Glenn F. Cota

Ecology of bottom ice algae: I. Environmental controls and variability

Abstract Over large ocean areas of the Arctic, Subarctic and Antarctic, which are covered by landfast sea ice during springtime, high concentrations of microalgae have been observed in the interstices of the lower margin of sea ice floes and, in some cases, in a thin layer of surface water immediately under the ice cover or associated with semi-consolidated frazil ice. Ice algal blooms enhance and extend biological production in polar waters by at least 1–3 months. Biomass accumulation of sea ice algal populations ultimately depends upon the duration of the growth season, which is largely a function of climatic and environmental variability. Growth seasons are shorter at lower latitudes because of abbreviated photoperiods, warmer air temperatures and earlier ablation and break up. Environmental factors, which regulate ice algal distributions and dynamics, display characteristic scales of time/space variance. Sea ice habitats are much more stable than planktonic environments, because ice is not subject to large vertical displacements in the irradiance field. Temperature and salinity are relatively constant over most of the growth period. However, nutrients must be supplied to relatively thin, dense layers of cells and fluxes are variable depending on ice growth and hydrodynamics. Although the occurrence of prolonged blooms of ice algae at the ice-water interface is a widespread phenomenon, there are important differences between the growth habits and environments of several well-studied sites. Recent observations from seasonal studies of these sites are compared and contrasted with an emphasis on how the dominant scales of environmental variability influence ice algal populations.

Retrieval of Aerosol Scattering and Absorption Properties from Photopolarimetric Observations over the Ocean during the CLAMS Experiment

The extensive set of measurements performed during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment provides a unique opportunity to evaluate aerosol retrievals over the ocean from multiangle, multispectral photometric, and polarimetric remote sensing observations by the airborne Research Scanning Polarimeter (RSP) instrument. Previous studies have shown the feasibility of retrieving particle size distributions and real refractive indices from such observations for visible wavelengths without prior knowledge of the ocean color. This work evaluates the fidelity of the aerosol retrievals using RSP measurements during the CLAMS experiment against aerosol properties derived from in situ measurements, sky radiance observations, and sunphotometer measurements, and further extends the scope of the RSP retrievals by using a priori information about the ocean color to constrain the aerosol absorption and vertical distribution. It is shown that the fine component of the aerosol observed on 17 July 2001 consisted predominantly of dirty sulfatelike particles with an extinction optical thickness of several tenths in the visible, an effective radius of 0.15 0.025 m and a single scattering albedo of 0.91 0.03 at 550 nm. Analyses of the ocean color and sky radiance observations favor the lower boundary of aerosol single scattering albedo, while in situ measurements favor its upper boundary. Both analyses support the polarimetric retrievals of fineaerosol effective radius and the consequent spectral variation in extinction optical depth. The estimated vertical distribution of this aerosol component depends on assumptions regarding the water-leaving radiances and is consistent with the top of the aerosol layer being close to the aircraft height (3500 m), with the bottom of the layer being between 2.7 km and the surface. The aerosol observed on 17 July 2001 also contained coarse-mode particles. Comparison of RSP data with sky radiance and in situ measurements suggests that this component consists of nonspherical particles with an effective radius in excess of 1 m, and with the extinction optical depth being much less than one-tenth at 550 nm.

Dimethyl sulfide in the Arctic atmosphere

Dimethyl sulfide (DMS), sulfur dioxide, non-sea-salt sulfate, and various aerosol properties were measured during three field programs (two airborne and one ground-based) near Barrow and Deadhorse (Prudhoe Bay), Alaska. The two airborne sampling programs took place in spring and early summer, and the ground-based measurements spanned an entire summer. DMS concentrations in the Arctic atmosphere ranged from a few parts per trillion by volume (pptv) in spring and fall to higher values in summer (generally a few tens of pptv with occasional peaks of 100 to 300 pptv). In addition, DMS concentrations were measured during the spring near Resolute in seawater below the ice and in ice-algae and kelp cultures. The seawater samples taken from below the ice in spring had DMS concentrations comparable to those in other oceanic regions. Taken together, these measurements show that the Arctic Ocean is potentially a substantial source of DMS, which likely becomes important as sea ice melts in the early summer. Local atmospheric concentrations increased throughout the summer, peaking in August. In regions where accumulation mode aerosols have been scavenged (e.g., by low-level stratus clouds, which are common during the Arctic summer), evidence of rapid new particle production was observed. The seasonal cycle of atmospheric DMS closely resembles that of fine particles observed at Barrow, Alaska, and Alert, Northwest Territories, Canada. This finding indicates that DMS is likely an important precursor to the types of particles that dominate the background arctic aerosol in summertime. These results, together with those from several recently published studies of arctic aerosol, are combined to yield a consistent picture of the role of locally emitted DMS in the production of atmospheric aerosols in the Arctic in summer.

Ecology of bottom ice algae: II. Dynamics, distributions and productivity

Abstract Spring blooms of bottom ice algae are a common feature of landfast congelation ice in polar regions. Because ice algae are usually associated with a substrate, their population dynamics can be followed with considerable confidence. Although ice algal dynamics are closely related to irradiance, their dynamics and distributions are influenced by other abiotic and biotic factors. Ice algal abundance varies horizontally over all scales examined. Factors such as grazing and nutrient availability may contribute to local and geographic differences. Loss terms for most sea ice assemblages are largely unquantified. Ice algal biomass is most concentrated near the ice-water interface in spring. Environmental factors affecting ice algal abundance and productivity are considered here, emphasizing recent results from several well-studied sites. Biomass accumulation, growth rates and productivity have been documented for spring blooms of bottom interstitial and sub-ice assemblages. On an areal basis biomass accumulation in bottom ice assemblages can be comparable with planktonic systems. At low ambient temperatures and irradiances average specific growth rates (≤ 0.25 d −1 ) and production rates (≤ 1.0 mg C mg Chl −1 h −1 ) for ice algae are low. Current methods of measuring productivity are compared. Results are consistently low but variable with little systematic difference among them. At present, apparent differences in productivity between bottom ice assemblages in the Arctic and Antarctic, or among different antarctic assemblages, are so confounded by methodological and other sources of variability that no firm differences can be detected.

Ecology of bottom ice algae: III. Comparative physiology

Abstract The physiological behavior of bottom ice algal assemblages has been studied intensively at several locations, particularly over the last decade. Ice algal populations can be studied for 1–3 months because they are stationary and their environmental conditions can also be manipulated in situ. Therefore, they present a model system for studies of the physiological ecology of natural microalgal populations. Physiological responses to major environmental variables, including temperature, salinity, irradiance and nutrients, have been characterized. Ice algae are physiologically similar to polar phytoplankton, but there are important differences which appear to reflect their respective environmental conditions. Photosynthesis vs. irradiance responses, photosynthate allocation and biochemical composition have been determined for vernal blooms. Ice algae and phytoplankton have similar gross biochemical compositions (e.g., C:N, C:Si, C:Chl), but lipid contents can be markedly higher in ice algae. Ice algae normally exhibit relatively low maximal assimilation numbers (except at subpolar latitudes) but markedly higher photosynthetic efficiencies than planktonic diatoms; large low frequency fluctuations in photosynthetic performance are common during the later phases of blooms. Ice algae have relatively low photoadaptive indices Ik and optimal irradiances Im, reflecting their average growth irradiance. Compared to phytoplankton, photosynthate allocation by ice algae is lower for protein, similar for lipid but higher for polysaccharide and metabolites at the same irradiance. In several respects the physiological behavior of ice algae appears to be fundamentally different than that of phytoplankton.

Modification of NO, PO, and NO/PO during flow across the Bering and Chukchi shelves: Implications for use as Arctic water mass tracers

The NO and PO tracers (9[NO3−] + O2 and 135[PO4−] + O2, respectively,) and their derivative NO/PO have found increasing use in Arctic water mass analyses for identifying the specific basin or shelf areas from which surface waters originate, based upon assumed differences in Pacific- and Atlantic-derived content and basin-to-basin differences within the Arctic. Following shipboard sampling in June-September 1993 and May-June 1994, both north and south of Bering Strait, we have found evidence that Pacific-derived waters flowing north to Bering Strait do not necessarily have any unique NO, PO, or NO/PO identity that would permit unequivocal use as a water mass tracer. In particular, NO/PO ratios in the Bering Sea continental shelf (<150 m) waters varied from 0.7 to 1.1, which encompasses ratios previously reported for Arctic continental shelf and Atlantic origin waters in the Arctic Ocean. The highest NO/PO ratios (∼1) in the Bering Sea were observed to the southwest of St. Lawrence Island, close to where high nutrient waters are first upwelled onto the shelf, and seasonally early in the biological production cycle. By contrast, later in the summer, north of Bering Strait, at the depth of the Arctic Ocean nutrient maximum, the highest concentrations of silica (∼60 μM) were associated with low NO/PO ratios (∼0.7). Apparent increases in the proportions of sea ice melt in these waters, inferred from 18O and salinity regressions, were associated with lower NO/PO ratios. This pattern, the potential for sea-air exchange, and a significant relationship between decreases in nitrate/phosphate ratios and both NO/PO ratios and silica concentrations indicate that biological and physical processes north and south of Bering Strait affect the fidelity of these nutrient-based tracers. These results indicate the need for consideration of shelf-based processes before NO/PO ratios and other nutrient-based tracers can be successfully applied as Arctic circulation tracers.

Remote-sensing reflectance in the Beaufort and Chukchi seas: observations and models.

Two semianalytical remote-sensing reflectance models were evaluated and validated by use of bio-optical data collected in the Beaufort and Chukchi seas. Both models were efficient at retrieving chlorophyll concentration, phytoplankton absorption coefficients,and particulate backscattering coefficients. In contrast, they were not accurate in predicting an absorption coefficient for colored dissolved organic matter plus nonpigmented particulates. The poor model performance is attributed to the high variability in the concentrations of these colored materials. A chlorophyll-dependent reflectance model was also assessed, and it proved to be highly successful in reproducing measured reflectance spetra. A four-component, case 2 model with mean absorption spectra for phytoplankton, soluble materials, and nonpigmented particulates was employed in Hydrolight radiative-transfer model simulations. The remote sensing reflectance spectra simulated inthe radiative-transfer model were in excellent agreement with field data. The similarity between the model and the measurement confirms the accuracy of the underlying bio-optical relationships and underscores the utility of modeling for better understanding of the variability of ocean color observations. The latest SeaWiFS algorithm (OC4V4) overestimated chlorophyll by approximately 1.5 fold across most of the observed range of biomass (0.07-9 mg chlorophyll m(-3)). Regionally tuned algorithms explained > 93% of the variability in the surface chlorophyll concentration.

Biogenic bromine production in the Arctic

Bottom ice microalgae and a common kelp, Agarum cribosum, have been shown to contain and release organohalides during spring in the high Canadian Arctic. Although a variety of brominated, chlorinated or mixed-halogen compounds are present in algal tissues, bromoform (CHBr3) dominates tissue content and emission. Tissue loads for ice algae (n = 24) averaged 679 ± 355 ng CHBr3 (g dry weight)−1, and kelp (n = 3) tissues had 1806 ± 1037 ng CHBr3 (g dry weight)−1. Ice algal release averaged 998 ± 1119 ng CHBr3 (g dry weight)−1h−1 for 31 experiments but values ranged from 124 to 5434 ng CHBr3 (g dry weight)−1h−1. In three experiments kelp released from 41 to 58 ng CHBr3 (g dry weight)−1h−1 with an average of 52 ± 8 ng CHBr3 (g dry weight)−1h−1. On a carbon-specific basis the rates of emission are similar for both algal groups. However, the fragments of kelp blades that we used may have had a ‘wound’ response with artificially high release. Emission of CHBr3 by ice algae in the light is about twice that in the dark while seawater ‘controls’ change little, if at all. Dark release of bromoform by kelp averaged 87% of light values. A positive relationship was found between CHBr3 emission by ice algae and carbon fixation which explains over half of the variance. Bromoform release was also reduced to varying degrees by additions of metabolic inhibitors. Time series of release by ice algae show that CHBr3 production in the light was largely linear over the span of our routine experiments. Biogenic release of CHBr3, particularly by ice algae, constitutes a globally significant source of organobromine.

Vertical profiles of bromoform in snow, sea ice, and seawater in the Canadian Arctic

Bromoform (CHBr3) was measured in vertical profiles from the snow surface through the snowpack, sea ice, and water column to the seafloor at Resolute Bay, Canada, in the spring of 1992. Elevated concentrations of bromoform were observed in both the ice (32–266 ng L−1 by liquid water volume) and seawater (∼20 ng L−1) at the ice-water interface, associated with bromoform emission from ice microalgae. A surprising finding was a second horizon of high bromoform concentrations (336–367 ng L−1) in sea ice at the snow-ice interface. Chlorophyll and salinity were also elevated in this upper ice layer, although chlorophyll was much lower than in the basal ice microalgal layer. We speculate that this upper bromoform-enriched layer may have originated from scavenging of the surface water layer by frazil ice during initial ice formation in the preceding autumn. Equally unexpected was the occurrence of yet higher bromoform concentrations in snowpack immediately overlying the sea ice (492–1260 ng L−1), declining in concentration (by about a factor of 2 or more) toward the snow surface. Snow of very recent origin, however, contained as little as 2 orders of magnitude less bromoform than the older snowpack. Possible origins for elevated bromoform in the snowpack include diffusion out of the bromoform-enriched upper ice layer and gradual concentration of bromoform out of the atmosphere by adsorption on to ice crystals. These are considered in turn. In one scenario, photolysis of bromoform from snow is considered, which might help account for atmospheric bromine-ozone chemistry. The possible contributions from snow, sea ice, and seawater to atmospheric bromoform levels during both the winter and spring are also considered, and it is concluded that surface seawater presents the most significant reservoir for atmospheric bromoform.

Radiative Transfer Modeling for the CLAMS Experiment

Spectral and broadband radiances and irradiances (fluxes) were measured from surface, airborne, and spaceborne platforms in the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) campaign. The radiation data obtained on the 4 clear days over ocean during CLAMS are analyzed here with the Coupled Ocean‐Atmosphere Radiative Transfer (COART) model. The model is successively compared with observations of broadband fluxes and albedos near the ocean surface from the Clouds and the Earth’s Radiant Energy System (CERES) Ocean Validation Experiment (COVE) sea platform and a low-level OV-10 aircraft, of near-surface spectral albedos from COVE and OV-10, of broadband radiances at multiple angles and inferred top-of-atmosphere (TOA) fluxes from CERES, and of spectral radiances at multiple angles from Airborne Multiangle Imaging Spectroradiometer (MISR), or ‘‘AirMISR,’’ at 20-km altidude. The radiation measurements from different platforms are shown to be consistent with each other and with model results. The discrepancies between the model and observations at the surface are less than 10 W m 22 for downwelling and 2 W m22 for upwelling fluxes. The model‐observation discrepancies for shortwave ocean albedo are less than 8%; some discrepancies in spectral albedo are larger but less than 20%. The discrepancies between low-altitude aircraft and surface measurements are somewhat larger than those between the model and the surface measurements; the former are due to the effects of differences in height, aircraft pitch and roll, and the noise of spatial and temporal variations of atmospheric and oceanic properties. The discrepancy between the model and the CERES observations for the upwelling radiance is 5.9% for all angles; this is reduced to 4.9% if observations within 15 8 of the sun-glint angle are excluded. The measurements and model agree on the principal impacts that ocean optical properties have on upwelling radiation at low levels in the atmosphere. Wind-driven surface roughness significantly affects the upwelling radiances measured by aircraft and satellites at small sun-glint angles, especially in the near-infrared channel of MISR. Intercomparisons of various measurements and the model show that most of the radiation observations in CLAMS are robust, and that the coupled radiative transfer model used here accurately treats scattering and absorption processes in both the air and the water.