F. Conversano

Water mass properties and chemical signatures in the central Mediterranean region

During the last 15 years, the knowledge of Mediterranean physical dynamics as well as of atmospheric forcing underwent a tremendous improvement because of the action within several international programs and the development of remote sensing and modelling approaches. Curiously, it is still very difficult to build up a climatological database for chemical and basic biological parameters for the whole basin because most of the data published in the open literature were preferentially related to meso- to small-scale processes. Within the European Union project Mass Transfer and Ecosystem Response (MATER), systematic measurements of routine chemical parameters, such as dissolved oxygen and nutrients, have been conducted. They will fill the existing gap between physical and chemical information. In this paper, we analyze the hydrographic data from a cruise conducted in the fall 1996 in the Central Mediterranean region and report, for the first time, on oxygen and nutrient concentrations, ranges and distributions. The joint analysis of T–S properties and chemical data also allows a better definition of water mass characteristics in this crucial area and hints at basic mechanisms relevant to water mass transformation and biological production in the basin. D 2002 Elsevier Science B.V. All rights reserved.

How diffusivity, thermocline and incident light intensity modulate the dynamics of deep chlorophyll maximum in Tyrrhenian Sea

During the last few years theoretical works have shed new light and proposed new hypotheses on the mechanisms which regulate the spatio-temporal behaviour of phytoplankton communities in marine pelagic ecosystems. Despite this, relevant physical and biological issues, such as effects of the time-dependent mixing in the upper layer, competition between groups, and dynamics of non-stationary deep chlorophyll maxima, are still open questions. In this work, we analyze the spatio-temporal behaviour of five phytoplankton populations in a real marine ecosystem by using a one-dimensional reaction-diffusion-taxis model. The study is performed, taking into account the seasonal variations of environmental variables, such as light intensity, thickness of upper mixed layer and profiles of vertical turbulent diffusivity, obtained starting from experimental findings. Theoretical distributions of phytoplankton cell concentration was converted in chlorophyll concentration, and compared with the experimental profiles measured in a site of the Tyrrhenian Sea at four different times (seasons) of the year, during four different oceanographic cruises. As a result we find a good agreement between theoretical and experimental distributions of chlorophyll concentration. In particular, theoretical results reveal that the seasonal changes of environmental variables play a key role in the phytoplankton distribution and determine the properties of the deep chlorophyll maximum. This study could be extended to other marine ecosystems to predict future changes in the phytoplankton biomass due to global warming, in view of devising strategies to prevent the decline of the primary production and the consequent decrease of fish species.

Growth and photophysiological responses of two picoplanktonic Minutocellus species, strains RCC967 and RCC703 (Bacillariophyceae)

Reaching up to 50% of the total biomass in oligotrophic waters and armed with a set of ecological and biological properties related to their small size, picophytoplankton (<3.0 µm) are a good model to address ecophysiological questions regarding phytoplankton biodiversity. Two picoplanktonic diatoms, one isolated from an upwelling ecosystem in the Pacific Ocean (Minutocellus sp., strain RCC967), and another from oceanic waters in the Indian Ocean (Minutocellus sp., strain RCC703) were used to test hypotheses on the functional relation between ecological niche adaptation and photosynthetic regulation capacity and efficiency. Cultures were subjected to five sine light climates, each one set to peak at a different photon flux density, respectively 10, 50, 100, 250 and 500 µmol photons m−2 s−1. Growth rate, photosynthesis, non-photochemical fluorescence quenching, pigment composition, and particulate organic carbon and nitrogen content were followed daily for 5 days. Growth rate and physiological response curves were different in the two species, in agreement with their distinct habitats of origin. Such differences could be related to the diverse photoacclimative strategies displayed by the two species, revealing a clear adaptive divergence despite their close taxonomic relationship. Photoacclimative strategies of the two picoplanktonic diatoms are discussed in the light of functional diversity and ecosystem adaptation.

Role of light and photophysiological properties on phytoplankton succession during the spring bloom in the north-western Mediterranean Sea

This study aimed to determine the role of light on the succession of the phytoplankton community during the spring bloom in the northwestern Mediterranean Sea. To this end, three successive Lagrangian experiments were carried out between March and April 2003. The three experiments correspond to distinct phases of the bloom development (pre-bloom, bloom peak and post-bloom, respectively) and therefore to different trophic conditions. Phytoplankton (sampled on a daily scale) was grouped in size-based classes (pico and nano+micro) each of them were characterised in terms of chemotaxonomic composition, primary production and photophysiological properties. The phytoplankton community evolved with time changing in both size-class dominance and specie/group dominance within each size class. The bloom peak was characterised by highly dynamic condition (i.e. vertical mixing) and by the dominance of both small (pico) and large (nano and micro) diatoms, as a result of their capacity to photoacclimate to changing light regimes (‘physiological plasticity’). Concluding, we suggest that the physiological adaptation to light is the main factor driving the succession of the phytoplankton community during the first phases of the bloom (until the onset of thermal stratification) in the western Mediterranean Sea.

The role of noise on the steady state distributions of phytoplankton populations

The spatio-temporal behaviour of total chlorophyll concentration is investigated in the middle of the Tyrrhenian Sea by using a stochastic approach. The study is based on a reaction–diusion–taxis model, which is used to analyse the dynamics of five phytoplankton groups, responsible for about 80% of the total chlorophyll a inside the euphotic zone of the water column. The analysis is performed by considering: (i) the intraspecific competition of the phytoplanktonic groups for limiting factors, i.e. light intensity and nutrient concentration, (ii) the seasonal changes of environmental variables, and (iii) the random fluctuations of the components of the velocity field and temperature. Specifically, we investigate the eects of external perturbations, both deterministic and random, on the dynamics of phytoplankton populations, by inserting a term of multiplicative noise into the dierential equation of the nutrient dynamics. The theoretical results of the phytoplankton abundances obtained by the stochastic model are converted in chlorophyll a concentrations, and compared with the experimental findings. The statistical checks, based on D Valenti et al The role of noise on the steady state distributions of phytoplankton populations Printed in the UK 054044 JSMTC6

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.