John Marra

Phytoplankton Photosynthetic Response to Vertical Movement in a Mixed Layer

Abstract14C uptake was measured in experimental bottles suspended at a series of fixed depths (using subsamples from one sample depth), and was compared with estimates obtained from bottles provided with vertical movement similar to that which might occur in Langmuir circulations. In most experiments, the vertically cycled bottles gave estimates of integral photosynthesis 19 to 87% higher than estimates from the control series. Evidence from laboratory experiments detailing the change in photosynthesis over time at various irradiance levels shows that a higher lightsaturated photosynthetic rate under variable light conditions could account for the enhanced integral photosynthesis found in the bottles provided with vertical movement.

Effect of short-term variations in light intensity on photosynthesis of a marine phytoplankter: A laboratory simulation study

Parameters of photosynthesis and growth were measured for a marine diatom (Lauderia borealis) grown in axenic continuous culture under three different light regimes: constant, simulated diurnal variation, and fluctuating. The light fluctuations were systematic increases and decreases in light intensity superimposed on the diurnal regime. In the first two regimes, a morning maximum and an afternoon depression in photosynthesis were observed. In the fluctuating light regime, the afternoon depression was less pronounced and the morning maximum was enhanced. The results may be explained by postulating a time-dependent value for the light-saturated rate of photosynthesis. Light utilization [mmol O2 cell-1 (E m-2)-1] was the same for the diurnally varying and fluctuating regimes, despite the fact that the peak light intensity in the fluctuating regime was double that of the diurnally varying regime.

Primary production at 47°N and 20°W in the North Atlantic Ocean: a comparison between the 14C incubation method and the mixed layer carbon budget

Abstract Primary production in the oceanshas been estimated mainly on the basis of in vitro incubation measurements. An implicit assumption is that the growth rate of phytoplankton observed in vitro represents that occurring in the freely circulating water of the euphotic zone. We have tested this assumption at 47°N-20°W in the eastern North Atlantic Ocean during the initial stages of a spring phytoplankton bloom. The daily primary production was measured by means of the 14C assimilation method, in which the incubation bottles were suspended in the ocean from dawn to dusk daily (about 14 h). The mean daily carbon assimilation rate in the photic zone and in the mixed layer was 107 ± 23 and 84 ± 18 mmol C m −2 day ∗−1 (where day ∗ = 14 daylight hours), respectively, during the 12-day period between 26 April and 7 May 1989. The mixed layer carbon assimilation data are found to be consistent with the in situ CO2 utilization rate of 82 ± 17 mmol C m − day ∗−1 estimated on the basis of the thickness of surface mixed layer, the CO2 concentration in it, and the air-sea CO2 flux. We conclude that primary production in the open ocean appears to be well represented by the in vitro measurements, if the samples are incubated under the in situ light and temperature conditions. The mean daily reduction rate of the total CO2 concentration observed in the mixed layer over the 12-day period is 2.3 μmol kg−1 day−1, about 75% of the rate, 3.1 μmol kg−1 day∗−1, expected from the rate of primary production. About 8.5% of this difference is explained by the atmospheric CO2 flux, and the remaining 16.5% may be attributed to the respiration and the influx of CO2-rich waters from the mixed layer.

Nutrients and mixing, chlorophyll and phytoplankton growth

During a cruise to the Sargasso Sea in April 1985, we observed an event of wind-induced mixing during a 4-day sequence of observations while operating in a Lagrangian sampling mode. The sharp increase in wind stress was followed by a sharp increase in nitrate concentration in the euphotic zone. The nitrate declined rapidly, and over the next 2 days the quantity of chlorophyll a in the euphotic zone increased by a factor of three. The phytoplankton community was dominated by diatoms; this and other evidence indicates that the events observed were part of the spring bloom in the north Sargasso Sea. These observations are interpreted in terms of laboratory models for nutrient-dependent phytoplankton growth. The cell-quota model of Caperon and Droop provides an internally consistent explanation of the observed data. The data also suggest the notion of “nutrient switching” (rather than a multiplicative form of nutrient interaction) in the interaction of nitrate and silicate, although this could not be verified.

Moored observations of upper-ocean response to the monsoons in the Arabian Sea during 1994-1995

The role of surface forcing in the semiannual evolution of the upper-ocean temperature, salinity, and velocity fields in the Arabian Sea is examined. To do so, variability in the upper ocean in the central Arabian Sea was observed from an array of moorings deployed from October 1994 to October 1995. The Northeast (NE) Monsoon was characterized by moderate winds, clear skies, and dry air; sea-surface temperature (SST) dropped by 31C; the ocean lost an average of 19.7 W m � 2 and the mixed layer deepened by 100 m in response. The Southwest (SW) Monsoon was accompanied by strong winds, cloudy skies, and moist air; the ocean gained an average of 89.5 W m � 2 but SST dropped by 5.51C and the mixed layer deepened to almost 80 m. The response to the NE Monsoon included daily cycling in the depth of the mixed layer in response to the diurnal cycle in the buoyancy forcing and a weak local, wind-driven response. Stronger windforcing during the SW Monsoon dramatically reduced diurnal restratification, and a clearer signal of local, wind-driven flow in the upper ocean was found. The strongest velocity signal in the upper ocean, however, was the flow associated with mesoscale geostrophic features that passed slowly through the moored array, dominating the current meter records in the first part of the NE Monsoon and again in the latter part of the SW Monsoon. One-dimensional heat and freshwater balances, which held at other times through the year, broke down during the passage of these features. r 2002 Published by Elsevier Science Ltd.

Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

Abstract The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the highest and lowest concentrations of plant biomass in the world’s ocean. During the US JGOFS Process Study in the Arabian Sea (September 1994–January 1996), the absolute maximum in biomass of epipelagic zooplankton in the entire study was observed during the Southwest Monsoon season inshore of the Findlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monsoon, as was the strongest onshore–offshore gradient in biomass. Lowest biomass throughout the study was observed at the most offshore station (S15), outside the direct influence of the monsoon forcing. The greatest day/night contrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exceeding nighttime in the Southwest Monsoon season. The diel vertical migration patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of zooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppress migration. Based on the distribution of biomass, we hypothesize that inshore of the Findlater Jet, zooplankton grazing on phytoplankton is the dominant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous, conditions that result in reduction of the carbon flux mediated by the zooplankton. Predation by mesopelagic fish, primarily myctophids, may equal daily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integrated, may have evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Primary productivity and its regulation in the Pacific Sector of the Southern Ocean

We measured primary productivity in the Pacific Sector of the Southern Ocean as part of the Joint Global Ocean Flux Study. We collected data along 170degrees W from 54 degreesS to 72 degreesS on four cruises during the austral growing season of 1997-1998. The cruises crossed the Subantarctic Front, the Antarctic Polar Front (APF), the Southern Antarctic Circumpolar Current (ACC) Front, and the Southern Boundary of the ACC. Primary productivity and chlorophyll a increased rapidly in spring, peaked in summer, and decreased rapidly in fall, following the seasonal pattern of irradiance. In early spring (October), primary productivity was 20 mmol C m(-2) d(-1) and increased to 54 mmol C m(-2) d(-1) within 3 weeks. During peak irradiance (December), productivity reached its maximum throughout the study area with values ranging from 33 to 93 mmol C m(-2) d(-1) depending on station location. By February, average productivity dropped to 20+/-1 mmol C m(-2) d(-1), and individual station values reached a minimum of 13 mmol C m(-2) d(-1). In early spring, chlorophyll was less than 0.5 mg Chl m(-3) throughout the study area. In late spring and early summer, chlorophyll values were between 0.15 and 1.5 mg Chl m(-3) depending on station location. By late summer, chlorophyll decreased to less than 0.3 mg Chl m(-3) across the study region. Highest absolute values of productivity and biomass occurred near the southward-propagating Si gradient (DeltaSi(max)). A spatial gradient in photosynthetic performance correlated with DeltaSi(max): photosynthetic performance was elevated in low silicic acid waters (less than 10 muM) to the north of DeltaSi(max) and was depressed in high silicic acid waters (greater than 30 muM) to the south of DeltaSi(max) Photosynthetic performance also was correlated with iron-enrichment response: when photosynthetic performance was low, iron-enrichment response was high, and when photosynthetic performance was high, iron-enrichment response was low. These results suggest that phytoplankton were iron sufficient north of DeltaSi(max) and iron limited south of DeltaSi(max). We argue that the southward-traveling DeltaSi(max), the APF, and the location of upwelling, iron-rich Upper Circumpolar Deep Water (UCDW) define three regions with differing iron sufficiency. Furthermore, we suggest that a winter recharge of upwelled, iron-rich UCDW within the Antarctic and Southern ACC Zones provides enough iron to support a diatom bloom that annually propagates poleward across the Antarctic and Southern ACC Zones to the Southern Boundary of the ACC, where the absence of UCDW prevents the blooms progression into the Subpolar Regime

The carbon balance during the 1989 spring bloom in the North Atlantic Ocean, 47°N, 20°W

We report on studies of the carbon balance of the upper water column, done as part of the JGOFS North Atlantic Bloom Experiment, over a 13-day period, at 47°N, 20°W, during the 1989 spring phytoplankton bloom. Gross carbon production was calculated from data on 18O gross O2 production and from 14C production as well. Net carbon production was calculated from net O2 production rates measured in vitro, as well as from changes in the inventories of nutrients and O2 along with O2 evasion rates by gas exchange. Gross carbon production during this period was measured to be 1.83 mol m−2, and net production was 0.68 mol m−2. Of this net carbon production, 0.30 mol m−2 was stored in the euphotic zone as particulate organic carbon, and 0.09 mol m−2 rained out to depths >150 m. The remainder was remineralized to DIC in the 50–150 m depth interval, with perhaps some DOC storage in the upper 150 m.

Constraining bacterial production, conversion efficiency and respiration in the Ross Sea, Antarctica, January–February, 1997

Bacteria consume dissolved organic carbon at rates averaging about 50% of primary production across a wide spectrum of marine ecosystems. However, total utilization rates are poorly constrained due to a lack of data on conversion e

Three-dimensional flow in the upper ocean.

ciencies and/or bacterial respiration rates. We estimated total community dark respiration rates (DCR) from in vitro oxygen utilization and estimated bacterial production from 3H-leucine incorporation during January}February 1997 in the Ross Sea, Antarctica. Bacterial respiration rates (BR) were estimated by assuming that BR was less than some fraction of DCR, and by choosing values for the bacterial growth e