Interactive effects of light and temperature on sporophyte production in the giant kelp Macrocystis pyrifera

Laboratory experiments were conducted in 1984 to test for an interaction between irradiance and temperature in controlling sporophyte production in the giant kelp Macrocystis pyrifera. Gametophytes from plants in the San Onofre kelp forest were cultured under a combination of seven irradiance (2.9 to 30 E m^-2 s^-1) and ten temperature (11° to 20°C) levels. The minimum daily quantum dose for fertility of female gametophytes during the 42 d culture-period was 0.25 E m^-2 d^-1. The saturation quantum dose was approximately 0.60 E m^-2 d^-1. Temperatures between 11° and 20°C had little effect on the fertilityirradiance relationship, except at irradiations near the threshold and saturation levels. Cultures at 0.60 E m^-2 d^-1 had significantly lower fertility after 6 wk at 20°C than at the lower temperatures and there was a trend toward lower fertility at temperatures greater than 15°C in cultures at 0.25 E m^-2 d^-1. These differences were also reflected in the development time for female gametophytes. The 11°C cultures reached 50% fertility slightly faster and the 20°C cultures slower than cultures at intermediate temperatures.     

Ammonium excretion in a temperate-reef community by a planktivorous fish,Chromis punctipinnis (Pomacentridae), and potential uptake by young giant kelp,Macrocystis pyrifera (Laminariales)

The blacksmith Chromis punctipinnis, an abundant planktivorous damselfish off southern California, USA, shelters along rocky reefs at night. While sheltered, blacksmiths excrete ammonium that could, in turn, be utilized by nearby benthic macrophytes. Laboratory experiments during the summer and fall of 1983 and 1984 indicate that ammonium excretion at night ranged from 18.1 mol h^-1 by a 8.5 g (dry) fish, to 89.1 mol h^-1 by a 27.3 g fish; excretion rates generally declined throughout the night. Field measurements at night indicate that ammonium concentrations were significantly higher in rocky crevices occupied by blacksmiths than in unoccupied shelters, and the ammonium level in one shelter dropped after a blacksmith was experimentally removed. Young kelp plants (Macrocystis pyrifera) are capable of taking up ammonium at night. Ammonium levels in chambers containing both a blacksmith and a young kelp plant were significantly lower than in chambers containing only a fish, and ammonium levels dropped in ammoniumspiked chambers that contained kelp plants. Nighttime ammonium uptake rates by young kelp plants, which averaged 1.6 mol g^-1 (dry) h^-1, were only slightly lower than those during the day. Daytime excretion by blacksmiths occasionally results in elevated ammonium levels in the water column. On two of six days, ammonium concentrations in midwater foraging aggregations were slightly but significantly higher than in upcurrent controls; since blacksmiths typically aggregate at the incurrent margin of kelp beds, the ammonium is swept downcurrent and may be utilized by large M. pyrifera that extend through the water column. Thus, the activities of blacksmiths may results in the importation of extrinsic, inorganic nitrogen to primary producers on temperate reefs.     

Seasonal growth and productivity ofMacrocystis integrifolia in British Columbia,Canada

The seasonal growth rates and nitrogen and carbon fluxes were estimated for two subtidalMacrocystis integrifolia Bory kelp forests in British Columbia, Canada from changes in population structure through time. Mean relative growth rates of the forests varied from a high of 4.3% d^-1 to a low of-3.6% d^-1. Mean net assimilatioon rates of carbon (a photosynthesis analog) varied from a high of 0.66 g C m^-2 of foliage d^-1 to a low of-0.87 g C m^-2 d^-1. The leaf area index ranged from 0.3 to 11.9. Annual carbon input on a foliage area basis was calculated at 250 g C m^-2 yr^-1. Annual carbon input to the forest was estimated at 1 300 g C m^-2 of ocean bottom yr^-1. The yearly nitrate nitrogen input to the forest was estimated at 60 g N m^-2 of ocean bottom yr^-1. The net ecosystem production varied from-520 to +31 g C m^-2 of ocean bottom yr^-1. The intra-forest, inter-forest and seasonal variabilities of these productivity parameters are discussed.     

Growth and utilization of internal nitrogen reserves by the giant kelp Macrocystis pyrifera in a low-nitrogen environment

An adult giant kelp plant (Macrocystis pyrifera), moved from an inshore kelp forest to an offshore, low-nitrogen environment near Santa Catalina Island, California (USA), maintained growth for 2 wk on internal nitrogen reserves. Frond elongation rates decreased significantly during the third week, and plant growth rate (wet wt) dropped from an initial inshore rate of 3.6 to 0.9% d^-1. During this 3 wk period, nitrogen contents and free amino acid concentrations decreased, while mannitol and dry contents increased in frond tissues. After depletion of internal nitrogen reserves, the nitrogen content of lamina and stipe tissues averaged 1.1 and 0.7% dry wt, respectively. The experimental plant was exposed to higher ambient nitrogen concentrations during the last 2 wk. Rates of frond elongation and plant growth increased, but nitrogen content and amino acids in frond tissues remained low. Of the total nitrogen contained in frond tissue of the plant before transplantation, 58% was used to support growth in the absence of significant external nitrogen supply. Amino acids constituted a small proportion of these internal nitrogen reserves. Net movement of nitrogen occurred within large fronds, but not between different frond size classes. The nitrogen content of holdfast tissue remained relatively constant at 2.4% dry wt and accounted for 18 to 29% of the total nitrogen. Holdfast nitrogen was not used to support growth of nitrogen-depleted fronds. In comparison to Laminaria longicruris, which is adapted to long seasonal periods of low nitrogen availability, M. pyrifera has small nitrogen-storage capacity. However, internal reserves of M. pyrifera appear adequate to make nitrogen starvation uncommon in southern California kelp forests.     

Seasonal growth in Laminaria longicruris: Relations with dissolved inorganic nutrients and internal reserves of nitrogen

Observations have been made on seasonal fluctuations in dissolved inorganic nutrients, internal reserves of nitrogen and growth rates in Laminaria longicruris. The onset of winter growth in shallow-water stations (6 and 9 m) correlated well with improved dissolved nitrate conditions in the sea. During the winter, reserves of NO ₃ ^- were accumulated by the plants and reached maximum values of 150 moles per g fresh weight in March. This represents a concentration factor of approximately 28,000 over the ambient levels, or an internal nitrogen reserve of 2.1% of the dry weight of the tissue. Depletion of this nitrogen pool followed the disappearance of the external NO ₃ ^- with a lag period of up to 2 months. Rapid kelp growth was measured during this period. Reserves of organic nitrogen also reached maximum values in March and declined slowly throughout the summer into autumn. It is suggested that the combined inorganic and organic nitrogen reserves sustain the rapid growth rates into July and at reduced rate through the late summer. Fertilization of an experimental perimental kelp bed with NaNO₃ increased the internal plant reserves of NO ₃ ^- and produced a much improved summer growth rate. The enriched plants developed very small reserves of carbohydrate during the rapid summer growth phase.NRCC No. 15549.     

In-situ measurements of nitrogenous nutrient uptake by kelp (Ecklonia maxima) and phytoplankton in a nitrate-rich upwelling environment

Nitrogen uptake by the kelp Ecklonia maxima Osbeck and phytoplankton was examined under different conditions of nutrient availability in a kelp bed off the Cape of Good Hope by measuring nutrient depletion in large plastic bags by the kelp and ¹⁵N uptake by phytoplankton. E. maxima took up nitrate and ammonia, but not urea, and showed only a weak preference for reduced nitrogen. Phytoplankton absorbed all three forms of nitrogen available, with a preference for ammonia and urea. Ambient nitrate concentration exhibited a marked and rapid decrease with northerly winds and an increase in response to offshore southerly winds. Nitrogen uptake by E. maxima was linearly related to ambient concentration and did not saturate even at nitrate concentrations >20g-at N l^-1, resulting in a significantly higher tissue nitrogen content under upwelling conditions. Nitrate imported by upwelling was the chief source of nitrogen utilised within the kelp bed. Locally regenerated nitrogen (ammonia and urea) was calculated to contribute only ca 4% of total nitrogen uptake during upwelling and 30% during the relaxation or downwelling phase.     

The association of N2-fixing bacteria with sea urchins

Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C₂H₂ reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6g N fixed g wet wt^-1 d^-1, with a yearly mean activity of 1.36 g N fixed g wet wt^-1 d^-1. Studies with ¹⁵N confirmed the C₂H₂ reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N₂ fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity.     

Nitrogenous nutrition in the euphotic zone of the Central North Pacific Gyre

The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August–September 1985. The measurements were performed by using ¹⁵N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 mol l^-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m^-2 d^-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d^-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands were very close to the rates found at the oceanic station. Organisms <3 m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.     

Evidence for facultative heterotrophy in cultured zooxanthellae

The locus of symbiotic dinoflagellates within host cells provides a habitat which could potentially be exploited by the alga through heterotrophic uptake of host-derived organic substrates. Using zooxanthellae (Symbiodinium sp.) isolated from the tropical sea anemone Aiptasia pulchella collected from Kaneohe Bay, Hawaii, the effect of various potential organic substrates on growth in vitro was assessed in Erdschreiber seawater medium (ES) supplemented with organic compounds. Zooxanthellae maintained at 5 to 7 E m^-2 s^-1 (below compensation irradiance) grew heterotrophically when supplied with 100 M glycerol, glycolate, acetate, malate, or propionate, and grew in darkness on 100 M propionate. Zooxanthellae exposed to irradiance below compensation were able to utilize carbon sources in the unsupplemented ES medium for slow growth, but generally the growth rate of cultured zooxanthellae was a function of incubation irradiance. Zooxanthellae incubated for 10 wk in unsupplemented ES at 5 to 7 E m^-2 s^-1 were capable of growth at this low irradiance, but were also capable of net photosynthetic oxygen production at higher irradiances. This suggests that zooxanthellae can be photoautotrophic or facultatively heterotrophic. An estimate for the duration of mitosis (t _d ) is made on the basis of growth rate of cultured zooxanthellae in log-phase; this estimate of t _d =4.88 h is less than half the estimated t _d for zooxanthellae in situ.     

Salinity dependence of sexual and asexual reproduction in the rotifer Brachionus plicatilis

A salinity dependent mictic response was observed in a clone of Brachionus plicatilis cultured in the 2 to 4 salinity range. This response was related to asexual exponential reproduction rates (G) and could be divided into three categories: (a) no mixis occurred at a salinity of 35 S and above, where G values were lower than 0.30 d^-1, (b) low mictic levels in rotifers cultured at 2 and 30 S, where G values ranged between 0.40 to 0.50 d^-1, and (c) high mictic levels in rotifers cultured at salinities ranging between 4 and 20 S, where G values ranged between 0.50 to 0.85 d^-1. Fluctuations in mictic levels varied with time during the course of the experiments. Results suggest that salinity conditions leading to optimal parthenogenic reproduction also support mixis.     

Effect of UV-B radiation on the marine diatoms Lauderia annulata and Thalassiorsira rotula grown in different salinities

The marine diatoms Lauderia annulata Cleve and Thalassiosira rotula Meunier were grown at different salinities (20, 35 and 45) and exposed to different levels of midultraviolet, UV-B) 439, 717 and 1230 J m^-2 d^-1, weighted) for 2 d. A low UV-B dose (439 J m^-2 d^-1) usually caused a slight increase in biomass production (dry weight) compared to non-UV-B irradiated cells. Enhanced UV-B radiation (717 J m^-2 d^-1) depressed protein and pigment content (chlorophyll a, chlorophyll c ₁+c₂ and carotenoids), especially in algae grown at 20 or 35 salt concentration of the nutrient solution. The effect of UV-B radiation (717 J m^-2 d^-1) on the pattern and concentration of amino acids was species-dependent. Aspartic acid was reduced in all tested diatoms. A drastic increase in glutamine and a reduction in glutamic acid pools could be observed in L. annulata samples, but no significant variation of the impact of UV-B was found in dependence on the salt concentration of the nutrient medium. T. rotula cells grown at 35 S showed an increase of glutamic acid and a decrease of glutamine levels after UV-B radiation. The results are discussed in relation to the impact of UV-B upon carbon and nitrogen metabolism.     

Growth of the kelp Ecklonia radiata near the northern limit of its range in Western Australia

The length, growth and survivorship of mature sporophytes of Ecklonia radiata (C.Ag.) J. Agardh were measured on two patch reefs within the lagoon of a high-latitude, coralreef atoll (Easter Group, Houtman Abrolhos; 28° to 29° S; 113°35 to 114°03E) for 1 yr (1982–1983). The sites differed in their proximity to the perimeter reef, but had similar regimes of temperature, light, nutrient concentration and water movement. Kelp length, growth rate and survivorship differed significantly between sites, although plant density was similar (less than l m^-2). At the site near the lagoon perimeter, the central lamina averaged 381 mm in length, grew at an annual mean rate of 1.28 mm d^-1, and 56% of tagged individuals had died after one year. The corresponding values for the site near the centre of the lagoon were: 257 mm, 0.75 mm d^-1 and 92%. Three other sites on the coastal limestone reefs near Perth (400 km to the south) were studied for various intervals over a 5 yr period (April 1979 to August 1984). The kelp from the low-latitude sites were smaller, grew more slowly, and suffered similar or greater mortality than their southern counterparts. Latitudinal comparisons were confounded by differences in kelp density between sites, but there is no evidence for density-dependent effects on the measured parameters across the range of natural densities observed. Growth rates at all sites were negatively correlated with ambient sea temperatures when these were above 20°C. There was no evidence of adaptation to the higher sea temperatures experienced at the Abrolhos, and temperature cannot be dismissed as a factor controlling the growth of the species near the northern limit of its distribution on the Western Australian coast. Other factors however must be involved in determining the latitudinal position of that limit.     

Growth rates of summer nanoplankton (<10 μm) populations in lower Narragansett Bay,Rhode Island,USA

Growth rates of summer (June–September) phytoplankton assemblages and constituent species were measured in 30 diffusion culture experiments. Size-fractionated (<10 m) phytoplankton assemblages were incubated in situ or under simulated in-situ conditions in outdoor tanks connected to a running seawater system. Doubling rates of important species and groups (such as microflagellates) were compared to community biomass doubling rates estimated from ¹⁴C uptake and changes in chlorophyll a concentrations. Division rates of dominant diatom species generally equalled or exceeded community biomass doubling rates, while those of flagellates and non-motile ultraplankters were slower. Maximum division rates of sixteen common diatom species exceeded 2.1 divisions d^-1, while nine had maximum division rates in excess of 3 d^-1. Mean division rates of 12 diatom species exceeded 1 d^-1. Maximum division rates of flagellated species, uncharacterized microflagellates and non-motile ultraplankton assemblages were 2.1, 1.5 and 1.4 d^-1, respectively. Microflagellate and non-motile ultraplankton assemblage doubling rates were less than 0.5 d^-1 in over half of all growth experiments.     

Stable isotope study of sedimentary carbon utilization by Capitella spp.: effects of two carbon sources and geochemical conditions during their diagenesis

Two experiments were conducted to determine the effect of the type of sediment organic matter and geochemical conditions during diagenesis on the stable carbon isotope ratio ¹³C of the deposit-feeding polychaete worms Capitella spp. Laboratory experiments showed ¹³C values of-13.5 to-13.9%. for worms grown on fresh and aged kelp, Macrocystis pyrifera. Field experiments on worms in the Santa Barbara Channel, California (USA), in 1986 revealed more negative ¹³C values (to-19%.) when oil and kelp were added to sediments. The more negative values suggest the incorporation of oil carbon into worm tissues, supporting previous indications of the importance of hydrocarbons in food webs near oil seeps.     

Effects of predators on sea urchin density and habitat use in a southern California kelp forest

Grazing sea urchins can reduce kelp abundance and therefore strongly affect kelp forest community structure. Despite the ecological importance of sea urchins, direct field studies on the role that urchin predators play in shaping urchin populations are rare for southern California. We conducted surveys and manipulative experiments within kelp forests near San Diego, CA, (32–51′28″N, 117–16′00″W) from 2006 to 2009 to determine whether predators such as sheephead (Semicossyphus pulcher) and spiny lobsters (Panulirus interruptus) may be linked to purple urchin (Strongylocentrotus purpuratus) and red urchin (Strongylocentrotus franciscanus) distribution and habitat use, as well as purple urchin density-dependent mortality. Purple urchins were less dense and more cryptic inside a local marine protected area (MPA) that contained high predator abundance than in nearby heavily fished areas, whereas red urchins rarely were found outside the MPA. Urchin proportional mortality was inversely density dependent during the day when sheephead were active, despite fish aggregations in plots of high urchin density, but was density independent during the night when lobsters were active. Urchin mortality was reduced under understory algal cover during the day, but not during the night. Examining whether urchin mortality from predation is density dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.     

Effect of algal bloom deposition on sediment respiration and fluxes

Using sediment cores collected in November 1989 from Aarhus Bight, Denmark, the fluxes of O₂, CO₂ (total CO₂), NH ₄ ⁺ , NO ₃ ^– +NO ₂ ^– and DON (dissolved organic nitrogen) across the sediment-water interface were followed for 20 d in an experimental continous flow system. On day 7, phytoplankton was added to the sediment surface, to see the result of simulated algal bloom sedimentation. Benthic O₂ consumption and CO₂ efflux, 38 to 41 mmol O₂ m^-2 d^-1 and 25 to 30 mmol CO₂ m^-2 d^-1, respectively, immediately increased by 39% and 100% after the algal addition, but gradually declined to the orginal level. Fluxes of NH ₄ ⁺ (1.0 to 1.2 mmol m^-2 d^-1) and DON (0.3 to 0.9 mmol m^-2 d^-1) increased due to the organic substrate addition. NH ₄ ⁺ and NO ₃ ^– flux changed direction, becoming an efflux and influx, respectively, for a few days and a large amount of DON (max. rate 4.0 mmol m^-2 d^-1) was immediately produced either by bacterial hydrolytic activity or from autolysis of the algae. DON was the most significant nitrogen component in pore water and in terms of N-flux from sediment. A temporary stimulation of anaerobic respiration processes (sulfate reduction and denitrification) and a decrease in nitrification were indicated. After the effect of the organic addition had declined, the fluxes gradually reverted to the original rates. The halflife of the added algal material, of which 20 to 30% was very labile, was estimated to be 2 to 3 wk.     

Vertical fluxes and ecological significance of organic materials during the phytoplankton bloom during austral summer in Breid Bay,Antarctica

A mooring system consisting of an in situ fluorometer at a depth of 50 m and a time-series sediment trap at a depth of 110 m was deployed at the sediment trap site (70°11.536'S; 24°18.679'E; water depth: 300 m) in Breid Bay, Antarctica in austral summer from 28 December 1985 to 13 February 1986. Sinking particles, consisting of diatoms (mainly Thalassiosira antarctica), were analyzed for organic materials, stable carbon and nitrogen isotopes. Vertical fluxes of organic carbon and nitrogen were determined to be within the ranges of 12.3 to 116 mg C m^-2 d^-1 and 1.79 to 15.4 mg N m^-2 d^-1, respectively, with maxima in the middle of January 1986, after which time the organic carbon and nitrogen fluxes tended to decrease with a steep gradient. High values of ¹³C were found in the organic matter of the sinking particles collected before the middle of January, indicating that the organic matter was derived from the diatoms in the logarithmic phase of their growth. Increased abundance of glucose was found in the water-extractable carbohydrate, which was one of the sinking particles collected in the middle of January. This fact clearly indicated that the diatoms were no longer in the growth phase but rather in the stationary phase, because reserved glucan as well as various cellular organic materials were reportedly accumulated within the algal cells in the stationary phase. Fifteen species of protein amino acids with trace amounts of -alanine, -aminobutyric acid and ornithine were found in the sinking particles upon acid hydrolysis, but the amino acid composition of these samples had not been affected much by biological agents. The essential amino acid index was calculated for the sinking particles collected in the course of the sediment trap experiment. The indices obtained indicated that the sinking particles collected in Breid Bay were more ecologically significant than the sinking and suspended particles from deep waters.     

Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (¹³C) were used to assess the importance of kelp carbon (-13.6 to-16.5) versus phytoplankton carbon (-25.5 to-26.5) to resident fauna of an isolated kelp bed community on Alaska's north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in ¹³C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2), a nonselective suspension feeder (an ascidian,-19.0) and a predatory gastropod (-17.6). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, ¹³C values identified four major feeding habits: deposit-feeders (-18.0), omnivores (-20.4), predators (-22.2) and microalgal herbivores (-23.0). Distinct seasonal changes in the ¹³C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.     

Nutrient regeneration in shallow-water sediments of the estuarine plume region of the nearshore Georgia Bight,USA

Benthic community respiration and the cycling of N and P were seasonally investigated in the unprotected, sandy sediments (Z5m) of the nearshore zone of the Georgia Bight, USA in 1981 and 1982. Nutrient exchange across the sediment-water interface was calculated from a diffusive model, measured by in-situ enclosure experiments and estimated from whole core incubations. Seasonally changing pore water profiles indicated that the sediments were not in steady-state with respect to N and P and showed the characteristics of enhanced interstitial water movement by benthic animals. Over an annual period the total flux of nitrogen measured in situ averaged 1812 mol m^-2 d^-1 from the sediments. NH ₄ ⁺ flux accounted for the vast majority of the total directly measured N flux (77%), followed by nitrate + nitrite (14%), and dissolved organic nitrogen (9%). Phosphorus flux averaged 537 mol m^-2 d^-1. A large ratio of in-situ fluxes to calculated diffusive fluxes (5.2:1) indicated flux enhancement due to benthic animal activity. ammonium fluxes measured in situ did not agree well with the rate of NH ₄ ⁺ produced in incubated whole cores (11.7 mmol m^-2 d^-1). Relative rates of C, N and P release throughout the year fluctuated considerably. Generally, nutrient fluxes were not simply related to respiration or temperature. As respiration was highly correlated with temperature, however, this suggested that respiration-regeneration was temporarily decoupled from exchange across the sediment-water interface. The annual C-N-P flux stoichiometry was 130:3.1:1. Using the rate at which NH ₄ ⁺ was produced in incubated cores the stoichiometry was 120:21:1. The anomalously low N flux measured in situ was attributed to a combination of denitrification and wave-and current-induced sediment nutrient flushing. The potential for sediment flushing is high as experiments showed that sediments were fluidized or resuspended down to 25 cm during large storms. Benthic nutrient flux contributed 40% to the annual P but only 11% to the annual N requirements of the pelagic primary producers.This is Contribution No. 558 from the University of Georgia Marine Institute. This work was supported by the Georgia Sea Grant College Program maintained by the National Oceanic and Atmospheric Administration, US Department of Commerce, under Grant No. NA80AA-D-00091     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.