Malgorzata Stramska

Initial results from the Bermuda Testbed Mooring program

The Bermuda Testbed Mooring (BTM) has been deployed since June 1994 and provides the oceanographic community with a deep-water platform for testing and intercomparing new instruments. The mooring is located about 80 km southeast of Bermuda. Surface instruments collect meteorological and spectral radiometric measurements from a buoy tower. Measurements at depth include: currents, temperature, conductivity, optical properties, and nitrate and trace element concentrations. Data have been sent to shore and to a nearby ship using a new inductive-link telemetry system. The high temporal resolution, long-term data collected from the mooring provide important information concerning episodic and periodic processes ranging in scale from minutes to years. For example, short nitrate pulses and associated biological events have been observed in the mooring data sets, which were not seen in the periodic ship-collected time-series data. Evaluation of undersampling and aliasing effects characteristic of infrequent sampling are also enabled with these data sets. The primary purposes of this report are to describe new systems and to illustrate early data resulting from the BTM program.

Bio-optical relationships and ocean color algorithms for the north polar region of the Atlantic

Received 30 October 2001; revised 30 October 2001; accepted 11 February 2003; published 13 May 2003. [1] Up to now, relatively few bio-optical measurements have been made in the high northern latitude waters, which allow sound relationships for ocean color remote sensing to be determined. We collected optical and chlorophyll a concentration, Chl, data in the north polar region of the Atlantic in summer season. The investigated region includes subarctic and arctic waters between 70� N and 80� N within the meridional zone between 1� E and 20� E. Our measurements show that the current NASA global algorithms, OC2, OC4, and chlor-MODIS, generally overpredict Chl in the investigated waters by a factor of about 2 at low pigment concentrations (<0.2 mg m � 3 ) and underpredict Chl at higher concentrations (20–50%at2–3mgm � 3 ).Forourdataset,thebesttwo-bandalgorithmforChlinvolvesthe ratio of remote-sensing reflectance, Rrs(442)/Rrs(555), at 442-nm and 555-nm light wavebands. We found that the general trend of variation in the blue-to-green reflectance ratio, Rrs(442)/Rrs(555) or Rrs(490)/Rrs(555), with Chl was driven primarily by Chldependent change in the green-to-blue ratio of absorption by pure seawater and particles. The effect of the blue-to-green backscattering ratio was of secondary importance. We observed acharacteristic opticaldifferentiation ofwaterswithin theinvestigated region. The majority of waters, which are here hypothesized to be dominated by diatoms, exhibited a relatively high blue-to-green reflectance ratio. The waters at several other stations, presumably dominated by dinoflagellates and/or prymnesiophytes, showed much lower reflectance ratio. Our data also show that the seemingly random variations in particulate absorption and backscattering coefficients at any given Chl are significant (more than a factor of 2) in the investigated waters. INDEXTERMS: 4552 Oceanography: Physical: Ocean optics; 4275 Oceanography: General: Remote sensingand electromagnetic processes (0689); 9325 Information Related to Geographic Region: Atlantic Ocean; 9315 Information Related to Geographic Region: Arctic region;

Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California

Beach, California, over a period of 1.5 years. Measurements included the hyperspectral inherent optical properties (IOPs) of seawater (particulate beam attenuation, particulate and CDOM absorption coefficients within the spectral range 300–850 nm), particle size distribution (PSD) within the diameter range 2–60 mm, and the mass concentrations of suspended particulate matter (SPM), particulate organic carbon (POC), and chlorophyll a (Chl). The particulate assemblage spanned a wide range of concentrations and composition, from the dominance of mineral particles (POC/SPM 0.25) with considerably greater contribution of larger‐sized particles. Large variability in the particulate characteristics produced correspondingly large variability in the IOPs; up to 100‐fold variation in particulate absorption and scattering coefficients and several‐fold variation in the SPM‐specific and POC‐specific coefficients. Analysis of these data demonstrates that knowledge of general characteristics about the particulate composition and size distribution leads to improved interpretations of the observed optical variability. We illustrate a multistep empirical approach for estimating proxies of particle concentration (SPM and POC), composition (POC/SPM), and size distribution (median diameter) from the measured IOPs in a complex coastal environment. The initial step provides information about a proxy for particle composition; other particulate characteristics are subsequently derived from relationships specific to different categories of particulate composition. Citation: Woźniak, S. B., D. Stramski, M. Stramska, R. A. Reynolds, V. M. Wright, E. Y. Miksic, M. Cichocka, and A. M. Cieplak (2010), Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California, J. Geophys. Res., 115, C08027, doi:10.1029/2009JC005554.

Variability of particulate organic carbon concentration in the north polar Atlantic based on ocean color observations with Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

POC throughout the season. The lowest values, generally less than 200 mg m 3 , and at some locations often less than 50 mg m 3 , were observed in April. In May and June, POC can exceed 300 or even 400 mg m 3 in some parts of the study region. Patterns of interannual variability are intricate, as they depend on the geographic location within the study region and particular time of year (month) considered. By comparing the results averaged over the entire study region and the entire season (April through August) for each year separately, we found that the lowest POC occurred in 2001 and the highest POC occurred in 2002 and 1999.

Variability of bio‐optical properties of the upper ocean associated with diel cycles in phytoplankton population

The variability of bio-optical properties of seawater associated with planktonic responses to the daily light cycle was examined. We performed a spectral analysiof a 60-day time series of open ocean mooring measurements of the scalar irradiance (photosynthetically available radiation, PAR), beam attenuation coefficient at 660 nm (c660) , Stimulated fluorescence, and dissolved oxygen concentration. The measurements were done from April through May 1989 in the North Atlantic south of Iceland, as a pan of the Marine Light in the Mixed Layer program. We have shown that the statistical significance of the daily cycles of bio-optical properties of theocean varies in time throughout the spring season. The diurnal periodicity of c660 and 0 2 was especially well pronounced during the development of the phytoplankton bloom in May. The fluorescence signal was dramatically affected by the ambient light intensity. The measurements at depths of 10 and 30 m showed fluorescence rhythms completely out of phase with each other. The comparison between the 10- and 30-m beam attenuation signals suggests that the 30-m signal was more sensitive to within day PAR variability. Further investigations are needed to determine how widespread the daily variations of bio-optical properties in the ocean are, what conditions favor this cycling, and how this variability may impact procedures for estimating the phytoplankton biomass and production.

Vertical structure of the upper ocean during the Marine Light‐Mixed Layers experiment

The Marine Light-Mixed Layers (MLML) experiments took place in the subarctic North Atlantic Ocean, approximately 275 miles south of Reykjavik, Iceland, during 1989 and 1991. The 1991 field program took place from April 30 to September 6 and included a central surface mooring to document the temporal evolution of physical, biological, and optical properties. In this paper we describe the physical variability observed at the 1991 mooring site, concentrating on the vertical structure of temperature and velocity in the upper 300 m of the water column and their changes in response to heat and momentum fluxes at the sea surface. The deployment period included the spring transition, when upper ocean re stratification was initiated after deep winter mixing, and the fall transition, when mixed layer deepening began again. The dominant signal in temperature was seasonal variation, with a 6°C increase observed at the sea surface from May to August. Prior to development of the seasonal stratification, a period dominated by near-surface temperature variability was observed in association with a 15-day mean flux of only 20 W m−2 into the ocean. Pronounced day/night oscillations of heat flux during this period resulted in alternating development and destruction of stratification and intense diurnal cycling of the mixed layer depth. A qualitative comparison of the observed temperature structure to the prediction of a one-dimensional mixed layer model showed that local processes dominated during the initiation of restratification and during most of the summer warming period. Nonlocal processes were important after the fall transition.

Phytoplankton bloom and the vertical thermal structure of the upper ocean

Local heating rate within the oceanic mixed layer (ML) depends not only on the amount of solar radiation incident on the sea surface, but also on the vertical distribution of the irradiance in the water column. We have evaluated the effect of a phytoplankton bloom on mixed layer depth and temperature at a high latitude site near Iceland. The level 21~ version of the Mellor-Yamada (1982) turbulence scheme has been modified to include the vertical distribution of irradiance. This has allowed the investigation of the ML temperature and stability structure resulting from both physical and biological effects. An important part of the model is the parameterization of pigment-dependence which affects the spectral attenuation coefficient for downwelling irradiance as proposed by Morel (1988). Concurrent, high temporal resolution time series of physical and bio-optical data were used for the model. These data were acquired using a mooring deployed during the spring of 1989. We have estimated that the Increase of phytoplankton abundance induced an increase of the sea surface temperature by about 0.2”C at the mooring site. This led to stronger near-surface thermal stratification and shallower mixed layers. The dependence of the upper layer thermal structure on biology is more important when vertical mixing is weaker and when phytoplankton concentrations are higher.

Modeling phytoplankton dynamics in the northeast Atlantic during the initiation of the spring bloom

Primary productivity in the North Atlantic (59°29′N, 20°50′W) was estimated by applying a “light-pigment” productivity model (Kiefer and Mitchell, 1983) to mooring data collected during the spring of 1989. We show that the choice of the parametrization of the light captured by phytoplankton cells in a turbulent mixed layer has a significant effect on the calculated productivity estimates. It appears that the quality of such estimations benefits largely from using high-resolution time series data (minutes). We also examined phytoplankton dynamics by incorporating the Kiefer-Mitchell model into a one-dimensional model of the turbulent mixed layer (Mellor and Yamada, 1982). The calculated time-depth distribution of phytoplankton biomass compares relatively well with that measured in situ. The model results indicate that small changes in the water column stability can be sufficient to initiate phytoplankton bloom even before the apparent formation of the seasonal thermocline. The model also describes the diel cycle of biomass concentration, suggesting that near the sea surface the daytime losses of biomass by vertical diffusion can be much larger than nighttime losses. Thus, if not accounted for, such losses may bias estimates of primary production from diel variations in biomass concentration, for example, when using a method based on beam attenuation measurements. These losses should also be considered for the proper interpretation of in situ primary production measurements by incubation methods.

Dependence of apparent optical properties on solar altitude: Experimental results based on mooring data collected in the Sargasso Sea

The in situ variability of the vertical attenuation coefficient for downward irradiance (Kd) and radiance reflectance (RL) in response to changing solar altitude has been investigated. Observations were made 18 times per day from a fixed position mooring deployed in the Sargasso Sea, The mooring included instrumentation to measure spectral downwelling irradiance (Ed) and spectral upwelled radiance (Lu) at 15- and 35-m depths. Time series data used for our analysis represent conditions of low, almost constant concentration of chlorophyll-like pigments in the water and sunny or partly cloudy sky conditions. During sunny days the dominant component of the observed diel variability of the apparent optical properties (AOPs) was caused by the variable solar altitude. The statistical analysis performed on the data indicates a significant correlation between Kd and the cosine of the zenith angle of the direct solar beam after refraction at the air/water boundary (μ). These changes were wavelength dependent; Kd at 412, 443, 490, 510, and 555 nm decreased by 18–30%, while Kd at 665 and 683 nm increased by more than 50% when solar altitude increased from about 17° to 60° (i.e., when μ increased from 0.69 to 0.93). The vertical attenuation coefficient for photosynthetically available radiation (KPAR) decreased by about 25%. The daily amplitudes of similar changes in RL at the blue/green light were much smaller, up to about 4 to 8% at 412, 443,490, and 510 nm and 11% at 555 nm. However, at 665 and 683 nm, RL changed by more than 100%. This large increase of RL and decrease of Kd at 665 and 683 nm with Sun altitude (with μ) can be explained by the fact that the light field at red wavelengths was strongly affected by inelastic processes such as Raman scattering and natural fluorescence. Remote-sensing techniques rely on the interpretation of reflectance and upwelled radiance ratios. We observed up to 8% changes in reflectance ratios and up to 17% changes in upwelling radiance ratios during the day. These maximum changes were observed for the ratios including Lu or RL at 555 nm, which were affected by inelastic scattering of light. Although our data illustrate the dependence of AOPs on the Sun altitude on a daily timescale, similar relationships are expected to be of importance on a seasonal scale and with latitude.

Short‐term variations of the bio‐optical properties of the ocean in response to cloud‐induced irradiance fluctuations

Variations in natural irradiance caused by changing cloud cover were examined as a potential source for the variability of the bio-optical properties in the open ocean. Time series of the scalar irradiance (PAR), beam attenuation coefficient (at 660 nm), stimulated chlorophyll fluorescence, concentration of dissolved oxygen, and water temperature were subject :o spectral analysis. These data were acquired in the Sargasso Sea at a depth of 23 m during the Biowatt mooring experiment in 1987. Three cloudy days, on which power spectra of PAR exhibited distinct maxima at periods of about 40 to 100 min, were selected for the analysis. Similar variations occurred in beam attenuation, fluorescence and dissolved oxygen as evidenced by relatively high coherence (> 0.5) between PAR and these variables. This is suggested to be related to rapid photoadaptive responses in the phytoplankton community on time scales of minutes. Possible explanations for these responses include changes in fluorescence yield, photosynthetic rate, cellular absorption, cell size and refractive index. No relationship between PAR and the bio-optical parameters was observed when hydrodynamical factors prevailed over time scales similar to those for cloud-induced irradiance fluctuations. The hydrodynamical effects were successfully traced by water temperature variations which, in further studies, may prove useful for separation of phenomena controlling within-day bio-optical vat:ability in the ocean.