Ulf Riebesell

Mass aggregation of diatom blooms: Insights from a mesocosm study

While the aggregation and mass settlement of diatoms at the termination of blooms results in significant export of carbon from the surface ocean, the mechanisms of bloom aggregation have been poorly understood. The aggregation of a multispecies diatom bloom was investigated under controlled conditions in a 1200 liter, nutrient-enriched, laboratory mesocosm in order to elucidate the parameters sufficient to accurately predict bloom aggregation. A diverse bloom of diatoms dominated by several species of Chaetoceros and Thalassiosira progressed through a classic pattern of exponential, stationary, and senescent phases in the mesocosm. Aggregates larger than 0.5 mm became detectable on the eighth day after inoculation, and aggregates >1 mm increased exponentially from Day 10 onward producing the appearance of a mass aggregation event late on Day 10. The bloom aggregated sequentially with Thalassiosira dominating early aggregates and Chaetoceros dominating later ones. Chaetoceros resting spores formed only in aggregates. Aggregation was not linked to nutrient depletion or to the physiological state of the cells since the onset of aggregation and the mass aggregation event occurred 1 to 3 days prior to nutrient depletion and while carbon:nitrogen ratios of cells were still very low and growth rates high. Moreover, visible aggregates did not form in the mesocosm until cell abundances were considerably higher than abundances observed to aggregate in nature, suggesting that aggregation was not strongly linked to phytoplankton cell concentration. Complementary studies in this volume clarify the role of non-phytoplankton particles in aggregation of the mesocosm bloom. The mesocosm approach proved highly effective in producing an aggregating diatom bloom under controlled conditions.

Quantifying the time lag between organic matter production and export in the surface ocean: Implications for estimates of export efficiency

The oceans potential to export carbon to depth partly depends on the fraction of primary production (PP) sinking out of the euphotic zone (i.e., the e-ratio). Measurements of PP and export flux are often performed simultaneously in the field, although there is a temporal delay between those parameters. Thus, resulting e-ratio estimates often incorrectly assume an instantaneous downward export of PP to export flux. Evaluating results from four mesocosm studies, we find that peaks in organic matter sedimentation lag chlorophyll a peaks by 2 to 15u2009days. We discuss the implications of these time lags (TLs) for current e-ratio estimates and evaluate potential controls of TL. Our analysis reveals a strong correlation between TL and the duration of chlorophyll a buildup, indicating a dependency of TL on plankton food web dynamics. This study is one step further toward time-corrected e-ratio estimates

Video of the sampling strategy to empty sediment traps of the “Kiel Off-Shore Mesocosms for future Ocean Simulations” (KOSMOS)

We deployed KOSMOS units in different climate zones and marine ecosystem types between 2011 and 2015, in order to study the influence of ocean acidification on the succession of plankton communities under in-situ conditions. Settling particulate matter within the mesocosms was quantitatively collected in sediment traps attached to the bottom of the mesocosms. We applied a low vacuum sampling strategy to empty these particle traps through silicon tubes reaching down from the sea surface outside of the mesocosm enclosures. The video shows the setup of the traps and the sample recovery from small boats attached to the KOSMOS units. n nThe video can be downloaded from the OceanRep server (GEOMAR) but is also available on the KOSMOS channel of the streaming platform YouTube.

Mating Clione limacina (Phipps, 1774)

In July 2010 the authors observed two mating specimens of Clione limacina (Phipps, 1774) in the surface water of the Arctic Kongsfjord, Svalbard (78° 57.6564 N, 12° 2.55558 E). Clione limacina is a holopelagic gastropod mollusc that belongs to the monophyletic group Pteropoda, which members are characterized by the possession of paired, wing-like appendages. These wings constitute a modified gastropod foot and are used for active swimming. Two subgroups of pteropods exist, the shelled Thecosomata and the unshelled Gymnosomata. Gymnosome species are hard to study for a variety of reasons and thus our knowledge on these animals is still very limited, in particular when it comes to behavioral observations. Here we present rare video footage of two mating specimens of Clione limacina, which have been transferred into a plastic bowl for closer observation. Specimens were facing each other on their ventral sides and kept swimming slowly during the whole period. The cephalic copulatory apparatus, which contains a long and slender accessory organ and a penis is everted in both specimens. The free accessory copulatory organs are wrapped around the partner and the penes are inserted into the matesO posterior genital opening. The transfer of a spermatophore from the anterior male genital opening along the penis is clearly visible. Fertilization is reported to be reciprocal in this species. Although the mating process has been described previously, the functional physiology of involved anatomical structures (i.e. complex glandular systems) in the formation of the spermatophores and spawn remains obscure.

Ozeanversauerung: Gewinner und Verlierer im Plankton

Durch die Verbrennung fossiler Energietrager und veranderte Bodennutzung steigt der Gehalt an Kohlendioxid in der Atmosphare. Ein groser Teil des Kohlendioxids wird von den Ozeanen aufgenommen und reagiert dort mit dem Meerwasser. Durch die Bildung von Kohlensaure versauern mit zunehmender Kohlendioxidaufnahme die Meere. Wahrend kalkbildende Organismen (Kalzifizierer) durch die Versauerung beeintrachtigt werden, profitieren moglicherweise andere, meist nicht-kalzifizierende, Organismen von der Ozeanversauerung. Es ist daher wahrscheinlich, dass sich als Folge der Versauerung die marinen Okosysteme verandern und ihre Artenvielfalt verringern wird.

Performance of herring larvae in a simulated future ocean food web, using the "Kiel Off-Shore Mesocosms for future Ocean Simulations"

We studied the combined direct physiological and indirect food web effects of ocean acidification on herring larvae inside pelagic mesocosms. A natural plankton community of the Gullmarsfjord, Sweden was enclosed in the Kiel Off-Shore Mesocosms for future Ocean Simulations (KOSMOS) for 113 days from March to June 2013 at ambient and projected end-of-the-century CO2 levels (~760 µatm pCO2). Herring eggs were introduced into the mesocosms, where they hatched in mid of May. The larvae developed inside the mesocosms for ~6 weeks, feeding on prey organisms that experienced treatment CO2 levels for ~9 weeks. This video is meant as an illustration of the herring larvae«s performance inside our mesocosm units.

Video of a plankton community enclosed in a “Kiel Off-Shore Mesocosm for future Ocean Simulations” (KOSMOS) during a study in Tvärminne Storfjärden (Finland) 2012

In June 2012 we deployed six 19 m long KOSMOS units for 8 weeks in the Tvarminne Storfjarden (59° 51.5’ N, 23° 15.5’ E), an open archipelago area on the south-west corner of Finland, in order to study the influence of ocean acidification on the succession of a plankton community under in-situ conditions in the Baltic Sea. At the beginning of the study (18th of June), we mounted an underwater camera on a diving torch and slowly lowered this setup to a depth of approximately 18.5 m. The video during camera descent shows the post-spring bloom plankton community that we studied within the KOSMOS system. In contrast to mesocosm studies in other regions, the plankton community was less lively: there was a comparatively high number of organic matter aggregations and low abundance of zooplankton. The conical mesocosm sediment trap can be seen at the end of the video in 19 m water depth. The results of this mesocosm study are presented in Biogeosciences in a Special Issue Effects of rising CO2 on a Baltic Sea plankton community: ecological and biogeochemical impacts, currently in Biogeosciences Discussion. nThe video can be downloaded on OceanRep but is also available on the KOSMOS channel of the streaming platform YouTube.

Video of a plankton community enclosed in a "Kiel Off-Shore Mesocosm for future Ocean Simulations" (KOSMOS) during the long-term study in Gullmar Fjord (Sweden) 2013

From January to July 2013 we deployed ten 19 m long KOSMOS units in a Swedish fjord (Gullmar Fjord, 58° 16’008 N, 11° 28’680E) close to the city of Lysekil, in order to study the influence of ocean acidification on a natural winter-to-summer succession of a plankton community under in-situ conditions. Towards the end of the study (21st of May), we mounted an underwater camera on a diving torch and slowly lowered this setup to a depth of approximately 18.5 m. A variety of different organisms (copepods, fish larvae, jelly fish) and detrital aggregates can be seen during the descent of the camera into the deep. The conical mesocosm sediment trap and particles collected in the funnel can be seen at the end of the video in 19 m water depth. The video shows that we enclosed and studied a lively plankton community within the KOSMOS system.

Video of a plankton community enclosed in a “Kiel Off-Shore Mesocosm for future Ocean Simulations” (KOSMOS) during the SOPRAN study in Raunefjord (Norway) 2011

In May 2011 we deployed nine 25 m long KOSMOS units for 5 weeks in a Norwegian fjord (Raunefjord, 60.265°N, 5.205°E) close to the city of Bergen, in order to study the influence of ocean acidification on the succession of a plankton community under in-situ conditions. Towards the end of the study (7th of June), we mounted an underwater camera on a diving torch and slowly lowered this setup to a depth of approximately 24.5 m. A variety of different organisms (copepods, fish larvae, jelly fish, appendicularians) and detrital aggregates can be seen during the descent of the camera into the deep. The conical mesocosm sediment trap and particles collected in the funnel can be seen at the end of the video in 25 m water depth. The video shows that we enclosed and studied lively plankton communities within the KOSMOS system. nThe video can be downloaded on OceanRep but is also available on the KOSMOS channel of the streaming platform YouTube.