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The most probable number (MPN) assay is used to enumerate viable phytoplankton in the 10–50 μm size range for Type Approval of ballast water management systems. However, its validation has been questioned by the United States Coast Guard, which does not accept it as an alternative to its required method. To address key elements of method validation, three research laboratories used the same experimental protocols to enumerate viable organisms from uniformly viable and heat-killed cultures of three species of phytoplankton. Statistical analysis of the ratio of MPN estimates of viable organisms to measurements from flow cytometry was consistent with predictions based on perfect adherence to the method’s assumptions. These include reliable detection of all viable organisms and no false-positive results. The validation confirmed established estimates of the MPN method’s precision and demonstrated reproducibility, having been transferred to and implemented by two of the laboratories for the first time.

Ballast water management systems (BWMS) must be tested to assess their compliance with standards for the discharge of organisms, for example in the ≥ 10- and 80% of cases, long before the assays were terminated. We conclude that the SDC-MPN assay as described is robust and adaptable for use on natural phytoplankton.

Regulations for ballast water treatment specify limits on the concentrations of living cells in discharge water. The vital stains fluorescein diacetate (FDA) and 5‐chloromethylfluorescein diacetate (CMFDA) in combination have been recommended for use in verification of ballast water treatment technology. We tested the effectiveness of FDA and CMFDA, singly and in combination, in discriminating between living and heat‐killed populations of 24 species of phytoplankton from seven divisions, verifying with quantitative growth assays that uniformly live and dead populations were compared. The diagnostic signal, per‐cell fluorescence intensity, was measured by flow cytometry and alternate discriminatory thresholds were defined statistically from the frequency distributions of the dead or living cells. Species were clustered by staining patterns: for four species, the staining of live versus dead cells was distinct, and live‐dead classification was essentially error free. But overlap between the frequency distributions of living and heat‐killed cells in the other taxa led to unavoidable errors, well in excess of 20% in many. In 4 very weakly staining taxa, the mean fluorescence intensity in the heat‐killed cells was higher than that of the living cells, which is inconsistent with the assumptions of the method. Applying the criteria of ≤5% false negative plus ≤5% false positive errors, and no significant loss of cells due to staining, FDA and FDA+CMFDA gave acceptably accurate results for only 8–10 of 24 species (i.e., 33%–42%). CMFDA was the least effective stain and its addition to FDA did not improve the performance of FDA alone.

Discharge standards for ballast water treatment (BWT) systems are based on concentrations of living cells, for example, as determined with vital stains. Ultraviolet radiation (UV) stops the reproduction of microorganisms without killing them outright; they are living, but not viable, and ecologically as good as dead. Consequently, UV-treated discharge can be compliant with the intent of regulation while failing a live/dead test. An alternative evaluation of BWT can be proposed based on the assessment of viable, rather than living, cells in discharge water. In principle, the serial dilution culture-most probable number (SDC-MPN) method provides the appropriate measure for phytoplankton. But, the method has been criticized, particularly because it is thought that many phytoplankton species cannot be cultured. A review of the literature shows that although SDC-MPN has been used for more than 50 years-generally to identify and count phytoplankton species that cannot be preserved-its application to enumerate total viable phytoplankton seems to be new, putting past criticisms of the method in a different light. Importantly, viable cells need to grow only enough to be detected, not to be brought into sustained culture, and competition between species in a dilution tube is irrelevant as long as the winner is detectable. Thorough consideration of sources of error leads to recommendations for minimizing and quantifying uncertainties by optimizing growth conditions and conducting systematic comparisons. We conclude that with careful evaluation, SDC-MPN is potentially an effective method for assessing the viability of phytoplankton after BWT.

The Stain-Motility (S-M) and Most Probable Number (MPN) methodologies may be used for enumerating viable organisms≥ 10 μm to

An equation for the rate of photosynthesis as a function of irradiance introduced by T. T. Bannister included an empirical parameter b to account for observed variations in curvature between the initial slope and the maximum rate of photosynthesis. Yet researchers have generally favored equations with fixed curvature, possibly because b was viewed as having no physiological meaning. We developed an analytic photosynthesis-irradiance equation relating variations in curvature to changes in the degree of connectivity between photosystems, and also considered a recently published alternative, based on changes in the size of the plastoquinone pool. When fitted to a set of 185 observed photosynthesis-irradiance curves, it was found that the Bannister equation provided the best fit more frequently compared to either of the analytic equations. While Bannister's curvature parameter engendered negligible improvement in the statistical fit to the study data, we argued that the parameter is nevertheless quite useful because it allows for consistent estimates of initial slope and saturation irradiance for observations exhibiting a range of curvatures, which would otherwise have to be fitted to different fixed-curvature equations. Using theoretical models, we also found that intra- and intercellular self-shading can result in biased estimates of both curvature and the saturation irradiance parameter. We concluded that Bannister's is the best currently available equation accounting for variations in curvature precisely because it does not assign inappropriate physiological meaning to its curvature parameter, and we proposed that b should be thought of as the expression of the integration of all factors impacting curvature.

Ocean iron fertilization (OIF) is being considered as a strategy for mitigating rising atmospheric CO2 concentrations. One model for implementation is the sale of carbon offsets. Model- ing studies predict that OIF has the potential to produce a material difference in the rise of atmos- pheric CO2 over the next several decades, but this could only be attained by alteration of the ecosys- tems and biogeochemical cycles of much of the world's oceans. The efficacy of OIF on this scale has not been proven. However, the consequences of successful implementation must be considered now, for 2 important reasons: (1) to determine if the environmental effects would be predictable and veri- fiable, and if so, acceptable; and (2) to establish whether the basis for valuing carbon offsets — an accurate audit of net reductions in cumulative greenhouse gas potential over 100 yr — can be met. Potential side-effects of widespread OIF that must be considered include a reduced supply of macronutrients to surface waters downstream of fertilized regions, increased emissions of the potent greenhouse gases nitrous oxide and methane, and changes in the extent or frequency of coastal hypoxia. Given the uncertainties inherent in ocean models, predictions of environmental effects must be backed up by measurements. Thus, to go forward with confidence that the effects of rising CO2 could indeed be mitigated through OIF over the next century, and to establish the foundations for auditing carbon offsets, it must be explicitly demonstrated that methods exist to predict and detect downstream effects of OIF against the background of both climate variability and global warming. We propose that until the side-effects of widespread OIF can be shown to be verifiable — and there is good reason to believe that they cannot — OIF should not be considered a viable technology for climate mitigation.

Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.

We discuss important sources of variability in sun-induced chlorophyll fluorescence and examine difficulties in deriving fluorescence data products from satellite imagery, with a focus on the MODerate-resolution Imaging Spectroradiometer (MODIS) sensor. Our results indicate that there are limitations in the present MODIS algorithms that could lead to biases in the interpretation of the fluorescence products across gradients of chlorophyll concentration. To avoid some of these limitations, we suggest replacing the calculation of absorbed radiation by phytoplankton (ARP) over a finite depth with integration over the entire water column, and including a term accounting for cellular reabsorption of fluoresced light. These suggestions are incorporated into two new algorithms, based on established bio-optical models for case 1 waters (most open ocean waters), to retrieve chlorophyll concentration and the quantum yield of fluorescence. We compare our results to the results using MODIS algorithms for two regions: one located off the coast of Central America, including the Costa Rica Dome, and the other in the Arabian Sea. The new algorithms provide a similar field for the quantum yield of fluorescence in the first region, while they provide a different and more uniform field in the second region. We suggest that this discrepancy originates from the use of the water leaving radiance at 412 nm in the MODIS standard algorithm, which is not used in our algorithm and can be problematic under certain environmental conditions (e.g., absorbing aerosols or highly scattering waters).

The application of new molecular and genomic techniques to the ocean is driving a scientific revolution in marine microbiology. Discoveries range from previously unknown groups of organisms and novel metabolic pathways to a deeper appreciation of the fundamental genetic and functional diversity of oceanic microbes. The “oceanic genotype” represents only the potential biological capacity and sets an upper constraint on possible pathways and ecosystem rates. The realized structure and functioning of marine ecosystems, the “oceanic phenotype”, reflects the complex interactions of individuals and populations with their physical and chemical environment and with each other. A comprehensive exploitation of the wealth of new genomic data therefore requires a close synergy with interdisciplinary ocean research. Incorporating the information from environmental genomics, targeted process studies, and ocean observing systems into numerical models will improve predictions of the ocean's response to environmental perturbations. Integrating information from genes, populations, and ecosystems is the next great challenge for oceanography.

Autonomous underwater vehicles (AUVs) can map water conditions at high spatial (horizontal and vertical) and temporal resolution, including under cloudy conditions when satellite and airborne remote sensing are not feasible. Applications of this technology are numerous, and harnessing the full potential of AUV platforms for oceanographic research will require innovative sampling and data processing techniques, particularly in shallow littoral environments. We deployed a passive radiometer on a small AUV called the Remote Environmental Monitoring UnitS (REMUS; Hydroid) to demonstrate a novel method that uses one optical sensor at depth to characterize variability in underwater clarity and the constituents of coastal seawater (i.e., dissolved organic material, algal biomass, and other particles). This approach uses spectral differences between attenuation coefficients that are computed from ratios of downwelling irradiance measured at one depth assuming a constant shape, but not magnitude, for the solar irradiance spectrum at the surface. A spectral inversion model provides estimates of the absorption coefficients (m−1) of the constituents above the sensor. There is no requirement for sensors at the ocean surface using this approach and, compared to inversion methods for subsurface reflectance, the effects of bottom reflectance in shallow waters are minimal. Maps of biooptical properties at high spatial resolution demonstrate that our approach can be used for characterizing shallow and highly variable coastal waters. This simple and robust technique is also applicable to other in situ sampling platforms (e.g., gliders, moorings, Argo floats).

We present a simple method for describing the influence of variable attenuation of spec- tral irradiance, Kd(λ), on water-column photosynthesis and its inhibition by ultraviolet radiation (UV). The approach is based on weighted water transparency, a calculation introduced by Vincent et al. 1998 (Ann Glaciol 27:691-696). Key results of a depth- and spectrally-resolved model of photosyn- thesis can be reproduced for a broad range of water types by simple parameterizations using a refer- ence solar irradiance spectrum at the surface and water transparency (1/Kd(λ)) weighted spectrally for biological effectiveness. Transparency that has been weighted spectrally by the normalized prod- uct of irradiance and photosynthetic absorption (PUR-weighted transparency, T W) describes spec- tral effects on photosynthesis in the water column. An empirical parameterization of transparency weighted by the product of surface irradiance and the biological weighting function for inhibition of photosynthesis (T W PIR), along with T W PUR, describes the inhibition of water-column photosynthesis rela- tive to the uninhibited rate. Our approach is directly compared with an analysis that used weighted transparency as an indicator of the potential for inhibition of photosynthesis by UV as influenced by variations in chromophoric dissolved organic matter associated with climate change over the past 6000 yr (Pienitz & Vincent 2000, Nature 404:484-487). Results demonstrate how weighted trans- parency, used as an indicator of potential inhibition, can be transformed into a predictor of biological effects.

Size-fractionated chlorophyll concentration and phytoplankton absorption spectra were compared for a wide variety of natural communities. We found that, in general, when phytoplankton abundance increases, larger sizeclasses are added incrementally to a background of smaller cells. Natural phytoplankton communities from surface waters were explicitly characterized according to their dominant cell size and taxonomic group, and the relationships between this classification and the spectral shape of the phytoplankton absorption coefficient for the whole assemblage was described. By specifying the cell size of the dominant organism (pico-, ultra-, nano-, or microplankton), more than 80% of the variability in spectral shape of the phytoplankton absorption coefficient from 400 to 700 nm could be explained. This is a result of the strong covariation of the size of dominant organisms and several factors controlling the spectral shape of the phytoplankton absorption coefficient, such as pigment packaging and concentration of accessory pigments. Consequently, the shapes of phytoplankton absorption spectra can be reproduced using a spectral mixing model, where two spectra, representing the normalized phytoplankton absorption coefficients for the smallest and the largest cells found in our data set, are combined additively, using a single parameter to specify the complementary contribution of each. The differences between reproduced and measured spectra contain taxonomic and physiological information. This parameterization provides a simple tool for extracting ecological information from optical measurements. It can also be used in sensitivity analyses to describe the influence of the dominant cell size of phytoplankton on optical properties of surface waters. Changes in phytoplankton species composition are a central feature of marine ecosystem dynamics. Description and prediction of these changes are important goals to many fields in oceanography. In recent years, great effort has been made to understand how changes in phytoplankton species composition can affect optical properties of surface waters (e.g., Morel 1997; Kahru and Mitchell 1998; Stuart et al. 1998; Stramski et al. 2001). A major application of these results is the use of in situ optical instruments or remote sensing to observe variability of phytoplankton continuously or synoptically. This is a complicated topic because phytoplankton communities include species differing in size, shape, external and internal structures, and pigment composition. All these characteristics influence their interaction with the light field to some degree, so many factors must be considered to completely describe the optical properties of different communities of phytoplankton. A central goal is 1

The nitrogen cycle is fundamental to Earth's biogeochemistry. Yet major uncertainties of quantification remain, particularly regarding the global oceanic nitrogen fixation rate. Hydrogen is produced during nitrogen fixation and will become supersaturated in surface waters if there is net release from diazotrophs. Ocean surveys of hydrogen supersaturation thus have the potential to illustrate the spatial and temporal distribution of nitrogen fixation, and to guide the far more onerous but quantitative methods for measuring it. Here we present the first transect of high resolution measurements of hydrogen supersaturations in surface waters along a meridional 10,000 km cruise track through the Atlantic. We compare measured saturations with published measurements of nitrogen fixation rates and also with model-derived values. If the primary source of excess hydrogen is nitrogen fixation and has a hydrogen release ratio similar to Trichodesmium, our hydrogen measurements would point to similar rates of fixation in the North and South Atlantic, roughly consistent with modelled fixation rates but not with measured rates, which are lower in the south. Possible explanations would include any substantial nitrogen fixation by newly discovered diazotrophs, particularly any having a hydrogen release ratio similar to or exceeding that of Trichodesmium; under-sampling of nitrogen fixation south of the equator related to excessive focus on Trichodesmium; and methodological shortcomings of nitrogen fixation techniques that cause a bias towards colonial diazotrophs relative to unicellular forms. Alternatively our data are affected by an unknown hydrogen source that is greater in the southern half of the cruise track than the northern.

water is nearly opaque to infrared energy (Figure 1),the study of ocean optics has been primarily concernedwith propagation of visible energy, i.e. light. This is alsoa natural starting point for the study of ocean ecologyas photosynthesis is driven (with a few exceptions) byenergy within the visible light spectrum, which has suf-ficient energy per photon to induce photochemistry. Asphytoplankton accumulations also impact the colorand clarity of the water column, there is a direct linkbetween the studies of ocean optics and ocean ecology(Yentsch and Phinney, 1989).

It is well established that ultraviolet radiation (UVR, 280–400 nm) harms aquatic organisms. Reductions in productivity have been reported for phytoplankton, ichthyoplankton, and zooplankton in incubations exposed to UVR. It is difficult, however, to estimate the effects of UVR in natural waters. Quantitative assessments of UVR effects on aquatic organisms require high-resolution measurements of solar irradiance and its attenuation in the water, spectral weighting functions for biological effects, and realistic descriptions of the distributions and vertical movements of particles in the water column. Using experimentally determined biological weighting functions for UV-induced mortality along with measurement-based models of solar irradiance and of vertical distributions of embryos as influenced by mixing, we modeled UVR-induced mortality in the early life stages of two key species in the upper estuary and Gulf of St. Lawrence, Atlantic cod (Gadus morhua) and the planktonic copepod, Calanus finmarchicus. G. morhua embryos are insensitive to UVR, with an average daily survival of ~99% over numerous environmental conditions. C. finmarchicus are considerably more vulnerable, with an average survival of 90% ± 12% (SD). Lowest modeled daily survival was 59% under ambient ozone and 49% under 50% ozone loss. A sensitivity analysis allowed us to examine the relative influences of hydrographic variability, meteorological conditions, and ozone depletion on UVR-induced mortality in C. finmarchicus embryos. The modeled hydrographic and meteorological conditions are a representative range of natural variability for the St. Lawrence region during the 1997 field season, with the exception of extreme ozone depletion (50%). Effects are expressed as relative change of survival normalized to survival under a reference simulation. Similar to other studies, water column mixing and water clarity have the most significant influence on embryo survivorship, with a 3%–80% increased chance of survival when in static, compared with mixed waters, and a 3%–46% increased chance of survival when in the darkest, compared with the clearest waters. Cloudy skies increase survivorship between 1%–30%, and ozone depletion of 50% can decrease survivorship by 9%. On average, ozone depletion decreases survival by 3% and of the factors considered has the smallest influence on mortality of C. finmarchicus embryos.

Knowledge of the spectral attenuation associated with clouds is important for accurate estimates of natural irradiance at the Earth's surface. We compare spectral measurements of visible downwelling irradiance, under varying sky conditions at Halifax, Nova Scotia, Canada, with results from a clear-sky model. The spectral effect of clouds is estimated by taking the ratio of the measurements to the modeled irradiances and removing spectrally consistent instrumental effects and errors in the model. Empirical relationships derived between the spectral cloud effect and both CF, the cloud factor (the ratio of measured to modeled irradiances at 490 nm), and ƒ, the fraction of sky covered by cloud, were found to follow a wavelength (λ) dependence of the form a(CF or ƒ) + b(CF or ƒ)( λ/490)−4 in the 412–700 nm wavelength range. Both this relationship and a previously published linear relationship were found to be inadequate for describing cloudy irradiance data from the Bering Sea, indicating that the spectral effect of clouds can vary with cloud type and location. We show here that the spectral cloud effect can be mimicked by using a clear-sky model and changing the magnitude of the sky reflectivity or the spectral shape and magnitude of the ground albedo within the model. An investigation of the effects of cloud-dependent changes in irradiance spectra on calculations of bio-optical properties is also presented. Estimates of chlorophyll concentration from near-surface radiances are found to vary by up to 30%, whereas the effects on estimates of photosynthetically available and usable radiation at the sea surface are negligible.

To assess the potential impacts of ozone depletion on photosynthesis in the Southern Ocean, we need to know more about effects of ultraviolet radiation (UV) on phytoplankton in Antarctic waters, where, in addition to variable stratospheric ozone, temporal and regional differences in vertical mixing might influence photosynthesis and photoacclimation of phytoplankton assemblages. Toward this end, we quantified the responses to UV of Antarctic phytoplankton in the Weddell-Scotia Confluence during the austral spring of 1993. Experimental results on spectral sensitivity of photosynthesis were fit statistically to a model that incorporated uninhibited photosynthesis as a function of photosynthetically available radiation (PAR), wavelength-dependence of inhibition, and the kinetics of photosynthesis during exposure to UV. In contrast to previous results on UV-induced photoinhibition in a diatom culture at 20°C, natural phytoplankton from open waters of the Antarctic showed no ability to counter UV-induced inhibition of photosynthesis during exposures of 0.5-4 h: the rate of photosynthesis declined exponentially as a function of cumulative exposure, and inhibition was not reversed during incubations for up to 3.5 h under benign conditions. The results suggest that nonlinear exposure-response relationships are necessary for modeling UV-dependent photosynthesis in the surface mixed layer of the springtime Weddell-Scotia Confluence. Consequently, we modified our laboratory-based model of photosynthesis and photoinhibition to describe photoinhibition as a nonlinear function of biologically weighted cumulative exposure to damaging irradiance. The model described ∼90% of the spectrally dependent experimental variation in photosynthetic rate, and yielded six biological weighting functions (BWFs) for phytoplankton in the Weddell-Scotia Confluence. Assemblages from different stations showed substantial variability in sensitivity to UV. Tolerance of UV was generally highest in assemblages from shallower mixed layers, which presumably had experienced higher irradiance, including UV, prior to sampling. The BWFs of assemblages that seemed acclimated to low irradiance showed the highest sensitivity to UV yet seen for Southern Ocean phytoplankton. The pattern of UV sensitivity was consistent with acclimation, but also with selection against less tolerant species.

Photosynthesis of Antarctic phytoplankton is inhibited by ambient ultraviolet (UV) radiation during incubations1,2,3,4, and the inhibition is worse in regions beneath the Antarctic ozone ‘hole’4. But to evaluate such effects, experimental results on, and existing models of, photosynthesis5,6,7 cannot be extrapolated directly to the conditions of the open waters of the Antarctic because vertical mixing of phytoplankton alters UV exposure and has significant effects on the integrated inhibition through the water column2,8,9. Here we present a model of UV-influenced photosynthesis in the presence of vertical mixing, which we constrain with comprehensive measurements from the Weddell-Scotia Confluence during the austral spring of 1993. Our calculations of photosynthesis integrated through the water column (denoted PT) show that photosynthesis is strongly inhibited by near-surface UV radiation. This inhibition can be either enhanced or decreased by vertical mixing, depending on the depth of the mixed layer. Predicted inhibition is most severe when mixing is rapid, extending to the lower part of the photic zone. Our analysis reveals that an abrupt 50% reduction in stratospheric ozone could, in the worst case, lower PT by as much as 8.5%. However, stronger influences on inhibition can come from realistic changes in vertical mixing (maximum effect on PT of about ±37%), measured differences in the sensitivity of phytoplankton to UV radiation (±46%) and cloudiness (±15%).

We present a numerical model of nutrient uptake and photosynthesis during migrations of the marine diatom Rhizosolenia that was developed to estimate fluxes of carbon and nitrogen due to these migrations in the open ocean. The predicted specific rate of increase of Rhizosolenia was 0.11-0.15 day1, whereas the total time for one migration cycle ranged between 3 and 5 days. Using published estimates of Rhizosolenia abundance, we estimate that new primary production due to Rhizosolenia migrations ranges between 0.018 and 0.033 mmol N m~2 day1. These values represent up to 17% of new production due to turbulent diffusive fluxes of nitrate into the euphotic zone and are of the same order of magnitude as new production due to nitrogen fixation in tropical oceans. Large-scale contributions of Rhizosolenia to oceanic new production are limited by their relatively low standing crop. Variations in the formulation of losses with depth greatly affected gross and net fluxes of carbon and nitrogen. Better characterization of losses of Rhizosolenia and improved estimates of its abundance will help determine more accurately the contributions of Rhizosolenia to global biogeochemical cycles.

Concerns about harmful algal blooms (HABs) have grown in recent years. There is a pressing need for robust, quantitative, and cost-effective methods to detect and characterize algal blooms. Critical applications of these methods include long-term monitoring of coastal waters to indicate the degree to which present trends of HABs and human activities are linked, early warning systems to protect aquaculture sites and to signal the need for further investigations, and systems to characterize synoptically the distributions and physiological state of phytoplankton in an oceanographic context. Because phytoplankton strongly influence the fate of light in the ocean, optical methods are well suited for HAB applications. Passive optical systems can measure ocean color or the penetration of solar irradiance in surface waters; both properties can be related to the constituents of natural waters, including phytoplankton. The sensors measure radiometric quantities: consequently, rigorous calibration is possible and measurements can be compared between sites and over long periods of time. One passive optical system-a radiometer buoy-is shown to be useful for characterizing biological variability in surface waters over scales from minutes to months. A red tide was easily observed in measures of ocean color from the buoy; sensors for downwelling irradiance detected a subsurface bloom. Some optical instruments use artificial illumination to determine optical properties such as the coefficients of absorption and scattering. These measures can be made continuously in situ and effectively related to phytoplankton. Several types of fluorometers can be used to characterize the abundance, pigmentation, and physiological state of phytoplankton. All of these optical technologies should be useful for the study of HABs, but biological interpretations of many optical measurements need further testing.

C. R. BOOTH AND J. H. MORROW, Biospherical Instruments, San Dlego, CA, USA THOMAS P. COOHILL, Ultraviolet Consultants, Bowling Green, KY, USA JOHN J. CULLEN, Dalhousie University, Halifax, Nova Scotla, Canada JOHN E. FREDERICK, University of Chicago, IL, USA DONAT-P. HADER, University of Erlangen-Nurnberg, Germany OSMUND HOLM-HANSEN, Scripps Institution of Oceanography, La Jolla, CA, USA WADE H. JEFFREY, University of West Florida, Pensacola, FL, USA DAVID L. MITCHELL, M.D. Anderson Cancer Center, Smithvllle, TX, USA PATRICK J. NEALE, Smithsonian Environmental Research Center, Edgewater, MD, USA IGOR SOBOLEV, Chemical and Polymer Technology, Inc., Orinda, CAI USA JAN VAN DER LEUN, University of Utrecht, The Netherlands ROBERT C. WORREST, CIESIN, Washington, DC, USA

[1] Depth-integrated models of primary production (DIMs) are used to estimate water column photosynthesis as a function of chlorophyll concentration, irradiance at the surface, the penetration of photosynthetically available radiation (PAR), and parameters of the relationship between photosynthesis and PAR. These models are inherently unable to account for variability in the ratio of photosynthetically utilizable radiation (PUR) to PAR with depth and water type, and they cannot account for the inhibition of photosynthesis by ultraviolet radiation, UVR. These important spectral effects — all sensitive to climate change — are readily described with numerical models that require many computations and are unsuitable for some important applications, including the estimation of aquatic productivity from remote sensing. We present a simple DIM that accounts for the spectral effects of irradiance on photosynthesis, including inhibition by UVR. Water column photosynthesis, normalized to surface chlorophyll and scaled to the maximum rate per unit chlorophyll, is described as a function of four dimensionless derived variables:E*PUR, PUR at the surface scaled to the saturation irradiance for photosynthesis; T*PUR, water transparency, normalized to a depth scale and weighted spectrally for photosynthetic absorption; E*PIR, surface irradiance weighted spectrally for inhibition of photosynthesis; and T*PIR, scaled transparency weighted for photosynthesis-inhibiting radiation. Simple functions of these variables closely approximate (within 6%) the results of a full-spectral numerical model of instantaneous and daily integrated water column photosynthesis with and without UVR for a broad range of water types, solar angles, stratospheric ozone concentrations and biological properties of phytoplankton. The spectral DIM is suitable for examining patterns in global ocean productivity and can be used to assess the biological effects of variations in solar radiation (e.g., ozone depletion) and water clarity in climate-change scenarios for lakes and oceans.

[1] The ocean is the dominant source of atmospheric methyl iodide (CH3I). The mechanisms of CH3I production in the marine environment are poorly understood. A previous laboratory and field study suggested Prochlorococcus marinus, a ubiquitous marine cyanobacterium, is a globally significant biological producer of CH3I. In this study, CH3I concentrations were measured in cultures of P. marinus (high-light-adapted MED4 and low-light-adapted CCMP 1427) and the marine cyanobacterium Synechococcus. Cell-normalized production rates from P. marinus cultures (MED4) averaged 1.6 (±0.9) molecules of CH3I cell−1 d−1; these rates were 1000-fold lower than production rates reported for the previous study, which reported comparable concentrations of CH3I in cultures but cell concentrations about 1000-fold lower than measured in this study. Extrapolating CH3I production rates from the current study yields a global production rate of 0.6 Mmol yr−1 which accounts for 0.03% of the estimated global CH3I production, suggesting P. marinus is not a globally significant source of CH3I.

Severe reduction of stratospheric ozone over Antarctica has focused increasing concern on the biological effects of ultraviolet-B (UVB) radiation (280 to 320 nanometers). Measurements of photosynthesis from an experimental system, in which phytoplankton are exposed to a broad range of irradiance treatments, are fit to an analytical model to provide the spectral biological weighting function that can be used to predict the short-term effects of ozone depletion on aquatic photosynthesis. Results show that UVA (320 to 400 nanometers) significantly inhibits the photosynthesis of a marine diatom and a dinoflagellate, and that the effects of UVB are even more severe. Application of the model suggests that the Antarctic ozone hole might reduce near-surface photosynthesis by 12 to 15 percent, but less so at depth. The experimental system makes possible routine estimation of spectral weightings for natural phytoplankton.

[1] While the global ocean is an important source of atmospheric methyl iodide (CH3I), the major producers of CH3I within the ocean remain unclear. During a seasonal study in the NW Atlantic, the relationship between CH3I and some characteristic phytoplankton pigments was examined in order to identify possible phytoplankton producers of CH3I. Although no characteristic pigments exhibited a strong positive correlation with CH3I, in the surface mixed layer, there was a weak correlation (R = 0.35, n = 70, p = 0.003) between the concentrations of CH3I and zeaxanthin, a pigment characteristic of cyanobacteria in the open ocean. In this study, a moderate correlation was observed between the surface mixed layer CH3I concentration and depth-averaged daily radiant exposure (R = 0.61, n = 15, p = 0.02), which indicates a positive influence of solar radiation on CH3I production. However, the results from this study do not conclusively show whether the influence was exerted through photochemistry or other pathways. A positive correlation between the CH3I concentration and sea surface temperature was also observed (R = 0.61, n = 79, p ≪ 0.001).

Ocean high-nutrient, low-chlorophyll waters are characterized principally by the persistence of major nutrients at the sea surface. This condition indicates control of autotrophic production by something other than NO{sub 3} or PO{sub 4}, but the nature of this control is at present unresolved. The range of hypotheses to explain the high-nutrient condition is illustrated by the grazing hypothesis (specific growth rates of phytoplankton are maximal and environmental stability allows development of a balanced food web that maintains low standing crops of phytoplankton) and the iron hypothesis (standing crop of plankton is constrained by availability of Fe: if more Fe were available, the standing crop of phytoplankton would increase and NO{sub 3} would be depleted, despite grazing). The iron hypothesis has been examined experimentally in the subarctic and equatorial Pacific and in Antarctic waters. In each environment, Fe enrichment enhanced the field yield of phytoplankton and that enrichment of high-nutrient waters with Fe would change the species composition of phytoplankton and food-web interactions, thereby enhancing utilization of NO{sub 3}. The magnitude of this enhancement cannot be predicted with confidence.

Numerous studies of the growth of phytoplankton in the laboratory have demonstrated the dependence of cellular pigment concentration and growth rate upon light intensity, photoperiod, temperature, and nutrient supply. These same environmental parameters vary with season in the polar seas and presumably affect the growth rate and cellular pigment concentration of the phytoplankton crop. Unfortunately, there has not been a complete mathematical description of the interaction of all four environmental parameters. This study presents an approach to describing these interactions. It can reasonably be assumed that the gross specific growth rate, g, is a function of the specific rate of light absorption. The dependent variables in this equation are g, the gross specific growth rate, ?, the maximum carbon-specific photosynthetic rate, and, ?, the ratio of carbon to chlorophyll. The value of all three dependent variables is constrained. The independent variables are E?, the light intensity (assumed constant during the photoperiod), and ?, the photoperiod (as a fraction of 24 hours) that the cells are illuminated, ? is the instantaneous capacity of the dark reactions to assimilate electrons, while the product ap?max E?/? is the instantaneous capacity of the light reactions to supply electrons. If the capacity for photochemistry exceeds the capacity for assimilation, dissipative processes occur, and the quantum yield is low. We have applied this equation to the analysis of the growth and light absorption by Skeletonema costatum cultured under light, temperature, and nutrient limitation. Decreases in nutrient supply and temperature cause decreases in ? and increases in ?; thus both the capacity for electron supply and utilisation decrease. However, decreases in temperature decrease the capacity for electron assimilation more rapidly than the capacity for supply; quantum yield drops. Decreases in nutrient supply cause the capacity for supply and assimilation to drop in parallel; quantum yeield is maintained. Decreases in day length cause decreases in ? and increases in ?. The capacity to assimilate electrons and the capacity to supply electrons increase in parallel; quantum yield is maintained. Decreases in light intensity cause decreases in both ? and the capacity to supply electrons. Although the changes in ? with light intensity arc difficult to assess, the capacity to assimilate electrons appears to be little changed by light limitation. Quantum yields increase with decreasing light levels.

[1] This study examines variability in the relationship between Sun-induced chlorophyll fluorescence and incident solar irradiance as a potential diagnostic of the nutritional status of phytoplankton. The study site is the Bering Sea, where two optical drifters were caught for more than 100 days in an anticyclonic eddy, while two others provided data from adjacent waters. Estimates of fluorescence emission normalized to the absorption of light by pigments were analyzed as a function of irradiance to describe variability of the quantum yield of fluorescence. Yields in bright sunlight and under lower light varied by a factor of 5 or more on the scale of days to weeks. For the one drifter that remained in the high-velocity region of the eddy, there was a lagged correlation between the eddy rotation period and fluorescence parameters, with higher fluorescence yields in both low and high irradiance associated with slower rotation. Since nutrient input to the photic zone may increase with increasing shear of the eddy flow, this is consistent with the established suggestion that Sun-induced fluorescence increases with nutrient stress in phytoplankton. Independent measurements of variable fluorescence (Fv/Fm, an indicator of photosynthetic efficiency) further support this interpretation. However, modeling shows that the established hypothesis of competition between photosynthesis and fluorescence for absorbed photons (i.e., photochemical quenching), with high fluorescence yields reflecting photosynthetic debility, does not apply near the sea surface, where photosynthesis is saturated, and dissipation of excess absorbed radiation by nonphotochemical quenching is the dominant influence on fluorescence yield.

[1] The absorption of ultraviolet and visible radiation by colored or chromophoric dissolved organic matter (CDOM) drives much of marine photochemistry. It also affects the penetration of ultraviolet radiation (UV) into the water column and can confound remote estimates of chlorophyll concentration. Measurements of ocean color from satellites can be used to predict UV attenuation and CDOM absorption spectra from relationships between visible reflectance, UV attenuation, and absorption by CDOM. Samples were taken from the Bering Sea and from the Mid-Atlantic Bight, and water types ranged from turbid, inshore waters to the Gulf Stream. We determined the following relationships between in situ visible radiance reflectance, Lu/Ed (λ) (sr−1), and diffuse attenuation of UV, Kd(λ) (m−1): Kd(323nm) = 0.781[Lu/Ed(412)/Lu/Ed(555)]−1.07; Kd(338nm) = 0.604[Lu/Ed(412)/Lu/Ed(555)]−1.12; Kd(380 nm) = 0.302[Lu/Ed(412)/Lu/Ed(555)]−1.24. Consistent with published observations, these empirical relationships predict that the spectral slope coefficient of CDOM absorption increases as diffuse attenuation of UV decreases. Excluding samples from turbid bays, the ratio of the CDOM absorption coefficient to Kd is 0.90 at 323 nm, 0.86 at 338 nm, and 0.97 at 380 nm. We applied these relationships to SeaWiFS images of normalized water-leaving radiance to calculate the CDOM absorption and UV attenuation in the Mid-Atlantic Bight in May, July, and August 1998. The images showed a decrease in UV attenuation from May to August of approximately 50%. We also produced images of the areal distribution of the spectral slope coefficient of CDOM absorption in the Georgia Bight. The spectral slope coefficient increased offshore and changed with season.

The objectives of the research program reported upon here were (1) to measure ambient levels of UV radiation and determine whichvariables most strongly affected its attenuation in the waters of the estuary and Gulf of St. Lawrence, Canada; and (2) to investigate the potential direct impacts of W radiation on species of crustacean zooplankton and fish whose early life stages are planktonic. In this geographic region, productivity-determining biophysical interactions occur in the upper 0 to 30 m of the water column. Measurements of the diffuse attenuation coefficients for ultraviolet-B radiation (W-B, 280 to 320 nm) at various locations in this region indicated maximum 10% depths (the depth to which 10% of the surface energy penetrates at a given wave- length) of 3 to 4 m at a wavelength of 310 nm. Organisms residing in this layer-including the eggs and larvae of Calanus fin- marchicus and Atlantic cod Gadus morhua-are exposed to biologically damaging levels of W radiation. As a result of these phys- ical and biological characteristics, this system offered a relevant opportunity to assess the impacts of UV on subarctic marine ecosystems. Eggs of C. finmarchicus were incubated under the sun, with and without the W-B and/or UV-A (320 to 400 nm) wave- bands. W-exposed eggs exhibited low percent hatchmg compared to those protected from W: W radiation had a strong nega- tive impact on C. finmarchicus eggs. Further, percent hatching in W-B-exposed eggs was not significantly lower than that in eggs exposed to UV-A only: under natural sunlight, UV-A radiation appeared to be more detrimental to C. finmarchicus embryos than was UV-B. In analogous experiments with Atlantic cod eggs, exposure to UV-B produced a significant negative effect. However, UV-A had no negative effect on cod eggs. Additional experiments using a solar simulator (SS) revealed high wavelength-depen- dent mortality in both C. finmarchicus and cod embryos exposed to UV. The strongest effects occurred under exposures to wave- lengths below 312 nm. At the shorter wavelengths (

Adding iron to the ocean is not an effective way to fight climate change, and we don't need further research to establish that, say Aaron Strong, Sallie Chisholm, Charles Miller and John Cullen.

never exhausted in surface waters, and phytoplankton biomass is less than expected. Martin (6, 7) suggested that it is the scarcity of biologically available iron in these high-nutrient, low-chlorophyll (HNLC) regions that makes it impossible for the phytoplankton to use the excess N and P. He also recognized that atmospheric dust from land is an important source of iron for the sea and that HNLC regions receive a relatively small dust flux. Furthermore, he noted that ice core records of atmospheric CO2 and dust concentrations over the past 180,000 years are anticorrelated: when dust was high, CO2 was low. This is consistent with the notion that during the arid glacial periods, dust transport was greater, more iron was available, and the biological pump delivered more CO2 to the deep sea. This “iron hypothe

It has been proposed that widespread deficits of nitrate in the ocean, like those observed today, are caused by iron limitation of marine nitrogen fixation1. That is, only when iron is sufficiently abundant to satiate nitrogen fixers will the ratio of nitrate to phosphate in the ocean increase to 16, the average for phytoplankton. Tyrrell2 developed a simple two-box model of oceanic nitrogen and phosphorus cycles to describe the regulation of both nitrate and phosphate concentrations in the global ocean. His criterion for nitrate deficit in the ocean, a molar ratio of N:P in surface waters (Rs) of less than 16, is satisfied without recourse to iron limitation, calling into question Falkowski's proposal1 about the biogeochemical significance of iron limitation as it relates to nitrogen fixation and oceanic levels of nitrogen and phosphorus. Here I show that small changes in the assumptions of Tyrrell's model, well within acknowledged uncertainty, can lead to values of Rs greater than 16. Consequently, the consistency of the model with the observed distributions of nutrients in the ocean is uncertain, and the influence of iron may still be considered important.

In phytoplankton of the eastern tropical Pacific Ocean from 25 to 90 percent of the biomass (measured as chlorophyll a) and 20 to 80 percent of the inorganic carbon fixation were attributable to particles that could pass a screen with a 1-micrometer pore diameter. Evidence is presented that these are indeed autotrophic cells and not cell fragments.

A comparison is made of the in vivo Chl a fluorescence per cell measured by the flow cytometer (1c,,,) and dark-adapted bulk fluorescence measured in a standard field fluorometer for the marine cryptomonad Chroomonas sp. (clone Chang 2). The bulk fluorescence protocol estimated the levels of the minimum (F,) and maximum (F’,,) fluorescence yields that are exhibited depending on the rcdox state of the photosystem II reaction center. Both I;0 and F,,, are known to be functions of cell irradiance history. During the illumination of control samples at growth irradiance (40 pmol quanta m-* s-l), F,, F,,,, and F,,, (EPICS V) all increased by about the same proportion. After exposure to photoinhibiting irradiance (1,700 pmol quanta m-2 s-l), F,,,,, decreased and F0 increased. Parameters of the photosynthesis-irradiance curve verified that photoinhibition had occurred, indicating less activity at all irradiances. In contrast to bulk fluorescence measurements, relative changes in F,,, in response to strong-irradiance treatment were much smaller than changes in FO, and F,,,,,,. We conclude that this is because FM is intermediate between F. and F,,,,,. Multiple regression analyses suggest that, under the flow cytometry conditions used, F,, exhibits -20% enhancement above F,,, i.e. an average of 20% of the increase from F. to F,,,. Time scales of photosystem II primary photochemistry are consistent with this amount of fluorescence enhancement occurring over the residence time of the cell in the laser beam. These results suggest caution in using oversimplified interpretations of Fcyt. The enhancement effect should also be considered in other instances where fluorescence is excited by a brief saturating flash, for example, some types of in situ fluorometers.

As bivalve aquaculture continues to grow, it is imperative to understand the spatially-explicit interactions between farmed bivalves and the environment. However, the ability of models to represent a large number of bivalve ecophysiology and environmental variables—in 3-D spatially-explicit domains—has been limited by computational constraints. To overcome some of these computational limitations, we developed an optimized hybrid model by two-way coupling a state-of-the-art Eulerian model (Regional Ocean Modeling System; ROMS) that simulates physical, planktonic and sediment dynamics, with an individual-based model (IBM) that simulates shellfish ecophysiology (SHELL-E). The IBM model efficiently represents sparsely-distributed variables that do not occur in every grid cell of the domain, and simplifies the representation of complex life-history and physiological processes, like spawning events. We applied the hybrid model to a mussel farm in Ship Harbour (Eastern Canada) and compared model results against measurements of physical variables, water samples (chlorophyll, nutrients, oxygen and suspended sediments) and mussel size distributions. The hybrid model reproduced the main dynamics of the physical, planktonic and sediment Eulerian variables, as well as the bivalve ecophysiology IBM variables. Prognostic limitations estimated by the model suggested that mussels were temperature-stressed during parts of the summer, and food-limited during parts of the winter. We also used the hybrid model to estimate the production carrying capacity of the farm and we found that the farm is not overstocked. However, we also found that the estimation of carrying capacity strongly depends on the inferred natural mortality, which is difficult to estimate accurately. This work aims to transfer sound and open-source oceanographic tools (i.e. ROMS) into the applied fields of aquaculture research and management.

There is a growing aggregation of papers related to the search for invariants, or first integrals, of time-dependent dynamical systems. The citation of all such work since the seminal papers by LewisIl•) would be prohibitively lengthy in a short note. Two short review papers by Ray3),.) contain many references to recent literature on this subject. We will refer here only to key papers selected to serve the main points of the present paper. Our purpose is to prove two theorems which pertain to the equations of motion of dynamical systems recently discussed in the literature. The proof of the theorems will show that an auxiliary variable is naturally associated with these equations of motion, will reveal the origin of useful rescaling variables and will lead to direct determination of an associated invariant of the motion, The discussion of time-dependent systems is frequently presented in terms of an auxiliary function p(t), say, where t denotes the independent variable time. For many such systems the function p (t) is any solution to the specific nonlinear differential equation