Áurea Maria Ciotti

Freshwater outflow and Subtropical Convergence influence on phytoplankton biomass on the southern Brazilian continental shelf

Abstract The present study discusses the role of different continental shelf water masses on inorganic nutrient levels and on phytoplankton biomass along the southern Brazilian coast during October 1987 and September 1988. In this productive area, variability of phytoplankton biomass has been related to the seasonal latitudinal displacement of the Subtropical Convergence and to the freshwater outflow of La Plata River and Patos Lagoon. Very distinct precipitation rates, as a consequence of the ENSO (El Nin˜o-Southern Oscillation) cycle, preceding the two sampled periods of this study, allowed a first evaluation of the impact of this event on freshwater outflow and shelf phytoplankton biomass. The amount of chlorophyll in shelf waters was directly related to nutrients supplied by the Coastal Water (i.e. freshwater outflow), Subtropical and Subantarctic Waters. Comparing the results of this study with previous cruises performed in the area, it was found that high chl a concentrations are common features during periods of elevated precipitation rates. Apart from nutrient input by freshwater and the consequent changes in the vertical profiles of density, the extreme precipitation rates are also associated with different wind patterns, which, in turn, control the presence of particular water masses in the euphotic zone. Our results suggest that ENSO events have an important impact on the variability of phytoplankton production, and thus should affect biogeochemical cycles in the southern Brazilian coastal areas.

A semi-analytical model of the influence of phytoplankton community structure on the relationship between light attenuation and ocean color

A model was developed to examine the influence of phytoplankton community structure on the relationship between diffuse attenuation and ratios of upwelling radiance. Shifts in phytoplankton communities were represented by changing mean optical properties as a function of chlorophyll (C, mg m−3), consistent with large data sets from the field and laboratory. The product of cell size and internal pigment concentration, dci, governs pigment packaging, which alters the specific absorption coefficients of phytoplankton ( aph*, m2 mgChl−1). Pigment packaging was parameterized as a function of C by combining the relationship between dci and aph* from phytoplankton cultures with that between aph* and C from the field, using data for 675 nm, where absorption by accessory pigments is low. Changes in accessory pigmentation were approximated by quantifying residual variability in aph* at other wavelengths, as functions of C, once the variability with dci was taken into account. Absorption by colored dissolved organic matter (CDOM), detrital absorption, and scattering by particles were also parameterized as functions of C, so that bio-optical relationships could be modeled as functions of trophic status. The model thus reconciled recognized relationships between optical properties and C with ecologically interpretable shifts in phytoplankton communities. Empirical relationships between diffuse attenuation and ocean color were well reproduced at low (0.5 mg m−3) to medium (10 mg m−3) C. Analysis of variability imposed by a range of dci suggests that it may be possible to recognize phytoplankton communities with cell sizes and intracellular pigment concentration different from the central tendency, given a set of wavelengths which minimizes the influence of CDOM and detrital absorption.

The spectral effects of clouds on solar irradiance

Knowledge of the spectral attenuation associated with clouds is important for accurate estimates of natural irradiance at the Earths 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.

PHYSIOLOGICAL AND OPTICAL PROPERTIES OF RHIZOSOLENIA FORMOSA (BACILLARIOPHYCEAE) IN THE CONTEXT OF OPEN‐OCEAN VERTICAL MIGRATION1

Cultures of Rhizosolenia formosa H. Peragallo were studied to assess whether or not physiological and optical characteristics of this large diatom were consistent with the ability to migrate vertically in the open ocean. Time‐course experiments examined changes in chemical composition and buoyancy of R. formosa during nitrate (N)–replete growth, N starvation, and recovery. Cells could maintain unbalanced growth for at least 53 h after depletion of ambient nitrate. Increases in C:N and carbohydrate: protein ratios observed during N starvation reversed within 24 h of reintroduction of nitrate to culture medium. Buoyancy was related to nutrition: Upon N depletion, the percentage of positively buoyant cells decreased to 4% from 11% but reverted to 9% within 12 h of nitrate readdition. Cells took up nitrate in the dark. Nitrogen‐specific uptake rates averaged 0.48 d−1; these rates were higher than N‐specific growth rates (0. 15 d−1), indicating the potential for luxury consumption of nitrate, which can be stored for later use. Measurements of photosynthesis vs. irradiance, chlorophyll‐specific absorption (aph*(λ)), and pigment composition showed that cells may be adapted for growth under a wide range of irradiances. Values of aph*(λ) were lower for N‐depleted cells than for N‐replete cells, and N‐depleted cells had higher ratios of total carotenoids to chlorophyll a. Aggregation of chloroplasts was more pronounced in N‐depleted cells. These are possibly photoprotective mechanisms that would be an advantage to N‐depleted cells in surface waters. Compounds that absorb in the ultraviolet region were detected in N‐replete cells but were absent in N‐depleted cultures. Overall, these results have important implications for migrations of Rhizosolenia in nature. Cells may survive fairly long periods in N‐depleted surface waters and will continue to take up carbon; then they can resume nitrate uptake and revert to positive buoyancy upon returning to deep, N‐rich water. Uncoupled uptake of carbon and nitrogen during migrations of Rhizosolenia is a form of new production that may result in the net removal of carbon from oceanic surface waters.

A Consumer's Guide to Satellite Remote Sensing of Multiple Phytoplankton Groups in the Global Ocean

Phytoplankton are composed of diverse taxonomical groups, which are manifested as distinct morphology, size and pigment composition. These characteristics, modulated by their physiological state, impact their light absorption and scattering, allowing them to be detected with ocean color satellite radiometry. There is a growing volume of literature describing satellite algorithms to retrieve information on phytoplankton composition in the ocean. This synthesis provides a review of current methods and a simplified comparison of approaches. The aim is to provide an easily comprehensible resource for non-algorithm developers, who desire to use these products, thereby raising the level of awareness and use of these products and reducing the boundary of expert knowledge needed to make a pragmatic selection of output products with confidence. The satellite input and output products, their associated validation metrics, as well as assumptions, strengths and limitations of the various algorithm types are described, providing a framework for algorithm organization to assist users and inspire new aspects of algorithm development capable of exploiting the higher spectral, spatial and temporal resolutions from the next generation of ocean color satellites.

Changes in phytoplankton composition in response to tides, wind-induced mixing conditions, and freshwater outflows in an urbanised estuarine complex.

Recent reports have shown an increase in potentially harmful phytoplankton in Santos bay (Southeastern Brazilian Coast), located in a highly urbanised estuarine complex. Prediction of blooms is, thus, essential but the phytoplankton community structure in very dynamic regions is difficult to determine. In the present work, we discriminate bloom forming microphytoplankton dominance and their relationship to physical and meteorological variables to look for patterns observed in different tides and seasons. Comparing 8 distinct situations, we found five scenarios of dominance that could be related to winds, tides and rainfall: i) Surfers, diatoms occurring during high surf zone energies; ii) Sinkers, represented by larger celled diatoms during spring tide, after periods of high precipitation rates; iii) Opportunistic mixers, composed of chain forming diatoms with small or elongate cells occurring during neap tides; iv) Local mixers, microplanktonic diatoms and dinoflagellates which occurred throughout the 298 sampling stations; and v) Mixotrophic dinoflagellates, after intense estuarine discharges. Results suggest alterations in the temporal patterns for some bloom-forming species, while others appeared in abundances above safe limits for public health. This approach can also illustrate possible impacts of changes in freshwater discharge in highly urbanised estuaries.

Experimental strategies to assess the biological ramifications of multiple drivers of global ocean change – a review

Marine life is controlled by multiple physical and chemical drivers and by diverse ecological processes. Many of these oceanic properties are being altered by climate change and other anthropogenic pressures. Hence, identifying the influences of multifaceted ocean change, from local to global scales, is a complex task. To guide policy-making and make projections of the future of the marine biosphere, it is essential to understand biological responses at physiological, evolutionary and ecological levels. Here, we contrast and compare different approaches to multiple driver experiments that aim to elucidate biological responses to a complex matrix of ocean global change. We present the benefits and the challenges of each approach with a focus on marine research, and guidelines to navigate through these different categories to help identify strategies that might best address research questions in fundamental physiology, experimental evolutionary biology and community ecology. Our review reveals that the field of multiple driver research is being pulled in complementary directions: the need for reductionist approaches to obtain process-oriented, mechanistic understanding and a requirement to quantify responses to projected future scenarios of ocean change. We conclude the review with recommendations on how best to align different experimental approaches to contribute fundamental information needed for science-based policy formulation.

Trichodesmium latitudinal distribution on the shelf break in the southwestern Atlantic Ocean during spring and autumn

We report Trichodesmium slicks in the South Atlantic associated with fronts produced by the southward displacement of the Brazil Current. For the first time, a relatively high spatial sampling grid of trichomes density is shown over an undersampled region with regard to Trichodesmium. In this study, the distributions of Trichodesmium along the southeastern Brazilian continental shelf break were characterized during four oceanographic cruises and related to seawater temperature, upper layer nutrient concentrations and wind dynamics. Meridional changes in the densities of trichomes followed an expected trend with higher (lower) values found in warmer (colder) and more saline (less saline) water masses. Between 23° and 28°S, large accumulations of trichomes were associated with high temperature, increased phosphate concentrations (>0.08 μM), nitrogen below 2.4 μM, and low wind speeds (<11 m s−1). Maximum densities of 212.6 × 105 trichomes L−1, which are considerably larger than those reported in other studies in subtropical oceans, occurred at stations showing weak water column stability and higher phosphate levels over the upper 25 m. We suggest that shelf break dynamics, which drive and modulate the intensity of the shelf break upwelling, as well as wind divergence at spatial scales of approximately 600 km2, associated with mesoscale activities, may favor Trichodesmium growth and accumulation. On the other hand, Trichodesmium densities were low at latitudes between 28° and 33°S, likely due to either or both the lowest sea surface temperatures and/or out-competition by the faster-growing nondiazotrophs.

Temporal variability of chlorophyll-a in the São Vicente estuary

Estuaries are extremely dynamic environments that are vulnerable to anthropogenic alterations. Thus, monitoring phytoplankton abundances and composition is an essential tool for the prediction of eutrophication and its effects on coastal ecosystems. Phytoplankton biomass, as chlorophyll-a, in the Sao Vicente estuary (Brazil) varies in response to tidal cycles and seasonal rainfall. Objectives. To present two datasets designed to assess the relationship between chlorophyll-a and changes in water turbidity driven by tide and rain. Methods. Weekly observations were made in the shallow embayment (February to September 2008; site 1) and observations recorded on alternate days (summer 2010, site 2). Results. At site 1, turbidity differed between high and low tides, but on most days was over 3000 RU, maintaining moderate chlorophyll-a levels (4 mg.m-3) and only two blooms developed during low turbidity. Site 2 mean turbidity was 1500 RU, nutrient level was higher during neap tides and phytoplankton blooms were mainly observed at the end of neap tides at 15-day intervals, dominated by chain-forming diatoms and occasionally flagellates and pennate diatoms. Conclusions. Taxonomic composition of the blooms was different and their frequency altered by events characterized by intense freshwater discharges from the Henry Borden Hydroelectric Dam (> 9*106.m3), inhibiting phytoplankton accumulation during neap tide periods.

Relationship between near-surface chlorophyll and solar-stimulated fluorescence: biological effects

The fluorescence of chlorophyll a (Chl) nm can be detected in water leaving radiance and related quantitatively to the concentration of Chl. Solar-induced fluorescence has also been related to photosynthesis in deeper waters. However, little is known about the relationships between Chl, fluorescence, photosynthesis, and irradiance near the sea surface. Quantum yields of fluorescence and photosynthesis, as well as the ratio of fluorescence to photosynthesis, change during exposures to bright light. Several physiological processes are at play. Consequently, it is difficult to construct models of near-surface quantum yields. Experimentation and comprehensive sampling in the field are required for critical information. Some approaches are presented here. Radiometer buoys that measure downwelling irradiance at 490 nm, Ed(490), and upwelling spectral radiance, Lu(lambda) are good tools for measuring solar-stimulated fluorescence during studies of near-surface biology. Results can be compared with experimental measurements using a fluorometer with a very weak measuring beam that does not perturb the balance between fluorescence and photosynthesis. Comparisons indicate that relationships between near-surface Chl, fluorescence, photosynthesis and irradiance can vary widely for reasons that are not yet well resolved. Still, Lu(683), corrected for backscatter and normalized to Ed(490), is a useful measure of near-surface Chl in many environments.