Search

Krill represent a major link between primary producers and higher trophic levels in polar marine food webs. Potential links to lower trophic levels, such as heterotrophic microorganisms, are less well documented. Here, we studied the kinetics of microbial degradation of sinking carcasses of two dominant krill species Thysanoessa raschii and Meganyctiphanes norvegica from Southwest Greenland. Degradation experiments under oxic conditions showed that 6.0-9.1% of carbon and 6.4-7.1% of nitrogen were lost from the carcasses after one week. Aerobic microbial respiration and the release of dissolved organic carbon were the main pathways of carbon loss from the carcasses. Ammonium release generally contributed the most to carcass nitrogen loss. Oxygen micro profiling revealed anoxic conditions inside krill carcasses/specimens, allowing anaerobic nitrogen cycling through denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates were up to 5.3 and 127.7 nmol N carcass-1 d-1 for T. raschii and M. norvegica, respectively, making krill carcasses hotspots of nitrogen loss in the oxygenated water column of the fjord. Carcass-associated DNRA rates were up to 4-fold higher than denitrification rates, but the combined activity of these two anaerobic respiration processes did not contribute significantly to carbon loss from the carcasses. Living krill specimens did not harbor any significant denitrification and DNRA activity despite having an anoxic gut as revealed by micro profiling. The investigated krill carcasses sink fast (1500-3000 m d-1) and our data show that only a small fraction of the associated carbon is lost during descent. Based on data on krill distribution, our findings are used to discuss the potential importance of sinking krill carcasses for sustaining benthic food webs in the Arctic.

Plankton succession was studied in a hyper-eutrophic stratified estuary, Mariager Fjord, Denmark. Above the pycnocline (15 m) pH increased from 8.5 to 9.2 and the oxygen increased to super saturation after 5 d of sunny weather due to high primary production. The protistan grazers were dominated by heterotrophic dinoflagellates and mixotrophic and heterotrophic ciliates. Metazooplankton was dominated by meroplankton, rotifers and the copepod, Acartia tonsa, all with a relatively low biomass. Cirriped nauplii occupied the upper strata while polychaete larvae populated the whole water column. Bivalve larvae occurred occasionally above the pycnocline even at very high pH. In pH challenge experiments, the mixotrophic ciliate Mesodinium rubrum was the least pH tolerant species, followed by Strombidium spp., which did not cope well with seawater pH > 8.5. Some heterotrophic dinoflagellates were more tolerant with net growth at pH > 9. The predominant rotifer Synchaeta sp. tolerated up to pH 9.5 and the copepod survived pH 10 but stopped producing eggs at pH 9.5 with unaffected egg hatching success. The polychaete and cirriped larvae tolerated pH 9.5, but bivalve larvae showed decreased survival already at pH 8.5. In situ distribution patterns and pH challenge experiments suggest that pH indeed contribute to structuring zooplankton distribution.

In Antarctica, summer is a time of extreme environmental shifts resulting in large coastal phytoplankton blooms fueling the food web. Despite the importance of the microbial loop in remineralizing biomass from primary production, studies of how microzooplankton communities respond to such blooms in the Southern Ocean are rather scarce. Microzooplankton (ciliate and dinoflagellate) communities were investigated combining microscopy and 18S rRNA sequencing analyses in the Amundsen Sea Polynya during an extensive summer bloom of Phaeocystis antarctica. The succession of microzooplankton was further assessed during a 15-day induced bloom microcosm experiment. Dinoflagellates accounted for up to 59 % of the microzooplankton biomass in situ with Gymnodinium spp., Protoperidium spp. and Gyrodinium spp. constituting 89 % of the dinoflagellate biomass. Strobilidium spp., Strombidium spp. and tintinids represented 90 % of the ciliate biomass. Gymnodinium, Gyrodinium and tintinnids are known grazers of Phaeocystis, suggesting that this prymnesiophyte selected for the key microzooplankton taxa. Availability of other potential prey, such as diatoms, heterotrophic nanoflagellates and bacteria, also correlated to changes in microzooplankton community structure. Overall, both heterotrophy and mixotrophy appeared to be key trophic strategies of the dominant microzooplankton observed, suggesting that they influence carbon flow in the microbial food web through top-down control on the phytoplankton community

Within zooplankton, copepods are the most abundant organisms in marine ecosystems. Copepods of the genus Oithona are present in polar, temperate, subtropical, and tropical waters; in epipelagic and mesopelagic environments; in estuaries, mangroves, continental shelves, and in the open ocean. They are often the most abundant copepod, and they play a key role in biogeochemical cycles in pelagic marine food webs. Despite their recognized ecological importance in pelagic environments, the genus Oithona is one of the least studied groups of copepods, and there are many unknown aspects of their biology, ecophysiology, and ecological role in marine ecosystems. Here we present results on the natural diet of Oithona spp. based on feeding experiments with adult females. We suggest that the ambush feeding behaviour of Oithona spp. defines the prey spectrum of their natural diet, since they feed on relatively few taxonomic groups comprising mainly motile organisms. In this presentation we will briefly review the known (lights) and unknown (shadows) aspects of the feeding ecology of Oithona spp., and add new insights based on comparative experiments conducted in tropical and polar waters

Intensive sampling in coastal waters of the central Red Sea during a period of thermal stratification showed that vertical patches of prokaryotes and nano- and microplankton developed and persisted for at least a week within the apparently well mixed layer. This vertical structure was most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioral aggregation. Adding nutrient-rich deep water simulating a mixing event triggered dense phytoplankton blooms in this nutrient-poor environment. These findings have implications for present bloom-forming theories because suggest that vertical structure within the mixed layer provides a critical seed mass that can rapidly exploit nutrient influx during early seasonal mixing.

Intensive sampling at the coastal waters of the central Red Sea during a period of thermal stratification, prior to the main seasonal bloom during winter, showed that vertical patches of prokaryotes and microplankton developed and persisted for several days within the apparently density uniform upper layer. These vertical structures were most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioural aggregation. Simulating a mixing event by adding nutrient-rich deep water abruptly triggered dense phytoplankton blooms in the nutrient-poor environment of the upper layer. These findings suggest that vertical structures within the mixed layer provide critical seeding stocks that can rapidly exploit nutrient influx during mixing, leading to winter bloom formation

The biomass, production and consumption of phytoplankton, bacteria and zoo- plankton in a shallow Danish estuary (Limfjord) were analysed during a 9-day period. The water column changed between stratified and mixed conditions which influenced the dominant processes in the pelagic system. During strong stratification, phytoplank- ton was mainly controlled by microzooplankton grazing. A mixing event, which homogenized the water column, possibly provided food to a mussel-dominated benthic community. Concomitantly, zooplankton feeding and reproduction decreased. However, the nutrient input to the upper part of the water column during mixing and the subsequent stabilization provided the ideal conditions for the recovery of phytoplankton from the loss processes from previous days. Microzooplankton, which was also a significant consumer of bacteria throughout the sampling period, was not the only consumer controlling phytoplankton. The microbial food web was an important route for total plankton carbon during the study. However, the shorter food web increased in importance during mixing, possibly due to stronger benthic–pelagic coupling than during stratified periods. Stratification-mixing cycles, occurring during short-time periods, should be a key mechanism maintaining the benthic and pelagic communities in this shallow water system

The small cyclopoid copepod Oithona is widely occurring in polar areas; however, knowledge of its biology and ecology is very limited. Here, we investigate the population dynamics, vertical distribution, and reproductive characteristics of Oithona spp. from late winter to summer, in a subarctic fjord of West Greenland. During winter-early spring, the abundance of Oithona spp. was low (1.8 × 103 ind. m-2) and the population was mainly composed of late copepodites and adults, whereas in summer, abundance peaked and younger stages dominated (1.1 × 106 ind. m-2). In general, all stages of Oithona spp. remained in the upper 100 m, with nauplii exhibiting a shallower distribution. Although no general seasonal migration was found, a deeper distribution of the adult females in winter was observed. The mean clutch size of Oithona spp. varied from 16 to 30 eggs per female, peaking in summer. Egg production rates (EPR) were low in winter-early spring (0.13 ± 0.03 eggs female-1 day-1) and reached maximum values in summer (1.6 ± 0.45 eggs female-1 day-1). EPR of Oithona spp. showed a significantly positive relationship with both temperature and protozooplankton biomass, and the development of the population seemed to be appreciably affected by temperature. Oithona spp. remained active throughout the study, stressing the key importance of these small copepods in high-latitude ecosystems, especially in periods when larger copepods are not present in the surface layer. © 2014 Springer-Verlag Berlin Heidelberg

The impact of microzooplankton on the plankton community was assessed during a bloom of Phaeocystis sp. in the Oosterschelde basin (SW Netherlands). Chlorophyll a concentration and Phaeocystis sp. abundance reached maxima values in the middle of the sampling period (~ 27 mg chl a m− 3 and 3.4 × 1010 cells m− 3, respectively). The bloom collapse coincided with the last sampling day, when accentuated decreases in chlorophyll a concentrations (~ 11 mg chl a m− 3) and Phaeocystis sp. cells numbers (~ 1.3 × 1010 cells m− 3) were recorded. Microzooplankton organisms were significant consumers of both phytoplankton and heterotrophic plankton. Although Phaeocystis sp. was the most consumed organism (336 ± 71 mg C m− 3 d− 1), microzooplankton impact on its standing stock was lower than on the stocks of other less abundant organisms. This impact was also lower during the peak of the bloom, when colonial forms of Phaeocystis sp. presumably predominated, than during the bloom collapse, when free-living cells were supposedly more abundant. The impact of microzooplankton on heterotrophic organisms was higher than on phytoplankton, and increased when the bloom collapsed. Picoheterotrophs experienced the highest impact on their standing stock (~ 75% d− 1) and production (~ 90% d− 1). These results demonstrate that during a Phaeocystis sp. bloom, the microbial food web was responsible for channelling a significant fraction of plankton biomass, either from direct consumption of Phaeocystis sp. cells or through consumption of heterotrophs, which would have been favoured by the high quantities of organic matter released during the bloom collapse.

Motility patterns and egg production were investigated in two populations of Acartia tonsa, field animals from the Öresund and laboratory animals from a 12-year-old (≈ 120 generations) culture. When observed in aquaria with a layer of Thalassiosira weissffogii in the middle, laboratory animals displayed weak aggregation behaviour, while field animals did not aggregate at all. Both populations made longer and more frequent feeding bouts inside the patch. Egg production measurements were in accordance with the behaviour of the laboratory population if no diel feeding rhythm was assumed. The field population produced fewer eggs than predicted from activity measurements, probably due to a diel feeding rhythm. It is concluded that laboratory reared A. tonsa can be used for experiments involving behaviour, but that the possible loss of diel rhythms should be a concern. Both populations differed considerably from field-caught A. tonsa from the eastern United States, where both behaviour and egg production changed consistently and in accordance with a strong aggregation in food patches. © 1995