Spring-summer transition in the Curonian lagoon (SE Baltic Sea) phytoplankton community

Abstract

1 - The Curonian lagoon is a shallow, mostly freshwater estuary. In the northern part it is connected to the south-eastern Baltic Sea, while the main inflow is dominated by the freshwater discharges from the Nemunas river and directed towards the sea. Phytoplankton development in the lagoon generally follows a pattern typical of eutrophic fresh waters: in the spring diatoms dominate, while during summer filamentous cyanobacteria prevail. 2 - Hydrological summer in the temperate waters starts when the thermocline is steady. However, the Curonian lagoon is very shallow, with a mean depth of 3.8 m, and no thermocline could be observed. Moreover, the exact timing of the shift between the spring and summer succession stages does not synchronise with calendar seasons and cannot be defined in calendar days. Therefore, the aim of this study was to describe spring and summer phytoplankton communities and delineate conditions when the shift between spring and summer seasons occurs in the eutrophic Curonian lagoon. This study was based on the structure of spring and summer phytoplankton communities according to the taxa, size and morphological features. 3 - Two main seasonal phytoplankton communities (spring and summer) were described as well as the timing of the transition between them in the temperate Curonian lagoon. Phytoplankton communities diverged into two groups according to the taxa and morphological features, and into three groups according to the size structure. The first "spring" group derived from all examined features contains mainly coccoid Centrales, with cells of 10-20 µm. The second (second and third according to the size structure) "summer" group was dominated by filamentous Cyanobacteria. Structure and taxonomical composition differences of two "summer" groups as well as mechanisms responsible for the seasonal shift were discussed. 4 - The shift from spring to summer communities was found to be decided by the cumulative sum of daily water temperatures and occurs when the sum of cumulative degrees is between 587 and 627.