The effect of microorganisms and seasonal factors on the isotopic composition of particulate organic carbon from the black sea

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the¹²C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO₂ fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC δ¹³C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Effect of microorganisms and seasonal factors on the isotope composition of organic carbon from Black Sea suspensions

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the 12C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO2 fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC delta 13C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Temporal variability of particulate organic carbon,nitrogen and phosphorus in the Northern Adriatic Sea

The particulate organic carbon (POC), nitrogen (PN) and phosphorus (PP) vertical distribution along the water column and temporal variability in coastal and offshore waters of the Northern Adriatic Sea were related to the hydrodynamic conditions and biological processes. Fresh water inputs from the Po and Adige rivers enhance primary production, resulting in high POC, PN and PP concentrations at the surface. In offshore waters, POC and PN concentrations were about 3–4 times less than in the coastal waters, while PP were up to 10 times lower, highlighting a marked phosphorus depletion. In the bottom layer, the POC content decreases due to the strong density gradients which separate bottom waters with prevailing degradation processes. Short term 48 h-variability of POC, PN and PP in the coastal waters was determined to a great extent by variations in the spreading of river plumes at the surface and by nepheloid layers and resuspension processes in the bottom waters. The particulate matter in the Adriatic offshore waters is extremely depleted as regards particulate phosphorus and is characterised by C_org:P and N:P ratios higher than the Redfield ratio.     

The composition and distribution of the particulate matter in the Strait of Magellan (Chile) during the 1991 and 1995 Italian campaigns

During 1991 and 1995, the Italian National Program for Antarctic Research carried out two oceanographic campaigns in the framework of the International (Strait of) Magellan Project. In this paper, we describe the distribution, biochemical composition, and mineralogical characteristics of particulate matter and the characteristics of the water masses defining microbasins in the Strait of Magellan. The data analyses highlighted differences in quality and quantity of the suspended matter and its organic component in the basins that make up the Strait and the Pacific Ocean. The westernmost basin is subject to an eastward flow from the Pacific Ocean, and continental runoff in the surface layers, which are consequently rich in organic matter with a high C:N ratio. The central basin, Isla Carlos III–Segunda Angostura, is characterized by the mixing of Sub-Antarctic Pacific waters, continental runoff, and glacio-fluvial waters: The basin has the lowest particulate matter concentration, but at Paso Ancho showed high concentrations of organic matter with a lower C:N ratio. The easternmost basin, Segunda Angostura-Atlantic entrance, is characterized by the mixing of the water column due to strong Atlantic tides and showed high concentration of particulate matter with a high detritic component. Generally, the organic matter concentrations showed significant differences during the cruises, being higher in 1991. Using the multivariate discriminant factor analysis to determine whether statistically significant differences existed between defined sampling areas, we determined that temperature, particulate organic carbon, and total particulate matter were the variables most important for the difference between areas.     

Particulate matter composition and bacterial distribution in Terra Nova Bay (Antarctica) during summer 1989–1990

Spatial distributions of particulate organic matter (POM) and microbes were investigated during the summer of 1989–1990 in the coastal waters of Terra Nova Bay (Antarctica). The elemental (organic carbon and nitrogen) and biochemical (lipids, proteins, carbohydrates, DNA and RNA) composition of organic matter was related to bacterioplankton abundance, and pico-phytoplankton density. The ATP concentrations were also measured to gather information about the relationships between particulate matter composition and microbial distribution in Antarctic waters. Total seston was characterized by little spatial variation and was unrelated to the distance from the coast. Suspended particulate matter included some terrestrial components but was mostly composed of autochthonous material. POM was characterized by a uniform distribution and homogeneous composition (mostly of phytoplanktonic origin), and was associated with a relatively scarce microbial community characterized at the surface by high picophytoplankton density. The increase with depth of the living carbon fraction suggested an increase in the microheterotrophic community in the deeper water layers. A significant positive relationship between total bacterioplankton density, and carbohydrate and RNA concentrations was found. Similar significant relationships between pico-phytoplankton abundance and lipids, proteins, carbohydrates and nucleic acids were observed. On the basis of the close coupling found between microbiological and chemical compartments, it seems that, in Terra Nova Bay, bacterial distribution depends on suspended matter and in particular to the labile fraction of the organic detritus.     

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system. Received: 17 June 1997 / Accepted 25 September 1997     

Effects of exported seston on aquatic insect faunal similarity and species richness in lake outlet streams in Montana,USA

The quantity of seston exported from a lake affected the qualitative composition and species richness of aquatic insects in 13 lake outlet streams in northwestern Montana. Mean values for chlorophyll and particulate organic carbon in lake effluent waters ranged from 0.5 to 2.7 and 80 to 601 mg m^-3, respectively. Correlations between a detrended correspondence analysis of faunal similarities in outlet streams and measures of organic export from the lake indicated that distributional patterns were primarily dependent upon the general trophic status of the lakes. A comparison of faunal assemblages at stream stations located at the outlet and 100 m downstream demonstrated that species composition at the two stations was more similar in productive systems.     

The distribution of macroinvertebrate assemblages in a reach of the River Allier (France), in relation to riverbed characteristics

Investigations on large canalised rivers, for example the Danube, have shown that transported particulate matter, which is typically inorganic, is predominantly deposited in waters near the river’s main channel. This investigation deals with the lower section of the River Havel (NE Germany), a canalised lowland river with a very flat floodplain. This river is highly polluted by nutrients from urban areas (Berlin) and a long chain of river lakes produces high concentrations of phytoplankton. Due to the high proportion of planktogenic detritus, it was hypothesised that greater quantities of nutrient-rich fine particulate organic matter (FPOM) would be deposited in floodplain waters located further from the main channel than has been reported for large rivers. The total nutrient, P-binding metal (Fe, Al, and Mn), organic and inorganic carbon (TOC, TIC) contents of the upper organic sediment layer (0–4 cm) were analysed in samples collected from 48 floodplain water and river sites. The sediment bulk density, calculated on the basis of dry mass content and loss on ignition, was used to characterise the waters according to the impact of the river current. The results showed that the variability of total phosphorus (TP) was best explained by the variability of total iron (TFe, R² = 0.52). The floodplain water sediments could clearly be separated into two groups on the basis of the sediment particle size composition, and of the element ratios TOC:TP, TN:TP, primarily TFe:TP. The sediments from impounded river sections and from mouth sections of backwaters (approx. 100–200 m) were characterised by a high proportion particles from the 0.1 to 0.5 mm size fraction and by homogeneous, low TFe:TP, TOC:TP and TN:TP ratios. Sediments from distal sections of backwaters and of oxbow lakes tended to exhibit high element ratios with much higher variability. These results were interpreted as a spatially limited impact of the river on the floodplain water sediments. Contrary to expectation, the phosphorus bound in river seston was predominantly and very homogeneously deposited in the impounded river and mouth sections of backwaters. This implies that the inundation of the floodplain waters during spring floods seems to have no important material impact on the sediments in waters of low hydrological connectivity with the River Havel.     

Is the copepod egg production in a highly turbid estuary (the Gironde, France) a function of the biochemical composition of seston?

The influence of the biochemical composition of particles originating from surface waters of the Gironde estuary on egg production rates of Eurytemora affinis zooplanktonic population was studied. In the high turbidity zone, suspended particulate matter had a low nutritional quality because the easily available organic fraction represented less than 15% of the overall particulate organic matter. In waters located seaward of the high turbidity zone, a slight increase in nutritional quality was observed. As a result, the sum of easily extractable organic macromolecules represented 15 to 33% of the overall particulate organic matter. The present study suggests that the low egg-production rate of Eurytemora affinis, occurring in the high turbidity zone, results from combined effects of temperature and bad feeding conditions in the area. Low copepod production can be explained by little phytoplankton growth due to light limitation and, therefore, restricted food availability, as well as difficulties in food selection, non-living particle may being dominant.     

Competition for Ammonium between Nitrifying and Heterotrophic Bacteria in Dual Energy-Limited Chemostats

The absence of nitrification in soils rich in organic matter has often been reported. Therefore, competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing species Nitrosomonas europaea and the heterotrophic species Arthrobacter globiformis was studied in the presence of Nitrobacter winogradskyi in continuous cultures at dilution rates of 0.004 and 0.01 h⁻¹. Ammonium limitation of A. globiformis was achieved by increasing the glucose concentration in the reservoir stepwise from 0 to 5 mM while maintaining the ammonium concentration at 2 mM. The numbers of N. europaea and N. winogradskyi cells decreased as the numbers of heterotrophic bacteria rose with increasing glucose concentrations for both dilution rates. Critical carbon-to-nitrogen ratios of 11.6 and 9.6 were determined for the dilution rates of 0.004 and 0.01 h⁻¹, respectively. Below these critical values, coexistence of the competing species was found in steady-state situations. Although the numbers were strongly reduced, the nitrifying bacteria were not fully outcompeted by the heterotrophic bacteria above the critical carbon-to-nitrogen ratios. Nitrifying bacteria could probably maintain themselves in the system above the critical carbon-to-nitrogen ratios because they are attached to the glass wall of the culture vessels. The numbers of N. europaea decreased more than did those of N. winogradskyi. This was assumed to be due to heterotrophic growth of the latter species on organic substrates excreted by the heterotrophic bacteria.     

Origin, biochemical composition and vertical flux of particulate organic matter under the pack ice in Terra Nova Bay (Ross Sea, Antarctica) during late summer 1995

Water samples and particulate materials settling under the pack ice were collected in an ice-covered area near the Terra Nova Bay Italian Station during late summer 1995, in order to study short-term changes in the biochemical composition of particulate organic matter. At the end of the study period the phytoplankton biomass increase (up to >3.0 μg chlorophyll-a l⁻¹) was probably related to the intrusion under the pack ice of chlorophylls-enriched surface waters coming from the near ice-free area. Such increase was associated also with a notable increase in particulate organic matter concentrations, as well as in particulate organic matter vertical fluxes (up to >100 mg C m⁻² day⁻¹). Proteins were the most abundant biochemical class of particulate organic matter (on average about 49%), followed by lipids (29%) and carbohydrates (22%). By contrast, organic matter collected in the sediment trap was characterized by the dominance of lipids (about 55% of the total biopolymeric carbon flux) over carbohydrates (28%) and proteins (17%). The hydrolizable particulate biopolymeric carbon accounted for about 23% of total biopolymeric carbon. This value was about one-half of that found in ice-free waters, suggesting that the suspended particulate organic material under the pack ice was less digestible than in ice-free waters or was already partially digested. Despite this, and the decay of labile organic compounds in the sediment trap during the deployment, material settling towards the sea bottom under the pack ice in Terra Nova Bay, owing to its high lipid content, might represent an important high-quality food source for benthic consumers. Finally, assuming as possible the intrusion under sea ice of primary organic matter-enriched waters, we hypothesize the occurrence of a “fertilization” effect deriving from ice-melting areas towards under-ice waters, supplying the latter with an additional rate of primary organic matter. Accepted: 18 February 1999     

Importance of Saprotrophic Freshwater Fungi for Pollen Degradation

Fungi and bacteria are the major organic matter (OM) decomposers in aquatic ecosystems. While bacteria are regarded as primary mineralizers in the pelagic zone of lakes and oceans, fungi dominate OM decomposition in streams and wetlands. Recent findings indicate that fungal communities are also active in lakes, but little is known about their diversity and interactions with bacteria. Therefore, the decomposer niche overlap of saprotrophic fungi and bacteria was studied on pollen (as a seasonally recurring source of fine particulate OM) by performing microcosm experiments with three different lake types. Special emphasis was placed on analysis of fungal community composition and diversity. We hypothesized that (I) pollen select for small saprotrophic fungi and at the same time for typical particle-associated bacteria; (II) fungal communities form specific free-living and attached sub-communities in each lake type; (III) the ratio between fungi or bacteria on pollen is controlled by the lake''s chemistry. Bacteria-to-fungi ratios were determined by quantitative PCR (qPCR), and bacterial and fungal diversity were studied by clone libraries and denaturing gradient gel electrophoresis (DGGE) fingerprints. A protease assay was used to identify functional differences between treatments. For generalization, systematic differences in bacteria-to-fungi ratios were analyzed with a dataset from the nearby Baltic Sea rivers. High abundances of Chytridiomycota as well as occurrences of Cryptomycota and yeast-like fungi confirm the decomposer niche overlap of saprotrophic fungi and bacteria on pollen. As hypothesized, microbial communities consistently differed between the lake types and exhibited functional differences. Bacteria-to-fungi ratios correlated well with parameters such as organic carbon and pH. The importance of dissolved organic carbon and nitrogen for bacteria-to-fungi ratios was supported by the Baltic Sea river dataset. Our findings highlight the fact that carbon-to-nitrogen ratios may also control fungal contributions to OM decomposition in aquatic ecosystems.     

Distribution and composition of particulate organic matter in the Ross Sea (Antarctica)

The biochemical composition and spatial distribution of particulate organic matter (POM) were studied in the Ross Sea (Antarctica) in summer 1989 to assess the quantitative role of organic carbon fractions in the cycling of organic matter in the water column. Large differences in chemical composition were observed between surface and deep layers. The results indicated that, despite large geographical differences, POM was quite homogeneous, of phytoplankton origin and mostly detrital. Different ratios were used to investigate the changes in biochemical composition of particulate organic matter in relation to the ice-melting: CN (organic carbonorganic nitrogen ratio) and C-POMPOC (sum of carbohydrate, protein and lipid carbontotal organic carbon ratio) were used to analyse the percentage of refractory organic material. PPRTPCHO (proteincarbohydrate ratio) were used to establish POM age and RNADNA ratios as a relative measure of particulate activity; POCChl a and N-PPRTChl a ratios were used to estimate the autotrophic contribution to the suspended particulate organic matter. Despite its low caloric value (5.3 Kcal g POM^–1), an high caloric content in the photic layer (1.6 Kcal m^–3 of POM and 2.5 Kcal m^–3 of POC) was found thus indicating that a large amount of food was available to higher trophic levels.     

Seasonal and site-specific variability in terrigenous particulate organic carbon concentration in near-shore waters of Lake Biwa, Japan

Identifying sources of particulate organic matter (POM) is important for clarifying fundamental mechanisms by which lake food webs are sustained. We determined carbon and nitrogen stable isotope ratios of POM in near-shore waters of Lake Biwa, a large, meso-eutrophic lake in Japan, to estimate relative contributions of terrigenous particulate organic carbon (T-POC), plankton-derived POC (P-POC) and epilithon-derived POC (E-POC) to POC in near-shore waters. Samples were collected during different months (November, February, May and July) at 29 sites located near the mouth of tributary rivers with different discharge and catchment land use. The data revealed that POC mainly consisted of P-POC and T-POC, with relative contributions varying widely over season and among locations. E-POC generally contributed little to the near-shore POC. Path analyses revealed that the concentration of riverine POC whose isotopic signatures were similar to those of rice straws increased with a larger %paddy field area in the catchment of tributary rivers, which subsequently enhanced T-POC inputs to near-shore waters through riverine transportation. Furthermore, our results suggested that T-POC contribution was influenced, with a time lag, by wave-driven turbulence and shore topography, which appear to affect sedimentation and resuspension of T-POC.     

Particulate DNA and protein relative to microorganism biomass and detritus in the Catalano-Balearic Sea (NW Mediterranean) during stratification

Using microscopic and biochemical approaches, the relative contribution of the main groups of pelagic microorganisms (bacteria, heterotrophic nanoflagellates, phytoplankton and ciliates) and detritus (<150 m) to total particulate protein and DNA was investigated at two stations of the Catalano-Balearic Sea (NW Mediterranean) during the stratified period. The two stations, one located in the shelf break front (S) and the other in the open sea, above the central divergence zone (D), were sampled twice in early summer 1993. Both of them showed a well-developed deep chlorophyll maximum (DCM). Maximum DNA concentrations were observed close to the DCM, while protein concentrations were fairly homogeneous from the surface to 60 m depth in all samplings. In general, the microorganism distribution showed maximum concentrations at or near the DCM depths. At both stations, bacteria were the most important contributors to living particulate DNA (22.5-32.6%), while phytoplankton and heterotrophic nanoflagellates were the main contributors to living particulate protein (3.8-24.4 and 2.9-29.1%, respectively). In addition, an important amount of detrital DNA and protein was estimated to occur at both stations. Detrital DNA accounted for 23.9-42.9% of the particulate DNA, while detrital protein represented from 63.5 to 84.7% of the particulate protein. Because both protein and DNA contain nitrogen and DNA is also a phosphorus source, these results indicate that heterotrophic organisms and detrital particles play an important role in the nitrogen and phosphorus cycles in the open sea waters of the NW Mediterranean.      

梯级水电站开发对香溪河悬浮颗粒物粒径及浓度的影响

通过对香溪河悬浮颗粒物粒度及浓度的实验分析,研究梯级水电站开发对河流悬浮颗粒物的影响,结果表明:(1)香溪河干流悬浮颗粒物粒径由上游到中游逐渐减小,并且在无水电站干扰时颗粒物粒径减小幅度较大;(2)从进水口至出水口河段,水电站悬浮颗粒物粒径总体变化不显著;(3)水电站从进水口河段至出水口河段,总悬浮颗粒物浓度(CTPM)在某些位置有所增加,有机颗粒物浓度(CPOM)无明显变化.悬浮颗粒物粒度及浓度的变化与水流流速及流量有着相当大关系.     

Summertime primary production and carbon export in the southeastern Beaufort Sea during the low ice year of 2008

Following the extreme low ice year of 2007, primary production and the sinking export of particulate and gel-like organic material, using short-term particle interceptor traps deployed at 100 m, were measured in the southeastern Beaufort Sea during summer 2008. The combined influence of early ice retreat and coastal upwelling contributed to exceptionally high primary production (500 ± 312 mg C m⁻² day⁻¹, n = 7), dominated by large cells (>5 μm, 73% ± 15%, n = 7). However, except for one station located north of Cape Bathurst, the sinking export of particulate organic carbon (POC) was relatively low (range: 38–104 mg C m⁻² day⁻¹, n = 12) compared to other productive Arctic shelves. Estimates indicate that 80% ± 20% of the primary production was cycled through large copepods or the microbial food web. Exopolymeric substances were abundant in the sinking material but did not appear to accelerate POC sinking export. The use of isotopic signatures (δ¹³C, δ¹⁵N) and carbon/nitrogen ratios to identify sources of the sinking material was successful only at two stations with a strong marine or terrestrial signature, indicating the limitations of this approach in hydrographically and biologically complex Arctic coastal waters such as in the Beaufort Sea. At these two stations influenced by either coastal upwelling or erosion, the composition and magnitude of particulate sinking fluxes were markedly different from other stations visited during the study. These observations underscore the fundamental role of mesoscale circulation patterns and hydrodynamic singularities on the export of particulate organic material on Arctic shelves.     

Suspended particulate matter dynamics in the shallow mesotidal urdaibai estuary (Bay of Biscay,Spain)

Seston quantity and quality was measured in the Urdaibai Estuary in summer, when river discharge is low and tidal flushing is the main driving force to transport and resuspend particulate matter in the estuary. The highest seston concentrations are found in the upper estuary, where more than 90% of total suspended particulate matter is made up of particles <20 μm. There is a temporal ebb-flood asymmetry and turbulent mixing is stronger at flood tide. During dry calm periods sediment resuspension is observed for a short period of time at floods during spring tides, decreasing drastically before slack waters. In spite of this, particulate organic matter mainly originates from anthropogenic sources (domestic wastewater discharge) and plankton production.     

Pigment patterns in suspended matter from the Elbe estuary,Northern Germany

In the Elbe hardly anything is known about the actual fate of phytoplankton and the resultant pigment composition of suspended matter. As part of a longterm study on the role of suspended matter in the cycles of nitrogen and associated oxygen consumption processes in the Elbe estuary in northern Germany (‘Sonderforschungsbereich 327 Elbe’ project) and the characterization of estuarine and coastal water bodies with regard to the transport of pollutants such as heavy metals, we have started to characterize suspended matter with regard to the pigments present therein. This will allow us to study phytoplankton turnover and to determine the role of phytoplankton decay mechanisms in the mobilization, binding, and transport of pollutants associated with particulate matter. Our first data on the pigment composition of suspended matter from the Elbe and particularly over the region of Hamburg harbour, obtained using a High Performance Liquid Chromatography (HPLC) method, is presented. The pigment concentrations varied considerably over the summer months. Values up to 250 mg l⁻¹ of chlorophyll a were observed upstream of Hamburg. We found that on its course through Hamburg the concentrations (μg g⁻¹ of particulate matter) of all pigments in the Elbe drop to under half those found upstream of Hamburg. The pigment concentrations in the turbidity maximum were significantly lower than in the rest of the estuary due to the decline of algae as a result of inadequate light conditions and the salinity gradient. The highest particulate matter concentrations were found in the turbidity maximum and just downstream of Hamburg. The highest particulate organic carbon values were found just downstream of Hamburg. The presence of of fucoxanthin and the chlorophyllsc andb, and the changes in pigment patterns were indicative for the dominant algal classes and the phytoplankton succession in the suspended matter. The ratio of chlorophylla to lutein was found to be a possible indicator of phytoplankton breakdown.     

Phytoplankton blooms in the vicinity of palmer station,Antarctica

Summary Fifteen oceanographic stations were occupied in the vicinity of Anvers Island, Antarctica, in January of 1985 and 1987. All stations showed high phytoplankton biomass (4.0 to 30 g chl-a/liter) which was either uniformly distributed in the upper mixed layer or showed a pronounced sub-surface maximum at 4–5 m depth. As phosphate was less than 0.02 m and nitrate about 2.0 m in surface waters, it appears that nutrient limitation of phytoplankton growth may be of importance during such blooms. This view is supported by chemical measurements of the particulate material which showed high chl-a/ATP ratios (about 7.7), as well as high POC/ATP ratios (about 700). Microscopical analysis revealed a dominance of large-celled diatoms and the near absence of heterotrophic protozoans. Size fractionation studies showed that the nanoplankton accounted for only 28% of the total phytoplankton biomass. When phytoplankton biomass reaches the levels found at these stations, it appears that the cells are light-limited and hence dark-adapted, which results in the high chl-a/ATP ratios and the low assimilation values (0.49–1.64) obtained in our studies. Under such conditions greater than 50% of the total phytoplankton biomass is found below the 1% light level.Contribution No. 2154 of the Hawaii Institute of Geophysics