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Photophysiological parameters provide useful insights into the effects of environmental forcings on phytoplankton physiology. We present data on the short-term photoacclimative responses of phytoplankton throughout the water column during a diel sampling (every 1.5–3 h for 33 h) at a coastal station in the Gulf of Naples (Italy) in November 1996. Liposoluble pigments (high performance liquid chromatography), variable fluorescence (Prim-Prod probe) and picoplankton cell counts and autofluorescence (flow cytometry) were investigated every 1.5–3 h over a period of 33 h. The phytoplankton was phased to the alternation of light and dark and also showed acclimation to the different light intensities. Photoprotective pigments were synthesized during the day at the surface (0–20 m) and were significantly correlated with light intensity changes, as well as with the effective quantum yield of fluorescence. At night, recovery of photosystems from excess light was observed as was redistribution of nutrients and algae due to vertical convective motions caused by thermal dissipation. Equations linking photobiological parameters and time or light evolution were inferred to obtain kinetic coefficients. These were then used as biological tracers of vertical mixing whose velocity in the surface layer was estimated to be ,0.05 cm s 21 .

Aspects of phytoplankton photophysiology were investigated in a coastal area of the Mediterranean Sea (Gulf of Naples) using high performance liquid chromatography pigment analysis, flow cytometric cellular fluorescence information and in vivo pump and probe measurements. Discrete sampling, continuous profiles and on-board kinetic experiments were used to estimate photoacclimation kinetics and to identify the time scales of these reactions in phytoplankton. Based on the relationship between the depth of the euphotic zone and the mean chlorophyll concentration, the area could be classified as Case 1 water, although, oligotrophy was not reflected in phytoplankton photosynthetic efficiencies, suggesting that it represented a transient feature induced by an eddy, originating offshore. Fronts at the edge of the eddy exhibited different photophysiological features, in which the phytoplankton showed greater physiological stress, brought about by the convergent motion within the front. Kinetic experiments allowed identification of time scales for photoacclimation. Using photoprotective pigments as tracers we estimated vertical mixing velocities in the upper layer of 0.07 cm s � 1 and from this we inferred kinetic coefficients for different photophysiological processes in the mixed layer as well as significant threshold values for photoacclimation rates to be 4% h � 1 . From the threshold value, the vertical eddy diffusivity in the area at the time of sampling was estimated as 1.75 � 10 � 2 m 2 s � 1 , a high value for vertical mixing.

Phytoplankton abundance, species composition and primary production of the Gulf of Naples were investigated during an autumn bloom in November 1985. Hydrographic data and surface phytoplankton samples were collected during a 3 day cruise, whereas investigations on in situ primary production and phytoplankton vertical distribution were conducted from a second boat on three different dates. A coast-offshore gradient was recorded for most of the chemical and biological parameters analysed. At stations closer to the coast, which were affected by land run-off, phytoplankton populations attained concentrations of 2.5 x lO* cells P'. with a marked dominance of diatoms belonging to the genera Thalassiosira and Chaetoceros. The most striking character of the system was a remarkably high carbon assimilation rate (up to 1.04 g C m'2 day"1) at stations closer to the coast. The causative mechanism for this bloom appeared to be land-derived nutrient enrichment, possibly enhanced by autumn rains, followed by a period of favourable meteorological conditions, which occurs recurrently in the region for a brief period around November, known locally as "St Martin's Summer'. We hypothesize that a similar mechanism can stimulate phytoplankton growth more than once every year, since high-stability periods following meteoro- logical perturbations can occur several times in temperate regions of the northern hemisphere in autumn.