Spring sea ice photosynthesis,primary productivity and biomass distribution in eastern Antarctica, 2002–2004

While it is known that Antarctic sea ice biomass and productivity are highly variable over small spatial and temporal scales, there have been very few measurements from eastern Antarctic. Here we attempt to quantify the biomass and productivity and relate patterns of variability to sea ice latitude ice thickness and vertical distribution. Sea ice algal biomass in spring in 2002, 2003 and 2004 was low, in the range 0.01–8.41 mg Chl a m⁻², with a mean and standard deviation of 2.08 ± 1.74 mg Chl a m⁻² (n = 199). An increased concentration of algae at the bottom of the ice was most pronounced in thicker ice. There was little evidence to suggest that there was a gradient of biomass distribution with latitude. Maximum in situ production in 2002 was approximately 2.6 mg C m⁻² h⁻¹ with assimilation numbers of 0.73 mg C (mg Chl a)⁻¹ h⁻¹. Assimilation numbers determined by the ¹⁴C incubations in 2002 varied between 0.031 and 0.457 mg C (mg Chl a)⁻¹ h⁻¹. Maximum fluorescence quantum yields of the incubated ice samples in 2002 were 0.470 ± 0.041 with E _k indices between 19 and 44 μmol photons m⁻² s⁻¹. These findings are consistent with the shade-adapted character of ice algal communities. In 2004 maximum in situ production was 5.9 mg C m⁻² h⁻¹ with an assimilation number of 5.4 mg C (mg Chl a)⁻¹ h⁻¹. Sea ice biomass increased with ice thickness but showed no correlation with latitude or the time the ice was collected. Forty-four percent of the biomass was located in bottom communities and these were more commonly found in thicker ice. Surface communities were uncommon.     

Phytoplankton community structure and primary production in small intertidal estuarine-bay ecosystem (eastern English Channel,France)

From May 2002 to October 2003, a fortnightly sampling programme was conducted in a restricted macrotidal ecosystem in the English Channel, the Baie des Veys (France). Three sets of data were obtained: (1) physico-chemical parameters, (2) phytoplankton community structure illustrated by species composition, biovolume and diversity, and (3) primary production and photosynthetic parameters via P versus E curves. The aim of this study was to investigate the temporal variations of primary production and photosynthetic parameters in this bay and to highlight the potential links with phytoplankton community structure. The highest level of daily depth-integrated primary production Pz (0.02–1.43 g C m⁻² d⁻¹) and the highest maximum photosynthetic rate P ^B _max (0.39–8.48 mg C mg chl a ⁻¹ h⁻¹) and maximum light utilization coefficient α^B [0.002–0.119 mg C mg chl a ⁻¹ h⁻¹ (μmol photons m⁻² s⁻¹)] were measured from July to September. Species succession was determined based on biomass data obtained from cell density and biovolume measurements. The bay was dominated by 11 diatoms throughout the year. However, a Phaeocystis globosa bloom (up to 25 mg chl a m⁻³, 2.5 × 10⁶ cells l⁻¹) was observed each year during the spring diatom bloom, but timing and intensity varied interannually. Annual variation of primary production was due to nutrient limitation, light climate and water temperature. The seasonal pattern of microalgal succession, with regular changes in composition, biovolume and diversity, influenced the physico-chemical and biological characteristics of the environment (especially nutrient stocks in the bay) and thus primary production. Consequently, investigation of phytoplankton community structure is important for developing the understanding of ecosystem functioning, as it plays a major role in the dynamics of primary production.     

Effects of light exposure on the retention of kleptoplastic photosynthetic activity in the sacoglossan mollusc <Emphasis Type="Italic">Elysia viridis</Emphasis>

The effects of light exposure on the photosynthetic activity of kleptoplasts were studied in the sacoglossan mollusc Elysia viridis. The photosynthetic activity of ingested chloroplasts was assessed in vivo by non-destructively measuring photophysiological parameters using pulse amplitude modulation (PAM) fluorometry. Animals kept under starvation were exposed to two contrasting light conditions, 30 μmol photons m⁻² s⁻¹ (low light, LL), and 140 μmol photons m⁻² s⁻¹ (high light, HL), and changes in photosynthetic activity were monitored by measuring the maximum quantum yield of photosystem II (PSII), F _v/F _m, the minimum fluorescence, F _o, related to chlorophyll a content, and by measuring rapid light-response curves (RLC) of relative electron transport rate (rETR). RLCs were characterised by the initial slope of the curve, α_RLC, related to efficiency of light capture, and the maximum rETR level, rETR_m,RLC, determined by the carbon-fixation metabolism. Starvation induced the decrease of all photophysiological parameters. However, the retention of photosynthetic activity (number of days for F _v/F _m > 0), as well as the rate and the patterns of its decrease over time, varied markedly with light exposure. Under HL conditions, a rapid, exponential decrease was observed for F _v/F _m, α_RLC and rETR_m,RLC, F _o not showing any consistent trend of variation, and retention times ranged between 6 and 15 days. These results suggested that the retention of chloroplast functionality is limited by photoinactivation of PSII reaction center protein D1. In contrast, under LL conditions, a slower decrease in all parameters was found, with retention times varying from 15 to 57 days. F _v/F _m, α_RLC and rETR_m,RLC exhibited a bi-phasic pattern composed by a long phase of slow decrease in values followed by a rapid decline, whilst F _o decayed exponentially. These results were interpreted as resulting from lower rates of D1 photoinactivation under low light and from the gradual decrease in carbon provided by photosynthesis due to reduction of functional photosynthetic units.     

Clearance rates of the great scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) and blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) at low natural seston concentrations

Great scallop, Pecten maximus, and blue mussel, Mytilus edulis, clearance rate (CR) responses to low natural seston concentrations were investigated in the laboratory to study (1) short-term CR variations in individual bivalves exposed to a single low seston diet, and (2) seasonal variations in average CR responses of bivalve cohorts to natural environmental variations. On a short temporal scale, mean CR response of both species to 0.06 μg L⁻¹ chlorophyll a (Chl a) and 0.23 mg L⁻¹ suspended particulate matter (SPM) remained constant despite large intra-individual fluctuations in CR. In the seasonal study, cohorts of each species were exposed to four seston treatments consisting of ambient and diluted natural seston that ranged in mean concentration from 0.15 to 0.43 mg L⁻¹ SPM, 0.01 to 0.88 μg L⁻¹ Chl a, 36 to 131 μg L⁻¹ particulate organic carbon and 0.019 to 0.330 mm³ L⁻¹ particle volume. Although food abundance in all treatments was low, the nutritional quality of the seston was relatively high (e.g., mean particulate organic content ranged from 68 to 75%). Under these low seston conditions, a high percentage of P. maximus (81–98%) and M. edulis (67–97%) actively cleared particles at mean rates between 9 and 12 and between 4 and 6 L g⁻¹ h⁻¹, respectively. For both species, minimum mean CR values were obtained for animals exposed to the lowest seston concentrations. Within treatments, P. maximus showed a greater degree of seasonality in CR than M. edulis, which fed at a relatively constant rate despite seasonal changes in food and temperature. P. maximus showed a non-linear CR response to increasing Chl a levels, with rates increasing to a maximum at approximately 0.4 μg L⁻¹ Chl a and then decreasing as food quantity continued to increase. Mean CR of M. edulis also peaked at a similar concentration, but remained high and stable as the food supply continued to increase and as temperatures varied between 4.6 and 19.6°C. The results show that P. maximus and M. edulis from a low seston environment, do not stop suspension-feeding at very low seston quantities; a result that contradicts previous conclusions on the suspension-feeding behavior of bivalve mollusks and which is pertinent to interpreting the biogeographic distribution of bivalve mollusks and site suitability for aquaculture.     

Interplay of host morphology and symbiont microhabitat in coral aggregations

The Belizean reef coral Agaricia tenuifolia Dana forms aggregations in which rows of thin, upright blades line up behind each other. On average, the spacing between blades increases with depth and hence with decreasing ambient irradiance. We designed and built a small, inexpensive light meter and used it to quantify the effect of branch spacing on light levels within colonies at varying distances from branch tips. Concurrently, we measured photosynthetic pigment concentrations and population densities of symbiotic dinoflagellates (zooxanthellae) extracted from coral branches of colonies with tight (≤3 cm) vs wide (≥6 cm) branch spacing, collected at 15 to 17 m and from colonies with tight branch spacing collected at 1 to 2 m. Light levels decreased significantly with tighter branch spacing and with distance from the branch tips. Total cellular pigment concentrations (chlorophylls a, c ₂ and peridinin) as well as chlorophyll a:c ₂ and chlorophyll a: peridinin ratios all increased significantly with distance from the branch tip, indicating very localized differences in photoacclimation within individual branches. Zooxanthellae from colonies with widely-spaced branches displayed significantly lower chlorophyll a:c ₂ and chlorophyll a:peridinin ratios, and were present at significantly higher population densities than those from colonies with tightly-spaced branches collected at the same depth (15 m). Tightly-spaced colonies collected from shallow environments (1 to 2 m) displayed pigment ratios similar to those from widely-spaced colonies from deeper water (15 m), but maintained zooxanthellae populations at levels similar to those in tightly-branched colonies from deeper water. Thus, variation in colony morphology (branch spacing and distance from branch tip) can affect symbiont physiology in a manner comparable to an increase of over 15 m of water depth. These results show that a host's morphology can strongly determine the microhabitat of its symbionts over very small spatial scales, and that zooxanthellae can in turn display steep gradients in concordance with these altered physical conditions. Received: 12 June 1997 / Accepted: 24 June 1997     

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.     

In situ net primary productivity and photosynthesis of Antarctic sea ice algal, phytoplankton and benthic algal communities

Primary production at Antarctic coastal sites is contributed from sea ice algae, phytoplankton and benthic algae. Oxygen microelectrodes were used to estimate sea ice and benthic primary production at several sites around Casey, a coastal area in eastern Antarctica. Maximum oxygen export from sea ice was 0.95 mmol O₂ m⁻² h⁻¹ (~11.7 mg C m⁻² h⁻¹) while from the sediment it was 6.08 mmol O₂ m⁻² h⁻¹ (~70.8 mg C m⁻² h⁻¹). When the ice was present O₂ export from the benthos was either low or negative. Sea ice algae assimilation rates were up to 3.77 mg C (mg Chl-a)⁻¹ h⁻¹ while those from the benthos were up to 1.53 mg C (mg Chl-a)⁻¹ h⁻¹. The contribution of the major components of primary productivity was assessed using fluorometric techniques. When the ice was present approximately 55–65% of total daily primary production occurred in the sea ice with the remainder unequally partitioned between the sediment and the water column. When the ice was absent, the benthos contributed nearly 90% of the primary production.     

Effects of diurnal variations in phytoplankton photosynthesis obtained from natural fluorescence

Effects of diurnal variation in phytoplankton photosynthesis on estimating daily primary production (DPP) were examined using field data from Sagami Bay, Japan. DPP at 5 m depth was calculated from the continuous data of chlorophyll a (Chl a) and light intensity monitored by a natural fluorescence sensor with and without considering time-dependent changes in the photosynthesis–irradiance (P–E) relationship. Chl a could be estimated from natural fluorescence examining the variations in the quantum yield of fluorescence (φ _f) and Chl a-specific light absorption coefficient (a*_ph), and relating them to Chl a. The P–E relationship was determined by water sampling three times daily. A distinct diurnal pattern was observed for the maximum photosynthetic rate (P*_max), being maximal at noon, while periodicity of the maximum light utilization coefficient (α*) was less obvious. The actual DPP was calculated by interpolating the P–E parameters from those obtained at dawn, noon, and dusk. For comparison, DPP was calculated by fixing the P–E parameters as the constants measured at dawn, noon or dusk for a day. The difference from the actual DPP was small when the P–E parameters measured at dawn (3% on average) and noon (5%) were used as the constants for a day. The difference was largest when the values at dusk were used (−43%). The medium values of P*_max at dawn, its low values at dusk, and the fact that a major part of the DPP was produced around noon were responsible for these results. The present study demonstrates that measurement of the P–E parameters at dawn or noon can give a good estimation of DPP from natural fluorescence.     

Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 10⁸ cell clam⁻¹, N = 6.6 · 10⁸ cell clam⁻¹, P = 5.7 · 10⁸ cell clam⁻¹, N + P = 5.7 · 10⁸ cell clam⁻¹; and C = 4.1 · 10⁸ cell clam⁻¹, N = 5.1 · 10⁸ cell clam⁻¹, P = 4.7 · 10⁸ cell clam⁻¹, N + P = 4.5 · 10⁸ cell clam⁻¹, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam⁻¹, N = 13.1 mg chl a clam⁻¹, P = 12.9 mg chl a clam⁻¹, N + P = 11.8 mg chl a clam⁻¹; and C = 8.8 mg chl a clam⁻¹, N = 12.8 mg chl a clam⁻¹; P = 11.2 mg chl a clam⁻¹, N + P = 11.3 mg chl a clam⁻¹, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association. Received: 2 June 1997 / Accepted: 9 June 1997     

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H _sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P _max) and daily hours of saturating irradiance (H _sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H _sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g⁻¹ leaf dry wt d⁻¹ in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g⁻¹ leaf dry wt d⁻¹ in July. Modeled net carbon gain was highest in July at 454 mg C m⁻² d⁻¹ (1.4 g dry wt m⁻² d⁻¹), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P _net:R _d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P _net:R _d ratios. Received: 13 August 1997 / Accepted: 6 March 1998     

Photosystem II photoinhibition and altered kinetics of photosynthesis during nutrient-dependent high-light photoadaptation in Gonyaulax polyedra

Gonyaulax poledra Stein was transferred at different cell densities from increasingly nutrient-limited low-light (LL, 80 E m^-2 s^-1) batch-cultures to high-light (HL, 330 E m^-2 s^-1) growth conditions. Several age-dependent differences in HL-adaptation strategies were apparent. Short-term (3h) susceptibility to photosynthetic photoinhibition increased with culture age, with light-limited rates of photosynthesis exhibiting greater photosuppression than light-saturated rates at all stages of growth. These shortterm changes were not accompanied by photobleaching of chlorophyll but were directly related to age-dependent photoinactivation of Photosystem II electron-transport rates. The capacity of electron transport by Photosystem I was only slightly affected. Prolonged exposure of LL log-phase cells to HL conditions did induce photobleaching of chlorophyll associated with increased cell volume, a transient decrease of organic carbon and nitrogen content, enhanced cellular-, carbon-and chlorophyll-based rates of light-saturated photosynthesis (P _max) and suppressed cellular rates of light-limited photosynthesis. As a result, the density of LL log-phase cells doubled and their cellular photosynthetic performance nearly tripled within 1 d of HL exposure while cellular respiratory demands remained unchanged. By contrast, prolonged HL incubation of LL stationary populations induced a transitory burst in cell division and a large reduction in cell volume, leading to a short-term increase in volume-based organic carbon and nitrogen content. Despite reduced cell volume and lowered carbon demand, the cellular-, carbon-and chlorophyll-based rates of P _max in nondividing populations fell by 64, 48 and 27%, respectively, over a 4 d exposure to HL, while light-limited rates were almost fully suppressed within 1 d and chlorophyll a content was reduced by 56%. As a result, the photosynthetic performance of LL-aged cells declined immediately under HL conditions. Addition of inorganic nutrients to LL stationary cultures at the time of HL transfer led to immediate and complete suppression of photosynthesis and cell lysis within 1 d. Addition of nutrients following transfer to HL induced cell responses intermediate to those described for LL log and aged cells exposed to HL. Results support the view that declining nutrient-status impairs HL photoadaptive responses in phytoplankton populations and that the rate and pattern of photoadaptive responses may be used as physiological growth indicators in field studies. The study was conducted from March 1981 to May 1983.     

Settlement of <Emphasis Type="Italic">Macoma balthica</Emphasis> larvae in response to benthic diatom films

The role of multi-species benthic diatom films (BDF) in the settlement of late pediveliger larvae of the bivalve Macoma balthica was investigated in still-water bioassays and multiple choice flume experiments. Axenic diatom cultures that were isolated from a tidal mudflat inhabited by M. balthica were selected to develop BDF sediment treatments characterized by a different community structure, biomass, and amount of extracellular polymeric substances (EPS). Control sediments had no added diatoms. Although all larvae settled and initiated burrowing within the first minute after their addition in still water, regardless of treatment, only 48–52% had completely penetrated the high diatom biomass treatments after 5 min, while on average 80 and 69% of the larvae had settled and burrowed into the control sediments and BDF with a low diatom biomass (<3.5 μg Chl a g⁻¹ dry sediment), respectively. The percentage of larvae settling and burrowing into the sediment was negatively correlated with the concentration of Chl a and EPS of the BDF. This suggests higher physical resistance to bivalve penetration by the BDF with higher diatom biomass and more associated sugar and protein compounds. The larval settlement rate in annular flume experiments at flow velocities of 5 and 15 cm s⁻¹ was distinctly lower compared to the still-water assays. Only 4.6–5.8% of the larvae were recovered from BDF and control sediments after 3 h. Nonetheless, a clear settlement preference was observed for BDF in the flume experiments; i.e., larvae settled significantly more in BDF compared to control sediments irrespective of flow speed. Comparison with the settlement of polystyrene mimics and freeze-killed larvae led to the conclusion that active selection, active secondary dispersal and, at low flow velocities (5 cm s⁻¹), passive adhesion to the sediment are important mechanisms determining the settlement of M. balthica larvae in estuarine biofilms.     

Age determination in the Antarctic shrimp Notocrangon antarcticus (Crustacea: Decapoda), using the autofluorescent pigment lipofuscin

Determination of basic population parameters in long-lived Crustacea is hindered by a lack of appropriate methods for age determination. This study uses the pigment lipofuscin as an age marker in the common Antarctic decapod Notocrangon antarcticus (Pfeffer, 1887) from the eastern Weddell Sea. Resin brain sections of the lateral somacluster of the olfactory lobe of 189 specimens were digitally recorded by confocal microscopy and images were subsequently analyzed. A modal progression analysis of the lipofuscin concentration–frequency distribution revealed a total of eight regularly spaced modes presumed to reflect consecutive annual age classes. All eight modes contained females, and the first four contained males. No regular modes were obvious in the comparable length-frequency distribution. The average yearly pigment accumulation was nearly linear and estimated as 0.02% area fraction year⁻¹, which is considerably lower than rates published for species from lower latitudes. This is probably explained by the effect of low water temperature on metabolism and lipofuscin accumulation rate. The growth parameters CL_∞ and k from the von Bertalanffy growth function were 22.3 mm and 0.79 year⁻¹ for females, respectively, and 16.9 mm and 0.64 year⁻¹ for males, respectively. Mortality, estimated from catch curves, amounted to 0.44 year⁻¹ in females and 0.92 year⁻¹ in males, whereas P/B, calculated from the mass specific growth rate method, was slightly lower. The results indicate that the lipofuscin-inferred population parameters are an improvement over what can be learned about N. antarcticus with traditional methods. Received: 1 August 2000 / Accepted: 21 September 2000     

Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host–symbiont system in darkness decreased the O₂ concentration at the shell surface to <70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h⁻¹ foraminifer⁻¹. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h⁻¹. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂h⁻¹, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h⁻¹). Experimental light–dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E _o curves) showed light saturation irradiances (E _k) of 75 and 137 μmol photons m⁻² s⁻¹ in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m⁻² s⁻¹. The light compensation point of the symbiotic association was 50 μmol photons m⁻² s⁻¹. Radial profile measurements of scalar irradiance (E _o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E _d). Received: 26 January 1998 / Accepted: 11 April 1998     

Effects of dissolved ammonium addition and host feeding with Artemia salina on photoacclimation of the hermatypic coral Stylophora pistillata

 Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PAR_o), zooxanthella population density and chlorophyll a+c per 10⁶ zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PAR_o). The P _max of CO₂ per 10⁶ zooxanthellae decreased, while P _max of CO₂ per 10³ polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella frequency (PZF, a measure of zooxanthella division) declined significantly at light levels <18 ± 3% PAR_o. At the lowest levels of illumination (<5% PAR_o), zooxanthella population density decreased, as did the PZF; chl a+c per 10⁶ zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth (65 ± 4% PAR_o) were incubated in aquaria under bright (80 to 90% PAR_o), reduced (20 to 30% PAR_o), and extremely low (2 to 4% PAR_o) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Artemia salina nauplii (SW + A). An increase in P _max of CO₂ per 10³ polyps was found in corals acclimated to reduced light (20 to 30% PAR_o) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PAR_o and SW + A at 20 to 30% PAR_o) increased zooxanthella population density, but had no effect on chl a+c per 10⁶ zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PAR_o) and extremely low (1 to 3% PAR_o) light intensities shifted ¹⁴C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and feeding with Artemia salina nauplii resulted in an increase in photosynthetic assimilation of ¹⁴C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water. Received: 19 June 1998 / Accepted: 13 June 2000     

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.     

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     

Effects of mussel (Perna canaliculus) biodeposit decomposition on benthic respiration and nutrient fluxes

Suspension-feeding bivalves increase the quantity and quality of sedimenting organic matter through the production of faeces and pseudofaeces that are remineralised in coastal sediments and thus increase sediment oxygen demand and nutrient regeneration. Bivalves are intensively cultivated worldwide; however, no bivalve biodeposit decay rates are available to parameterise models describing the environmental effects of bivalve culture. We examined sediment biogeochemical changes as bivalve biodeposits age by incubating coastal sediments to which we added fresh mussel (Perna canaliculus) biodeposits and measured O₂ and nutrient fluxes as well as sediment characteristics over an 11-day period. Biodeposits elevated organic matter, chlorophyll a, phaeophytin a, organic carbon and nitrogen concentrations in the surface sediments. Sediment oxygen consumption (SOC) increased significantly (P=0.016) by ∼1.5 times to 1,010 μmol m⁻² h⁻¹ immediately after biodeposit addition and remained elevated compared to control cores without additions for the incubation period. This increase is in the range of observed in situ oxygen demand enhancements under mussel farms. To calculate a decay rate for biodeposits in sediments we fitted a first-order G model to the observed increase in SOC. The significant model fit (P=0.001, r ²=0.72) generated a decay rate of 0.16 day⁻¹ (P=0.033, SE=0.05) that corresponds to a half-life time of 4.3 day. This decay rate is 1–2 orders of magnitude higher than published decay rates of coastal sediments without organic enrichment but similar to rates of decaying zooplankton faecal pellets. NH₄⁺ release increased rapidly on the day of biodeposit addition (P=0.013) and reached a maximum of 144 μmol m⁻² h⁻¹ after 5 days which was 3.6 times higher compared to control cores. During this period NH₄⁺ release was significantly (P<0.001 to P=0.043) higher in the cores with biodeposit additions than in control cores.     

Ferrate(VI) oxidation of ibuprofen: A kinetic study

The kinetics of ferrate(VI) (Fe^VIO₄ ²⁻, Fe(VI)) oxidation of an antiphlogistic drug, ibuprofen (IBP), as a function of pH (7.75–9.10) and temperature (25–45°C) were investigated to see the applicability of Fe(VI) in removing this drug from water. The rates decrease with an increase in pH and the rates are related to protonation of ferrate(VI). The rates increase with an increase in temperature. The E _a of the reaction at pH 9.10 was calculated as 65.4±6.4 kJ mol⁻¹. The rate constant of the HFeO₄ ⁻ with ibuprofen is lower than with the sulphur drug, sulfamethoxazole. The use of Fe(VI) to remove ibuprofen is briefly discussed.     

Irradiance and temperature regulation of oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

 Short-term effects of temperature and irradiance on oxygenic photosynthesis and O₂ consumption in a hypersaline cyanobacterial mat were investigated with O₂ microsensors in a laboratory. The effect of temperature on O₂ fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m⁻² s⁻¹) in the temperature range from 15 to 45 °C. Areal rates of dark O₂ consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol⁻¹ and the corresponding Q ₁₀ value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O₂ consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q ₁₀ value (20 to 30 °C) of 2.8. Both O₂ dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q ₁₀ of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m⁻² s⁻¹) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E _k = 148 to 185 μmol photons m⁻² s⁻¹) estimated from P versus E _d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m⁻² s⁻¹, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m⁻² s⁻¹ at 25 °C to 77 μmol photons m⁻² s⁻¹ at 40 °C, due to increased rates of O₂ consumption relative to gross photosynthesis. Areal rates of O₂ consumption in the illuminated mat were higher than dark O₂ consumption at corresponding temperatures, due to an increasing O₂ consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O₂ cycling within hypersaline cyanobacterial mats. Received: 30 November 1999 / Accepted: 11 April 2000