Shifting Diatom—Dinoflagellate Dominance During Spring Bloom in the Baltic Sea and its Potential Effects on Biogeochemical Cycling

Abstract

The Baltic Sea is affected by a range of human induced environmental pressures such as eutrophication. Here we synthesize the ongoing shift from diatom dominance toward more dinoflagellates in parts of the Baltic Sea during the spring bloom and its potential effects on biogeochemical cycling of key elements (e.g., C, N, and P). The spring bloom is the period with the highest annual primary production and sinking of organic matter to the sediment. The fate of this organic matter is a key driver for material fluxes, affecting ecosystem functioning and eutrophication feedback loops. The dominant diatoms and dinoflagellates appear to be functionally surrogates as both groups are able to effectively exhaust the wintertime accumulation of inorganic nutrients and produce bloom level biomass that contribute to vertical export of organic matter. However, the groups have very different sedimentation patterns, and the seafloor has variable potential to mineralize the settled biomass in the different sub-basins. While diatoms sink quickly out of the euphotic zone, dinoflagellates sink as inert resting cysts, or lyse in the water column contributing to slowly settling phyto-detritus. The dominance by either phytoplankton group thus directly affects both the summertime nutrient pools of the water column and the input of organic matter to the sediment but to contrasting directions. The proliferation of dinoflagellates with high encystment efficiency could increase sediment retention and burial of organic matter, alleviating the eutrophication problem and improve the environmental status of the Baltic Sea.

Functional Differences in the Blooming Phytoplankton Heterosigma akashiwo and Prorocentrum donghaiense Revealed by Comparative Metaproteomics

“…Long-term field investigations have revealed that Prorocentrum donghaiense, Heterosigma akashiwo, and other phytoplankton species cause extensive blooms from spring to early summer in this area (7,36,37). Similar bloom events and distinct ecological niches of these HAB species have also been reported in other coastal areas (38)(39)(40)(41). However, the mechanisms that drive bloom occurrence of different phytoplankton species are unclear.…”

Climate Driven Changes in Timing, Composition and Magnitude of the Baltic Sea Phytoplankton Spring Bloom

“…Third, if climate becomes more cloudy and windy this could effectively delay the spring bloom, and thus increase the phytoplankton-zooplankton overlap period. A lower energy input to the benthic system would reasonably reduce benthic production above the halocline, but cause less pressure on benthic oxygen consumption and potentially decreasing anoxic zones (Spilling et al, 2018). However, higher water temperatures have been suggested to increase the oxygen consumption and anoxia below the halocline (Kabel et al, 2012;Meier et al, 2012).…”

Decadal shifts of coastal microphytoplankton communities in a semi-enclosed bay of NW Mediterranean Sea subjected to multiple stresses

“…during P 2 suggested a nutrient depletion condition or a change in nutrient supply ratios. Also, as mixotrophic dinoflagellates were known to outcompete diatoms in organic nutrient conditions (Spilling et al, 2018), the change observed within the community may also suggest the impact of organic matter input from the Eygoutier River. It is interesting to note that such hypothesis contrast with the few available studies investigating the impact of freshwater runoff on phytoplankton communities in coastal northwestern subbasin due to specific trophic state conditions.…”