Limnology and Oceanography

Ryan, John P.; Yoder, James A.; Cornillon, Peter C.

doi: 10.4319/lo.1999.44.1.0001pmid: N/A

In 8 yr (1979–1986) of Coastal Zone Color Scanner (CZCS) imagery, we find annual enhancement of chlorophyll at the shelfbreak of the Mid‐Atlantic Bight (MAB) and Georges Bank during the spring transition from well‐mixed to stratified conditions. Spatial and temporal extents of enhancement vary interannually, and expression is intermittent intraannually. This feature can span the entire MAB and southern flank of Georges Bank (∼1,100 km) and can be expressed for as long as 10 weeks (mid‐April to late June). Pigment concentrations within the feature average more than two times that of adjacent shelf and slope waters. Enhanced shelfbreak chlorophyll consistently coincided with the shelf‐slope front and often extended inshore of the surface outcrop of the front a few to ∼10 km. In all years except 1986, it coincided with seaward entrainment of shelf water by Gulf Stream warm‐core rings (WCRs) or meanders. Shelfbreak chlorophyll enhancement was most pronounced during 1980. Using satellite and in situ observations, we found that during 1980, it coincided with the shelf‐slope front for 10 weeks, and, unlike the spring bloom, it was dominated by the nanophytoplankton (<20 um) size fraction. During the peak of the 1980 occurrence, four WCRs simultaneously interacted with shelf water, and chlorophyll enhancement inshore of one WCR coincided with a slope‐water intrusion onto the shelf. Empirical orthogonal function (EOF) decomposition of CZCS images for late March–June 1980 showed that shelfbreak enhancement was strongly pronounced in an EOF that accounted for >10% of the variance about the mean. This annual biological feature, brought to light in satellite ocean color imagery, is an important aspect of the shelf‐slope ecology of the MAB and Georges Bank.

Paul, John H.; Pichard, Scott L.; Kang, Jordan B.; Watson, Gregory M. F.; Tabita, F. Robert

doi: 10.4319/lo.1999.44.1.0012pmid: N/A

The factors affecting the regulation of photosynthetic carbon fixation in diverse phytoplankton populations are not yet understood. To this end, we have measured the expression of the gene (rbcL) for the major carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase, in coastal phytoplankton populations off Cape Hatteras and in oligotrophic oceanic picoplankton near Bermuda. Using gene probes specific for the cyanobacterial/chloro‐phytic clade and the chromophytic clade (diatoms, chrysophytes, prymnesiophytes, and others) of Form I rbcL genes (“cyano” and “chromo” probes, respectively), we have measured rbcL messenger ribonucleic acid (mRNA) levels in size‐fractionated coastal waters, in a decktop diel incubator and a Lagrangian drifter study, and in vertical profiles in stratified, oligotrophic ocean water. In coastal waters influenced by estuarine plumes, an equal distribution of carbon fixation between the picoplankton and the micro/nannoplankton occurred, with cyano rbcL mRNA coinciding with Synechococcus counts in the <1‐μm fraction, with the majority of the chromo rbcL mRNA expression occurring in the larger sized phytoplankton fraction. In profiles of oligotrophic oceanic waters, the cyano rbcL mRNA was found in the upper water column (∼50‐m depth) and coincided with peaks in Synechococcus counts. The chromo rbcL mRNA was concentrated at the subsurface chlorophyll a (Chl a) maximum (∼85 m) and corresponded to red‐fluorescing cell counts, thought to be picoeucaryotes and diatoms. Photosynthetic carbon fixation and RUBISCO enzyme activity encompassed both cyano and chromo rbcL mRNA peaks, suggesting a near equal contribution to carbon fixation in the water column by these two phytoplankton clades. Both decktop diel incubator studies and a Lagrangian drifter study in coastal waters indicated cyano rbcL transcription in the morning and chromophytic rbcL transcription in the late afternoon/early evening. Thus, the two major clades of RUBISCO‐containing phytoplankton occupy separate niches in time, space, and cell size in the waters off Cape Hatteras. The factors determining such clade‐specific niches may include efficiency of nutrient utilization, differences in relative carboxylase/oxygenase activity (τ values) of cyano (Form IB) and chromophytic (Form ID) RUBISCOs, and differences in pigment composition/adaptation to light regimes. Additionally, we propose that chromo rbcL mRNA may be indicative of new production, whereas cyano rbcL mRNA correlates with recycled production in stratified, oligotrophic oceanic environments.

Culver, Mary E.; Perry, Mary Jane

doi: 10.4319/lo.1999.44.1.0024pmid: N/A

The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient for phytoplankton grown in culture and sampled from Puget Sound, Washington. The difference spectrum between total phytoplankton and photosynthetic absorption should be equivalent to photoprotective absorption. For cultures, the difference spectra exhibit peaks near 460 and 490 nm and broad‐band absorption between 400 and 450 nm. However, for field samples an additional pronounced peak is observed around 440 nm, similar in shape to the chlorophyll a Soret peak. If the 440‐nm peak were associated with photosystem I chlorophyll a, the photosynthetic absorption coefficient will be underestimated by <15% for these samples. Variability in both coefficients is predictable as a function of irradiance. The photosynthetic coefficient varies inversely with growth irradiance, and the photoprotective coefficient varies directly with irradiance. This direct relationship with irradiance accounts for much of the variability in the spectral shape of the total phytoplankton absorption coefficient. The ratio of the photosynthetic absorption coefficient to the total phytoplankton absorption coefficient increases as a function of decreasing irradiance for cultures and for field samples collected from stratified regions of the water column. This ratio is a photoadaptive parameter that can serve to integrate physiological response to irradiance and has the potential to provide estimates of mixed layer dynamics.

Lim, Ee Lin; Dennett, Mark R.; Caron, David A.

doi: 10.4319/lo.1999.44.1.0037pmid: N/A

The geographical distribution and seasonal abundance of the cosmopolitan heterotrophic flagellate Paraphysomonas imperforata in several coastal waters was examined using species‐specific oligonucleotide hybridization probes which target small subunit ribosomal RNA. P. imperforata was found to occur in several coastal environments, but at very low abundances (typically <50 cells ml−1). The seasonal abundance of P. imperforata examined at one sampling site remained consistently low and constituted no more than 1% of the total nanoplankton at any time during a 17‐month sampling period. In contrast to the low abundances observed in natural water samples, P. imperforata frequently dominated heterotrophic enrichment cultures prepared from these same samples, comprising up to 98% of the total nanoplankton. Based on these findings, we conclude that P. imperforata is an opportunistic species capable of growing rapidly to high abundances when prey concentrations are high. Water and enrichment temperature as well as the temperature tolerance range of P. imperforata appear to have played a role in the seasonal differences observed in P. imperforata dominance. Experiments with enrichment cultures indicated that the absolute abundances of P. imperforata in the water samples and the activity of consumers of nanoplankton also influenced the degree to which P. imperforata dominated the heterotrophic nanoplankton assemblages of enrichment cultures. Seasonal changes in water temperature might also affect these latter factors, and, as a consequence, indirectly influence the ability of P. imperforata populations to dominate enrichments. Our results support the notion that enrichment cultivation of heterotrophic flagellates, and perhaps incubations in general, can select for species such as P. imperforata that may not be representative of nanoplanktonic protists that numerically dominate natural assemblages.

Christaki, Urania; Jacquet, Stéphan; Dolan, John R.; Vaulot, Daniel; Rassoulzadegan, Fereidoun

doi: 10.4319/lo.1999.44.1.0052pmid: N/A

The two most abundant marine autotrophic prokaryotes, Prochlorococcus and Synechococcus, often have different distributions in the ocean. For example, Synechococcus is restricted to the first 100 m, whereas Prochlorococcus extends much deeper in oligotrophic waters. This is in part explained by differences in adaptation to nutrient and light regimes. However, they could also be subjected to different predation rates. To explore this hypothesis, we compared the consumption of these two picoplankters by an algivorous ciliate, Strombidium sulcatum, and a bactivorous ciliate, Uronema sp. For both ciliate species, removal rates were higher, by a factor of 3 to 10, for Synechococcus compared to Prochlorococcus when prey items were presented alone or together. The growth of the two ciliates fed Synechococcus and/or Prochlorococcus also differed. S. sulcatum grew well on both prey items, whether alone or together, whereas Uronema sp. grew slowly when fed Synechococcus and very poorly when fed Prochlorococcus either alone or with Synechococcus. Our results suggest that Prochlorococcus may be less subject to ciliate predation than Synechococcus.

Thingstad, T. F.; Havskum, H.; Kaas, H.; Nielsen, T. G.; Riemann, B.; Lefevre, D.; Williams, P. J. le B.

doi: 10.4319/lo.1999.44.1.0062pmid: N/A

An enclosure experiment was performed in the brackish layer of a Norwegian fjord, a system where both phytoplankton and bacterial growth have previously been hypothesized to be phosphorus (p) limited. All enclosures had a succession pattern characterized by an initial autotrophic phase with positive net community production, increasing phytoplankton primary production, and chlorophyll, followed by a heterotrophic phase with decline in primary production, negative net community production, increased community respiration, bacterial production, and biomass. Daily additions of phosphate increased the amplitudes of both the autotrophic and the heterotrophic phases of this succession. Because no stimulating effect of glycine addition on bacterial production or on community respiration was observed, the delay in bacterial response could not easily be explained as a delay in production or availability of carbon substrates for the bacteria. An alternative hypothesis is that bacterial production is regulated by a combination of P‐limited bacterial growth rate and predatory control of bacterial biomass. A simple numerical model based on this hypothesis was able to reproduce the main features of the observed pattern of succession.

Fasham, Michael J. R.; Boyd, Philip W.; Savidge, Graham

doi: 10.4319/lo.1999.44.1.0080pmid: N/A

A size‐structured ecosystem model was developed and fitted to an extensive set of observations of size‐fractionated biomass, primary and heterotrophic bacterial production, respiration, particle flux, and zooplankton herbivory obtained on a cruise in the NE Atlantic during 20 days of the spring bloom. Rates of water‐column‐integrated primary production and heterotrophic bacterial production were used to force the model, and the model parameters were determined by optimizing the model to the remaining observations. A unique model solution was not possible in the absence of dissolved organic carbon (DOC) measurements. By setting a weak constraint on the DOC concentrations (that they should remain broadly constant), a solution was obtained that predicted a small DOC increase during the bloom period; this increase was similar in magnitude to observations in other areas and produced a good fit with the other data. Major conclusions: (1) gross primary production (GPP) was over twice the measured 14C production estimates; (2) phytoplankton exudation was a major source of DOC, and 35% of the GPP flowed through the DOC and was utilized by the bacteria, with a low gross growth efficiency (17%); (3) 62% of the community respiration came from organisms <5 µm in size; (4) the major source of food for microzooplankton was phytoplankton 1–5 µm in size, but the mesozooplankton ingested more microzooplankton than phytoplankton; and (5) only 8% of the total net primary production was grazed by mesozooplankton. These results provide quantitative support to the emerging consensus on the relative roles of large and small organisms in the euphotic zone and demonstrate the necessity of measuring DOC to constrain the carbon budget within the euphotic zone.

Olsson, Kristina; Anderson, Leif G.; Frank, Markus; Luchetta, Anna; Smethie, William

doi: 10.4319/lo.1999.44.1.0095pmid: N/A

Production and remineralization of carbon in the Eurasian sector have been estimated based on a combined data set of the Oden—91 and Polarstern—93 (ARK IX—4) expeditions. This sector includes the deep Nansen and Amundsen Basins and their linked shelf seas, i.e., the Barents, Kara, and Laptev Seas. The water masses in this region are composed of Atlantic water, river runoff, and sea ice‐melt water. The fractionation between these source waters is elucidated from the δ18O—salinity relation and conservation of mass. By combining preformed nitrate concentrations of the source waters with the fractionation model and the measured nitrate concentrations, nitrate deficits and excesses are calculated. These concentrations are then converted to carbon equivalents by applying a C/N ratio, whereby a measure of apparent carbon utilization (ACU) is obtained. From the relative inventory of ACU along the slope and deep basin sections, we conclude that the shelf areas are the dominant productivity sites and that the productivity signal is transported to all water masses in the Eurasian Basin. The flux of utilized carbon from the Barents‐Kara and Laptev Seas is about 0.022 Gton C yr−1.

Vidal, Montserrat; Duarte, Carlos M.; Agusti, Susana

doi: 10.4319/lo.1999.44.1.0106pmid: N/A

The variation in the concentration and the turbulent fluxes of dissolved organic nitrogen and phosphorus (DON and DOP) across the thermocline in the central Atlantic were studied along a quasi‐meridional transect from the Canary Islands to Argentina (22°N to 31°S). In general, DON concentrations were high in surface waters and declined toward the thermocline, whereas DOP concentrations were less variable with depth. Vertical, gradient‐driven fluxes of DON and DOP generally involved a downward flux, with a median DON:DOP ratio of 28:1. The downward flux of DON was closely correlated with the nitrate supply to the mixed layer and sufficed to remove an amount of nitrogen equivalent to that associated with the upward nitrate influx into the mixed layer (median = 104%). The downward flux of DON exceeded the supply of nitrate off the NW African coast, where Trichodesmium sp. was abundant. The downward flux of DOP was also very closely correlated with the phosphate supply to the mixed layer (r = 0.87; P = 0.0007), but the downward export of phosphorus as DOP accounted for only 9% of the upward phosphate supply to the mixed layer. There was also evidence of a deep upward flux of DON and DOP from below the thermocline, which, combined with the general downward flux in the upper waters, indicates the presence of a strong sink for DON and DOP within the thermocline, tentatively identified to be microplankton respiration. Our results point to a dominant role of downward DOM fluxes as a path for the removal of nutrients, and probably carbon, from the biogenic layer of the ultraoligotrophic central Atlantic.

Planas, Dolors; Agustí, Susana; Duarte, Carlos M.; Granata, Tim C.; Merino, Martín

doi: 10.4319/lo.1999.44.1.0116pmid: N/A

The latitudinal variation (35° to 28°N) in the rate of diffusive nitrate supply across the thermocline and the associated variation in the uptake rate of nitrate and ammonium in the Central Atlantic was studied. The calculated diffusive nitrate flux showed a sharp latitudinal gradient, with the lowest nitrate supply (0.00037 μmol m−3 d−1) in the South Atlantic subtropical gyre and the highest values (23.5 μmol m−3 d−1) between the Equator and 15°N. The uptake rate of nitrate was inhibited at high irradiance at most stations. Both nitrate and ammonium uptake rates were lowest (about 3 and 10 umol m−3 d−1, respectively) at the southern end of the transect and increased (about 20 and 55 μmol m−3 d−1, respectively) towards the Equator, with this increase being much greater for ammonium than for nitrate uptake. The f‐ratio was highest (≈0.4) just south of the Equator and lowest (≈0.03) at the southern end of the transect. The slope between total uptake rate of dissolved inorganic nitrogen and gross primary production, calculated from O2 ‐based measurements, in surface waters (4.72 ± 1.54) was somewhat lower, but not significantly so, than the expected C/N ratio of 6.6. The average uptake rate of nitrate did not differ significantly from the average estimated diffusive supply of nitrate to the biogenic layer over the Central Atlantic. However, the nitrate uptake rate increased as the ⅓ power of the diffusive nitrate flux to the biogenic layer. As a result, nitrate uptake far exceeded (by up to 100‐fold) the nitrate flux to the biogenic layer in the stations where the supply of nitrate was lowest. The excess nitrate uptake averaged 0.65 ± 0.24 mmol NO3 m−2 d−1 (range, 0.05–1.9 mmol NO3 m−2 d−1), which must be supplied through atmospheric deposition and other perturbation events. This excess nitrate uptake is relatively large compared to the diffusive supply in the most unproductive areas, where external nitrate inputs fuel the new production. In contrast, these sources of nitrate are far less significant where high diffusive fluxes suffice to maintain high nitrate uptake rates.