mg C m⁻² were calculated to produce about 20 mg C m⁻² day⁻¹, comparable to other major sources of vertical flux of organic material in the Southern Ocean. Measured sinking rates for salp fecal pellets indicated that the majority of this organic material could reach deep sediments within a few days, providing a fast and direct pathway for carbon to the deep ocean.     

Seasonal variations of organic-carbon and nutrient transport through a tropical estuary (Tsengwen) in southwestern Taiwan

This paper reports the fluvial fluxes and estuarine transport of organic carbon and nutrients from a tropical river (Tsengwen River), southwestern Taiwan. Riverine fluxes of organic carbon and nutrients were highly variable temporally, due primarily to temporal variations in river discharge and suspended load. The sediment yield of the drainage basin during the study period (1995–1996, 616 tonne km^–2 year^–1) was ca. 15 times lower than that of the long-term (1960–1998) average (9379 tonne km² year^–1), resulting mainly from the damming effect and historically low record of river water discharge (5.02 m³ s^–1) in 1995. The flushing time of river water in the estuary varied from 5 months in the dry season to >4.5 days in the wet season and about 1 day in the flood period. Consequently, distributions of nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC) were of highly seasonal variability in the estuary. Nutrients and POC behaved nonconservatively but DOC behaved conservatively in the estuary. DOC fluxes were generally greater than POC fluxes with the exception that POC fluxes considerably exceeded DOC fluxes during the flood period. Degradation of DOC and POC within the span of flushing time was insignificant and may contribute little amount of CO₂ to the estuary during the wet season and flood period. Net estuarine fluxes of nutrients were determined by riverine fluxes and estuarine removals (or additions) of nutrients. The magnitude of estuarine removal or addition for a nutrient was also seasonally variable, and these processes must be considered for net flux estimates from the river to the sea. As a result, nonconservative fluxes of dissolved inorganic phosphorus (DIP) from the estuary are –0.002, –0.09 and –0.59 mmol m^–2 day^–1, respectively, for dry season, wet season and flood period, indicating internal sinks of DIP during all seasons. Due to high turbidity and short flushing time of estuarine water, DIP in the flood period may be derived largely from geochemical processes rather than biological removal, and this DIP should not be included in an annual estimate of carbon budget. The internal sink of phosphorus corresponds to a net organic carbon production (photosynthesis–respiration, p–r) during dry (0.21 mmol m^–2 day^–1) and wet (9.5 mmol m^–2 day^–1) seasons. The magnitude of net production (p–r) is 1.5 mol m^–2 year^–1, indicating that the estuary is autotrophic in 1995. However, there is a net nitrogen loss (nitrogen fixation–denitrification < 0) in 1995, but the magnitude is small (–0.17 mol m^–2 year^–1).     

Benthic oxygen uptake and carbon cycling under aphotic and resource-limiting conditions in a submarine cave

To establish relationships between organic input to the benthos and decreases in benthic population biomass and density, benthic oxygen uptake was measured in an oligotrophic submarine cave in the northwestern Mediterranean Sea (Marseille, France), on seven separate occasions in 1987, using an in situ bell-jar respirometer. Oxygen uptake was measured in both the outer twilight section and the dark inner section of the cave during an annual survey (seven recording periods from February 1987 to November 1988). The mean annual benthic oxygen uptake was 80.9 litres O₂ m^–2 yr^–1 for the twilight outer section and 15.5 litres O₂ m^–2 yr^–1 for the dark inner section. The results are discussed and the biogeochemical budget for particulate organic carbon at the sediment-water interface calculated. Respiration rates (expressed as carbon equivalents), together with previously published data on vertical fluxes and burial of organic carbon, revealed that anaerobic pathways accounted for 14% and aerobic pathways for 86% of the total benthic metabolism in the outer part of the cave. In the inner section of the cave, degradation of organic carbon occurred only through aerobic degradation, indicating a strongly carbon-limited ecosystem. The low respiration rates recorded in the dark section were similar to values recorded for some oligotrophic deep-sea environments (1 000 to 2 000 m). Such budgets are essential preliminary steps in order to accurately model benthic metabolic pathways. The determination of annual fluxes linked to the acquisition of long-term data will yield better knowledge of the recycling processes at the water-sediment interface.     

RETRACTED ARTICLE: Removal of lead and cadmium ions from aqueous solutions using the macroalga Caulerpa racemosa

Sediment bulk parameters and fatty acid biomarkers were used to investigate the relative contribution of major sources of particulate organic matter during upwelling and non-upwelling conditions, at organic enriched sites on the south-eastern Brazilian coast. The degradation state of the organic material and its implications for benthic food webs were also evaluated. Temperature and salinity values indicate the intrusion of the South Atlantic Central Water in spring with a strong influence at station 4S. C:N ratios and fatty acid biomarkers suggest that sedimentary particulate organic matter is mainly autochthonous and originated from phytoplankton, zooplankton and bacteria, with a minor terrestrial contribution. Short-chain saturated fatty acids predominated, indicating the prevalence of partially degraded detritus. An exception was station 4S, where polyunsaturated fatty acids dominated, indicating the input of labile organic matter linked to upwelling of the South Atlantic Central Water. Fatty acid concentrations and the organic carbon content of the sediments suggest that food quantity is not a limiting factor for benthic fauna. Despite the high organic background of the sediments, the sporadic inputs of labile organic matter associated with South Atlantic Central Water intrusions may produce an important impact on benthic fauna and on the biogeochemistry of the sediments.     

Analyse des paramètres hydrobiologiques dans la remontée de Cabo Frio (Brésil)

South Atlantic Central Waters (SACW) upwell close to Cabo Frio (Brazil, Rio de Janeiro) shore. The resultant typical hydrobiological conditions were studied at an anchorage station over an annual cycle from February 1973–February 1974. Multivariate analyses of hydrological and planktonic data revealed the structure of the ecosystem and the factorial relations governing its dynamics. Alternation, superposition or mixing of the water masses of different origin (Brazil current, coastal, SACW) constitute the most important factors responsible for the great hydrobiological variability observed in the study area. Spasmodic changes in wind direction and force are superimposed on a seasonal trend of more frequent upwelling in summer than during the rest of the year. The deep water is characterized by temperatures of <18°C, nitrate contents of 10 g-at l^-1 and by organic matter mainly composed of detritic elements from the shelf. Temperature variations together with nutrient contents (NO₃ or PO₄) reflect variations in primary biomass at the surface but not at 50 m depth, where detrital matter precluded valid measurements. Water of the Brazilian Current (salinity 36.0) frequently mixes with deep water of the thermal front, or with coastal water (<35.0) which invades the area when south-west winds prevail. This lower-salinity water is rich in seston particles. During the study period, primary biomass was relatively low due to eutrophication. We observed less than 3 g l^-1 chlorophyll and 10⁶ phytoplankton cells per litre: the phytoplankton populations were highly diversified, indicating an advanced degree of complexity and evolution of this ecosystem. The observed formation of a thermocline constitutes an important inducing factor for an algal bloom. Simultaneous phyto- and zooplankton maxima would induce an increased grazing rate by herbivorous zooplankton which would also partly explain the relatively low level of primary biomass. Zooplankton is as abundant here as in other great upwelling regions: 100 organisms l^-1 and 200 mg organic matter m^-3.     

Particulate protein-nitrogen in North Atlantic surface waters

Depth profiles of particulate protein-nitrogen at 4 oceanic and 2 upwelling stations in the North Atlantic Ocean were measured by a new fluorometric method. The protein-nitrogen in the upper 20 m ranged from 0.19 to 1.61 μg-at N/1 at the oceanic stations and from 0.43 to 3.54 μg-at/1 at the upwelling stations. The mean values in the euphotic zone were 0.54 μg-at N/1 for the oceanic stations and 1.70 μg-at N/1 for the upwelling stations. The ratio of protein-nitrogen to chlorophyll at the two sets of stations was 2.83 and 0.54 μg-at N/μg chlorophyll, respectively. Regression analysis of the pooled data yielded a detritus and zooplankton-free ratio of 0.38 μg-at N:μg chlorophyll. Calculations of the phytoplankton protein-nitrogen, based on this ratio, suggest that in the oceanic water only 20% of the sestonic protein-nitrogen is associated with the phytoplankton. In the upwelling waters, the phytoplankton may account for 65% of the sestonic proteinnitrogen.     

Decomposition of faecal matter and somatic tissue of Mytilus galloprovincialis: changes in organic matter composition and microbial succession

Variations in the biochemical composition of pseudo-faeces and faeces egested by Mytilus galloprovincialis (Lamarck) and in detritus derived from the somatic tissue of mussels during the decomposition process were investigated by means of two intensive experiments. During the degradation process, the biochemical composition of pseudo-faeces and faeces showed a clear increase in protein content related to the microbial colonization. Changes also occurred in the biochemical composition of particulate organic matter (POM) in the surrounding water due to faecal matter decomposition, heterotrophic utilization and conversion of particulate carbohydrates and proteins to the dissolved pool. The study of production and heterotrophic utilization of the POM derived from the somatic tissue of M. galloprovincialis collected in the Gulf of Tigullio, Italy in 1990 indicates that this kind of material is rapidly decomposable and largely available for benthic consumers. Bacteria utilized selectively the different compounds, and proteins proved to be the most suitable substrate for bacterial growth. The input of organic detritus into the experimental system resulted in an enhancement of bacterial activity and consequently of the RNA/DNA ratio. Bacterial DNA accounted on average for 17% of particulate DNA. During decomposition processes, nutrient release was about ten times higher than the value calculated from individual excretion rates, indicating that mussel beds may be important sites for nutrient regeneration. Carbon conversion efficiency for bacteria growing on faecal matter was, on average, 17.2%. The potential importance of faecal output and bacterial production as a carbon resource for benthic communities near mussel culture areas is discussed.     

Influences of Suspended Particles on the Runoff of Pesticides from an Agricultural Field at Askim, SE-Norway

Field and laboratory experiments were conducted to study the loss of particles from agricultural fields, and the role of suspended particles in carrying pesticides in surface runoff and drainage water. Propiconazole, a widely used fungicide was applied to experimental fields located at Askim, SE-Norway. Samples from surface runoff and drainage water were collected and analyzed for sediment mass, pesticides, particulate and dissolved organic carbon through a whole year. The surface soil and the runoff material were characterized by its particle size distribution, organic carbon content in size fractions and its ability to bind propiconazole. The results show that (1) particle runoff mostly occurred during the rainfall event shortly after harrowing in autumn. The highest particle concentration observed in the surface runoff water was 4600 mg l^–1, and in the drainage water 1130 mg l^–1; (2) the erosion of surface soil is size selective. The runoff sediment contained finer particle/aggregates rich in organic matter compared to its original surface soil; (3) the distribution coefficient (K _d) of propiconazole was significantly higher in the runoff sediment than in the parent soil. According to our calculation, particle-bound propiconazole can represent up to 23% of the total amount of propiconazole in a water sample with a sediment concentration of 7600 mg l^–1, which will significantly influence the transport behavior of the pesticide.     

Decomposition of urea associated with photosynthesis of phytoplankton in coastal waters

Decomposition of urea in seawater was studied in Mikawa Bay, a shallow eutrophic bay on the southern coast of central Japan. The urea concentration in seawater ranged from 1.3 to 5.9 μg-at. N/1 and comprised 12 to 40% of the dissolved organic nitrogen. Using ¹⁴C labelled urea, the rate of CO₂ liberation from urea and the incorporation rate of urea carbon into the particulate organic matter were determined. For the surface samples, high rates of CO₂ liberation from urea as well as the incorporation of urea carbon into the particulate organic matter were observed in the light, while much lower rates were obtained in the dark. Incubation experiments with exposure to different light intensities revealed that the rate of CO₂ liberation from urea and the incorporation of urea carbon into particulate organic matter changed with light intensity, showing a pattern similar to that of photosynthesis. The highest liberation and incorporation rates were observed at 12,000 lux. Incubation in light and in dark produced marked decreases and increases, respectively, in urea and ammonia, while no appreciable changes were observed for nitrate and nitrite. It is suggested that urea decomposition associated with photosynthetic activity of phytoplankton is one of the major processes of urea decomposition, and that it plays a significant role in the nitrogen supply for phytoplankton in coastal waters.     

Release of ultraviolet-absorbing compounds by the red-tide dinoflagellateLingulodinium polyedra

We tested the hypothesis that ultraviolet-absorbing compounds known as mycosporine-like amino acids (MAAs) are not only synthesized but also excreted by marine phytoplankton. An experiment was performed with cultures of the marine dinoflagellateLingulodinium polyedra (previously known asGonyaulax polyedra) exposed to visible (photosynthetically available, PAR, 400 to 700 nm) and ultraviolet (UV, 290 to 400 nm) radiation. Absorption properties of both particulate and dissolved organic matter pools (POM and DOM, respectively) showed maxima in ultraviolet absorption at 360 nm. Chromatographic analysis confirmed the presence of MAAs in both pools. Release of organic matter byL. polyedra, as measured spectrophotometrically by changes in UV absorption in the surrounding medium, showed a differential increase at 360 nm in cultures exposed to UV-B + PAR radiation. The changes in absorption in the DOM fraction were inversely proportional to intracellular UV absorption. Photodegradation experiments in which the DOM fraction was exposed to visible and UV-B radiation showed a decrease in absorption with dose. First-order photooxidation decay rates varied between – 0.005 and – 0.26 m² (mol quanta)^–1 and were also a function of the initial optical density (OD). These results indicate that UV-absorbing compounds synthesized by phytoplankton, such as certain dinoflagellates, may be a component of the DOM pool in surface waters of the ocean and contribute to the attenuation of UV radiation in the water column. Photooxidation consumes only 3 to 10% of the daily production of the DOM absorbing between 280 and 390 nm (including MAAs). This suggests that MAAs dissolved in seawater may contribute to the decrease of UV transmission through the water column on a time scale representative of phytoplankton growth (days) and bloom development (weeks).     

Modelling organic matter dynamics in headwater streams of south-western British Columbia,Canada

A mass-balance model was developed to simulate organic matter (OM) dynamics in headwater stream ecosystems of south-western British Columbia, Canada. Empirical data from two streams were used to structure and test a mass-balance model of the riparian–stream system. The model was driven by data on inputs, outputs, processing rates, discharge and water temperature. Statistical sub-models were derived for different processes (e.g. decomposition rates and periphyton growth). Inputs and outputs of OM were modelled on the basis of a series of assumptions of system properties, such as temperature and hydrological regimes. Major uncertainties identified through Monte-Carlo simulations of model predictions and variables important in controlling OM dynamics in these streams were dissolved OM (DOM) import and export, stream area and litterfall import. DOM was quantitatively the most important source of OM, accounting for 80% of total export of OM, followed by export of fine particulate organic matter (FPOM) at 15%. Different scenarios of logging and temperature regimes on the system were simulated to predict how these factors would affect standing stock of OM in the stream. When inputs of riparian litterfall were simulated to mirror reductions predicted from forest harvesting in the riparian area particulate OM (POM) standing stock was reduced by almost 80%. In comparison, a 3 °C increase in water temperature resulted in only a 20% reduction of POM standing stock due to enhanced mineralisation.     

Meiobenthos distribution pattern in the tropical East Atlantic: indication for fractionated sedimentation of organic matter to the sea floor?

To test the extent to which sea surface productivity governs the distribution pattern of benthic organisms, meiobenthic standing stocks were investigated on the shelf, continental margin and the adjacent abyssal plains off the western African coast between Guinea (10°N) and Angola (18°S). The area of investigation is characterized by gradients in surface productivity due to spatially and seasonally varying coastal upwelling. Reflecting the dependency of deep-sea organisms on organic matter input from the euphotic zone, similar gradients ought to be expected within the benthos. Meiofaunal abundances and biomasses (including Foraminifera) from a total of 57 stations along 13 transects across the continental margin showed a fairly close correlation with sediment-bound chloroplastic pigment concentrations, indicating the sedimentation of particulate organic matter from phytoplankton production. However, certain discrepancies in faunal and pigment distribution patterns were found in regions apart from the centres of enhanced primary␣productivity, i.e. apart from the upwelling centres: whereas pigment concentrations in the sediments were still comparably high, meiofaunal numbers in those peripheral areas were generally lower. It is suggested that smaller/lighter phytodetritial matter, transported over long distances by subsurface currents and exposed to ongoing microbial degradation during its passage, probably does not have the same energy content as the relatively fast-sinking, larger phytodetritus aggregates (“marine snow”), which are found in centres of enhanced primary productivity and support extensive benthic stocks. Thus, meiobenthic abundances in relation to sediment-bound pigment concentrations on the western African continental margin may indicate fractionated sedimentation of organic matter to the sea floor. Received: 28 May 1997 / Accepted: 14 July 1997     

Significance of cellular nitrate content in natural populations of marine phytoplankton growing in shipboard cultures

Phytoplankton intracellular nitrate concentrations have been monitored in a 56-h experiment on a shipboard culture of surface sea water from an upwelling region. These measurements were related to parameters of biomass (particulate nitrogen) and nitrate assimilation using the ¹⁵N isotope technique and the nitrate reducase (NR) assay. The procedure for measuring cellular nitrate concentrations is described. This parameter exhibited diurnal variations, ranging from 3.1 to 20.6 ng-at nitrate per g-at particulate nitrogen, and could be correlated positively with NR activity. Nitrogen budgets show that NR activity represents only 12% of nitrate incorporation in organic phytoplankton material when nitrate is available in the sea water. However, upon depletion of the environmental nitrate (zero uptake), NR activity can fully account for the decrease of internal nitrate. From the results, it seems that internal nitrate content is a better index of nitrate consumption by marine phytoplankton than the external concentration of nitrate-nitrogen.     

Particulate organic matter in the Northern and Central Adriatic

The particulate organic matter distribution and its elemental composition in the northern and central Adriatic Sea during different seasonal periods are shown, highlighting the principal processes and factors influencing their distribution and characteristics. In the low salinity waters the concentrations of particulate carbon, nitrogen and phosphorus were higher and more variable than in the dense waters, mainly due to dilution effects which induce an abundant phytoplankton growth. Generally in summer the particulate organic matter distribution followed the trophic gradient while in winter resuspension events often became more important. Differences between summer and winter were more evident in the diluted waters and were mainly due to the seasonal heat exchanges and to the fresh water inputs. Marked differences in C/P ratios were observed in the POM: high ratios in the northern diluted waters and low in the more saline waters and in the central Adriatic.     

Seasonal Fluctuations in the Nutritional Value of Particulate Organic Matter in A Lagoon

Water samples were collected on a fortnightly basis in the lagoon of S. Gilla (Sardinia, Mediterranean Sea) in order to study seasonal nutritional fluctuations of particulate organic matter. the lagoon is characterized by high quantities of suspended matter throughout the year. Thermohaline conditions had no effect on particulate matter quantity and composition, but the quantity as well as quality of suspended particles was drastically affected by the wind, the major effecter of sediment resuspension. As a result of sediment resuspension, seston was always richer in inorganic fraction. However, throughout the year of investigation, most particulate organic carbon was quite appealing for filter feeding communities, although the best POM quality was available during phytoplankton blooming. the phytoplankton pool of suspended matter was just a small fraction of the bulk, accounting for only 13% on average of particulate organic carbon. in terms of energy available in the seston, the highest amount was stored in organic matter heterotrophic fraction, whilst the smallest was to be found in living phytoplankton.     

Hydrodynamic and chemical conditions during onset of a red-tide assemblage in an estuarine upwelling ecosystem

The hydrodynamics and nitrogen/silicon biogeochemistry accompanying the development of a red-tide assemblage were examined in the Ría de Vigo (northwest Spain), a coastal embayment affected by upwelling, during an in situ diel experiment in September 1991. Despite a low N:Si molar ratio (0.5) of nutrients entering the surface layer, which was favourable for diatom growth, the diatom population began to decline. Limited N-nutrient input, arising from moderate coastal upwelling in a stratified water column, restricted net community production (NCP = 630 mg C m⁻² d⁻¹). In addition, light-limitation of gross primary production (GPP = 1525 mg C m⁻² d⁻¹) was observed. The relatively high f-ratio (= NCP:GPP) recorded (0.41, characteristic of intense upwelling conditions) would have been as low as 0.15 had not GPP been limited by light intensity. Temporal separation of carbohydrate synthesis during the photoperiod from protein synthesis in the dark could be inferred from the time-course of the C:N ratio of particulate organic matter. Severe light-limitation would lead to diatom collapse were the diatoms not able to meet all their energy requirements during the hours of darkness. Under the hydrodynamic, nutrient and light conditions of the experiment, an assemblage of red-tide-forming species began to develop, aided by their ability to migrate vertically and to synthesize carbohydrates during the light in surface waters and protein during the dark at the 4 m-deep pycnocline. Thermal stratification, reduced turbulence, intense nutrient mineralization, and the limited nitrogen input through moderate upwelling were all favourable to the onset of a red-tide assemblage. Received: 15 February 1997 / Accepted: 26 September 1997     

Suspended particulate organic matter in a Mediterranean submarine cave

Submarine caves display a paucity of benthic density and biomass that may be related to low trophic resources. Analysis of organic carbon, organic nitrogen, carbohydrate, protein and lipid content of suspended particulate matter was made during the period July 1985–July 1987 in a Mediterranean cave (Marseille, France) in order to determine any differences in the particulate organic matter composition along an horizontal transect. Particulate organic matter content clearly declined from the entrance of the cave to the dark inner area. This impoverishment could not be explained by a simple decrease in a few organic compounds, but appeared to be related to the combination of a decrease in both the amount and the composition of the suspended particles. Three progressive levels of impoverishment were identified towards the dark inner area of the cave: (i) decreasing amounts of seston; (ii) decreasing organic content of particles; (iii) increasing proportions of the geopolymeric (i.e., humic) components in the remaining organic matter, indicating increased degradation. The cave appeared to be sharply divided into two distinct sections — a twilight outer section whose waters contained slightly lower amounts of particulate organic matter than the open sea, and a dark inner section, 8 to 10 m higher, separated from the outer section by a steep rise and with waters of low organic matter content. The water in the twilight section appeared to be in thermal equilibrium with the open sea, and that in the dark inner section displayed thermal stratification. These differences indicated the presence of two distinct water bodies with contrasting average residence times, estimated as 1 d in the outer twilight section and 8 d in the dark inner section. The joint action of sedimentation and degradation resulted in an abrupt depletion of particulate organic matter in the dark inner section accompanied by a decrease in the benthic fauna. The decline in benthic heterotroph populations is probably related to the abrupt transition to oligotrophic conditions.