Mesoscale features of phytoplankton and planktonic bacteria in a coastal area as induced by external water masses

The effects of different water masses and their physical structures on phytoplankton and bacteria were investigated in a coastal area of the Mediterranean Sea, the Gulf of Naples (Italy) on 9 and 10 November 1995. A small eddy (8 km in diameter), generated from offshore waters, occupied the inner part of the Gulf at surface, while the Modified Atlantic Water (MAW) intruded between 60 and 80 m depth. Phytoplankton communities as identified by HPLC pigment analysis (total) and flow cytometry (ultraphytoplankton) showed peculiarities in the different subsystems. Cryptophytes dominated by pigment biomass were everywhere, while diatoms were more abundant at the coast and nanoflagellates at offshore stations. In the MAW, a peculiar phytoplankton community, different to that in the other water masses, occurred, and 2 populations of prochlorophytes were distinguished based on scatter and red fluorescence observations. Total bacterial numbers were significantly correlated to chlorophyll a (chl a) concentrations. Different populations of heterotrophic bacteria were identified based on apparent DNA content. Percentages of 2 such populations were significantly correlated to total chi a. Therefore, phytoplankton appears to regulate not only bacterial quantity but also composition. Although our study area was characterized by a marked oligotrophy, heterotrophic bacterial biomass never exceeded autotrophic biomass, as derived using constant conversion factors of bacteria and chi a to C. Therefore, no inverted pyramid was observed and bacterial biomass was still controlled by phytoplankton biomass. Mesoscale variability is of great relevance for energy and matter budgets of coastal areas and its study is therefore fundamental. For example, eddies can be very important in coastal areas, influencing distribution of nutrients and organisms. Our approach, coupling HPLC and flow cytometry, represents a valuable tool for obtaining information on both phytoplankton composition and ecophysiology on a large number of samples, necessary for a valid representation of the space scales involved.