‘St Martin's Summer’: the case of an autumn phytoplankton bloom in the Gulf of Naples (Mediterranean Sea)

Abstract

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 day1) 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.