Caterina Nuccio

Evaluation of empirical and semi-analytical chlorophyll algorithms in the Ligurian and North Tyrrhenian Seas

Abstract. The estimation of chlorophyll concentration in marine waters is fundamental for a number of scientific and practical purposes. Standard ocean color algorithms applicable to moderate resolution imaging spectroradiometer (MODIS) imagery, such as OC3M and MedOC3, are known to overestimate chlorophyll concentration ([CHL]) in Mediterranean oligotrophic waters. The performances of these algorithms are currently evaluated together with two relatively new algorithms, OC5 and SAM_LT, which make use of more of the spectral information of MODIS data. This evaluation exercise has been carried out using in situ data collected in the North Tyrrhenian and Ligurian Seas during three recent oceanographic campaigns. The four algorithms perform differently in Case 1 and Case 2 waters defined following global and local classification criteria. In particular, the mentioned [CHL] overestimation of OC3M and MedOC3 is not evident for typical Case 1 waters; this overestimation is instead significant in intermediate and Case 2 waters. OC5 and SAM_LT are less sensitive to this problem, and are generally more accurate in Case 2 waters. These results are finally interpreted and discussed in light of a possible operational utilization of the [CHL] estimation methods.

Spatial and Temporal Distribution of Phytoplankton Assemblages in the Ross Sea

The temporal evolution and spatial distribution of phytoplankton assemblages were investigated, in coastal and open waters of the western Ross Sea, during three austral summers. Terra Nova Bay shows a first massive bloom (up to 107 cell |-1) of Fragilariopsis cf. curta between December and January in the receding ice-edge zone and another increase in February also with the contribution of different species. Phytoplankton shows a patchy distribution, with areas of bloom dominated by diatoms (Fragilariopsis, Nitzschia) and Phaeocystis sp., and less rich zones, mainly dominated by dinoflagellates and other flagellates. The different phytoplankton assemblages show characteristics corresponding to different stages that alternate during the summer season in the different areas, but which are related to the temporal development of environmental conditions after the melting of the ice.

Phytoplankton Biomass Related to Environmental Factors in the Ross Sea

Three years of investigation in the western Ross Sea reveal that despite the general abundance of phytoplankton biomass, the ratios between biomass and nutrient availability are low due to high nutrient concentrations. The N/P ratio is practically identical to that of Redfield; however, during blooms the ratio can be severely altered due to very low phosphate concentrations. Phytoplankton biomass during summer shows a bimodal cyclic pattern with two blooms in early and late summer. The spatial and temporal distribution of these blooms follows the latitudinal distribution of daily irradiance, which determines the advance of sea-ice melting from south to north forming an upper mixed layer rich in nutrients. This mixed layer is well illuminated and warmer than the deeper layer, thereby favouring the development of the early bloom. Afterwards, in the mixed water column, biomass decreases and reaches the summer minimum. A new stabilization of the water column is followed by a second bloom, less extensive than the first. In all 3 years, similar cycles have been recorded, giving corroborative evidence for the existence of a second bloom.

Photosynthetic Parameters, Irradiance, Biooptical Properties and Production Estimates in the Western Ross Sea

The primary production characteristics of phytoplankton coenoses were investigated during three summer periods (1987–88, 1989–90 and 1994–95), in Terra Nova Bay. The basic photosynthetic parameters of the P vs. E curves were analyzed, as well as the in vivo spectral absorption and fluorescence properties, which allow the maximum quantum yield for carbon uptake and for PSII fluorescence to be appreciated and compared, in order to evaluate the physiological conditions and the photoacclimation performance of this phytoplankton. Low light acclimation features (elevated values of αB, PB max and φm, with Ek between 11 and 102µE m-2 s-1) are clearly present in these natural populations. High daily productions of the water column have been estimated through a biooptical model (0.1–3.1 g C m-2 day-1), and a seasonal integrated value of 125g C m-2 has been appreciated at Terra Nova Bay, for the period from December through February.

Analysis of Chlorophyll-a and Primary Production Dynamics in North Tyrrhenian and Ligurian Coastal–Neritic and Oceanic Waters

ABSTRACT Marchese, C.; Lazzara, L.; Pieri, M.; Massi, L.; Nuccio, C.; Santini, C., and Maselli, F., 2015. Analysis of chlorophyll-a and primary production dynamics in north Tyrrhenian and Ligurian coastal–neritic and ocean waters. Properly tuned algorithms based on optical remote sensing data can provide estimates of chlorophyll-a (as a proxy for phytoplankton biomass) concentration in near real time, allowing the monitoring of phytoplankton dynamics for both neritic and oceanic areas. The main objective of this study was, through the use of ocean color satellite images, to offer a description of the interannual variability of chlorophyll-a and primary production both in coastal–neritic and in oceanic areas of the North Tyrrhenian and Ligurian Seas (NW Mediterranean). The second objective was to highlight the possible influence of land runoff on phytoplankton biomass variability in coastal–neritic waters. The results indicate that seasonal cycles of phytoplankton biomass and production were quite different in neritic areas potentially affected by freshwater runoff compared to offshore waters. Neritic areas are characterized by an anticipated bloom (winter–spring) and by higher spatial variability that appears to be linked with the distance from shore. Meanwhile, oceanic areas are dominated by a marked seasonal cycle and the typical bloom occurs in spring (March–April) in relation with vertical mixing. Finally, linear regression analysis suggests the influence of freshwater runoff in modulating the variability of chlorophyll-a in coastal–neritic areas. Overall, the results confirm previous observations on the dynamics of phytoplankton biomass and contribute to more realistic and lower estimates of both chlorophyll-a concentration and annual primary production.

Structural and ultrastructure changes show an increase in amoeboid forms in Heterosigma akashiwo (Raphidophyceae), during recovery after nutrient depletion

Abstract Heterosigma akashiwo shows remarkable ultrastructural changes during the recovery from a late stationary phase (“aged” culture) induced by nutrient depletion. H. akashiwo cells showed different morphological types in “aged” cultures, with an increase in irregular cells and cell fragments. The irregular cells mostly corresponded to an amoeboid shape of the cell. Many of these cells showed chloroplasts with a homogeneous matrix of medium electron density lacking most thylakoids and condensed nucleus, probably as a result of cyst/resting cells germination. In other cells, we observed nuclear blebbing without chromatin condensation and changes in mitochondrion ultrastructure. Some vegetative cells in active phase (“young” culture) were connected to each other, apparently phagocytizing cytoplasmic fragments and intact chloroplasts in the medium. An explanation for the phenomenon may reside in the need of acquiring organic material after nutrient reduction for a faster recovery. On the basis of our observations, we conclude that some ultrastructural features, normally used to distinguish between different species and strains of Raphidophyceae, may be related to different physiological states and should be used with caution for systematic purposes.

SYMPLEX experiment: first results on oceanic mesoscale dynamics and related primary production from AVHRR and SeaWIFS satellite data and field experiments

Upper ocean dynamics is characterized by a strong variability, at different scales, both in direction and structure of the flow. Mesoscale variability, which is ubiquitous in the world ocean, is often the dominant component in the variance spectrum of velocity with relevant implications on water mass mixing and transformation and on the carbon transfer in the marine food web. Mesoscale activity is manifested through the formation of instabilities, meanders and eddies. Eddies generate either a doming of isopycnals (cyclones) or a central depression (anticyclones). This in turn modifies, among the others, nutrient and organism distributions in the photic zone eventually enhancing or depressing photosynthetic activity and other connected biological responses. The mechanism is similar to what has been thoroughly studied for the warm and cold core rings but at different spatial and temporal scales. The enhancement of phytoplankton growth and the modification of photosynthetic parameters has been shown to occur in situ by means of a modulated fluorescence probe. More recently, an attempt to estimate the magnitude of this specific forcing on nutrient fluxes and primary production has also been conducted at different scales by modeling exercises, though with contrasting estimates the relative importance concerns. Because phytoplankton growth takes place when light, nutrients and cells are found at the same place, the increase in primary production favored by mesoscale eddies cannot be easily predicted. The incident light, the seasonality, the life-time of the structure, its intensity etc. can all influence the final yield. In addition, it has still to be determined which component of the community reacts faster and takes advantage of the new nutrients and how efficiently the new carbon is channeled in the food web. For what remote sensing is concerned, the detectability form the space of such structures is certainly dependent on the depth at which the upward distortion of isopycnals takes places. It can be supposed that a change in bio-optical signature of the whole structure could occur because of the 3-D dynamics of the eddy. If this holds true, then color remote sensing coupled with sea level topography and sea surface temperature should be a powerful tool to track such transient structures. The ALT-SYMPLEX program has been designed to better understand the relationship between short living eddies and carbon transfer in the food web. This is based on several experiments aimed to integrate remote sensing data (ocean color and surface topography) and in situ data in order to evaluate the relationship between surface and sub-surface physical dynamics and its relations on chemical and biological aspects in presence of mesoscale features.

Macro-grazer herbivory regulates seagrass response to pulse and press nutrient loading

Coastal ecosystems are exposed to multiple stressors. Predicting their outcomes is complicated by variations in their temporal regimes. Here, by means of a 16-month experiment, we investigated tolerance and resistance traits of Posidonia oceanica to herbivore damage under different regimes of nutrient loading. Chronic and pulse nutrient supply were combined with simulated fish herbivory, treated as a pulse stressor. At ambient nutrient levels, P. oceanica could cope with severe herbivory, likely through an increase in photosynthetic activity. Elevated nutrient levels, regardless of the temporal regime, negatively affected plant growth and increased leaf nutritional quality. This ultimately resulted in a reduction of plant biomass that was particularly severe under chronic fertilization. Our results suggest that both chronic and pulse nutrient loadings increase plant palatability to macro-grazers. Strategies for seagrass management should not be exclusively applied in areas exposed to chronic fertilization since even short-term nutrient pulses could alter seagrass meadows.

Organic aggregates formed by benthopleustophyte brown alga Acinetospora crinita (Acinetosporaceae, Ectocarpales)

This work presents the elemental, polysaccharide, and fatty acid compositions of benthic aggregates formed by the filamentous brown alga Acinetospora crinita, which are widely spread on the rocky bottoms of the Mediterranean Sea. The aggregates can be characterized as mineralized centers in which regeneration of nutrients and recycling of dissolved organic matter actively occur and favor the development of an abundant phytoplankton community. Analyses of the stable isotopes of C and N display their marine origin and could provide evidence of the processes that occur inside/outside of the aggregates. The monosaccharide compositions of Adriatic and Tyrrhenian mucilages produced by brown alga A. crinita were quite similar. In particular, the Adriatic sample compositions resembled the average composition of the Tyrrhenian high molecular weight exopolymers, and the observed differences could be ascribed to different degradation stages. The fatty acid patterns found for the aggregates were similar to those observed in the isolated A. crinita algae with variable contributions from embedded diatom species. The bacterial contribution to the fatty acid pool was quite low, most likely due to the known poor conditions for their heterotrophic growth.

On the discrimination of multiple phytoplankton groups from light absorption spectra of assemblages with mixed taxonomic composition and variable light conditions

According to recommendations of the international community of phytoplankton functional type algorithm developers, a set of experiments on marine algal cultures was conducted to (1) investigate uncertainties and limits in phytoplankton group discrimination from hyperspectral light absorption properties of assemblages with mixed taxonomic composition, and (2) evaluate the extent to which modifications of the absorption spectral features due to variable light conditions affect the optical discrimination of phytoplankton. Results showed that spectral absorption signatures of multiple species can be extracted from mixed assemblages, even at low relative contributions. Errors in retrieved pigment abundances are, however, influenced by the co-occurrence of species with similar spectral features. Plasticity of absorption spectra due to changes in light conditions weakly affects interspecific differences, with errors <21% for retrievals of pigment concentrations from mixed assemblages.