A. Malej et al., PHYTOPLANKTON RESPONSES TO FRESH-WATER INPUTS IN A SMALL SEMIENCLOSEDGULF (GULF OF TRIESTE, ADRIATIC SEA), Marine ecology. Progress series, 120(1-3), 1995, pp. 111-121
Phytoplankton variations in response to freshwater inputs and water co
lumn structure in a shallow (< 25 m) semi-enclosed Mediterranean gulf
[Gulf of Trieste, Adriatic Sea) are described. Hydrographic profiles a
nd in situ fluorescence were recorded at high resolution, while nutrie
nts and phytoplankton were analysed at 5 pre-set depths. Analyses of t
he phytoplankton community structure were completed by determinations
of the pigment pool using reverse-phase high-performance liquid chroma
tography and size-fractionated primary production. The phytoplankton s
tanding crop, community structure and primary production were profound
ly influenced by freshwater inputs and water column stratification. Th
e water column was well mixed and supported modest phytoplankton conce
ntrations during winter. A spring diatom bloom (Skeletonema costatum,
Nitzschia spp., Chaetoceros spp.) developed in response to major fresh
water input to the surface layer; a similar increase in diatoms (Rhizo
solenia delicatula, R. fragilissima) followed October-November freshet
s. A shift from diatom-dominated winter-spring phytoplankton to a flor
a with increased numbers of flagellates in the stratified water column
during late spring-summer was reflected in the composition of accesso
ry pigments [lower fucoxanthin: 19'-hexanoyloxyfucoxanthin ratio, an i
ncrease of chlorophyll b (chi b) and peridinin concentration]. Primary
productivity was correlated with chi a biomass and showed surface pea
ks in April and June. The 2 to 10 mu m phytoplankton contributed signi
ficantly to the total productivity in January and June, while on an an
nual basis the most important producers were > 10 mu m. The smallest f
raction (< 2 mu m) seemed to contribute little, except in July. Our da
ta indicate that the new production resulting from land-born nutrient
inputs was the main cause of major phytoplankton blooms, while the nut
rient supply from the bottom became comparatively more important durin
g dry summer months.