Vertical distribution and settling of spring phytoplankton in the offshore NW Baltic Sea proper

Abstract

We studied the vertical distribution and settling of dominant diatoms and dinoflagellates during the 1996 spring phytoplankton bloom in the offshore NW Baltic Sea proper. We sampled phytoplankton at 11 depths (to 80 m) and collected settling cells in sediment traps at 25, 50 and 100 m depth, every week from March 26 to May 7. Phytoplankton were counted and sized from both water and trap samples, to estimate the share of phytoplankton in bulk settling carbon. Diatoms, mainly Chaetoceros spp. and Achnanthes taeniata, dominated the early bloom, but were replaced by the dinoflagellates cf. Scrippsiella hangoei and Peridiniella catenata when inorganic nitrogen was depleted above the seasonal pycnocline at ca. 10 m depth. By late April, vertically migrating dinoflagellates had depleted inorganic nitrogen down to 30 m, well below the seasonal pycnocline. We found clear species-specific sedimentation patterns. Scrippsiella hangoei and Chaetoceros spp., which dominated in the water column, were clearly underrepresented in the traps, while Thalassiosira baltica and T. levanderi, which were sparse in the water column, were overrepresented in sediment traps. Only 4, 3 and 0.5 g C m -2 (or 16, 12 and 2% of phytoplankton primary production) settled as intact phytoplankton cells at 25, 50 and 100 m, respectively, although these numbers may be overestimated due to migrating P. catenata. The settling bulk carbon was ~3 g C m -2 or 12% of the primary production at all depths. This is low compared to other estimates from coastal waters and suggests additional loss mechanisms, e.g. disintegration in the water column and grazing by zooplankton overwintering in the permanent halocline area.

Biomarkers in the stratified water column of the Landsort Deep (Baltic Sea)

“…The latter species was previously reported to produce dinosterol (Leblond et al, 2007). However, P. catenata as well as S. hangoei are virtually absent below 50 m water depth (Höglander et al, 2004) and can thus not account for the second peak of dinosterol at the suboxic-anoxic transition zone. An accumulation of surface-derived dinosterol at the bottom of the suboxic zone is unlikely, as the pycnocline, and thus the strongest density discontinuity, is located at 60-70 m water depth, i.e., about 20 m further above.…”

“…The strong peak in the surface layer of the oxic zone probably represents contributions from phototrophic dinoflagellates. Plausible candidates are Peridiniella catenata and Scrippsiella hangoei, both of which are involved in the spring phytoplankton blooms in the central Baltic Sea (Wasmund et al, 1998;Höglander et al, 2004). The latter species was previously reported to produce dinosterol (Leblond et al, 2007).…”

The Diatom/Dinoflagellate Index as an Indicator of Ecosystem Changes in the Baltic Sea 1. Principle and Handling Instruction

“…Figure 2), and from March to May in the Baltic Proper (see also Carstensen et al, 2004). This period covers also the spring bloom in the northern Baltic Proper (Höglander et al, 2004), Gulf of Finland (Niemi and Ray, 1977;Jaanus and Liiva, 1996), Gulf of Riga (Jurgensone et al, 2011), and the Bothnian Sea (Andersson et al, 1996). In the Bothnian Bay, the spring bloom concept is not directly applicable because the phytoplankton (diatom) growth may start rather late and reaches its peak usually only in June or July (Alasaarela, 1979).…”

“…They prefer stratification of the water column, which develops as the water temperature increases (Smayda and Reynolds, 2001). Therefore, a succession from diatoms to dinoflagellates is found within the spring bloom (Bralewska, 1992;Heiskanen, 1998;Wasmund et al, 1998;Höglander et al, 2004;van Beusekom et al, 2009;Klais et al, 2011) and the timing of this transition may have consequences for food availability to different consumers.…”

The Importance of Mesodinium rubrum at Post-Spring Bloom Nutrient and Phytoplankton Dynamics in the Vertically Stratified Baltic Sea

“…The late phase of the spring bloom (in May) in the Gulf of Finland is dominated by vertically migrating dinoflagellates together with ciliates (Heiskanen, 1995;Höglander et al, 2004;. Ciliates are an important trophic link between primary producers and metazoa consuming a significant fraction of smallsized phytoplankton and bacterioplankton production and are important in remineralization of macronutrients (Rivkin et al, 1999;Calbet and Landry, 2004).…”