Phytoplankton abundance and pigment biomarkers in the oligotrophic, eastern Adriatic estuary

Abstract

Phytoplankton distribution and environmental characteristics were determined in a shallow, highly stratified and oligotrophic estuary (Zrmanja, eastern Adriatic). Samples were collected in two contrasting seasons; winter (February 2000), when river discharge was high, and in summer (July 2003), a period of drought. Phytoplankton distribution was closely related to salinity gradients, nutrient levels, and water residence time. Microscopic analysis revealed that phytoplankton was composed mainly of marine diatoms, dinoflagellates, cryptophytes, green flagellates, and coccolithophorids. The dominant biomarker pigments were fucoxanthin, alloxanthin and 19'-hexanoyloxyfucoxanthin, while lower, but indicative contributions of peridinin and chlorophyll b were also noted. Maximum abundance and biomass were found in the middle estuary in winter and in the upper estuary in summer. The estuary is mostly P-limited. Development of chain-forming marine diatoms was evident in winter. Due to the reduced nutrient input in summer, the biomass accumulated in the upper estuary (1,000 ng chlorophyll a l(-1)) was composed mostly of nanoplanktonic unicellular diatoms, nanoplanktonic marine dinoflagellates, cryptophytes, and chlorophytes. The concentrations of about 200 ng l(-1) hex-fuco, suggested that the contribution of prymnesiophytes to total biomass was comparable to that of diatoms and dinoflagellates. In the middle estuary and coastal sea, PO(4) and TIN were 3.5 times lower, resulting in a fivefold decrease in biomass (<100 ng chlorophyll a l(-1)). The oligotrophic Zrmanja and other karstic rivers discharging in the eastern Adriatic Sea, provide insufficient source of nutrients and low productivity of the eastern Adriatic Sea.

Distribution of phytoplankton community in Kotor Bay (south-eastern Adriatic Sea)

“…The chlorophyll a values in this study were higher than those in the more saline and less nutrient-enriched waters of the north-eastern coastal Adriatic [10], the eastern Mediterranean [6], and coastal north-western Mediterranen [43].…”

“…and Thalassionema nitzschioides. Species that prefer nutrient-enriched water were also recorded in the northern Adriatic Sea [10,12], the eastern Adriatic [51] and the middle Adriatic [13], also with differences in their abundance and frequency of appearance.…”

“…In the Adriatic Sea [4,10], phosphate was detected as a limiting factor all year round and nitrogen sporadically in the summer. Although NO 3 -is generally considered to limit primary productivity in most of the world's oceans, some studies [38] have suggested the Mediterranean Sea may be an exception.…”

“…The north-eastern part of the Adriatic Sea is influenced by currents and oligotrophic karstic rivers which bring waters poor in nutrients and with higher salinity [10]. In the north-eastern Adriatic Sea, diatoms such as Cerataulina pelagica, Chaetoceros socialis, and Pseudo-nitzschia spp., often dominate the plankton community, as first noticed by Viličić et al [11].…”

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“…(Drakulović, Pestorić, Cvijan, Krivokapić, & Vuksanović, 2012). Generally, in the Adriatic Sea (Burić et al, 2007;Viličić et al, 2008), limited nutrient is phosphate almost throughout the year, while nitrogen availability is sporadically limited in the summer. Negative significant correlation between nitrate and silicate on one side and salinity on other indicates an influence of river runoff during periods of high precipitation.…”