Abstract
The algal spring bloom in the Baltic Sea represents an anomaly from the winter-spring bloom patterns worldwide in terms of frequent and recurring dominance of dinoflagellates over diatoms. Analysis of approximately 3500 spring bloom samples from the Baltic Sea monitoring programs revealed (i) that within the major basins the proportion of dinoflagellates varied from 0.1 (Kattegat) to >0.8 (central Baltic Proper), and (ii) substantial shifts (e.g. from 0.2 to 0.6 in the Gulf of Finland) in the dinoflagellate proportion over four decades. During a recent decade (1995–2004) the proportion of dinoflagellates increased relative to diatoms mostly in the northernmost basins (Gulf of Bothnia, from 0.1 to 0.4) and in the Gulf of Finland, (0.4 to 0.6) which are typically ice-covered areas. We hypothesize that in coastal areas a specific sequence of seasonal events, involving wintertime mixing and resuspension of benthic cysts, followed by proliferation in stratified thin layers under melting ice, favors successful seeding and accumulation of dense dinoflagellate populations over diatoms. This head-start of dinoflagellates by the onset of the spring bloom is decisive for successful competition with the faster growing diatoms. Massive cyst formation and spreading of cyst beds fuel the expanding and ever larger dinoflagellate blooms in the relatively shallow coastal waters. Shifts in the dominant spring bloom algal groups can have significant effects on major elemental fluxes and functioning of the Baltic Sea ecosystem, but also in the vast shelves and estuaries at high latitudes, where ice-associated cold-water dinoflagellates successfully compete with diatoms.
Highlights
On a geological time scale, the development of ocean and atmosphere chemistry has been highly integrated with the evolution of photosynthesis in the ocean, and still today, approximately half the global C-fixation takes place in the sea [1]
We used standard deviations of the predicted annual values from the generalized additive models (GAM) models to compare which of the two groups revealed higher long-term biomass variability
In the Gulf of Finland, the biomass of both groups varied with almost equal amplitude over time, while in the other basins diatoms were the more variable component of the spring bloom (Fig. 2 a panels)
Summary
On a geological time scale, the development of ocean and atmosphere chemistry has been highly integrated with the evolution of photosynthesis in the ocean, and still today, approximately half the global C-fixation takes place in the sea [1] This production is mainly carried out in the free water masses by phytoplankton, which is a highly heterogeneous group of microscopic algae. 250 Myr ago [4], and in the contemporary oceans they contribute a major part of the primary production These two phylogenetic groups exhibit unique and distinct, often contrasting adaptive ecologies, explaining their global niche partitioning on the turbulence-nutrient matrix of habitats and onshore-offshore gradient [5]. Dinoflagellate blooms, in contrast, have low species diversity, and exhibit a rudimentary species succession, if any [9]
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