Dierdre A. Toole

Modes and mechanisms of ocean color variability in the Santa Barbara Channel

Characterizing the substances, processes, and mechanisms that regulate ocean color variability is crucial for assessing marine resources and impacts of land-ocean interactions with remote sensing data. Bimonthly optical and in situ water column observations from a 3-year field program within the Santa Barbara Channel, California, are used to assess sources of ocean color variability in a semiarid coastal region. Correlation analyses demonstrate that remote sensing reflectance variability is tightly coupled to biologically and terrestrially derived particles throughout the visible spectrum. An empirical orthogonal function analysis indicates that nearly two thirds of the observed variability in remote sensing reflectance is contained in a backscattering mode, whereas phytoplankton-driven absorption processes contribute only ∼30% of the observed variance. Observations from the Santa Clara River outflow during a period of high discharge demonstrate that under extreme conditions, ocean color spectra are regulated almost entirely by backscattering processes. A mechanistic partitioning of ocean color variability confirms the dominance of backscattering processes and a strong coupling between the roles of backscattering and absorption at this site. A similar analysis of data from the Sargasso Sea demonstrates a lack of coupling between the role of absorption and backscattering. Application of the Seaviewing Wide Field-of-view Sensor (SeaWiFS) operational chlorophyll algorithm highlights the degraded predictive power of band ratio algorithms in coastal settings such as the Santa Barbara Channel. For this case II environment, backscattering is the dominant driver of ocean color variability and must be correctly incorporated into future ocean color modeling efforts.

Effects of solar radiation on the fate of dissolved DMSP and conversion to DMS in seawater

Abstract.The effect of ultraviolet radiation (UVR) and photosynthetically-active radiation (PAR) on the conversion of dissolved dimethylsulfoniopropionate (DMSPd) to dimethylsulfide (DMS) was studied in coastal, shelf and open ocean waters. Unfiltered and 0.8 μm filtered seawater samples were incubated in the dark or exposed to solar radiation for ~6 h followed by post-exposure, dark incubations with tracer additions of 35S-DMSPd. End-products resulting from 35S-DMSPd metabolism were quantified, including 35S-DMS, total volatile 35S and particle-assimilated 35S. Exposure of productive coastal and shelf waters of the Gulf of Mexico to UVR+PAR inhibited the initial rates of 35S-DMSPd consumption and the rates of 35S assimilation into cellular macromolecules by 12 to 87% and 13 to 81% respectively, compared to dark controls. After 24 h of post-exposure, dark incubation, however, the assimilation of 35S in the UVR+PAR treatments was the same as observed in dark controls. In contrast, the 35S-DMS yield from DMSPd consumption was always higher in UVR+PAR treatments than in dark controls after 24 h post-exposure, dark incubation. Exposure of mesotrophic Mediterranean Sea or oligotrophic Sargasso Sea water samples to UVR+PAR resulted in variable effects on DMS yields, with two out of four experiments showing lower, and two out of four showing higher DMS yields from 35S-DMSP compared with dark controls. In the Gulf of Mexico and Sargasso Sea, the higher 35S-DMS yields caused by UVR+PAR exposure were offset by strong inhibitory effects of UVR+PAR on 35S-DMSPd consumption rates, leading to lower 35S-DMS production overall. When DMS production from DMSPd was compared to DMS production from total DMSP, we found that only 20 to 75% of the produced DMS came from DMSPd, in one case with the lowest contributions from DMSPd in UVR+PAR treatments. Our results suggest that UVR exposure is likely an important factor promoting higher DMS yields from DMSPd in productive coastal waters, and that a substantial fraction of DMS production comes from non-DMSPd-derived sources.

Chemical “light meters” for photochemical and photobiological studies

Abstract.Nitrate and nitrite solar actinometers or chemical ‘light meters’ were used to quantify light doses in photochemical and photobiological experiments involving dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycling. Light doses were calculated based on the photochemical production of salicylic acid (SA) from benzoic acid in these actinometers, with SA quantified by either spectrofluorometry or high performance liquid chromatography. Nitrate and nitrite actinometers were modified for deployment at low temperatures in Antarctic waters by addition of sodium chloride as a freezing point depressant. The addition of salt did not affect the solar response of the actinometers; however, the solar response did change slightly with latitude. In the Antarctic, peak response wavelengths (and bandwidths) for the Mylar D-wrapped actinometers in quartz tubing were 326 nm (319 – 333 nm) and 353 nm (325 – 380 nm) for nitrate and nitrite, respectively, and these were 2 – 5 nm blue shifted compared to the peak response wavelengths and bandwidths observed in the Sargasso Sea. Excellent agreement was observed when comparing the integrated irradiance determined with the actinometers to that determined with a spectroradiometer. Likewise, diffuse attenuation coefficients for downwelling irradiance (Kd(λ)) calculated from water column actinometer measurements agreed well with Kd(λ) values calculated from irradiance measurements determined with a Biospherical PUV-511 profiling radiometer. Actinometers were used to measure light doses in experiments involving DMS and DMSP transformations during several field campaigns in the Ross Sea, Antarctica and the Sargasso Sea. Based on actinometer measurements, it was determined that DMS photolysis was dependent on UV irradiation between approximately 325 – 380 nm, while biological consumption rates of DMS and DMSP were inhibited by radiation at wavelengths less than approximately 333 nm. When DMS photolysis rate constants were expressed in terms of light dose rather than time, it was possible to 1) directly determine photolysis rate constants in the water column and 2) directly compare photolysis rate constants across diverse oceanographic regions.

Revising upper-ocean sulfur dynamics near Bermuda: new lessons from 3 years of concentration and rate measurements

Environmental context Microscopic marine organisms have the potential to influence the global climate through the production of a trace gas, dimethylsulfide, which contributes to cloud formation. Using 3 years of observations, we investigated the environmental drivers behind the production and degradation of dimethylsulfide and its precursor dimethylsulfoniopropionate. Our results highlight the important role of the microbial community in rapidly cycling these compounds and provide an important dataset for future modelling studies. Abstract Oceanic biogeochemical cycling of dimethylsulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP), has gained considerable attention over the past three decades because of the potential role of DMS in climate mediation. Here we report 3 years of monthly vertical profiles of organic sulfur cycle concentrations (DMS, particulate DMSP (DMSPp) and dissolved DMSP (DMSPd)) and rates (DMSPd consumption, biological DMS consumption and DMS photolysis) from the Bermuda Atlantic Time-series Study (BATS) site taken between 2005 and 2008. Concentrations confirm the summer paradox with mixed layer DMS peaking ~90 days after peak DMSPp and ~50 days after peak DMSPp:Chl. A small decline in mixed layer DMS was observed relative to those measured during a previous study at BATS (1992–1994), potentially driven by long-term climate shifts at the site. On average, DMS cycling occurred on longer timescales than DMSPd (0.43±0.35 v. 1.39±0.76 day–1) with DMSPd consumption rates remaining elevated throughout the year despite significant seasonal variability in the bacterial DMSP degrader community. DMSPp was estimated to account for 4–5% of mixed layer primary production and turned over at a significantly slower rate (~0.2 day–1). Photolysis drove DMS loss in the mixed layer during the summer, whereas biological consumption of DMS was the dominant loss process in the winter and at depth. These findings offer new insight into the underlying mechanisms driving DMS(P) cycling in the oligotrophic ocean, provide an extended dataset for future model evaluation and hypothesis testing and highlight the need for a reexamination of past modelling results and conclusions drawn from data collected with old methodologies.