Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Biomass partitioning of benthic microbes in a Baltic inlet: relationships between bacteria, algae, heterotrophic flagellates and ciliates

 The structure of a benthic microbial food web and its seasonal changes were studied in the shallow brackish waters of the island of Hiddensee, northeastern Germany, at two sites in close proximity by monthly or bimonthly sampling from July 1995 to June 1996. Abundance and biomass of phototrophic and non-phototrophic bacteria, heterotrophic flagellates (HF) and ciliates as well as the biomass of microphytobenthos were determined in the upper 0.3 cm sediment layer. Abundance of organisms showed strong positive correlation with water temperature, with the exception of the bacteria. Non-phototrophic bacterial numbers ranged from 7 × 10⁸ to 6.7 × 10⁹ cells cm⁻³ and phototrophic bacterial abundance from 4 × 10⁷ to 2.7 × 10⁸. Heterotrophic protist abundance ranged from 8 × 10³ to 104 × 10³ ind cm⁻³ for HF and from 39 to 747 ind cm⁻³ for ciliates. The biomass partitioning demonstrated the primary importance of non-phototrophic bacteria (min. 0.83, max. 84.87 μg C cm⁻³), followed by the microphytobenthos (min. 1.32, max. 50.93 μg C cm⁻³). The heterotrophic protists contributed roughly the same fraction to the total microbial biomass, with the biomass of the HF being slightly higher (HF 0.23 to 1.76 μg C cm⁻³, ciliates 0.04 to 1.17 μg C cm⁻³). Taxonomic classification of the benthic HF revealed the euglenids to be the most important group in terms of abundance and biomass, followed by thaumatomastigids and kinetoplastids. Other important groups were apusomonads, cercomonads, pedinellids and cryptomonads. The structure of the HF assemblage showed strong seasonal changes with euglenids being the most abundant taxa in summer, while apusomonads and thaumatomastigids were predominant in winter. Similar to the pelagic microbial food web, benthic picophototrophic bacteria were occasionally abundant, and the feeding modes of heterotrophic protists exhibited a great variety (predominantly omnivores, bacterivores, herbivores or predators). Filter-feeding HF were of little importance. Contrary to the pelagic environment, a top-down control on total benthic bacterial numbers by HF seemed unlikely at the studied stations which were characterised by muddy sand. Received: 6 January 1999 / Accepted: 21 October 1999     

Feeding of Clausocalanids (Calanoida,Copepoda) on naturally occurring particles in the northern Gulf of Aqaba (Red Sea)

A total of 12 feeding experiments were conducted in the northern Gulf of Aqaba during spring (March/April) and autumn (September/October) 2002 at the Marine Science Station (MSS) in Aqaba. Females of three species of clausocalanids were selected: Clausocalanus farrani, C. furcatus and Ctenocalanus vanus. Natural occurring particle (NOP) larger than 5 μm were investigated as food source. The ambient chlorophyll a concentration at sampling depth (∼70 m) ranged between 0.15 and 1.00 μg chl a l⁻¹ and NOP concentrations ranged between 1.78 and 14.0 × 10³ cells l⁻¹ during the sampling periods. The division of particles into five size classes (5–10, 10–20, 20–50, 50–100 and >100 μm) revealed that most of the particles were found in the size classes below 50 μm (81–98%), while most of the natural occurring carbon (NOC) was concentrated in the size classes larger than 20 μm (70–95%). Ingestion rates were food density dependent rather than size dependent ranging between 0.02 and 1.65 × 10³ NOP ind⁻¹ day⁻¹ and 0.01 and 0.41 μg NOC ind⁻¹ day⁻¹, respectively, equivalent to a body carbon (BC) uptake between 0.4 and 51.8% BC day⁻¹. The share of the size classes to the total ingestion resembled in most cases the size class composition of the natural particle community.     

Changes in desert steppe soil culturable bacteria from northwestern China and correlation with physicochemical parameters

Seasonal variations effects on desert steppe soil culturable bacteria in northwestern China and physicochemical parameters were investigated. Soil temperature ranged from 5.37 to 23.73 °C. Moisture content varied from 3.22% to 7.62%. The pH was between 7.98 and 8.72. Soil nutrients were altered in different seasons. Total potassium (K) remained the same, but available K levels ranged from 101.29 to 227.55 mg/kg. The culturable bacterial counts in January, April, July and October were 0.13 × 10⁷, 4.09 × 10⁷, 5.33 × 10⁷ and 1.8 × 10⁷ colony forming units (cfu)/g, respectively. Bacterial populations were most abundant in surface layers and declined with increase of soil depth. Seventy-two bacterial strains were isolated and classified according to 27 genera of 5 phyla groups: Firmicutes, Actinobacteria, Alphaproteobacteria, Gammaproteobacteria and Bacteroidates. The bacterial diversities varied according to seasons. In spring, only Firmicutes and Actinobacteria were isolated. In summer, Firmicutes, Actinobacteria and Bacteroidetes were detected with Firmicutes the dominant species. In autumn and winter, Firmicutes, Actinobacteria, α-Proteobacteria and γ-Proteobacteria predominated. Our results indicated that climatic variations, vegetation coverage and soil physicochemical parameters are critical factors for maintaining culturable bacterial populations and diversity in the desert steppe. Soil moisture content and temperature exerted marked influence on bacterial quantities and diversities.     

Allocation of nitrogen and carbon in an estuarine salt marsh in Portugal

Above and below-ground biomass and nitrogen and carbon composition ofSpartina maritima, Halimione portulacoides andArthrocnemum perenne, dominating species in plant communities of the lower, middle and higher salt marsh, respectively, were compared in an estuarine salt marsh in Portugal. Plant and soil nitrogen and carbon pools were estimated. For all three species root biomass was significantly higher (70–92% of total biomass) than above-ground biomass. The percentage of root biomass was related to the location of the plants in the marsh: higher values were found in plants growing in the lower salt marsh where the sediment was more unstable and subject to tidal action, which stresses the role of the roots as an anchor. For all three species nitrogen concentrations were highest in leaves, reflecting the photosynthetic role of the tissue. For carbon higher concentrations were found in the stems, with the exception ofS. maritima. In general, lower nitrogen concentrations were found in summer, which can be explained by dilution processes due to plant growth. For both nitrogen and carbon, higher concentrations were found in the soil surface layers. Higher soil nitrogen and carbon levels were associated with higher organic matter contents. Most of the nitrogen in the salt marsh occurred in the sediments (0–40 cm) and only ca. 5.7–13.3% of the total was found in the plants. The greater portion (76.5%–86%) of carbon was found in the sediment.     

Effects of salinity on soil bacterial and archaeal community in estuarine wetlands and its implications for carbon sequestration: verification in the Yellow River Delta

Soils from two typical tidal salt marshes with varied salinity in the Yellow River Delta wetland were analysed to determine possible effects of salinity on soil carbon sequestration through changes in soil microbiology. The mean soil respiration (SR) of the salt water–fresh water mixing zone (MZ) was 2.89 times higher than that of the coastal zone (CZ) (4.73 and 1.63?μmol?m^?2?s^?1, respectively, p?Pseudomonas sp. and Limnobacter sp. that might have led to its higher dehydrogenase activity and respiratory rates. Additionally, the CZ possessed more Halobacteria and Thaumarchaeota with the ability to fix CO₂ than the MZ. Significantly lower soil salinity in MZ (4.25?g?kg^?1) was suitable for β-Proteobacteria, but detrimental for Halobacteria compared with CZ (7.09?g?kg^?1, p?相似文献   

Metabolism and elemental composition of four oncaeid copepods in the western subarctic Pacific

Respiration rates and elemental composition (carbon and nitrogen) were determined for four dominant oncaeid copepods (Triconia borealis, Triconia canadensis, Oncaea grossa and Oncaea parila) from 0–1,000 m depth in the western subarctic Pacific. Across the four species of which dry weight (DW) varied from 2.0 to 32 μg, respiration rates measured at in situ temperature (3°C) increased with DW, ranging from 0.84 to 7.4 nl O₂ individual⁻¹ h⁻¹. Carbon (C) and nitrogen (N) composition of the four oncaeid species ranged from 49–57% of DW and 7.0–10.3% of DW, respectively, and the resultant C:N ratios were 4.8–8.3. The high C contents and C:N ratios were reflected by large accumulation of lipids in their body. Specific respiration rates (SR, a fraction of body C respired per day) ranged between 0.5 and 1.3% day⁻¹. Respiration rates adjusted to a body size of 1 mg body N (i.e. adjusted metabolic rates, AMR) of the four oncaeid species [0.6–1.1 μl O₂ (mg body N)^−0.8 h⁻¹ at 3°C] were significantly lower than those (1.7–5.1) reported in the literature for oithonid and calanoid copepods at the same temperature. The present results indicate that lower metabolic expenditure due to less active swimming (pseudopelagic life mode) together with rich energy reserve in the body (as lipids) are the characters of oncaeid copepods inhabiting in the epi- and mesopelagic zones of this region.     

Selective feeding of littoral harpacticoids on diatom algae: hungry gourmands?

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm², so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.     

Trophic role of small cyclopoid copepod nauplii in the microbial food web: a case study in the coastal upwelling system off central Chile

Copepod grazing impact on planktonic communities has commonly been underestimated due to the lack of information on naupliar feeding behaviour and ingestion rates. That is particularly true for small cyclopoid copepods, whose nauplii are mainly in the microzooplankton size range (<200 μm). The trophic role of Oithona spp. nauplii was investigated off Concepción (central Chile, ~36°S) during the highly productive upwelling season, when maximum abundances of these nauplii were expected. Diet composition, ingestion rates, and food-type preferences were assessed through grazing experiments with different size fractions of natural planktonic assemblages (<3, <20, <100, and <125 μm) and cultures of the nanoflagellate Isochrysis galbana. When the Oithona spp. nauplii were offered a wide range of size fractions as food (pico- to microplankton), they mostly ingested small (2–5 μm) nanoflagellates (5–63 × 10³ cells nauplius⁻¹ day⁻¹). No ingestion on microplankton was detected, and picoplankton was mainly ingested when it was the only food available. Daily carbon (C) uptake by the nauplii ranged between 28 and 775 ng C nauplius⁻¹, representing an overall mean of 378% of their body C. Our relatively high ingestion rate estimates can be explained by methodological constraints in previous studies on naupliar feeding, including those dealing with “over-crowding” and “edge” effects. Overall, the grazing impact of the Oithona spp. nauplii on the prey C standing stocks amounts up to 21% (average = 13%) for picoplankton and 54% (average = 28%) for nanoplankton. These estimates imply that the nauplii of the most dominant cyclopoid copepods exert a significant control on the abundances of nanoplankton assemblages and, thereby, represent an important trophic link between the classical and microbial food webs in this coastal upwelling system.     

Environmental geochemistry study of arsenic in Western Hunan mining area,P.R. China

The geochemical characteristics of arsenic in the soil of the Western Hunan mining area of P.R. China were systematically studied. The results show that the strata of Western Hunan are rich in arsenic and that Western Hunan is a geochemically abnormal region for arsenic. The experimental study on speciation in the strata also indicates that the speciation of arsenic in the Neoproterozoic-Cambrian strata are mainly easily transferred speciation (exchangeable, carbonate-bound, sulfides-bound), which are approaching or exceed 60%. Arsenic content in the main soil of Western Hunan is in the range of 8.8–22.8 μg g⁻¹, the mean value is 16.1 μg g⁻¹, which is larger than the arsenic background value of Hunan soil. The distribution of rock with high arsenic content or high easily transferred arsenic speciation is consistent with the distribution of high arsenic content soil. In the mining region, part soils and river/brook waters were polluted by mine tailings and mining/smelting waste water. The arsenic content in polluted paddy soils and river/brook water is 46.26–496.19 μg g⁻¹, 0.3–16.5 mgL⁻¹, respectively. The positive abnormality and pollution of arsenic in the soil and water affects the arsenic content of the crop and the inhabitants’ health.     

A sensitive spectrophotometric method for the determination of arsenic in environmental samples

We developed a cost-effective and sensitive spectrophotometric method for the determination of arsenic at trace level using a new reagent, leuco malachite green. Here we show that, arsenic reacts with potassium iodate in acidic conditions to liberate iodine, and the liberated iodine selectively oxidizes leuco malachite green to malachite green dye. We studied the Beer’s law at 617 nm, which showed linearity over the concentration range 0.09–0.9 μg ml⁻¹ of arsenic. We show that the molar absorptivity, Sandell’s sensitivity and detection limit of the method are 6.1 × 10⁴ l mol⁻¹ cm⁻¹, 0.0012 μg cm⁻² and 0.025 μg ml⁻¹, respectively. We applied the developed method for the determination of arsenic in environmental samples.     

Strong variability in bacterioplankton abundance and production in central and western Bay of Bengal

With large influx of freshwater that decreases sea-surface salinities, weak wind forcing of <10 m s⁻¹ and almost always warm (>28°C) sea-surface temperature that stratifies and shallows the mixed layer leading to low or no nutrient injections into the surface, primary production in Bay of Bengal is reportedly low. As a consequence, the Bay of Bengal is considered as a region of low biological productivity. Along with many biological parameters, bacterioplankton abundance and production were measured in the Bay of Bengal during post monsoon (September–October 2002) along an open ocean transect, in the central Bay (CB, 88°E) and the other transect in the western Bay (WB). The latter representing the coastal influenced shelf/slope waters. Bacterioplankton abundances (<2 × 10⁹cells l⁻¹) were similar to those reported from the HNLC equatorial Pacific and the highly productive northern Arabian Sea. Yet, the thymidine uptake rates along CB (average of 1.46 pM h⁻¹) and WB (average of 1.40 pM h⁻¹) were less than those from the northwestern Indian Ocean. These abundances and uptake rates were higher than those in the oligotrophic northwestern Sargasso Sea (<7 × 10⁸ cells l⁻¹; av 1.0 pM h⁻¹). Concentrations of chlorophyll a (chl a), primary production rates and total organic carbon (TOC) were also measured for a comparison of heterotrophic and autotrophic production. In the WB, bacterioplankton carbon biomass equaled ∼ 95% of chl a carbon than just 31% in the CB. Average bacterial:primary production (BP:PP) ratios accounted for 29% in the CB and 31% in the WB. This is mainly due to lower primary productivity (PP) in the WB (281 mg C m⁻² d⁻¹) than in the CB (306 mg C m⁻² day⁻¹). This study indicates that bacteria–phytoplankton relationship differs in the open (CB) and coastal waters (WB). Higher abundance and contrastingly low bacterial production (BP) in WB may be because of the riverine bacteria, brought in through discharges, becoming dormant and unable to reproduce in salinities of 28 or more psu. Heterotrophic bacteria appear to utilize in situ DOC rather rapidly and their carbon demand is ∼50% of daily primary production. It is also apparent that allochthonous organic matter, in particular in the western Bay, is important for meeting their carbon demand.     

Dietary contribution of the microphytobenthos to infaunal deposit feeders in an estuarine mudflat in Japan

The food sources of benthic deposit feeders were investigated at three stations in an estuarine mudflat (Idoura Lagoon, Sendai Bay, Japan) during July and August 2005, using δ¹³C and δ¹⁵N ratios. Sediment at the stations was characterized by low chlorophyll (chl) a content (0–1 cm depth, <4 μg cm⁻²) and the dominance of riverine–terrestrial materials (RTM) in the sediment organic matter (SOM) pool. Surface-deposit feeders (Macoma contabulata, Macrophthalmus japonicus, and Cyathura muromiensis) exhibited much higher δ¹³C values (−18.4 to −12.4‰) than did the SOM pool (<−25‰). A δ¹³C-based isotopic mixing model estimated that benthic diatoms comprised 45–100% (on average) of their assimilated diet, whereas RTM comprised a lesser fraction (29% maximum). The major diet of the deep-deposit feeding polychaetes Notomastus sp. and Heteromastus sp. was benthic diatoms and/or marine particulate organic matter (POM), with little RTM assimilated (39% maximum). The consumers appeared to lack specific digestive enzymes and to use detritus-derived carbon only after its transfer to the microbial biomass. The isotopic mixing model also showed that the dietary contribution of RTM increased slightly (15% maximum) in the vicinity of freshwater input, suggesting that spatial changes in RTM supply affect the dietary composition of deposit feeders. These results clearly demonstrate that deposit feeders selectively ingest and/or assimilate the more nutritious microalgal fractions in the SOM pool. Such adaptations may allow enhanced energy gain in estuarine mudflats that are rich in vascular plant detritus with low nutritive value.     

Photolytical property of L(+)-α-phenylglycine in aqueous solution

A wide range of pharmaceutical compounds have been identified in the environment, and their existence is a topic of growing concern, both for human and ecological health. The work described here has investigated the photolytic properties of L(+)-α-phenylglycine (L-α-PG-H) in aqueous solution as it can be degraded by photo-catalysis. In 266 nm laser flash photolysis of aqueous solution of L-α-PG-H saturated with nitrogen, two transient absorption bands are observed at 280–330 nm and 450–800 nm, respectively, due to L-α-PG-H radical cation and hydrated electrons (e_aq⁻). Then e_aq⁻ reacts with L-α-PG-H to form the L-α-PG-H radical anion. Decaying rate constants of e_aq⁻ observed at 720 nm is to be 8.9 × 10⁸ dm³ mol⁻¹ s⁻¹. The rate constant for oxidation of L-α-PG-H by SO₄ ⁻ is calculated as 4.5 × 10⁸ and 4.3 × 10⁸ s⁻¹ mol⁻¹ dm³, respectively. The dissociation constants (pKa) of L-α-PG-H is 3. Excited triplet of L-α-PG-H in solution is formed by laser flash photolysis. The quench rate constant of L-α-PG-H excited triplet (k _s) is determined to be 1.3 × 10⁷ dm³ mol⁻¹ s⁻¹ and k ₀ is equal to 1.7 × 10⁵ s⁻¹.     

Spatial and temporal variation in the elemental and stable isotopic content of the seagrasses <Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis> from the Illes Balears,Spain

Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and δ¹³C and δ¹⁵N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the δ¹³C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ¹⁵N. Mean annual δ¹⁵N was 4.0‰; δ¹⁵N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g⁻¹ and a maximum of 167.7 μg g⁻¹. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 μg g⁻¹ was eight times as high as the minimum of 0.20 μg g⁻¹. There were interspecific differences in δ¹³C of the two species; C. nodosa was consistently more enriched (δ¹³C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ¹³C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ¹³C and δ¹⁵N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses.     

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). II. Abundance, biomass and production

 The abundance and biomass of Corophium multisetosum Stock, 1952 were determined from benthic corer samples collected monthly over 1 yr in the upper reaches of Canal de Mira (Ria de Aveiro, Portugal). Both density and biomass over the sampling period were negatively correlated with water temperature and positively correlated with chlorophyll a concentration in the sediment. C. multisetosum density was significantly negatively correlated with plant biomass and positively correlated with salinity. The nature of the sediment, favourable environmental conditions, high availability of food and low interspecific competition allowed the population to reach a maximal density of 200 × 10³ individuals m⁻² and a maximal biomass (ash-free dry wt, AFDW) of 62 g_AFDW m⁻². The population was highly productive, especially during the autumn/winter period. Production, estimated by two different methods (Hynes method: 251 g_AFDW m⁻² yr⁻¹; Morin–Bourassa method: 308 g_DW m⁻² yr⁻¹), was much higher than the values reported for other Corophium species. The annual P:Bˉ ratio (10) was high, but similar to values reported for Swedish populations of C. volutator and lower than the values estimated from Mediterranean populations of C. insidiosum. Received: 8 October 1999 / Accepted: 22 June 2000     

Reproduction in Balanus amphitrite Darwin (Cirripedia: Thoracica): influence of temperature and food concentration

Balanus amphitrite, an acorn barnacle, is distinctly euryhaline, eurythermal and a dominant fouling organism found in warm and temperate waters throughout the world. In this study, the influence of temperature and food concentration on the reproductive biology of this species collected from a tropical habitat was evaluated. Adult barnacles were maintained at 20, 25 and 30°C temperatures at different concentrations of food (50, 100, 150 and 200 Artemia ind⁻¹ day⁻¹). In this previously believed obligatory cross-fertilizing hermaphrodite, self-fertilization was observed. The rise in temperature from 20 to 30°C resulted in a longer interbreeding interval (6–7 days, 200 Artemia ind⁻¹ day⁻¹; 11–13 days, 50 Artemia ind⁻¹ day⁻¹). Computed carbon gained through feeding during the interbreeding interval indicated an inverse relationship to the temperature. At 20°C, although a greater amount of carbon was gained through feeding, the numbers of larvae produced were fivefold less when compared to those raised at 30°C. At 20°C, 2.3 μg C was required to produce a single larva, whereas at 30°C it was 0.4 μg C. A rise in rearing temperature also influenced the molting rate positively. Observations on temporal variation in the gonad development of this species in a tropical coastal environment influenced by the monsoons indicated gonad development to be positively related to chlorophyll a concentration.     

Contribution of salps to carbon flux of marginal ice zone of the Lazarev Sea, southern ocean

In order to estimate the in situ grazing rates of Salpa thompsoni and their implications for the development of phytoplankton blooms and for the sequestration of biogenic carbon in the high Antarctic, a repeat-grid survey and drogue study were carried out in the Lazarev Sea during austral summer of 1994/1995 (December/January). Exceptionally high grazing rates were measured for S. thompsoni at the onset of a phytoplankton bloom (0.2 to 0.8 μg chlorophyll a l⁻¹) in December 1994, with up to ≃160 μg of plant pigments consumed by an individual salp of 7 to 10 cm length per day. Dense salp swarms extended throughout the marginal ice zone, consuming up to 108% of daily phytoplankton production and 21% of the total chlorophyll a stock. Due to the much faster sinking rates and higher carbon content of salp faecal pellets, the efficiency of downward carbon flux through salps is much higher than through the other major grazers, krill and copepods. S. thompsoni can thus export large amounts of biogenic carbon from the euphotic zone to the deep ocean. With the observed ingestion rates during December 1994, this flux could have attained levels of up to 88 mg C m⁻² d⁻¹, accounting for the bulk of the vertical transport of carbon in the Lazarev Sea. However, in January 1995, when phytoplankton concentrations exceeded a threshold level of 1.0 to 1.5 μg chlorophyll a l⁻¹, salps experienced a drastic reduction in their feeding efficiency, possibly as a result of clogging of their filtering apparatus. This triggered a dramatic reversal in the relationship, during which a dense phytoplankton bloom developed in conjunction with the collapse of the salp population. Increases in the biomass and geographic range of the tunicate S. thompsoni have occurred in several areas of the southern ocean, often in parallel with a rise in sea-surface temperature during sub-decadal periods of warming anomalies. Received: 10 August 1997 / Accepted: 21 October 1997     

Microbial activity and carbon, nitrogen, and phosphorus content in a subtropical seagrass estuary (Florida Bay): evidence for limited bacterial use of seagrass production

Bacterial abundance, production, and extracellular enzyme activity were determined in the shallow water column, in the epiphytic community of Thalassia testudinum, and at the sediment surface along with total carbon, nitrogen, and phosphorus in Florida Bay, a subtropical seagrass estuary. Data were statistically reduced by principle components analysis (PCA) and multidimensional scaling and related to T. testudinum leaf total phosphorus content and phytoplankton biomass. Each zone (i.e., pelagic, epiphytic, and surface sediment community) was significantly dissimilar to each other (Global R = 0.65). Pelagic aminopeptidase and sum of carbon hydrolytic enzyme (esterase, peptidase, and α- and β-glucosidase) activities ranged from 8 to 284 mg N m⁻² day⁻¹ and 113–1,671 mg C m⁻² day⁻¹, respectively, and were 1–3 orders of magnitude higher than epiphytic and sediment surface activities. Due to the phosphorus-limited nature of Florida Bay, alkaline phosphatase activity was similar between pelagic (51–710 mg P m⁻² day⁻¹) and sediment (77–224 mg P m⁻² day⁻¹) zones but lower in the epiphytes (1.1–5.2 mg P m⁻² day⁻¹). Total (and/or organic) C (111–311 g C m⁻²), N (9.4–27.2 g N m⁻²), and P (212–1,623 mg P m⁻²) content were the highest in the sediment surface and typically the lowest in the seagrass epiphytes, ranging from 0.6 to 8.7 g C m⁻², 0.02–0.99 g N m⁻², and 0.5–43.5 mg P m⁻². Unlike nutrient content and enzyme activities, bacterial production was highest in the epiphytes (8.0–235.1 mg C m⁻² day⁻¹) and sediment surface (11.5–233.2 mg C m⁻² day⁻¹) and low in the water column (1.6–85.6 mg C m⁻² day⁻¹). At an assumed 50% bacterial growth efficiency, for example, extracellular enzyme hydrolysis could supply 1.8 and 69% of epiphytic and sediment bacteria carbon demand, respectively, while pelagic bacteria could fulfill their carbon demand completely by enzyme-hydrolyzable organic matter. Similarly, previously measured T. testudinum extracellular photosynthetic carbon exudation rates could not satisfy epiphytic and sediment surface bacterial carbon demand, suggesting that epiphytic algae and microphytobenthos might provide usable substrates to support high benthic bacterial production rates. PCA revealed that T. testudinum nutrient content was related positively to epiphytic nutrient content and carbon hydrolase activity in the sediment, but unrelated to pelagic variables. Phytoplankton biomass correlated positively with all pelagic components and sediment aminopeptidase activity but negatively with epiphytic alkaline phosphatase activity. In conclusion, seagrass production and nutrient content was unrelated to pelagic bacteria activity, but did influence extracellular enzyme hydrolysis at the sediment surface and in the epiphytes. This study suggests that seagrass-derived organic matter is of secondary importance in Florida Bay and that bacteria rely primarily on algal/cyanobacteria production. Pelagic bacteria seem coupled to phytoplankton, while the benthic community appears supported by epiphytic and/or microphytobenthos production.     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000