Nitrogen fixation by cyanobacteria stimulates production in Baltic food webs.

Filamentous, nitrogen-fixing cyanobacteria form extensive summer blooms in the Baltic Sea. Their ability to fix dissolved N2 allows cyanobacteria to circumvent the general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for the food web. However, the fate of the nitrogen fixed by cyanobacteria remains unresolved, as does its importance for secondary production in the Baltic Sea. Here, we synthesize recent experimental and field studies providing strong empirical evidence that cyanobacterial nitrogen is efficiently assimilated and transferred in Baltic food webs via two major pathways: directly by grazing on fresh or decaying cyanobacteria and indirectly through the uptake by other phytoplankton and microbes of bioavailable nitrogen exuded from cyanobacterial cells. This information is an essential step toward guiding nutrient management to minimize noxious blooms without overly reducing secondary production, and ultimately most probably fish production in the Baltic Sea.

Bloom-forming cyanobacteria support copepod reproduction and development in the Baltic Sea.

It is commonly accepted that summer cyanobacterial blooms cannot be efficiently utilized by grazers due to low nutritional quality and production of toxins; however the evidence for such effects in situ is often contradictory. Using field and experimental observations on Baltic copepods and bloom-forming diazotrophic filamentous cyanobacteria, we show that cyanobacteria may in fact support zooplankton production during summer. To highlight this side of zooplankton-cyanobacteria interactions, we conducted: (1) a field survey investigating linkages between cyanobacteria, reproduction and growth indices in the copepod Acartia tonsa; (2) an experiment testing relationships between ingestion of the cyanobacterium Nodularia spumigena (measured by molecular diet analysis) and organismal responses (oxidative balance, reproduction and development) in the copepod A. bifilosa; and (3) an analysis of long term (1999-2009) data testing relationships between cyanobacteria and growth indices in nauplii of the copepods, Acartia spp. and Eurytemora affinis, in a coastal area of the northern Baltic proper. In the field survey, N. spumigena had positive effects on copepod egg production and egg viability, effectively increasing their viable egg production. By contrast, Aphanizomenon sp. showed a negative relationship with egg viability yet no significant effect on the viable egg production. In the experiment, ingestion of N. spumigena mixed with green algae Brachiomonas submarina had significant positive effects on copepod oxidative balance, egg viability and development of early nauplial stages, whereas egg production was negatively affected. Finally, the long term data analysis identified cyanobacteria as a significant positive predictor for the nauplial growth in Acartia spp. and E. affinis. Taken together, these results suggest that bloom forming diazotrophic cyanobacteria contribute to feeding and reproduction of zooplankton during summer and create a favorable growth environment for the copepod nauplii.

Responses of phyto- and zooplankton communities to Prymnesium polylepis (Prymnesiales) bloom in the Baltic Sea.

A large bloom of Prymnesium polylepis occurred in the Baltic Sea during the winter 2007-spring 2008. Based on numerous reports of strong allelopathic effects on phytoplankton exerted by P. polylepis and its toxicity to grazers, we hypothesized that during this period negative correlations will be observed between P. polylepis and (1) main phytoplankton groups contributing to the spring bloom (i.e., diatoms and dinoflagellates), and (2) zooplankton growth and abundance. To test these hypotheses, we analyzed inter-annual variability in phytoplankton and zooplankton dynamics as well as growth indices (RNA:DNA ratio) in dominant zooplankton in relation to the Prymnesium abundance and biomass. Contrary to the hypothesized relationships, no measurable negative responses to P. polylepis were observed for either the total phytoplankton stocks or the zooplankton community. The only negative response, possibly associated with P. polylepis occurrence, was significantly lower abundance of dinoflagellates both during and after the bloom in 2008. Moreover, contrary to the expected negative effects, there were significantly higher total phytoplankton abundance as well as significantly higher winter abundance and winter-spring RNA:DNA ratio in dominant zooplankton species in 2008, indicating that P. polylepis bloom coincided with favourable feeding conditions for zooplankton. Thus, primary consumers, and consequently also zooplanktivores (e.g., larval fish and mysids), may benefit from haptophyte blooms, particularly in winter, when phytoplankton is scarce.

Assessing impacts of invasive phytoplankton: the Baltic Sea case.

There is an increasing understanding and requirement to take into account the effects of invasive alien species (IAS) in environmental quality assessments. While IAS are listed amongst the most important factors threatening marine biodiversity, information on their impacts remains unquantified, especially for phytoplankton species. This study attempts to assess the impacts of invasive alien phytoplankton in the Baltic Sea during 1980-2008. A bioinvasion impact assessment method (BPL - biopollution level index) was applied to phytoplankton monitoring data collected from eleven sub-regions of the Baltic Sea. BPL takes into account abundance and distribution range of an alien species and the magnitude of the impact on native communities, habitats and ecosystem functioning. Of the 12 alien/cryptogenic phytoplankton species recorded in the Baltic Sea only one (the dinoflagellate Prorocentrum minimum) was categorized as an IAS, causing a recognizable environmental effect.

GYMNODINIUM COROLLARIUM SP. NOV. (DINOPHYCEAE)-A NEW COLD-WATER DINOFLAGELLATE RESPONSIBLE FOR CYST SEDIMENTATION EVENTS IN THE BALTIC SEA(1).

A naked dinoflagellate with a unique arrangement of chloroplasts in the center of the cell was isolated from the northern Baltic proper during a spring dinoflagellate bloom (March 2005). Morphological, ultrastructural, and molecular analyses revealed this dinoflagellate to be undescribed and belonging to the genus Gymnodinium F. Stein. Gymnodinium corollarium A. M. Sundström, Kremp et Daugbjerg sp. nov. possesses features typical of Gymnodinium sensu stricto, such as nuclear chambers and an apical groove running in a counterclockwise direction around the apex. Phylogenetic analyses based on partial nuclear-encoded LSU rDNA sequences place the species in close proximity to G. aureolum, but significant genetic distance, together with distinct morphological features, such as the position of chloroplasts, clearly justifies separation from this species. Temperature and salinity experiments revealed a preference of G. corollarium for low salinities and temperatures, confirming it to be a cold-water species well adapted to the brackish water conditions in the Baltic Sea. At nitrogen-deplete conditions, G. corollarium cultures produced small, slightly oval cysts resembling a previously unidentified cyst type commonly found in sediment trap samples collected from the northern and central open Baltic Sea. Based on LSU rDNA comparison, these cysts were assigned to G. corollarium. The cysts have been observed in many parts of the Baltic Sea, indicating the ecologic versatility of the species and its importance for the Baltic ecosystem.

Particle-water partitioning of PCBs in the photic zone: a 25-month study in the open Baltic Sea.

From previous laboratory and field studies, it remains unclear whether partitioning of hydrophobic organic contaminants (HOCs) to phytoplankton from water is kinetically limited or may be treated as an equilibrium process. Here, we report on the partitioning of polychlorinated biphenyls (PCBs) to particulate organic carbon (POC), dominated by planktonic primary production, in the open Baltic Sea during a 25-month period. The organic carbon-normalized partition coefficient (Koc) was corrected for temperature, salinity, and sorption to filter-passing organic carbon. At all 21 sampling occasions, the log Koc-log Kow regression was significantly linear, despite a large variation in biogeochemical parameters such as POC concentration and composition, primary production, and phytoplankton species composition. These data strongly suggest that partitioning of PCBs to POC in temperate surface waters is equilibrated and therefore not kinetically limited by factors such as rapid phytoplankton growth rate or large cell size. The partitioning of PCBs to the POC was described throughout seasonal cycles by log Koc = 0.88 +/- 0.07 log Kow + 0.90 +/- 0.47 (95% confidence interval). The slope of the log Koc-log Kow regression for the single sampling occasions varied between 0.56 and 1.25, and there was a seasonal variation in the POC sorbent quality (e.g., log Koc for PCB 28 varied between 5.5 and 6.9; median 5.9). These variations reflect the variability in structural composition of the POC pool in such pelagic waters. Being able to predict particle-water partitioning of HOCs significantly reduces the required complexity of both food web uptake models and predictions of POC-mediated export of HOCs to the deep ocean.