engleski

Effect of thermohaline circulation on diatom distribution in the open South Adriatic Sea

Dynamics of a diatom community was studied during winter-spring period in span of two years that differed with respect to the oceanographic conditions induced by specific hydroclimatic events in southern Adriatic. Sampling station was situated in the South Adriatic Pit (SAP) with depth of 1200 m. Altogether, 16 research cruises were undertaken and in total 232 bottle samples were observed for physical-chemical and phytoplankton analizes. Two specific phenomena were recorded in SAP during investigated periods: high abundances of diatoms in the aphotic zone associated with strong downward flow, and intense surface diatom bloom in 2017 induced by strong vertical convective mixing which increase nutrient availability in surface layer. Diatoms were the most diverse and abundant group of microphytoplankton (MICRO) during investigated periods, classified into 102 taxa of 48 genera. Contribution of diatoms in MICRO abundance ranged from 8.3 to 97.9% in 2016 and from 0.4 to 99.6% in 2017 with intense development and contribution during bloom in March 2017. At that time (3th and 20th of March) diatom abundances ranged from 6.1x102 to 5x104 cells L-1 and 2x102 to 1.9 x105 cells L- 1, respectively. In the begining of bloom (3th of March), intensive development of diatoms was recorded in surface layer (0-50 m). During the peak (20th of March) high abundances spread to 300 m. Four days later (24th of March), diatom abundance decreased in the surface layer and maintained in the reduced values in the deep samples (under 75 m). The diatom community during the winter-spring period was characterized by atypical mix of benthic, epilitic and planktonic species with Asterionellopsis glacialis (Castrac.) Round (2.4x104 cells L-1), Chaetoceros affinis Lauder (2.5 x104 cells L-1), Chaetoceros lorenzianus Grunow (2.3 x104 cells L-1), unidentified Chaetoceros Ehrenb. spp. (2.2 x 104 cells L-1), and Pseudo-nitzschia H. Perag. spp. (4.5 x104 cells L-1) as the most abundant taxa. The great contribution of coastal species at significant distance from the coast suggests the strong influence of different water masses in this area. Such a high abundances of diatoms, accompanied by a high concentration of chlorophyll a (>1 µg L-1), indicated that area of SAP is not exclusively oligotrophic in the winter. Winter- early spring in the open water of South Adriatic could be considered as a productive season with significant phytoplankton activity.

marine diatoms ; winter-spring period ; South Adriatic Pit (SAP) ; diatom bloom ; vertical convective mixing

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